SELVAM COLLEGE OF TECHNOLOGY, NAMAKKAL -03
DEPARTMENT OF MECHANICAL ENGINEERING
HYDRAULICS AND PNEUMATIC SYSTEMS
NOTES OF LESSON
Prepared by G. Selvaraj, AP/Mech.
Approved by Prof.S.Malikraj, HOD/Mech.
ANNA UNIVERSITY OF TECHNOLOGY, COIMBATORE (R – (R – 2008) HYDRAULICS AND PNEUMATIC SYSTEMS SYLLABUS HYDRAULICS AND PNEUMATIC SYSTEMS
UNIT I
FLUID POWER SYSTEMS AND FUNDAMENTALS
3
0
0
100
3 9
Introduction to fluid power, Advantages of fluid power, Application of fluid power system. Types of fluid power systems, Properties of hydraulic fluids – fluids – General types of fluids – fluids – Fluid power symbols. Basics of Hydraulics-Applications of Pascals Law- Laminar and Turbulent flow – flow – Reynold’s number – Darcy’s equation – equation – Losses in pipe, valves and fittings. UNIT II
HYDRAULIC SYSTEM AND COMPONENTS
9
Sources of Hydraulic Power: Pumping theory – Pump classification – Gear pump, Vane Pump, piston pump, construction and working of pumps – pumps – pump performance – performance – Variable displacement pumps. Fluid Power Actuators: Linear hydraulic actuators – actuators – Types of hydraulic cylinders – cylinders – Single acting, Double acting special cylinders like tanden, Rodless, Telescopic, Cushioning mechanism, Construction of double acting cylinder, Rotary actuators – actuators – Fluid motors, Gear, Vane and Piston motors.
UNIT III
DESIGN OF HYDRAULIC CIRCUITS
9
Construction of Control Components : Director control valve – valve – 3/2 way valve – valve – 4/2 way valve –Shuttle –Shuttle valve – valve – check valve – valve – pressure control valve – valve – pressure reducing valve, sequence valve, Flow control valve – valve – Fixed and adjustable, electrical control solenoid valves, Relays, ladder diagram. Accumulators and Intensifiers: Types of accumulators ac cumulators – – Accumulators circuits, sizing of accumulators, intensifier – Applications of Intensifier – – Intensifier circuit. UNIT IV
PNEUMATIC SYSTEMS AND COMPONENTS
9
Pneumatic Components: Properties of air – – Compressors – Compressors – Filter, Regulator, Lubricator Unit – Unit – Air control valves, Quick exhaust valves, pneumatic actuators. Fluid Power Circuit Design, Speed control circuits, synchronizing circuit, Penumo hydraulic circuit, Sequential circuit design for simple applications using cascade method.
UNIT V
DESIGN OF PNEUMATIC CIRCUITS
9
Servo systems – systems – Hydro Mechanical servo systems, Electro hydraulic servo systems and proportional valves. Fluidics – Fluidics – Introduction to fluidic devices, simple circuits, Introduction to Electro Hydraulic Pneumatic logic circuits, ladder diagrams, PLC applications in fluid power control. Fluid power circuits; failure and troubleshooting.
UNIT I FLUID POWER SYSTEMS AND FUNDAMENTALS CONTENTS Si. No.
Topics
1
Introduction to fluid power
2
Advantages of fluid power
3
Application of fluid power system
4
Types of fluid power systems
5
Properties of hydraulic fluids
6
General types of fluids
7
Fluid power symbols
8
Basics of Hydraulics
9
Applications of Pascals Law
10
Laminar and Turbulent flow
11
Reynold’s number
12
Darcy’s equation
13
Losses in pipe, valves and fittings
OBJECTIVES 1. Understand the fluid power technology 2. Understand the basic principles of fluid power 3. Identify the different areas of application 4. Understand the importance of hydraulic fluids 5. Know the different properties of hydraulic fluids and their properties 6. Understand the importance of Reynolds Number 7. Evaluate the head losses in a pipe line using Darcy’s equation
PART – A 1. Define fluid power . AU-Ch, Dec 2005 Fluid power technology is a means to convert, transmit, control and apply fluid energy to perform useful work. Fluid power includes hydraulics and pneumatics. 2. What are the advantages of fluid power? AU-Ch, Dec 2007 (i) Ease and Accuracy of control. (ii) Multiplication of Forces. (iii) Constant force or torque. (iv) Simplicity, safety, and economy. 3. List any four advantages of using the Fluid power System. AU-Ch, May 2008 (i) Hydraulic power is easy to generate, transmit, store, regulate and control, maintain and transform. (ii) The hydraulic power systems provide reversible, infinitely variable speed and load control. (iii) They can achieve accurate feedback control of load, position, etc. (iv) They offer cushioning for shock loads. Hence the noise and vibration produced are minimal. 4. Write any four applications of Fluid power systems. AU-Ch, June 2007 Agriculture, Aviation, Fabrication industry, Machine tools, Oil industry and Pharmaceuticals. 5. State Pascal’s law. AU-Ch, Dec 2009 According to Pascal law, the pressure generated at one point in a confined liquid acts equally in all directions. 6. What are the major components of a Hydraulic System? AU-Ch, Dec 2007 (i) Reservoir (or tank), 2. Pump, 3. Prime mover, 4. Valves, 5. Actuator, and 6. Fluid-transfer piping. 7. List few properties of a Hydraulic Fluid. AU-CBE, Dec 2011 (i) Viscosity (ii) Viscosity Index (iii) Oxidation stability (iv) Demulsibility (v)Lubricity (vi)Rust prevention (vii) Flash point and fire point (viii) Neutralization Number. 8. Define viscosity and bulk modulus of a hydraulic fluid. AU-Ch, June 2006 Viscosity may be defined as the shearing force required to move two plane surfaces relative to one another with a film of fluid between them. Bulk modulus is the measure of compressibility of a fluid. It is the reciprocal of compressibility. 9. What is Viscosity Index? What do you infer from V.I Number of Oil? AU-Ch, May 2008 The rate of change of viscosity with temperature is indicated on an arbitrary scale called Viscosity Index (vi). The lower the viscosity index, the greater the variation in viscosity with changes in temperature and vice versa. 10. Name any four Hydraulic Fluids that are commonly used. AU-Ch, May 2008 (i) Water, (ii) Petrolium oil (iii) Fire resistant fluids, (a) Water-in-oil emulsion (invert emulsion) (b) Water glycol emulsion (c) Synthetic fluids (d) High water content fluids(HWCF)
11. Define the term “Reynold’s Number”. AU-CBE, Dec 2011 It is defined as the ratio of the inertia force to the viscous force of a flowing fluid denoted by Re. Inertia force VD Re = ---------------------- = ------------Viscous force µ 12. What are the minor energy Losses in Pipes. AU-Ch, June 2007 1. Loss of energy due to sudden enlargement. 2. Loss of head due to sudden contraction. 3. Loss of energy at the exit from the pipe. 4. Loss of energy due to gradual contraction enlargement. 5. Loss of energy due to an obstruction in pipe. 6. Loss of energy at the entrance to a pipe. 7. Loss of energy in bends. 8. Loss of energy in various pipe fittings. 13. Why should a hydraulic fluid have good lubricating ability? AU-Ch, June 2007 If the hydraulic fluid does not have good lubricating ability it will not lubricate moving parts properly, causing wear. Wear results in increased clearance, which leads to all sorts of operational difficulties including fall of efficiency. 14. Distinguish between Hydraulic and Pneumatic Systems. AU-Ch, June 2007 Si. No. 1 2 3 4 5 6 7 8
Hydraulic System
Pneumatic System
It employs a pressurized liquid as a fluid It employs a compressed gas usually air as a fluid Generally, Hydraulic systems are designed as closed Pneumatic systems are usually designed as open system system System get slow down if leakage occurs Leakage does not affect the system much more Valve operations are difficult Easy to operate the valves Heavier in weight Light in weight Pumps are used to provide pressurized liquid Compressors are used to provide compressed gas System is unsafe to fire Hazards System is free from fire hazards Special arrangements are made for lubricating the Automatic lubrication is provided parts
15. What is neutralization number of hydraulic fluid and Give its significance? AU-Ch, May 2008 The neutralization number is a measure of acidity or alkalinity of a hydraulic fluid. High acidity causes the oxidation rate in an oil to increase rapidly. PART – B
1. (i) What are the functions of a fluid in any fluid power system? (4) AU-Ch, June 2007 Functions of a Fluid in any Fluid Power System: 1. To transmit power (Basic purpose) 2. To lubricate moving parts; 3. To seal gaps and clearances between mating components 4. To dissipate heat generated by internal friction. 5. To prevent rust and Corrosion. 6. To remove unwanted and harmful impurities from the system. (ii) Compare the various characteristics of the liquid and gaseous fluid in fluid power system. (4) AU-Ch, June 2007
S.NO
Liquid
1
Posses a definite volume for a given mass, but conforms to the shape of the container
2
Since it posses a definite volume, it forms a free surface if the volume of the container is grater than the volume of the liquid filled
3 4 5
Incompressible fluid Spacing between the molecules is relatively large when compared to solids, but relatively very less when compared to gas Can be contained in open vessel
Gas Has a definite mass, but does not posses a definite volume and conforms to the shape of the container Since it does posses the definite volume, it expends and occupies the whole volume of the container. Compressible fluid Spacing between the molecules is extremely large when compared to solids and liquids. Always need to be contained in a closed vessel
(iii) Enumerate the properties that a good hydraulic fluid should posses. (8) AU-Ch, June 2007 Good hydraulic fluid posses the following properties: 1) High corrosive resistant properties 2) Good lubrication properties to reduce friction and wear 3) Good sealing properties 4) Good heat transfer capabilities 5) Non effected by temperature changes 6) Free from acidity and should be non-toxic 7) High Flash point and low pour point 8) Chemically and environmentally stable 9) Less volatile 10) High degree of incompressibility 11) Ideal viscosity 12) Good fire and foam resistant properties 13) Low density\readily available 14) Inexpensive 2. i) what are the advantages of oil hydraulic systems over other methods. (8) AU-Ch, Dec 2006 1) Multiplication and variation of force: Linear or rotary force can be multiplied from a fraction of an ounce to several hundred tons of output. 2) Easy, accurate control: You can start, stop, accelerate, decelerate, reverse or position large forces with great accuracy. Analog (infinitely variable) and digital (on/off) control are possible. Instantly reversible motionwithin less than half a revolution can be achieved. 3) Multi-function control: A single hydraulic pump or air compressor can provide power and control for numerous machines or machine functions when combined with fluid power manifolds and valves. 4) High horsepower, low weight ratio: Pneumatic components are compact and light weight. You can hold a five horsepower hydraulic motor in the palm of your hand. 5) Low speed torque: Unlike electric motors, air or hydraulic motors can produce large amounts of torque (twisting force) while operating at low speeds. Some hydraulic and air motors can even maintain torque at zero speed without overheating. 6) Constant force or torque: This is a unique fluid power attribute. 7) Safety in hazardous environments: Fluid power can be used in mines, chemical plants, near explosives and in paint applications because it is inherently spark-free and can tolerate high temperatures. 8) Established standards and engineering: The fluid power industry has established design and performance standards for hydraulic and pneumatic products through NFPA, the National Fluid Power Association and ISO, the International Organization for Standardization.
ii) What are the desirable properties of hydraulic fluids? Discuss any eight of them in detail. (8) AUCh, Dec 2007 a) Viscosity: Viscosity is a measure of the fluid’s internal resistance offered to flow.
b) Viscosity Index: The rate of change of viscosity with temperature is indicated on an arbitrary scale called viscosity Index (V.I) Viscosity Index = L – U / L – H × 100 Where, L – SSU viscosity of reference oil at 38° C with a viscosity index of 0. H – SSU viscosity of reference oil at 38° C with a viscosity index of 100. U – SSU viscosity of oil at 38° C whose viscosity index is to be calculated. c) Oxidation Stability: Chemical stability is another property which is exceedingly important in the selection of a hydraulic liquid. It is defined as the liquid’s ability to resist oxidation and deterioration for long periods. All liquids tend to undergo unfavourable changes under severe operating conditions. This is the case, for example, when a system operates for a considerable period of time at high temperatures. d) Demulsibility: The ability of hydraulic fluid to separate rapidly from moisture and successfully resist emulsification is known as ‘demulsibility’. e) Lubricity: Wear results in increased clearance which leads to all sorts of operational difficulties including fall of efficiency. At the time of selecting a hydraulic iol, care must be taken to select one which will be able to lubricate the moving parts efficiently. f) Rust Prevention: The moisture entering into the hydraulic system with air causes the parts made ferrous materials to rust. This rust if passed through the precision made pumps and valves may scratch the nicely polished surfaces. So, additives named ‘inhibitors’ are added to the oil to keep the moisture away from the surface. g) Pour Point: The temperature at which an oil will congeal is referred to as the pour point, the lowest temperature at shich the oil is able to flow easily. It is great importance in cold countries where the systems are exposed to very low temperatures. h) Flash Point and Fire Point: Flash point is the temperature at which a liquid gives off vapour in sufficient quantity to ignite momentarily or flash when a flame is applied. The minimum temperature at which the hydraulic fluid will catch fire and continue burning is called the fire point. i) Neutralisation Number: The neutralisation number is a measure of the acidity or alkalinity of a hydraulic fluid. This is referred to as the PH value of the fluid. High acidity causes the oxidation rate in an oil to increase rapidly. Total ml of Titrating solution Neutralisation Number = ------------------------------------------ × 5.61 Weight of sample used
3. Explain the hydraulic and pneumatic fluid power system. (16) AU-Ch, Dec 2006 Hydraulic Fluid power systems [Fluid- Liquid]: Hydraulic systems are power –transmitting assemblies employing liquid as a fluid for transmitting energy from an energy – generating source to an energy – use area to accomplish work. Figure shows the simple circuit of a hydraulic systems with basic components.
Functions of Components: Hydraulic Actuator: It is a device used to convert the fluid power into mechanical power to do useful work. Hydraulic pump: It is used to force the fluid to the rest of the hydraulic circuit from the reservoir. Valves: valves are used to control the direction, pressure and flow rate of a fluid flowing through the circuit. External Power supply (Motor): It is required to drive the pump. Reservoir: It is used to hold the hydraulic liquid usually hydraulic oil. Piping systems: It carries the hydraulic oil from one place to anther. Fitters: Fitters are used to clean the hydraulic oil used in that circuit. Pressure regulator: Pressure regulator regulates i.e maintains the required level of pressure in the hydraulic fluid.
Working Principle: 1. It is a closed loop type with fluid transferred from storage tank to one side of the piston and returned from the other side of the piston to the tank.
2. Fluid is drawn from the tank by a pump which produces fluid flow at the required level of pressure. 3. If the fluid pressure exceeds the required level, then the excess fluid will return back to the reservoir until the pressure regulates to the required level. 4. When the position of valve is changed to raiser position, the pipe pressure line is connected to port A and thus the load is raised. 5. When the position of valve is changed to lower position, the pipe pressure line is connected to port B and thus the load is raised. 6. When the valve is in centre position, the valve locks the fluid into the cylinder causing all the pump output fluid to return the tank through pressure regulator. Advantages: 1) Large load capacity with almost high accuracy and precision. 2) Smooth movement 3) Automatic lubricating provision to reduce wear. 4) Division and distribution of hudraulic power is simpler and easier than other forms of energy. 5) Limiting and balancing of hydraulic forces are easily performed. Disadvantages: 1) Hydraulic Elements needs to be machined to a high degree of precision. ) Leakage of Hydraulic Oil poses problems to hydraulic operators. 3) Special treatment is needed to protect them from rust, corrosion, dirt etc 4) Hydraulic oil may pose problems if it disintegrates due to aging and chemical deterioration. 5) Hydraulic oils are messy and almost highly flammable. Pneumatic Fluid power systems [Fluid- Compressed Air]: Pneumatic system carries power by employing compressed gas generally air as a fluid for transmitting the energy from an energy – generating source to an energy –use area to accomplish work. Figure shows the simple circuit of a pneumatic system with basic components. Functions of Components: Pneumatic actuator: It converts the fluid power into mechanical power to do useful work. Compressor: It is used to compress the fresh air drawn from the atmosphere. Storage reservoir: It is used to store a given volume of compressed air. Valves: It is used to control the direction, flow rate and pressure of compressed air. External Power supply (Motor): It is used to drive the compressor. Piping system: It carries the pressurized air from one location to another.
Working Principle: 1. It is a open loop type as the air after the work done is simply vented to the atmosphere. 2. Air is drawn from the atmosphere through air filter and raised to required pressure by an air compressor. 3. As the pressure rises, the temperature also rises and hence air cooler is provided to cool the air with some preliminary treatment to remove the moisture. 4. Then the treated pressurized air needs to get stored to maintain the pressure. With the storage reservoir, a pressure switch is fitted to start and stop the electric motor when pressure falls and reached the required level, respectively. 5. The three position change over control valve delivering air into the cylinder operates in a similar any to its hydraulic circuit. Advantages: 1) Low inertia effect of pneumatic components due to light density of air. 2) Systems is light in weight. 3) Comparatively easy operations of valves. 4) Power losses and leakages are less in pneumatic systems. 5) Low Cost. Disadvantages: 1) Suitable only for light loads or small loads. 2) Availability of the assembly components is doubtful. 4. State Pascal’s law; explain any application of Pascal’s law with neat sketch. (12) AU-CBE- Dec 2011. Pascal’s law: The pressure generated on a confined fluid at rest is transmitted equally undiminished in all directions throughout the fluid and acts at right angles to the containing surfaces. P1 = P2 F1 /A1 = F2 /A2 100/10 = F 2 /100 F2 = 1000N
Application of pascal’s law: 1) Hand operated Hudraulic Jack:
This system uses a piston-type hand pump to power a hydraulic load cylinder for lifting loads, It consists of handle ABC, pivoted about point C and a piston rod is pinned to it at point B. When handle is pulled up at A, the piston rises and valve creates a vaccum in the space below it thus draws the oil from the tank through check valve1.This check valve allows flow to pass in only one direction, as indicated by the arrow. When the handle is pushed down at A, the pump cylinder ejects the oil to the load cylinder through check valve2. Pressure builds up below the load piston and thus lifts the load. Bleed valve is a hand-operated valve, which opened to lower the load by bleeding the oil from the load cylinder back to the oil tank.
2) Air-to-Hydraulic pressure Boosters: The device uses shop air to increase hydraulic pressure needed for operating hydraulic cylinders requiring small to medium volumes of higher-pressure oil. It consists of a large air piston cylinder driving a small hydraulic cylinder to clamp a workpiece to a machine tool table by supplying high-pressure oil.
5. Compare and contrast between hydraulic, pneumatic, and Electromechanical power systems. (16) AU-Ch, June 2007 S. NO 1
DESCRIPTION Energy
HYDRAULIC SYSTEM Electrical energy is used to drive the hydraulic pumps, which pressurizes the liquid.
PNEUMATIC SYSTEM Electrical energy is used to drive the motor of the compressor, which compresses the air/gas
2
Medium
Pressurized liquid
Compressed air/ gas
3 4
Energy storage Regulators
5
Transmitters
Accumulator(Limited) Hydraulic valves Transmitted through hydraulic cylinders, and hydraulic rotary actuators.
6
Distribution system
Limited, basically a local facility. Up to 100 m flow rate(v)= 2-6 m/s
Reservoir(Good) Pneumatic valves Transmitted through Pneumatic cylinders, Pneumatic rotary drives, and Pneumatic rotary actuators. Good, can be treated as a plant wide service. Up to 1000 m, flow rate(v) = 20-40 m/s
7
Operating speed
V=0.5 m/s
8
Positioning accuracy
9
Stability
Electrical energy is used to drive the electric motors There is no medium used in this system, rather the energy is transmitted through the mechanical components. Batteries(Limited) Variable frequency drives. Transmitted through the Mechanical components like gears, cams, screw-jack, etc Excellent, with minimal loss
V=1.5 m/s
10
Forces
11
Energy cost
Precision up to ±µm can be achieved depending on expenditure. High, since oil is almost incompressible and pressure level is considerably high Protected against overload, with high system pressure of up to 600 bars, very large forces can be generated. F<3999 kN Medium
12
Linear actuators
Hydraulic cylinders. It can produce very high force.
Pneumatic cylinders. It can produce medium force.
Rotary actuators
Hydraulic rotary drives and hydraulic rotary actuators. 1. Low speed 2. High turning moment 3. Good control
Pneumatic rotary drives and Pneumatic rotary actuators. 1. Wide speed range 2. Accurate speed 3. Difficult to control
13
ELECTRICAL/ELECTRO – MECHANICAL SYSTEM
Without load change, precision of 1/10 mm is possible Low, since air is compressible Protected against overload. Forces are limited by pneumatic pressure and cylinder diameter. F < 30 k N at 6 bar Highest
Precision to up to ±µm and easier to achieve. Very good values can be achieved using mechanical links Not over-loadable. Poor efficiency due to downstream mechanical elements. Very high forces can be realized. lowest Short motion via solenoid, otherwise via mechanical conversion. AC and DC motors – Simple and Powerful. AC motors – Cheap DC motors – better control
4. Motion can be stalled 14
Controllable force
Controllable, high force
Controllable, medium force
15
Work environment
Dangerous, unsightly and fire hazardous because of leakage
Noisy
Possible with solenoid and DC motors. Needs cooling, hence complicated. Danger, because of electric shock
6. (i) How to calculate the frictional losses in the valves and fittings . (8) AU-Ch, May 2008
Pressure drops are due to valves, expansions, contractions, bends, elbows, tees and pipe fittings. The losses in valves and fittings in hydraulic systems are frequently computed in terms of equivalent length of hydraulic tube. Equivalent lengths can then be substituted in the Darcy-Weisbach equation to solve for total pressure loss in the system. The formula for computing equivalent length is Equivalent length L e = KD/f (m) Where, K – Factoe for valve/fitting (no dimension) D – Inner Diameter of fitting (m) F – Friction factor (no dimension) The K factor value for several fittings and valves is given in below Valve or fittings Globe valve : Wide open ½ open Gate valve : Wide Open ¾ open ½ open ¼ open Return bend Standard tee Standard Elbow 45° Elbow 90° Elbow Ball check valve Poppet check valve
K- Factor 10.0 12.5 0.19 0.90 4.5 24.0 2.2 1.8 0.9 0.42 0.75 4.0 3.0
Calculating pressure drop for a hydraulic circuit involves : 1) 2) 3) 4) 5) 6) 7)
Determining the floe velocity through the circuit plumbing Computing the Reynolds number(Re) Determining the friction factor(f) Calculating the equivalent length for each of the fittings and valves. Combining the series of equivalent lengths into a total equivalent length(Le) Calculation of head loss for total equivalent length using Darcy – Weisbach equation Final calculation to determine the total pressure drop pressure drop in the circuit
(ii) Define Reynolds Number. (4) AU-Ch, Dec 2007
It is dimensionless number for predicting the nature of flow (laminar or turbulent) in a given situation and expressed as
vD vD Re = ----------- = -------- µ Where, Re = Reynolds Number (non-dimensional) ρ = density (kg/m 3) v = velocity of flow (m/s) D Inside diameter of pipe (m) μ = Absolute viscosity of the fluid (Ns/m 2) = kinematic viscosity (m 2/s) i) For Laminar flow, R e value is less than 2000. ii) For Turbulent flow, R e value is Greater than 4000. iii) For critical flow, R e value is varies between 2000 and 4000 and assumed as turbulent flow for calculations. This is an unpredicted flow type. (iii) Differentiate between laminar and turbulent flow. (4) AU-Ch, May 2008 Si. No.
Laminar flow
Turbulent flow
1.
Fluid particles move in layers with one layer of fluid sliding smoothly over an adjacent layer.
2.
Fluid particle move in well defined paths and they remain same relative position at successive cross section of the flow passage.
3.
This type of flow generally occurs in smooth pipe when velocity of flow is low.
4.
There is no eddies or vortices present.
5.
Laminar flow occurs in the liquids having high viscosity.
Fluid particles move in an entirely haphazard or erratic manner. Fluid particles move in an unpredicted path that results in a rapid and continuous mixing of the fluid leading to momentum transfer as flow occurs. This type of flow occurs in rivers, canals, streams, water supply pipes etc. Eddies or vortices present in this type of flow. This flow occurs in liquids having low viscosity.
6.
7) Draw any six fluid power symbols? (12) AU-Ch, Dec 2007 Lines Continuous line - flow line
Dashed line - pilot, drain
Envelope - long and short dashes around two or more component symbols. Circular
Large circle - pump, motor
Small circle
- Measuring devices
Semi-circle - rotary actuator
Square One square - pressure control function Two or three adjacent squares - directional control
Diamond Diamond - Fluid conditioner (filter, separator, lubricator, heat exchanger)
Miscellaneous Symbols Spring
Flow Restriction Triangle Solid - Direction of Hydraulic Fluid Flow Open - Direction of Pneumatic Flow
8. A hydraulic pump delivers at 60 bars, 120 lpm into a circuit laid on a horizontal plane. There are 4 elbows (k=0.75), one globe valve fully open (k=10), and a direction control valve (pressure drop=3 bar) with the inside dia of the pipe is 30mm .The total length of the straight run pipe is 20m and the specific gravity of the oil is 0.9. The kinematics viscosity of oil is .0001 m2/s. Determine the pressure in bar at the exist point of the pipe . AU-Ch, Dec 2006 Solutions: Velocity of flow V = Q / A = (120 x 10 3 / 60) Π /4 x 0.03 3 = 2.83 m/s Reynolds number Re= V D/ v = 2.83 x 0.03/ 0.0001 = 849 The Reynolds number is less than 2000. So the flow is laminar. Since the flow is laminar, Friction factor f = 64/ Re = 64 /849 = 0.075 K factors for the valves and fittings 4 elbows (K=0.75) = 3 1 Globe valve (K= 10) = 10 / 13 Equivalent length for fittings KD/f = (13x 0.03)/0.075= 5.2 m Total equivalent length Le = Pipe length + Equivalent length for fittings= 20+ 5.2 =25.2m
Head Loss H L =f (Le/D) (v2/2g) = 0.075 × (25.2/0.03) × (2.83 2 / 2×9.81) = 25.72 m Pressure drop in pipe and fittings = HL = (0.9× 1000 × 9.81) × 25.72 = 2.27 × 10 5 = 2.27 bar Pressure drop in direction control valve = 3 bar Pressure at the exit of the pipe = 60 – (2.27+3) = 54.73 bar.
Formula used Viscosity Index = L – U / L – H × 100 P1 = P2, F1/A1 = F2/A2 Velocity of flow V = Q / A Reynolds number Re= V D/ v Friction factor f = 64/ Re Equivalent length for fittings = KD/f Head Loss H L =f (Le/D) (v2/2g)
Pressure drop in pipe and fittings = HL
Assignment Topics 1. Explain the Advantages and Applications of Fluid Power Systems. 2. Explain the Properties of hydraulic fluids? 3. State Pascal’s law; explain any application of Pascal’s law with neat sketch . 4. Derive the Darcy’s Equation. 5. Explain the hydraulic and pneumatic fluid power system.
UNIT II HYDRAULIC SYSTEM AND COMPONENTS CONTENTS Si. No.
Topics
1
Pumping theory, pump classification
2
Construction and working of Gear, Vane, Piston pumps
3
Pump performance
4
Variable displacement pumps
5
Fluid power actuators
6
Linear hydraulic actuators
7
Single, Double acting and special type cylinders
8
Cushioning mechanism of a cylinder
9
Rotary actuators
10
Fluid motors like gear, vane, piston motors
11
Pump performance problems
OBJECTIVES 1. Identify the elements of an oil hydraulic system 2. Differentiate positive and non-positive displacement pumps 3. Compute the efficiencies of the pumps 4. To know about linear and rotary actuators 5. Select a mounting configuration for a particular application
PART – A
1. What is the function of a Hydraulic Pump? AU-CBE, Dec 2011 In a hydraulic system, a pump converts mechanical energy into hydraulic energy. Mechanical energy is given to the pump via a prime mover such as an electric motor. 2. What is positive displacement Pump? AU-Ch, Dec 2007 The positive displacement pump discharges a fixed quantity of fluid unto the hydraulic system per revolution of pump shaft rotation. Since the flow of fluid is guaranteed on every revolution of the shaft, this type of pump is named as ‘positive’ displacement pump. 3. Draw the characteristic curve for pump. AU-CBE, Dec 2011
4. List different types of Pumps used in Hydraulics. AU-Ch, June 2007 (i) Gear pumps (Fixed displacement only) a) External gear pump, b) Internal gear pump, c) Lobe pump, d) Screw pump, e) Gerotor pump. (ii) Vane pumps (Fixed or Variable displacement) a) Balanced vane pump, b) Unbalanced vane pump. (iii) Piston pumps (Fixed or Variable displacement) a) Axial design, b) Radial design. 5. What are the advantages of positive displacement pumps? AU-Ch, June 2007 i) Positive displacement pumps can operate at very high pressure. ii) They are small and compact in size. iii) They have very high power-to-weight ratio. iv) They achieve high volumetric efficiency. 6. List the factors to be considered in the pump selection. AU-Ch, May 2005 1. Actuator type based on load carrying capacity. 2. Flow rate requirements. 3. Type of application. 4. Pressure rating. 7. When is Lobe pump preferred? AU-Ch, Dec 2009 Lobe pump is preferred for high volumetric displacement places.
8. What do you mean by non-positive displacement pumps? State its applications. AU-Ch, Dec 2006 In non-positive displacement pumps, the fluid flow is achieved using the inertia of the fluid in motion. Since these pumps cannot withstand high pressures, they are used only for transpoting from one place to another. For the same reason, they are not preferred in the fluid power industries. 9. Differentiate pressure compensated and non-pressure compensated pumps. AU-Ch, Dec 2006 In pressure compensated pump, the pump flow can be made to zero. Such a pump has its own protection against excessive pressure build up. Also there is no power waste and reduced fluid heading. Whereas non-pressure compensated pumps are hydraulically unbalanced and cause undesirable side load on the bearings of the pump. 10. What is Pump Cavitation? How can you avoid it? AU-Ch, May 2008 The formation, growth, and collapse of vapour filled cavities or bubbles in a flowing liquid dew to local fall in fluid pressure is called pump cavitation. Pump cavitation can be avoided by ensuring that the suction pressure is always greater than that required by the pump. 11. A positive displacement pump has an overall efficiency of 87% and volumetric efficiency of 93%. What is its mechanical efficiency? AU-Ch, Dec 2008 Overall efficiency = Volumetric efficiency X Mechanical efficiency
Mechanical efficiency
87 93
100 93.5%
12. List the different types of Actuators. AU-Ch, May 2008 (i) Hydraulic motor--continuous rotary motion. (ii) Semi-rotary actuator-limited angle movement. (iii) Hydraulic cylinder-linear motion. 13. What are the advantages of double acting cylinder over a single acting cylinder? AU-Ch, May 2007 (i) In a single acting cylinder, the fluid is fed only on one side. Hence the Cylinder can produce work in one direction only. But the fluid moves the Piston in two directions in a double acting cylinder, so they perform work in both directions. (ii) In a single acting cylinder, the stroke is limited by the compressed length of the spring. But in principle, the stroke is unlimited in a double acting cylinder. (iii) While the piston moves forward in a single acting cylinder, the fluid has to overcome the pressure of the spring and hence some power is lost before the actual stroke of the piston starts. But this problem is not present in a double acting cylinder. 14. Give one method by which a quick return mechanism of a double acting cylinder could be converted into equal speed operation. AU-Ch, Dec 2007 (Or) Why is end cushioning provided in Hydraulic cylinder operations? AU-Ch, Dec 2009 Due to the inertia forces of the moving parts at the end of piston travel, the piston will hit the cylinder head at full speed. To overcome this, the designers provide an arrangement by which the hydraulic
cylinder can be slowly retarded or cushioned during the last portion of the stroke. This is known as cylinder cushioning. 15.
Write the classifications of Hydraulic Motors. AU-Ch, June 2007 (i) Gear type hydraulic motors (iii) Piston type hydraulic motors.
(ii) Vane type hydraulic motors
PART – B
1. i) What are the factors to be selected in selection of a pump for automobiles lift. (8) AU-Ch, May 2005 i) ii) iii) iv) v) vi) vii) viii) ix) x) xi) xii) xiii) xiv)
Safe and maximum system working pressures. Allowable pump speeds. Rated pump performance. System flow rate requirements. Suitability of variable displacement control. Volumetric and / overall efficiency. Operating reliability and durability. Life expectancy at variable loads and speeds. Cost and economic factors for overall system compatibility. Oil characteristics and their relationship to rate of pump wear. Pump noise generation under different speeds, pressures and flow rates. System temperature. Manufacturing characteristics: Component clearances and fit. Compactness and power –to- weight ratio.
ii) How will you measure the pump performance? Explain each with suitable Examples? (8) AU-Ch, May 2005 The performance characteristics of a pump can be represented in terms of overall efficiency. Overall efficiency, in turn, has two components: volumetric efficiency and mechanical efficiency. These 3 efficiencies are presented below Volumetric Efficiency: It is the ratio between the actual flow rate produced by the pump and the theoretical flow rate that the pump should produce. Volumetric efficiency = actual flow rate produced by the pump/ theoretical flow rate that the pump should produced x 100 Volumetric efficiency = Q A/ QT x 100 ηvol =Q A/ QT x 100 Significance : the volumetric efficiency indicates the amount of leakage within the pump. The lower the internal slip losses, the higher the volumetric efficiency. For zero slip, the volumetric efficiency is 100. Mechanical Efficiency: It is the ratio between the theoretical power required to operate the pump and the actual power delivered to the pump Mechanical efficiency = theoretical power required to operate the pump/ actual power delivered to pump x 100 ηmech =(P x QT/ TA x ω )100 Where, P = Pump discharge pressure in N/m 2 QT= Theoretical flow rate of the pump in m 3 / s T A = Actual torque delivered to pump in N – m
ω = Angular speed of the pump in rad/s = 2 π N/ 60, and N = Speed of the pump in r.p.m Overall Efficiency : It is the ratio between the actual power delivered by pump and the actual power delivered to pump overall efficiency = (Power output by pump/ actual power given by pump ) x 100 Mathematically the overall efficiency can also be written as η0 = Volumetric efficiency x Mechanical efficiency η0 = ηvol x ηmech Substituting equations we get η0=(P . Q A /T A . ω ) x 100 Significance : the overall efficiency indicates the amount of energy losses by all means. 2. i) A pump has a displacement volume of 98.4 cm³ .It delivers 0.00152 m³/s of oil at at 1000rpm and 70 bars. If the prime mover input torque is 124.3 N-m a) What is the overall efficiency of the pump? b) What is the theoretical torque required to operate the pump? AU-Ch, May 2005 Given Data: V D = 98.4 cm 3 = 98.4 x 10 -6 m3 Q A= 0.00152 m 3 /s N = 1000 rpm P = 70 bar = 70 x 10 5 N / m2 T A = 124.3 N – m Solution: ω= 2 π N /60 =2 π (1000) / 60 ω = 104.72 rad/s To find over all efficiency of the pump ( η0 ) We know that, η0 =(P . Q A /T A . ω ) x 100 =(70x10 5( 0.00152) x100 =81.74 % 124.3 x 104.72 To find the theoretical torque required to operate the pump (T T ); We know that, T T = V D x P / 2π = (98.4 X 10 -6 )(70 x 10 5 )/ 2 π = 109.62 N – m ii) What is cylinder cushioning? Explain with diagram. (or) With a neat sketch explain the “end cushion” provided hydraulic cylinders. Au, Ch Dec 2007 End caps are generally cast (from iron or aluminium) and incorporate threaded entries for ports. End caps have to withstand shock loads at extremes of piston travel. These loads arise not only from fluid pressure, but also from kinetic energy of the moving parts of the cylinder and load. These ends of travel shock loads can be reduced with cushion valves built into the end caps. In the cylinder shown in Figure, for example, exhaust fluid flow is unrestricted until the plunger is removed.
Cylinder Cushioning
3. i) A pump has a displacement volume of 0.0819 *10-3 m3 it delivers 0.0758 m3 /min at 1000rpm at 67 bar if the prime mover input torque is 100 N- m a) What is the overall efficiency, b) What is the theoretical torque required to operate the pump. Au, Ch Dec 2005 Given Data: V D = 0.0819 x 10 3 m3 / min N = 1000 rpm 98.4 x 10-6 m3 Q A= 0.0758m3 / min N = 1000 rpm P = 67 bar = 67 x 10 5 N / m2 T A = 100 N – m Solution: ω= 2 π N /60 =2 π ( 1000 ) / 60 ω = 104.72 rad/s To find over all efficiency of the pump( η0 ) We know that, η0 =(P . Q A /T A . ω ) x 100 = (67 x10 5( 0.0758/60) x100 =80.83 % 100 x 104.72 To find the theoretical torque required to operate the pump(T T ); We know that, T T = V D x P / 2π = (0.0819X 10 -3 )(67 x 10 5 )/ 2 π = 87.33 N - m (ii) Explain the construction and working of gear pump. Au, Ch Dec 2007 Gear pumps: The simplest and most robust positive displacement pump, having just two moving parts, is the gear pump. Its parts are non-reciprocating, move at constant speed and experience a uniform force. Internal construction, shown in Figure 1, consists of just two close meshing gear wheels which rotate as shown. The direction of rotation of the gears should be carefully noted; it is the opposite of that intuitively
expected by most people. As the teeth come out of mesh at the centre, a partial vacuum is formed which draws fluid into the inlet chamber. Fluid is trapped between the outer teeth and the pump housing, causing a continual transfer of fluid from inlet chamber to outlet chamber where it is discharged to the system.
Figure.1. External Gear Pump Pump displacement is determined by: volume of fluid between each pair of teeth; number of of fluid from inlet port teeth; and speed of rotation. Note the pump merely delivers a fixed volume determined solely by design of the rest of to outlet port for each rotation; outlet port pressure is the system. Performance of any pump is limited by leakage and the ability of the pump to withstand the pressure differential between inlet and outlet ports. The gear pump obviously requires closely meshing gears, minimum clearance between teeth and housing, and also between the gear face and side plates. Often the side plates of a pump are designed as deliberately replaceable wear plates. Wear in a gear pump is primarily caused by dirt particles in the hydraulic fluid, so cleanliness and filtration are particularly important. The pressure differential causes large side loads to be applied to the gear shafts at 45~ to the centre line as shown. Typically, gear pumps are used at pressures up to about 150 bar and capacities of around 150 gpm (6751 min -1). Volumetric efficiency of gear pumps at 90% is lowest of the three pump types.
Figure. 2 Internal Gear Pump
The Figure 2 is another variation called the intemal gear pump, where an extemal driven gear wheel is connected to a smaller internal gear, with fluid separation as gears disengage being performed by a crescent-shaped moulding. Yet another variation on the theme is the gerotor pump of Figure 2.3, where the crescent moulding is dispensed with by using an internal gear with one less tooth than the outer gear wheel. Internal gear pumps operate at lower capacities and pressures (typically 70 bar) than other pump types. 4 (i) With a neat sketch explain the principle construction working advantages, limitations and applications of a non-pressure compensated reciprocating vane pump. Au, Ch Dec 2006 (Balanced Vane pump) Balanced vane pump:
A balanced vane pump is one that has two inlet and two outlet ports which are diametrically opposite to each other and thus complete hydraulic balance is achieved. These two inlet and outlet ports are combined as a common inlet and outlet within the housing as shown in figure.
When the rotor rotates inside a elliptical shaped cam ring, the vanes strokes twice per revolution of pump shaft. Thus pumping chambers undergo an increase or decrease in volume twice per cycle due to the above effort. For every time volume increase, the fluid is drawn into the pump through the inlet port. Conversely, for every time volume decrese, the fluid is forced out of the pump through the outlet port.
(ii) How the capacity of a variable displacement vane pump is adjusted? Explain with a Diagram. Au, Ch Dec 2007 (Unbalanced vane pump) Unbalanced vane pump:
This pump has a cylindrical rotor with radial slots in which flat radial vanes(blades) can slide freely like plunger in cylindrical barrel. The rotor is keyed with the drive shaft and rotates inside a cam ring. The rotor is mounded eccentrically within the cam ring as shown in figure. The delivery rate of the pump depends on the degree of offset of the rotor with respect to the cam ring. Hence, vane pump is also known as variable displacement pump.
As the rotors rotates, the vanes are pushed out against the surface of the cam ring by centrifugal force, which in turn, the vanes are kept in contact with the cam surface(cam ring). The vanes divide the space between the rotor and the cam ring into a series of small pumping chambers. During the revolution of one half of rotation, the pumping chambers between the vanes are opened near the inlet. This action creates a partial vacuum at its inlet. So, the fluid from the reservoir is drawn into the pump through the inlet port and fills the void. The fluid is carried between the vanes to the outlet. During second half of rotor rotation, the vanes are pushed back to their slots and thus the pressure is increased. This action forces the fluid out of the pump through the outlet port.
5. Classify the hydraulic actuators and explain any two types of special actuators. AU-CBE, Dec 2011 Single acting cylinder Double acting cylinder Telescoping cylinder SINGLE ACTING CYLINDER: A single acting hydraulic cylinder is designed to apply in only one direction.
It consists of a piston inside a cylinder housing called a barrel. Attached to one end of the piston is a rod which extends outside. At the other end (blank end) is a port for the entranced and the exit of oil. A single acting cylinder can exetrt a forc e only in the extending direction, as fluid from the pump enters through the blank end of the cylinder. Single acting cylinder do not hydraulic retract. Retraction is accomplished by using gravity or by the inclusion of a compression spring at the rod end.
DOUBLE ACTING CYLINDER:
A double acting cylinder capable of delivering forces in both directions. The barrel is made of seamless steel tubing, honed to a fine finish on the inside surface. The piston which is made of ductile iron contains U cup packings to seal the leakage between the piston and the barrel. The port are located in the end caps which are secured to the barrel by tie rods. The load of the piston rod at the neck is taken by a rod bearing, which is generally made of brass or bronze. A rod wiper is provided at the end of the neck to prevent foreign particals and dust from entering into the cylinder along with the piston rod.
TANDEM CYLINDER:
Tandem cylinder design has two cylinders mounted in line with piston connected by a Common piston rod. These cylinders provide increased output force when the bore size of a cylinder is limited.
But the length of the cylinder is mere than a standard cylinder and also requires a larger flew rate to achieve a speed because flow must go to both pistons.
6. A pump has a displacement of 80 cm3. It delivers 1.25 Lps at 1200 rpm and 75 bar. If the prime mover input torque is 110 N-m, (1) Find the overall efficiency of the pump. 2) What is the theoretical torque required to operate the pump? Au, Ch June 2007 Given Data: V D = 80cm3 = 80 x 10 -6 m3 Q A= 1.25 Lps =1.25x 10 -3 m3/s N = 1200 rpm P = 75 bar = 75 x 10 5 N / m2 T = 90 N . m Solution: ω= 2 π N /60 =2 π ( 12 00 ) / 60 ω = 125.66 rad/s To find over all efficiency of the pump( η0 ) We know that the volumetric efficiency of a pump η0 =(P . Q A /T A . ω ) x 100 = (75 x10 5( 1.25x 10-3 ) x100 =82.89 % 90 x 125.66 To find the theoretical torque required to operate the pump(T T ); We know that theoretical torque T T = V D x P / 2π = (80X 10 -6 )(75 x 10 5 )/ 2 π = 95.49 N - m 7. (i) A double acting cylinder is hooked up in regenerative circuit as shown in fig.(a).The relief valve setting is 105 bar .The piston area is 130 cm 3 and the rod area is 65 cm 3. If the pump flow is 0.0016 m 3 /s, find the cylinder speed and load carrying capacity for (1) Extending stroke, (2) Retracting stroke. Au, Ch June 2007
Given Data: P = 105 bars
A P =130 cm2 N = 1000 rpm 98.4 x 10 -6 m3 A r = 65 cm2 Q P = 0.0016m3/s Solution :
Cylinder speed and load carrying capacity for extending stroke: Cylinder Speed: (vp) ext
= Q P / A r = 0.0016/ 65 X 10 – 4 = 0.246 m/s
Load carrying capacity (F load) ext = P . A r = (105 x 10 5 ) 65 X 10 – 4 =68.25 kN Cylinder speed and load carrying capacity for retracting stroke: Cylinder Speed: (vp) ret
= Q P / A p - A r = 0.0016/ (130 X 10 – 4 -65 X 10 – 4 ) = 0.246 m/s
Load carrying capacity (Fload) ret = P . A p - A r = (105 x 10 5 ) (130 X 10 – 4 -65 X 10 – 4 ) =68.25 kN (ii) Explain the working principle of telescoping cylinder with neat diagram. Au, Ch June 2007 Two-stage telescopic Cylinder:
The stroke of a simple cylinder must be less than barrel length, giving at best an extended/retracted ratio of 2:1. Where space is restricted, a telescopic cylinder can be used. Figure 1 shows the construction of a typical double-acting unit with two pistons. To extend, fluid is applied to port A. Fluid is applied to both sides of piston 1 via ports X and Y, but the difference in areas between sides of piston 1 causes the piston to move to the right. To retract, fluid is applied to port B. A flexible connection is required for this port. When piston 2 is driven fully to the left, port Y is now connected to port B, applying pressure to the fight-hand side of piston 1 which then retracts. The construction of telescopic cylinders requires many seals which makes maintenance complex. They also have smaller force for a given diameter and pressure, and can only tolerate small side loads.
Figure. 1.Two-stage telescopic Cylinder Pneumatic cylinders are used for metal forming, an operation requiting large forces. Pressures in pneumatic systems are lower than in hydraulic systems, but large impact loads can be obtained by accelerating a hammer to a high velocity then allowing it to strike the target. 8. (i) Explain the construction and working of Piston pump. Au, Ch June 2007
Piston pumps: A piston pump is superficially similar to a motor car engine, and a simple single cylinder arrangement was shown earlier in figure. Such a simple pump, however, delivering a single pulse of fluid per revolution, generates unacceptably large pressure pulses into the system.
Figure. 1 Radial Piston Pump Practical piston pumps therefore employ multiple cylinder and pistons to smooth out fluid delivery, and much ingenuity goes into designing multicylinder pumps which are surprisingly compact. Figure 1 shows one form of radial piston pump. The pump consists of several hollow pistons inside a stationary cylinder block. Each piston has spring-loaded inlet and outlet valves. As the inner cam rotates, fluid is transferred relatively smoothly from inlet port to the outlet port.
Figure. 2 Bend axis type Piston Pump
Figure. 3 Swash plate type Piston Pump (ii) Explain about Rotary motors with sketch. Au, Ch May 2008 Rotary actuators: Rotary actuators are the hydraulic or pneumatic equivalents of electric motors. For a given torque, or power, a rotary actuator is more compact than an equivalent motor, cannot be damaged by an indefinite stall and can safely be used in an explosive atmosphere. For variable speed applications, the complexity and maintenance requirements of a rotary actuator are similar to a thyristor-con-trolled DC drive, but for fixed speed applications, the AC induction motor (which can, for practical purposes, be fitted and forgotten) is simpler to install and maintain.
Figure. 1 Rotary actuator symbols
Formula used
Volumetric efficiency η vol
= Q A/ QT x 100
Mechanical efficiency η mech
=(P x QT/ TA x ω )100
overall efficiency η 0= ηvol x ηmech=(P . Q A /T A . ω ) x 100
ω
= 2 π N /60
Theoretical torque T T
= V D x P / 2π
Cylinder Speed: (v p) ext
= Q P / A r
Load carrying capacity (F load) ext = P . A r
Cylinder Speed: (v p) ret
= Q P / A p - A r
Load carrying capacity (F load) ret = P . A p - A r
Assignment Topics 1. Explain with diagram: a) Gear pump b) Vane pump c) Piston Pump. 2. Explain about Fluid Power Actuators. 3. Explain about Cushioning mechanism of a cylinder. 4. Explain about Rotary Actuators. 5. What are the Double acting special cylinders? Explain
UNIT III
DESIGN OF HYDRAULIC CIRCUITS CONTENTS Si. No.
Topics
1
Director control valve
2
Shuttle valve
3
Check valve
4
Pressure control valve
5
Pressure reducing valve
6
Sequence valve
7
Flow Control valve
8
Fixed and adjustable, electrical control solenoid valves
9
Relays, ladder diagram
10
Types of accumulators – Accumulators circuits, sizing of Accumulators
11
Intensifier – Applications of Intensifier – Intensifier circuit
OBJECTIVES 1. Understand the various control elements of hydraulic systems. 2. Describe the working principle of different valves. 3. Know various relays and ladder diagram. 4. Classify the types of accumulators and their functions and applications. 5. Know the application of a intensifier.
PART – A
1. What are the control functions of different valves in hydraulic systems? AU-Ch, Dec 2007 Directional Control Valves: To control the direction of flow by which the direction of actuator is controlled. Pressure control values: To control the pressure of the fluid in the circuit by which the force exerted by actuator is controlled. Flow controlled values: To control the flow rate of fluid in the circuit by which the speed of the actuator is controlled. 2. Give the hydraulic symbol for a 3 position 4 way closed centre solenoid operated direction control valve. AU-Ch, June 2006 A
B
T
3. Draw the ANSI symbol for (a) Pilot operated check valve (b) Shuttle valve. AU-Ch, May 2008 (a) Pilot operated check valve
(b) Shuttle valve
_ _ _ _ _ _ 4. Draw the symbol of a simple pressure relief valve. AU-Ch, May 2008, 2 007
5. Distinguish between pressure reducing valve and pressure relief valve giving the graphical symbol. AU-Ch, May 2006 Si. No. 1. 2. 3.
Pressure reducing valve It is type of hydraulic pressure control valve that controls the maximum pressure in a branch of a circuit. The reducing valve reads the pressure downstream. The pressure reducing valve has an external drain. Graphical Symbol
Pressure relief valve It is a type of pressure control valve that limits the maximum pressure in a hydraulic or pneumatic circuit. The relief valve reads the pressure upstream. The pressure relief valve does not have an external drain. Graphical Symbol
4.
6. What is the advantage of using an unloading valve? AU-Ch, Dec 2007
The unloading valve is used to permit a pump to operate at minimum load and is therefore at minimum horsepower. The unloading valve is useful to control the amount of flow at any given time in systems having more than one fixed delivery pump.
7. Draw the symbols for a time delay valve and one-way flow control valve? AU-Ch, June 2007 a) Time delay valve
b) One-way flow control valve
8. Distinguish between meter-in and meter-out hydraulic circuits. In meter-in type, the flow control valve is located in the pressure line leading to the work cylinder. This type is used when load characteristics are constant and positive. In meter-out type, the flow control valve is installed in the return side of the cylinder. This type is used in machine tools like drills and mills in which the tool is dragged suddenly. 9. What is the use of a temperature switch? AU-Ch, May 2008 A temperature switch is an instrument that automatically senses a change in temperature and opens or closes an electrical switching element when a predetermined temperature point is reached. 10. What is the purpose of synchronised hydraulic circuits? AU-Ch, Dec 2009 There are many industrial applications which require nearly perfect synchronization of movement of two or more cylinders in order to complete some phase of operation synchronization is required where exact movement is required at both end of a span, such as on rolling equipment, power shears, power brakes, large pressures. 11. What is the purpose of regenerative circuits? AU-Ch, Dec 2005 The regenerative circuit is used to speed up the extending speed of a double acting hydraulic cylinder. 12. Mention the function of solenoid valves. AU-CBE, Dec 2011 A solenoid is an electromagnetic mechanical transducer that converts an electrical signal into a mechanical output force. Solenoids provide a push or pull force to remotely operate fluid power valves. 13. What is a ladder diagram? AU-Ch, Dec 2005 The ladder diagram is a representation of hardware connections between switches, relays and solenoids etc., which constitute the basic components of an electrical control system. The left leg of the ladder connected to the power and the right to the ground. 14. What is the function of accumulators? AU-Ch, May 2008, AU-CBE, Dec 2011 Leakage compensation, auxiliary power source, emergency power source, shock suppressor, thermal expansion compensator.
15. What is an intensifier? AU-Ch, Dec 2007
Intensifier, also known as pressure intensifier or pressure booster, is a device used to compress the liquid in a hydraulic system to a valve above the pump discharge pressure. It is analogous to a step – up electrical transformer. PART – B 1. (i) Explain the operational feature of check valve. (8) AU-CBE, Dec 2011 Check Valve: Check Valve or one way valves are direction control elements that allow flow in one direction through them and blocks flow in opposite direction.
The one way valve which consists of a ball with a light bias spring the spring holds the ball against the valve seat. Flow coming into the inlet pushes the ball off the valve seat against the light force of the spring and goes to the outlet. If the flow tries to enter from the opposite direction, the pressure pushes the ball against the seat and the flow cannot pass through. A check valve is modified and designed as pilot check valve as shown in figure. Pilot lines are hydraulic lines that are used for control purposes and shown in dashed line on schematics. It is a two way valve. When no pressure is applied to the pilot line, the pilot check valve allows the flow in one direction but not in reverse. When sufficient pressure is applied to pilot line, the poppet is pushed to open, thereby allows flow in both direction. The drain port prevents the oil from creating a pressure buildup on the bottom of the piston.
(ii) Write short notes on shuttle valve. (8) AU-Ch, Dec 2007 Shuttle Valve: Shuttle valves are direct-acting double check valves with a cross bleed. They are essentially OR logic elements which allow two alternate flow sources to be connected to one branch circuit. They have two inlets P and Q and one outlet R. Outlet R receives flow from either inlet P or Q whichever is at higher pressure. Figure shows the operations of a shuttle valve.
If the pressure at the inlet P is greater than the pressure at another inlet Q, then the ball slides to the low resistance portion (towards right) and open the inlet P to send flow to outlet R. The reverse operation also takes place, if the pressure at inlet Q is greater than that of P.
2. (i) Explain the working of 3/2 and 4/3 direction control valve. (8) AU-CBE, Dec 2011
3/2 Directional Control Valves:
The 3/2 valves have 3 ports and 2 positions. The construction and operation of a typical sliding-spool type, solenoid-actuated 3/2 DC valve is illustrated in Fig. It has three ports or openings: pressure supply port (P), Output port(A), and exhaust port (R)
As shown in (a) and (b), the valve is actuated by a current passing through a solenoid and, returned to its original position by a spring. The spool slides over the finely finished valve bore inside the valve housing. Spool position 1: In the original or neutral position of the spool Fig.(a), the presssurized fluid flows from port P to port A to move the actuator, the exhaust port (R)renig closed. Spool position 2: When the solenoid is activated Fig (b), the spool moves to the extreme left. In the extreme spool position, the fluid from port P gets closed and hence the idispermitted to flow from port A to port R. Thus the valve alternately connects and disconnects fluid supply to the cylinder by the sliding spool.
4/3 Directional Control Valves:
The 4/3 valves have 4 ports and 3 distinct positions. The construction and operation of a typical sliding-spool type 4/3 DC valve is illustrated in Fig. This valve also has four ports P, A, B, and T.
Spool position 1: For the spool position shown in Fig (a), the fluid can flow from port P to port A and return from port B to port T.
Spool position 2: During the neutral position shown in Fig (b), aII the ports are blocked.
Spool position3: For the spool position show in fig (c) the reverse flow is permitted i. e fluid can flow from port P to port B and return from port A to port T. (ii) How does a pilot operated direction control valve function? Explain with a diagram. (8) AU-Ch, Dec 2007
3. (i) Discuss the construction and working of a solenoid actuated valve with a diagram. (8) AU-Ch, Dec 2009 Electrical Control Solenoid Valves: Solenoids are electromagnets that operate by converting the input electrical signal into mechanical force which is used to actuate the spool valves in fluid power systems. Solenoids are powered either by 24VDC or 11 OVAC/60Hz. DC solenoids are not suitable for large system as it involves high cost. Current through a DC solenoid is set by the winding resistance. DC solenoids works at low voltage having a switching time of about 40 to 50 milliseconds with an average life of about 15000 operations per hour. AC solenoids current is set by inductance of the windings to provide high inrush current and hence suitable for large systems. Its switching time is about 25 to 30 milliseconds with an average life of 7200 to 7500 operations per hour. Solenoids operate on the principle that when electric current is sent through a coil of wires, a magnetic field is generated within the coil. This magnetic field attracts the movable element called a plunger, which is made up of magnetized material such as iron. This attraction shifts the valve spool for actuation. Hydraulic solenoid valves are usually of wet armature type, which means that the hydraulic fluid is allowed to circulate through the solenoid case. This carries away the heat generated by the current flowing through the coil.
Figure shows a simple four-way, two-position single solenoid, spring return directional control valve. The primary components of the solenoid are the coil and the plunger. In this normal position, the spring holds the valve when no current is applied for energizing the solenoid. When the solenoid is energized by applying the current, a magnetic field is generated within the coil which pulls the iron plunger into the coil, shifting the valve spool. Normally, a solenoid can exert a pull or push of about 5 to 10 kg which is adequate only for pneumatic systems, but too low for large capacity hydraulic systems. Hence, pilot operated solenoids are used in hydraulic circuits. This is essentially two valves in one package. Solenoid actuates a small pilot valve, which in turn uses the pressure of the system to shift the main valve.
(ii) Discuss the functioning of an unloading valve with a diagram. (8) AU-Ch, Dec 2009 UNLOADING VALVE:
In the case of pressure relief valve, the pump delivers full pump flow at the pressure relief valve setting and thus operates at maximum horse power conditions. The unloading valve is used to permit a pump to operate at minimum load and is therefore at minimum horsepower.
The 'In' port of the unloading valve is connected to the line which is to be unloaded. The pilot port is connected to the line which is supposed to send the pressure impulse for unloading the valve. As soon as the system pressure reaches the setting pressure which is available at the pilot port, it lifts the spool against the spring force. The valve is held open by pilot pressure and the delivery from the pump starts going into the reservoir. When the pilot pressure is released, the spool is moved down by the spring and the flow is directed through the valve into the circuit. The unloading valve is useful to control the amount of flow at any given time in systems having more than one fixed delivery pump.
Application circuit: The circuit uses a high pressure low flow pump in conjunction with low pressure high flow pump. In hydraulic press, the hydraulic ram must extend rapidly over large distances with low pressure high flow requirements. During the punching operation, the pressure requirements are high due to the punching load. Since the cylinder travel is small during punching operations, the flow rate required is also low. So at that time, the expensive low pressure, high flow pump is eliminated by using an unloading valve.
During rapid advance, both the pumps are supplying oil into the circuit. When the punching operation begins, the increased pressure opens the unloading valve to unload the low pressure high volume pump. The pressure relief valve is to protect the high pressure pump from over pressure at the end of the cylinder stroke. The check valve protects the low pressure pump from high pressure which occurs during punching operation.
(4) (i) Draw and explain the working of a direct acting pressure reducing valve with neat diagram. AU-Ch, May 2007 Pressure Reducing Valve:
Pressure reducing valve is a normally open valve which is used to maintain reduced pressures in specified locations of hydraulic systems. It is actuated by downstream pressure and tends to close at this pressure reaches the valve setting. Figure shows the operation of PRV with spring loaded spool. If downstream pressure is below the valve setting, fluid will flow freely from the inlet side to outlet side. An internal passage way is provided from the outlet, which transmits outlet pressure to the spool end opposite the spring. When the outlet pressure increases to the valve setting, the spool moves to the right to partially block the outlet port. Just enough flow is passed to the outlet to maintain its preset level. Drain is provided to prevent the pressure buildup above the valve setting.
(ii) With a neat sketch, Explain the construction and operation of a pilot operated sequence valve. (8) AU-Ch, May 2007 Sequence Valve: Sequence valve finds application in a two cylinder circuit where the two cylinders works in predetermined sequence.
Figure shows the normally closed position sequence valve. In its normal position, the fluid flow from pump port to cylinder 1to extends it. When the work is done by the cylinder, further movement is not possible and resistance is met. This increases the pressure in the system. When the pressure in the system overcomes the valve setting, the valve spool moves up and allows the flow to cylinder 2 through secondary port. Sequence valve may be either direct or remote pilot operated.
5. (i) With neat sketch describe the construction and operation of pressure compensated flow control valve. (10) AU-Ch, May 2008 Pressure Compensated Flow Control Valve:
The flow through the valve varies as the square root of the pressure drop across it. So any change in pressure at the outlet and inlet changes the flow through the valve. But pressure compensated flow control automatically adjusts to pressure changes and maintains a constant pressure drop from inlet to outlet, thus providing constant flow. The valve has two main parts: 1. The throttle valve which is nothing but a non-pressure compensated flow control valve or a needle valve. It is an orifice whose area can be adjusted by an external knob setting. The orifice area setting determines how the flow rate is to be controlled. 2. The pressure compensator automatically controls the size of the restriction in the flow passage and maintains a constant pressure drop across the throttle valve. The size of the inlet orifice is determined by the position of the pressure compensator which automatically varies the orifice size to compensate for the system pressure variation. When the fluid passes through the adjustable varying orifice, pressure builds up in the spring side of the ·compensator. So the fluid force in the spring side and the thrust of the spring combine to oppose the force on the other side of compensator. If P1 and P2 are the inlet and outlet pressures. Ts is the spring thrust and A is the exposed area of compensator, for the pressure compensator to be in a balanced position.
So the pressure drop through the inlet and outlet depends upon the thrust of the spring and compensator area. Since the drop also remains constant. Due to constant pressure drop, whatever be the fluctuation of pressure at inlet or outlet, the flow is constant. Pressure compensated flow control valves are available with integral reverse flow check valves which allow fluid to flow unrestricted in the opposite direction.
The temperature variation is another factor which affects the flow rate. As the viscosity of hydraulic oil varies with temperature, the oil becomes less viscous when temperature increases. As less viscous fluid flows more readily through an orifice, the increase in temperature causes increase in flow for the valve setting. A temperature compensator is used to offset the effect of such temperature variations. The temperature compensator is a temperature sensitive element with a high coefficient of thermal expansion. When the temperature varies, temperature compensator adjusts the orifice opening to correct the effects of viscosity changes caused by temperature fluctuations of the fluid.
(ii) Enumerate four methods of actuation of hydraulic control valves. (6) AU-Ch, May 2008
Actuation Devices: Actuation devices are components used in hydraulics/pneumatic circuits that are used for shifting the valve spool from one position to another. Many actuation devices are in common use. Some of the devices are discussed below Manual Actuation Devices: It includes push button, lever, spring; foot pedal etc., In this, the valve spool is shifted manually by applying a force through the movement of handle, pressing of push button or stepping on foot pedal. When the lever is released, the spring returns the spool to its original position as shown in figure
Hand leaver Bush Button Foot Pedal Mechanical Actuation Devices: It includes cam, roller, plunger, roller tappet etc; In figure the spool end containing a roller that is typically actuated by a earn-type mechanism. These valves shift when depressed by some mechanical component of the machine.
Manual Actuation Device Pilot operated actuation devices:
These valves are shifted with system pressure. When a pilot signal (oil or air) is applied against a piston at either end of the valve spool, it pushes the piston to shift the spool. Solenoid Operated Actuation Devices:
Figure shows the very common way of actuating a spool valve which is by using a solenoid. They are widely used in electronically controlled machineries. When an electric current is passed through a solenoid, it gets energized and induces a magnetic force. This force pulls the armature into the coil and pushes the spool to move.
6. (i) Explain the accumulator. Weight-loaded
neat sketch of weight loaded (8) AU-Ch, Dec 2009 Accumulator:
A weight loaded accumulator is basically a vertically mounted cylinder with a large weight attached at its top. It incorporates a piston with packaging to prevent leakage as shown in figure. When hydraulic fluid is pumped into this accumulator, the weight is raised. This exerts a force to the piston, which generates a pressure on the fluid side of the piston. The advantage of this type is that it applies a constant pressure on the fluid throughout its volume. Its usage is limited by a very large weight requirement.
(ii) An accumulator is loaded with 400 KN weight. The ram has a diameter of 300 mm and a stroke of 6m. It’s friction may be taken as 5 %. It takes two minutes to fall through its full stroke. Find the total work supplies and power delivered to the hydraulic appliance by the accumulator, 0.075 m3 /sec of liquid is being delivered by a pump, while the accumulator descends with the stated velocity. Take the density of oil as 1000 N/m 3. (8) AU-Ch, Dec 2009 Given Data: W= 400 KN D= 300 mm = 0.3 m L= 6 m Friction = 5% A
=
=
D2 0.32
= m2 Total work supplied to Hydraulic machine: Net load on Accumulator when it descends = Actual load x (1- friction) = 400 x 0.95 = 380 KN Time taken by ram to fall t = 2 min = 120 sec Velocity of ram/ Distance moved by ram/sec = = 0.05 m/s
Work done by accumulator/sec
= Net load x V = 380 x 0.05 = 19 kNm/s
Intensity pressure of liquid P
=
=
Pressure Head
= 5376.3 x 10 3 P = gh =
= Work supplied by pump/sec
= 548 m = Wt of water supplied/sec x pressure head = Wqh = 9810 x 0.0075 x 548 = 40.319 kNm/sec
Total work supplied To hydraulic machine
Power hydraulic Machine
= work supplied by accumulator + Work supplied by pump = 19 + 40.319 = 59.319 Knm/sec = 59.319 kW
7. With the help of circuit diagrams, illustrate the application of accumulators. (16) AU-Ch, May 2008 Accumulator Circuits: Accumulator circuits depict the application of accumulator. Accumulator finds four basic applications. Auxiliary power source, Emergency power source, leakage compensation and shock absorber. Auxiliary Power Source: One common application for an accumulator is to supplement the pump flow during intermittent periods of high flow demand. Many applications require large amounts of flow to generate fast actuator speeds, but only for a portion of the work cycle. For this, we need large capacity pump. So, to avoid this, accumulator can be used to serve as a secondary power source to supplement the pump flow during the high demand period, thereby allowing a smaller pump to be used. Figure shows such an application circuit.
When the four way valve is actuated, oil flows from the accumulator to the blank end of the cylinder to extend the piston up to the end of its stroke. While the desired operation is occurring in its fully extending position, the pump charges the accumulator. Then, the four-way valve is deactivated to retract the cylinder. Oil from the pump and accumulator flows to retract the cylinder rapidly. The unloading valve opens when the accumulator is fully charged and allows pump delivery to return to the reservoir at very low pressure. The accumulator maintains its charge, since it is isolated by the check valve. Emergency Power Source:
Emergency condition indicates the event of loss of power failure. Figure shows such application circuit. When the three way valve is energized, oil flows from pump to the blank end of the cylinder and also to the accumulator through check valve and rod end of the cylinder. The accumulator charges as the cylinder extends. When electrical failure occurs, the solenoid will deenergize, shifting the valve to its spring off-set mode. Then the oil stored in the accumulator is forced to the rod end of the cylinder and retracts the cylinder to its starting position. Leakage Compensator: In some circuit, it may be desirable to shut down the pump completely once the accumulator has been fully charged. This can be accomplished by incorporating a pressure switch that stops the electric motor that drives the accumulator as shown in figure
When the valve is actuated, the oil flows to the blank end of the cylinder and also filling the accumulator simultaneously, When the accumulator is fully charged, the pressure in the accumulator reaches the setting of the pressure switch, an electrical signal is sent that will cause the pump to stop. The accumulator then supplies enough flow to compensate for leakage. Finally, when system pressure drops to the minimum pressure setting of the pressure switch, it closes the electric circuit of the pump motor until the accumulator has been recharged. The relief valve is a safety backup to the pressure switch and would be set to a slightly higher pressure than the pressure switch so that it does not open during the normal operation of the circuit. Surge or Shock Suppressor: Another application of an accumulator is to dampen out high pressure spikes or hydraulic shock. A pressure spike will occur due to sudden stoppage or deceleration of a hydraulic fluid flowing at relatively high velocity in a pipeline. This can damage the system components by inducing vibrations. To suppress the pressure spikes, an accumulator can be installed near the rapidly closing valve. A dead weight accumulator is always recommended for this application, because the surge causes the dead weight to ride up and down freely as shown in figure.
8. (i) What is Relay? Explain with a sketch working or relay. (6) AU-Ch, May 2007
Relay: A relay is a electrically actuated switch which open or close when its corresponding coil is energized. These relays are commonly used for energizing and de-energizing the solenoids as they require high current to operate. This low voltage circuit can be used to energize relay coils that control high-voltage contacts used to open and close circuits containing the solenoids. When relay coils are energized, the contacts that are normally closed will open and vice versa. They are usually spring loaded to assume their normal positions when the coil is de-energized. Figure shows the symbol of electrical relay contacts.
Use of relays permit interlocking capability to avoid accidental actuation of two solenoids at the opposite ends of a valve spool, which prevents the burnout of one or both of the solenoids. (ii) Make a Circuit, Showing an intensifier in a punching press application. (10) AU-Ch, May 2007 Intensifier in a Punching Press: The application of intensifier in a punch press is shown in figure. The intensifier should be installed near the cylinder to shorten the pipelines to avoid heavy losses.
When the DCV is shifted to right mode, the pump flow reaches the blank end of the cylinder through the check valve. When the pressure in the cylinder reaches the pressure setting of the sequence valve, the intensifier starts to operate. The pressure gets intensified and this high pressure in turn pressurizes the blank end of the cylinder to perform the punching operations. When the DCV is shifted to left mode, the pump flow reaches the rod end of the cylinder. So, once the pressure builds-up, the pilot signal opens the check valve and the piston retracts to its original position.
Assignment Topics 1. Explain with diagram: a) 3/2 way valve b) 4/2 way valve 2. Explain about Shuttle valve and check valve. 3. Difference between pressure control valve and pressure reducing valve. 4. Write about different types of accumulators. 5. Write the applications of Intensifier.
UNIT IV PNEUMATIC SYSTEMS AND COMPONENTS CONTENTS Si. No.
Topics
1
Properties of Air
2
Compressors
3
Filter, Regulator, Lubricator Unit
4
Air Control Valves, Quick Exhaust valves
5
Pneumatic Actuators
6
Fluid power circuit design
7
Speed control circuits
8
Synchronizing circuit
9
Penumo hydraulic circuit
10
Sequential circuit design for simple applications using cascade method
OBJECTIVES 1. Understand the basic components of pneumatics. 2. To know the basic components of pneumatic system. 3. To categorize the hydro pneumatic applications. 4. Compare the hydraulic, pneumatic and hydro pneumatic systems. 5. To understand the common methods of designing logic circuits.
PART – A
1. How can you specify an air compressor? AU, Ch, April 2008. A compressor is a device used to compress air (or any gas) from a low inlet pressure (usually atmospheric pressure) to a higher desired pressure level. 2. How are the air compressors classified? AU, Ch, Dec 2007. 1. Reciprocating type – Piston type and diaphragm type compressors. 2. Rotary types – Screw types, flow type and vane type compressors. 3. How do you rate a compressor? AU, Ch, Dec 2006. Air compressors are generally rated by the following: (i) Outlet pressure of air that can be delivered by the compressor, (ii) Capacity or volume of air that can be delivered by the compressor, and (ii) Compressor configuration and cylinder geometry. 4. What are the functions of FRL unit? AU, Ch, Dec 2007. The compressed air is first filtered and then regulated to the specific pressure and made to pass through a lubricator for lubricating the oil. 5. Give the standard graphical symbol for FRL unit. AU, Ch, May 2006.
6. Name the various types of filters used in the pneumatic system. AU, Ch, Dec 2006. 1. Dry filters (with replaceable cartridges), and 2. Wet filters 9where the incoming air is bubbled through an oil bath then passed through a wire mesh filter). 7. What is the need of lubricator unit in the pneumatic system? AU, Ch, Dec 2006. The function of the lubricator unit is to add a controlled amount of oil with air to ensure proper lubrication of internal moving parts of pneumatic components. 8. What are the basic components of a pneumatic system? AU, Ch, Dec 2007. 1. Compressor, 2. Primary Air Treatment, 3. Receiver, 4. FRL unit, 5. Pneumatic valves, 6. Actuator, 7. Muffler. 9. What are the reasons for pressure drop in pneumatic lines? AU, Ch, Dec 2007. The reasons include sudden change in volume, leakage exist in the circuit line, change in temperature, bends, and restrictions present in the pipe line. 10. What is the purpose of regenerative circuit and how is it attained? AU, Ch, Dec 2007.
A regenerative circuit is used to speedup the extending speed of a double acting cylinder. This is attained by connecting the return flow from cylinder to pump flow. 11. What is the function of reservoirs? reservoirs? AU, Ch, April 2005. 2005. 1. Store the hydraulic fluid, 2. Provide heat exchange, 3. Allow contaminants to settle, 4. Allow air to escape. 12. What is the function of bleed-off circuit? AU, Ch, April 2005. Bleed off circuits control the fluid flow rate by bleeding off the excess flow back to the tank. This is accomplished by providing a additional line parallel to the system pressure line. To slow down the actuator, some of the flow is bled off through this line, there by reducing the flow to the actuator. It may be noted that, opening a bleed off FCV, slows down the actuator, whereas, opening a meter out FCV increases the actuator speed. 13. What is air-oil intensifier? AU, Ch, Dec 2005. An air-oil intensifier circuit, which drives a cylinder over a large distance at low pressure and then over a small distance at high pressure. 14. What is meter in circuit? What is its limitations? AU, Ch, May 2006. In this system the flow control valve is placed in the line loading to the inlet port of the cylinder. Limitations: Meters in systems are used primarily when the external load opposes the direction of motion of the hydraulic cylinder. An example of the opposite situation is the case of a weight pulling downward on the piston rod of a vertical cylinder. In this case the weight would suddenly drop by pulling the piston rod down if a meter in system is used even if the flow control valve is completely closed. Hence meter out system is generally preferred over the meter in type. 15. Compare hydraulic and pneumatic circuit with respect to their characteristics. AU, Ch, April 2004. Si. No. 1 2 3 4 5 6
Hydraulics
Pneumatics
The operating pressure of the hydraulic circuit is 35 bar to 350 bar. Very precise operation can be performed. The fluid used will act as self lubricant. Slow, smooth motion. Fluid used is incompressible incompres sible liquid. Not clean due to leakage.
The operating pressure of the pneumatic circuit is limited to 7 bar. Not as precise as hydraulic circuits. Separate lubricant should be used. Quick and jumpy motion. Fluid used is compressible gas. Generally cleaner.
PART – PART – B 1. (i) Discuss the construction construction and working principle principle of a Rotary vane air compressor. (8) AU, Ch, May 2007. Figure shows the vane design compressor. In this design, the cylindrical slotted rotor is off centre to the housing and turns inside the housing. Each rotor slot is fitted with a rectangular vane. As the rotor spine, the vanes are kept in contact with the housing by centrifugal forces and therefore slides in and out. turns, air is trapped trapped between the vanes during during one-half of each each revolution and As the rotor turns, compressed between the vanes. Air is pushes out during the the other half of each revolution revolution as the the space decreases. decreases.
(ii) (1) Calculate the required size of a receiver that must supply air to a pneumatic system consuming 0.85 standard m 3 /min for 8 min between 10 bar and 7 bar before before the compressor compressor resumes operation. (2) What size of receiver is required if the compressor is running and delivering air at 0.2 standard m3 /min. (8) AU, Ch, Ch, May 2007. Given Data: Qr = Qr = 0.85 m3 / min; t = t = 8 min; P max max = 10 bar = 1000 kPa; Pmin = 7 bar = 700 kPa. Solution: (i) Required receiver size before the compressor starts operation: Since compressor has not started its operation, the output flow rate of compressor,Qc= 0. Receiver size, Vr =
= 101x8x(0.85 – 101x8x(0.85 – 0) =2.289m3 1000 -700 (ii) Required receiver size when the compressor is running and delivering air at 0.2 Standard m 3 /min : Given that Qc Q c = 0.2 m 3/min Receiver size, Vr V r = = 101x8x(0.85 – 101x8x(0.85 – 0.2) 1000 -700 =1.75 m3 2. Describe the operation of an FRL unit. (16) AU, Ch, Dec 2007, AU, Ch, June 2012. 1. Filters:
The most common device installed in hydraulic systems to prevent foreign particles and contamination from remaining in the system are referred to as filters. They may be located in the reservoir, in the return line, in the pressure line, or in any other location in the system where the designer of the system decides they are needed to safeguard the system against impurities. Filters are classified as full flow and proportional or partial flow. In the full-flow type of filter, all the fluid that enters the unit passes through the filtering element, while in the proportional-flow type, only a portion of the fluid passes through the element.
2. Pressure Regulator:
The main function of this valve is to regulate the incoming pressure to the system so that the desired air pressure is capable of flowing at a steady condition. The valve has a metallic body with primary and secondary openings. The pressure regulation is obtained by opening the poppet valve to a measured amount for achieving the desired pressure level. This is done by an adjustable screw. The adjusting screw will move the diaphragm upward and thus will make the poppet to unseat, creating an opening to allow air to flow from the primary to secondary side. The opening of the valve and thereby the pressure of air flowing through it will be directly proportional to the compression of the spring underneath the diaphragm.
3. Lubricator:
The function of the lubricator is to insert drops of oil into the air stream. A small amount of air from the mainstream is diverted through the bowl pressure control valve into the bowl or reservoir. This air pushes the oil through the siphon tube past the adjustment screw to the drip tube. The drops of oil are then transferred through the centre of the variable orifice and enters the mist generator, mixing with the oil deliver by the drip tube. The air-oil mixture then rejoins the remaining air and continues toward its final destination.
3. (i) Discuss the construction and function of a Quick Exhaust valve with a diagram. (8) AU, Ch, Dec 2009. Book o. 54, p. No. 4.11
Quick exhaust valve is a special purpose three way pneumatic valve, that increases the cylinder rod speed by dumping the exhaust air directly to the atmosphere from the cylinder. Use of quick exhaust valves, permits increased cylinder velocities and needs smaller, less expensive DCV. This eliminates the need for exhaust air to travel from the cylinder to the main control valve through long restricted pipe lines. Quick exhaust valves are usually closely installed with the cylinders as shown in figure.
Air passes through the inlet port pushes the diaphragm to bend against the exhaust port, thus closing the exhaust port. Further Air inlet pressure still bends the diaphragm to open a passage drilled around the diaphragm. So, the air from the DCV passes to the cylinder through the small passage after passing the inlet port. When the DCV is shifted, inlet air pressure-is reduced and creates a difference in pressure on the diaphragm. This pressure difference forces the diaphragm to lift from the exhaust port and seals back the inlet port. This opens the exhaust port, and air is allowed freely to the atmosphere through exhaust port from the outlet port. However, operation is noisy which is overcome by using specially designed diaphragm.
(ii) Describe various pneumatic actuators with neat sketches?(8) AU, CBE, DEC 2011 Pneumatic systems make use of actuators (linear and rotary) in a fashion similar to that of hydraulic systems. But these differ very little from the hydraulic applications because Air is used rather than hydraulic oil; pressures are either low or medium and a lighter construction is encountered. Pneumatic cylinder construction makes extensive use of aluminium and other non-ferrous alloy materials to reduce the weight and the corrosive Effects of air and to improve heat transfer capabilities.
Pneumatic Cylinders: The pneumatic power is converted into straight-line reciprocating motions by pneumatic cylinders. The various industrial applications for which air cylinders are used can be Divided duty-wise into three groups: light duty, medium duty and heavy duty. According to the operating principle, air cylinders can be classified as follows. Single acting cylinder: It has a single air inlet line. When this line is pressurised, the piston extends. The return movement of the piston is affected by a built-in spring mounted on the rod side of the piston or by application of an external force like gravity. Single acting cylinders with a return spring are limited in stroke, because the spring force that must be overcome while extending increases with the stroke. The advantage of a single acting cylinder lies in its reduced air consumption, since air is not wasted while retracting the piston. Double acting cylinder : The force exerted by the compressed air moves the piston in two directions in a double acting cylinder. The double acting cylinder produces less to force during retraction, because the piston rod's cross-sectional area is subtracted from the piston area under pressure. They are used particularly when the piston is required to perform work not only to the advance movement but also on the return movement. In principle, the stroke Length is unlimited, although buckling and bending must be considered before we select particular size of piston diameter, rod length and stroke length. Advantages of a double acting cylinder over a single acting cylinder. In a single acting cylinder, the compressed air is fed only on one side. Hence, this cylinder can produce work only in one direction. But the compressed air moves the piston in two directions in a double acting cylinder, so they work in both directions. Tandem cylinder: It consists of two pistons operating in separate sections along the same axis with a common piston rod. Since the available force is doubled, this design is useful when larger forces are required, but a single cylinder with a larger diameter cannot be accommodated. Three position cylinder: It is quite similar to the tandem cylinder, except that the left piston rod is not connected to the right piston and the left cylinder is shorter than the' right one. With the left piston extended, the retraction of the right piston is limited to an intermediate position which is determined by the ability of the right piston to retract fully. Through rod cylinder: Here the piston rod extends to both ends of the piston. This will ensure equal force and speed on both sides of the cylinder.
Basic Cylinder Types a) Single Acting Cylinder b) Double ActingCylindlf c) Tandem Cylinder d) Three Position Cylinder e) Through Rod Cylinder I) Adjustable Stroke Cylinder g) Telescoping Cylinder 4. (i) Discuss in detail the considerations taken while designing pneumatic circuits. (8) AU, Ch, Dec 2009. Safety Consideration Any system should be designed to operate in a safe manner. For this, the designer may consider the following factors: 1. Environment conditions 2. Operating pressure ratings of system components 3. Operator skill and maintenance facilities. 4. Component suppliers/manufacturers. 5. Sequence of operations 6. Temperature ratings of system components 7. Power failure locks or safety pins and protective devices for operators and servicemen. 8. Emergency shutdown features at fire hazards. 9. Compatibility between system components. 10. Suitable operating speeds Consideration of System Performance for Desired Function The system should be designed to perform the desired function. For this, the designer may consider the following factors 1. Stringency of the specifications of the selected components. 2. Life of the components and also the machine. 3. Charging and discharging period of accumulator. 4. Margin of safety on operational hazards basis. 5. Maintenance of the selected components. Consideration of Efficiency of Operation The system must be efficient to perform the operations efficiently. For this, one may consider the following factors. 1. Reliability 2. Working environment 3. Use of standardized components. 4. Clean design 5. Availability of replacement of parts.
All the above three groups of design consideration should move with the cost analysis based on the knowledge and skill of the designer.
(ii) Highlight the advantages of an air over oil circuit and explain with suitable applications. (8) AU, Ch, Dec 2009.
Figure shows the air-over oil circuit. The main advantage of this system is the elimination of cost of pump and tank unit. Atmospheric air after passing through the compressor flows through the FRL unit after which it passes into the surge tank. Now, the surge tank gets pressurized by the air. This forces the oil out the bottom of the surge tank. Then, the flow is being bypassed through the check valve and also through the metering orifice. This extends the cylinder to lift a load.
5. (i) Explain the working of a pneumatic speed control circuit. (8) AU, Ch, Dec 2007.
The motion and speed of a double acting cylinder can be controlled by using flow Control valves. In pneumatics, meter-out flow control is preferred in all situations. This is because meter-out flow control provides a stabilising back pressure on the piston, which smoothes out the jumping motion of pneumatic actuators. The meter-out flow Control circuit is shown in figure.
(ii) What is the time delay circuit? Discuss with an example. (8) AU, Ch, Dec 2007.
In certain engineering applications, a predetermined time delay may be required to actuate the pilot operated direction control valve. For example, a pneumatic cylinder actuates material handling platform. At the end of the extension stroke, the platform has to wait for a predetermined time for material removal and then it has to return back.' For such application, a time delay valve is an ideal solution. When the push button of 3/2 DC valve (1) is actuated, the pilot signal goes to chamber (X) of 4/2 DC valve (4). Now, the cylinder extends and actuates the roller operated pilot valve (2). But, it is connected to a time delay valve (3). So, the 4/2 DC valves not actuated immediately. The air is accumulated in the reservoir of time delay valve. When the pressure exceeds the spring value of3/2 DC valve of time
delay valve, the valve opens and the pilot signal is fed to the chamber (Y) of 4/2 DC valve. Now, the cylinder returns. The return of the cylinder is delayed by a certain time. The amount of delay ti me is the time required to buildup pressure in the reservoir.
6. (i) What is compressor? Explain the working principle of piston type compressor with neat sketch. (8) AU, Ch, Dec 2007. A compressor gives high pressure air by changing its volume. As the volume is reduced, the pressure increases. Piston compressor:
Piston compressors are by far the most common type of compressor, and a basic single cylinder form is shown in Figure. As the piston descends during the inlet stroke, the inlet valve opens and air is drawn into the cylinder. As the piston passes the bottom of the stroke, the inlet valve closes and the exhaust valve opens allowing air to be expelled as the piston rises in Figure b, implies that the valves are similar to valves in an internal combustion engine. In practice, spring-loaded valves are used, which open and close under the action of air pressure across them. One common type uses a 'feather' of spring steel which moves above the inlet or output port, as shown in Figure c.
Normally two stages are used for pneumatic pressures of 10 to 15 bar, but multistage compressors are available for pressures up to around 50 bar. Multistage compressors can be manufactured with multi cylinders as shown in Figure. There is contact between pistons and air, in standard piston compressors, which may introduce small amounts of lubrication oil.
(ii) What is the synchronizing? Explain the synchronizing circuit with suitable approaches? AU, CBE, DEC 2011, AU, Ch, May 2007.
Synchronizing Circuits Some situation require to lift the platform by using two or more cylinders. While lifting, number of cylinders performing the work should work at same speed. The process of performing tins kind of work is called synchronization. Synchronization process may be accomplished by either series piping or parallel piping connection. Cylinders Connected in Parallel
Figure shows the parallel piping cylinder synchronization. For parallel cylinder synchronization system, there are two basic requirements. 1.Twocylinder must be identical, but no cylinder are really identical, as manufacturing tolerances may vary. 2. Load should be divided equally for both cylinders to extend in exact synchronization When the DVC is shifted to left envelope, pump flow goes to the blank end of the cylinder extending it to perform work at the same speed. The reverse is possible, if the DVC is shifted to right envelope. 7. With neat sketch, explain the working principle of a screw compressor. (6) AU, Ch, June 2012.
A rotary screw compressor compresses air between two intermeshing screws and oil is used as a coolant
to remove some of the heat of compression.
Figure shows the top view of single stage screw compressor. Compression is done by allowing the trapped air in the small space between meshing elements, as the screws rotate. Screws rotates in opposite direction.
(ii) Develop an electro pneumatic circuit for the following sequence A+B+A-B- where A & B stand for cylinder (+) indicates extension and (-) indicates retraction of cylinders. (10) AU, Ch, May 2007, AU, Ch, May 2006. Solution: Group = A+B+ A-BI
II
No of pressure lines =2 No of limit valves = No of Cylinders x2 =4(a 0, a1, b0, b1) No of group valves = No of groups-1 =2-1=1 No of Cylinder actuating valves = 2(VA,VB)
8. Three pneumatic cylinders A,B,C are used in an automatic sequence of operation. A cylinder extends, B cylinder extends, B cylinder retracts and and then A cylinder retracts, C cylinder extends and C cylinder retracts. Develop pneumatic circuit by cascade method. Sketch also travelstep diagram and explain briefly. (16) AU, Ch, May 2008.
Assignment Topics 1. Explain in detail about FRL unit. 2. Draw the diagrams of Air Compressors types. 3. Explain about Quick exhaust valves. 4. Explain about Speed control circuits, synchronizing circuit. 5. Draw the Sequential circuit design for simple applications using cascade method.
UNIT V DESIGN OF PNEUMATIC CIRCUITS CONTENTS Si. No.
Topics
1
Servo systems – Hydro Mechanical servo systems
2
Electro hydraulic servo systems
3
Proportional valves
4
Fluidics – Introduction to fluidic devices
5
Introduction to Electro Hydraulic Pneumatic logic circuits
6
ladder diagrams
7
PLC applications in fluid power control
8
Fluid power circuits
9
Failure and troubleshooting
OBJECTIVES 1. To understand the working principle of different types of servo valves and proportional valves. 2. To understand the concept of pneumatic logic control. 3. To know the various sensors and switching elements used in pneumatic logic control systems. 4. To understand the use of microelectronic devices, PLC and microprocessor in fluid power circuits. 5. To know the troubleshooting procedures.
PART – A
1. Define Relay. AU-Ch, APR 2006. (or) What is meant by interlock contacts? AU-Ch, APR 2005. Interlock contacts are nothing but relay contacts. Electrical relay is a switch whose contacts open or close when its coil is energised. Relays are used for energising and de-energising solenoids. 2. What is Ladder diagram? AU-Ch, Dec 2005. The ladder diagram is a representation of hardware connections between switches, relays and solenoids etc., which constitute the basic components of an electrical control system. The left leg of the ladder connected to the power and the right to the ground. 3. Mention any two roles of pneumatic systems in low cost automation. AU-Ch, APR 2005. Pneumatic systems are popularly used for low cost automation (LCA) applications due to their low cost, ease of fabrication, and safe operation. 4. What is power pack? AU-Ch, Dec 2005. Power pack consists of a pump, electric motor, reservoir and associated valving assembled to one unit to supply pressurized fluid. They are relatively small in size and provide functions of pressure, direction and flow control within the basic package. 5. How do microprocessors differ from PLCs? AU-Ch, Dec 2005. The advantages of PLCs over microprocessors are given below: 1. PLCs are rugged and designed to withstand vibrations, temperature, humidity and 2. The interfacing for inputs and outputs is inside the controller. 3. They are easily programmed and have an easily understood programming language.
noise.
6. What is microprocessor? AU-Ch, June 2006. Microprocessor is the central processing unit (CPU) of a micro-computer. It is the heart of the microcomputer. It is a semiconductor device. It includes arithmetic and logic unit, register arrays, and control circuits on a single chip. 7. State the role of PLC in fluid power industry. AU-Ch, Dec 2006. PLCs are designed as a replacement for hard-wired electro-mechanical relays to control fluid power systems. 8. What are the important components of a hydraulic power pack? AU-Ch, Dec 2006. Hydraulic power pack consists of the pump, drive motor, mechanical couplings, oil reservoir, strainers, filters, coolers, etc. 9. What is low cost automation? AU-Ch, Dec 2007 Low cost automation is defined as a technology that creates some degree of automation around the existing equipment, tools and methods, using mostly the standard equipments and softwares available in the market.
10. Define Fluidics. AU-Ch, Dec 2007 Fluidics is the technology that utilizes fluid flow phenomena in components and circuits to perform a wide variety of control functions (such as sensing, memory, timing and interfacing).
PART – B 1. (i) Explain the hydro mechanical servo valve with an industrial example. (12) AU-Ch, June 2012. Book No. 54 page no 5.2
(ii) What is Coanda effect? Explain with a diagram.
AU-Ch, June 2007.
A free jet of air is emitted into a confined region or orifice at a velocity high enough to produce turbulent flow. The free jet of air will continue in a given direction, pulling in with it the available air from its surroundings as it leaves the orifice.
If there is greater availability of this entraining air from one side, a small vortex area (low pressure area) is created near the nozzle exit. This low pressure area then tends to attract the free jet, distorting it and pulling it towards the wall, because the atmospheric pressure on the other side forces the jet to cling to the surface. This free jet attachment continues until a small air supply is fed to the low pressure area, thus relieving the Attraction of the jet to the wall. When this signal is injected, the free jet then detaches itself from the wall and resumes its normal uninterrupted flow path.
2. Explain with a neat sketch construction and operation of an Electro hydraulic servo valve. Also deduce the expression for its open loop gain. (16) AU-Ch, June 2007. Book No. 54 page no 5.3
3. i)What is the selection criterion for pneumatic components? (4) AU-Ch, Dec 2007 Jayakumar book, page No. s6 ii) What are the factors considered during the installation of pneumatic system? (6) AU-Ch, Dec 2007 Jayakumar book, page No. s24 iii) What are advantages of using fluidics system? (6) AU-Ch, Dec 2007 Jayakumar book, page No. 15.2
4. (i) Explain the working principle of a PLC with a neat block diagram. What are the advantages of PLC? (12) AU-Ch, June 2007. Book No. 54, page No. 5.22 (ii) How does PLC differs from Micro processor? (4) AU-Ch, June 2007. Book No. 204, page No. 352
5. i) Briefly explain the various electrical devices used in the control fluid power systems. AU-Ch, May 2008. Book No. 54, page No. 5.13 ii) Draw the basic standard symbols that are used in ladder diagram? Also show rungs in a ladder diagram? AU-Ch, May 2008. Book No. 54, page No. 5.25 6. (i) Discuss with a circuit diagram how reciprocation of a cylinder is achieved by using pressure switches in pneumatic circuits. (12) AU-Ch, Dec 2009. Book No. 204. Page No. 335
(ii) How is AND function achieved in a fluidic circuit? (4) AU-Ch, Dec 2009. Which is similar to the NOR gate except that the NOR gate is used to determine when none of the control signals is present, whereas the AND gate is used to determine when all the control signals are present, whereas the AND gate is used to determine when all the control signals are present. Both the C1 and C2 control signals are present to get an output at the O1 port. The absence of either or bot signals will result in the stable output at the O2 port.
7. i)List out various operating problems associated with pumps and valves and the corresponding possible causes and suitable remedy for each problem. (10)AU-Ch, APR 2008. Book No. 204. Page no. 365
ii) Write sort notes on Bistable Flip-flops. (6) AU-Ch, JUNE 2012. In this, the air flow is going from the input Ps down through the o channel or the 02 channel depending upon the signal from either C1or C2.
A truth table tel1s how a particular device behaves. Number 0 means 'OFF' and number 1 means 'ON' for al1 devices. Therefore when control signal C1 is 'ON' and control signal C2, is 'OFF' the output is at o1.
