Nuclear Power Corporation of India Limited (A Government of India Enterprise)
TRAINING MANUAL ON TURBINE HYDRULIC COMPONENTS COURSE NO. : SA-M-FH-3.4
Prepared by
:
Prashant Puri, STO (M)
Reviewed by
:
C.M. Mishra, ENC (MT)
Approved by
:
N. Nagaich Training Superientendent, RAPS 1 to 4
Nuclear Training Centre RAJASTHAN ATOMIC POWER STATION Revision (1) July, 2002
Next Revision due : July, 2007
PREFACE
This training manual on Hydraulic Componenets is intended to be used for basic skill training for Scientific Assistant trainees on Oil Hydraulic Equipments. This manual is prepared with a view of imparting training effectively on Function, Construction, Material of parts, and testing aspects of the Oil Hydraulic Components. This training manual will also be used for Tradesman trainees. It has been prepared in view the important aspect of the subject and bring the competency in the new entrants so that they can do maintenance right at the first attempt. This Manual also meets requirement of Hydraulic Components (Oil, D2O components) course of Fuel Handling Operation & Control Maintenance Scientific Assistant & Tradesman Trainees. I express my sincere thanks to Shri C.M. Mishra, ENC(MT) for his guidance and encouragement to complete this task. I owe my sincere gratitude to Shri N.Nagaich, Training Superintendent, RAPS-1 to 4 for his kind cooperation and motivation in preparation of this manual.
PRASHANT PURI Senior Training Officer Nuclear Training Center
CONTENTS DESCDRPTION
PAGE No.
Chapter - 1 PRINCIPLES OF HYDRAULICS
1
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 Chapter - 2
Fluid Power and Hydraulics Pascal's law Force, Weight and Mass Pressure Work Power Torque Speed Transmission of the Fluid Power Advantage of Hydraulic System HYDRAULIC FLUID
1 1 2 2 3 3 4 4 4 5 7
2.1 2.2 2.3 2.4 2.5
Introduction Purpose of the Hydraulic Fluid Quality Requirements Fluid Properties Types of Hydraulic Fluids
7 7 7 8 11
Chapter - 3 STRAINER AND FILTER
14
3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8
14 14 14 15 15 16 17 18
Introduction Settling Degree of Filtration Strainer and Filter Location of Strainer and Filters Types of Filtering Material Type of Filter Elements Magnetic Strainer
Chapter - 4 ACTUATORS
19
4.1 4.2 4.3
19 19 22
Introduction Liner Actuators Hydraulic Motors
Chapter - 5 DIRECTION CONTROL VALVE
26
5.1 5.2
26 26
Introduction Check Valves
53
5.3 5.4 5.5
Spool Valves Pilot Pressure Sources Deceleration Valves
29 32 32
Chapter - 6 FLOW CONTROL VALVES
34
6.1 6.2 6.3 6.4 6.5
34 34 35 36 37
Introduction Flow Control Methods Types of Flow Control Valve Pressure Compensation in Flow Control Valves Temperature Compensation of the Flow Control Valve
Chapter - 7 PRESSURE CONTROL VALVES
39
7.1 7.2
39 39
Introduction Types of the Pressure Control Valves
Chapter - 8 STANDARD GRAPHICAL SYMBOLS
47
Chapter - 9 PRACTICAL ON HYDRAULIC TRAINER
50
CHECK LIST FOR MVD PRACTICE
71
CHECK LIST FOR RELIEF VALVE PRACTICE
73
CHECK LIST FOR LINER ACTUATOR PRACTICE
75
CHECK LIST FOR FLOW CONTROL VALVE PRACTICE
77
54
CHAPTER - 1
PRINCIPLES OF HYDRAULICS 1.1
FLUID POWER AND HYDRAULICS: Hydraulics and fluid power have become every day terms in the modern industrial plant. These fluid power system that transmit force through a fluid to perform work. The fluid can either be a liquid such as oil or water, or a gas, such as compressed air, nitrogen, or carbon dioxide. A fluid power system that uses a gas as the transmitting medium is called PNEUMATIC SYSTEM. A system that uses liquid as the transmitting force is called a HYDRAULIC SYSTEM. The word hydraulic is derived from the Greek words " hydro" (meaning water) and "aulis"( which means pipe). Originally, hydraulic means only to the flow of the water in pipes. Today it includes the flow of any liquid in the system. Some common examples of the hydraulic systems include automobile braking system, power steering, hydraulic elevators, and hydraulic lifts in the gasoline stations. Industrial examples are like operation of the lifts, jacks, lifts, hoists, presses, riveting machines torque converters etc.
1.2
PASCAL'S LAW When pressure is exerted on a confined liquid, the pressure is transmitted equally in all the direction through the liquid, as shown in fig.1.1. If the hammer strikes the solid block of the wood, the force is only transmitted via straight line, but if hammer strikes a fluid the force is transmitted in all the direction. Similarly, the pressure
Fig. 1.2
Fig. 1.1
1
exerted on the liquid in fig.1.2, is equally distributed by the liquid through the system. The pressure in the tubing and containers acts with equal force in all directions.
1.3
FORCE, WEIGHT, AND MASS A force is a push or pull that is exerted on an object in order to change its position or the direction of the movement. In the hydraulic system force must be present at all the times in order for the system to function. As shown in fig.1.3, a pump exerts a force on a stream of hydraulic fluid. This force must be sufficient both to overcome the resistance to the fluid flow by the tubing and to do the work of the system. The more work the system must do, the more force required. An object, or the substance, has a weight as a result of the gravitational force, or pull on the object. Weight is always a downward force. In hydraulic system, the fluid in the reservoir, the lines or any of the components, has weight. This is true whether fluid is standing still or moving. All objects or substances also have MASS. The mass represent the amount of the matter in an object and its inertia, or resistance to movement. The mass of the object determines its weight on the earth, or in any other gravitational field. The inertia determines how much force is required to start, stop or cause a change in the movement of an object. The greater its mass the more force is required to overcome its inertia.
Fig. 1.3
1.4
PRESSURE Pressure is the amount of the force exerted on an object or a substance derived by the area over which the force is exerted. In hydraulic system, we are concerned with two types of the pressure - atmospheric and hydraulic. Atmospheric pressure is at all 2
the times on the fluid reservoir which is vented to the atmosphere. Hydraulic pressure is created by the fluid pump and acts on all internal passages on the discharge side. On the intake side a negative pressure exists, which is more properly referred as a partial vacuum. In most of the cases the pressure gauges which are mounted on the hydraulic systems do not indicate the absolute pressure. Pressure results whenever the flow of the fluid is resisted. The resistance may come from (1) a load on the actuator or (2) a restriction (or orifice) in the piping. The tendency to cause flow ( or the push) may be supplied by the mechanical pump or may be caused simply by the weight of the fluid. In most of the time in hydraulic system positive displacement pump is used. Function of positive displacement pump is to cause the flow only. Pressure is created when flow is restricted. Pressure can be lost when there is a leakage path that will divert all flow from the pump. It is well known that in the water, pressure increases with the depth. The pressure is always equals at any particular depth due to the weight of the water above it. An orifice is a restricted passage in a hydraulic line or a component, used to control the flow or create a pressure difference (pressure drop). In order for oil to flow through an orifice, there must be a pressure difference or pressure drop through the orifice. If there is no flow then there will not be any pressure drop.
1.5
WORK Work takes place when a force is moved through a distance. WORK = FORCE X DISTANCE In hydraulic system, force is exerted by the fluid pressure acting over the flow area. The flow area of the cylinder can be expressed as the product of the piston area to the stroke. Work done by the hydraulic cylinder will be given in the following formula : Hydraulic work = Pressure X Piston area
1.6
POWER Power is defined as an amount of the work done in a given amount of the time. Power = Force X Distance/Time In hydraulic system speed and distance is indicated by flow and force is indicated in terms of the pressure. Power = Flow X Pressure Normally power is expressed in term of the H.P.
3
H.P. = GPM (flow) X PSI(pressure) X .000583
1.7
TORQUE Torque is the term used in the case of rotary actuator. Torque =
C. X H. P. R. P. M.
where C is the dimension conversion factor.
1.8
SPEED In hydraulic system speed of the actuator depends on its size and rate of the oil flow into it. For the same actuator size actuator speed depends on the flow only.
SPEED =
So we can conclude that the force or the torque of an actuator is directly proportional to the pressure and independent of the flow. Speed and the rate of the travel depend on the amount of the fluid flow.
1.9
˚ ¸POWER ˇ ı TRANSMISSION OF THE FLUID
ˇ
ˇ As shown in the fig.1.4(A), if a force of the 10 pounds is applied to the piston 1, it will be transmitted through the liquid in the cylinder to the piston 2. Pascal's law states that pressure developed in the confined fluid is equal at all the point. Therefore internal fluid pressure developed in a confined fluid is equal at every point. Therefore, the internal pressure developed by piston 1 acts on the piston 2. If area of the each piston is same, the force developed on piston 2 is same as the force applied by the piston 1, discounting the friction losses. This is the principle upon which hydraulic power systems are based. The cylinder in the fig.1.4(A) has been replaced by the two individual cylinders in the fig.1.4.(B). Both are of the same diameter and connected with suitable hydraulic line. The conditions present in fig.1.4.B) are not changed, because the hydraulic system has not been changed. The force applied to piston 1 is transmitted through the fluid to the piston 2. Some frictional losses are present in any operating system, but we are not considering it. A similar arrangement of the two pistons connected by a tube is shown in fig.1.5 However the pistons are placed in a vertical position and are different sizes. If a force of 100 pounds is applied to the 10 square inches of the area of piston 1, a hydraulic pressure of 10 psi. is built up under piston 1, in the connecting tubing and 4
under the 50 square inch area of the piston 2. The 10 psi. therefore exert a total force of 500 pounds on piston 2. This increase in the power is hydraulic leverage, and all in the similar applications. However, if the applied force is reversed and the 500 pounds in fig.5. is applied against the piston 2, the out put force on the piston 1 is reduced to 100 pounds. Both of these examples demonstrate how force can be increased or decreased in a hydraulic system by leverage. There is another principle of leverage that must be remembered. That is, for every force increase in a two piston system, there is a corresponding movement decrease. If piston 1 in the fig.1.5 moves 5 inches, it displaces 50 cubic inches of fluid. The 50 cubic inches of the hydraulic fluids is transmitted through the system to the piston 2. The 50 cubic inches of fluid acts on the 50 square inch area of the piston 2, causing it to move 1 inch. The arrangement of pistons shown in fig.1.5 provides a ratio of the 5 to 1 for any force applied on piston 1. At the same time the amount of the piston, movement of the piston 2 is 1/5 the movement of the piston 1. The velocity or the speed of the piston 2 is also 1/5 the velocity of the piston 1. No matter what the ratio, if you want to multiply the hydraulic force of the system, you will reduce the amount and the speed of movement. On the other hand, if force is applied to the larger piston, you increase the amount and speed of the movement, but you reduce the force exerted by the system.
Fig. 1.4
1.10
Fig. 1.5
ADVANTAGES OF HYDRAULIC SYSTEM Variable speed : Most electric motors runs at the constant speed. It also desirable to operate an engine at a constant speed. The actuator of the hydraulic system, however, can be driven at infinitely variable speeds by varying the pump delivery or using a flow control valve.
5
Reversible : Few prime movers are reversible. Those that are reversible must be slow down to a complete stop before reversing them. A hydraulic actuator can be reversed instantly while in full motion without any damage. A four way directional valve or a reversible pump provides a reversing control, while a pressure relief valve protects the system components from excess pressure. Overload protection : The pressure relief valve in a hydraulic system protects it from overload damage. When the load exceeding the valve setting, pump delivery is directed to the tank with definite limits to the torque or force out put. The pressure relief valve also provides a means of setting a machine for specified amount of torque or force, as in chucking and clamping operation. Small packages : Hydraulic components, because of the their high speed and pressure capabilities, can provide high power output with very small size and weight. Stalling : A hydraulic actuator can be stalled without damage when overloaded, can be restarted immediately as soon as the load is reduced. During stalling the relief value diverting the delivery from pump to tank.
6
CHAPTER - 2
HYDRAULIC FLUID 2.1
INTRODUCTION Without fluid, a hydraulic system will not function at all. With the wrong kind of fluid the system may or may not function, depending on the type of the fluid that is used. If the system work with the wrong type of fluid it is doubtful if it will operate for a long time without any problem or with any degree of accuracy.
2.2
PURPOSE OF THE HYDRAULIC FLUID The hydraulic fluid has four primary functions : a.
To transmit power
b.
To lubricate moving parts
c.
To seal, clearances between parts
d.
To cool or dissipate heat.
Power transmission : As a power transmitting medium, the fluid must flow easily through lines and components passages. Too much resistance to flow creates considerable power loss. The fluid must be as incompressible as possible so that action is instantaneous when the pump is started or a valve shifts. Lubrication : In most hydraulic components, internal lubrication is provided by the fluid. Pump elements and other wearing parts slide against each other on a film of fluid. For long component life the oil must contain the necessary additives to ensure high anti wear characteristics. Sealing : In many instances, the fluid is the only seal against pressure inside a hydraulic component. The close mechanical fit and oil viscosity determines leakage rate. Cooling : Circulation of the oil through lines and around the walls of the reservoir gives up heat that is generated in the system.
2.3
QUALITY REQUIREMENTS In addition to these primary functions, the hydraulic fluid may have a number of other quality requirements. These are as given below : 7
2.4
D
Prevent rust
D
Prevent formation of sludge, gum and varnish.
D
Depress foaming.
D
Maintain its own stability and thereby reduce fluid replacement cost.
D
Maintain relatively stable over a wide temp. range.
D
Prevent corrosion and pitting.
D
Separate out water
D
Compatibility with seals and the gaskets.
D
It should have moderate viscosity.
FLUID PROPERTIES These properties of the hydraulic fluid which enable it to carry out its primary functions and fulfill some or all of its quality requirements. 2.4.1 VISCOSITY It is the measure of the fluid resistance to flow; or an inverse measure of the fluidity. If a fluid flows easily, its viscosity is low. This fluid is thin or low in the body. A fluid that flows with difficulty has high viscosity. It is thick or high in body. a)
Effect of high viscosity : Too high viscosity increases friction, resulting in :
D
High resistance to flow.
D
Increased power consumption due to frictional loss.
D
High temp. caused by the friction.
D
Increased pressure drop because of the resistance.
D
Possibility of sluggish or slow operation.
D
Difficulty in separating air from oil in reservoir.
D
Greater vacuum at the pump intake, causing cavitation and reduce pump efficiency.
D
Higher system noise level.
b)
Effect of the low viscosity :
D
To low viscosity decreases the overall efficiency in the following ways :
D
Increased leakage within valves and actuators, resulting in the loss of 8
precision control and some power. D
External leakage developing at gaskets, mechanical connections, and seals.
D
Increased pump slippage resulting in the loss of pressure and lower volumetric efficiency.
D
Increased wear on the moving parts, especially control valves and actuators.
D
Increased fluid temps. and shortened fluid service life.
2.4.2
VISCOSITY INDEX
Viscosity is an arbitrary measure of a fluid's resistance to viscosity change with temperature changes. A fluid that has a relatively stable viscosity at temperature extremes has high viscosity index (VI). A fluid that is very thick in cold and very thin when hot has a low VI. An example chart shows that how viscosity varies with temperatures.
VI
degree F
100 degree F
210 degree F
50
12,000 SUS
150 SUS
41 SUS
90
8,000 SUS
150 SUS
43 SUS
In this example 90 VI oil is thinner at zero degrees and thicker at 210 degrees, while both have the same viscosity at 100 degrees. 2.4.3
POUR POINT
Pour point is the lowest temperature at which a fluid will flow. It is very important specification if the hydraulic system will be exposed to extremely low temperature. For a thumb rule pour point should be 20 degree F below the lowest temp. to be encountered. 2.4.4
LUBRICATION ABILITY
To have a proper lubrication full film lubrication should be there. However, in certain high performance equipment, increased speeds and pressure, coupled with lower clearances, cause the film of fluid to be squeezed very thin and condition of the boundary lubrication occurs. 2.4.5
OXIDATION RESISTANCE
This is the most important property of the hydraulic fluid. Oxidation is the chemical 9
union of the oxygen with the hydrogen or carbon of oil. Formation of oxidation products take place. In reaction formation of acids also take place. Additional reactions cause the formation of gum, sludge and varnish. First stage products which stay in oil (acidic in nature) cause corrosion throughout the system, in addition to increasing the viscosity of the oil. Insoluble gums, sludge, and varnish plug orifices, valves openings, increase wear and cause valve to stuck. There are always number of oxidation catalyst or helpers in a hydraulic system. Heat pressure, contaminants, water, metal surfaces all accelerate oxidation once it starts. Additives are used to reduce the oxidation of the oil. 2.4.6 RUST AND THE CORROSION PREVENTION : As previously mentioned that corrosion is the chemical reaction of acid with metal of the system. Rust is formed by the chemical reaction of the water with metal. During corrosion, particles of the metal are dissolved and washed away. Both rust and corrosion contaminate the system and promote wear. Rust and corrosion can be inhibited by incorporating additives that plate on the metal surfaces to prevent their being attacked chemically. 2.4.7
DEMULSIBILITY
Small quantities of water can be tolerated in most systems. In fact, some anti-rust compounds promote a degree of emulsification, or mixture with any water that gets into the system. This prevents the water from settling and breaking through the antirust film. However, very much water in the oil will promote the collection of contaiminants and can cause sticky valves and accelerated wear. With proper refining, a hydraulic oil can have a high degree of demulsibility or ability to separate out water. 2.4.8 FOAMING Although all fluids are susceptible to foaming, the amount of foam in a system can be reduced to a minimum by the addition of the chemical foam depresents. These additives do not prevent air from dissolving in a fluid or prevent foam from foaming when air pressure is suddenly reduced. A foam depresent forms an unstable foam with larger bubbles that break up faster once it has broken out of the fluid. This reduces the total amount of the foam that is formed. As a result, the amount of the foam that accumulates in the reservoir can be held down to a comparatively thin layer.
10
2.5
TYPES OF HYDRAULIC FLUIDS 2.5.1
PETROLEUM OIL
Petroleum oil is still by far the most highly used base for hydraulic fluids. The characteristics or properties of petroleum oil fluids depend on three factors : D
The type of the crude oil is used.
D
The degree and method of the refining.
D
The additives used.
Advantages : D
Excellent lubrication.
D
Oil naturally protects against rust; seals well; dissipates heat easily.
D
Easy to keep clean by filtration or gravity separation of contaiminants.
Disadvantages : D
It can catch fire.
2.5.2
WATER-GLYCOL TYPE FLUIDS
It contains (A) 35% to 40% water to provides resistance to burning. (B) A glycol (C) Additives used for preventing rust, corrosion, foaming, increasing viscosity. Advantages : D
Good wear resistance
D
Fire resistant.
Disadvantages : D
Fluid has high gravity which can create vacuum at the pump inlet.
D
Not compatible with zinc, cadmium, magnesium.
D
Asbestos, leather and cork should not be used as these material absorbs the water.
D
It is necessary to measure the quantity of the water continually and make up for evaporation to maintain required viscosity. Evaporation also cause the loss of additives, there by reducing the life of fluid and of the hydraulic components.
D
Cost is greater than for conventional oils.
11
2.5.3 Water oil Emulsion (a)
Oil in water : Oil in water contains tiny droplets of specially refined oil dispersed in water. In addition to these it contains emulsifiers, stabilizers and other additives to hold the two liquids together.
Advantages : D
Highly fire resistant.
D
Excellent cooling properties.
D
Compatible with seals and metals.
Disadvantages : D
Poor lubricity but can be increased with the help of additives.
D
Rusting problem may be there, but can be decreased with additives.
D
Has greater affinity for contamination so it required extra attention for filtration.
(b)
Water in oil : Water in oil contains tiny droplets of the water dispersed in oil. In addition to these it contains emulsifiers, stabilizers and other additives to hold the two liquids together.
Advantages : D
Good fire resistant.
D
Excellent lubricity.
D
Better cooling ability.
D
Compatible with seals and metals.
Disadvantages : D
Rusting problem may be there but can be reduced with the help of additives.
D
Have affinity for the contamination and required extra attention for filtration.
D
Foaming problem may be there but can be reduced with the help of additives.
2.5.4
Synthetic Fire Resistant Fluids
Synthetic fire resistant fluids are laboratory synthesized chemicals. Typical of these are : (1) phosphate esters (2) chlorinated hydrocarbons (3) Silicate esters (4) Combination of these.
12
Advantages : D
Fire resistant.
D
Since they contain no water or other types of volatile materials so they can be used at high temp. without loss of any material.
D
They are also suitable for high pressure system.
Disadvantages : D
As these fluids have high gravity so chances of cavitation is there at pump inlet.
D
Not well for low temp. system.
D
Not compatible with the materials like Nitrile (Buna) and Neoprene.
13
CHAPTER - 3
STRAINER AND FILTER 3.1
INTRODUCTION Clean hydraulic fluid is very important if a fluid power system is to function properly for acceptable period of time. During operation, a hydraulic system picks up and generates a lot of contaiminants. As much contamination as possible should be removed or treated while system is in operation. Otherwise, equipments begin to wear, valves begin to stick, seals begin to leak, and hydraulic system no longer functions properly.
3.2
SETTLING The larger or heavier contaiminants usually settle to the bottom of a fluid that is moving slowly or standing still. As shown in fig 3.1, returning fluid enters the reservoirs through the return line, and flows slowly around and over the baffle which divides the reservoir in half. The larger and heavier solid and sludge particles settle to the bottom of the reservoir. Sludge should be removed at regular intervals.
Fig. 3.1 3.3
DEGREE OF FILTRATION Strainer and filter are made so they pass particles of one size and stop those of another, with the strainer removing the larger particles and the filter removing smaller particles. In addition to their opening size, strainers and filters are describe by their degree of filtration. For instance, a strainer with a degree of filtration of 25 microns or more. This is known as its NOMINAL RATING. This same strainer, however may remove 100 percent of the particles from a fluid if they are all larger than 50 microns. This is called its ABSOLUTE RATING. 14
3.4
STRAINER AND FILTER Filter A device whose primary function is the retention, by some porus medium, of insoluble contaiminants from a fluid. Strainer A course filter.
3.5
LOCATION OF STRAINER AND FILTERS 3.5.1
Inlet strainer and filters :
Both filters and strainers are available for inlet lines. Filter alone are generally used in other lines. Refer fig.3.2 illustrates a typical strainer of the type installed on the pump inlet lines inside the reservoir. It is relatively coarse as filters and being constructed of fine mesh wire. A 100-mesh strainer, suitable for thin oil, protects the pump from the particles above about 150 microns in size. There are also inlet lines filters. These are usually mounted outside the reservoir near the pump inlet. They too must be relatively coarse. A fine filter creates more pressure drop than can be tolerated in an inlet line.
Fig. 3.2 3.5.2
Pressure line filters :
These filters are designed to trap much smaller particles than inlet line filters. Such a filter might be used where system components such as valves are less dirt tolerant than the pump. The filter thus would trap this fine contamination from the fluid that it leaves the pump. Pressure line filter of course must be able to withstand the operating pressure of the system, refer fig.3.3
15
Fig. 3.3 3.5.3
Return line filter :
Return line filters also can trap very small particle before the fluid returns to the reservoir. They are particularly useful in the systems which do not have large reservoir to allow contaiminants to settle out of the fluid. A return line filter is nearly a must in the system with a high performance pump which has very close clearances and usually can not be sufficiently protected by an inlet filter, refer fig. 3.4
Fig. 3.4 3.6
TYPES OF FILTERING MATERIAL Filter materials are classified as mechanical, absorbent and adsorbent.
3.6.1
Mechanical : Mechanical filters operate by trapping particles between closely woven metal screens or discs. Most mechanical filters are relatively coarse.
3.6.2
Absorbent : Absorbent filters are used for most minute particle filtration in hydraulic systems. They are made of wide range of porous materials, including paper, wood pulp, cotton yarn and cellulose. Paper filters are usually resin impregnated for strength.
16
3.6.3
Adsorbent or active filters : These type of filters have surface absorption. Cleaning of these filters is very difficult. These types of filters should be avoided in hydraulic system, since they may remove essential additives from the hydraulic fluid.
3.7
TYPES OF FILTER ELEMENTS 3.7.1
Surface filter :
Surface type filters are made of closely woven fabric or treated paper with pores to allow fluid to flow through. Very accurate control of pore size is a feature of surface type elements. See fig.3.5
Fig. 3.5 3.7.2
Depth type filter :
This types of filters are composed of layers of fabrics or fibres which provides many tortous paths for the fluid to flow through. The pores are or passages vary in the size, and degree of filtration depends on the flow rate. Increases in flow rate tends to dislodge trapped particle. This type of element is generally limited to low flow low pressure drop conditions, refer fig. 3.6
Fig. 3.6 17
3.7.3
Edge type filter :
An edge type filter separates particles from oil flowing between the finely spaced plates. The filter as shown in fig.3.7 features stationary cleaner blades which scrape out the collected contaiminants when the handle is twisted to turn the element.
Fig. 3.7 3.8
MAGNETIC STRAINER These magnetic plug or strainer are used to pick up magnetic particles generated from equipment and process line wear, refer fig.3.8
Fig. 3.8
18
CHAPTER - 4
ACTUATORS 4.1
INTRODUCTION : Two types of actuators are there in the hydraulic system (1)
Linear actuator i.e. known as piston- cylinder actuator
(2)
Rotary actuator i.e. known as motor
Actuators are used to perform work i.e. to convert the pressure energy of hydraulic fluid into the mechanical work. 4.2
LINEAR ACTUATORS : 4.2.1
Linear actuators and of different
a)
Single acting actuator
Single acting actuator has a power stroke in one direction only - usually on the out or extending stroke because the piston has a larger surface area on the cap end. The load or some other force is used to return the piston to its original position when the work is completed. Ram type actuator is also a example of the single acting actuator. These are mostly mounted vertically and retract by the force of gravity on the load. Practical for long stroke, ram type actuator are used in elevators, jacks and automobile hoists. Refer fig.4.1 for single acting actuator. Telescopic actuator A telescopic actuator is used where the collapsed length must be shorter than could be obtained with standard actuator. Up to 4 and 5 sleeves can be used, these actuators can be double acting or single acting. Refer fig.4.3 for telescopic actuator arrangement.
Fig. 4.2a
Single Acting Fig. 4.1
Fig. 4.3
Fig. 4.2b 19
b)
Double acting actuator
The double acting actuator is so named because it is operated in both directions. This type of actuator is having the power stroke in both the directions. This type of actuator is classified in two types (1) differential (2) non-differential. Differential type double acting actuator : These type of actuator is having unequal areas on the both sides so subjected to different forces during the forward and return movements. The difference being a function of the cross sectional area of the rod. Refer to fig.4.2a, extending stroke is slower, but capable of exerting a greater force than can be obtained when piston and rod are being retracted. Non-differential type double acting actuator : This is also known as double rod actuator, these are used where it is advantageous to couple a load to each end, or where equal speed and equal force are required in both directions. Refer fig.4.2b for non-differential type double acting actuator. 4.2.2
CYLINDER RATINGS
The rating of a cylinder include its size and pressure capability. Most come with standard rod size although intermediate and heavy duty rods are available. Cylinder size is piston diameter and stroke length. The speed of the actuator, the output force available and a pressure for a given load all depend on piston area. The area of the piston rod must be substracted when piston is being retracted. 4.2.3 CONSTRUCTION OF CYLINDER Refer fig.4.4. for detailed construction, Piston rod : one end of the piston rod is connected to the piston and other end is connected to the device which do work, depending upon the requirement of the job. Piston rod must be extremely durable, since they are always subjected to grinding compounds and other adverse conditions. Piston rods are made of good grade of steel that is ground and polished to an extremely smooth finish, and they may be hardened and chrome plated to resist wear. Stainless steel is often used to resist corrosion. Rod wiper : A road wiper (made of durable synthetic material) is used to clean the piston rod as it is retracted into the cylinder. All foreign material in the rod must be removed before the rod is drawn back into the packing.
20
Cylinder cover : Each cylinder has two covers - the front or rod end cover and the blank or blind end cover. Sometimes the blind end cover is a part of the cylinder body or tube. The functions of cylinder cover are : to seal the ends of the cylinder tubes; to provide a method of mounting; to provide a housing for seals, rod bearing, and rod packing (front cover); to provide the ports of entry for fluid; to absorb the impact of the piston; and to provide the room for the cushioning arrangement. Cylinder tube : Piston move inside the cylinder tube. The sealing action of the piston depends largely on the finished obtained in the cylinder used. The covers are often fastened to the cylinder tube with cover screws. Piston assembly : The piston must fit closely to the cylinder to provide a suitable bearing and to eliminate any possibility of extrusion of synthetic seals. Since the function of piston is to act as a bearing, it must be made of the material that will not score the cylinder tube. Piston assembly has piston rings similar to the automotive tube rings. Piston with automotive rings has relief at each end, so that any small particle of dirts which may get into the system will not spring the end land cause the piston ring to be crushed or frozen. Sometimes lip seals and rings are also used. Piston lock nut : A locking-type nut prevents the piston from coming loose on the piston rod; it is compact and secure. Tie rods : Tie rods are used to hold the cylinder together. They must be strong enough to absorb the shock loads as the piston contacts the cylinder cover. Piston rod bearing. The piston rod bearing not only houses the rod packing but also acts as a bearing and as a guide for the piston rod. Rod packing : The rod packing seals the piston rod , so that the fluid cannot escape around the rod. Piston rod packings are of various types such as lip ring, O ring etc. Cover gaskets : Cover gaskets act as a seal between the cylinder cover and the cylinder tube. Normally 'O' rings are used. 4.2.3
CYLINDER CUSHIONS
Cylinder cushions (refer fig.4.5) are often installed at either ends of the cylinder to slow it down near the end of the stroke and prevent the piston from hammering against the end cap. Deceleration begins when the tapered cushion ring or plunger enters the cap and begins to restrict exhaust flow from the barrel to the port. During the final fraction of the stroke, exhaust oil must discharge through and adjustable orifice. The cushion feature also includes a check valve to bypass the orifice on the return stroke. This type of arrangement is used in the operation of vault doors. 21
Fig. 4.4
Fig. 4.5 4.3
HYDRAULIC MOTORS Motor is a rotary type of actuator. Motor very closely resemble pump in construction. Instead of the pushing on the fluid as pump does, as output member in the hydraulic system, they are pushed by the fluid and develop torque and continuous rotating motion. 4.3.1 MOTOR RATINGS Hydraulic motors are rated according to displacement (size), torque capacity and maximum pressure limitations. Displacement It is the amount of fluid which motor will accept in turning one revolution (fig.4.6); or in other words, capacity of one chamber multiplied by number of chambers the mechanism contains.
22
Fig. 4.6 Torque Torque is the force component of the motor's output. It is defined as turning or twisting effort. Motion is not required to have torque but motion will result if torque is sufficient to overcome friction and resistance to load. Pressure Pressure required in hydraulic motor depends on the torque load and displacement. A large displacement motor will develop a given torque with less pressure than smaller unit. Torque = PRESSURE X DISPLACEMENT
/ 2 X PIE
There are various types of motor actuators like vane motors, gear motors, piston motors etc. but we will concern only with bent axis type piston motor (used in fuel handling system) and balanced and unbalanced sliding vane type piston motors. 4.3.2
TYPES OF HYDRAULIC MOTORS
a.
Vane Motor
In a vane motor, torque is developed by the pressure on exposed surfaces of the rectangular vanes which slide in and out of slots in a rotor splined to the drive shaft (fig.4.7) As the rotor turns, the vanes follow the surface of cam ring, forming sealed chambers which carry the fluid from inlet to outlet. In unbalanced design a thrust exist in the direction from inlet to outlet of the fluid. In the balanced design as shown fig.4.6 pressure built up at either port is directed to two interconnected chambers within the motor located 180 degree apart. Any side loads which are generated oppose and cancel each other. 23
Figure : 4.7 b.
Bent Axis Piston Motor
Refer fig. 4.8, in this motor the shaft bearings and bearing housing hold the output shaft and prevent any axial movement as fluid force is applied against the pistons. All pistons and piston rods for a given size motor are the same size and length and extend a fixed distance from the flange of the output shaft. The cylinder block is so mounted that it is free to rotate at a fixed angle in respect of the out put shaft. At the inside of this angle, the cylinder block is closest to the output shaft flange therefore the piston make their deepest penetration into the bores of the cylinder block. At the outside of this fixed angle, the cylinder block of farthest from the flange of the output shaft, therefore there is least penetration by the piston into the bores of the cylinder block. The application of hydraulic force through the valve plate inlet slot to the piston which is deep in the cylinder block tends to force this piston away from the valve plate, however the only manner the piston can move away from the valve plate is by rotating the output shaft and cylinder block. Therefore piston is forced through a stroke by input flow until it is around to the outside of the fixed angle and its outer most position in the cylinder block. At this point cylinder block opening is momentarily blocked until continued rotation open the cylinder bore by rotating shaft ( which is being driven by succeeding pistons) so that fluid is forced out of the outlet connection. Shortly before the piston is again reaches its deepest penetration, the piston bore is again blocked off by the valve plate until further rotation being it to resister with the valve plate inlet slot.
24
Figure : 4.8 Housing angle : The displacement of a rotary motor is the amount of the fluid necessary to rotate the output shaft through one rotation. In this, displacement is determined by the size of cylinder bores and the length of the piston strokes. In any piston motors the longer the piston stroke greater the displacement. Increased housing angle increases the length of stroke and provide greater displacement. Relation between speed and work potential (torque) of the motor depends upon its displacement. If the displacement were increased to increase work potential with input flow and pressure remaining constant, output speed would be proportionally reduced. If the displacement were decreased to increase output speed keeping pressure differential constant across the motor, torque would be proportionally reduced. Thus a motor with a 30 degree housing has a higher torque to speed ratio than a motor with 23 degree housing, because former has a greater angle.
25
CHAPTER - 5
DIRECTION CONTROL VALVE 5.1
INTRODUCTION
Directional valves, as the name implies, are used to control the direction of flow. Though sharing this common function, direction valves vary considerably in construction and operation. They are classified according to their principle characteristics, as such : D
Type of the internal valve element Poppet (piston or ball), rotary spool and sliding spool.
D
Method of actuation Cams, manual lever, mechanical, electrical solenoid, hydraulic pressure (pilot operated).
D
Number of flow paths Two way, three way, four way, etc.
D
Size Nominal size of pipe connections to valve or its mounting plate, or rated gpm flow.
D
Connections Pipe thread, straight thread, flanged.
5.2
CHECK VALVES Check valves are unidirectional two way valves. A check valve permits free flow in one direction only and blocks flow in the other direction. Normally ball or poppet checks are held against the check valve seat by a spring. Hydraulic fluid under pressure forces the ball or poppet of check valve and make the opening for flow through the valve to the outlet. When the inlet pressure is reduced, pressure in the outlet closes the check valve against the seat and shut off the reverse flow through the check valve. Inline check valve Inline check valves are so named because they are connected into the line and the oil flows straight through. The valve body is threaded for pipe or tubing connector, 26
and is machined to form a seat for the poppet or ball ( as shown in fig. 5.1). A light spring holds the poppet seated in the normal closed position permitting the valve to be mounted in any attitude. In the free flow direction, the spring will be overcome and the valve will crake open at about 5 psi pressure drop. The spring are not adjustable.
Figure 5.1 Right angle check valve A heavier duty unit, the right angle valve, has a steel poppet and a hardened seat pressed into the iron body (fig.5.2). It gets its name from the angle between the flow passage to the poppet and the passage away from the poppet. These valves are built in threaded, flanged connected types.
Figure 5.2 27
Restriction check valve A restriction check valve (refer fig.5.3) is a modification of a simple check valve. An orifice plug is placed in the poppet to permit a restricted flow in the normally closed position. This valve is used where a free flow of fluid in one direction and a controlled amount in the other. One example would be in controlling the rate of decompression in a large press.
Figure 5.3 Pilot operated check valve Pilot operated check valve is designed to permit free flow in one direction and to block return flow, until opened by the pilot signal, refer fig.5.4 for operation of this type of check valve.
Figure 5.4 28
5.3
SPOOL VALVES 5.3.1 Sliding type Two-way spool valves In two-way valves, two ports are there, one port is connected to pressure line and other port is connected to the actuator. A spring is there to keep the valve in one position when not actuated. These are of two positions one is normally opened and other is normally closed, refer fig.5.5 is self explanatory. The normally open spool valve is unbalanced because inlet pressure acts on both spool surfaces only when the valve is closed, inlet pressure has to be overcome to open the valve. A normally closed valve is balanced in both position.
Figure : 5.5 Three-way spool valves Three-way spool valves have three working connections. Three way valves can be used as diverter valve (selector valve) and as a directional valve. In fig.5.6 spool of the diverting valve is held in the non- actuated position by the spring. Hydraulic fluid at P is able to flow through the valve and out port 1 to the cylinder A. When the valve is actuated Fig. 5.7, spring is compressed and hydraulic fluid flows from P to port 2 to cylinder B. In this the diverter valve operates only the cylinders. Returns of the cylinders and the actuating fluid to the reservoir is accomplished with another three way valve. The differences between three-way diverting valve and directional valve are in the piping connections and the internal construction. Three-way directional valve in fig.5.8 is in nonactuated position. The position of the spool allows fluid from cylinder to flow through the valve and out of the port T. When the valve is actuated, fluid under pressure flows through the valve and out of the port 1 raising the cylinder. The spool may be the spring loaded to hold the valve in the drain or fill position depending on the position. In the above examples in diverter valve, three way valve is used for raising the cylinder and other for lowering the cylinder. For this application the cylinder movements alternate. In the case of directional valve each cylinder can be operate together, then only one three way 29
directional control valve is required. Three-way valves, like two-way valves, are classified as normally closed valve has the flow blocked from the pressure inlet P to the actuator line when valve is in nonactuated position. A normally open position of valve permits flow from the pressure inlet P to the actuator line when the valve is in a nonactuated position.
Figure : 5.7
Figure : 5.6
Figure : 5.8 Four way valves : A four way valve has four primary working connections, as shown in fig.5.9. It has a pressure fluid inlet P, a return line to the reservoir T, and connections 1 and 2. Connections 1 and 2 connect the fluid outlet port to the actuator, which is usually a cylinder or motor. This valve has four external connections while it has five internal connections (the two exhaust port are directed to a common external connections ). Number of the external connection determines the valve classification. Also pilot lines are not considered as primary lines and are indicated or shown separately with their own connections. Most four way valves are designed to efficiently perform as many functions as possible. To accomplish this most valves are manufactured with three operating positions : middle (nonactuated), actuated to the left and actuated to the right. If the center position is not used, it is called a cross over position and the valves become two position valve. When the middle non actuated position is used, spool is centered by two springs, then this type of the valve is known as the spring centered sliding spool type directional valve. In this manner the valve has to be 30
actuated in both directions from the middle position. If the middle position is not used, a spring is usually placed on one end of the spool and valve is only actuated in one direction ( this is known as spring offset sliding spool type directional valve). If spring is not there and directional valve is actuated in both the positions with actuator like solenoid, hydraulic pressure (pilot pressure), mechanical lever etc, then this type of directional valve is known as " no spring" sliding spool directional valve.
Figure : 5.9 Five way directional valve Five way valve is almost identical to the four way valve only difference is that it has five external ports. Two external exhaust ports are not connected to each other as in the case of four way valves. Separate exhaust connections are used so the fluid leaving each side of the actuator can be regulated. This allows the actuator to travel at different speeds, hold its position, or move in steps. (Refer Fig. 5.10)
Figure : 5.10
31
Rotary valves Rotary valves are made with round core that has passages in it that line up with various openings are ports in the side of valve body. Instead of the valve shifting right or left, it rotates. fig.5.11 shows a four way three position rotary spool directional valves. In position A, spool valves directs hydraulic fluid from the system pressure line to port 1, allowing fluid to enter the system. When the spool is shifted towards the right, fluid is directed to the port 2. In the centre position, C, the ports to the spool valve and the actuators are closed and no fluid flows.
Figure : 5.11 Above spool valves (either rotary or sliding) are actuated by various devices (as explained in the classification or directional valve). Rotary spool valves can also be two way, three way, four way and five way valves. 5.4
PILOT PRESSURE SOURCES In this type, an additional directional valve is used to change the direction of the pilot supply which is going to the main directional valve for shifting the position. This pilot supply is used for shifting large directional valve (main directional valve) as solenoid force is not sufficient to shift the main directional valve.
5.5
DECELERATION VALVES Hydraulic cylinders often have cushions built in to slow down the cylinder pistons at the extreme end of their travel. When it is necessary to decelerate a cylinder at some intermediate position or to slow down or stop a rotary actuator, an external valve is required. Most deceleration valve are cam operated valves with tapered spools. They are used to gradually decrease flow to or from an actuator for smooth stopping or deceleration. A normally open valve cuts of the flow when its plunger is depressed by a cam. Some applications require a valve to permit flow when it is actuated and to cut of the flow 32
when the plunger is released. In this case a normally closed valve is used. This type valve often is used to provide an interlocking arrangement where by flow can be directed to an other branch of the circuit when actuator or load reaches to certain position. Both normally closed valve and the normally opened valve are available with integral check valves to permit reverse free flow, refer fig.5.12.
Figure : 5.12
33
CHAPTER - 6
FLOW CONTROL VALVES 6.1
INTRODUCTION : Volume or flow control valves are used to regulate speed. The speed of an actuator depends on how much oil is pumped into it per unit of the time. It is possible to regulate the speed with the help of variable displacement pump, but in many circuits it is more practical to have a fixed type of pump and have a flow control valve for regulating the speed of the actuator.
6.2
FLOW CONTROL METHODS There are three basic methods of applying volume control by control valves to control actuator speeds. They are meter-in, meter-out and bleed-off. Meter-In Ckt. In this operation, the flow control valve is placed between pump and actuator (fig.6.1). In this way, it controls the amount of the fluid going into the actuator. Pump delivery in excess of the metered amount is directed to the tank over the relief valve. If it is desired to control the flow in both the directions, the flow control can be installed in the pump outlet line prior to the directional valve.
Figure : 6.1 The meter-in method is highly accurate. It is used in application where the load continually resist the movement of the actuator, such as raising a vertical cylinder under load or pushing a load at controlled speed. Meter-Out Ckt. Meter-out control (fig.6.2) is used where the load might tends to run away. The flow control valve is located where it will restrict the exhaust flow from the actuator. To regulate speed in both the directions, the valve is installed in the tank line from the 34
directional valve. If control is needed in one direction then flow control valve is placed in the line between the actuator and the directional valve.
Figure : 6.2 Bleed-Off Ckt. In a bleed off arrangement (fig.6.3), the flow control is bleed off the supply line from the pump and determines the actuators speed by metering a portion of the pump delivery to the tank. The advantages is that the pump operates at a pressure required by the work, since excess flow returns to the tank through the flow control instead of the relief valve. Its disadvantage is some loss of accuracy because the measured flow is to the tank rather than into the cylinder, making the latter subject to the variations in the pump delivery due to changing work loads. Bleed of ckt. should not be used in applications where there is possibility of load running away. 6.3
TYPES OF FLOW CONTROL VALVE The most common way of controlling fluid flow is by a fixed or variable orifice in the hydraulic line. As the area of an orifice increases, the pressure drop and fluid velocity decrease slightly. Fluid viscosity increases, so does the flow resistance. A fixed orifice usually consists of disc with a small hole in it as shown in fig.6.4
35
Figure : 6.3 Systems having varying flow requirements usually are equipped with some type of adjustable flow control valve. Three different types are also shown in fig.6.5. Valve A is very similar to globe valve. Valve B controls the flow with the stem adjustment with the notch in the stem. The valve marked C is slightly different in construction. In this valve size of the slots in the spool sleeve are present. Flow control is regulated by the spool within the sleeve. On many valves, spool may also be moved in or out to change the position of the flow opening.
Fig. 6.4 6.4
Fig. 6.5
PRESSURE COMPENSATION IN FLOW CONTROL VALVES : Flow through orifice is proportional to the square root of the pressure drop across it. If pressure at down stream side changes say increases (as load on the actuator increases), pressure drop will decrease across the orifice, hence it will reduce the flow and vice-versa in the case of reduced load on the actuator. Increased pressure at upstream side increases pressure drop across the orifice, hence increases flow and vice-versa in case of decreased flow at upstream side. To have a constant flow we require pressure compensation in flow control valve.
36
In fig.6.6 a variable flow valve is connected to a two way spool valve to allow for pressure changes. In operation, fluid above (upstream) of the variable orifice acts on the lower face of the spool. The pressure below (downstream) the orifice acts on the upper spring loaded face of the spool. The adjustment on the spring determines the average pressure differential setting for the compensator. The spool is shifted to allow the desired amount of the fluid to flow through the regulator.
Figure : 6.6 If the down stream pressure drops more than the upstream, the spool shifts upwards closing the valve, and restricting fluid flow. If the downstream pressure rises, spool shifts downwards and allow more fluid flow. If the downstream pressure drops greatly, the spool blocks the inlet until the downstream pressure increases enough to allow the spool to return to its pressure modulating position. The valve is set to modulate because of its small movements while regulating the fluid pressure. 6.5
TEMPERATURE COMPENSATION OF THE FLOW CONTROL VALVE With increase in temperature viscosity decreases and hence increases the flow. Viceversa in the case of the reduced temperature. Our requirement is to have a constant flow. For these conditions, a special variable flow valve is used. The one shown in fig.6.7 uses a stem that has a high rate of expansion combined with a tapered notch to obtain fluid temperature flow compensation. When operating cold, the expanding element contracts, allowing increased fluid flow. As the fluid temperature rises, the expanding element lengthens and the opening between the notch and the sleeve becomes smaller, thereby reducing the fluid flow. There are other types of temperature compensation valves, but they are operate on the same principle.
37
Figure : 6.7
38
CHAPTER - 7
PRESSURE CONTROL VALVES 7.1
INTRODUCTION
Pressure control valve performs functions such as limiting maximum system pressure or regulating reduced pressure in certain portion of the circuit, and other function where in actuation is the result of a change in operating pressure. There operation is based on the balanced between the pressure and spring force. Most of them are infinite positioning, that is the valve can be assumed various position between the fully open and fully closed position, depending upon the pressure differential and flow rate.
Figure : 7.1 7.2
TYPES OF THE PRESSURE CONTROL VALVES D
RELIEF VALVE
D
SEQUENCE VALVE
D
PRESSURE REDUCING VALVE
D
COUNTER BALANCED VALVE
D
UNLOADING VALVE
7.2.1
RELIEF VALVES
The relief valve is found virtually in every hydraulic system. It is a normally closed valve connected between the pressure line and the reservoir. Its purpose is to limit the pressure in the system to a present maximum by diverting some or all of the pumps output to tank when the pressure setting is reached. Simple relief valve A simple or direct acting relief valve (fig.7.1) may consist of ball or poppet held seated in the valve body by a heavy spring. When the pressure at the inlet is insufficient to overcome the force of the spring, the valve remains closed. When the 39
preset pressure is reached, the ball or poppet is force off its seat and allows flow through the outlet to tank as long as the pressure is maintained. In most of the valve an adjusting screw is provided to carry the spring force. Thus valve can be set at any pressure to open within the specified range. The pressure at which the valve first begins to divert the flow is called the cracking pressure. As flow through the valve increases, the poppet is forced further off its seat causing increased compression of the spring. Thus, when the valve is bypassing its full rated flow, the pressure can be considerably higher than the cracking pressure. Pressure at the inlet when the valve is passing its maximum volume is called full flow pressure. The difference between the full pressure and the cracking pressure is known as the override pressure. In some cases, pressure override may not be objectional but in many cases considerable wasted power due to the fluid lost through the valve before its maximum setting is reached. It can permit the maximum pressure to exceed the rating of the other components. Where it is desirable to minimize override pressure, a compound relief valve should be used. Compound relief valve A compound relief valve (fig.7.2) operates in two stages. The pilot stage in the upper body contains in the pressure limiting valve, a poppet held against a seat by an adjustable spring. The ports connection are made with lower body, and diversion of full flow is accomplished by the balanced piston in the lower body.
.Figure : 7.2 Balanced piston : The balanced piston is so named because in the normal operation (fig.7.3), it is in hydraulic balance. Pressure at the inlet port acting under the piston is also sensed on its top by means of an orifice drilled through the large land. At any pressure less than the valve setting the piston is held on its seat by light spring. When pressure reaches the setting of the adjustable spring, the poppet is forced off its seat limiting pressure in the upper chamber. 40
Figure : 7.3 The restricted flow through the orifice in the upper chamber results in an increase in pressure in the lower. This unbalanced the hydraulic forces and tends to raise the piston off its seat. When the difference in the pressure between the upper and lower chambers is sufficient to overcome the force of light spring (approx. 20 psi), the large piston unseats permitting flow directly to tank. Increased flow through the valve causes the piston to lift further off it seat but since this compresses only the light spring very little override is encountered. Spool Type Relief Valve These types of valves are direct acting sliding spool type pressure control valve. The spool operate within the body and is held in the closed position by an adjustable spring. Operating pressure sensed through a passage in the bottom cover opposes the spring load. Spool area is such that with heavier spring normally used, the valve would open at approx. at 125 psi. To extend their pressure range, most models include a small piston or plunger in the bottom cover to reduce the bottom pressure reacting area to 1/8 (1/16 in the 2000 psi range) of the area of the spool end. When operating pressure exceeds the valve setting, the spool is raised and oil can flow from the primary to the secondary port. A drain passage is provided in the top cover to drain the spring chamber. This drain also removes leakage oil from the space between the spool and piston by means of passage drilled lengthwise through the spool. Depending on the assembly of the top and bottom covers, this valve can be used as relief valve sequence valve and unloading valve. It can also built with an integral check valve to permit reverse flow when used as sequence and counterbalance valve.
41
As shown in the fig. 7.4 the pressure line is connected to the primary port and secondary port is connected to the tank. This application permits the valve to be internally drained and the upper cover is assembled with the drain passage and aligned with the secondary port. The lower cover is assembled so that the operating pressure is sampled internally from the primary port making necessary to maintain max. system pressure to keep the valve open. In view A, the system pressure against the piston is too low to overcome the spring and the valve remains closed. In view B, pressure has shifted the spool to allow flow to the secondary port and to the tank at the pressure determined by the spring setting.
Figure : 7.4 With the small piston, this valve is capable of operation at higher pressures. However, because of its high override characteristics, it is not recommended for use as a relief valve above 500 psi. 7.2.2
UNLOADING VALVE To use same valve (spool type relief valve) as unloading valve (fig.7.5), the lower cover is assembled to block the internal operating pressure passages. An external pressure source is used to move the spool and divert the flow to the secondary port. The drain connection remains, internal since the secondary port is connected to the tank. The relief valve operates in balance, being held open at one of an infinite number of positions by the flow of oil through it. Max. pressure maintained at the primary port is determined by the spring adjustment. With the unloading valve, however, the primary port pressure is independent of the spring force because the 42
remote pressure source operates the spool. As long as the control pressure is atleast 150 psi above the spring setting, free flow is permitted from the primary to secondary port. 7.2.3
SEQUENCE VALVE A sequence valve is used to cause actions to take place in the system in a definite order, and to maintain predetermined minimum pressure in the primary line while the secondary operation occurs. Fig.7.6 shows the spool type sequence valve. The fluid flows freely through the primary passage to operate the first phase until then pressure setting of the valve is reached. As the spool lifts, flow is diverted to the secondary port to operate a second phase.
Fig. 7.5
Fig. 7.6
To maintain pressure in the primary system, the valve is internally operated. However the drain connection must be external, since the secondary port is under pressure when the valve sequences. If the pressure were allowed in the drain passage, it would add to spring force and raised the pressure required to open the valve. To provide reverse free flow in sequence valve a provision of bypass check valve is provided, as shown in fig.7.7
43
Fig. 7.7 7.2.4
COUNTER-BALANCE VALVE A counter balance valve is used to maintain control over a vertical cylinder so that it will not fall freely because of gravity. The primary port of the sequence valve (with inbuilt check valve) is connected to the lower cylinder port and the secondary port to the directional valve (fig.7.8). The pressure setting is slightly higher than is required to hold the load from falling. When the pump delivery is directed to the top of the cylinder, the cylinder piston is force down causing pressure at the primary port to increase and raise the spool, opening a flow path for discharge through secondary port to the directional valve and subsequently to tank. In cases, where it is desired to remove back pressure at the cylinder and increase the force potential at the bottom of the stroke, this valve can be operated remotely. When the cylinder is being raised (view B), the integral check valve opens to permit free flow for returning the cylinder.
Figure : 7.8 44
The counterbalanced valve can be drained internally. In the lowering position (view A), when the valve must open, its secondary port is connected to the tank. In the reverse condition, it does not matters that load pressure is effective in the drain passage, because the check valve by passes the spool. 7.2.5
PRESSURE REDUCING VALVES Pressure reducing valves are normally-open pressure controls used to maintain reduced pressures in certain portions of the system. They are actuated by pressure sensed in the branch circuit and tends to close as it reaches the valve setting, thus preventing further buildup. Direct-acting pressure reducing valve In the fig.7.9, direct- acting valve has spring loaded spool to control the downstream pressure. If the main supply pressure is below the valve setting, fluid will flow freely from the inlet to the outlet. An internal connection from the outlet passage transmits the outlet pressure to the spool end opposite the spring. When the outlet pressure rises to the valve setting, the spool moves to partly block the outlet port. Only enough flow is passed to the outlet to maintain the preset pressure. If the valve closes completely, leakage past the spool could cause the pressure to build up in the branch circuit. Instead a continuous bleed to tank is permitted to keep it slightly open and prevent downstream pressure from rising above the valve setting. A separate drain passage is provided to return this leakage flow to the tank.
A. BELOW VALVE SETTING Fig. 7.9
45
B. AT VALVE SETTING
Pilot operated pressure reducing valve The pilot-operated pressure reducing valve (fig.7.10) has wider range of adjustment and generally provides more accurate control. The operating pressure is set by an adjustable spring in the pilot stage in the upper body. The valve spool in the lower body function in essentially the same manner as the direct acting valve. Refer fig.7.10, view A shows the condition when supply pressure is less than the valve setting. The spool is hydraulically balanced through and orifice in its center, and the light spring hold it in the wide-open position. In view B, pressure has reached the valve setting and pilot valve is diverting flow to the drain passage limiting pressure above the spool. Flow through the orifice in the spool creates a pressure difference that moves the spool up against the spring force. The partially closes the outlet port to create a pressure drop from the supply to the branch system.
Figure : 7.10 Again, the outlet port is never entirely closed. When no flow is called in the branch system, there is still a continuous flow through the spool orifice and pilot valve to drain.
46
CHAPTER - 8
STANDARD GRAPHICAL SYMBOLS THE SYMBOLS SHOWN CONFORM TO THE AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI) SPECIFICATIONS. BASIS SYMBOLS CAN BE COMBINED IN ANY COMBINATION. NO ATTEMPT IS MADE TO SHOW ALL CONBINATIONS. LINES AND LINE FUNCTIONS
PUMPS
LINE, WORKING
PUMP, SINGLE FIXED DISPLACEMENT
LINE, PILOT (L>20W) LINE, DRAIN (L<5W) CONNECTOR
PUMP, SINGLE VARIABLE DISPLACEMENT
LINE, FLEXIBLE
MOTORS AND CYLINDERS
LINE, JOINTING
MOTOR, ROTARY, FIXED DISPLACEMENT
LINE, PASSING
MOTOR, ROTARY VARIABLE DISPLACEMENT
DIRECTION OF FLOW, HYDRAULIC PNEMATIC
MOTOR, OSCILLATING
LINE TO RESERVIOR ABOVE FLUID LEVEL BELOW FLUID LEVEL
CYLINDER, SINGLE ACTING
CYLINDER, DOUBLEACTING
LINE TO VENTED MANIFOLD
CYLINDER, DIFFERENTIAL ROD
PLUG OR PLUGGED CONNECTION RESTRICTION, FIXED
CYLINDER, DOUBLE END ROD
RESTRICTION, VARIABLE
CYLINDER, CUSHIONS BOTH ENDS. 47
BASIC VALVE SYMBOLS (CONT.)
MISCELLANEOUS UNITS DIRECTION OF ROTATION (ARROW IN FRONT OF SHAFT
VALVE, SINGLE FLOW PATH, NORMALLY OPEN
COMPONENT ENCLOSURE
VALVE, MAXIMUM PRESSURE (RELIEF)
RESERVOIR, VENTED
BASIC VALVE SYMBOL, MULTIPLE FLOW PATHS
RESERVOIR, PRESSURIZED
FLOW PATHS BLOCKED IN CENTRE POSITION
PRESSURE GAGE
MULTIPLE FLOW PATHS (ARROW SHOWS FLOW DIRECTION)
TEMPERATURE GAGE
VALVE EXAMPLES UNLOADING VALVE, INTERNAL DRAIN, REMOTELY OPERATED
FLOW METER (FLOW RATE ELECTRIC MOTOR
DECELERATION NORMALLY OPEN
ACCUMULATOR, SPRING LOADED
VALVE,
ACCUMULATOR, GAS CHARGED
SEQUENCE VALVE, DIRECTLY OPERATED, EXTERNALLY DRAINED
FILTER OR STRAINER
PRESSURE REDUCING VALVE
HEATER COUNTER BALANCE VALVE WITH INTEGRAL CHECK
COOLER TEMPERATURE CONTROLLER
TEMPERATURE AND PRESSURE COMPENSATED FLOW CONTROL WITH INTEGRAL CHECK
INTENSIFIER PRESSURE SWITCH
DIRECTIONAL VALVE, TWO POSITION, THREE CONNECTION
BASIC VALVE SYMBOLS CHECK VALVE
DIRECTIONAL VALVE, THREE POSITION, FOUR CONNECTION
MANUAL SHUT OFF VALVE BASIC VALVE ENVELOPE
VALVE, INFINITE POSITIONING (INDICATED BY HORIZONTAL BARS)
VALVE, SINGLE FLOW PATH, NORMALLY CLOSED 48
MISCELLANEOUS UNITS
BASIC VALVE SYMBOLS (CONT.)
PRESSURE COMPENSATOR
LEVER
DETENT
PILOT PRESSURE
MANUAL
SOLENOID
MECHANICAL
SOLENOID CONTROLLED, PILOT PRESSURE OPERATED
SPRING
PEDAL OR TREADLE
SERVO
PUSH BUTTON
49
CHAPTER - 9
PRACTICAL ON HYDRAULIC TRAINER Hydraulic trainer is a unit used for demonstration of the various hydraulic operations. It consist of a balanced type sliding vane motor which provides pressurised oil to linear and rotary actutors. It has different valves mounted on panel. Following valves are mounted in the hydraulic trainer. l
One relief valve.
l
One sequence valve.
l
One decceleration valve.
l
Two linear actuators.
l
One rotary motor.
l
One rotary pump.
l
One flow control valve.
Ten practicals on this hydraulic trainer are detailed on subsequent pages.
50
