1
Steel Authority of India Limited Durgapur Steel Plant Centre for HRD
Reading Material of Industrial Hydraulics -- I
INDEX
Sl. No. 1
Subject Introduction to Hydraulics
1
2
Basic Principles of Industrial Hydraulics
4
3
Basic Elements of a Hydraulic System
11
4
ISO Symbols
24
5
Hydraulic Oil
38
6
Pressure Relief Valve
43
7
Hydraulic Pump
52
8
Actuators
67
9
Directional Control valves
75
10
Definitions of Technical Terms
86
2
Page
Steel Authority of India Limited Durgapur Steel Plant Centre for HRD
Reading Material of Industrial Hydraulics -- I
INDEX
Sl. No. 1
Subject Introduction to Hydraulics
1
2
Basic Principles of Industrial Hydraulics
4
3
Basic Elements of a Hydraulic System
11
4
ISO Symbols
24
5
Hydraulic Oil
38
6
Pressure Relief Valve
43
7
Hydraulic Pump
52
8
Actuators
67
9
Directional Control valves
75
10
Definitions of Technical Terms
86
2
Page
DURGAPUR STEEL PLANT CENTRE FOR HRD READING MATERIAL ON INDUSTRIAL HYDRAULICS HYDRAULICS
OIL OR INDUSTRIAL HYDRAULICS The field of hydraulics which deals with the principles of power transmission and control of mechanical motions is generally known as oil or INDUSTRIAL HYDRAULICS. To cope up with the the rapid advancement of application application of oil hydraulics components to various types of machines, a good knowledge on this subject is necessary, for their faultless designs and long trouble free operations . For want of clear understanding of basic principles, many times maintenance personnel take longer time to locate fault in a hydraulic equipment, resulting in undesired production loss and labour cost etc. Minimum knowledge which which is necessary for smooth operation and maintenance of oil hydraulic equipments will the covered in the training programme. The name "Hydraulics" infact comes from the Greek word "HYDOR" meaning water & "AULOS" meaning pipe. The importance of of "hydraulics" is known to mankind since prehistoric time. Between 200 and 100 BC, man realized that the flowing water of river has great energy in it. That time the man converted the energy of the flowing water into useful mechanical energy, by means of a waterwheel, and first time "hydraulics" was put to the service of the mankind. After the invention of W att's steam engine, there arose a need for transmission of power, from the point of generation to the point of use, more efficiently. Gradually many types of mechanical devices were developed e.g. line shaft, belting, gearing, pulleys and chains etc. Then the scope of transmitting power through fluid under pressure was thought of. This was a new field of of hydraulics, dealing with power transmission, transmission, control of motions, multiplying force and characteristics of fluids under pressure. 3
HYDRAULIC SYSTEM Function of the hydraulic system is to transmit power by converting mechanical energy to fluid energy (through pump) and again converting this fluid energy to mechanical energy (through actuator) for doing useful work. Hydraulic :
HYDOR + AULOS
(Water + Pipe)
Principles: §
Pascal's Law - Pressure applied applied on a confined fluid is transmitted undiminished in all directions, and acts at right angle to the contacting surface.
§
Pressure is is created by providing providing resistance in the flow path of fluid.
§
Flow through various resistances in in series. Here the resistances add up.
§
§
Flow through various resistances connected parallely. Here flow occurs through the path of minimum resistance. Pressure difference creates flow.
Typical application of Hydraulics : § § § § § § § §
Presses, Jacks Motor vehicle brakes Mobile material handling machines : Hydraulic cranes, Bulldozer rams, Excavators, Fork lift trucks Aeroplane under carriages and wing flap Cold saw machine Special purpose machine tools Movable throat armour of Blast furnace Reclaimer Jacks
4
Some important application of Hydraulics in Durgapur Steel Plant : § § § § §
Stacker cum reclaimer - RMHP Side arm charger & Wagon tippler - RMHP & COCC Tap hole mud gun & drilling machine - BLAST FURNACE Skirt Lifting hydraulics for converter - BOF Slide gate hydraulics - BOF & CCP § 42" Mill Roll Balance - BLOOMING MILL § 63 MN Wheel press - WHEEL & AXLE PLANT Advantages : § § § § § § § § § § § § § § § § §
For long distances - Force & Torque Infinitely variable speeds Overload protection Complete automation Power to weight ratio, 1/5 th of electro High response Mechanically stiff Self lubricating - no wear and rusting Self cushioning - anti vibration pads, damping devices Linkage in inaccessible position - confined areas Safe for operation & machine Stalling at maximum load and speed reversible Power and torque can be changed Instant reversibility Compact Simple in operation Accurate position control
Limitations : § § § § §
Initial cost high Maintenance cost high (components) Fault diagnosis is difficult Housekeeping, leakage's & fire hazard Dirt contamination
5
Pressure (Force per Unit Area) is Transmitted Throughout a Confined Fluid
Energy Can Neither be Created Nor Destroyed
6
Hydraulic Leverage 7
Pressure Caused by Restriction and Limited by Pressure Control Valve 8
Parallel Flow Path
9
Series Resistance Add Pressure
10
Pressure Drop and Flow Through an Orifice
11
Pressure Loss Requires Full Loss of Pump Output
12
Basic Elements of a Hydraulic System The most essential basic elements with the help of which a Hydraulic System may be run, consists of the following items. (1) Prime Mover :- The Prime Mover is the Primary source of energy. It may come from a v v v v
Diesel engine Internal Combustion engine Steam engine Electric motor etc. v Manual (2) Medium :- The medium of power transmission in the hydraulic system is the hydraulic oil. Through this oil mechanical energy from pump is converted into fluid energy of the incompressible Hydraulic oil and again converted to mechanical energy at actuator. Oil may be :v Mineral oil v Petroleum oil v Water v Emulsion etc. (3) Reservoir :- The reservoir is the place where Hydraulic oil is stored. Primarily it is a storage place but it also has some more important functions e.g. cooling, filtering, sedimentation etc. (4) Pump :- Pumps in Hydraulic System are used to create the oil flow. It is the input component of the hydraulic system & converts mechanical energy to hydraulic energy. They are of various types mainly rotary & reciprocating e.g. gear pump, vane pump, piston pump etc. (5) Piping :- The hydraulic fluid is conducted through the pipe lines, which are of cold drawn seamless steal tubes, pipes & flexible hoses. Different kinds of fittings are used to arrest external leakages in the hydraulic system. (6) Pressure control valves :- These control the maximum pressure in a hydraulic circuit there by providing safety & regulating the force or torque output of an actuator.
13
(7) Direction control valves :- The necessary direction of movement of the actuator is controlled by its actuation. (8) Actuator :- The Actuator is the output element in a H YDRAULIC System which converts the hydraulic power available through the Medium into fruitful mechanical output. This handles the largest amount of face. The design of a hydraulic system starts with an actuator as the hydraulic system is designed to do certain work & the work to be done will decide the type of actuator to be used.
Hydraulic system has primarily 2 parts 1) Signal control section. 2) Power section. The Power section of a Hydraulic system primarily consists of 3 main parts. v
Power supply section v Energy control section v Drive section In the power supply section hydraulic power is generated & pressure medium is prepared. The following component are used to for energy conversion i.e. electrical energy to mechanical energy & then to hydraulic or fluid power. v v v v
Electric Motor IC engine Coupling Pump v Pressure indicator v Protective circuitry
14
To maintain the medium or to condition the hydraulic fluid following are used :v v v v v v v v
Filter Cooler Heater Thermometer Pressure gauge Hydraulic fluid Reservoir Filling level indicator
Hydraulic fluid power is supplied to drive section through energy control section as per control parameters. Following components are involved v v v v
Directional Control Valves Flow Control Valves Pressure Valves Non-return Valves
The drive section is the Executor of the Hydraulic System. It utilizes the ultimate available Hydraulic power to provide the desired output by movement of various machine parts this generating force. The main components are v v
Hydraulic Cylinders (Linear actuators) Hydraulic Motors (Rotary actuators)
The signal control section deals with the operation of Hydraulic system. This is again divided into two parts 1) Signal input section i.e. sensing which may be v v v v
Manually Mechanically By observing By other means
15
2) Signal Procuring which may be done v v v v v
By manually / operator By electricity By electronics By pneumatics By mechanics v By hydraulics Hydraulic Symbols : Symbols are clear representation of Hydraulic components & circuits which are basically simple & graphic in nature. A symbol identifies the function of the component but provides no information on its design. After Second World war a graph of top industries formed what was called the JIC - JOINT INDUSTRY CONFERENCE who laid down the first set of standardized hydraulic component & circuit representation. Later on another European body CETOP - improvised on the existing symbols. Presently the DIN ISO 1219 standards is the most modern version of Hydraulic symbols. ISO meaning International Standard Organization. Location of various important component in a Hydraulic Circuit : The structure of a Hydraulic system design is represented in a Hydraulic Circuit. The power supply section is shown at the bottom comprising of the reservoir, oil, pump, motor, strainer, indicators etc. Above this is shows the power control section in which there are directional control valves, pressure control valves, pressure relief valve & flow control valves. These components control & modulate the output characteristics of the hydraulic power package as per requirement of the customer & manufacturers design. On the top is represented the "Drive Section" i.e. the Actuator.
16
In a hydraulic circuit fluid power flows from bottom to the top i.e. from the input (the pump) & through various controls to the output (i.e. the actuators).
Reservoir : The reservoir generally performs the following functions. A B C D
- It is a store house for the hydraulic fluid. - It provides a place for air to separate out of the fluid. - Allows contaminates to settle down. - Allows for heat dissipation.
Normally the reservoir is designed so that maintenance is easy. However in aerospace & where there are space problems, the reservoir is made compact in size. Construction : The tank is constructed by steel plate welded together, with end plates supporting the unit on the floor. The inside of the tank is painted with a sealer to reduced rust (which may form due to condensed moisture). The paint used should be as per manufacture recommendation (due to compatibility). The bottom is sometimes slanted or dished & has a drain plug at the lowest point for draining purposes. Removable covers or side walls allow for access for cleaning & maintenance. Sight glass allow for oil level checking. In some modules there are filler holes on the top with a fine mesh screen for popping up hydraulic fluid. The capacity of the tank is 3 to 4 times of pump delivery per minute.
17
Hydraulic Reservoir
18
Breather : A vented breather cap on top of the reservoir allows for atmospheric pressure to act on the oil surface for the proper functioning of the pump. The breather has a fine oil filtering screen. In dirty atmospheric an oil bath air filter is sometimes used, for checking the ingress of external pollutants present in the atmospheric air. The breather size should be of adequate size. Larger the flow rate, longer the breather required. In many power packs the breather acts as the filler. A choked breather may starve the pump & call for unwanted problems in the hydraulic system. Dirty air filters are to replaced from time to time. On a pressurized reservoir there is no breather & is replaced by & air valve which maintains pressure in the tank to certain limit.
Baffle Plate : A baffle plate extend along the length through the centre of the tank. Usually it is 2/3 rd of the oil level & separates pump inlet line from return line, so that same fluid is not be circulated continuously, but must take route fenced by the baffle plate. The baffle 1 2 3 4
- Prevents local turbulence in the tank. - Allows foreign material to settle to bottom. - Gives the fluid an opportunity to get rid of entrapped air. - Helps to increase heat dissipation through take walls.
Line connections & Fittings : Pump inlet & return lines must be well below the fluid level. Otherwise oil may get aerated & foamy. Line connections at tank top cover are often sealed by slip-joint type flanges, preventing dirt ingress through these openings & makes it easy remove inlet line strainer for cleaning. Drains lines terminate above the fluid level, to avoid pressure build up in drain passages or siphoning oil through them. 19
Lines terminating near the tank bottom not equipped with strainers are 0 generally at 45 angle. This present the line opening from 'bottoming' in the tank & cutting of flow. On a return line the angled opening is pointed. So that flow is directed at the tank walls & away from pump inlet lines. Size : The tank size should allow for enough oil the system requires of the level should be high enough to avoid 'whirlpool' effect at pump inlet opening so that air is not taken in with fluid. Heat expansion of fluid, charges in fluid level due to system operation inside tank area exposed to water condensation are factors which are considered to determine the size. Industrial thumb rule. Tank size = Flow rate / minute x 3 or 4. Filters & Strainers : Contamination is very dangerous for a hydraulic system. Researches have indicated that even particles of 1-5 microns have a degrading effect causing failures in SERVO system & oil deterioration in many cases. Task : It is the task of a filter to reduce contamination to an acceptable level in order to protect various component from excessive wear. Magnetic plugs are used in some tanks to trap iron & steel particles carried by fluid.
20
Filter & Strainer : National Fluid Power Association defines filter as a device whose primary function is the retention of insoluble contaminates from a fluid, by some porous medium. Strainer is a course filter. Rating of a Strainer : A simple screen of wire mesh strainer is rated by mesh number or its near equivalent Standard Sieve number. Higher the mesh no. or Sieve no. finer the screen. Filter Rating : -6
A filter is rated by micron size. A micron is one millionth (10 ) of a meter. A grain salt is about 70 microns across. The smallest a sharp eye can see is about 40 microns. Absolute filter fineness indicates the largest particle size able to pass through a filter. Nominal filter fineness indicate particles of nominal pore size are arrested on passing through it several times. Usually a filter is rated by it nominal filter size. System are often flushed by economical filters before commissioning. Selection of filter should as per manufacturers recommendation. Every filter causes a pressure drop. Pressure filter temperature.
: Pressure difference (∆p) 1 to 1.5 bar at operating
Return line filter : Pressure difference (∆p) temperature. Intake / suction filter
0.5 bar at operating
: Pressure difference (∆p) 0.05 bar to 0.1 bar at operating temperature.
21
Filter
22
Filter materials & Design : Type Surface type :- This is the most common type & are made of closely woven fabric or treated paper with pores to allow fluid flow. Very accurate control of pore size is possible. These are disposable types. Deep - bed filter (Depth type) These are made of compressed or multi layered fabric, cellulose, plastic etc. These filters have a high dirt retention capacity across the same filter area. FILTERING MATERIALS are mainly two types : ADSORBENT.
ABSORBENT &
Absorbent filters are made of porous materials like paper, wood pulp, cotton yarn & cellulose. Paper filters are usually resin in impregnated for strength. They provide very minute particle filtration. Adsorbent filters (or Active filters) each as charcoal, filters earth should be avoided in hydraulic system since they may remove essential additives from oil. The full flow filter :- Most common in all types of hydraulic system. Full flow means all the flow into the filter inlet port passes through the filtering system. The unfiltered oil from the outside of the element is filtered to the inside of the filter element from where it goes to the system. However there is a by-pass valve which is a spring loaded NON RETURN VALVE / CHECK VALVE, or a poppet which lifts up at a given pressure drop & allows flow by-passing the filter. Changing of filter element :Certain filters have mechanical indicators, which indicate by a change of colour that element needs a change. In all cases manufacturers recommendation which may be in running hours or usually in the range of maximum allowable pressure drop, the element has to be replaced.
23
Location of filters :There are three general areas in the system for locating a filter. Pump inlet line strainers & filters :Pump inlet line strainers & filters are relatively course in size as because pressure drop should not be very high. Typically a 100 mesh strainer protects the pump from particles of size 150 microns. Inlet line filters are generally mounted outside the tank, near pump inlet for easy access. Some lines they are also found in side reservoir. Pressure line filters : The filtration size is much less compared to inlet line. The trap finer size particles for healthy operation of valves and other system components. Pressure line filter must be able to with - stand the operating pressure of the system. Return line filters : These can trap small particles before fluid returns to the tank. They are very useful in systems without a large reservoir to allow contaminates settle out of the fluid. Systems using high performance pumps, return line filter is a must because inlet filter can not sufficiently protect the pump which has fine clearances. Results of filtration : § § § § § § §
bearing life increase upto 20 times 4 to 10 10 fold increase in pump and motor life 4 to 10 fold increase in hydrostatic hydrostatic transmission life 5 to 100 100 fold increase in valve spool life eliminates spool sticking extends fluid life through reduced oxidation 10 fold extension of sliding bearing life efficiency of hydraulic systems is assessed by the term " HYDRAULIC FLUID INDEX " (HFI). 24
HFI is determined by dividing the total amount of hydraulic fluid consumed in a year by the hydraulic oil reservoir capacity, for example, if a plant with a total reservoir capacity of 10 kl consumes 50 kl of hydraulic fluid in a year, it has a HFI of 5 as a thumb rule, a properly maintained system should have a HFI 1.5.
Typical Dynamic Clearances Component
Clearances (micron)
Roller elements bearings Journal Bearings Hydrostatic bearings Gears Dynamic seal
0.1 - 1 0.5 - 100 1.25 0.1 - 1 0.05 - 0.5
Pump Gear Tooth to side plate Tooth tip to ease
0.5 - 5 0.5 - 5
Pump vane Vane sides Vane tip
5 -13 0.5 - 1
Pump piston Piston to bore Valve plate to cylinder
5 - 40 0.5 - 5
Servo valves Orifice Flapper wall Spool to sleeve
130 - 450 18 - 63 1-4
Actuators
50 - 250
Ref : ASTM Wear Control Handbook.
25
ISO Symbology
26
ISO Symbology
27
ISO Symbology
28
Summery of Directional Control Valves
29
Types of Valve Actuation
30
Types of Valve Actuation
31
ISO Symbols
32
ISO Symbols
33
ISO Symbols
34
ISO Symbols
35
ISO Symbols
36
ISO Symbols
37
ISO Symbols
38
ISO Symbols
39
Oil Hydraulic fluid or oil has many important features :(1) Power transmission : As a medium for transmission of power oil should be able to flow easily through the pipe line & the components. Too much resistance to the flow of oil will cause power loss. Hydraulic oil is incompressible (0.5% compressible is 1000 psi). So the action is instantaneous when the pump is started or valve is shifted. (2) Lubrication :Hydraulic oil acts as a lubricant. Pump elements, valve parts & other wearing parts slide against each other on a film of fluid. For lasting life of components certain additives are added to hydraulic oil which ensure high anti-wear characteristics. (3) Sealing :In many places, the hydraulic oil acts as a sealant against pressure inside a hydraulic component. There is no seal ring in between a valve spool & body to minimize leakage from high pressure passage to low pressure passages. The tolerance the spool & valve body & oil viscosity determines the leakage rate. (4) Cooling :Oil circulates in the system & carries heat from the (place were heat is generated) hot zone & dissipates the heat at a cooler zone, i.e.. in the cooler & reservoir. Fluid Properties : Viscosity : Viscosity is the measure of the fluids resistance to flow, or an inverse measure of fluidity. A fluid that flows with difficulty has a high viscosity. It is a thick oil. A fluid that flows easily has a low viscosity. It is thin oil. Viscosity technically is defined in various ways namely, absolute viscosity, kinetic viscosity & relative viscosity. 40
Absolute viscosity =
Shear stress ---- Rate of shear
1 poise
1 dyne second -----Square c.m.
Kinetic viscosity
1 Centistoke
=
=
Absolute viscosity ------- Density
=
1 centipoise ----------------Density
Relative viscosity unit : SUS (Say bolt Universal System) is determined by timing the flow of a given quantity of the fluid through a standard orifice. Viscosity a compromise :When a manufacturer determines the oil viscosity to be used in a system, he actually compromises. A high viscosity is desirable for maintaining sealing between two meeting surfaces but high viscosity increases friction resulting in v v v v
High resistance to flow Increased power consumption due to frictional loss. High temperature caused by friction. Increased pressure drop due to resistance. v Possibility of sluggish or slow operation. v Difficulty in separating air from oil in the reservoir. If viscosity is too low : v v
Internal leakage increases Excessive wear or even actuator may seize under heavy load due to breakdown of oil film between parts. v Pump efficiency may decrease causing less flow. v Increased temperature resulting from leakage losses. 41
Viscosity Index : Viscosity Index is a measure of fluids resistance to change in viscosity with temperature. A fluid having a high Viscosity Index (VI) has a stable viscosity over a wide range of temperature. This is important for a machine which runs under temperature extremes. Pour point : Pour point is the lowest temperature at which hydraulic oil will
flow.
Lubricating ability : Oil generally has very good lubricating properties. But where clearance between wear parts is very small (Boundary lubrication) and high speeds & pressure has to be encountered, there chemical additive are used to enhance thus property. Oxidation resistance : Oxidation or chemical union with oxygen of oil drastically cuts down oil life. Petroleum oils are susceptible to oxidation. The oxidation products further reacts with oil to form soluble & insoluble matter. The soluble part are acidic in nature & causes corrosion in the system. The insoluble part like gums, sludge plug orifices, increase wear & cause valves to stick. Catalysts : In a hydraulic system, heat, pressure, contaminant, water, metal surfaces & agitation accelerate oxidation. Temperature is very vital. 0
0
Since below 135 F oil oxidizes very slowly but for every 18 F rise oxidation rate is doubled. Demulsibility :Is the ability of the oil to separate out of water. Small quantities of water in emulsification can be tolerated in a hydraulic system. But larger amounts of water will cause contamination, rust & sticky valves. 42
Rust & corrosion prevention : Since air or moisture cannot be kept out of a hydraulic system, rust & corrosion may occur in the system. Rust & corrosion contaminates (corrosion caused due to reaction between metal & acid) the whole system & reduces the life of component. Oil additives : In order to maintain a healthy system & important positive properties to the hydraulic fluid, chemical additives are used. 1) Oxidation Inhibitor 2) Rust Inhibitor 3) Extreme pressure agent 4) Pour depressant
- Reduces & controls the fluid oxidation. - Prevents formation of rust. - Improves pressure with standing property. - Very low temperature.
Fluid specification : Maintenance hydraulic fluid should be used as per recommendation of manufacturer. Oil of different types & grades should not be allowed to mix. Two oils of same viscosity but different grades may not be equally compatible with the system. Oil should be stored in a clean place & utmost care should be taken whole pouring or topping up oil in a hydraulic system, for avoiding contaminations. Various grades of oil in use are :- Servo System 32, 46, 57 where the 0
last 2 digits is the viscosity in centistokes at 40 C. 2
Ideal viscosity rating = 15 to 100 mm /sec Upper limit
2
= 750 mm /sec
Viscosity Index : (VI) is used as a reference characteristics.
valve for viscosity
temperature
Higher the viscosity index : Less the viscosity changes or greater the temperature range in which the oil can be used. ISO VG10 : Means, Kinetic viscosity 2
2
0
9.0 minimum - mm /sec at 40 2
2
0
11.0 Maximum - mm /sec at 40 . 43
Contamination Level According to NAS 1638 ( Maximum number of dirt particles in 100 ml oil) Class
Size range in mm 5 - 15
15 – 25
25 - 50
50 - 100
>100
00
125
22
4
1
0
0
250
44
8
2
0
1
500
89
16
3
1
2
1000
178
32
6
1
3
2000
356
63
11
2
4
4000
712
126
22
4
5
8000
1425
253
45
8
6
16000
2850
506
90
16
7
32000
5700
1012
180
32
8
64000
11400
2025
360
64
9
128000
22800
4050
720
128
10
256000
45600
8100
1440
256
11
512000
91200
16200
2880
512
12
1024000
182400
32400
5760
1024
44
Pressure Relief Valve Function : A pressure relief valve, or safety valve, performs three important functions in a hydraulic circuit. The first function is to limit the maximum system pressure, providing protection for the various components, piping, and tubing used in the system. The second function is to allow hydraulic fluid from the pump to bypass the system to tank. This provides a means of varying the quantity of fluid flowing in a system using a constant displacement pump. The valve also allows adjustment of the maximum force generated in a hydraulic system. Types : 1. Direct acting relief valve 2. Pilot operated relief valve Direct acting relief valve : In a direct acting relief valve (simple relief valve), the spring tension is set by an adjusting screw. When the system pressure exceeds the spring force on the poppet, the "excess pressure" will shift the poppet, allowing fluid to flow back to the tank. Changes in maximum system pressure is made by increasing or decreasing the spring tension. The pressure at which the valve begins to divert 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. Pressure at the inlet when the valve is passing its maximum volume is called full-flow pressure. Pilot operated relief valve : A pilot operated relief valve (also known as compound relief valve or balance piston relief valve) operates in two stages. The pilot stage in the upper valve contains the direct type of pressure relief valve. The port connections are made to the lower body and diversion of the full flow volume is accomplished by the balanced piston in the lower body. The balanced piston is so named because in normal operation, 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 45
land. At any pressure less than the valve setting, the piston is held on its seat by a light spring. (If this orifice gets clogged the system will not develop any pressure). When pressure reaches the setting of the adjustable spring, the poppet is forced of its seat limiting pressure in the upper chamber. The restricted flow through the orifice into the upper chamber results in an increase in pressure in the lower chamber. This unbalances the hydraulic forces and tends to raise the piston off its seat. When the difference in pressure between the upper and lower chambers is sufficient to overcome the force of the light spring (approximately 20 psi), the large piston unseats permitting flow directly to tank. Increased flow through the valve causes the piston to lift further off its seat but since this compresses only the light spring very little override is encountered. Vent connection : Pilot relief valve may be remotely controlled by means of an outlet port from the chamber above the piston. When this chamber is vented to tank, the only force holding the piston on its seat is that of the light spring, and the valve will open fully at approximately 20 psi. (In case of constant delivery pumps, the pump is "unloaded" by using automatic venting facility of pilot operated relief valves).
Unloading Valve It is a normally closed type of pressure control valve. The pilot line is connected remotely, from an external pressure source which is used to move the spool in the unloading valve and diverts pump delivery to the tank line. The operational difference between the unloading valve and pressure relief valve is that the relief valve operates in balance, being held open at one an infinite number of positions by the flow of oil through it. Maximum pressure maintained in the pressure port is determined by the spring adjustment. With the unloading valve the primary port pressure is independent of the spring force because the remote source pressure operates the spool. Thus when the unloader valve opens the pressure at the primary port is negligible. Thus it is used to unload the pump when actuator movement has stopped enabling the pump to operate under minimum resistance. 46
Pressure Relief Valve
47
Pilot Operated Relief Valve
48
SYMBOL
Pilot operated relief valve with directional valve unloading 49
Operation of Pilot Operated Relief Valve
50
Venting the Relief Valve
Pressure Relief Valve Pilot Operated 51
Pressure Limiting Valve (PLV)
52
3-Way Pressure Control Valve
53
Hydraulic Pump Function : To create flow, convert mechanical energy to hydraulic energy by pushing the hydraulic fluid into the system. Two Categories : §
Hydrodynamic or Non positive displacement pump.
No positive seal between inlet & outlet ports and pressure capability is a function of drive speed operate by centrifugal force output is reduced when pressure is increased. §
Hydrostatic or Positive displacement pump. Hydrodynamic or Non positive displacement pump (Uses impact or kinetic energy)
v
v v v v v
Operate by centrifugal force, where by fluids entering the center of the pump housing are thrown to the outside by means of a rapidly driven impeller . No positive seal between inlet and outlet ports. Pressure capability is a function of speed. Provide smooth continuous flow. Outpu t is reduced as resistance is increased. (for this reason non positive pumps not used in hydraulic system). This centrifugal or turbine designs are used primarily in the transfer of fluids where the only resistance encountered is that created by fluid itself & friction. Hydrostatic or Positive displacement pump (Force applied to a confined fluid)
v
As the name implies, These pump PROVIDE a given amount of fluid for every stroke, revolution or cycle. v Output, except for leakage losses is independent of outlet pressure, making them well suited for use in the power transmission.
54
Pump Ratings : Maximum operating pressure capability and their output (volume) at a given drive speed. Pressure Ratings : Determined by the manufacture based upon reasonable service life expectancy under specified working condition. Operating at higher pressure may result in reduced pump life or more serious damage. Displacement : Flow capacity of a pump can be expressed as its displacement per revolution or by its output in GPM. Volume of liquid transferred in one revolution. It is volume of one pumping chamber multiplied by the no. of chambers that pass the outlet per revolution. Fixed displacement : Most of the pumps are fixed displacement type, those flow rate cannot be changed except replacing certain components. Variable displacement : In some pumps, it is possible to vary the size of the pumping chamber and thereby the displacements by means of external controls. Volumetric Efficiency = Actual Output / Theoretical Output Hydraulic power is calculated from pressure and flow rate. Power = Pressure X Flow rate i.e. P = p.Q. where, P = Power in Watts = [ Nm/s]
2
p = Pressure in Pascal = [ N/m ] 3
Q = Flow Rate m /sec
55
2
Power = N/m
3
x m /sec = Watt.
Power is rate of doing work = Workdone / time = Force x distance / time = Newton x Meter / time = Nm / sec = Joules / sec Efficiency = Output Power / Input Power Volumetric power loss caused by leakage losses and hydro-mechanical power loss caused by friction. nv (volumetric efficiency): Covers the losses resulting from internal & external leakage losses in pumps, actuators and valves. nhm (Hydro-mechanical efficiency): Covers the losses resulting from friction in pump, motors and cylinders. During Power conversion Total Efficiency = Efficiency Volumetric x Efficiency Hydro-Mechanical
Pump Cavitation : v
When does it occur?
Cavitation occurs when the oil does not entirely fill the space provided for it in the pump. This leaves air cavities in the oil which can be detrimental to the pump. Suppose the pump inlet line is narrowed, it then causes the pressure of the incoming oil to drop. When the pressure is lowered, oil cannot flow into the pump as fast as it is pumped out. The result is that cavities or spaces are formed in the incoming oil. Air in the oil : This pressure drop tends to release any dissolved air in the oil and the air fills cavities. Air in oil in the form of bubbles also fills the cavities. When the air-filled cavities, which have been formed in a low pressure area, are conveyed to a high pressure area in the pump, they are forced to collapse by pressure. This creates an action similar to an implosion which disintegrates or chips away small particles of the metal parts of the pump, and causes excessive noise and pump vibration.
56
Fffects of the implosion : The collapse occurs quite instantaneously and a very great implosion takes place. 2
This implosion reaches as much as 1000 kg/cm , and it chips away small particles off the metal part of the pump. If a pump is allowed to continually cavitate, it will be seriously damaged. Atmospheric pressure charges the pump : The inlet of a pump normally is charged with fluid by a difference in pressure between the reservoir and the pump inlet. u sually the pressure in the reservoir is atmospheric pressure, which is 14.7 psi on absolute gauge (1.013 bar ) it is then necessary to have a partial vacuum or reduced pressure at the pump inlet to create flow. In a reciprocating pump, on the intake stroke, the piston / ram creates a partial vacuum in the pumping chamber. in a rotary pump, successive pumping chambers increase in size as they pass the inlet, effectively creating an identical void condition. Atmospheric pressure 1.013 bar in the reservoir pushes oil into the chamber to fill the void. 2
2
1 BAR = 100000 PA = 105 N/M = 10 N/CM 1 KG 9.81 N Caution :
Available pressure difference should be much less as liquid vaporize in a vacuum and puts gas bubbles into the system. Vacuum no more than 12.2 psi absolute, 0.841 bar (5" of mercury). Hence (14.7 - 12.2)= 2.5 psi pressure difference to push oil into system (1.013 - 0.841)= 0.172 bar. Aeration : The pressure of dispersed air bubbles in the system’s hydraulic fluid. Aeration can result in increased heating and sound in the pump. The air bubbles may reach the actuators csusing jerky motion. In such case air bleeding has to be done through bleed screws in actuator or by suitably loosening the nearest port. Cavitation usuallycan be distinguished from aeration by the sound. Aeration producs an intermittent sound, whereas 57
the sound of cavitation is more constant. Gear pumps : v
Develops flows by carrying fluid between the teeth and pump housing. v One gear is driven by the drive shaft and turns the other. Pumping chambers formed between the gear teeth, are enclosed by the pump housing and side plates (often called wear or pressure plates) v A partial vacuum is created at the inlet as the gear teeth unmesh. v Because of the difference of pressure on oil level of reservoir and pump inlet, fluid flows into fill the space from the reservoir and is carried around the outside of the gears. v As the teeth mesh again at the outlet the fluid is forced out. High pressure at the pump outlet imposes an unbalanced load on the gears and the bearing supporting them.
Characteristics : v v v
Fixed displacements Delivery range (output) 3.5 cc to 100 cc / rev very low to high. Usually low pressure because of shaft side loading although some may be used upto 3000 psi (250 bar). v Internal leakages increase with wear. v Fairly durable and more dirt tolerant than other pumps. Vane pumps : v
In the unbalanced design (only used for variable volume design) the throw or eccentricity between the ring and rotor changes the size of pumping chamber. v Most fixed displacement pumps today utilize balanced cartridge design developed by Mr. Harry Vickers in 1920's). In this design cam ring is elliptical rather than a circle and permits two sets of 0
internal ports. Two outlet ports are 180 apart so that pressure forces on the rotor are canceled out preventing side loading of the drive shaft and bearing. v A slotted rotor is splined to the drive shaft and turns inside a cam ring. v Vanes are fitted to the rotor slots and follow the inner surface of the ring as the rotor turns. 58
v
Centrifugal force and pressure under the vanes hold the vanes out against the cam ring. v Pumping chambers are formed between the vanes and are enclosed by the rotor, ring and two side plates. Vane pump :- (Operation) v
At the pump inlet a partial vacuum is created as the space between the rotor and ring increases.
v
Oil entering here is trapped in the pumping chambers and then is pushed into the outlet as space decreases. v The displacement of the pump depends on the width of the ring and rotor and on the throw of the ring. The cost rebuilding these pumps, when they sufficiently wear out or are damaged, may be high compared to the original cost. So, when they wear out, they are usually replaced. Installation and operation cares : The installation of the pump with respect to its reservoir is very important because it affects the performance of the pump. The higher the pump is mounted above the fluid level, the greater is the resistance faced by the pump, in drawing fluid into it. So, generally the vane pumps are mounted just above the reservoir cover, so that the gap between the fluid level and the suction-port of the pump is not more than one meter. The pumps should be mounted on strong foundation so that alignment is well maintained. They should be located such that they can be easily removed for repairs or replacement. Entrance of air into the pump should be well prevented because this will reduce the life of the pump and may lead to air locks into the system. This air also creates abnormal sound in the pump. Hence the pipe connections should be air tight and oil seals should be changed whenever a vane pump is dismantled for any repair. A vane pump should not be run at its peak pressure continuously for a long period. However, it can be worked at 75 to 80 percent of its peak pressure. In no case should a vane pump be used at a pressure higher than the pump rating. Since it is positive displacement unit, the relief valve must be set correctly to its maximum limit. Though the speed range for vane pumps is 1200 RPM to 1800 RPM 59
manufacturer's recommendations should be always followed. The idle vane pumps should be run at least once a week. During normal operation of a hydraulic system, it should be protected against dirt and dust. Recommended viscosity of the hydraulic fluid falls 0
0
in the range of 150 SSU to 225 SSU at 37.5 C (100 F). In summer it should be on the higher side and in winter on the lower side. The 0
0
operating temperature above 66 C (150 F) reduces the pump life too fast.
High Performance Double Pump Construction
60
Maintenance cares : The internal construction of a vane pump is much more delicate than a gear pump, So, more care is required for the maintenance of vane pumps than for a gear pumps. For a long life of vanes and cam ring, good filtration of hydraulic fluid is very important . The rubbing or sliding between rotor, vanes and cam ring does not tolerate the abrasive contamination of unfiltered fluid. It is possible to change the delivery rate of the pump by changing the contour of the cam ring. So, at the time of replacing a damaged cam ring, proper care should be taken for checking the dimension of the new cam ring. Some times a particular dimension of a cam ring is expressed by saying a particular ring number. Since the lubrication of the internal mechanism is done by the system pressure, the speed of the actuator should not be raised to its maximum limit immediately after starting. This precaution allows the pump to develop its priming, while the speed is picking up. This precaution is specially important at the time of starting a new or newly assembled pump, because in this case working tolerances are critical and adequate internal lubrication is vital. A new or over hauled pump is started under load on the first. This creates a back pressure to assure adequate internal lubrication. The alignment of the drive shaft of vane pump with its driving mechanism is very important. A misalignment of shaft couplings will cause too much frictional loss and severe wear of the moving parts thus creating unbalance and undue leakage. In vane pump there is the provision of changing the direction of flow by changing the direction of rotation. So, the assembly of the internal mechanism is done in such a manner that each part matches with the desired direction of rotation of the shaft. Generally the arrows showing the direction of rotation are marked on the top of the body, cam ring and the sides of the bushings. While fitting the pump head, head bolts are tightened alternatively 1800 from each other, until the head seats evenly. The manufacturer's recommendations should be strictly followed in applying torque to the head bolts. Before starting the pump, the freedom of movement of the internal parts should be checked by avoidance of binding should not be started. 61
To protect the pump from over heating, the fluid level of the reservoir should be checked from time to time. Make up fluid is poured through a 200 mesh wire screen. If a pump is driven in the wrong direction, it will be unable to draw fluid even for its lubrication and as a result it will seize after a short period of operation. The pipe fittings of the suction line of the pump should be always kept tight. If it is not, the air enters through the joints into the pump ans. is carried away into the system. The air fluid mixture causes noise, but its effect is different from that of cavitation. When exposed to load pressure, this undissolved air gets compressed, forming a cushioning effect, and does not collapse violently. It passes into the system in from of compressed bubbles, causing erratic valve and actuator operations. Overhauling of pumps : Before breaking any circuit connection, it should be made sure that the electrical power is put off and the system is relieved from trapped pressure. All exposed parts and openings should be covered with cloth. The following are the steps to be followed for overhauling of a pump. i ) After dismantling, the internal parts of the pump should be kept on a clear paper. ii) Vanes should be inspected carefully for wear and sticking, in the rotor slots. When dry, they should be able to move freely into the slots by their own weight. Defective vanes should be replaced. Sharp edges of new vanes can be removed by rubbing then on oil stone. If the wear is only on the outer edges of the vanes,. they should be reversed, because both sides are generally chamfered in the same manner. Before reversing, their surfaces should be smoothened to remove burrs caused by wear on the outer edge. iii) If the cam ring is scored or has cross-grooved surface, it should be replaced. Minor scratches can be removed by an oil stone. iv) If the faces of rotor and bushings are worn and scored, they should be removed by lapping. if the scoring is very heavy, the rotor should be replaced.
62
v) Bearings should be inspected for cracked or pitted races and balls. They should be replaced if found defective. vi) The drive shaft should also be inspected for any wear or damage. vii) The shaft seal, O-rings and head packing should be replaced. viii) Bending of the locating pin should be checked by keeping on a horizontal smooth surface. ix) All the internal parts are coated with a compatible fluid and lip of the shaft seal is lightly lubricated to prevent any damage during assembling. Then the reassemble of the whole pump is done in the reverse order. Installation and operation cares : The piston pumps are mounted on strong base plates to avoid mechanical vibrations. The inlet should be mounted, such that they are free from restrictions as far as possible. They should be of sufficient size to meet the full requirement of the suction chamber of the pump. The height of the pump's inlet above the reservoir, should be as minimum as possible. For the ranges of operating pressures, temperatures and speeds; manufacturer's recommendation should be always should followed. The maximum temperature of fluid in the system should normally be not 0
more then 60 C. The control of the temperature is important, because the moving parts of the pump have very narrow clearances and high temperature will try to expand the moving parts, resulting in fast wear and may be seizure. 0
The lower limit of the operating system temperatures and around 12 C. In case of operation below this, it is advisable to install a heater to raise the operating temperature. When a heater to raise the operating temperature. When a heating device is not available, the system should be started with "inching" (i.e. rapid intermittent starts and stops) for warming up. After a suitable operating temperature has been reached, normal operation should be taken up.
63
Before putting the pump into normal operation the direction of rotation of the pump should be taken checked. Before installation, the pipes should be pickled and examined for scale and residue. The lubrication of the internal moving parts is very important. Since the lubrication of the bearings and other internal parts in done by the fluid present inside the pump, initial start up of the pump should not be attempted unless the pump's case has been filled up with the fluid. During normal operation, the hydraulic system should be well protected against entrance of dirt and foreign particles. While starting a new pump for the first time, the following sequence of steps should be maintained:1) Start the pump by keeping the main relief valve setting low. 2
2) Set the main relief valve slightly higher, say by 1 N/mm . 3) Vent air from all parts of the circuit. Ensure that there is no leakage of oil in any part. 4) Then increases the speed of the prime mover if desirable. 5) Adjust the main relief valve setting as per recommendation, by adding load to the system. 6) Ensure that there is no leakage anywhere in this condition of pressure. 7) Stop the prime mover immediately and investigate if any abnormality like high temperature rise in bearings or noise etc. is found. Maintenance cares : Dirt, grit and foreign particles are the biggest enemies of a piston pump. If not filtered properly, they scratch and score the highly polished surfaces of the moving parts and the whole pump may be ruined by its own worn out particles. So a high degree of cleanliness is essential for the hydraulic system, for a smooth running and long life of a piston pump. The fluid of the hydraulic system should be periodically checked, cleaned and changed if necessary.
64
Whenever the pump is dismantled for overhauling, the shaft seal should be replaced with a new one and the condition of the shaft at the point of seal contact should be checked. For detailed information about the operation and maintenance cares of he pump, manufacturer's instruction's manual must be consulted. When the piston pumps are dismantled, the parts would be carefully marked so that they can be replaced or reassembled properly. Before starting this type of pump for the first time, it is important that the housing be completely filled with hydraulic fluid for lubrication. A drain line should be connected to the housing drain port so that the housing remains full of fluid the operation.
Pressure Compensated Variable Vane Pump with Angular vanes 65
Swash plate Causes Piston to Reciprocate
Bent Axis Piston Pump
66
External Gear Pump
Internal Gear Pump
67
Unbalanced Vane Pump Operation
Construction of Round Type Pump 68
Actuators Actuators are the Output component of hydraulic systems. The design of a hydraulic system starts from the actuator after determining the type of job to be done and its power requirement. Only after actuator is chosen and sized, the remaining circuit component can be designed / selected. The actuators has to handle large forces and converts fluid energy / hydraulic energy to mechanical energy. Classification v v
Linear actuators Rotary actuators
Cylinders : Cylinders are linear actuators which means that the output of a cylinder is straight line motion and/or force. Different type of cylinders commonly used are: v v
Single acting cylinder: Ram type cylinder, Spring loaded cylinder Double acting cylinder: Standard double acting cylinder which can be differential type or non-differential, double rod cylinder v Telescoping cylinder : Telescoping cylinders increases stroke length upto 4 or 5 sleeves while collapsed length is less. v Tandem cylinder : where diameter of cylinder is restricted but stroke is not. Cylinder Construction : The main parts are a barrel, a piston and rod, end caps and suitable seals. Barrels usually are seamless steel tubing, honed to a fine finish on the inside. The piston usually cast iron or steel incorporates seals to reduce leakage between it and the cylinder barrel. Step cut automotive type piston rings are used where some leakage can be tolerated. For supporting loads or very low feed rates, a T-ring or "O" ring with 2 heavy duty back-up rings is often used. The ports of the cylinder are in the end caps, which may be attached directly to each end of the barrel, or secured by tie-bolts. The rod packing is a cartridge type including both the seal and wiper for easy replacement. Cylinder Mountings : Various cylinder mountings provide flexibility in anchoring the cylinder. Rod ends are usually threaded for attachment directly to the load or to accept a clevis, yoke or similar coupling device.
69
Single Acting Cylinder
70
Double Acting Cylinder 71
Cylinder Ratings : The rating of a cylinder considers its size and pressure capability. Cylinder size is specified by diameter of piston and stroke length, rod size is standard though intermediate and heavy duty rods are also used. The speed, output force and pressure required are all related to the piston diameter. Effects on cylinder applications for changes in input flow, size, and pressure. Change
Speed
Effect on Operating Pressure Increase Pressure Setting No Effect No Effect Decrease Pressure Setting No Effect No Effect Increase GPM Increases No Effect Decrease GPM Decreases No Effect Increase Cylinder Diameter Decreases Decreases Decrease Cylinder Diameter Increases Increases
Output Force Available Increases Decreases No Effect No Effect Increases Decreases
Above table assumes a constant work load. Cylinder cushions : Cushioning of cylinders are used at both 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 starts when the tapered cushion ring or plunger enters the cap and the return line flow is restricted. During the final part of the stroke, the exhaust oil is discharged through an adjustable orifice, a check valve to bypass the orifice on the return stroke is also provided. Hydraulic Motor : Hydraulic Motor is the name given to rotary hydraulic actuator and are similar in construction to pump. The hydraulic motor is driven by the fluid and develop torque and continuous rotating motion. Since both outlet ports may be pressurised, most motors are externally drained. Motor Ratings : Hydraulic motors are rated according to displacement (size), torque capacity and maximum pressure limitation. Displacement is the quantity of fluid which the motors require in turning one revolution. Torque is the force component of the motor's output. It is defined as a turning or twisting effort. Pressure required in a hydraulic motor depends on the torque load and the displacement. Types of Motor : Gear motors, Vane motors, High torque motor, Inline piston motors, Bent-axis piston motors etc. 72
Double Acting Cylinder With End Position Cushioning 73
Flange Mounted Double Acting Cylinder
74
Cylinder Mounting Methods
75
Tandem Cylinder
Telescopic Cylinder
76
Directional Control Valve Function : The control of direction of flow is achieved by changing the position of internal movable parts of the Directional Control Valve, by actuation. ♦ Most of the Direction Control Valves are finite positioning. ♦ The control of the direction of flow is made possible by opening or closing certain flow paths by the internals of the directional valve in definite valve positions. ♦
Classification : They are classified according to the principle characteristics as below :Type Internal Valving element ♦ ♦ ♦
Poppet (Ball or Piston) Rotary Spool Sliding Spool
Method of Actuation : The switching position of a Directional Control Valve i.e. spool positioning can be obtained by following actuation methods. v v v v v
Manually operated (Lever, Push Button, foot Pedal) Mechanically (Cam, Plunger, Roller) Electrically (Solenoid) Hydraulically (Hydraulic pressure) Pneumatically (Pneumatic pilot pressure)
No. of Flow paths v v v
Two way Three way Four way
77
Size : Valves are rated v
By the size of the pipe connections to valves or to its mounting plate. v By the flow (LPM, GPM) Designation : The designation of Directional Control Valve refers to the number of service ports / working ports / main ports (pilot control, seal drain not included) and the number of switching positions. A valve with 3 service ports and 2 switching positions is thus designated as 3/2 way valve. Hence when labeling Directional Control Valves, it is first necessary to specify the number of ports than the number of switching positions. Symbols : v v
Each different switching position is shown by a squared envelope. The ports are marked in the envelope / square of the normal position of the valve. Type of valve
Normal condition
For spring off - set valve : Spring side envelope. For spring control valve : Neutral / centre position envelope. Pump delivery / pressure port : P or 1. Working port / cylinder conection port : A, B or 2,4. Tank port : T, R or 3,5. Pilot port : X, Y, Z v
Flow direction is indicated by arrows in all the switching position.
v
Blocked ports are shown by horizontal lines.
v
Lines indicate how the ports are interconnected in the various switching positions.
v
Seal drain ports are drawn as broken lines and distinguish them from Control / pilot ports.
78
labeled (L), to
v
The actuation symbol (depending on method of actuation) is outside the extreme squares, as per position of the actuating part.
v
Depending on the flow occurring or not occurring with respect to the pressure / delivery (P,1) line the valve is said to be normally open or closed.
v
Depending on the flow path in - the normal position i.e. neutral / centre position of a 3 switching position valve, with respect to pressure / delivery (P,1) line, the directional valve is according named as open centre valve, closed centre valve, tandem centre valve, float centre valve etc.
Manually operated Four-way valve
79
Switching Overlap
Poppet and slide principle 80
2/2- Way Directional Control Valve
81
3/2 – Way Directional Control Valve (Slide Principle)
82
3/2 – Way Directional Control Valve (Poppet Principle)
83
4/2- Way Directional Control Valve (with three piston)
84
4/3 - Way Directional Control Valve with Recirculation Mid-position
85
Pilot choke mounts on DG 3 or DG 5 valves
86
Solenoid Controlled Pilot Operated Directional Control Valve
87
S o m e i m p o r t an t d e f i n i t io n s Aeration : Air in the hydraulic fluid. Excessive aeration causes the fluid to appear milky and components to operate erratically because of the compressibility of the air trapped in the fluid. Annular area: A ring shaped area -often refers to the net effective area of the rod side of a cylinder piston Atmospheric pressure: Pressure exerted by the atmosphere at any specific location.( Sea level pressure is approx. 14.7psi 1 bar 1 atm.) Breather : A device which permits air to move in and out of a container or component to maintain atmospheric pressure. Closed centre valve : One in which all ports are blocked in the centre or neutral position Compensator control : A displacement control for variable pumps and motors which alters displacement in response to pressure changes in the system as related to its adjusted pressure setting. Cooler : A heat exchanger used to remove heat from the hydraulic fluid. Counter Balance valve : A pressure control valve which maintains back pressure to prevent a load from falling. Cracking Pressure : The pressure at which a pressure actuated valve begins to pass fluid. Drain : A passage in, or a line from, a hydraulic component which returns leakage fluid independently to reservoir or to a vented manifold. Hydrodynamics : Engineering science pertaining to the energy of liquid flow and pressure. Enclosure : A rectangle drawn around a graphical component or components to indicate the limits of an assembly. Heat Exchanger : A device which transfers heat through a conducting wall from one fluid to another. Hydrostatics : Engineering science pertaining to the energy of liquids at rest. Laminar flow : A condition where the fluid particles move in continuous parallel paths. Manifold : A fluid conductor which provides multiple connection ports. Manual Override : A means of manually actuating an automatically controlled device. Micron : One millionth of a meter or about .00004 inch. Open centre valve : One in which all ports are interconnected and open to each other in centre or neutral position. Pilot pressure : Auxiliary pressure used to actuate or control hydraulic components. Poppet : The part of certain valves which prevents flow when it closes against a seat. 88
