FUNDAMENTAL OF HYDRAULICS : FUNDAMENTAL OF HYDRAULICS Prepared by
Training Officer HYDRAULIC SYSTEMS : HYDRAULIC SYSTEMS Slide 3: The advantages of hydraulic systems over other methods of power transmission are-
Simpler design. In most cases, a few pre-engineered components will replace complicated mechanical linkages.
Flexibility. Hydraulic components can be located with considerable flexibility. Pipes and hoses instead of mechanical elements virtually eliminate location problems.
Smoothness. Hydraulic systems are smooth and quiet in operation. Vibration is kept to a minimum.
Control. Control of a wide range of speed and forces is easily possible.
Cost. High efficiency with minimum friction loss keeps the cost of a power transmission at a minimum.
Overload protection. Automatic valves guard the system against a breakdown from overloading.
The main disadvantage of a hydraulic system is maintaining the precision parts when they are exposed to bad climates and dirty atmospheres. [Protection against rust, corrosion, dirt, oil deterioration, and other adverse environmental conditions is very important] BASIC SYSTEMS Slide 4: BASIC HYDRAULIC SYSTEMS. HYDRAULIC JACK
In this system (Figure 2-1), a reservoir and a system of valves has been added to Pascal's hydraulic lever to stroke a small cylinder or pump continuously and raise a large piston or an actuator a notch with each stroke.
Diagram A shows an intake stroke.
Diagram B shows the pump stroking downward. Slide 5: B. MOTOR-REVERSING SYSTEM. Figure 2-2 shows a power-driven pump operating a reversible rotary motor.
A reversing valve directs fluid to either side of the motor and back to the reservoir.
A relief valve protects the system against excess pressure and can bypass pump output to the reservoir, if pressure rises too high. Slide 6: C. OPEN-CENTER SYSTEM. In this system, a control-valve spool must be open in the center to allow pump flow to pass through the valve and return to the reservoir.
To operate several functions simultaneously, an open-center system must have the correct connections, An open-center system is efficient on single functions but is limited with multiple functions. neutral position Slide 7: Series Connection.
Figure 2-4 shows an open-center system with a series connection.
Oil from a pump is routed to the three control valves in series. The return from the first valve is routed to the inlet of the second, and so on.
In neutral, the oil passes through the valves in series and returns to the reservoir, as the arrows indicate.
When a control valve is operated, the incoming oil is diverted to the cylinder that the valve serves. Return liquid from the cylinder is directed through the return line and on to the next valve. C. OPEN-CENTER SYSTEM. This system is satisfactory as long as only one valve is operating at a time. Slide 8: 2) Series/Parallel Connection.
Figure 2-5 shows a variation on the series connection.
Oil from the pump is routed through the control valves in series, as well as in parallel.
In neutral, a liquid passes through the valves in series, as the arrows indicate.
However, when any valve is operating, the return is closed and the oil is available to all the valves through the parallel connection. C. OPEN-CENTER SYSTEM. This ability to operate two or more valves simultaneously is an advantage over the series connection. Slide 9: 3) Flow Divider.
Figure 2-6 shows an open-center system with a flow divider.
A flow divider takes the volume of oil from a pump and divides it between two functions.
For example, a flow divider might be designed to open the left side first in case both control valves were actuated simultaneously. Or, it might divide the oil to both sides, equally or by percentage.
With this system, a pump must be large enough to operate all the functions simultaneously. It must also supply all the liquid at the maximum pressure of the highest function, meaning large amounts of hp are wasted when operating only one control valve. C. OPEN-CENTER SYSTEM. Slide 10: In this system, a pump can rest when the oil is not required to operate a function.
This means that a control valve is closed in the center, stopping the flow of the oil from the pump. D. CLOSED-CENTER SYSTEM. Slide 11: Fixed-Displacement Pump and Accumulator.
Figure 2-8 shows a closed-center system.
In this system, a pump of small but constant volume charges an accumulator. When an accumulator is charged to full pressure, an unloading valve diverts the pump flow back to a reservoir.
A check valve traps the pressured oil in the circuit.
When a control valve is operated, an accumulator discharges its oil and actuates a cylinder. As pressure begins to drop, an unloading valve directs the pump flow to an accumulator to recharge the flow.
This system, using a small capacity pump, is effective when operating oil is needed only for a short time. However, when the functions need a lot of oil for longer periods, an accumulator system cannot handle it unless the accumulator is very large. D. CLOSED-CENTER SYSTEM. Slide 12: With a closed-center system, the quantity of oil to each function can be controlled by line or valve size or by orificing compare to open-center system easily. Slide 13: COLOR CODING The figures that show oil-flow conditions or paths are prepared with industrial standardized color codes.
Table 2-1 lists the colors for the hydraulic lines and passages that are in many of the figures: RESERVOIRS : RESERVOIRS Slide 15: A reservoir stores a liquid that is not being used in a hydraulic system. It also allows gases to expel and foreign matter to settle out from a liquid.
A properly constructed reservoir should be able to dissipate heat from the oil, separate air from the oil, and settle out contaminates that are in it.
Reservoirs range in construction from small steel stampings to large cast or fabricated units.
The large tanks should be sandblasted after all the welding is completed and then flushed and steam cleaned. Doing so removes welding scale and scale left from hot-rolling the steel)
The inner surface then should be sealed with a paint compatible with the hydraulic fluid. Non bleeding red engine enamel is suitable for petroleum oil and seals in any residual dirt not removed by flushing and steam cleaning. Slide 16: B. SHAPE: Figure 2-11 shows some of the design features of a reservoir.
It should be high and narrow rather than shallow and broad.
The oil level should be as high as possible above the opening to a pump's suction line.
This prevents the vacuum at the line opening from causing a vortex or whirlpool effect, which would mean that a system is probably taking in air.
Aerated oil will not properly transmit power because air is compressible. Aerated oil has a tendency to break down and lose its lubricating ability. Slide 17: C. SIZE.
Reservoir sizes will vary. However, a reservoir must be large enough so that it has a reserve of oil with all the cylinders in a system fully extended.
An oil reserve must be high enough to prevent a vortex at the suction line's opening. A reservoir must have sufficient space to hold all the oil when the cylinders are retracted, as well as allow space for expansion when the oil is hot.
Example: A common-size reservoir on a mobile machine is a 20- or 30-gallon tank used with a 100-GPM system.
A large-size tank is highly desirable for cooling. The large surface areas exposed to the outside air transfer heat from the oil. Also, a large tank helps settle out the contaminates and separates the air by reducing recirculation.
D. LOCATION. Most mobile equipment reservoirs are located above the pumps. This creates a flooded-pump-inlet condition. This condition reduces the possibility of pump cavitation-a condition where all the available space is not filled and often metal parts will erode. Flooding the inlet also reduces the vortex tendency at a suction pipe's opening.
A reservoir's location affects heat dissipation. Ideally, all tank walls should be exposed to the outside air. Heat moves from a hot substance to a cold substance; heat transfer is greatest when there is a large temperature difference. Reservoirs that are built into front-end loader arms are very effective in transferring heat. Slide 18: E. VENTILATION AND PRESSURIZATION.
Most reservoirs are vented to the atmosphere. A vent opening allows air to leave or enter the space above the oil as the level of the oil goes up or down.
This maintains a constant atmospheric pressure above the oil.
A reservoir filter cap, with a filter element, is often used as a vent.
Some reservoirs are pressurized, using a simple pressure-control valve rather than a vented one.
A pressure-control valve automatically lets filtered air into a tank but prevents air release unless the pressure reaches a preset level.
A pressurized reservoir takes place when the oil and air in a tank expand from heat. Slide 19: F. LINE CONNECTIONS.
A pump suction and a tank's return lines should be attached by flanges or by welded heavy-duty couplings. Standard couplings usually are not suitable because they spread when welded.
If a suction line is connected at the bottom, a coupling should extend well above the bottom, inside the tank; residual dirt will not get in a suction line when a tank or strainer is cleaned.
A return line should discharge near a tank's bottom, always below the oil level. A pipe is usually cut at a 45-degree angle and the flow aimed away from a suction line to improve circulation and cooling.
A baffle plate is used to separate a suction line from a return line.
This causes the return oil to circulate around an outer wall for cooling before it gets to the pump again.
A baffle plate should be about two-thirds the height of a tank. The lower corners are cut diagonally to allow circulation.
They must be larger in area than a suction line's cross section.
Maintenance procedures include draining and cleaning a reservoir.
A plug fitting should be flush with the inside of a tank to allow for full drainage.
A reservoir should have a sight gauge or dipstick for checking the oil level to prevent damage from lubrication loss.
When a reservoir is pressurized by compressed air, moisture can become a maintenance problem. A tank should have a water trap for moisture removal; it should be placed where it can be inspected daily. STRAINERS & FILTERS : STRAINERS & FILTERS Slide 21: To keep hydraulic components performing correctly, the hydraulic liquid must be kept as clean as possible. (Foreign matter and tiny metal particles). Strainers, filters, and magnetic plugs are used to remove foreign particles from a hydraulic liquid and are effective as safeguards against contamination.
A strainer is the primary filtering system that removes large particles of foreign matter from a hydraulic liquid. (Screening action).
A strainer usually consists of a metal frame wrapped with a fine-mesh wire screen or a screening element made up of varying thicknesses of specially processed wire.
Figure 2-12 shows a strainer in three possible arrangements for use in a pump inlet line. If one strainer causes excessive flow friction to a pump, two or more can be used in parallel. Strainers and pipe fittings must always be below the liquid level in the tank. Slide 22: FILTERS.
A filter removes small foreign particles from a hydraulic fluid and is most effective as a safeguard against contaminants.
Filters are located in a reservoir, a pressure line, a return line, or in any other location where necessary. They are classified as full flow or proportional flow. Slide 23: The general classes of filter materials are mechanical, absorbent inactive, and absorbent active.
Mechanical filters, contain closely woven metal screens or discs. They generally remove only fairly coarse particles.
Absorbent inactive filters, such as cotton, wood pulp, yarn, cloth, or resin, remove much smaller particles; some remove water and water-soluble contaminants. The elements often are treated to make them sticky to attract the contaminants found in hydraulic oil.
Absorbent active materials, such as charcoal and fuller's earth (a claylike material of very fine particles used in the purification of mineral or vegetable-base oils), are not recommended for hydraulic systems.
The three basic types of filter elements are surface, edge, and depth.
A surface-type element is made of closely woven fabric or treated paper. Oil flows through the pores of the filter material, and the contaminants are stopped.
An edge-type filter is made up of paper or metal discs; oil flows through the spaces between the discs. The fineness of the filtration is determined by the closeness of the discs.
A depth-type element is made up of thick layers of cotton, felt, or other fibers. FILTERING MATERIAL.: ACCUMULATORS : ACCUMULATORS Slide 25: Like an electrical storage battery, a hydraulic accumulator stores potential power, in this case liquid under pressure, for future conversion into useful work.
This work can include operating cylinders and fluid motors, maintaining the required system pressure in case of pump or power failure, and compensating for pressure loss due to leakage.
Accumulators can be employed as fluid dispensers and fluid barriers and can provide a shock-absorbing (cushioning) action.
Example:Accumulators are used mainly on the lift equipment to provide positive clamping action on the heavy loads when a pump's flow is diverted to lifting or other operations.
An accumulator acts as a safety device to prevent a load from being dropped in case of an engine or pump failure or fluid leak.
On lifts and other equipment, accumulators absorb shock, which results from a load starting, stopping, or reversal. ACCUMULATORS: Slide 27: a. Spring-Loaded Accumulator. It uses the energy stored in springs to create a constant force on the liquid contained in an adjacent ram assembly.
Figure 2-15 shows two spring-loaded accumulators.
The load characteristics of a spring are such that the energy storage depends on the force required to compress a spring.
The free (uncompressed) length of a spring represents zero energy storage.As a spring is compressed to the maximum installed length, a minimum pressure value of the liquid in a ram assembly is established.
As liquid under pressure enters the ram cylinder, causing a spring to compress, the pressure on the liquid will rise because of the increased loading required to compress the spring. TYPES OF ACCUMULATOR Slide 28: b. Bag-Type Accumulator. This accumulator (Figure 2-16) consists of a seamless, high-pressure shell, cylindrical in shape, with domed ends and a synthetic rubber bag that separates the liquid and gas (usually nitrogen) within the accumulator.
The bag is fully enclosed in the upper end of a shell. The gas system contains a high-pressure gas valve. The bottom end of the shell is sealed with a special plug assembly containing a liquid port and a safety feature that makes it impossible to disassemble the accumulator with pressure in the system.
The bag is larger at the top and tapers to a smaller diameter at the bottom. As the pump forces liquid into the accumulator shell, the liquid presses against the bag, reduces its volume, and increases the pressure, which is then available to do work. d. Maintenance. Before removing an accumulator for repairs, relieve the internal pressure: in a spring-loaded type, relieve the spring tension; in a piston or bag type, relieve the gas or liquid pressure. : d. Maintenance. Before removing an accumulator for repairs, relieve the internal pressure: in a spring-loaded type, relieve the spring tension; in a piston or bag type, relieve the gas or liquid pressure. c. Piston-Type Accumulator. This accumulator consists of a cylinder assembly, a piston assembly, and two end-cap assemblies. The cylinder assembly houses a piston assembly and incorporates provisions for securing the end-cap assemblies.
An accumulator contains a free-floating piston with liquid on one side of the piston and precharged air or nitrogen on the other side (Figure 2-17).
An increase of liquid volume decreases the gas volume and increases gas pressure, which provides a work potential when the liquid is allowed to discharge. Slide 30: Pressure gauges are used in liquid-powered systems to measure pressure to maintain efficient and safe operating levels. Pressure is measured in psi.
Flow measurement may be expressed in units of rate of flow-GPM or cubic feet per second (cfs). It may also be expressed in terms of total quantity-gallons or cubic feet.
a. Pressure Gauges. Figure 2-18 shows a simple pressure gauge. Gauge readings indicate the fluid pressure set up by an opposition of forces within a system. Atmospheric pressure is negligible because its action at one place is balanced by its equal action at another place in a system. PRESSURE GAUGES AND VOLUME METERS.: Slide 31: b. Meters. Measuring flow depends on the quantities, flow rates, and types of liquid involved. All liquid meters (flowmeters) are made to measure specific liquids and must be used only for the purpose for which they were made. Each meter is tested and calibrated.
In a nutating-piston-disc flowmeter, liquid passes through a fixed-volume measuring chamber, which is divided into upper and lower compartments by a piston disc (Figure 2-19).
During operation, one compartment is continually being filled while the other is being emptied. As a liquid passes through these compartments, its pressure causes a piston disc to roll around in the chamber.
The disc's movements operate a dial (or counter) through gearing elements to indicate that a column of fluid that has passed through the meter. PUMPS : PUMPS PUMPS: : PUMPS: Hydraulic pumps convert mechanical energy from a prime mover (engine or electric motor) into hydraulic (pressure) energy.
The pressure energy is used then to operate an actuator.
Pumps push on a hydraulic fluid and create flow.
All pumps create flow. They operate on the displacement principle.
Pumps that discharge liquid in a continuous flow are nonpositive-displacement type.
Pumps that discharge volumes of liquid separated by periods of no discharge are positive-displacement type. Slide 34: a) Nonpositive-Displacement Pumps. With this pump, the volume of liquid delivered for each cycle depends on the resistance offered to flow.
A pump produces a force on the liquid that is constant for each particular speed of the pump. Resistance in a discharge line produces a force in the opposite direction.
When these forces are equal, a liquid is in a state of equilibrium and does not flow.
If the outlet of a nonpositive-displacement pump is completely closed, the discharge pressure will rise to the maximum for a pump operating at a maximum speed.
A pump will churn a liquid and produce heat. Figure 3-1 shows a nonpositive-displacement pump. A water wheel picks up the fluid and moves it. Slide 35: b. Positive-Displacement Pumps. With this pump, a definite volume of liquid is delivered for each cycle of pump operation, regardless of resistance, as long as the capacity of the power unit driving a pump is not exceeded.
If an outlet is completely closed, either the unit driving a pump will stall or something will break.
Therefore, a positive-displacement-type pump requires a pressure regulator or pressure-relief valve in the system.
Figure 3-2 shows a reciprocating-type, positive-displacement pump. Slide 36: Figure 3-3 shows another positive-displacement pump.
This pump not only creates flow, but it also backs it up. A sealed case around the gear traps the fluid and holds it while it moves.
As the fluid flows out of the other side, it is sealed against backup. This sealing is the positive part of displacement.
Without it, the fluid could never overcome the resistance of the other parts in a system. Slide 37: c. Characteristics. The three contrasting characteristics in the operation of positive- and non positive-displacement pumps are as follows:
Non positive-displacement pumps provide a smooth, continuous flow; positive displacement pumps have a pulse with each stroke or each time a pumping chamber opens to an outlet port.
Pressure can reduce a non positive pump's delivery. High outlet pressure can stop any output; the liquid simply recirculates inside the pump. In a positive-displacement pump, pressure affects the output only to the extent that it increases internal leakage.
Non positive-displacement pumps, with the inlets and outlets connected hydraulically, cannot create a vacuum sufficient for self-priming; they must be started with the inlet line full of liquid and free of air. Positive displacement pumps often are self-priming when started properly. Slide 38: Pumps are usually rated according to their volumetric output and pressure.
Volumetric output (delivery rate or capacity) is the amount of liquid that a pump can deliver at its outlet port per unit of time at a given drive speed, usually expressed in GPM or cubic inches per minute.
Changes in pump drive affect volumetric output,
Pumps are sometimes rated according to displacement, that is the amount of liquid that they can deliver per cycle or cubic inches per revolution.
Pressure is the force per unit area of a liquid, usually expressed in psi. (Most of the pressure in the hydraulic systems is created by resistance to flow.)
The pressure developed in a system has an effect on the volumetric output of the pump supplying flow to a system. As pressure increases, volumetric output decreases.
This drop in output is caused by an increase in internal leakage (slippage) from a pump's outlet side to its inlet side.
Slippage is a measure of a pump's efficiency and usually is expressed in percent.
Some slippage is designed into pumps for lubrication purposes.
If pressure increases, more flow will occur through a leakage path and less from an outlet port. Any increase in slippage is a loss of efficiency. PERFORMANCE Slide 39: a. External. Figure 3-6 shows the operating principle of an external gear pump.
It consists of a driving gear and a driven gear enclosed in a closely fitted housing. The gears rotate in opposite directions and mesh at a point in the housing between the inlet and outlet ports.
As the teeth of the two gears separate, a partial vacuum forms and draws liquid through an inlet port into chamber A. Liquid in chamber A is trapped between the teeth of the two gears and the housing so that it is carried through two separate paths around to chamber B. As the teeth again mesh, they produce a force that drives a liquid through an outlet port. GEAR PUMPS: Slide 41: b. Internal. Figure 3-7 shows an internal gear pump. The teeth of one gear project outward, while the teeth of the other gear project inward toward the center of the pump.
The two gears mesh on one side of a pump chamber, between an inlet and the discharge. On the opposite side of the chamber, a crescent-shaped form stands in the space between the two gears to provide a close tolerance.
The rotation of the internal gear by a shaft causes the external gear to rotate.
Since the two are in mesh. Everything in the chamber rotates except the crescent, causing a liquid to be trapped in the gear spaces as they pass the crescent.
Liquid is carried from an inlet to the discharge, where it is forced out of a pump by the gears meshing. As liquid is carried away from an inlet side of a pump, the pressure is diminished, and liquid is forced in from the supply source.
The size of the crescent that separates the internal and external gears determines the volume delivery of this pump. A small crescent allows more volume of a liquid per revolution than a larger crescent. Slide 42: c. Lobe. Figure 3-8 shows a lobe pump. It differs from other gear pumps because it uses lobed elements instead of gears. The element drive also differs in a lobe pump. In a gear pump, one gear drives the other. In a lobe pump, both elements are driven through suitable external gearing. Slide 43: In a vane-type pump, a slotted rotor splined to a drive shaft rotates between closely fitted side plates that are inside of an elliptical- or circular-shaped ring.
Polished, hardened vanes slide in and out of the rotor slots and follow the ring contour by centrifugal force.
Pumping chambers are formed between succeeding vanes, carrying oil from the inlet to the outlet. A partial vacuum is created at the inlet as the space between vanes increases. The oil is squeezed out at the outlet as the pumping chamber's size decreases.
The normal wear points in a vane pump are the vane tips and a ring's surface, the vanes and ring are specially hardened and ground. A vane pump is the only design that has automatic wear compensation built in. As wear occurs, the vanes simply slide farther out of the rotor slots and continue to follow a ring's contour. Thus efficiency remains high throughout the life of the pump. VANE PUMPS: Unbalanced Vane Pumps:Unbalanced design, (Figure 3-9), a cam ring's shape is a true circle that is on a different centerline from a rotor's.
Pump displacement depends on how far a rotor and ring are eccentric.
The advantage of a true-circle ring is that control can be applied to vary the eccentricity and thus vary the displacement.
A disadvantage is that an unbalanced pressure at the outlet is effective against a small area of the rotor's edge, imposing side loads on the shaft. Slide 45: Balanced Vane Pumps. In the balanced design (Figure 3-10), a pump has a stationary, elliptical cam ring and two sets of internal ports.
A pumping chamber is formed between any two vanes twice in each revolution.
The two inlets and outlets are 180 degrees apart.
Back pressures against the edges of a rotor cancel each other.
Recent design improvements that allow high operating speeds and pressures have made this pump the most universal in the mobile-equipment field. Slide 46: Vane-type double pumps: (Figure 3-11) consist of two separate pumping devices.
Each is contained in its own respective housing, mounted in tandem, and driven by a common shaft. Each pump also has its own inlet and outlet ports, which may be combined by using manifolds or piping.
Design variations are available in which both cartridges are contained within one body. An additional pump is sometimes attached to the head end to supply auxiliary flow requirements.
Double pumps may be used to provide fluid flow for two separate circuits or combined for flow requirements for a single circuit.
Separate circuits require separate pressure controls to limit maximum pressure in each circuit. TWO-STAGE PUMPS: : TWO-STAGE PUMPS: Two-stage pumps consist of two separate pump assemblies contained in one housing.
The pump assemblies are connected so that flow from the outlet of one is directed internally to the inlet of the other. Single inlet and outlet ports are used for system connections.
In construction, the pumps consist of separate pumping cartridges driven by a common drive shaft contained in one housing. A dividing valve is used to equalize the pressure load on each stage and correct for minor flow differences from either cartridge. PISTON PUMPS: : PISTON PUMPS: Piston pumps are either radial or axial.
a. Radial. In a radial piston pump (Figure 3-14), the pistons are arranged like wheel spokes in a short cylindrical block.
A drive shaft, which is inside a circular housing, rotates a cylinder block. The block turns on a stationary pintle that contains the inlet and outlet ports.
As a cylinder block turns, centrifugal force slings the pistons, which follow a circular housing.
A housing's centerline is offset from a cylinder block's centerline. The amount of eccentricity between the two determines a piston stroke and, therefore, a pump's displacement.
Controls can be applied to change a housing's location and thereby vary a pump's delivery from zero to maximum. Slide 50: Figure 3-15 shows a nine-piston, radial piston pump. When a pump has an uneven number of pistons, no more than one piston is completely blocked by a pintle at one time, which reduces flow pulsations. With an even number of pistons spaced around a cylinder block, two pistons could be blocked by a pintle at the same time.
If this happens, three pistons would discharge at one time and four at another time, and pulsations would occur in the flow. A pintle, a cylinder block, the pistons, a rotor, and a drive shaft constitute the main working parts of a pump. Slide 52: INTERNAL RADIAL PISTON MOTOR: The barrel with the eight radial mounted pistons rotates over a fixed shaft which has the function of a sleeve valve. At the right moment a piston is pushed outwards and the roller which is connected to the piston, has to 'follow' the curved and fixed mounted ring.
By changing the direction of oil supply to the motor the direction of rotation can be changed. Slide 53: THE RADIAL PISTON MOTOR AS A WHEEL MOTOR: The barrel with the eight radial mounted pistons is fixed; the housing and the central sleeve valve rotate. The central sleeve valve takes care for the distribution of the oil.
By changing the direction of oil supply to the motor the direction of rotation can be changed. Slide 54: The axial piston pump with rotating swashplate.
In hydraulic systems with a workingpressure above aprox. 250 bar the most used pumptype is the pistonpump.
The pistons move parallel to the axis of the drive shaft. The swashplate is driven by the shaft and the angle of the swashplate determines the stroke of the piston.
The valves are necessary to direct the flow in the right direction. This type of pump can be driven in both directions but cannot be used as a hydromotor. THE AXIAL PISTON PUMP: Slide 55: THE AXIAL PISTON PUMP WITH ROTATING BARREL: This axial piston pump consists of a non rotating swashplate (green) and a rotating barrel (light blue).
The advantage of this construction is that the pump can operate without valves because the rotating barrel has a determined suck and pressure zone.
The animation shows the behaviour of only one piston; normally this pump has 5, 7, 9 or 11 pistons.
The pump in the animation can also be applied as a hydraulic motor. Slide 56: The animation shows how the displacement of an axial piston pump can be adjusted. In this example we use an axial piston pump with a rotating cylinder barrel and a static' swashplate.
The cylinder barrel is driven by the drive shaft which is guided through a hole in the swashplate. The position (angle) of the swashplate determines the stroke of the pistons and therefore the amount of displacement (cm3/omw) of the pump.
By adjusting the position of the swashplate the amount of displacement can be changed. The more the swashplate turns to the vertical position, the more the amount of displacement decreases.
In the vertical position the displacement is zero. In that case the pump may be driven but will not deliver any oil. Normally the swashplate is adjusted by a hydraulic cylinder built inside the pumphousing. THE AXIAL PISTON PUMP WITH VARIABLE DISPLACEMENT: Slide 57: BENT-AXIS AXIAL PISTON PUMP: Pumping action is the same as an in-line pump.
The angle of offset determines a pump's displacement, just as the swash plate's angle determines an in-line pump's displacement.
In fixed-delivery pumps, the angle is constant. In variable models, a yoke mounted on pintles swings a cylinder block to vary displacement.
Flow direction can be reversed with appropriate controls. PUMP OPERATION: : PUMP OPERATION: The following graphs address some of the problems that could occur when a pump is operating:
a. Overloading. One risk of overloading is the danger of excess torque on a drive shaft.(You may need a larger pump)
b. Excess Speed. Running a pump at too high a speed causes loss of lubrication, which can cause early failure.
Excess speed also runs a risk of damage from cavitation. (use a higher displacement pump) Slide 59: c. Operating Problems. There are common operating problems in a pump.
(1) Pressure Loss. Pressure loss means that there is a high leakage path in a system.(relief valve, cylinders, motors, & A badly worn pump).
(2) Slow Operation. This can be caused by a worn pump or by a partial oil leak in a system. Pressure will not drop, however, if a load moves at all. Therefore, hp is still being used and is being converted into heat at a leakage point.
(3) No Delivery. If oil is not being pumped, a pump-
Could be assembled incorrectly.
Could be driven in the wrong direction.
Has not been primed. The reasons for no prime are usually improper start-up, inlet restrictions, or low oil level in a reservoir.
Has a broken drive shaft.
(4) Noise. If you hear any unusual noise, shut down a pump immediately. Cavitation noise is caused by a restriction in an inlet line, a dirty inlet filter, or too high a drive speed. Air in a system also causes noise. Noise can be caused by worn or damaged parts, which will spread harmful particles through a system, causing more damage if an operation continues. Slide 60: d. Cavitation. Cavitation occurs where available fluid does not fill an existing space.
Most of the time cavitation occurs in the suction part of the system. When cavitation takes place the pressure in the fluid decreases to a level below the ambient pressure thus forming 'vacuumholes' in the fluid.
When the pressure increases, for example in the pump, these 'vacuumholes' implode.
cavitation can be caused by:
acceleration of the oil flow behind a throttle /
when the oil contains water or air
high fluid temperature
a resistance in the suction part of the system
a suction line which is to small in diameter
a suction hose with a damaged inside liner
a suction filter which is saturated with dirt (animation)
high oil viscosity
insufficient breezing of the reservoir HYDRAULIC ACTUATORS : HYDRAULIC ACTUATORS Slide 62: A hydraulic actuator receives pressure energy and converts it to mechanical force and motion.
An actuator can be linear or rotary.
A linear actuator gives force and motion outputs in a straight line. It is more commonly called a cylinder but is also referred to as a ram, reciprocating motor, or linear motor.
A rotary actuator produces torque and rotating motion. It is more commonly called a hydraulic motor or motor. HYDRAULIC ACTUATORS: Slide 63: A cylinder is a hydraulic actuator that is constructed of a piston or plunger that operates in a cylindrical housing by the action of liquid under pressure.
Figure 4-1 shows the basic parts of a cylinder.
A cylinder housing is a tube in which a plunger (piston) operates.
In a ram-type cylinder, a ram actuates a load directly. In a piston cylinder, a piston rod is connected to a piston to actuate a load.
An end of a cylinder from which a rod or plunger protrudes is a rod end. The opposite end is a head end. The hydraulic connections are a head-end port and a rod-end port (fluid supply). CYLINDERS a. Single-Acting Cylinder. This cylinder (Figure) only has a head-end port and is operated hydraulically in one direction.
When oil is pumped into a port, it pushes on a plunger, thus extending it. To return or retract a cylinder, oil must be released to a reservoir. A plunger returns either because of the weight of a load or from some mechanical force such as a spring.
In mobile equipment, flow to and from a single-acting cylinder is controlled by a reversing directional valve of a single-acting type. Slide 64: b. Double-Acting Cylinder. This cylinder (Figure 4-2 must have ports at the head and rod ends.
Pumping oil into the head end moves a piston to extend a rod while any oil in the rod end is pushed out and returned to a reservoir.
To retract a rod, flow is reversed. Oil from a pump goes into a rod end, and a head-end port is connected to allow return flow.
The flow direction to and from a double-acting cylinder can be controlled by a double-acting directional valve or by actuating a control of a reversible pump. Slide 65: c. Differential Cylinder. In a differential cylinder, the areas where pressure is applied on a piston are not equal. On a head end, a full piston area is available for applying pressure. At a rod end, only an annular area is available for applying pressure. A rod's area is not a factor, and what space it does take up reduces the volume of oil it will hold.
Two general rules about a differential cylinder are that-
With an equal GPM delivery to either end, a cylinder will move faster when retracting because of a reduced volume capacity.
With equal pressure at either end, a cylinder can exert more force when extending because of the greater piston area. In fact, if equal pressure is applied to both ports at the same time, a cylinder will extend because of a higher resulting force on a head end.
d. Nondifferential Cylinder. This cylinder (Figure 4-3) has a piston rod extending from each end. It has equal thrust and speed either way, provided that pressure and flow are unchanged. A nondifferential cylinder is rarely used on mobile equipment. Slide 66: e. Ram-Type Cylinder. A ram-type cylinder is a cylinder in which a cross-sectional area of a piston rod is more than one-half a cross-sectional area of a piston head. In many cylinders of this type, the rod and piston heads have equal areas.
A ram-type actuating cylinder is used mainly for push functions rather than pull.
Figure 4-4 shows a telescoping, ram-type, actuating cylinder, which can be a single- or double-acting type.
In this cylinder, a series of rams are nested in a telescoping assembly.
Except for the smallest ram, each ram is hollow and serves as a cylinder housing for the next smaller ram.
A ram assembly is contained in a main cylinder housing, which also provides the fluid ports. Although an assembly requires a small space with all of the rams retracted, a telescoping action of an assembly provides a relatively long stroke when the rams are extended. Slide 67: f. Piston-Type Cylinder. In this cylinder, a cross-sectional area of a piston head is referred to as a piston-type cylinder. A piston-type cylinder is used mainly when the push and pull functions are needed.
A single-acting, piston-type cylinder uses fluid pressure to apply force in one direction.
In some designs, the force of gravity moves a piston in the opposite direction. However, most cylinders of this type apply force in both directions.
Fluid pressure provides force in one direction and spring tension provides force in the opposite direction.
Figure 4-5 shows a single-acting, spring-loaded, piston-type cylinder. In this cylinder, a spring is located on the rod side of a piston. In some spring-loaded cylinders, a spring is located on a blank side, and a fluid port is on a rod end of a cylinder.
Figure 4-6 shows a double-acting piston-type cylinder. This cylinder contains one piston and piston-rod assembly and operates from fluid flow in either direction.
If this is an unbalanced cylinder, which means that there is a difference in the effective working area on the two sides of a piston.
Figure 4-6 shows a balanced, double-acting, piston-type cylinder. The effective working area on both sides of a piston is the same, and it exerts the same force in both directions. Slide 68: g. Cushioned Cylinder. To slow an action and prevent shock at the end of a piston stroke, some actuating cylinders are constructed with a cushioning device at either or both ends of a cylinder.
This cushion is usually a metering device built into a cylinder to restrict the flow at an outlet port, thereby slowing down the motion of a piston. Figure 4-7 shows a cushioned actuating cylinder. h. Lockout Cylinders. A lockout road wheel moves up, a control lever forces the respective cylinder to ccylinder is used to lock a suspension mechanism of a tracked vehicle when a vehicle functions as a stable platform. A cylinder also serves as a shock absorber when a vehicle is moving. Each lockout cylinder is connected to a road arm by a control lever. When each ompress. Hydraulic fluid is forced around a piston head through restrictor ports causing a cylinder to act as a shock absorber. When hydraulic pressure is applied to an inlet port on each cylinder's connecting eye, an inner control-valve piston is forced against a spring in each cylinder. This action closes the restrictor ports, blocks the main piston's motion in each cylinder, and locks the suspension system. Slide 69: THE LIMITED ANGLE ROTARY ACTUATOR: The limited angle rotary actuator is applied when the shaft has to rotate over a limited angle.
The animation shows how this simple actuator works: in this case the shaft can rotate over an angle of about 270 degrees.
This type of actuator is, among others, used as a rotator actuator on (small) cranes and excavators. Slide 70: CYLINDERS FOR LONG-STROKE APPLICATIONS: SPECIFICATIONS TO BE CONSIDERED WHILE PURCHASING A HYDRAULIC CYLINDER: : SPECIFICATIONS TO BE CONSIDERED WHILE PURCHASING A HYDRAULIC CYLINDER: The specifications that need to be considered while purchasing a hydraulic cylinder are:
Bore Diameter: It is the diameter of the cylinder bore.
Maximum operating pressure: The maximum working pressure a cylinder can carry is known as maximum operating pressure.
Rod Diameter: It is the diameter of the piston or the rod that are used in hydraulic cylinders.
Stroke: The distance traveled by a piston in a hydraulic cylinder is known as stroke. The length of a stroke could be several feet, or a fraction of an inch.
Type Of Cylinder: The different types of cylinders are tie-rod cylinder, ram cylinder and welded cylinder.
1)Tie-rod cylinder: These types of hydraulic cylinders make use of a single or multiple tie-rods to provide extra stability to the cylinder. The tie-rods are mostly installed on the exterior diameter of the cylinder. The tie-rods carry most of the load in this type of hydraulic cylinder.
2)Welded cylinder: There are heavy-duty welded cylinders used to balance the cylinder. The welded cylinders are smooth hydraulic cylinders.
3)Ram cylinders: As the name suggests, this cylinders act as a ram. The cross-section of the moving components is half of the cross-section area of the piston rod. These hydraulic ram cylinders are not used to push and are mostly used to pull. The ram cylinder is a hydraulic cylinder that is used in applications of high pressure. Slide 72: A cylinder is constructed of a barrel or tube, a piston and rod (or ram), two end caps, and suitable oil seals.
A barrel is usually seamless steel tubing, or cast, and the interior is finished very true and smoothly. A steel piston rod is highly polished and usually hard chrome-plated to resist pitting and scoring. It is supported in the end cap by a bushing or polished surface.
The cylinder's ports are built into the end caps, which can be screwed on to the tubes, welded, or attached by tie bolts or bolted flanges.
If the cylinder barrel is cast, the head-end cap may be integral with it. Mounting provisions often are made in the end caps, including flanges for stationary mounting or clevises for swinging mounts.
Seals and wipers are installed in the rod's end cap to keep the rod clean and to prevent external leakage around the rod.
Other points where seals are used are at the end cap and joints and between the piston and barrel. Depending on how the rod is attached to the piston, a seal may be needed.
Internal leakage should not occur past a piston. It wastes energy and can stop a load by a hydrostatic lock (oil trapped behind a piston). CONSTRUCTION AND APPLICATION Slide 74: MAINTENANCE: Hydraulic cylinders are compact and relatively simple. The following lists service tips in maintaining cylinders:
a. External Leakage. If a cylinder's end caps are leaking, tighten them. If the leaks still do not stop, replace the gasket. If a cylinder leaks around a piston rod, replace the packing. Make sure that a seal lip faces toward the pressure oil. If a seal continues to leak, check paragraphs 4-3e through i.
b. Internal Leakage. Leakage past the piston seals inside a cylinder can cause sluggish movement or settling under load. Piston leakage can be caused by worn piston seals or rings or scored cylinder walls. The latter may be caused by dirt and grit in the oil.
NOTE: When repairing a cylinder, replace all the seals and packings before reassembly.
c. Creeping Cylinder. If a cylinder creeps when stopped in midstroke, check for internal leakage (paragraph 4-3b). Another cause could be a worn control valve.
d. Sluggish Operation. Air in a cylinder is the most common cause of sluggish action. Internal leakage in a cylinder is another cause. If an action is sluggish when starting up a system, but speeds up when a system is warm, check for oil of too high a viscosity (see the machine's operating manual). If a cylinder is still sluggish after these checks, test the whole circuit for worn components.
e. Loose Mounting. Pivot points and mounts may be loose. The bolts or pins may need to be tightened, or they may be worn out. Too much slop or float in a cylinder's mountings damages the piston-rod seals. Periodically check all the cylinders for loose mountings. Slide 75: f. Misalignment. Piston rods must work in-line at all times. If they are side-loaded, the piston rods will be galled and the packings will be damaged, causing leaks. Eventually, the piston rods may be bent or the welds broken.
g. Lack of Lubrication. If a piston rod has no lubrication, a rod packing could seize, which would result in an erratic stroke, especially on single-acting cylinders.
h. Abrasives on a Piston Rod. When a piston rod extends, it can pick up dirt and other material. When it retracts, it carries the grit into a cylinder, damaging a rod seal. For this reason, rod wipers are often used at the rod end of a cylinder to clean the rod as it retracts. Rubber boots are also used over the end of a cylinder in some cases. Piston rods rusting is another problem. When storing cylinders, always retract the piston rods to protect them. If you cannot retract them, coat them with grease.
i. Burrs on a Piston Rod. Exposed piston rods can be damaged by impact with hard objects. If a smooth surface of a rod is marred, a rod seal may be damaged. Clean the burrs on a rod immediately, using crocus cloth. Some rods are chrome-plated to resist wear. Replace the seals after restoring a rod surface.
j. Air Vents. Single-acting cylinders (except ram types) must have an air vent in the dry side of a cylinder. To prevent dirt from getting in, use different filter devices. Most are self-cleaning, but inspect them periodically to ensure that they operate properly. Slide 76: HYDRAULIC MOTORS: Hydraulic motors convert hydraulic energy into mechanical energy.
In industrial hydraulic circuits, pumps and motors are normally combined with a proper valving and piping to form a hydraulic-powered transmission.
A pump, which is mechanically linked to a prime mover, draws fluid from a reservoir and forces it to a motor. A motor, which is mechanically linked to the workload, is actuated by this flow so that motion or torque, or both, are conveyed to the work.
Figure 4-9 shows the basic operations of a hydraulic motor. The main types of motors are gear, vane, and piston. They can be unidirectional or reversible. (Most motors designed for mobile equipment are reversible.) HYDRAULIC MOTOR APPLICATIONS: : HYDRAULIC MOTOR APPLICATIONS: Compact and extremely efficient, small hydraulic motors can be used for various machining operations like boring, reaming, drilling etc. Due to their small size they are tools of choice for applications like:
Electric motor coil winding
Oil pipeline inspection equipment
Undersea camera manipulation
Jumbo jet maintenance jacks
Milling and sawing applications
Dynamite blast hole pump drive
Textile washing agitators
Orange peeling machines
Diamond wheel dresser
Drill and tap machine tool
Chicken processing machinery
Conveyor drives VALVES : VALVES Slide 79: Valves are used in hydraulic systems to control the operation of the actuators.
Valves regulate pressure by creating special pressure conditions and by controlling how much oil will flow in portions of a circuit and where it will go.
The three categories of hydraulic valves are pressure-control, flow- (volume-) control, and directional-control (see Figure 5-1).
Some valves have multiple functions, placing them into more than one category.
Valves are rated by their size, pressure capabilities, and pressure drop/flow. VALVES: Slide 80: FLOW-CONTROL VALVES: Flow-control valves are used to control an actuator's speed by metering flow. Metering is measuring or regulating the flow rate to or from an actuator.
Some of these valves are gasket-mounted, and some are panel-mounted. Slide 81: a. Gate Valve. In this type of valve, a wedge or gate controls the flow. To open and close a passage, a hand wheel moves a wedge or gate up and down across a flow line.
Figure 5-30 shows the principal elements of a gate valve.
When the valve is opened, the gate stands up inside the bonnet with its bottom flush with the wall of the line.
When the valve is closed, the gate blocks the flow by standing straight across the line where it rests firmly against the two seats that extend completely around the line.
A gate valve allows a straight flow and offers little or no resistance to the fluid flow when the valve is completely open. Sometimes a gate valve is in the partially open position to restrict the flow rate. However, its main use is in the fully open or fully closed positions. Slide 82: b. Globe Valve. A disc, which is screwed directly on the end of the stem, is the controlling member of a globe valve.
A valve is closed by lowering a disc into a valve seat. Since fluid flows equally on all sides of the center of support when a valve is open, there is no unbalanced pressure on a disc to cause uneven wear. Figure 5-31 shows a globe valve. Slide 83: c. Needle Valve. A needle valve is similar in design and operation to a globe valve. Instead of a disc, a needle valve has a long, tapered point at the end of a valve stem.
Figure 5-32 shows a sectional view of a needle valve. A long taper allows a needle valve to open or close gradually. A needle valve is used to control flow:
Into delicate gauges, which could be damaged if high-pressure fluid was suddenly delivered.
At the end of an operation when work motion should halt slowly.
At other points where precise flow adjustments are necessary.
At points where a small flow rate is desired. Slide 84: d. Restrictor. A restrictor is used in liquid-powered systems to limit the movement speed of certain actuating devices by limiting flow rate in a line.
Figure 5-33 shows a fixed restrictor. Figure 5-34 shows a variable restrictor, which varies the restriction amount and is a modified needle valve.
This valve can be preadjusted to alter the operating time of a particular subsystem. Also, it can be adjusted to meet the requirements of a particular system. Slide 85: e. Orifice Check Valve. This valve is used in liquid-powered systems to allow normal speed of operation in one direction and limited speed in another.
Figure 5-35 shows two orifice check valves. Slide 86: f.pressure compensated flow control: To control the velocity of a hydraulic motor or cylinder one has to control the flow to these components. This can be done with a simple flow controlThe flow through a flow control is determined by:a) The area of the flow control: a larger area means a higher amount of flow andb) the pressure drop across the flow control: an increase of the pressure drop means an increase of flow.
When the pressure drop across the flow control decreases as a result of an increase of the load on the cylinder the flow and velocity of the cylinder will decrease. If the velocity has to remain constant and independent of the load one has to use a pressure compensated flow control The plunger finds it's balance when:p2 = p3 + pspring ==> p2 - p3 = pspring and because of the fact that pspring= constant (8 bar) the pressure compensator keeps the pressure drop across the needle valve on a constant value of 8 bar. This means that the flow through the needle valve remains constant! Slide 88: DIRECTIONAL-CONTROL VALVES: Directional-control valves also control flow direction. However, they vary considerably in physical characteristics and operation. The valves may be a
Poppet type, in which a piston or ball moves on and off a seat.
Rotary-spool type, in which a spool rotates about its axis.
Sliding-spool type, in which a spool slides axially in a bore. In this type, a spool is often classified according to the flow conditions created when it is in the normal or neutral position.
Directional-control valves may also be classified according to the method used to actuate the valve element.
A poppet-type valve is usually hydraulically operated.
A rotary-spool type may be manually (lever or plunger action), mechanically (cam or trip action), or electrically (solenoid action) operated.
A sliding-spool type may be manually, mechanically, electrically, or hydraulically operated, or it may be operated in combination.
Directional-control valves may also be classified according to the number of positions of the valve elements or the total number of flow paths provided in the extreme position.
For example, a three-position, four-way valve has two extreme positions and a center or neutral position. In each of the two extreme positions, there are two flow paths, making a total of four flow paths. Slide 89: a. Spool valves (see Figure 5-11) are popular on modern hydraulic systems because they-
Can be made to handle flows in many directions by adding extra lands and oil ports.
Can be precision-ground for fine-oil metering.
Stack easily into one compact control package, which is important on mobile systems.
Spool valves, however, require good maintenance. Slide 90: b. Poppet Valve. Figure 5-12 shows a simple poppet valve. It consists primarily of a movable poppet that closes against a valve seat.
Pressure from the inlet tends to hold the valve tightly closed. A slight force applied to the poppet stem opens the poppet.
The action is similar to the valves of an automobile engine. The poppet stem usually has an O-ring seal to prevent leakage.
In some valves, the poppets are held in the seated position by springs. The number of poppets in a valve depends on the purpose of the valve. Slide 91: c. Check Valves. Check valves are the most commonly used in fluid-powered systems. They allow flow in one direction and prevent flow in the other direction.
They may be installed independently in a line, or they may be incorporated as an integral part of a sequence, counterbalance, or pressure-reducing valve.
The valve element may be a sleeve, cone, ball, poppet, piston, spool, or disc. Force of the moving fluid opens a check valve; backflow, a spring, or gravity closes the valve.
Figures 5-14, 5-15 and 5-16 show various types of check valves. Slide 93: Pilot-Operated Type: (Figure 5-19). In diagram A, the valve has poppet 1 seated on stationary sleeve 2 by spring 3. Slide 94: d. Two-Way Valve: A two-way valve is generally used to control the direction of fluid flow in a hydraulic circuit and is a sliding-spool type.
Figure 5-21 shows a two-way, sliding-spool, directional-control valve. As the spool moves back and forth, it either allows or prevents fluid flow through the valve.
In either shifted position in a two-way valve, a pressure port is open to one cylinder port, but the opposite cylinder port is not open to a tank.
A tank port on this valve is used primarily for draining. Slide 95: e. Four-Way Valves. Four-way, directional-control valves are used to control the direction of fluid flow in a hydraulic circuit, which controls the direction of movement of a work cylinder or the rotation of a fluid motor.
These valves are usually the sliding-spool type. A typical four-way, directional-control valve has four ports:
One pressure port is connected to a pressure line.
One return or exhaust port is connected to a reservoir.
Two working ports are connected, by lines, to an actuating unit.
Ports that are sealed off from each other in one position may be interconnected in another position. Spool positioning is accomplished manually, mechanically, electrically, or hydraulically or by combing any of the four. Slide 96: Figure 5-22 shows how the spool position determines the possible flow conditions in the circuit. The four ports are marked P, T, A, and B:
P is connected to the flow source;
T to the tank; and
A and B to the respective ports of the work cylinder, hydraulic motor, or some other valve in the circuit.
In diagram A, the spool is in such a position that port P is open to port A, and port B is open to port T. Ports A and B are connected to the ports of the cylinder, flow through port P, and cause the piston of the cylinder to move to the right.
Return flow from the cylinder passes through ports B and T. In diagram B, port P is open to port B, and the piston moves to the left. Return flow from the cylinder passes through ports A and T. Slide 97: PRESSURE-CONTROL VALVES: A pressure-control valve may limit or regulate pressure, create a particular pressure condition required for control, or cause actuators to operate in a specific order.
All pure pressure-control valves operate in a condition approaching hydraulic balance.
Usually the balance is very simple: pressure is effective on one side or end of a ball, poppet, or spool and is opposed by a spring.
Most pressure-control valves are classified as normally closed.
a. The pressure relief valve: The pressure relief valve is mounted at the pressure side of the hydraulic pump.
It's task is to limit the pressure in the system on an acceptable value. In fact a pressure relief valve has the same construction as a spring operated check valve.
When the system gets overloaded the pressure relief valve will open and the pump flow will be leaded directly into the hydraulic reservoir. Slide 98: b. Sequence Valves. Sequence valves control the operating sequence between two branches of a circuit.
The valves are commonly used to regulate an operating sequence of two separate work cylinders so that one cylinder begins stroking when the other completes stroking. Sequence valves used in this manner ensure that there is minimum pressure equal to its setting on the first cylinder during the subsequent operations at a lower pressure.
Figure 5-7, diagram A, shows how to obtain the operation of a sequencing pressure by adjusting a spring's compression Slide 100: e. Pressure Switches. Pressure switches are used in various applications that require an adjus-table, pressure-actuated electrical switch to make or break an electrical circuit at a predetermined pressure
An electrical circuit may be used to actuate an electrically controlled valve or control an electric-motor starter or a signal light.
Figure 5-10 shows a pressure switch. Liquid, under pressure, enters chamber A. If the pressure exceeds the adjusted pressure setting of the spring behind ball 1, the ball is unseated. The liquid flows into chamber B and moves piston 2 to the right, actuating the limit to make or break an electrical circuit.
When pressure in chamber A falls below the setting of the spring behind ball 1, the spring reseats ball 1.
The liquid in chamber B is throttled past valve 3 and ball 4 because of the action of the spring behind piston 2. The time required for the limit switch to return to its normal position is determined by valve 3's setting. Thank you : Thank you M.Ganesh Murugan