HYDRAULIC SYSTEM AND COMPONENTS
This section explain about hydraulic system in detail.
Sources of Hydraulic system power: Pumping Theory – Pump Classification- Construction, Working, Design, Advantages, Disadvantages, Performance, Selection criterion of Linear, Rotary- Fixed and Variable displacement pumps, Hydraulic Actuators: Cylinders – Types and construction, Hydraulic motors Control Components: Direction control, Flow control and Pressure control valves- Types, Construction and Operation- Applications – Types of actuation. Accessories: Reservoirs, Accumulators, Intensifiers, Pressure Switches- Applications- Fluid Power ANSI Symbol.
A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy (hydrostatic energy i.e. flow, pressure). It generates flow with enough power to overcome pressure induced by the load at the pump outlet.
A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action.
The basic operating principle that moves fluid through a pump is similar in all pumps.
–Enlarging the volume of a chamber allows fluid to enter the pump
–Reducing the chamber volume moves fluid to the system.
–Inlet and discharge valves or ports control fluid movement through the pump.
There are two broad classifications of pumps as identified by the fluid power industry. They are described as follows.
–Hydrodynamic or non-positive pumps
- They are used for low-pressure, high-volume flow applications.
- Normally their maximum pressure capacity is limited to 250-300 psi.
–Hydrostatic or positive pumps (Gear, vane, piston pumps)
- High pressure capability (up to 10,000 psi or higher)
- Small compact size
- High volumetric efficiency
A positive displacement pump makes a fluid move by trapping a fixed amount and forcing (displacing) that trapped volume into the discharge pipe. Some positive displacement pumps use an expanding cavity on the suction side and a decreasing cavity on the discharge side.
A non–positive–displacement pump. produces a continuous flow. However, because it does not provide a positive internal seal against slippage, its output varies considerably as pressure varies. Centrifugal and propeller pumps are examples of non–positive–displacement pumps.
Centrifugal pumps are a sub-class of dynamic axisymmetric work-absorbing turbomachinery, Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow.
The rotational energy typically comes from an engine or electric motor. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from where it exits.
Common uses include water, sewage, petroleum and petrochemical pumping.
Energy Transfer: the transfer of energy from the shaft to the impellor and from the impeller to water
Centrifugal Force: the force used to throw the water from the impeller.
Three different configurations:
End Suction Centrifugal. – Center of the suction line is centered on the impeller eye.
Split case pumps. – Volute Case is split horizontally.
Vertical Turbines. – Primarily mounted with a Vertical Shaft.
Axial Flow (Propeller) Pump.
An axial–flow pump, or AFP, is a common type of pump that essentially consists of a propeller (an axial impeller) in a pipe.
In axial flow centrifugal pumps the rotor is a propeller. Fluid flows parallel to the axis as illustrated in Figure. Diffusion vanes are located in the discharge port of the pump to eliminate the rotational velocity of the fluid imparted by the propeller.
There is also the axial flow centrifugal pump which uses a curved propeller-shaped impeller, whereas the impeller on a radial flow centrifugal pump looks more like a fan. Axial flow pumps move fluid by drawing fluid into their axis and using the impeller to send fluid out on the other side of the pump
A gear pump (Fixed Displacement only by Geometrical Necessity) uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for hydraulic fluid power applications. Gear pumps are also widely used in chemical installations to pump high viscosity fluids.
Internal (Gerotor) Gear Pump.
External Gear Pump.
A rotary vane pump is a positive-displacement pump that consists of vanes mounted to a rotor that rotates inside of a cavity. In some cases these vanes can have variable length and/or be tensioned to maintain contact with the walls as the pump rotates.
Balanced Vane Pump. ( Fixed Displacement only)
Unbalanced Vane Pump. (Fixed or Variable Displacement)
Piston pumps (Fixed or Variable Displacement) and plunger pumps use a mechanism (typically rotational) to create a reciprocating motion along an axis, which then builds pressure in a cylinder or working barrel to force gas or fluid through the pump. The pressure in the chamber actuates the valves at both the suction and discharge points.
1.Volumetric efficiency (ᾐvol ):
volumetric efficiency indicates the amount of leakage that takes place within the pump. This involves considerations such as manufacturing tolerances and flexing of the pump casing under design pressure operating conditions:
Volumetric efficiencies typically run from 80% to 90% for gear pumps, 82% to 92% for vane pumps, and 90% to 98% for piston pumps.
- Mechanical efficiency (ᾐmech):
Mechanical efficiency indicates the amount of energy losses that occur due to reason other than leakage. This includes friction in bearings and between other mating parts.
It also includes energy losses due to fluid turbulence. Mechanical efficiencies typically run from 90% to 95%.
- Overall efficiency (ᾐoverall):
The overall efficiency considers all energy losses and is defined mathematically as follows:
|FLOW CAPACITY (GPM)||COST (DOL
LARS PER HP)
–Select the actuator (hydraulic cylinder or motor) that is appropriate based on the loads encountered.
–Determine the flow-rate requirements. This involves the calculation of the flow rate necessary to drive the actuator to move the load through a specified distance within a given time limit.
–Determine the pump speed and select the prime mover.
–Pump type based on the Application & Cost.
–Select the system pressure. This ties in with the actuator size and the magnitude of the resistive force produced by the external load on the system.
Select the reservoir and associated plumbing, including piping, valving, hydraulic cylinders, and motors and other miscellaneous components.
–An actuator is a component of a machine that is responsible for moving or controlling a mechanism or system.
–A hydraulic actuator is used for converting hydraulic energy into mechanical energy.
–A hydraulic actuator consists of cylinder or fluid motor that uses hydraulic power to facilitate mechanical operation. The mechanical motion gives an output in terms of linear, rotatory or oscillatory motion. As liquids are nearly impossible to compress, a hydraulic actuator can exert a large force. The drawback of this approach is its limited acceleration.
Types: Linear actuators (also called ‘hydraulic cylinders’).
Rotary actuators (also called ‘hydraulic motors’).
The hydraulic cylinder is used to convert fluid power into linear mechanical force and motion.
Applications of hydraulic cylinders.
–The hydraulic cylinders are basically used for performing work such as pushing, pulling, tilting, and pressing in a variety of engineering applications such as in material handling equipment, machine tools, construction equipment, and automobiles.
Different types of hydraulic cylinders are
–single acting cylinders.
–Double acting cylinder.
A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder as a linear actuator.
The design of a hydraulic motor and a hydraulic pump are very similar. For this reason, some hydraulic pumps with fixed displacement volumes may also be used as hydraulic motors.
A hydraulic motor works the other way round as it converts hydraulic energy into mechanical energy: a rotating shaft.
One of the most important functions in any fluid power system is control. If control components are not properly selected, the entire system will fail to deliver the required output.
Elements for the control of energy and other control in fluid power system are generally called “Valves”.
The selection of these control components not only involves the type, but also the size, the actuating method and remote control capability. There are 3 basic types of valves.
- Directional control valves.
- Pressure control valves.
- Flow control valves
Directional Control Valve.
Directional control valves control start/stop, directions, and acceleration/deceleration of hydraulic cylinders and motors. They can be used in a various applications, and a wide range of products is available.
They can be categorized into three types: spool, poppet, and ball.
The spool type can be either a sliding type or a rotary type. The former is the most popular for pressure balancing and high capacity.
The poppet type offers excellent leak-tight capability (zero leak) for its poppet-seat contact.
The ball type is an alternative for the poppet; a ball is used instead of a poppet.
Classifications of DC Valve
According to number of working ports :
Two- way valves
Three – way valves
Four- way valves.
According to number of Switching position:
Two – position
Three – position
According to type of construction:
Designation of DC Valve
–The designation of the directional control valve refers to the number of working ports and the number of switching positions.
–A valve with 2 service ports and 2 switching positions is designated as 2 / 2 way valve.
–A valve with 3 service ports and 2 position is designated as 2 / 3 way valve.
–A valve with 4 service ports and 2 position is designated as 2 / 4 valve.
–A valve with 4 Service ports and 3 Switching position is designated as 3 / 4 way valve.
2/2 DC Valve
–The simplest type of directional control valve is a check valve which is a two way valve because it contains two ports.
–These valves are also called as on-off valves because
–They allow the fluid flow in only in one direction and the valve is normally closed.
In Fig, the port P is blocked by the action of spring as the valve is unactuated (absence of hand force). Hence the flow from port P to A is blocked. When actuated ( Presence of hand force ) the valve is opened, thereby connecting port P to A.
2/3 DC Valve
–A directional control valve primary function is alternatively to pressurize and exhaust one working port is called three-way valve. Generally, these valves are used to operate single- acting cylinders.
–These valves are normally closed valves ( i.e. the pump port is blocked when the valve is not operating ). The three-way valve ports are inlet from the pump, working ports , and exhaust to tank.
–Spool position 1: When the valve is actuated, the spool moves towards justify . In this position flow from pump enters the valve port P and flows out through the port A as shown by the straight- through line and arrow ( fig a). In this position, port T is blocked by the spool.
–Spool position 2: When the valve is un-actuated by the absence of hand force, the valve assumes this position by the action of spring In this position, port P is blocked by the spool. Flow from the actuator can go to the tank from A to T as shown by straight – through line and arrow.
2/4 DC Valve
–These valves are also used to operate double acting cylinder. These valves are also called as impulse valve as 2 / 4 DCV has only two switching positions, i.e it has no mid position.
–These valves are used to reciprocate or hold and actuating cylinder in one position.
–They are used on machines where fast reciprocation cycles are needed.
–Since the valve actuator moves
–such a short distance to operate the valve from one position to the other, this design is used for punching, stamping and for other machines needing fast action. Fig (a) and (b) shows the two position of 2 / 4 DCV
3/4 DC Valve
–These type of DCV consists of three switching position.
–Most three- position valves have a variety of possible flow path configurations, but has identical flow path configuration in the actuated position and different spring centered flow paths.
–It should be noted that a three-position valve is used whenever it is necessary to stop or hold a actuator at some intermediate position within its stroke range, or when multiple circuit or functions must be accomplished from one hydraulic power source.
- Open center 3 / 4 DCV
- Closed Center 3 / 4 DCV
- Tandem centered 3 /4 DCV
- Regenerative Center 3 / 4 DCV
- Floating Center 3 / 4 DCV
3/4 DC Valve. – Open Centered
–open- center circuit, all ports are open to each other in the center position.
–When the valve is in open center position, the pump flow is directed to the tank at atmospheric pressure.
–At the same time the actuator can be moved freely by applying an external force. Open center valve help to prevent heat buildup, and no work can be done by any part of the system.
3/4 DC Valve. – Close Centered.
–The working of the valve is similar to open center DCV and in actuated position the port connection is identical.
–In closed center DCV all ports are closed to each other. Hence the actuator connected to ports A and B is hydraulically locked and cannot be moved by an external force.
–In this position the pump flow must go over the relief valve when flow is not being used for any other parts of the circuit.
–This promotes oil oxidation , viscosity drop, which further raises the wear of parts and increased leakage.
3/4 DC Valve. – Tandem Centered.
–In the center configuration, the working ports A and B are blocked , and the pump port P is connected to tank port T.
–The tandem center also results in a locked actuator. However, it also unloads the pump at atmospheric pressure.
–The application of this design may be to hold a cylinder or fluid motor under load or to permit the pump flow to be connected to a series of valves for multiple circuitry.
3/4 DC Valve. – Regenerative.
–This is another type of common center configuration used in hydraulic circuits. Regenerative means the flow generated from the system itself.
–Regenerative center is used whenever the actuator movement in one direction requires two different speeds eg., Part of length of extending stroke of actuator requiring fast movement during no load condition and remaining length slow controlled motion.
During fast extending the DCV is un-actuated thereby by spring forces it comes to the mid position. This center saves on additional pump capacity required.
–Fig. shows double acting single rod cylinder connected to 3 / 4 DCV having regenerative center.
– The piston area(A) is more than ring area (B) . When the cylinder is actuated by this type of DCV , in the center configuration, the pump flow is directed towards A, since the piston area is more it offers least resistance( fluid flows always in the path of least resistance ) and hence the fluid flows towards A.
–The return flow from B also joins the pump flow, which increased the piston speed due to increased flow.
3/4 DC Valve. – Floating Centered
–In this position the pump port is blocked and the two working ports A and B are connected to tank port T.
–Since the working ports A and B are connected to tank T, the actuator can be moved freely without any external force and hence the name floating.
2/2 DC Valve. (Poppet design)
–It is essentially a check valve as it allows free flow of fluid only in one direction (P to A) as the valve is opened hydraulically and hence the pump Port P is connected to port A.
–In the other direction the valve is closed by the ball poppet ( note the fluid pressure from A pushes the ball to its seat) and hence the flow from the port A is blocked
–DC Valves may be actuated by a variety of methods. Actuation is the method of moving the valve element from one position to another.
–A manually actuated DCV uses muscle power to actuate the spool. Manual actuators are hand lever , push button, pedals.
–The DCV spool can be actuated mechanically, by roller and cam, roller and plunger. The spool end contains the roller and the plunger or cam can be attached to the actuator (cylinder).
–When the cylinder reaches a specific position the DCV is actuated. The roller tappet connected to the spool is pushed in by a cam or plunger and presses on the spool to shift it either to right or justify reversing the direction of flow to the cylinder.
– When the electric coil (solenoid) is energized, it creates a magnetic force that pulls the armature into the coil.
–This caused the armature to push on the spool rod to move the spool of the valve
The advantage of a solenoid lies with in its less switching time
Hydraulic Actuated. (Pilot Actuated)
– The hydraulic pressure may directly used on the end face of the spool . The pilot ports are located on the valve ends.
Fluid in the Y end (right end ,not shown in the figure) is passed through the adjustable needle valve and exhausted back to tank. The amount of fluid bled through the needle valve controls how fast the valve will shift
– When air is introduced through the justify end passage ( X), its pressure pushes against the piston to shift the spool to the right.
–Removal of this justify end air supply and introduction of air through the right end passage (Y) causes the spool to shift to the justify
–Unshaded arrow represent pneumatic signal .
Pressure Control Valve.
–These are the units ensuring the control of pressure.
–A throttling orifice is present in the valve and by variation of orifice, the pressure level can be controlled or at a particular pressure, a switching action can be influenced.
–Basically one differentiates between pressure regulating and pressure switching valves.
–Pressure regulation valves are for maintaining a constant pressure in a system. Pressure switching valves, apart from a definite control function they also perform a switching action.
Such valves not only provide a switching signal, as in the case of pressure switches, but also operate themselves as a DCV type of switching within the hydraulic system.
Opening and Closing Pressure Difference:
– The minimum pressure at which the valve action starts is called as the opening or cracking pressure. The difference between the cracking pressure (commencement of flow) and the pressure obtained at maximum flow ( normal flow without change of spring force ) is referred as the “opening pressure difference”.
– Similarly the difference between the pressure corresponding to nominal flow and no flow during closing of the valve is referred as “closing pressure difference”. This is larger than the opening due to the flow forces acting in the opening direction as also the hysterisis in the spring.
Classifications of PC Valve
–Pressure Relief valve
–Pressure Reducing Valve
Pressure Relief Valve
–Its primary function is to limit the system pressure.
–It is normally a closed valve whose function is to limit the pressure to a specified maximum value by diverting pump flow back to the tank.
–The poppet is held in position by spring force plus the dead weight of spool.
–When pressure exceeds this force, the poppet is forced off its seat and excess fluid in the system is bypassed back to the reservoir
–A unloading valve is used to permit a pump to operate at minimum load.
–It operates on the principle that pump delivery is diverted to the tank, when sufficient pilot pressure is applied to move the spool against the spring force.
–The valve is held open by pilot pressure until the pump delivery is again needed by the circuit.
–The pilot fluid applied to move the spool upwards becomes a static system.
–In other words, it merely pushes the spool upward and maintains a static pressure to hold it open. When the pilot pressure is relaxed, the spool is moved down by the spring, and flow is diverted through the valve into the circuit.
–A sequence valve’s primary function is to divert flow in a predetermined sequence.
–A sequence valve may be direct or remote pilot- operated. These valves are used to control the operational cycle of a machine automatically.
–It consists of 2 ports, one main port connecting the main line and other (secondary port) connected to the secondary circuit. Usually the secondary port is closed by the spool.
–The pressure is effective on the end of the spool. This pressure will urge the spool against the spring force and at the preset value of the spring it allows a passage from the primary to the secondary port.
–For remote operation it is necessary to close the passage used for direct operation by plugging and provide a separate pressure source as required for the operation of the spool in the remote operation mode.
–A Counterbalance valve is used to maintain back pressure to prevent a load from failing.
–Application in vertical presses, lift trucks, loaders and other machine tool that must position or hold suspended loads. valve is used to maintain back pressure to prevent a load from failing.
–It has two ports, one connected to load and the other to the tank A small opening connecting the tank is provided in the control chamber to drain the oil that may collected due to leakage, thereby preventing the failure of valve.
–Counterbalance valve acts on the principle that fluid is trapped under pressure until pilot pressure.
–counterbalance valve is normally closed valve and will remain closed until acted upon by a remote pilot pressure source. Therefore, a much lower spring force is sufficient to allow the valve to operate at a lesser pilot pressure.
Pressure Reducing Valve
–Pressure reducing valve is used to limit its outlet pressure. Reducing valves are used for the operation of branch circuits, where pressure may vary from the main system pressures.
–it can be visualized that if the spring has greater force, the valves open wider and if the controlled pressure has greater force, the valves moves towards the spring and throttles the flow.
Flow Control Valve
–Flow Control Valves are used to regulate the rate of flow to the actuators.
–Control of flow is extremely important because the speed of the hydraulic actuator depends on the rate of flow of the fluid.
–Non Pressure Compensated FC Valve.
–Pressure Compensated FC Valve.
Non Pr. Compensated FC Valve
–Non-pressure-compensated flow-control valves are used when the system pressure is relatively constant and motoring speeds are not too critical.
–Basic Principle is the rate of flow through an orifice depends on the pressure drop across it.
–Disadvantages: The inlet pressure is the pressure from the pump that remains constant. Therefore, the variation in pressure occurs at the outlet that is defined by the work load. This implies that the flow rate depends on the work load. Hence, the speed of the piston cannot be defined accurately using non-pressure-compensated flow-control valves when the working load varies.
Pr. Compensated FC Valve
–Pressure-compensated flow-control valves overcome the difficulty caused by non-pressure-compensated valves by changing the size of the orifice in relation to the changes in the system pressure.
–If the pressure drop across the valve increases, that is, the upstream pressure increases relative to the downstream pressure, the compensator spool moves to the right against the force of the spring. This reduces the flow that in turn reduces the pressure drop and tries to attain an equilibrium position as far as the flow is concerned.
Fluid Power Accessories
–The hydraulic reservoir is a container for holding the fluid required to supply the system, including a reserve to cover any losses from minor leakage and evaporation. The reservoir can be designed to provide space for fluid expansion, permit air entrained in the fluid to escape, and to help cool the fluid.
–The purpose of the hydraulic reservoir is to hold a volume of fluid, transfer heat from the system, allow solid contaminants to settle and facilitate the release of air and moisture from the fluid. The hydraulic pump transmits mechanical energy into hydraulic energy.
–A hydraulic accumulator is a pressure storage reservoir in which a non -compressible hydraulic fluid is held under pressure that is applied by an external source. The external source can be a spring, a raised weight, or a compressed gas.
–Raised weight accumulator
Bladder type accumulator
Diaphragm type accumulator
Piston type accumulator
Metal bellow type accumulator
Raised Weight Accumulators
–A raised weight accumulator consists of a vertical cylinder containing fluid to the hydraulic line.
–The cylinder is closed by a piston on which a series of weights are placed that exert a downward force on the piston and thereby energizes the fluid in the cylinder.
–Gravity acts on the weight to pressurize the hydraulic system fluid, thus storing energy.
–It uses the energy stored in springs to create a constant force on the liquid contained in an adjacent ram assembly.
–The load characteristics of a spring are such that the energy storage depends on the force required to compress s spring.
–The free (uncompressed) length of a spring represents zero energy storage.
–As a spring is compressed to the maximum installed length, high 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.
Compressed Gas Accumulators
–It is widely used accumulator in present scenario.
–It is popularly known as “hydro-pneumatic accumulator”. It apply force to the liquid by using a compressed gas that acts as the spring.
–It uses inert gas (nitrogen) under pressure that provides the compressive force on fluid.
–Oxygen is not used because oxygen and oil can form an explosive mixture when combined under pressure
–As the volume of the compressed gas changes the pressure of the gas, and pressure of the fluid, changes inversely.
BLADDER TYPE ACCUMULATOR
–A bladder accumulator consists of seamless high-pressure cylinder with an internal elastomeric bladder with pressurized nitrogen on it and hydraulic fluid on the other(external) side.
–The accumulator is charged with nitrogen through a valve installed on the top. The accumulator will be pre-charged to nominal pressure when the pumps are not operating.
–The maximum flow rate of the accumulator is controlled by the opening orifice and the pressure difference across the opening.
–Bladder material widely used are epichlorohydric rubber(ECO) and Acrylonitrile butadiene rubber (NBR).
DIAPHRAGM TYPE ACCUMULATOR
–A similar to bladder type, expect an elastomeric diaphragm is used in place of a bag.
–This would typically reduce the usable volume of the accumulator, so the diaphragm accumulator may not have volume capacity of a bladder accumulator.
–Diaphragm accumulator may be spherical or cylindrical.
–The main difference with bladder accumulator is an increased maximum compression ratio of approximately 8:1
–It is low weight, compact design and good for shock applications (good response characteristics)
PISTON TYPE ACCUMULATOR
–This accumulator consists of a cylinder assembly, a piston assembly, and two end-cap assemblies.
–An accumulator contains a free-floating piston with liquid on one side of the piston and pre-charged air or nitrogen on the other side.
–An increase of liquid volume decreases the gas volume and increases gas pressure, which provides a work potential when the liquid is allowed to dis-charged.
METAL BELLOW ACCUMULATOR
–The metal bellows accumulator is similar to bladder type, expect the elastic is replaced by a hermitically sealed welded metal bellows.
–Fluid may be internal or external to the bellows.
–It is used when a fast response time is not critical, yet reliability is important.
–Metal bellow types are pre-charged by supplier and then permanently sealed leading to a maintenance free accumulator.
–A hydraulic intensifier is a hydraulic machine for transforming hydraulic power at low pressure into a reduced volume at higher pressure.
–A hydraulic intensifier is a device which is used to increase the intensity of pressure of any hydraulic fluid or water, with the help of the hydraulic energy available from a huge quantity of water or hydraulic fluid at a low pressure.
–In most of the hydraulic machinery used, the usual pressure of 80 to 100-psi may not be sufficient to operate certain spool valves and other mechanisms.
–To cater to the need for a high pressure requirement for a comparatively short period of time, pumps and accessories are definitely not the solution.
–The hydraulic intensifiers which can increase the pressure from 100 psi to 40,000 psi, using small volumes of fluid.
There are different types based on the medium of hydraulic fluids used and the number of strokes used to intensify to the desired pressure. They are
–Differential Cylinder Intensifiers.
Recent developments are so vast that huge pressures are achieved by using combinations of the above types.
–A pressure switch is a form of switch that closes an electrical contact when a certain set fluid pressure has been reached on its input. The switch may be designed to make contact either on pressure rise or on pressure fall.
–Pressure switches are widely used in industry to automatically supervise and control systems that use pressurized fluids.
–Another type of pressure switch detects mechanical force. For example, a pressure-sensitive mat is used to automatically open doors on commercial buildings. Such sensors are also used in security alarm applications such as pressure sensitive floors.
–A pressure switch for sensing fluid pressure contains a capsule, bellows, Bourdon tube, diaphragm or piston element that deforms or displaces proportionally to the applied pressure.
–The resulting motion is applied, either directly or through amplifying levers, to a set of switch contacts.
– Since pressure may be changing slowly and contacts should operate quickly, some kind of over-center mechanism such as a miniature snap-action switch is used to ensure quick operation of the contacts.
–Hydraulic pressure switches have various uses in automobiles, for example, to warn if the engine´s oil pressure falls below a safe level, or to control automatic transmission torque converter lock-up.