Circuits

A/214

Using schematicsymbols as building

blocks, you canconstruct a functional

diagram showingpiping arragnementsand operation of any

hydraulic orpneumatic circuit.

Basic circuitsBasic circuits

Anumber of circuits are usedfrequently in fluid power sys-tems to perform useful func-

tions. For example, metering circuitsoffer precise control of actuatorspeed without a lot of complicatedelectronics, decompression circuitsreduce pressure surges within a hy-draulic system by controlling the re-lease of s tored f luid energy, andpump-unloading and regenerativecircuits make a system more energyefficient. Other circuits are designedfor safety, sequencing of operations,and for controlling force, torque, andposition. Still other circuits may en-

into new or retrofit applications mayinvolve integrating electronic feed-back and control into the circuit as amodern alternative to mechanicalcontrol. However, many existing andnew applications still gain the great-est benefit from mechanical control— especial ly those applicat ionswhere electricity could pose a threatto health and safety.

However, whether using mechani-cal control or electronic, perhaps thegreatest benefit may be gained bycustomizing one of these circuits toserve the specific requirements of anapplication.

hance the application of specificcomponents, such as pumps, motors,accumulators, filters, and airline lu-bricators.

The circuits appearing on the follow-ing pages are provided as a resource ofgeneral ideas. They may be used as: ● an educational resource to aid under-standing of circuits already in use● a starting point for new designs, and ● as a modification to enhance opera-tion of existing equipment.

They certainly do not have to beimplemented as shown. In fact, manyof the circuits use purely mechanicalcomponents, so incorporating them

Accumulator circuits . . . . . . . . . . . . A/215

Air lubrication circuits . . . . . . . . . . . A/217

Hydraulic filter circuits . . . . . . . . . . . A/218

Hydraulic motor circuits . . . . . . . . . A/219

Pressure-control circuits . . . . . . . . . A/221

Pump-unloading circuits . . . . . . . . . A/223

Regenerative circuits . . . . . . . . . . . . A/224

Sequencing circuits . . . . . . . . . . . . . A/225

Speed-control circuits . . . . . . . . . . . A/226

Synchronizing circuits . . . . . . . . . . . A/227

Basic circuits contents

2003 Fluid Power Directory A/215

Handpump

Dieselengine

Mainpump

Startingmotor

§ Traverse and clampThis arrangement of a large and small accumulator acts

similar to a hi-lo circuit for rapid traverse and clamp. Fluid fromthe large accumulator combines with pump output to extend thecylinder rapidly. Fully extending the cylinder trips the limitswitch to actuate solenoid (c). The small accumulator thenmaintains high clamping pressure on the cylinder for a timedperiod, during which the pump recharges the large accumulator.Any fluid lost by the small accumulator will also be replacedduring this time.

¶ Standby power source In this diesel engine starting circuit, maximum power

is required only for a short period, and time between op-erations is long. Power for starting the engine is stored inthe accumulators. During operation of the engine, themain pump charges the accumulators to the pressure set-ting of the unloading valve. The pump is then unloadedfor the remainder of engine running time. For starting, themanual valve is opened, connecting the combined outputfrom the accumulators to drive the fluid motor. The handpump serves as a means of recharging the accumulators incase of leakage over a long period of inactivity.

¶ Reduced pump capacity Frequently, pumps can be downsized if the circuit uses

an accumulator. The accumulator volume adds to that ofboth pumps to speed downward travel of the press ram.When the ram meets sufficient resistance, the pressureswitch is actuated, shifting the solenoid valve. This directsfluid from the large pump to recharge the accumulator,while the small pump continues to supply high-pressurefluid to the ram. When the manually operated, 4-wayvalve is shifted for the return stroke, pressure is relieved,the solenoid valve is de-energized, and both pumps andthe accumulator deliver fluid for rapid return.

MM

Press ram

Pressureswitch

High-pressurerelief valve

Low-pressure

reliefvalve

10 gal

1 gal

c

ba

LS

Accumulators storefluid energy and are

used to reduce pumpcapacity

requirements, speedoperation, reduce

pressure surges, andas standby power

sources.

AccumulatorAccumulatorcircuitscircuits

A/216

MMMM

Vise jaws

Unloadingvalve

MM

Unloadingvalve

Pressureloaded

rolls

MM

¶ Safety deviceThese mill rolls are loaded by hydraulic pressure.

Using an accumulator al lows running the pumpunloaded most of the time, which saves power. Theaccumulator also protects the rolls from damage if alarge piece of foreign matter enters the mil l byabsorbing fluid displaced when the roll rises. Thisfluid returns to the circuit when the foreign matter haspassed through.

¶ Increased speed Using a pilot-operated check valve allows adding fluid

from the accumulator to pump output at the proper timewithin a cycle. Operating the manual valve directs fluid toretract the cylinder, exerting a pulling force. Whenpressure increases, the check valve opens, connecting theaccumulator to the cylinder for fast action. Releasing themanual valve allows the pump to recharge the accumulatorto the pressure setting of the unloading valve.

¶ Surge reduction Operating the 4-way, closed-center valve in this circuit

can cause the formation of shock pressures several timesthe value of the maximum pressure setting on the reliefvalve. Because the relief valve cannot act fast enough todrain off fluid, the high pressures can be dangerous topersonnel and equipment. The accumulator in this circuitabsorbs the surge pressures generated when the valve isplaced in the neutral position.

¶ Clamping Holding pressure, leakage compensation, and power

savings are obtained by using the accumulator in this visecircuit. While the vise jaws are in the clamp position,pressure is held by the accumulator, and pump output isunloaded at low pressure. The accumulator compensatesfor any leakage past the piston seals in the cylinder.When clamping pressure drops below the setting of theunloading valve, the valve closes, and the pumprecharges the accumulator.

BASIC CIRCUITS


Injector

¶ Downstream lubrication

The upper drawing shows a conventional circuit: the lubricator mounts ahead of the control valve. In the lowerdrawing, air flow through the directional valve lubricatesthe cylinder. The valve is lubricated when the cylinderexhausts.

§ Oil injection

In the upper circuit, oil is injected into the head end of thecylinder each time the control valve cycles. In the bottom cir-cuit, oil is injected into the cylinder through a small tube insidethe air line.

Ä Lubricating short-stroke cylinders

In this circuit, the lubricator bypasses the control valveand lubricates the cylinder while its rod retracts. Whenthe rod extends, exhaust air from the head end lubricatesthe control valve.

For more informationFor more information on airline lubrication, see pages A/37-39 in this handbook.

Valves, cylinders, andair motors can be

lubricated by injectingoil into the air stream

powering them.Properly locating the

lubricator in the circuitis important to ensure

proper lubrication.

Air lubricationAir lubricationcircuitscircuits

¶ Pressure line filterThis circuit has the advantage of

full-flow filtration but the disadvan-tage of high pressure drop. Often itcan be used instead of a low-pres-sure return line filter to preventbuilding up backpressure.

A/218

¶ Suction line strainer or filter To prevent pump cavitation, com-

ponents in this system must havelow pressure drop and high capac-ity. Filter elements should be sub-merged in reservoirs so no part ofthe filter surface is exposed to air.

¶ Independent circuitSometimes called a kidney loop,

this circuit filters full flow from aseparate filter pump. These circuitsoften incorporate a heat exchangerand multiple filters.

¶ Return line bypass filter A backpressure check valve

forces fluid through an orifice tothe filter. The only limitation hereis that the backpressure must notinterfere with circuit operation.

¶ Pressure line bleed-off filter An orifice between the pump dis-

charge and filter assembly main-tains a constant flow through the fil-ter. This circuit generally is used onhigh-flow circuits because flowthrough the filter is lost as effectivepump output.

¶ Discharge line filterAn advantage of this circuit is that

it filters oil immediately as it returnsfrom the work station. Return linefilters can tolerate a higher pressuredrop than those in a suction line.

High-performancecircuits require cleanfluids. Filters may beinstalled in pressurelines, return-to-tanklines, or bleed-off

lines.

Hydraulic Hydraulic filter circuitsfilter circuits

MM Brakevalve

(b)

X

MM(a)


Driving a fixed-displacement fluid motor at constant pres-sure produces a constant torque drive, left-hand drawing. Usedwith a variable-volume pump to vary flow, the horsepoweroutput of the motor varies with speed. If the load becomes ex-cessive, pressure rises to actuate the pressure switch, de-ener-gizing the solenoid valve — pump unloads, fluid motor stops.

In the right-hand drawing, a fixed-displacement pump sup-plying a variable-displacement motor at constant pressure pro-duces a constant-horsepower drive. The motor produces low-est speed and highest torque when displacement is maximum.Highest speed and lowest torque are produced when the dis-placement is minimum.

MM

Brake valve

MM

Brake

When pump delivery is cut off from a motor, it continues ro-tating because of its inertia and that of the connected load. Themotor then acts as a pump, and a source of fluid must be avail-able to prevent it from cavitating. In (a) the manual valve al-lows coasting to a stop and braked stop, as well as the normaldriving condition of the motor. With the valve spool up, thepump output drives the fluid motor. With the spool centered,pump output and both sides of the fluid motor are connected totank, so that the motor coasts to a stop. With the valve spool

down, the pump is unloaded and the motor, acting as a pump,forces fluid through the relief valve, which brakes it to a stop.Circuit (b) shows a brake valve that is a modified sequencevalve. It supplies braking force as well as control of a negativework load. Under normal conditions, system pressure holds thebrake valve open for free discharge from the motor. A negativeload reduces pressure at the motor inlet, and the brake valvecloses to throttle motor discharge and create a backpressure. Tostop the motor, the 4-way valve is shifted to neutral.

¶ Constant torque & constant power

Ä Braking

Hydraulic pressureregulation controlsmotor torque, while

flow regulationcontrols speed. Using

a variable-displacement motorprovides constant

horsepower.

Hydraulic Hydraulic motor circuitsmotor circuits

A/220

MM

Brakevalve

Brakevalve

Super-chargerelief

Brakevalve

MM

When a fluid motor and pump are connected in a closed cir-cuit, make-up fluid to compensate for the leakage must be sup-plied through replenishing valves. These valves also supplyfluid to the motor during braking. In the left-hand circuit, a non-reversing variable volume pump is used and control of fluid mo-tor direction is by the 4-way valve. The fixed displacement

pump provides supercharge pressure. The network of check andrelief valves provides for replenishing and braking in either di-rection. Braking pressure in each direction can be set indepen-dently on the two brake valves. In the right-hand circuit, a re-versing pump is used. Although the replenishing network issimpler, brake pressure must be the same in both directions.

¶ Series connectionConnecting two fluid motors in series minimizes pump

size and eliminates the need for a flow divider. Line sizes arealso smaller than in a comparable parallel circuit, and pipingis usually simpler, with only one pressure line and one returnline required. Maximum torque at each motor is adjustablewith the relief valves. The speed of motor A is controlled bythe bleed-off flow control valve. Direction of motor B is con-trolled by the 4-way valve, which has an integral 2-wayvalve, which vents the relief valve when the motor isstopped. Total system pressure is then available at motor A.

¶ Parallel connectionPump pressure can be lower in a parallel circuit because in a

series circuit the pressure at the pump must be the sum of thepressure drops across the motors. However, where motor pres-sures vary widely, there is a loss of efficiency in supplying themotors requiring lower pressures. This circuit is most efficientwhere the load on each motor is the same. Raising the pressureon one motor renders the others less efficient and may disruptthe speed relationship. In parallel circuits, the only way to in-crease torque of the highest-pressure motor is to increase sys-tem pressure.

Ä Replenishing

Vent Line

BASIC CIRCUITS

§ Pilot pressureWhen open- or tandem-center valves are used in circuits requiring pilot pres-

sure to shift the valves, there must be a means of maintaining pressure when thevalves are in neutral. One method is to install a backpressure check valve in thetank line. The check valve maintains a backpressure of, say, 50 psi.

¶ Two relief valvesUsing two relief valves in this circuit gives two working

pressures. On the up stroke of the cylinder, the low-pres-sure relief valve limits system pressure. During the downstroke, the high-pressure valve limits maximum press ton-nage for doing work. Using the low-pressure relief on theup stroke saves power by supporting the cylinder with low-pressure fluid.

Pilot reliefvalve

MM

Low-pressurerelief valve


¶ Low-pressure retractionEnergizing the solenoid extends the cylinder at a maximum

pressure corresponding to the main system relief valve setting.De-energizing the solenoid valve retracts the cylinder andholds it retracted at the reduced pressure setting of the pilot re-lief valve. The check valve prevents the pilot relief valve fromoperating during cylinder extension.


See pages A/172-A175 for informationon operation of pressure-control valves.

Controlling pressurecontrols the outputforce of cylinders orthe output torque offluid motors or rotary

actuators.

Pressure- Pressure- control circuitscontrol circuits

¶ Two pressures Pilot-operated relief valves provide two pressures for the

mold-close cylinder and the injection cylinder of this plas-tic molding machine circuit. With the manual valve in neu-tral, the air-operated valves are actuated to extend the mold-close cylinder at a maximum pressure of 2000 psi.Operating the manual valve to extend the injection cylindervents the 400-psi relief to tank and vents the 2000-psi reliefat 400 psi. High-pressure fluid is held by the check valve.

A/222

¶Remote controlRegulating pump pressure from a remote station can be

accomplished by using small, pilot relief valves connectedto the system’s main pilot-operated relief valve. With the3-way solenoid valve de-energized, system pressure is lim-ited to 1500 psi in this circuit. Energizing the 3-waysolenoid valve permits venting the relief valve to either1000 psi or 500 psi, depending on the position of the 4-wayvalve, which is determined by the pilot signal it receives.

MM

Relief valve setat 1500 psi

500 psi

To work cylinders

1000 psi

MMRelief valve

set at 800 psi

Weld

Clamp

Pressure-reducing valveset at 400 psi

Mold close

Airsupply

400 psi

2000 psi

Injection

§ Reduced pressure

In a system with only one pump, reduced pressure forone branch of the circuit can be obtained with a pressure-re-ducing valve. This circuit is typical for a welder, which re-quires high clamping force to be set by the relief valve andreduced force on the welding gun to be set by the pressure-reducing valve. Placing the check valve in parallel with thepressure-reducing valve allows free return flow when theweld cylinder retracts.

BASIC CIRCUITS


© Hi-lo circuit Many systems require a high volume at low pressure for

rapid traverse of a vise or tool, and then low volume, highpressure for clamping or feeding. This can be accomplished bya hi-lo circuit using two pumps. During rapid traverse, bothpumps supply the system. When pressure rises during clamp-ing or feed, the large-volume main pump unloads, and thesmall pump maintains pressure. Output flow of the smallpump is low enough to prevent appreciable heating of the oil.Instead of pilot operation, the unloading valve can be solenoidcontrolled and actuated by a pressure switch.

MM


Unloadingvalve

PressureMaintaining

pump

Mainpump

When no flow isrequired in a hydraulicsystem, pump output

can be returned totank at low pressureinstead of dumpinghigh-pressure fluidover a relief valve.

Pump unloadingPump unloadingcircuitscircuits

¶ Pressure-compensated pumpA pressure-compensated, variable-vol-

ume pump is controlled by system pres-sure. As pressure increases, displacementof the pump decreases so that pump outputat the preset pressure is only sufficient tomake up for leakage. Used with a closed-center valve, the pump is stroked to mini-mum (zero) displacement when the valveis centered.

MM

¶ Open-center system When the open-center system is in neutral,

pump output flows through the directionalcontrol valve to tank. When this simple cir-cuit handles only low flows and the direc-tional valve spool has tapered lands, it provesto be very efficient. If several cylinders areused, the valve can be connected in series —that is, the tank port of one valve is connectedto the pressure port of the next.

§ Circuit 4When electrical control is desired, a

limit or proximity switch can be used toactivate and de-activate a regenerativecircuit. In the circuit shown, energizingsolenoids a and b extends the cylinderin a differential circuit. The limitswitch de-energizes solenoid b, direct-ing cylinder discharge fluid to tank.

X

Work

Work

Work

1

2

3

a

b

c

4

A/224

§ Circuit 1Rapid rod extension can be achieved

by returning the flow of oil from a cyl-inder’s head end back into its cap end.With no load, pressure in both head andcap ends is equal, so when the load isencountered, available working forcedepends on the differential area.

§ Circuit 2Combining the rapid extension of

Circuit 1 with full force in response toan applied load takes full advantage ofa regenerative circuit. The circuitshown produces a rapid approachstroke of the piston rod. When the rodencounters resistance (workpieceload), pressure rises on the cap end toopen the sequence valve and allow oilfrom the head end to flow to tankthrough the 4-way valve. Once this oc-curs, full effective force on the work-piece becomes available.

§ Circuit 3This circuit provides a costly means

to accomplish the same end as Circuit2. Instead of a sequence and built-incheck, an orifice and check are used.There is some backpressure remainingin the cylinder’s head end because ofthe orifice resistance during finalsqueeze. But with the cylinder ex-tended, no fluid flows across the ori-fice, so total available force acts on thecap end. Whether or not this circuit isappropriate for a given application de-pends on working force requirements.

If applications requirehigh force for only partof a cylinder’s stroke,regenerative circuits

provide reduced-forceactuation with theadvantage of faster

speed, which reducesover-all cycle times.

RegenerativRegenerativ eecircuitscircuits


¶ Sequence valvesSeveral cylinders can be connected to move in se-

quence on forward and return strokes. In this circuit,a clamp must close before a drill descends. On the re-turn stroke, the drill must pull out of the work beforethe clamp opens. The sequence valves are arrangedto cause pressure buildup when one cylinder com-pletes its stroke, the valve opens to allow flow to theother cylinder.

1 2 § Backpressurecheck valves

Cylinders may besequenced by restrict-ing flow to one cylin-der. One method of re-stricting flow is withbackpressure checkvalves. They preventflow until a set pres-sure is reached. In thiscircuit, cylinder 1 ex-tends and retractsahead of cylinder 2.

MM

Clamp1

4

32

Drill

Electrical, mechanical,and pressure signals

can sequenceoperation of fluidpower circuits. Insome automatic

sequencing systemsthese methods are

combined.

SequencingSequencingcircuitscircuits

1

3

2

4

a b c d

Momentarilyenergizedsolenoidvalves

§ Electrical controlLimit switches momentarily actuated by the

cylinders control the solenoid valves to sequencethis circuit. Solenoid a is energized by a pushbut-ton to initiate movement 1. At the completion ofmovement 1, limit switch E is actuated to energizesolenoid c, initiating movement 2. At the end ofmovement 2, limit switch F is actuated to energizesolenoid b, initiating movement 3. At the end ofthis movement, limit switch G is actuated to ener-gize solenoid d, initiating movement 4. The se-quence valves prevent a pressure drop in eithercylinder while the other operates.

© Variable feedMany machines require intermittent fast and slow feed during

their cycles. This can be accomplished by having a cam-oper-ated 2-way valve in parallel with a meter-out flow control valve.Rapid forward movement takes place any time the 2-way valveis open. Closing off the valve slows down cylinder speed. Prop-erly positioning the cams obtains the required speeds in se-quence. The check valve in parallel with the flow control per-mits free return flow, allowing the cylinder rod to return rapidly.

§ Variable-volume pumpPump flow can be controlled by various means such as manual, electric motor, hy-

draulic, or mechanical. How closely flow output actually matches command depends, inpart, on slip, which increases with load. With a pressure-compensated, variable-volumepump, output flow decreases with the increasing pressure. This type of pump can be usedfor traverse and clamp operations. An external relief valve is usually unnecessary when apressure-compensated pump is used. For details on the different types of pumps, their op-eration, and how they vary flow, refer to the pumps section of this Handbook.

MM

MM

A/226

¶ Bleed-off circuitFlow to the cylinder is regulated by

metering part of the pump flow to tank.This circuit is more efficient than meter-inor meter-out, as pump output is only highenough to overcome resistance. However,it does not compensate for pump slip.

¶ Meter-out circuitRegulating flow from the cylinder

is another way to control speed. Thiscircuit maintains a constant backpres-sure during rod extension and pre-vents lunging if the load dropsquickly or reverses.

¶ Meter-in circuitSpeed control during a work stroke

can be accomplished by regulatingflow to the cylinder. The check valveallows free reverse flow when the cyl-inder retracts. It normally gives finerspeed control than a meter-out circuit.

One of the greatadvantages of fluid

power is the ability tocontrol speed of acylinder, motor, or

rotary actuator. Speedis primarily a functionof oil flow and size of

the actuator.

Speed-controlSpeed-controlcircuitscircuits

© ReplenishingOne of the considerations in synchronizing cylinders is leakage

replacement. Under normal pressure, leakage can be practicallyzero over one stroke. Accumulated error is the main concern. A re-plenishing circuit, which replaces leakage after each cylinderstroke, eliminates this trend. In the circuit, the cylinders are con-nected in series and controlled by the 4-way manual valve. Thecylinders actuate limit switches, which control valves A and B. Onthe return stroke, if cylinder 1 bottoms first, valve A is actuated toopen valve C, permitting excess fluid from cylinder 2 to flow totank. If cylinder 2’s piston returns first, valve B is actuated to directfluid to retract cylinder 1.


21

Theoretically, twocylinders will move

together when flow toeach is the same.

Variations in leakage,friction, and cylinder

size cause them to getout of step.

SynchronizingSynchronizingcircuitscircuits

MM

Controlvalve

MM

Controlvalve

§ Rack and pinionMechanically tying two

cylinders together by in-stalling a rack on each pistonrod and fastening the pinionsto a single shaft works wellwhen the linkage is rigid andthe mesh is proper. A chainand sprocket arrangementcan be used if synchronizedmotion is required in onlyone direction.

ÄFluid motor flow dividerAn effective flow divider can be

made up of two fluid motors of the samesize coupled together. Both motorsmust rotate at the same speed and,therefore, deliver equal volumes offluid. Variations in load or friction donot greatly affect synchronization, butmotor slip is a factor.

Documents

Circuits