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Rexrothmannesmann

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Hydraulic-TrainingAxial Piston Units Basic Principles

RE 90600/01.98

Axial Piston Units

1 Basic Principles

1.1 Types of 2- 3Hydraulic Circuit

1.1.1 Open circuit1.1.2 Closed Circuit

2 Principles of Function

2.1 Bent-Axis 4- 9

2.1.1 Bent-Axis Principle2.1.2 Description of Function2.1.3 Principles of Calculation2.1.4 Rotary Group Forces2.1.5 40o-Tapered Piston,

Bent-AxisRotary Group

2.1.6 Examples of Types2.1.7 Symbols

2.2 Swashplate 10-15

2.2.1 Swashplate Principle2.2.2 Description of Function2.2.3 Principles of Calculation2.2.4 Rotary Group Forces2.2.5 Swashplate Rotary Group2.2.6 Examples of Types2.2.7 Symbols

3 Components 16-28A Selection of TypicalModels from theindividual Product Groups

3.1 Standard Modelsto of Bent-Axis and3.10 Swashplate Design

3.11 Summary of Control Devices

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Open CircuitThe Way to the complete Hydraulic System:(explanation of symbols, see page 3)

Basic system with hydraulic pump and hydraulic motor (orcylinder). Single direction of input, output and lift.

The directional control valve allows reversal of the direction ofrotation or of movement at the user.

Variable output speed is achieved by installation of a flowcontroller for variable flow. The pressure relief valve (DBV)protects the system from overload.

Here, the fixed pump and flow controller have been replaced bya variable pump. Further valve functions have been added, e.g.free-wheeling of the user. Filter, cooler and other accessoriesare also fitted.

1 Basic Principles

1.1 Types of Hydraulic Circuit

1.1.1 Open Circuit

Open circuit normally means the case where the pumpsuction line leads below a fluid level whose surface isopen to atmospheric pressure. Maintenance of a pres-sure balance between the air in the hydaulic tank andthe air in the atmosphere guarantees good pump suctioncharacteristics. There must be no resistance in the inletline which might cause pressure to drop below theso-called suction head/suction limit.

Axial piston units are self-priming; in certain specialcases, however, a low pressure is applied to the suctionside.

In open circuit hydraulic fluid is fed to the user via direc-tional control valves and returned to the tank in thesame way.

Typical features of the open circuit are:

n suction lines - short length, large diametern directional control valves - flow-related sizesn filter / cooler - flow-related sizesn tank size - a multiple of the max. pump flow in litresn pump arrangement - adjacent to or below the tank

n drive speeds - limited by the suction headn load maintained in return by valves

The open circuit is standard in many industrial andmobile applications - from machine tools, through

Basic Principles

For the hydraulic engineer, there are three basic typesof circuit to consider:

open circuitclosed circuitsemi-closed circuit

In the following we look at open and closed circuits insome details. The semi-closed circuit is a mixture of thesetwo types of circuit and is used in applications wherevolume compensation via prefill valves is necessary(e.g. when using a single rod cylinder).

flowQ = constant

output speedn = constant

input speedn = constant

suction line

tank size (litres)

lifting speedv = speed


output speedn = constant

lifting speed

flowQ = constant


output speedn = variable lifting speed

v = variable

directionof lifting

flowQ = variable

operating position

directionof position

lifting speedv = variable

flowQ = variable

output speedn = variable


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1.1.2 Closed Circuit

A hydraulic system is described as closed when thehydraulic fluid is returned from the user direct to thepump.

There is a high pressure and a low pressure side,depending on the direction of load (take-off torque atthe user).

The high pressure side is protected by pressure reliefvalves which unload to the low pressure side. Thehydraulic fluid remains in the circuit.

Only the continuous leakage from pump and motor(dependent on operating data) must be replaced.

This fluid is normally replenished by an auxiliary pumpflanged direct onto the main pump which delivers acontinuous, adequate supply of fluid (boost fluid) froma small tank via a check valve into the low pressureside of the closed circuit. Any surplus flow of the boostpump, which operates in open circuit, is returned via aboost-pressure relief valve to the tank. The boostingof the low pressure side enhances the pump-operatingcharacteristics.

Typical features of the closed circuit for axial pistonunits are:

n directional control valves - small sizes for pilotoperation

n filter/cooler - small sizesn tank size - small, dimensioned to suit boost

pump flow and volume of systemn speed - high limiting values through boostn arrangement/mounting - position-flexible/optionaln drive - completely reversible through centre

positionn Ioad maintained - via the drive motorn feedback of braking power

Explanation of Symbols / Colour Code for Open and Closed Circuits

An auxiliary pump for replenishment of leakage oil and control ofthe pump. Boost check valve RV. Boost pressure relief valve DBV.Fitted filter, cooler and accessories.

Basic Principles

αα

α

α

Closed CircuitA Step-by-Step Guide(explanation of symbols, see below left)

Basic system with variable pump and variable motor. Single pumpinput drive direction. Motor power take-off in both directions. Thepump can be swivelled smoothly over centre, i.e. the direction offlow is reversible.

∆

∆

Pressure relief valves, one each for the high and low pressuresides, prevent the maximum permissible pressure from beingexceeded.

The leakage from pump and motor is led back to a small tank andmust be replenished!

Q = variable

pump motor

oil tank

leak

age

oil

boost checkvalve

for pump control

auxiliarypump

filter

cooler

red (high) pressure line

blue (low, boost) pressure lineblue suction line

green return line (tank, bypass pressure)

green leakage line

yellow hydraulic components (pump, motorcylinder, valves, accessories)

orange control element (solenoids, springs)

boostcheckvalve

boostpressurevalve

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Schematic Diagram of a Bent-Axis Unitwith fixed or variable swivel angle α

2 Principles of Function

2.1 Bent Axis

Example:

Fixed Displacement Unitwith tapered piston rotary group

h = piston strokeA = piston areaDT = pitch diameter of

drive shaftα = swivel angle (e.g. 25o)Vg = geom. displacement

[cm3/rev.]x = number of pistons (e.g. 7)h = DT • sin αVg = x • A • hVg = x • A • DT • sinα

Principles of Function

α

α

α

α

drive shaft

tapered piston

cylinder

port plate for

α = constant

kidney-shapedcontrol slots

control lens for

α = variable

center position

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rotation of the drive shaft also causes the cylinder torotate without the need for a Cardan coupling. Thepistons execute a stroke within the cylinder boresdependent on the angle of inclination of the bent axis.The hydraulic medium is fed to the low pressure (inlet)side of the pump and pumped out by the pistons onthe high pressure (outlet) side into the system.

Motor Function:In motor operation, the process is reversed andpressure oil is fed to the inlet side of the unit. Thepistons perform a stroke which is converted via theflexible piston mounting on the drive flange into a rotarymovement. The cylinder rotates with the pistons,generating an output torque on the drive shaft. Oilexiting on the outlet side flows back into the system.

Swivel Angle:The tilt/swivel angle of the fixed displacement unit isdetermined by the housing and is therefore fixed. On avariable unit, this angle is infinitely variable withinspecific limits. Changing the swivel angle changes thepiston stroke, thus allowing variable displacement.

2.1.1 Bent-Axis Principle

Description

The axial piston units of bent-axis design with fixed orvariable displacement can operate as hydraulic pumpsor hydraulic motors.

1 = drive shaft2 = piston3 = piston area4 = cylinder5 = pressure stroke6 = suction stroke

7 = port plate8 = upper dead point OT9 = lower dead point UT

10 = control slot, pressure side(for direction of rotation shown)

11 = control slot, suction side(for direction of rotation shown)

2.1.2 Description of FunctionExample: Fixed Displacement Unit

The bent-axis rotary group is a displacement unitwhose displacement pistons are arranged at an angleto the drive shaft.

Pump Function:Through the flexible piston/piston rod arrangement,


When used as a pump, the flow is proportional to theinput speed and the swivel angle. If the unit is used asa motor, the output speed is proportional to the flowthrough the unit. The input (pump) or output (motor)torque increases with the pressure drop between thehigh and low pressure sides. In pump operation,mechanical energy is converted into hydrostatic power,while in motor operation, inversely, hydrostatic poweris converted into mechanical energy. By adjusting theswivel angle of a variable pump or motor it is possibleto vary the displacement and thus the flow.

Function... as a pump in open circuit:On rotation of the drive shaft, the cylinder is caused torotate by seven pistons flexibly mounted in a circulararrangement on the drive shaft. The cylinder slides onthe spherical port plate which has two kidney-shapedcontrol slots. As the cylinder rotates, each of the sevenpistons moves from the upper dead point OT to thelower dead point UT and back, thereby executing astroke dependent on the swivel angle. The pistonmovement from the lower to the upper dead point inthe cylinder bore produces the suction stroke, wherebya quantity of oil relative to the piston area and pistonstroke is sucked in through the control slot on thesuction side.

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Q = (l/min)

Drive speed n = (rpm)

Flow Q = (l/min)

On further rotation of the drive shaft, as the pistonsmove from the upper to the lower dead point, oil ispushed out through the other control slot (pressureside). The pistons are held against the drive shaft byhydraulic pressure.

... as a motor:The motor function is the reverse of the pump function.In this case, hydraulic oil is fed via the connection platethrough a control slot to the cylinder bores. 3 or 4cylinder bores are located over the pressure sidecontrol slot, 4 or 3 over the return-line side, with onebore possibly being covered by part of the port platedirectly at the dead point. The force generated as aproduct of pressure and piston area acts on the driveshaft to produce the output torque.

Control Function: (with control devices fitted)The swivel angle of the bent axis can be changed, forexample, mechanically via an adjusting spindle orhydraulically via an adjusting piston. The hydraulicsection of the rotary group cylinder complete withcontrol lens (port plate) is swivelled out and, dependingon the type of circuit and function, is held in the zeroor starting position by either mechanical or hydraulicmeans. Increasing the swivel angle increasesdisplacement and torque; decreasing the angle givesa corresponding reduction in these values. If the swivelangle is zero, the displacement is also zero.

Mechanical or hydraulic control devices are normallyfitted, which can themselves be controlled andregulated by mechanical, hydraulic or electrical means.Well-known types of control are: handwheel control,electro-proportional control, constant horsepowercontrol.

GeneralBecause of the bent-axis design, in both pump andmotor operation, the torque is generated direct at thedrive shaft. The radial loading of the pistons on thecylinder is very low, giving low wear, high efficiencyand good starting torque. The spherical design of theport plate means a torque-free cylinder bearing sinceall forces acting on the cylinder pass through one point.Axial movement caused by elastic deformation doesnot increase the leakage losses between cylinder andport plate. When idling and during the start-up process,the cylinder is held against the port plate by the built-in cup springs. As pressure increases, hydraulic forcesbalance the cylinder so that, even with high loadingon the control face between cylinder and port plate, apermanent oil film is maintained and leakage is kept toa minimum. Mounted on the drive shaft is the bearingset which absorbs axial and radial forces. Externalsealing of the rotary group is by means of radial sealand O-rings. A retaining ring holds the complete rotarygroup in the housing.

2.1.3 Principles of Calculation

Calculating Pump Size

Where:Q = flow (l/min)M = drive torque (Nm)P = drive power (kW)Vg = geometric displacement per rev. (cm3)Vg max = max. geom. displacement per rev. (cm3)n = speed (rpm)

Vg • n • ηvol1000

Q • 1000

Vg • ηvol

Vg • ∆p20 π • ηmh

Drive torque M = = (Nm)1,59 • Vg • ∆p100 • ηmh

2 π • M • n60 000

Drive power P = = (kW)

Q • ∆p600 • ηvol • ηmh

P = = (kW)

Fixed displacement bent-axis pump

M • n

9549

Q • ∆p600 • ηt

n = (rpm)

M = = (Nm)

2 π • M • n60 000

P = = (kW)


P = = (kW)

Variable displacement bent-axis pump

M • n

9549

Q • ∆p600 • ηt

Vg max • n • sinα • ηvol1000 • sin αmax

Q • 1000 • sin αmaxVg max • ηvol • sinα

Vg max • ∆p • sinα20 π • ηmh • sin αmax

1,59 • Vg max • ∆p • sinα100 • ηmh • sin αmax


αmax = max. swivel angle (varies according to design)α = set swivel angle (between 0 and αmax)ηvol = volumetric efficiencyηmh = mechanical - hydraulic efficiencyηt = overall efficiency (ηt = ηvol • ηmh)∆p = differential pressure (bar)

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Q • 1000 • ηvolVg

Vg max • ∆p • ηmh • sinα20 π • sin αmax

P = • ηvol • ηmh = (kW)P = • ηvol • ηmh = (kW)Q • ∆p • ηt600

M • n

9549P = = (kW)

1,59 • Vg max • ∆p • ηmh • sinα100 • sin αmax

Q • 1000 • ηvol • sin αmaxVg max • sinα

Output torque M = = (Nm) M = = (Nm)

Resolution of Forces on Pump Drive Flange

Q = (l/min)

Output speed n = (rpm)

Consumption (Flow) Q = (l/min)

Calculating Motor Size

Es ist hierbei:Q = consumption (flow) (l/min)M = output torque (Nm)P = output power (kW)Vg = geometric displacement per rev. (cm3)Vg max = max. geom. displacement per rev. (cm3)n = speed (rpm)

Vg • n

1000 • ηvol

Vg • ∆p • ηmh20 π

2 π • M • n60 000

Output power P = = (kW)

Q • ∆p600

n = (rpm)

2 π • M • n60 000

Variable displacement bent-axis motor

M • n

9549

Vg max • n • sinα1000 • ηvol • sin αmax

2.1.4 Rotary Group ForcesParallelogram illustrating forces in a Fixed Displacement Unit.

The resolution of forces takes place on the drive flange, i.e. direct on the drive shaft.This conversion from torque into piston force in the pump, and in reverse in the motor,guarantees the best possible efficiency. A single resolution means a single efficiency loss!

Fixed displacement bent-axis motor

1,59 • Vg • ∆p • ηmh100

Q • ∆p • ηt600

Q • ∆p600



Resolution of Forces on Motor Drive Flange

High pressure

Piston force(high pressure force)

Bearing force(support force)

Input torque force

Low pressure(suction pressure)

High pressure

Low pressure(return linepressure)

Output torqueforce



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For this torque illustration, a segment of the hydraulicrotary group has been cut out and shown in simplifiedform in pure static condition with swivel angle 0o.

In practice, when the rotary group is swivelled out,dynamic loading is present since 3 or 4 of the pistonareas are always under high pressure.

Resolution of Forces on Port Plate with Spherical Control Face

M = centre of assumed (theoretical) spherer = radius of this sphereHS = focal point of hydrostatic bearing

pressure fieldFK = sum of the force of 3 or 4 pistonsFZ = force of hydrostatic pressure field of

cylinderFM = resulting force on centre pin

2.1.5 40o Tapered Piston, Bent-Axis Rotary Group

Shown here in housing with fixed swivel angle

n central swivel point

n cardanless cylinder drive

n torque-free cylinder bearing

n self-centring rotary group

n spherical port plate

n tapered roller bearings

n one-piece tapered pistonwith 2 piston rings

n automatic bearing lubrication

n piston-force resolutiondirect on drive flange


housing

taperedpiston

piston rings

drive flange

cylinder

fixed portplate

tapered rollerbearing

drive shaft

connection plate

center point

spherical sliding facewith hydrostatic pressurefields

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2.1.6 Examples of Types

Fixed Displacement Unit A2F(fixed swivel angle)

Variable Displacement Unit A7VO(variable swivel angle)

2.1.7 SymbolsSymbolic illustration of the best-known types

For operation as a pump with infinitely adjustabledisplacement in open circuit

For operation as pump or motorin open or closed circuit

A,B Pressure portsS Suction portT,R Case drain portsU Flushing port

A6VMVariable motor

Fixed displacement motor for open orclosed circuits; fixed swivel angle; bothdirections of rotation of power take-off.

Variable displacement motor for open andclosed circuits, swivel to one side only;infinitely variable swivel angle; bothdirections of rotation of power take-off.

Variable displacement pump for opencircuits, swivel to one side only; infinitelyvariable swivel angle; single direction ofrotation of drive.

Variable displacement pump for closedcircuits, swivel to both sides; swivel angleinfinitely var iable over centre; bothdirections of rotation of drive.

A2FMFixed motor

A7VOVariable pump

A2VVariable pump


A,B S

R

U R

T

T

A,B

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2.2 Swashplate

Example:Variable Displacement Pump with electro-hydraulic control,speed-related closed loop control and built-on auxiliary pump

Schematic Diagram of a Swashplate (with parallel-axis pistons)with fixed or variable swivel angle α

h = DT • tan αVg = x • A • hVg = x • A • DT • tanα

h = piston strokeA = piston areaDT = pitch diameter,

when α = 0α = swivel angle (e.g. 20o)Vg = geom. displacement

[cm3/rev]x = number of pistons (e.g. 9)

α


drive shaft

swashplate(rocker arm) slipper pad cylinder A

through drive

kidney-shapedcontrol slots

centerposition piston

port plate

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2.2.1 Swashplate Principle

The swashplate rotary group is a displacement unitwhose displacement pistons are arranged axially tothe drive shaft and are supported against a tilted plate(swashplate).

Pump Function:Through the gearing, rotation of the drive shaft alsocauses the cylinder to rotate. The pistons execute astroke within the cylinder bores dependent on the tiltangle of the swashplate. The hydraulic medium is fed

2.2.2 Description of FunctionExample: Variable Displacement Pump

DescriptionThe axial piston units of swashplate design, with fixedor variable displacement, can operate as hydraulicpumps or motors.When used as a pump, the flow is proportional to theinput speed and the swashplate angle. If the unit isused as a motor, the output speed is proportional tothe flow through the unit.The input (pump) or output (motor) torque increaseswith the pressure drop between the high and lowpressure sides.In pump operation, mechanical energy is convertedinto hydrostatic power while in motor operation,

1 = drive shaft 8 = through drive2 = piston 9 = port plate3 = piston area 10 = upper dead point OT4 = piston stroke 11 = lower dead point UT5 = swashplate 12 = control slot,pressure side6 = angle of (for direction of rotation shown)

adjustment 13 = control slot,suction side7 = cylinder (for direction of rotation shown)


to the low pressure (inlet) side of the pump andpumped out by the pistons on the high pressure (outlet)side into the system.

Motor Function:In motor operation, the process is reversed andpressure oil is fed to the inlet side of the unit. Thepistons perform a stroke, taking the cylinder with them.Via the gearing, this, in turn, causes the drive shaft torotate. Oil exits on the low pressure (outlet) side andflows back into the system.

Swashplate Angle:In the fixed displacement unit, the angle of theswashplate is fixed within the housing. In the variableunit, this angle is infinitely variable within specific limits.Changing the swashplate angle changes the pistonstroke, thus varying the displacement.

inversely, hydrostatic power is converted intomechanical energy.

By adjusting the swashplate angle of a variable pumpor motor it is possible to vary the displacement andthus the flow.

Function. . .as a pump:Driven by the prime mover, (e.g. diesel or electricmotor), the drive shaft rotates and, via the gearing,also causes the cylinder to rotate, taking with it thenine pistons. The pistons are held against the slidingsurface of the swashplate by the slipper pads and carryout a stroke. The slipper pads are held against thesliding surface and guided by means of a return device.As the cylinder rotates, each piston moves throughthe lower or upper dead point and back to its startingposition. A movement from one dead point to the other(where the direction of movement is reversed)constitutes one complete stroke during which a volumeof hydraulic fluid, corresponding to the piston area andthe stroke, is either sucked in or pumped out via thetwo control slots in the portplate.

During the suction stroke, the hydraulic fluid is sucked- in reality compressed - by atmospheric pressure inopen circuits, and by the boost pressure in closedcircuits, into the piston area as it increases in size.During the pressure stroke, fluid is pushed out throughthe piston bores on the opposite side into the hydraulicsystem.

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... as a motor:The motor function is the reverse of the pump function.In this case, hydraulic fluid is fed from the system tothe hydraulic motor. Oil flows via the connection platethrough a control slot to the cylinder bores. 4 or 5cylinder bores are located over the pressure sidecontrol slot. The remaining cylinder bores are locatedover the other control slot and are either connectedwith the return-line side, or may be closed when directlyabove the connecting strip between the two controlslots. Application of pressure to the piston causes it toslide down the swashplate, taking the cylinder with it.The cylinder and the nine connected pistons rotatewith the drive shaft and the pistons carry out a stroke.Hydraulic pressure generates a torque at the cylinderand thus the rotation of the drive shaft. The outputspeed is determined by the volume of flow into themotor.

Control Function: (with control devices fitted)The angle of the swashplate can be changed, forexample, mechanically via a pivot pin or hydraulicallyvia an adjusting piston. The swashplate/rocker arm ismounted on roller or plain bearings and the zeroposition is spring-centred. Increasing the swivel angleincreases displacement and torque, decreasing theangle gives a corresponding reduction in these values.If the swivel angle is zero, the displacement is also

Q = (l/min)

Drive speed n = (rpm)

Flow Q = (l/min)

2.2.3 Principles of Calculation

Calculating Pump Size

Where:Q = flow (l/min)M = drive torque (Nm)P = drive power (kW)Vg = geometric displacement per rev. (cm3)Vg max = max. geometric displacement per rev. (cm3)n = speed (rpm)

Vg • n • ηvol1000

Q • 1000

Vg • ηvol

Vg • ∆p20 π • ηmh

Drive torque M = = (Nm)1,59 • Vg • ∆p100 • ηmh

2 π • M • n60 000

Drive power P = = (kW)


P = = (kW)

Fixed displacement swashplate pump

M • n

9549

Q • ∆p600 • ηt

n = (rpm)

M = = (Nm)

2 π • M • n60 000

P = = (kW)


P = = (kW)

Variable displacement swashplate pump

M • n

9549

Q • ∆p600 • ηt

Vg max • n • tanα • ηvol1000 • tan αmax

Q • 1000 • tan αmaxVg max • ηvol • tanα

Vg max • ∆p • tanα20 π • ηmh • tan αmax

1,59 • Vg max • ∆p • tanα100 • ηmh • tan αmax


zero. Mechanical or hydraulic control devices arenormally fitted, which can themselves be controlledand regulated by mechanical, hydraulic or electricalmeans. Well-known types of control are: electro-proportional control, constant pressure control (zerostroke control), constant horsepower control.

GeneralSwashplate-type pumps and motors are suitable forapplication in open and closed circuits. Because of theirdesign, they are used predominantly as pumps inclosed circuits. The possibility of mounting auxiliary orsecondary pumps on the through drive and of makinguse of the integrated design of controls and valvesoffers a number of advantages. Through this compactand lightweight arrangement, longer service life maybe expected since the slipper pads are mounted onhydrostatic bearings (plain bearings). The resolutionof forces (piston forces/torque) takes place on theswashplate via the slipper pad. Forces within thehydraulic section of the rotary group, ie. cylinder withpistons and port plate, are balanced. The drive shaftbearings allow external forces to be absorbed. Theprinciple of the spherical control area, its lubrication,the pre-tensioning of the cylinder via cup springs, etc.,are similar to that of the bent-axis rotary group.


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Vg max • ∆p • ηmh • tanα20 π • tan αmax

M = = (Nm)

n = (rpm)

P = • ηvol • ηmh = (kW)P = • ηvol • ηmh = (kW)Q • ∆p • ηt600

M • n

9549P = = (kW)

1,59 • Vg max • ∆p • ηmh • tanα100 • tan αmax

Q • 1000 • ηvol • tan αmaxVg max • tanα

Output torque M = = (Nm)

Resolution of Forces on the Pump Swashplate Resolution of Forces on the Motor Swashplate

Q = (l/min)

Output speed n = (rpm)

Consumption (Flow) Q = (l/min)

Calculating Motor Size

Where:Q = consumption (flow) (l/min)M = output torque (Nm)P = output power (kW)Vg = geometric displacement per rev. (cm3)Vg max = max. geom. displacement per rev. (cm3)n = speed (rpm)

Vg • n

1000 • ηvol

Q • 1000 • ηvolVg

Vg • ∆p • ηmh20 π

2 π • M • n60 000

Output power P = = (kW)

Q • ∆p600

2 π • M • n60 000

Variable displacement swashplate motor

M • n

9549

Vg max • n • tanα1000 • ηvol • tan αmax

2.2.4 Rotary Group ForcesParallelogram illustrating forces in a variable displacement unit, e.g. with piston angled inwards

The resolution of forces takes place on the swashplate in the slipper pads and the cylinder.The piston slipper pads are on hydrostatic bearings, and guarantee long service lifeof the rotary groups.

Fixed displacement swashplate motor

1,59 • Vg • ∆p • ηmh100

Q • ∆p • ηt600

Q • ∆p600





Input torque force



Output torque force

High pressure

Low pressure(suction pressure)

Low pressure(return linepressure)

High pressure

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2.2.5 Swashplate Rotary GroupSimplified illustration

e.g. as a motor:As explained in the Description of Function, the pistonis pressurised by oil from the pump and pressed againstthe tilted plate.

The basic components of a swashplate rotary groupare cylinder , piston and swashplate .

2.2.6 Examples of Types

Variable Pump for Closed CircuitsA4VG

High pressure range up to 450 bar

Variable Pump for Open CircuitsA10VO

Medium pressure range up to 315 bar


cylinder

piston

swashplate(tilted plane)

The resolution of the piston forces at the point ofcontact (plain bearing) with the tilted plate produces abearing-force and a torque-force component (F

N and

FT). The piston slides down the tilted plate and carries

out a stroke, taking with it the cylinder and drive shaft.However, because of the movement allowed by thepiston tolerance within the cylinder bore, at the pointof break-away (start-up), a greater frictional resistanceoccurs (static friction) than during a normal stroke(sliding friction). This 2-fold resolution of forces is thereason for the slightly lower starting efficiency of theswashplate unit as compared to the single resolutionof forces in the case of the bent-axis unit. In practice,this can be important in motor operation, but is of littlesignificance in pump operation.

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Axial Piston Units

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2.2.7 SymbolsSymbolic illustration of the best-known types

A4VGVariable Pump

Variable displacement pump forclosed circuits, swivel to both sides;swivel angle infinitely variable overcentre; both directions of rotationof drive; tandem version possible bymounting additional pump at thethrough drive.

Variable displacement pump foropen, closed or semi-closedcircuits; swivel angle infinitelyvariable; single or both directions ofrotation of drive and flow direction,depending on the type of circuit;auxiliary pump may be mounted atthe through drive.

Fixed displacement motor for openor closed circuits; fixed swivel angle;both directions of rotation of powertake-off; possibility of through drivefor mounting brake.

Variable displacement pump foropen circuits, swivel to one sideonly; infinitely variable swivel angle;single direction of rotation of drive.

Variable displacement pump foropen circuits, swivel to one sideonly; infinitely variable swivel angle;single direction of rotation of drive;with built-on charging pump atthrough drive.

A4VSO/G/HVariable Pump

A4FMFixed Motor

A10VOVariable Pump

A11VLOVariable Pump

A,B Pressure portsS Suction portT,R,L Case drain portsU Flushing port


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Axial Piston Units

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3 Components

3.1 Fixed displacement motorsand pumps of bentaxis design

With high performance, modernaxial- tapered piston rotary groups,series 6

Important Features:

n Cardanless cylinder drive via tapered pistons

n tapered pistons with piston ring seals

n robust tapered roller bearings with longservice life

n flange and shaft end to ISO or SAE standards

n two drain ports and flushing port for bearings as standard

n direct mounting of brake valve possible

n models available for special applications

n nominal pressure up to 400 bar,peak pressure up to 450 bar

3.1.1 Fixed Motor

Operation as a motor in open and closed circuits;used in mobile and industrial applications, wherever aconstant displacement is required for hydrostatic powertransmission.

3.1.2 Fixed Pump

With a suitable connection plate fitted, the A2FM motorbecomes the A2FO pump (not illustrated) which issuitable for open circuits and is noted for its robustness,reliability, long service life and quiet operation.

3.1.3 Fixed Pump for Trucks

Pump with special characteristics and dimensions forapplications in mobile trucks; it is designed for apressure range of 250/350 bar. If it is necessary tochange the direction of rotation (e.g. with change ofgearbox), this is simply carried out on the open-circuitpump by rotating the connection plate.

400/450 bar

400/450 bar

250/350 bar

A2FM

KFA

Components

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3.2 Variable Motor of Bent-AxisDesign for Open andClosed Circuits

Important Features:

n gives greater control range for hydrostatictransmissions

n fulfils the requirement for high speedand high torque

n cost-saving through elimination of gearbox,and possibility of using smaller pumps

n Iow unit power

n good starting characteristics

n various control and regulating devices

n swivel to one side only

n nominal pressure 400 bar, peak pressure 450 bar

3.2.1 Automatic Control,High-Pressure-related

The A6VM variable motor has a bent-axis rotary group.The torque is therefore generated direct on the driveshaft and the cylinder is driven without a Cardancoupling directly by the tapered pistons. The swivelangle of the rotary group may be changed by movingthe control lens along a spherical surface by means ofthe adjusting piston. Provided that the pump flow andhigh pressure remain constant, then- reducing the angle gives higher speed

and lower torque- increasing the angle gives higher torque

and lower speed.The code ... HA refers to the high-pressure-relatedcontrol of this motor. The displacement is setautomatically dependent on operating pressure. Whenthe operating pressure set with the regulating valve -measured internally via A or B - is reached, the motoris swivelled from Vg min to Vg max. Below this pressure,the motor remains at minimum swivel angle.

HA curveAutomatic controlhigh-pressure-related

pB = operating pressureVg = displacement

regulating valve adjusting cylinder with setting spring (adjusting piston)

swivelrange

400/450 bar

A6VM...HA

Components

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3.3 Variable Pump ofBent-Axis Design,for Open Circuits

Important features:

n axial-tapered piston rotary group

n Cardanless cylinder drive via tapered pistons

n robust bearings with long service life

n displacement variable between Vg0

and Vg max

n constant horsepower controlwith exact hyperbolic curve

n constant pressure control, hydraulic and electriccontrol devices, load-sensing operation possible

n high pressure range up to 350/400 bar

n for use in mobile and industrial applications

3.3.1 High Pressure Applications

The A7VO is a variable pump with internal drain foropen hydraulic circuits. The bent-axis rotary group ischaracterised by its robustness and good self-primingproperties. The drive shaft bearings also allow externalforces to be absorbed. For particularly demandingrequirements in terms of force absorption and runningtime, type A7VLO has a rotary group with speciallystrengthened bearings.

The swivel angle of the rotary group is changed bymoving the control lens along a spherical surface bymeans of an adjusting piston. Increasing the angleincreases the pump flow and the required drive torque.

Reducing the angle reduces the pump flow and therequired drive torque. The maximum angle is e.g. 25o

or 26,5o. The minimum 0o. The pump is controlled as afunction of the operating pressure, and adjusted bymeans of external control signals. The necessarycontrol pressure is taken from the pressure side via acheck valve.

A7VO ... LRDwith fitted constant horsepower controlin the port plate

350/400 bar

Components

swivelrange

pressurecontrol

adjusting cylinder(adjusting piston)

horsepowercontrol

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3.3.2 Constant Horsepower Control LR(see Spring-Loaded Control/HyperbolicControl)

The control holds the pump input torque M [Nm] constant.The constant horsepower control is a function of thisconstant torque in combination with a constant inputspeed n [1 /min]. On one side we have the inputmechanical drive power P = M • n [kW], on the other thehydraulic output power P = Q • p [kW]. While operatingpressure p [bar] is dependent on load, flow Q [I/min]can be varied by changing the swivel angle.Similar to a computer, the control continuouslymultiplies pressure and flow, and compares the resultwith the set value. Any upward drift is corrected byreducing the swivel angle and, inversely, by increasingthe swivel angle. The control is adjustable (screwing inthe setting screw increases the set value).Start of control is at max. swivel angle. The position atend of control is dependent on the maximum pressure.Alternatively, both end values can be limited by thestop screws. Warning: if the maximum set angle isincreased there is a danger of cavitation in the pumpand of over-speeding of the hydraulic motor! Increasingthe minimum set angle can lead to overloading of theprime mover in the high pressure range.

HydraulicPower Formula : P = Q' • p [kW] = constant

n optimum power matcing by infinitely variablespring force, settable externally

n zero swivel, i.e. no residual flow,generating less heat

HydraulicPower Formula : P = Q • p [kW] = constant

n power matching possibleby changing the spring package

n power losses in the rangeshown by shaded section

n no zero swivel, i.e. residual flowagainst high pressure generates heat

Hyperbolic Control with ideal hyperbolic curve

Spring-loaded Control with approximated curve

Components

α α

Operating pressure acts via a measuring piston in theadjusting piston on a rocker arm. The opposed springforce, externally adjustable, determines the power setting.If operating pressure p exceeds the permissible value,calculated from the formula P = Q • p [kW], therocker arm actuates the control valve and the pump isdestroked. The pump flow is reduced until the productof Q • p once again corresponds to the available power.The ideal power hyperbola is achieved, the drive isnot overloaded, because of the "constant HP control".Inversely, the pump flow can, depending on operatingpressure and supported by a return spr ing, beincreased up to its maximum value.

control valve lever arm (fixed)

rocker arm

adjusting cylinderwith adjusting piston

measuringpiston

spring forces(adjustable)

swivel anglelever arm (variable)

operating pressure

powerhyperbola

theoreticalpowerhyperbola

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3.3.3 Variable Double Pumpwith 2 parallel Bent AxisRotary Groups

A8VO...SR

350/400 bar

Double pumpwith summated HP control

Q = flowp1 = operating pressure 1 . pump at A1p2 = operating pressure 2 . pump at A2

Hyperbolic ControlSummated HP control curve

p1 + p2 [bar]

Components

Q [l/min]P = Q • p [kW]

powerhyperbole

averaged highpressure signal

Two variable pumps - one drive. An advantageouscombination of two individual pumps with integraldistributor gear.

Models with secondary drive and/or auxiliary pump forthe supply of additional hydraulic circuits are todaystandard, particularly in mobile applications.

As an extension of the constant horsepower controlfor a single pump (A7VO..LR - see pages 18/19), thedouble pump A8VO..SR with summated horsepowercontrol is used, for example, for two parallel hydrauliccircuits where the total drive power is split betweenthe two circuits proportional to the pressures in bothcircuits.

The high pressure signal is averaged in the summationvalve and used as the reference value. The idealhyperbolic power curve is achieved when the torqueforces acting on the rocker arm of the constanthorsepower control are in balance.The max. hydraulic torque, which is the product of highpressure force FH and swivel stroke s, may not exceedthe mechanical torque occurring as a product of theadjustable spring force FF and the fixed lever arm a.

Since the operating pressure p is predetermined bythe hydraulic system and the pump can only changeits flow Q, if the max. allowable power is exceeded,the pump is automatically swivelled back to a smallerangle. Swivel stroke s is reduced until the resultinghydraulic torque is once again equal to the givenmechanical torque.

In practice, both individual or combined controls findapplication. Common variations are load-limit control,three-circuit control, load sensing, etc.

constantHP control:hyperbolic control

adjustablespring force

summated pressurevalve(p1 + p2)

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A4VSO

3.4 Variable Pump ofBent Axis Designfor universal industrial applications

O = open circuitG = closed circuitH = semi-closed circuit

350/400 bar

3.4.1 Pump Control Devicesof modular design for a varietyof control and regulating functions

Examples:

HDhydr. controlpressure-regulated

pumphousing

pumpcontrol

PHD = high pressure PST = pilot pressureVg = displacement U = control voltage

HSHydr. displacementcontrol(servo valve)

EO1Hydr. displacementcontrol(proportional valve)

Components

The A4VS.. variable pump is universally suitable foruse in the various types of circuit (O-C-S). A wide rangeof controls and pump models are available.For industrial applications the A4VS..G pump e.g.(operating in closed circuit) can be extended by theaddition of relevant adjusting devices and built-on valveblock, auxiliary pumps at the through drive, tank andcooler to form a complete hydraulic drive unit.A semi-closed circuit ..H.. may also be extended bythe use of anti-cavitation check valves. This willcompensate e.g. for differences in volume whenoperating single rod cylinders.

In addition to the recognised advantages of swashplatedesign the A4VSO pump, developed specially for usein industrial applications, also offers an extremely longbearing service life. Load-sensing control and mooringoperation, as well as secondary control, can beachieved with this pump.The system of closed loop secondary speed control,in combination with a pressure-controlled pump and asecondary-controlled motor, guarantees high controldynamics, accurate speed control, minimal power lossand energy recuperation.Speed control DS1 regulates the variable unit toproduce the necessary torque for the required speed.This torque (in the power supply with impressedpressure) is proportional to the displacement and thusalso proportional to the swivel angle. The swivel angle(adjusting distance) is recorded by means of aninductive positional transducer, and the speed isrecorded by a tachogenerator.

LRconst. horse-power control

DFRconst. pressureand flow control

DRconst. pressurecontrol

g min g maxg max g min g max g min

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3.5 Variable Pumpof Swashplate Designfor medium pressure rangein open circuits

Axial piston pump A10V.O is suitable for use in mobileand industrial applications at pressures of up to 280/350 bar. Compared to fixed pump systems it savesenergy, e.g. through automatic matching to force(pressure) and speed (flow requirements) by meansof a combined constant pressure and flow control.

Besides its compact build, other advantages of theswashplate design are its low unit power, long servicelife and quiet operation. The through-drive option formounting of additional pumps is a particularly usefulfeature.

As a fixed unit at an operating pressure of 280 bar,this is a real alternative to fixed displacement gear orvane type pumps.

3.6 Variable Pumpof Swashplate Designfor simple mobile applicationsin closed circuit

The A10VG is a variable pump of swashplate designfor hydrostatic transmissions in a closed circuit. Allnecessary valves and an auxiliary pump are integrated.The pump design is such that it can easily be extendedto a multiple pump.

The swivel angle of the rotary group is changed, e.g.directly by turning the pivot pin manually.

If the swivel angle is zero, the pump flow is also zero.Swivelling the pump over centre smoothly reverses thedirection of flow.

With the manual pivot pin control, the pin is connecteddirect to the rocker arm of the rotary group. The angleof rotation of the pin corresponds to the pump swivelangle. The necessary adjustment torque, usuallyapplied by hand or foot force, is influenced by the highpressure and the swivel angle. Limiting of the stroke,or angle of the stroke, or angle of the controlmechanism, or possible zero-centring, must be carriedout within the control mechanism itself.

In addition to the manual pivot-pin control, hydrauliccontrols can also be fitted to this pump.

Medium High Pressure Range

PHD = operating pressureVg = flow

DFRconstant pressureand flow control

Medium High Pressure Range

A, B = service linesT = tank, S = suction line

300/350 bar

A10VG

e.g.pivot pincontrol

B

A

A10VSO

280/350 bar

Components

g min g max

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3.7 Variable Pumpof Swashplate Designfor mobile applications in opencircuit with charging pump

The control functions of the A11VLO illustrated herecan be operated either super-imposed or as individualfunctions:■ constant horsepower control with hyperbolic curve■ constant pressure control via sequence valve■ load-sensing control via ∆p-control of load-pressure signa

High Pressure Range 350/400 bar

A11VLO

3.8 Variable Pumpof Swashplate Designhigh-pressure mobiletransmissions in closed circuit

We have here in the A4VG DA pump a power pack,complete with all components for closed circuitoperation, similar to that described in 3.6. Throughhydraulic control with various control devices, we havethe typical "mobile pump". This is combined with a fixedor variable motor to give the automatic "mobiletransmission".Illustrated here is the "speed-related automotivecontrol". The pump is controlled by means of the drivespeed, operating pressure and, electrically, via 2switching solenoids. The control energy is taken fromthe boost circuit. The pump control time is retarded bymeans of throttles.The DA control is designed for mobile transmissionswith combustion engines. It takes account of the factthat in combustion engines the torque increases withthe speed, and that with loads around the max. torquelimit a loss of speed occurs. The power capacity of acombustion engine can, therefore, more or less beequated with its current speed. Through correspondingadjustments on the hydraulic side, optimum automotivecontrol is achieved.

L = charging pumpp = operat. pressureVg = displacement

LRThe constant horsepowercontrol regulates the pumpdisplacement in relation to theoperating pressure so that agiven drive power at constantdrive speed is not exceeded.

DRThe constant pressure controlcauses the pump to swivel backtowards Vg=0, once the maxi-mum operating pressure hasbeen reached. This function issuper-imposed on the constanthorsepower control.

LRDSThe load-sensing control operatesas a load-pressure dependentflow controller and matches thepump displacement to the flowrequirement indicated by theoperator. Pump flow is dependenton the opening cross-section of

Example: Load-Sensing control

High Pressure Range 400/450 bar

A4VG DA

control LRDS

Components

the directional valves, but is not affected by the loadpressure in the range below the power curve. The con-stant power and pressure controls are super-imposed on the load-sensing function.

g

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3.9 Fixed Motorof Swashplate Design

The A4FM is of compact, space-saving design,offering the following technical advantages with:

Connection in series (summated pressure)Mounting of brake possibleInsensitive to oscillations

This fixed motor for high pressures (350/400 bar) is suitable for connection in series.The axial pressure compensation of thehydrostatic bearing on the slipper padsensures high loading capability and a longservice life.

3.10 Variable Motor of SwashplateDesign

The A10VM is a switching motor in swashplatedesign that can be switched between twopositions.It operates in the medium pressure range(250/315 bar) in open or closed circuits.

Variable motor/switching motorHydraulic or electrical two-point adjustmentwith mounted switching valve

Adjustment range 1 : 2,5

Mechanical holding brake can be built on

Also available as a small compact plug-inmotor

Components

350/400 bar

A4FM

A10VM

250/315 bar

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3.11.1 Pump Controls

n Mechanical - manual, proportional to:MA - control stroke s, or

MD - control angle ß for pumpsin reversible operation

n Mechanical/electrical, withEM - electric gear motor

n Hydraulic - mechanical, proportional to:DG - pilot pressure pSt

HW - control angle ß, orcontrol stroke s for pumpsin reversible operation

3.11 Summaryof the most commoncontrol optionson pumps and motors

The electronic components (amplifiers),used to amplify the signals, are not listedhere.

Vg = specific displacement s = control stroke ß = control angle

MAmanualcontrol

DGdirect operatedhydraulic controlpressure related

EMelectric motorcontrol

HWhydrauliccontrolstroke related

MDmechanicalpivot pincontrol

HWhydrauliccontrolstroke related 1)

Components

Vg = specific displacement pst = pilot pressure 1) = dead band in zero position

Different features of the controls are:n type of circuitn force transfer

(hydraulic or mechanical)n operation of control

(direct or pilot operation)n curve (position and adjustability)n open loop control (without feedback)

- mechanical/manual- mechanical/electrical- hydraulic/mechanical- hydraulic/electrical- hydraulic/hydraulic

n closed loop control (with feedback)- hydraulic/mechanical- hydraulic/electrical

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nHydraulic - hydraulic:HD - proportional to pilot pressure p

St

for pumps in open circuitor reversible operation

nHydraulic - electrical:EZ - with switching solenoids (not illust.)

EP - with proportional solenoids,proportional to control current Iin open or closed circuit

ES - with servo valve in closed circuit

nHydraulic, flow-related:HM - proportional to pilot oil flow VS,

in reversible operation

HS - electrical/hydraulicwith built-on servo valve,proportional to control current I

nHydraulic, flow-related:EO - with built-on proportional valve

in reversible operation;with electronic amplifier;closed loop control possible.

HDhydr. controlpressure related

HDhydr. controlpressure related 1)

HDhydr. controlpressure related

Vg = specific displacement pst = pilot pressure 1) = dead band in zero position

EPelectric controlwith proportionalsolenoid

Vg = specific displacement I = control current

EPelectric controlwith proportionalsolenoid

ESelectric controlwith servo valve

HMhydr. controlflow-related

HShydr. controlwith servo valve

EOelectroniccontrol

U = control voltage pHD = high pressure

Vg = specific displacement VS = pilot oil flow

Components

g

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3.11.2 Closed-Loop Pump Controls

nHydraulic:DR - constant system pressure through

suitable adjustment of pump flow

FR - constant pump flow with variabledrive speed and/or flow variationto meet changing user requirement

DFR- a combination of both the abovecontrols;the constant flow control is super-imposed on a mechanicallysettable constant pressure control

LR - constant drive torque control;power = torque x speed,P = M x n = constant.

SR - for parallel operation of two pumpsfrom one prime mover;automatic power distributionby summation of pressure

DFLR - combined constant pressure/flowcontrol with super-imposedconstant horsepower control

DRS - flow variation according to userrequirement (load sensing) withsuper-imposed constant pressurecontrol; pump is swivelleddependent on load pressure

LRDS - drive torque is limited on max.by addition of constant horse-power control; pump flow is variedaccording to user requirement

nElectronic:DFE - alternative electronic version of

combined control DFR.

Vg = specific displacement Q = flow pHD = high pressure

LRconstantHP control

DFLRconst. pressure,HP and flowcontrol

SRsummatedHP control

DRconstantpressure control

DFRconst. pressureand flow control

FRconstantflow control

Components

Q = flow pHD = high pressure

DRSconst. pressurecontrol withload sensing

DFEelectronic const.pressure andflow control

LRDSconst. HP control withpressure cut-offand load sensing

Vg = specific displacement pHD = high pressure = electric signal

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Components

3.11.3 Motor Controls

nHydraulic - hydraulic:HD - proportional to pilot pressure pST.

HZ - hydraulic/hydraulic,two-point control

EP - hydraulic/electric,with proportional solenoid

EZ - with switching solenoid,two-point control.

3.11.4 Closed-Loop Motor Controls

nHydraulic:HA - automatic high-pressure-related

control; automatic adjustment tocurrent torque requirement

DS - pumps with DS control are used asmotors for "secondary control"

DA - speed-related hydraulic control isthe basis for "mobile transmission"with "automotive control".

HDhydr. control, pilot-pressure-related

EPelectrical controlwith proportional solenoid

HZhydr. two-point control

EZelectrical two-pointcontrol withswitching solenoid

Vg = specific displacement pst = pilot pressure I = control current

HAautomatic controlhigh pressurerelated

DAhydr. controlspeed related

DSconstant speedcontrol, withsecondary control

Vg = specific displacement pB = operating pressure n = speed

currentconnected

nocurrent

g ming min g maxg max g maxg min

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