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Principle of Operation Eaton
®
Medium Duty Piston Pump
No. 3-205
January 1998
Principle of Operation
Model 72400 Variable Displacement Piston Pump
Servo Controlled 2
Model 72400
Medium Duty
Hydrostatic Drive
Principle of Operation
There are many methods used to transmit engine power to the driven wheels of a vehicle. These methods range from the
standard manual shift to the more sophisticated automatic transmission and now to the latest method of propelling vehicles,
hydrostatic propulsion drive.
The main advantage of the hydrostatic propulsion drive is the infinite speed ratios that can be obtained by being able to control
the piston pump camplate angle from the neutral position to full camplate forward or reverse position. The direction and speed of
the vehicle can be changed without changing the rotation of the pump
Once the fundamentals of the closed loop propulsion system are understood, the operating principles of the more complicated
propulsion systems can be readily understood.
The type of propulsion drive principle described consists of a manual servo controlled variable piston pump with a gerotor charge
pump and a fixed displacement motor.
Cam Angle Neutral Position
The Eaton PV-MF closed circuit (loop) hydrostatic transmission is considered to be in neutral when there is no output flow being
generated by the variable displacement pump as its shaft, internal components, and charge pump are being driven. Therefore,
with no flow available to the motor, the pump is at rest with no transmission output. Because of the variable displacement pump
being driven continuously, several functions must be maintained.
Hydraulic fluid used in the hydrostatic transmission is made available from a reservoir, through a filter, and then to the inlet of
the pump's charge pump. The charge pump is driven by the main pump's input shaft.
Flow rates of the charge pump are dictated by its displacement and driven speed.
The charge pump provides several functions to the hydrostatic circuits:
a) Provides flow to keep circuit primed and make up internal leakages.
b) Provides flow under pressure for maintaining back pressure on pump/motor pistons.
c) Provides flow under pressure for hydraulic control purposes.
d) Provides cooled and cleansed fluid for temperature control and flushing.
Fluid from the charge pump is directed through the two dual purpose system relief and check valve located in the pump
backplate.
The charge pump/check valve combination introduces fluid to both sides of the hydrostatic circuits and fills or primes all lines,
valves, etc. between the pump and motor. When the circuit is primed, the charge pump flow dumps across the charge pump
relief valve to the pump housing to aid in cooling and flushing the pump. The fluid then returns to the reservoir through a heat
exchanger (optional).
The charge pump relief valve maintains a minimum charge pressure level (17.24 to 20.68 bar [250 to 300 PSI]).
The connection between the servo piston and the camplate allows the variable displacement pump to be held in a positive neutral
position by the neutral centering spring located in the servo piston. The variable camplate must be maintained in a true neutral
location to prevent reciprocation of the pistons in the pump cylinder barrel. 3
Model 72400
Figure 1
Cam Angle Neutral Position
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Gerotor
Charge
Pump
Charge
Pump
Suction
Charge Pump
Relief Valve
Port "B"
To Motor
Port "A"
To Motor
High Pressure
Relief Valve - Forward
High Pressure
Relief Valve - Reverse
Servo Control Supply Oil
Control
Lever
Neutral
Manual
Servo
Control
Servo Piston
Auxiliary Charge Flow
Case
Drain
Camplate
Piston
Assembly
Motor
Servo Controlled
Piston Pump
CW
Auxiliary
Charge
Flow
Charge
Pump
Suction
Case Drain
Relief
Valve
Charge
Pump
R/V
Port "A"
Port "B"
Orange
Charge Pressure
Blue
Case Pressure
Green
Inlet Oil
Servo
Piston
Manual Servo
Control
Neutral
Centering Spring 4
Model 72400
Servo Control Function
Transmission output is initiated by movement of the variable displacement pump control lever (Figure 2). As the lever is
rotated, it moves the servo control spool first from its spring centered position, allowing pressurized charge pump flow or
control pressure past the spool to one side of the servo piston. The pressurized servo piston pushes against the camplate,
causing the camplate to rotate to a position out of neutral (Figure 3). Fluid in the servo piston side with the spring under
compression is exhausted through the control valve spool to the pump housing.
The position taken by the variable camplate is determined by the location of the control lever from its neutral position.
Maximum displacement of the pump is attained when the control lever is moved to its extreme position. As the camplate
attains any position selected by the control lever by the connecting feedback link and servo piston (Figure 2a), it relocates the
control valve spool to a metering position. The spool meters the fluid between the spool and servo piston to hold the camplate
in its desired location. Speed of camplate movement can be varied by controlling the flow rate to the servo piston by the
appropriate selection of control orifice diameter.
As the variable camplate is moved from its neutral position, a reciprocating action is created by the pistons in the pump
cylinder barrel, generating a flow. Flow rate is determined by length and frequency of strokes.
Figure 2a - Detail of Control Lever and Camplate Connection
Feedback Link
Servo Piston
Spool
Control
Lever
Camplate
Servo Piston Follower 5
Model 72400
Figure 2 Servo Control Function
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Gerotor
Charge
Pump
Charge
Pump
Suction
Charge Pump
Relief Valve
Port "B"
To Motor
Port "A"
To Motor
High Pressure
Relief Valve - Forward
High Pressure
Relief Valve - Reverse
Servo Control Supply Oil
Control
Lever
Forward
Manual
Servo
Control
Servo Piston
Auxiliary Charge Flow
Case
Drain
Camplate
Piston
Assembly
Motor
Servo Controlled
Piston Pump
CW
Auxiliary
Charge
Flow
Charge
Pump
Suction
Case Drain
Relief
Valve
Charge
Pump
R/V
Port "A"
Port "B"
Orange
Charge Pressure
Blue
Case Pressure
Green
Inlet Oil
Yellow
Control Pressure
Servo
Piston
Manual Servo
Control
Spool
Feedback
Link
Neutral
Centering Spring 6
Model 72400
Cam Angle Forward Position
As the control lever is slowly moved forward the vehicle starts a forward movement (Figure 3). As the camplate begins to
move, the piston assemblies start to reciprocate in the piston block. As the control lever continues a forward movement, the
cam angle increases, the pistons reciprocate further, more oil is being pumped, and the speed of the vehicle is increased. The
control lever can be moved forward until a full cam angle is reached, which also achieves maximum volume of oil being
discharged from the pump. When the camplate begins to move, the relief valve on the discharge or pressure side seats
because of the higher pressure differential. The other relief valve serves as a charge check, remaining open on the intake or
low pressure side to continue supplying the closed loop system with charge oil.
The motor in this case is a fixed displacement type delivering a constant output torque for a given pressure throughout the
speed range of the motor. The speed of the motor is dictated by the volume of oil discharged from the pump. A low camplate
angle delivers a small volume of oil, causing the motor to turn slowly. As the camplate angle increases, the speed of the motor
increases till the maximum camplate angle is reached.
The movement of the pump camplate in either the forward or reverse position controls the direction of the motor rotation. 7
Model 72400
Figure 3 Cam Angle Forward Position
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Gerotor
Charge
Pump
Charge
Pump
Suction
Charge Pump
Relief Valve
Port "B"
To Motor
Port "A"
From Motor
High Pressure
Relief Valve - Forward
High Pressure
Relief Valve - Reverse
Servo Control Supply Oil
Control
Lever
Forward
Manual
Servo
Control
Servo Piston
Auxiliary Charge Flow
Case
Drain
Camplate
Piston
Assembly
Motor
Servo Controlled
Piston Pump
CW
Auxiliary
Charge
Flow
Charge
Pump
Suction
Case Drain
Relief
Valve
Charge
Pump
R/V
Port "A"
Port "B"
Orange
Charge Pressure
Blue
Case Pressure
Green
Inlet Oil
Red
High Pressure
Yellow
Control Pressure
Servo
Piston