The direction of output shaft rotation can be
reversed in variable setups by shifting either the pump
or the swash plate of the motor over center.
Remember three factors control the operation of a
hydrostatic drive. These factors are as follows:
RATE of oil flowgives the speed
DIRECTION of oil flowgives the direction
PRESSURE of the oilgives the power
The pump is driven by the engine of the machine
and is linked to the speed set by the operator. It pumps a
constant stream of high-pressure oil to the motor.
Since the motor is linked to the drive wheels or tracks
of the machine, it gives the machine its travel speed.
The advantages ofhydrostatic drive are as follows:
Infinite speeds and torque
Easy one-lever control
Shifts "on the go"
High torque available for starting up
Reduces shock loads
Low maintenance and service
Flexible locationno drive lines
Eliminates clutches and large gear trains
Hydrostatic Drive Operation
For you to understand how a hydrostatic drive
operates, we will explain the operation of a typical
system. The system we will use has an axial piston
pump and motor which is the most common
hydrostatic drive system. The pump has a variable
displacement, while the motor has a fixed
displacement. Now look at the complete system in
operationforward, neutral, and reverse.
FORWARD (fig. 6-15).When the operator
moves the speed control lever forward, the spool in the
displacement control valve, also known as the FNR
valve (Forward, Neutral, and Reverse), moves from its
NEUTRAL position. This action allows pressure oil to
flow into the upper servo cylinder forcing the swash
Figure 6-15.Forward operation.