To provide a pumping action for the pistons, a
plate, called a SWASH PLATE, is located in both the
pump and motor (fig. 6-13). The pistons ride against
the swash plates. The angle of the swash plates can
be varied, so the volume and pressure of oil pumped
by the pistons can be changed or direction of the oil
reversed. A pump or motor with a movable swash
plate is called a variable-displacement unit. A pump
or motor with a fixed swash plate is called a fixed
displacement unit. There are four pump-motor
combinations, which are as follows (fig. 6-14):
Fixed displacement pump driving a fixed
displacement motor (fig. 6-14, view A). This
setup will give you constant horsepower and
Figure 6-13.Connected cylinders with swash plates.
torque at the output with a steady input speed. If
input speed varied, horsepower and speed will
vary but torque will remain constant. Because
both the pump and motor are fixed displacement,
this system is like a gear drive; it transmits power
without altering the speed or horsepower
between the engine and the load.
Variable displacement pump driving a fixed
displacement motor (fig. 6-14, view B). Since
the pump is variable, output speed is variable and
torque output is constant for any given pressure.
This setup provides variable speed and constant
Fixed displacement pump driving a variable
displacement motor (fig. 6-14, view C). In this
setup changing the motor displacement varies
output speed. When motor displacement
decreases, output speed increases, but output
torque drops. When the setup is balanced, it
gives a constant horsepower output.
Variable displacement pump driving a variable
displacement motor (fig. 6-14, view D). This
setup gives an output of both constant torque and
constant horsepower. It is the most flexible of all
the setups, but it is also the most difficult to
Figure 6-14.Pump and motor combinations for hydrostatic drives.