BASIC OPERATION:
HYDROSTATIC TRANSMISSION
Axial Piston, Slipper Pad Design
Sundstrand hydrostatic pumps and motors are an axial
piston, slipper pad design. There are nine (9) pistons
mounted in the cylinder block. As the cylinder block rotates,
these pistons are forced in and out of their bores by the
angle of the swashplate. This results in a specific amount of
fluid being displaced for every revolution of the cylinder
block. In a pump, the fluid is forced out as the angle of the
swashplate pushes the pistons into the bores. In a motor,
system pressure against the piston causes it to slide down
the inclined face of the swashplate resulting in output
rotation.
The slipper pad attaches to a spherical ball on the end of the
piston forming a ball and socket joint. This allows the
slipper pad to tilt at any angle and make contact with the
swashplate. The face of the slipper pad slides on a
hydrostatic fluid film which uses fluid pressure to balance
internal forces.
Variable Pump Tilting Swashplate
The variable displacement pumps use a tiltable swashplate
to vary displacement (output flow). The swashplate is
mounted on trunnion bearings and is connected to hydraulic
control (servo) cylinders. The control directs fluid to and
from the servo cylinders causing the swashplate to tilt and
change displacement of the pump. The swashplate can be
tilted in either direction from 0 angle and provide pump flow
in either direction.
Since the angle of the swashplate causes the pistons to
stroke in and out of the cylinder block bores as it is rotated,
changing this angle varies the piston stroke and, therefore,
the amount of fluid being displaced (pumped) to the motor.
This results in a change in the output speed of the motor.
Tilting the swashplate in the opposite direction reverses fluid
flow to the motor and its direction of rotation. Since each
servo control cylinder is spring loaded, loss of control
pressure or charge pressure will cause the swashplate to
return to neutral position.
Hydrostatic Transmission
The
hydrostatic
transmission
consists
of
a
variable
displacement pump connected by hydraulic lines to either a
fixed or variable displacement motor. The pump rotates in
one direction and requires a change of internal parts to
rotate in the opposite direction. The motor rotates in either
direction.
With the hydrostatic transmission. machine control can be
achieved with a single operator control which provides
smooth, stepless speed and direction changes. Placing the
control in neutral (0 swashplate angle) stops transmission
(motor) output which usually eliminates the need for
clutching mechanisms. This feature, however, does not
eliminate the need for a service brake or parking brake.
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