oil to both servo cylinders and keeps the swash plate
tilted. The swash plate will remain in position until the
speed control lever is moved again by the operator.
With the swash plate tilted forward and the pump
drive shaft and cylinder block rotating clockwise, the
ports reverse and the inlet port becomes the outlet and
the outlet port becomes the inlet. As the pump cylinder
block rotates past the pump inlet port, a check valve
opens and oil is forced by the charge pump into the
piston bores that align with the inlet port of the pump.
As rotation continues, the oil is pressurized and forced
out of the outlet port of the pump by each of the pistons,
as they align with the outlet port. This action forces oil
to flow to the motor, and as high-pressure oil from the
pump enters the inlet port of the motor, the pistons are
pushed against the swash plate. The pistons slide down
the inclined surface of the swash plate, rotating the
cylinder block. This action rotates the drive shaft
counterclockwise, driving the piece of equipment in
reverse. As the motor cylinder block continues to
rotate, oil is forced out the outlet port at low pressure
and returns to the pump.
The PUMP DRIVE SHAFT and cylinder
block always rotate clockwise, but the
MOTOR DRIVE SHAFT and cylinder block
rotate in clockwise and counterclockwise
directions, depending on the direction of the
oil entering the pump.
Maintenance of Hydrostatic Drives
As with any hydraulic system, the hydrostatic
drive system is fairly easy to maintain. The fluid
provides a lubricant and protects against overload.
Like any other mechanism, it must be operated
properly; too much speed, too much heat, too much
pressure, or too much contamination will cause
Before removing any part of the system, ensure
that the area is clean. Use steam-cleaning equipment if
available; however, do NOT let any water into the
system. Ensure that all hose and line connections are
tight. If steam cleaning is not possible, diesel fuel or a
suitable solvent may be used. Be certain to remove all
loose dirt and foreign matter that may contaminate the
system. Impurities, such as dirt, lint, and chaff, cause
more damage than any one thing. Always seal
openings when doing work to prevent foreign matter
from entering the system.
C l e a n t h e w o r k b e n c h o r t a b l e b e f o r e
disassembling any hydrostatic system component for
servicing. Be sure that all tools are clean and free of
dirt and grease.
NEVER perform internal service work on
the shop floor or ground or where there is a
danger of dust or dirt being blown into the
Before disassemble of any system component for
internal service, certain items must be available. These
items include the following:
Clean plastic plugs of various sizes to seal the
openings when removing hydraulic hoses and
Clean plastic bags to place over the ends of the
lines and hoses. Secure the bags to the line and
hoses with rubber bands.
A container of solvent to clean internal parts.
Ensure that all parts are clean before replacing
them. Compressed air may be used to dry the
parts after cleaning.
A container of hydraulic fluid to lubricate the
internal parts as they are reassembled.
A container of petroleum jelly to lubricate
surfaces where noted by the manufacturer
Anytime the components are serviced and
reassembled, always install new O rings, seals, and
gaskets. This provides tight seals for mating parts and
For instructions on the disassembly and
reassembly of hydrostatic components, refer
to the manufacturers service manual.
Never operate the hydraulic system empty.
Always check the fluid supply after servicing the
system. If fluid is to be added to the system. use ONLY
the fluid recommended in the service manual.
REVIEW 1 QUESTIONS
Q1. A power shift transmission has what total
number offorward and reverse speeds?