Hydrostatic transmission operation – John Deere 318 User Manual
HYDROSTATIC TRANSMISSION OPERATION
Transfers power from the input (pump) shaft to the
gear drive components of the transmission. It also
provides infinitely variable speed and torque within a
range in forward and reverse directions.
Also supplies pressurized oil to the hydraulic lift and
Theory of Operation:
The transmission consists of a variable displacement,
axial piston pump (B) connected in a closed loop to a
fixed displacement, axial piston motor (I). A charge
pump (C) and valve system is used to charge and
lubricate the transmission.
The charge pump is a gerotor-type, fixed
displacement pump. It continually pumps oil
throughout the entire hydrostatic/hydraulic system
whenever the engine is running.
As the input (pump) shaft (A) is turned by the engine
drive shaft, the pump rotating group consisting of the
variable hydrostatic pump and charge pump also turn.
The charge pump draws pressure-free oil from the
reservoir (F) through the filter (D) and pressurizes it
to approximately 620—1240 kPa (90—180 psi). The
charge oil pressure is enough to unseat the forward
and/or reverse check valve (E and M), supplying
charge pressure oil to pump (B).
The transmission also supplies pressurized oil to the
lift and hydraulic steering systems. If pressure in this
line becomes excessive, the implement relief valve
(N) will open allowing the oil to flow into the reservoir.
With the transmission in the neutral position, springs
in the pump cylinder bores force the pump pistons
against the variable position swashplate, which is
parallel to the pump body. With the swashplate
parallel to the pump body, the pistons do not
reciprocate in the cylinder block, they merely rotate,
no oil is being drawn in or discharged from the pump.
The pump is in a zero displacement position and the
machine remains stationary.
As the swashplate control arm is moved to the
forward position, the variable position swashplate is
moved from the neutral position to a forward angle
position. Springs inside the cylinder bores force the
pistons against the swashplate. As the pump rotates
the pistons follow the contour of the swashplate they
move outward, drawing oil into their bores. As the
pistons continue to rotate, the swashplate angle
forces the pistons back into the bores, forcing oil out
of the bores through the valve plate.
High pressure oil from the pump forces the forward
check valve (E) closed and supplies pressurized oil to
the drive motor (I). The motor works in conjunction
with a fixed position swashplate. Oil enters the piston
bore through a port in the valve plate at a point
where the piston is compressed in its bore.
As the oil fills the piston bore, the piston is forced out
and follows the contour of the swashplate. This
causes the motor to rotate. Oil pressure within these
components is directly proportional to the load
encountered. This is known as the high pressure side
of the system.
As the motor continues to rotate, the piston is now
compressed by the angle of the swashplate and oil is
forced from the piston bore into the other port in the
valve plate. This oil is directed back to the suction
side of the pump. There is minimal oil pressure from
the back to the motor and this is referred as the low
pressure side of the system.
The reverse check (freewheel) valve (M) remains
open to allow charge pressure oil to flow to the low
pressure side of the system to make up for any oil
lost due to internal lubrication/leakage.
The operation of the transmission in reverse is similar
to forward operation, except that the reverse
swashplate position causes the pressurized oil flow to
be reversed. When oil flow is reversed, the reverse
check (freewheel) valve (M) is forced closed suppling
pressurized oil to the drive motor. The forward check
valve (E) remains open to allow charge pressure oil
to flow to the low pressure side of the system to
make up for any oil lost due to internal
When the forward/reverse check (freewheel) valves
are manually engaged, the valves are forced off their
seats and allows oil to flow from both sides of the
motor to the reservoir. Normally the motor would
have excessive resistance to movement due to
dynamic braking of the hydrostatic closed loop.
Theory of Operation/Hydrostatic Transmission Operation
316, 318 & 420 Lawn and Garden Tractors