2-M10
ZD18(F) · ZD21(F) · ZD28(F), WSM
TRANSAXLE
[Serial No. Affected: above 20001]
 Neutral
With the control levers in the NEUTRAL position, the piston springs in the pump block in the pumps force the swash
plates to a position that is parallel to the pump body. With the swash plates parallel to the pump body, the pistons do
not reciprocate in the cylinder block, they merely rotate, and no oil is being drawn in or discharged from the pump.
The machine is in a zero displacement position and the machine remains stationary.
Oil returning from the HST housing and hydraulic pump is directed through the regulating valve before returning to
the transmission case.
 Forward
As the control levers are pushed forward, the swash plates in the pumps move from the neutral position (parallel
to the pump body) to a forward angle position. Piston springs inside the cylinder bores force the pistons against the
swash plates.
As the cylinder block rotates, the pistons follow the contour of the swash plate, moving outward, drawing oil into
their bores. As the cylinder block continues to rotate, the pistons are forced into their bores, discharging oil under
pressure.
High-pressure oil from the pumps is routed to the motors, driving the machine forward.
 Reverse
Reverse operation is accomplished by reversing the angle applied to the pump swash plates, reversing the flow of
high-pressure oil to the motors.
(1) Check and Relief Valve (with
Neutral Orifice)
(1)-1 Relief Valve
(1)-2 Check Valve
(1)-3 Neutral Orifice
(2) Oil Filter
(3) Regulator Valve
(4) Motor
(5) Lubricating Orifice
(6) Pump
(7) Hydraulic Pump
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