Section E - Hydraulics
Operation Description
Transmission Pump Operation - Series 1 Machines
E-17 E-17
9803-9340-8
Transmission Pump Operation - Series 1 Machines
The illustration K Fig 10. ( T E-18) shows a section
through the transmission pump. The pump unit consists of
a series of parallel axial pistons mounted in a barrel which
is rotated by the engine driven shaft. The pistons are
operated by a swiveling plate known as a “swash plate”.
The stroke of the pistons and therefore the displacement
of the pump is varied by increasing or decreasing the angle
of the swash plate. Also by reversing the angle of the
swash plate the flow of oil from the pump is reversed which
reverses the rotation of the drive motor.
Slipper pads are fitted to the ball ends of the pistons and
these slide on the surface of the swash plate, adjusting to
the angle of the surface. The end face of the cylinder barrel
runs against a valve plate which directs the oil flow into and
out of the bores as the barrel rotates.
The angle of the swash plate is varied by a mechanical
linkage connected to the forward/reverse pedal. Thus the
pump’s displacement is varied from full displacement in
one direction to full displacement in the opposite direction.
The components of the pump are contained within the
pump casing and are fully submerged in oil which is
supplied by the charge circuit. Charge flow is required to
make up internal leakage, maintain positive pressure in the
main circuit, provide flow for cooling and for operation of
the pump control system.
Charge Pressure Valve
This valve is mounted within the pump casing and controls
the charge pressure to a predetermined limit. Oil is
supplied by the charge pump to the variable displacement
pump via the valve plate. When the charge pump reaches
the maximum charge or ‘boost’ pressure the charge
pressure relief valve (Pa) opens and dumps excess oil into
the pump casing.
High Pressure Relief Valves
There are two of these valves, one for each output port of
the pump. They provide protection against high transient
pressures in the motor circuits.
By-pass (Tow) Valve
The by-pass valve is fitted across the motor feed/return
lines and is normally closed. The valve may be opened to
allow oil to circulate within the pump/motor loop so that the
machine can be towed for a short distance.
Fig 9.
Key
A Piston
B Swash Plate
C Cooler
D Valve Plate
E By-pass (Tow) Valve
F Auxiliary Port (not used)
Ma Motor Feed/Return
Mb Motor Feed/Return
P1 Pump Unit
P2 Charge Pump
Pa Charge Valve
Pb High Pressure Relief Valve
Pc High Pressure Relief Valve
Pd Non-return Valve
Pe Non-return Valve
S Suction Strainer
T
Tank
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