The drum propel pump is mounted in tandem with the axle
propel pump. The charge pressure for the propel system is
supplied by the steering pump. When the engine is operating,
drive shaft (1) and barrel assembly (10) are rotating. There are
nine pistons (13) in the barrel assembly. Port plate (5) and
swashplate (14) are fastened to case (9). The port plate and
the swashplate do not rotate. Spring washers (12) keep a
force on the barrel assembly in order to make a high pressure
seal between the barrel assembly and the pod plate. When
the barrel assembly is rotating, each piston (13) follows the
angle of the swashplate. If the swashplate angle is at zero, the
pistons do not move in and out of the barrel assembly and
there is no oil flow.
The position of the swashplate is controlled by the manual
directional control valve (4) and servo valve (3). Movement of
the propel control lever moves the directional control valve.
The directional control valve routes charge oil into the servo
valve. The servo valve controls the direction and the amount
of the swashplate angle.
The steering pump maintains charge oil in the propel pump in
order to keep the barrel assembly full of oil. The charge oil
lubricates the pump components. The internal loss of oil due
to leakage is replenished by the charge oil. The charge oil is
also used in order to release the brakes and the charge oil will
provide the shift valve with oil.
Oil flows from the drum propel pump to the drum propel motor
and back to the drum propel pump by way of inlet/outlet ports
(6) and (8). The position of the swashplate determines the
direction of flow as well as the high pressure side of the two
Servo Valve (Piston Pump)
SMCS Code: 3205
Location of Servo Valves on the Propel Pumps
(1) Servo valves.
(2) Springs. (3) Slot for control valve. (4) Servo piston. (5)
Slot for swashplate control.
Oil from the control valve goes to either the right or the left end
of servo piston (4). Servo piston (4) also receives a
mechanical input from the lever that is connected to slot (3).
The swashplate control is attached to the servo piston in slot
(5). When the servo piston moves, the lever moves the
swashplate to the angle that corresponds to the position of the
When the charge pressure is relieved by the POR valve,
springs (2) move servo piston (4) toward the centered position.
This reduces the angle of the swashplate in the pump which
causes the pump output to be reduced.