7-136. When the Direction-Throttle Control Bail is
main circuit reverses the direction of the motor output
released, the displacement control spool tends to be
returned to neutral by a spring.
7-147. The motor shaft and cylinder block assembly
rotate together. The direction of rotation depends on the
direction of the high-pressure oil entering the motor from
7-137. SERVO CONTROL CYLINDERS.
the hydrostatic pump. High-pressure oil enters one of
7-138. The hydrostatic pump is equipped with two (2)
the motor main ports to push the cylinder pistons out of
servo cylinders. They are controlled by low-pressure oil
their cylinder and firmly against the motor swashplate.
directed from the displacement
This action causes each piston slipper, which is
protected by a thin film of oil to slide down the face of
7-139. When the pump is stroked, oil under charge
the thrustplate, rotating the entire cylinder block
pressure is ported to the servo cylinder. The piston
assembly. The cylinder block is splined to, and rotates
moves the swashplate against the opposite servo spring.
the motor output shaft. As the cylinder block rotates and
Both servo springs are constrained so that they can only
the filled cylinders pass the other main port, return oil
force the swashplate toward neutral. When the swash-
flows to the hydrostatic pump at low-pressure. The
plate has moved to the angle set by the control handle,
speed at which the cylinder block rotates is determined
the feed-back link returns the displacement control spool
by the volume of pump output oil.
almost to neutral where it ports just enough oil to the
servo cylinder to keep the swashplate at the proper
angle. 7-140. When the Direction-Throttle Bail is
released, the displacement control spool is returned to
7-149. The manifold is connected across the
neutral by a spring. This allows oil from both servo
high-pressure and low-pressure sides of the
cylinders to flow into the case through the small
main circuit. It includes:
underlaps. Both servo cylinders are thus exhausted
(1) two high-pressure relief valves
and one of the servo springs mechanically forces the
(2) a shuttle valve
swash- plate to neutral.
(3) a low-charge-pressure relief valve.
7-141. HYDROSTATIC MOTOR ASSEMBLY (see
7-150. HIGH-PRESSURE RELIEF VALVES.
7-151. Two high-pressure relief valves are part of the
manifold. They serve to prevent abnormal pressure
7-142. The fixed displacement motor is an axial-piston
surges in either of the two branch lines of the main
motor that converts fluid power (received from the
circuit. They accomplish this by dumping oil from the
variable displacement pump) into rotary mechanical
high-pressure line to the low-pressure line. Situations
7-143. The fixed displacement motor is attached to the
application of load will activate the relief valves.
unit final drive mechanism at the right-hand side of the
drive drum (as viewed from the rear of the unit). The
7-152. SHUTTLE VALVE.
ports of the variable displacement pump are connected
7-153. At other than neutral setting, the shuttle valve
to the motor through high-pressure hoses.
establishes a connection between one of the low-
charge-pressure relief valves and whichever branch of
7-144. FIXED DISPLACEMENT HYDROSTATIC
the main circuit is at low-pressure. This connection
provides a control of charge pressure and a path for
7-145. The fixed displacement hydrostatic motor output
excess oil to escape to the cooling circuit. When the two
is coupled to a final drive by a shaft extending through
branches of the main circuit are reversing their high-
the center of the drive drum.
pressure and low-pressure functions, the spring-
7-146. The hydrostatic motor converts high- pressure
centered design of the shuttle valve closes the valve to
hydraulic power into rotary mechanical power. The
prevent loss of
motor shaft can rotate in either direction. The action of
the hydrostatic pump reversing the direction of oil in the