TM 5-3895-346-14
HYDROSTATIC SYSTEM
CONTROLS
CONTROLS
Various types of controls are available: i.e., displacement, pressure override and rotary by-pass.
DISPLACEMENT CONTROL OPERATION
The Sundstrand heavy-duty variable pumps incorporate a powered servo system to control the swashplate
position with a correspondingly low operator effort.
The valve assembly is a closed center four-way valve with the servo pressure ports exhausted at the center
position. The valve is operated through internal linkage connections with both the swashplate and the external
control handle. (Refer to Fig. 1.)
To put the pump in stroke, the control handle (A) moves the displacement control spool (B) through a spring
(C). The spool ports oil under charge pressure (D) to a servo cylinder (E). The piston moves the swashplate
(F) against the opposite servo spring (G). Both servo springs are constrained so that they can only force the
swashplate toward neutral. When the swashplate has moved to the angle set by the control handle, the feed-
back link (H) returns the displacement control spool almost to neutral where it ports just enough oil to the servo
cylinder to keep the swashplate at the proper angle.
The orifice (K) restricts the incoming charge supply to limit the maximum servo response rate. Spring (C)
allows the operator to rapidly pre-select the desired speed setting without waiting for the swashplate to follow-
up.
When the control handle is released, the displacement control spool is returned to neutral by a spring (J). This
allows oil from both servo cylinders to flow into the pump case through the small underlaps (I). (Refer to Fig. 2.)
Both servo cylinders are thus exhausted and one of the servo springs mechanically forces the swashplate to
neutral.
PRESSURE OVERRIDE CONTROL OPERATION
The pressure override control is used in conjunction with Sundstrand heavy-duty variable pumps equipped with
a displacement control valve. The pressure override control will override the displacement control at a
predetermined system pressure, known as override pressure.
The pressure override control valve assembly is a three-way valve with charge pressure (supply pressure)
ported to the displacement control in the normally open position. In the closed position, the supply pressure is
cut off to the displacement control and the control is allowed to drain to tank.
During normal operation, the transmission system pressure is below the override pressure, and the pressure
override valve is in the open position and therefore, does not affect the displacement of the variable pump.
Thus, the displacement of the variable pump is controlled by the displacement control during normal operation.
The pressure override control valve will override the displacement control and will control the displacement of
the variable pump when the load re- quires a system pressure above the override pressure. During this mode
of operation, a ball check senses the pressure from both sides of the loop and ports the higher pressure into
cavity (A) (refer to Fig. 3) and against signal piston (B). The signal piston (B) forces the controlling spool
against a spring (D). As system pressure increases, the spool (C) moves toward the spring (increased force on
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