HYDRAULIC SUPPORT SYSTEM
The Sundstrand hydrostatic transmission is easy to install
requiring no adjustments and few auxiliary components. It
has its own hydraulic support system which is discussed in
Basic Closed Circuit
The main ports of the pump are connected by hydraulic lines
to the main ports of the motor. Fluid flows, in either
direction, from the pump to the motor then back to the pump
in this closed circuit. Either of the hydraulic lines can be
under high pressure. The position of the swashplate
determines which line is high pressure as well as the
direction of fluid flow.
Case Drain and Heat Exchanger
The pump and motor require case drain lines to remove hot
fluid from the system. The motor should be drained from its
topmost drain port to insure the case remains full of fluid.
The motor case drain is then connected to the lower drain
port on the pump housing and out the upper port.
A heat exchanger, with a bypass valve, is required to cool
the case drain fluid before it returns to the reservoir.
Case pressure should not exceed 40 PSIG.
Charge System and Inlet Filter
A fixed displacement (gear type) charge pump is mounted
on the variable displacement pump and driven off the main
pump shaft. The charge pump supplies cool fluid to the
system, keeps the system charged and supplies fluid to
operate the control system. Charge pressure, with the pump
in neutral (O flow), is limited by a relief valve which is
normally factory set for 190-210 DPSI (above case
Since either of the main hydraulic lines can be high
pressure. two (2) charge check valves are used to direct the
charge supply into the low pressure line. The check valves
are contained in the pump end cap beneath the charge
The charge pump draws the fluid from the reservoir. An
inlet filter is required to insure that only clean fluid enters the
system. This filter should have a 10 micron (nominal) rating
and should not have a bypass.
The inlet vacuum, measured at the
charge pump inlet should not exceed 10
in. hg. except during cold starts.