Hydraulic Fluid Heater
strainer, located in the reservoir, into the pump where it
is pressurized to 1400 psi and applied to the hydraulic
The fluid heater is a 175-watt immersion-type
pressure line. A fluid flow filter removes solid impurities
heater. The fluid must be kept at approximately 70°F or
greater than 3 microns in size. In the event the filter
greater to prevent it from becoming too viscous and
becomes clogged, it is bypassed. The filter output then
causing servo errors. The heater is a Calrod type with a
flows past the pressure gauge, the pressure switch, and
built-in thermostat. The thermostat is normally factory
the bypass valve. The pressure gauge should indicate
set but may be adjusted if necessary. To adjust the heater,
1400 psi in normal operation, and the pressure switch
unscrew the cover plate by turning counterclockwise
should be closed for pressures above 1200 psi. The
and use the internal screwdriver adjustment to set the
bypass valve is normally closed and will open only if
temperature. It will take about a half hour for the tem-
the pressure exceeds 1800 psi.
perature to stabilize.
If the pump is operating normally, the bypass valve
will be closed and the fluid will flow through the check
valve and out the gate valve to the system. The check
valve is a one-way valve. The fluid returning from the
The overtemperature switch is a mechanically
system flows through the return gate valve and check
adjustable immersion-type thermoswitch. It is used to
valve into the reservoir. The return check valve only
indicate overheating of the pump oil. It does not indicate
allows fluid to flow in one direction and requires 75 psi
a direct failure. In a warm environment of approxi-
of pressure before it will open. This maintains the return
mately 85°F the oil temperature will be about 120°F. An
line pressure at 75 psi.
increase in oil temperature will most likely be due to
For the pump, heater, and overtemperature switch
increased fluid viscosity or a clogged pump falter. If this
to operate properly, the fluid reservoir must be properly
is the case, the pump should be drained and flushed with
filled. Too little fluid may actually cause the pump to
warm water, and the fluid and filter replaced.
The overtemperature switch is adjusted by prying
the cap off the protruding stem and inserting a screw-
Pump Motor Contactor
driver in the stem.
The motor controller usually has 440 volts ac
Hydraulic Pressure Switch
applied to it. The pump is actuated by applying 115 volts
ac to the motor controller relay. The pump motor is
The hydraulic pressure switch is a single-pole,
protected by thermal overloads, located in the motor
double- throw, pressure-actuated switch. It is used to
controller. A thermal overload is a relay that is actuated
turn on the system electronics when there is enough
by heat. Motor current flows through a low-value
pressure to stabilize the system. It is normally set to
resists, generating a small amount of heat. If the current
actuate at 1200 psi.
increases beyond a specified value (3.7 amps), the heat
The pressure switch is adjusted by turning the label
which holds back a spring-loaded relay. This will cut the
until the inner body is exposed. It can be turned with a
pump power by opening the circuit to the motor control
screwdriver or other instrument inserted in the inner
body holes. The pressure switch setting is decreased by
relay. The thermal relay should then be allowed to cool
turning the inner body counterclockwise as viewed from
before pushing the reset button on the pump controller.
the connector end.
The pump motor is factory wired for 440-volts ac
operation and should not be changed as the motor con-
The hydraulic pressure switch is a nonrepairable
item that must be replaced if it is not operating properly.
troller current limits are set for 440-volts ac operation.