Steering Control Unit
1600 SRM 974
Steering Control Unit Assembly
DESCRIPTION
The steering disk is attached to the shaft of the SCU.
This shaft engages the spool of the SCU. The SCU is
a load-sensing, closed-center, rotary-type valve. Dur-
ing the steering operation, the signal generated is
proportional to the rotation of the steering disk. The
unit controls both the direction and regulation of oil
flow to operate the hydraulic motor. If the steer-
ing pump fails to operate, a check valve closes in
the SCU. The SCU then operates as a hand pump to
make steering possible. The lift truck is difficult to
steer when the power steering is not operating. The
SCU is mounted to a bracket that is mounted to the
lift truck frame.
REMOVE
WARNING
Make sure the brake pedal is released and
the brake is applied. Put blocks on both sides
(front and back) of the drive/steer tire and
the caster wheels to prevent movement of the
lift truck. Put a block on each side (front and
back) of the load wheels to prevent movement
of the lift truck.
1.
Disconnect the battery.
Block the drive/steer
tires, caster wheels, and load wheels on both
sides to prevent movement of the lift truck.
Open the drive unit compartment cover. Remove
the battery compartment cover and the drive
unit cover.
2.
Install labels on the hydraulic lines for proper in-
stallation at assembly. Disconnect the hydraulic
lines and install caps on the lines and fittings.
3.
Remove the shoulder screw, lockwasher, and nut
that hold the steering control handle to the SCU.
Remove the steering control handle.
4.
Remove the two capscrews and lockwashers re-
taining the steering control unit to the mounting
bracket, and remove the steering control unit.
Steering Control Unit
DESCRIPTION
The steering system is a hydraulic system that does
not have a mechanical connection between the steer-
ing disk and the steering axle. The steering is con-
trolled through a hydraulic circuit.
If the steering pump fails to operate, a check valve
permits the Steering Control Unit (SCU) to operate
as a hand pump and to make steering possible. The
lift truck is difficult to steer when the hydraulic
pump is not operating, but the SCU can operate the
hydraulic motor and make steering possible.
OPERATION
The SCU is a rotary valve that is operated by the
steering disk.
During the steering operation, the
SCU controls the direction of flow and amount of
oil that flows to the hydraulic motor. The hydraulic
motor actuates the steering chain or gear to turn the
drive unit. Hydraulic oil returns from the hydraulic
motor to the SCU and then returns to the hydraulic
tank.
Turning the steering disk actuates three main parts
of the SCU: (1) the spool for the control section, (2)
the sleeve for the control section, and (3) the rotor
in the metering section. When the steering disk is
not moving, the spool and sleeve are held in the neu-
tral (center) position by springs. During this time, oil
flows freely through the SCU. The oil does not flow
to the steering motor.
As the steering disk is turned, the spool just begins to
rotate. The springs try to move the sleeve to keep the
neutral position between the spool and sleeve. How-
ever, the force necessary to turn the rotor is greater
than the pressure of the springs. The springs begin
to compress, letting the spool move a small amount
within the sleeve. The spool stops moving when it
touches the center pin. In this position, the holes in
the sleeve and the spool are aligned. Oil coming into
the control unit flows to the metering section.
More rotation of the steering disk causes the spool to
rotate the pin. This action causes the rotation of the
sleeve and the rotor in the metering section. The oil
then flows to one side of the steering cylinder. Hy-
draulic oil from the other side of the steering motor
returns through the control section of the SCU.
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