SERVICE REPAIR
HR45-27 HR45-31 HR45-40S [C227]
1600 SRM 786
Steering Control Unit Repair
Steering Control Unit Repair
DESCRIPTION
The steering control unit is a hydrostatic unit con-
trolled by the steering wheel. See Figure 3. It is
mounted in the steering housing. The steering con-
trol unit has a shaft assembly, control section, and a
metering section. The hydraulic hoses for the steer-
ing system are fastened to a manifold block.
1. CONTROL SECTION
2. METERING SECTION
3. CHECK BALL (MANUAL OPERATION)
4. SLEEVE
5. NEUTRAL POSITION SPRINGS
6. SPOOL
Figure 3. Steering Control Unit
OPERATION
Turning the steering wheel actuates three main
parts of the steering control unit (see Figure 3): (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 wheel is not moving, the
spool and sleeve are held in the NEUTRAL (center)
position by springs. During this time, oil flows freely
through the steering control unit. The oil does not
flow to the steering cylinder.
As the steering wheel is turned, the spool just be-
gins to rotate. The springs try to move the sleeve to
keep the NEUTRAL position between the spool and
sleeve. However, the force necessary to turn the ro-
tor is greater than the pressure of the springs. The
springs begin to bend, 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 com-
ing into the control unit flows to the metering section.
More rotation of the steering wheel 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 the flow amplifier and then to one
side of the steering cylinder. Oil from the other side
of the steering cylinder flows back through the flow
amplifier and the control section of the steering con-
trol unit.
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