Steering Control Unit (SCU)
Cross Section Of Steering Control Unit.
(1) Outlet port. (2) Check valve (not shown). (3) Inlet port. (4) Internal
pump gear. (5) External pump gear. (6) Spool. (7) Centering springs.
(8) Cross pin. (9) Left turn port. (10) Right turn port. (11) Sleeve.
(12) Body. (13) Drive. (A) Control section. (B) Metering section.
The steering control unit is spring centered (no turn), non-load
The steering control unit has two main sections. They are
control section (A) and metering section (B). The two sections
are connected inside the unit, both hydraulically and
mechanically. These two sections work together to send oil to
the steering cylinders.
Oil goes through inlet port (3) into control section (A). When
the steering wheel is turned, the control section sends oil to
metering section (B). Metered oil from section (B) is directed
by control section (A) to either left turn port (9) or right turn port
Metering section (B) is a small hydraulic pump. It produces a
specific amount (metered) of oil flow. This metered oil is then
directed by control section (A) to the steering cylinders.
The direction and the speed of a turn are controlled by the
steering control unit. As the steering wheel is turned faster,
there is an increase in the flow of oil from the steering control
unit to the steering cylinders. This increased flow causes the
cylinders to move farther and faster, which turns the machine
Oil flows through inlet port (3). When the steering wheel is
stationary (NEUTRAL), the oil is stopped by spool (6). The oil
cannot flow through the steering control unit to the steering
cylinders until the steering wheel is turned.
The steering wheel is connected to spool (6) by shaft assembly
and splines. When the steering wheel is turned, spool (6) turns
a small amount until centering springs (7) are compressed.
Then, sleeve (11) starts to turn. As long as the steering wheel
is turned, the spool and sleeve both turn as a unit, but they turn
a few degrees apart.
When the spool and sleeve are a few degrees apart, oil
passages are opened between them. This allows oil to flow
from inlet port (3) through passages in body (12) to the
Pump Gears In Metering Section.
(4) Internal pump gear. (5) External pump gear. (8) Cross pin. (13)
When the steering wheel is turned, cross pin (8) turns with
sleeve (11) and causes drive (13) to turn also. The drive
causes a rotation of internal pump gear (4) inside external
pump gear (5). This rotation of the gear sends a controlled
(metered) flow of pilot oil back through body (12). This oil
flows to right or left turn ports (9) or (10) and then to the
steering cylinders. The right or left turn port that is not used for
pressure oil to the steering cylinders, is used for return oil from
the other ends of the steering cylinders.
If the steering wheel rotation is stopped, centering springs (7)
move sleeve (11) back in alignment with spool (6). This closes
the passages between the metering section and control
section. The steering control unit is then in the neutral
When the engine is off, the steering control unit can be
manually operated. The metering section works as a pump.
Return oil from the steering cylinders is not returned to the
hydraulic oil filter. The suction of the metering section opens
check valve (2) and let return oil from the steering cylinders go
to the inlet side of the metering section. During power
operation, supply pressure keeps the check valve closed.