Section 14: STEERING
PA1562
5
2. POWER STEERING GEAR
FIGURE 3: POWER STEERING GEAR 14035
2.1 DESCRIPTION
The power steering gear is located in the lower
part of front service compartment (Figs. 3 & 4).
The housing of the ZF-Servocom contains a
control valve, working cylinder and a complete
mechanical steering gear. The pressure oil for the
steering is delivered by a motor-driven oil pump
which is supplied with oil from an oil tank.
The housing is designed as a cylinder for the
piston, which converts the rotation of the steering
shaft and the worm into an axial movement and
transfers this to the steering worm sector shaft.
The serration of the sector shaft is straight-cut
with a high surface quality in such a way that it is
only possible to set a unique setting without play
on installation in the straight-ahead driving area
by means of the two eccentrically designed lateral
housing covers.
The piston and worm are connected via a ball
chain. When the worm is turned, the balls are
collected by a circulating pipe at one end of the
chain and fed in again at the other end, thus
producing an endless ball chain.
The control valve consists of the valve slide in a
needle bearing in the worm, with six control
grooves on the circumference and the control
sleeve on the worm, which also has six control
grooves. The valve slide, designed with steering
shaft connection, turns together with the worm as
the steering wheel is turned.
FIGURE 4: FRONT SERVICE COMPARTMENT 18611
A torsion bar, which is pinned with the valve slide
and the worm, keeps the control valve in the
neutral position as long as no opposing force is
applied to the steering wheel. The steering
housing contains a pressure relief valve, which
limits the discharge pressure of the oil pump to the
maximum value required. A replenishing valve can
also be used, through which oil is sucked from the
return if steering is not hydraulically boosted.
Compared with constant ratio, steering versions
with variable ratio are more directly designed in
the center area than outside the center area. The
resulting smaller steering corrections benefit
steering behavior in straight-ahead driving. At the
same time, the indirect transmission means that
there is a higher hydraulic torque available at the
steering arm in parking movement. If the
hydraulic assistance fails, the operating forces on
the steering wheel are correspondingly lower in
this area. This is achieved through a
piston/steering worm sector shaft serration with
differing modulus and angle of pressure.
Upon transfer of a torque from the steering shaft
to the worm, or vice versa, the torsion bar is
deformed in the elastic area so that there is
torsion between the valve slide and the control
sleeve. When the steering wheel is released, the
torsion bar ensures that the valve is returned to
the neutral position.
Refer to the "ZF-SERVOCOM Repair Manual" and
“ZF-SERVOCOM Operating, Servicing
/Maintenance and Inspection Instructions”
annexed to this section for the functional aspects
and maintenance procedure of the steering gear.
NOTE
Also available is the ZF-Servocomtronic, which
provides variable assistance in function of
speed.
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