Iveco EuroCargo (12 to 26 t). Manual

URO

ARGO

ECTOR

12-26 t

STEERING

Base - February 2003

PART

HYDRAULIC

POWER

STEERING

ZF 8095

ZF 8098

TRW/TAS 55

Screw for securing hydraulic power steer-
ing to support

Nm (kgm)

520

± 52 (53 ± 5.3)

560

± 17 (57 ± 1.7) 700 ± 3.5 (71 ± 0.3)

Nut for securing lever on shaft

Nm (kgm)

520

± 52 (53 ± 5.3)

550

± 55 (56 ± 5.6) 700 ± 3.5 (71 ± 0.3)

Screw for securing the support to chassis

Nm (kgm)

655

± 65 (67 ± 6.6)

655

± 65 (67 ± 6.6)

Nut for securing hydraulic steering limiter
adjustment screws on hydraulic power
steering

Nm (kgm)

± 5 (1.5 ± 0.5)

± 7.5 (7.3 ± 0.7)

Nut for securing M22x1.5 steering wheel

Nm (kgm)

± 7 (7.4 ± 0.7)

TOOLS

TOOL No.

DENOMINATION

99347042

Driving steering wheel extractor.

99347068

Extractor for steering tie-rods head pins.

99374393

Tool with manometers for checking hydraulic ZF hydraulic power
steering pressure.

99374398

Graded sector and index for checking steering wheel clearance
(use with 99374393).

This solely mechanical steering movement is servo-assisted
by pressurised oil supplied by an engine-operated ZF pump.

The control valve is composed of rotating distributor,
supported on rollers in the worm screw and equipped with
six control grooves on perimeter and of worm screw end
supported in the steering case and equipped with six control
grooves as well.

The rotating distributor at the same time operates also as
lower connection element of steering shaft and rotates with
the worm screw till the steering wheel is rotated.

Description

The ZF hydraulic power steering

with ball circulation

control and a compact shape, is mainly composed of a case
and an embedded power steering mechanical part, of a
control valve and an operating cylinder. As can be seen in
Figure 4 and Figure 5, the rotating movement given to the
steering wheel is transmitted without friction from power
steering shaft to piston through an infinite sequence of balls
and then transformed into an axial piston movement. The
toothing of piston driven

into case

meshes into

toothed-sector shaft toothing and makes it rotatingly move.
The steering control arm fixed onto this shaft transmits the
torque to wheel steering tie-rods.

62596

Figure 3

HYDRAULIC POWER STEERING ZF-TYPE

URO

ARGO

ECTOR

12-26 t

STEERING

Base - February 2003

CONTROL VALVE IN NEUTRAL POSITION

A. Case — B. Piston — C. Rotating distribution/steering shaft — D. Control case/worm screw — E. Torsion bar —

F. Toothed-sector shaft — G. Pressure limiting valve — H. Re-suction valve — Q. Oil tank — R. ZF blade pump — S. Flow-rate

limiting valve.

This synchronous rotary motion is due to the fact that the
worm screw and the rotating distributor (C) are connected
through a torsion bar (E, Figure 4) that keeps the control
valve in a neutral position (rectilinear running) till the steering
wheel is rotated.

When a torque is transmitted from steering wheel or steering
wheels to worm screw, the torsion bar is subjected to a
distortion in its elastic area, so that between rotating
distributor (C) and worm screw end, that operates as control
case (D), a relative movement occurs. This causes a
distributor control grooves displacement with respect to
worm screw ends grooves, so that the control valve passes
from neutral position to operating position. The pressurised
oil by the control valve (G) can now cross the open control
grooves and penetrate into one of the two operating cylinder
chambers, thereby assisting the steering movement through
the pressure on one of piston surfaces.

If the hydraulic steering servoassistance is lacking, it is always
possible to steer, even if with a higher effort on the steering
wheel.

To avoid that with the whole hydraulic pressure it is possible
to steer till the right and left limit stop and in some cases
damage the driving tie-rods, the ZF-Servocom is equipped
with hydraulic steering limiting (Figure 6).

A re-suction valve (H, Figure 4) is assembled into the drive
case with which return circuit oil can be sucked when it is
necessary to steer without hydraulic servoassistance.

Moreover, according to the driving system execution, a
pump delivery pressure limiting valve is also assembled
according to a pre-established maximum value.

In the operating drawings (Figure 4 and Figure 5) the control
valve and oil flow are schematically shown. The valve (S) is
shown in a transverse section, so that its operation and
connection to cylinder chambers can be seen.

The pressurised oil coming from the pump flows into the
central annular groove of the control case and reaches,
through three radial holes, the arcuate control grooves of the
rotary distributor.

Figure 4

27200

URO

ARGO

ECTOR

12-26 t

STEERING

Base - February 2003

The mutual position of these grooves and the worm screw
end grooves allows, in a neutral valve position, the
pressurised oil to pass through admission ports till it reaches
the equally arcuate grooves in the control case. These latter
ones are connected through radial holes with both operating
cylinder chambers.

Therefore, in the control valve neutral position, the
pressurised oil can penetrate into the two operating cylinder
chambers and also into the three return circuit grooves of the
rotary distributor and from there reflow into the oil tank.

the

steering

wheel

rotated

rightwards,

the

rightward-threading piston moves rightward (Figure 5). Since
this piston movement must be hydraulically servoassisted,
the pressurised oil must penetrate into the left cylinder
chamber. The three control grooves of the rotating
distributor are displaced clockwise, so that the admission
ports (K) are further opened to let the pressurised oil flow.
The admission ports (J) are closed and stop the pressurised
oil flow towards the axial grooves (O) of the control case.

Through the admission ports (K), the pressurised oil
penetrates into the axial grooves (N) of the control case and
from there, after having crossed the worm screw balls
circulation threading, comes into the left cylinder chamber.
The hydraulic servoassistance is then turned on, while the
admission ports (I) closure prevents oil from returning into
the tank. The oil being present in the right cylinder chamber
is made go out and flows, through the open return ports (M),
towards the return grooves (P) of the rotating distributor and
returns, through its central hole, to the oil tank.

If the steering wheel is rotated leftwards (not shown), the
operating cylinder piston is displaced leftwards. The
distributor control grooves rotate counterclockwise. The
pressurised oil penetrates through admission ports (J) into
axial grooves (O) and then into right cylinder chamber.

The oil being present in the left cylinder chamber reflows into
the tank through the ball circulation threading, the return
ports (L), the return grooves (P) and the central rotating
distributor hole.

CONTROL VALVE IN OPERATING POSITION FOR CLOCKWISE ROTATED STEERING WHEEL

J. Admission port — K. Admission port — L. Return port — M. Return port — N. Axial groove — O. Axial groove —

P. Return groove.

Figure 5

27201

URO

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12-26 t

STEERING

Base - February 2003

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Iveco EuroCargo (12 to 26 t). Manual - part 160