SAFARI

POWER STEERING

ZF hydraulic ball and nut power steering Short

version

II. Design and function

1. Design

1.1 Worm :

Both radial and axial roller bearings are mounted

in the cover (128). Worm Shaft with grooved

channel for ball circulation is mounted in this

cover.

1.2 Piston :

The valve bush which accommodates the valve

spool is pressed into the cross bore hole. The

underside of the piston is machined as a gear

rack, which engages in the toothed segment of

the sector shaft.The steering nut and finger passes

into the longitudinal bore hole of the piston, thus

establishing the connection with the valve spool

through the worm.

The valve is centered by means of centering

springs, leaf springs or bending bar.

1.3 Steering nut :

Longitudinal bore hole for worm engagement with

grooves for ball circulation are provided. The

groove on the external diameter is milled and

ground for accepting the finger, which engages in

the cross bore hole of the valve spool .This

establishes the connection to the worm by means

of the ball chain between steering nut which

depends on the selection of the corresponding

type of ball.

The steering nut is mounted in piston, play-free,

between two roller bearings. Adjustment is carried

out by turning the threaded ring (118) which is

secured on the open side of the piston.

1.4 Housing :

The housing has integrated passages for the

pressure and return lines and, a built-in pressure

limiting valve. Oil passages which are required

within the steering system are arranged as bore

holes in the housing, housing cover / plate or in

the piston. No external connecting lines are

required due to this design. The housing has

mounting holes and machined surfaces for

securing the steering gear box on the mounting

bracket of the vehicle. It supports the sector shaft

(30) with bearings (5).

1.5 Sector shaft :

The sector gearing is cut with a flat angle to the

shaft axis in order to facilitate, service adjustment

in the axial direction. The center tooth is shaped

more convex, so that the readjustment can be

carried out in straight ahead position (SAP), since

this is the area where the greatest amount of wear

occurs, without the following gear teeth, both left

and right, becoming jammed.

The forces in the axial direction are taken up, by

means of an adjustment screw (31); the screw is

held into the bore hole of the sector shaft, play-

free and passes through threading in housing cover.

Adjustment in axial direction is carried out by

utilizing this thread pitch and locking is done by

means of a lock nut.

1.6 Housing cover :

It houses the adjustment screw and bearings (41)

for supporting the sector shaft.

2. Function :

In order to obtain hydraulic support during steering

of vehicle, spool valve must be moved out of the

neutral position.

The spool valve is held in the neutral position by

means of a centering spring or bending bar.

Steering forces must therefore be applied in order

to overcome this initial setting.

The piston which is positively connected with the

sector shaft and the steering wheels, offers

resistance to rotation.The steering nut is therefore

rotated by the worm and the ball chain during the

steering process thus overcoming the initial setting

of the centre spring. The pressurized oil flowing

into the gearbox housing from the Engine driven

pump is then routed into the chambers fromwhich

the steering procedure is hydraulically supported.

The oil flows in from the housing side into a

longitudinal groove of the piston and beneath the

valve. It also flows into a longitudinal groove of

the same size on the opposite side in order to

assist the pressure compensation and then

reaches, by means of cross bore holes, at the

front surfaces of the valve pistons which are

separated from the chambers by means of seals.

In the neutral positions of the valve the oil-flows

to the centre of the valve piston after first flowing