10 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, playfree 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
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