6 ZF Servocom ZF Servocom, type 8098, control valve in working position. Steering wheel turned clockwise. 1 Housing 2 Piston 3 Valve slide 4Worm 5 Torsion bar
ZF Servocom®
Mechanical construction The ZF Servocom, a compact-design recirculating-ball power steering gear, basically comprises a sturdy cast-iron housing (1) with integrated mechanical steering gear, control valve and power cylinder. A turning movement at the steering wheel is transmitted via the output shaft and the valve slide (3) in the control valve to the worm (4) and, via an endless ball chain (7), transformed into an axial movement of the piston (2). At the same time, the sector shaft (6), which is arranged at right angles to the longitudinal axis of the piston, is caused to rotate by the meshing of teeth. The drop arm mounted on the sector shaft moves the steering linkage which goes to the steering arms, thus causing the wheels to be turned. The basic components of the Servocom control valve are the valve slide, with six control grooves on its surface area, and a worm in whose valve bore axial grooves are provided which are matched to the control grooves. Centralizing the valve slide (neutral position) is done by a torsion bar (5) which provides at the same time the connection between
Figure at top: ZF Servocom, type 8098. Figure on page 5: ZF Servocom, type 8098, control valve in neutral position.
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ZF Servocom
the valve slide and the worm. The valve slide and the worm run in antifriction bearings to ensure the precision of operation and the functional safety of the control valve even at high pressures. If a torque is transmitted to the valve slide or the worm from the steering wheel or the steered wheels, a relative rotary motion influenced by the torsion bar will occur between the valve slide and the worm. The valve slide is thereby caused to change its position in relation to the worm valve bore surrounding it, so that the relative positions of the control grooves are changed, too. Pressurized oil can now flow through connecting bores to one of the two power cylinder chambers (ZL or ZR) and assists the axial movement of the piston. When the steering wheel is released, the
1 2 3 4 5 6 7 8
Housing Piston Valve slide Worm Torsion bar Sector shaft Ball chain Feed oil radial groove
9 10 11 12 13 14 15 16
action of the twisted torsion bar makes the control grooves return to the neutral position, and the same system pressure will exist in both of the power cylinder chambers.
Basic hydraulic function of the Servocom control valve The hydraulic fluid delivered by an engine-driven steering pump (21) flows through a connecting bore in the housing bottom, via the feed oil radial groove (8) and transverse bores in the valve portion of the worm (4), onward to the three feed oil control grooves (9) of the valve slide (3). In the valve neutral position (see fig. on page 5), the oil flows, over the open feed oil control edges (10), to all axial grooves (11) of the worm head and from there, over the open return oil control edges (13), also to the return oil control grooves (12) of the valve slide. From
Feed oil control groove Feed oil control edge Axial groove Return oil control groove Return oil control edge Return oil chamber Radial groove Radial groove
17 18 19 20 21 22 ZL ZR
Hydraulic steering limiter Pressure relief valve Replenishing valve Flow limiting valve Steering pump Oil reservoir Power cylinder, left Power cylinder, right
14 13 11
22
8
21
10
20
1 15
12
9
16
3 18
14
ZL
2
17
7
4
5
6
ZR
16
15 8 19
ZF Servocom
5
14 13 11
22
8
21
10
20
1 15
12
9
16
3 18
14
ZL
2
17
7
4
5
6
ZR
16
15
8 19
ZF Servocom, type 8098, control valve in working position. Steering wheel turned clockwise.
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ZF Servocom
1 2 3 4 5 6 7 8
Housing Piston Valve slide Worm Torsion bar Sector shaft Ball chain Feed oil radial groove
9 10 11 12 13 14 15 16
Feed oil control groove Feed oil control edge Axial groove Return oil control groove Return oil control edge Return oil chamber Radial groove Radial groove
17 18 19 20 21 22 ZL ZR
Hydraulic steering limiter Pressure relief valve Replenishing valve Flow limiting valve Steering pump Oil reservoir Power cylinder, left Power cylinder, right
these grooves the oil flows back, via bores, to the return oil chamber (14) inside the worm and from there to the oil reservoir (22). At the same time, the radial grooves (15 and 16) of the control valve and their associated connections provide for a connection between the right-hand (ZR) and lefthand power cylinder chambers (ZL). When turning the steering wheel clockwise (fig. on page 6), the piston (2) will move to the right in the piston bore if it is a right-hand ball thread. Due to the simultaneous rotation of the valve slide (3) to the right, the pressurized oil is directed, over the further opened feed oil control edges (10), to the three associated axial grooves (11), via bores to the radial groove (16), and, via a connection, to the left-hand cylinder chamber (ZL), whereby the piston movement is hydraulically assisted. An individually adaptable pressure build-up is achieved by the fact that the partially or fully closed feed oil control edges (10) restrict or prevent a connection between the pressure oil inlet and the other three axial grooves (11) connected to the radial groove (15). At the same time, the pressure oil outlet toward the pressurized axial grooves is restricted or prevented, too, by the closing return oil control edges (13).
The oil displaced by the piston (2) from the right-hand power cylinder chamber (ZR) first flows through a connection to the radial groove (15) and, through transverse bores, to the associated axial grooves and onward to the return oil control grooves (12) over the further opened return oil control edges (13). From here, the further return flow of the oil to the oil reservoir (21) takes places via the connecting bores leading to the return oil chamber (14). When the steering wheel is turned counterclockwise, the operating sequence will be analogous to the above.
Hydraulic steering limiter To protect the steering linkage, the wheel lock stops and the steering pump from excessive loads at the maximum wheel lock angles, the ZF Servocom has a mechanically adjustable or automatically on the vehicle adjusting hydraulic steering limiter (17). This device integrated in the piston (2) is always closed due to the
oil pressure in the right-hand or lefthand power cylinder chamber. It is only just before each piston end position that the steering limiter valve is opened by the contact of the valve pin with the adjusting screw or sleeve. Thus, that cylinder chamber which is under high pressure is connected with the opposite return oil chamber. As a result of the pressure drop, hydraulic assistance is reduced heavily. The steering wheel can only be turned up to the road wheel lock stop by using increased effort.
Further features The ZF Servocom is fitted with a pressure relief valve (18) which limits the delivery pressure of the steering pump at the maximum specified pressure. Also, a replenishing valve (19) can be fitted to the housing or the valve slide, if required. This valve allows to suck oil from the return flow if the vehicle must be steered without hydraulic assistance.
Mechanically adjustable (top) and automatically adjusting steering limiter (bottom). Steering limiter valve opened, oil pressure greatly reduced.
ZF Servocom
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