
404 LIGHT AND HEAVY VEHICLE TECHNOLOGY
Worm head Inner and outer
engages sleeves of spool
outer sleeve control valve
Hydraulic pump
and reservoir
Steering column
engages inner
sleeve
Torsion bar connection
Figure 24.12 Schematic arrangement and action of rotary-type spool control valve PAS
A from pump C to cylinder
B to reservoir D from cylinder
Another version of the rotary control valve principle
is that used since 1964 in the German ZF (Zahnradfabrik
Friedrichshafen) PAS system. This similarly embodies
a valve centring torsion bar but, instead of influencing
the relative angular position of two ported sleeves, it
actuates a cross-valve spoo ...