[200q20v] Re: [200q20v]PS Rack R&R

[Kneale Brownson]

I'm not familiar with the rack workings, just operating on suspicions that 
the system relies on a rod being moved in and out of a cylinder just like 
the hydraulics that make a backhoe or loader arm move.  Is this 
incorrect?  My impression of a stainless steel sleeve was that Jorgen 
sleeved the rod so that a stainless surface was what rode in and out on a 
seal at the outboard end of a cylinder.  I've resealed numerous hydraulic 
cylinders on tractors where the piston (rod) has had its chrome coating 
scratched so that rust can develop and provide a rough surface that tears 
up the seal.   I was thinking the stainless sleeve would eliminate that 
source of seal deterioration.


At 02:38 AM 12/12/2000 -0800, Bernie Benz wrote:




>The 44 chassis PS differs from the 89 chassis PS system in that the 44
>operates at a higher pressure, derived from a piston pump, as opposed to the
>89's lower pressure, vane type pump.  Accordingly, the 44 rack cylinder can
>be, and is of smaller diameter than that of the 89.  Further, I expect that
>the 44 system's higher average pressure most probably is the primary cause
>of the apparent higher failure rates for the 44's rack and hoses relative
>that of the 89s.
>
>I have only "overhauled" one Audi rack, but all ZF racks are of the same
>basic design, and I've done many when I was into the late '70s and early
>'80s Lancias.  IMO, the major, if not the only failure mode is the failure
>of the only external high pressure dynamic seal in these racks, the piston
>rod seal.  Further, I have never seen a wear scratch or any other type flaw
>in the fine chrome plated piston rod surface that could be considered the
>cause of this seal failure.  The long term sealing of high pressure linear
>motion hydraulics is just technically very difficult, and seals fail.  So
>for long rack, hose and pump life, keep the peak system pressure down, i.e.
>don't turn and hold the steering wheel against the limit stops.  Back off a
>notch.
>
>IMO, Jorden's SS sleeve is a sales gimmik to dupe those unfamilar with this
>rack design.  What would they sleeve and why?  The cylinder sees no metallic
>contact with the the piston, only a plastic piston ring (or was it an
>O-ring?) contacts the cylinder walls.  The cylinder is just not subject to
>wear, even in the presence of foreign particles, which are pushed outside of
>the active area.  If you must buy a rebuilt rack, demand a ZF rebuild.  ZF
>has a rebuild plant in the US, and has a good warranty, without sleeves.