Fwd: servotronic rack rebuild

[Bernie Benz]

Begin forwarded message:

> From: Bernie Benz <b.benz at charter.net>
> Date: March 14, 2006 4:41:48 PM PST
> To: <dennis at seaweedmonster.com>
> Cc: "Miller, Chris" <chris.miller at infofoundry.com>, 200q20V mailing  
> list <200q20v at audifans.com>
> Subject: Re: servotronic rack rebuild
>
>
>> From: "Dennis Stehlik" <djsj35 at hotmail.com>
>> Reply-To: dennis at seaweedmonster.com
>> Date: Tue, 14 Mar 2006 15:27:53 -0600
>> To: b.benz at charter.net
>> Subject: Re: servotronic rack rebuild
>>
>> Bernie,
>>
>> Thanks for the E-mail.  I have thought about replacing that pesky  
>> hard blue
>> o-ring.
> That seal element is not an O-ring. It is a machined or molded ring of
> special cross section and it has an O-ring around its OD to both  
> seal and
> add compliant squeeze pressure (unsuccessfully) to this primary  
> plastic seal
> element.
>>  Was actually talking with my father-in-law and his machine shop to
>> design something different.  Seems to me there should be 2 o-rings  
>> there,
>> but I am not an o-ring design expert so I hesitate.
> No, using an elastomeric O-ring as the primary sealing element in a  
> dynamic
> linear motion rod seal application is a bad design under any pressure,
> inasmuch as the ring will be subject to spiral failure due to the  
> linear
> motion. A Quad section ring will prevent this type failure, why I  
> used this
> type elastomeric ring for the primary seal element.
>
> But further, at 1000 psi any elastomeric ring material will fail by
> extrusion through the relatively large rod to seal housing  
> clearance without
> the use of a well sized back up ring. This is why ZF used the hard  
> plastic
> seal material, to prevent extrusion failure.
>> I have been reading up
>> on it lately and looking at alternatives available via the web and  
>> the
>> McMaster Carr catalog I have.
> Get a Parker O-ring design handbook, and a Minnesota Rubber Quad  
> ring design
> hand book.
>>
>> Can you give me more details on the replacement you used?  Where  
>> you got the
>> o-rings, the exact size, shap, part number, and material it is  
>> made of?  I
>> would love to look into it.
> You can have your machine shop cut the stakes from the old seal  
> assembly,
> but the two halves are still a press fit together and thus are  
> still a bitch
> to get apart, but doable. Heat may help, you’re not trying to save the
> plastic.
>
> Not many design options within the limited space of that seal  
> housing, but I
> found one that worked, as follows:
>
> 1. The primary sealing element is a Q4111 Quad ring, any available  
> material
> is OK, N70 most common. Bum a sample from MR.
>
> 2. The back up ring is a Micro Plastics #17W06881 molded nylon washer,
> .688 x .390 x .031. They’ll give you a bag of samples. The ID is a  
> little
> tight for the .392D piston rod so, having a taper reamer, I reamed  
> it out to
> about .395 placing the larger D of the taper toward the seal  
> housing bore
> and the smaller supporting the Quad.
>
> 3. There is also an O-ring used to load the Q ring radially and to  
> seal the
> OD of the back up ring to the housing, a generic 2-015. Assemble  
> these parts
> in the seal housing and press it back together, no need for the  
> stakes.
> Other than these new parts, my rebuild reuses all of the other  
> parts and
> O-rings.
>
> ZF uses a tapered insertion tool (100/200 Bentley 48.13) to protect  
> the seal
> from the sharp edge of the piston rod during assembly. You can make  
> one or
> just round off the sharp edge of the rod end.
>
> I’ve been using Ford Tractor hydraulic fluid in my Audi PS systems  
> for years
> now with no problems.
>>
>> As part of the rebuild o-ring kit I got a whole bunch of o-rings  
>> and seals,
>> but also got the metal insert with the blue ring.
> Wow! ZF told me a year+ ago that they most probably would never  
> sell this
> part because of liability in a critical steering application. BS!  
> Nothing
> critical about a little leak. IMO, they were just protecting their  
> rebuild
> business.
>
> Regards,
>
> Bernie Benz
> Gardnerville, NV
>
> '90 90Q-20V  160K
> '91 200Q-20V 210K
> ’02 A6 2.7T 6spd 70K (wife’s new ride)
> '88 Ford "Quatro" Tractor
> Other Misc. Toys
>
>
>
>> That insert has to be
>> placed into the carrier that I pressed out with the old insert.   
>> You are
>> right that there is some lateral play in that assembly.  I know I  
>> didnt have
>> to take the entire thing apart and redo all the rings, but I had  
>> them, was
>> interested, had the time....so I did.  It was more about the  
>> challenge of it
>> rather than actually fixing any problem.
>>
>> Thanks,
>> Dennis Stehlik
>> 630-456-4119
>>
>>