[TheForge] OT - Grease/oil seals for a gear box

[Bruce Freeman]

Well, I am surprised!  I always thought of packing seals for plumbing
applications like valve stems, where the rotating rate is about once
per day!  I've looked up a little and find some of them are good for
"high speed" applications, but I don't have enough info yet to
determine what RPM that means.

Does anyone know what considerations go into selecting design criteria
and packing material for an application with speeds upwards of 2000
RPM?  (I'm shooting for 10kRPM, but doubt I'll get there with this
prototype, for reasons other than the packing seals.)

It happens I might be able to modify my apparatus to incorporate
packing seals, depending upon the requisite design parameters.

So I have some questions I'm hoping some of you can field:
1) I presume that I want to box in the packing material with
close-fitting "washers".  Does it matter >how< close-fitting?  Or is
it OK if the packing can swell into  a gap between shaft and "washer"?
2) Likewise, does the packing need to be completely contained, or is
it OK to compress it with actual (close-fitting) washers, rather than
the more elaborate "box" used in plumbing fixtures.  (Washers would
allow the packing to swell outwards from the shaft.)
3) What kind of material would be appropriate?  (If you can reference
the McMaster webpage, all the better:
http://www.mcmaster.com/#packing-seals/=2ofm9j )  The shafts are
steel.  The housing is aluminum.
4) The packing will be adjacent to a ball bearing in every case.  I
presume I'll need a "washer" to isolate packing from bearing?  (Lest
there be friction between inner race of bearing and packing.)

Again, your advice will be appreciated.

On Thu, Jul 9, 2009 at 1:57 PM, Peter Fels & Phoebe
Palmer<artgawk at thegrid.net> wrote:
> Bruce..there should be a huge selection of standard oil/grease seals
> available if your shafting is standard sizes.
> Alternately, you could take some teflon packing cord and fake up a
> compression seal that ought to do fine. A home made O ring seal isn't
> all that hard but will have a shorter life, depending on the finish of
> the shaft.
>
> Bruce Freeman wrote:
>> I'm constructing a simple gearbox for a mechanism I'm playing with.
>> The gearbox itself is of no importance except as to provide power to
>> the mechanism, and the nature of the gearbox is such that I cannot
>> simply buy a commercial unit.
>>
>> Perhaps naively, I simply mounted sealed ball bearings into
>> appropriate recesses in the housingn, counting on the small clearance
>> between bearings, shafts and housing to  seal against oil leakage.  No
>> go.  The oil leaks right out pretty quickly.  Grease works better, but
>> also leaks.  Loss of grease is not a problem - the potential for
>> contamination of the driven device is the problem.
>>
>> Next I tried sealing between bearings and shafts or housing with a
>> medium-strength "lock-tight" product.  No go.  Oil leaks right out
>> when gearbox is run.
>>
>> I could go to a product like RTV rubber, but I'd probably never be
>> able to change out the bearings again if I did that.  Still I may
>> consider it.
>>
>> I'm looking for other ideas.  I'd prefer not to remanufacture the
>> housings, but  could do that if necessary.  For example, I'd have to
>> do something like that if I went to O-ring seals.  However, I'm not
>> convinced O-rings would do a better job, so am not ready to take that
>> particular step.
>>
>> How is this done in commercial mechanisms?
>>
>> Any suggestions would be appreciated.
>>
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-- 
Bruce
NJ

The total lack of evidence is the surest sign that the conspiracy is working.