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

[Peter Fels & Phoebe Palmer]

Bruce;
  Is there a reason not to use the sort of seals found on automobile and 
truck rear axles?

Bruce Freeman wrote:
> 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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> 
> 
>