[SEL] OT - expansion of metals in oil pumps

[Listerdiesel]

On Tue, 29 Mar 2005 12:12:13 -0500, Curt <curt at imc-group.com> wrote:
> Bill,
> A bit of quick math will shed some light on the question I think you are
> asking.
> Assume you have to design an oil pump that will operate at -50F up to
> the operating temp of the engine, say 250F.
> That is a Delta T of 300F.
> Cof. of expansion for steel is .00000633
> Cof. of expansion for alum is .00001244
> 
> Most pumps I've seen are about 1" long, but to amplify the problem of
> expansion a little lets assume the rotor is a long rotor of 1 1/2".
> 
> Change in length of the steel rotors:  .00000633(300F)*1.5" = .00284
> Change in length of the alum housing: .00001244(300F)*1.5" = .00554
> 
> Subtract the two and the difference is .00275
> 
> If you build the pump for a clearance of a few tenths at -50 you will
> only have a couple of thou clearance at operating temp. Don't think this
> is really a problem in terms of the leakage thru this gap. Especially
> considering that this clearance will be split between both ends. That
> would be .0015 each end.
> 
> Just my .002 cents worth.
> Curt Holland
Here's  another 2/100th of a dollar's worth:

If the Ali is Lo-ex or similar, then the expansion rate difference
becomes vanishingly small,  so my money is definitely on the wear rate
being a significant factor.

A lot of companies used Hoborn-Eaton concentric rotor pumps over here,
especially the Mini and that range of engines. They used an Ali
housing with steel rotors. Concentric rotor pumps use a rotating
driven inner and a rotating driven outer, having a tooth form of
contact but with unequal numbers of teeth to keep wear down.

Bit like a very simplified Wankel in fact...

Peter
-- 
Peter A Forbes
Email: listerdiesel at gmail.com
Web: www.oldengine.org/members/diesel