[SEL] OT - expansion of metals in oil pumps

[Curt]

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
Gastonia, NC

Bill Dickerson wrote:

> 
>For the engineers - rob, etc.
>
>Engine oil pump - automotive gear type. Gears are steel (or some sort of
>steel product) housing is aluminum - timing chain cover with pump body in
>the timing cover.
>The gear to housing end clearance is supposed to be VERY close to get the
>pump to put out good pressure.
>A lot of folks say this is a weak design - that the aluminum will expand
>more and open clearances between the gears and the housing.
>They claim the fix is to use aluminum gears in the alum housing so they
>expand at the same rate, meaning a more accurate and tight pump, longer
>life, higher pressure, etc.
>
>Thoughts?
>
>Bill
>
>
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