Ed Anderson wrote:

 > Ed how did you deal with the belt side loads in the pump? The pump

on my Continental O-470 is driven off an accessory drive that has the

bearings

built in. The pump itself has no bearings to speak of.

The range of prop RPM should be the same in both a geared
Mazda and a direct drive AC engine. Unless of course you are cruising
the Mazda at way less than 50% power. I think something else was going
on with your AC pump failure situation. There is a guy from Ontario
Canada that is running a V6 in a pusher that uses a belt drive AC
vacuum pump. He was at OSH this year. He had to fabricate a bearing
housing for the AC vacuum pump. I have a picture of the pump
around here someplace.

Paul Lamar

Hi Paul,

    My fitting had a bearing pressed into it for the the drive shaft/pulley.
The metal spline shaft for the vacuum pump fit into the shaft, so there was
no direct side load on the vacuum pump drive itself, just the metal drive
shaft.

While I agree with your statement that the range of the prop rpm is the same
for both a geared Mazda and a direct drive AC engine, the engine rpm is of
course not.  Since  my vac pump  pulley was driven off the eccenteric shaft
pulley the engine rpm range was approx twice that of a direct drive AC
engine.  Now if you drive the Vac pump after the PSRU reduction then of
course the rpm is the same.  However, I believe that to be harder to do
unless the PSRU is designed for it from the git-go. My Vac pump fitting
simply bolted on with the air/smog pump block holes.

The vac pump used in the V6 that uses a belt drive may survive longer as the
rpm range probably does not extend to 6000.  Like I said, it worked to a
degree, but when I used larger pulley to lower rpm so it would survive at
the higher engine rpm it still (less than a year) reduce to graphite dust
with the vac slowly decreasing.  On the other end of the rpm spectrum, at
low engine rpm, the pump did not put out enough vac such as when you might
pull the power back for decent through a cloud layer (not the time to have
your AI lay over on you {:>).

The weight of the Mazda smog pump is the only real disadvantage that I have
seen.  It produces great vacuum at low and high rpm.  I have heard that they
can freeze up, but I have never had a smog pump on a car do that.  I have
had them wear out, but not freeze up.

Ed

Well Ed if you are pulling it back to 500 engine RPM your prop RPM would be far
lower than any AC engine prop RPM. The drag maybe, however desirable, would be 
far more than you could get out of an aircraft engine. 

Does your Ross Gearbox not shutter at less than 1500 engine RPM?  

You could pull it back to not less than 1800 or so and then the prop RPM range 
and vacuum pump RPM range  would be roughly the same as a direct drive aircraft
engine. 
I think there was something else going on in the failure of your aircraft vacuum pump 
set up. I am just trying to understand what it was. Perhaps a vibration of
some sort. Do you have a drawing of the bearing housing?

This is not to say I am against smog pumps used as vacuum pumps. I love low 
cost solutions like that.


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