vacuum ala home made

just today i made a vacuum from a little .5hp lathe motor and an ac
compressor off a 78 cougar (thats whats painted on its side).

its about 50 degrees in the garage and i cant get it to 'boil' water
unless i heat it to about 90degrees (F) (takes about 10 sec)...bubbles
up then rolls till i break suction.

im using a small mouth quart jar and my sore hand to hold the suction
(just testing)

can anyone tell me if it sounds like the compressor is not strong
enough?

the gauge reads 26-27" max, never goes higher even when i dont use the
jar and just stop the hole up.

On Fri, 03 Dec 2004 22:39:57 -0800, in ?? "walt" wrote:

just today i made a vacuum from a little .5hp lathe motor and an ac
compressor off a 78 cougar (thats whats painted on its side).

its about 50 degrees in the garage and i cant get it to 'boil' water
unless i heat it to about 90degrees (F) (takes about 10 sec)...bubbles
up then rolls till i break suction.

im using a small mouth quart jar and my sore hand to hold the suction
(just testing)

can anyone tell me if it sounds like the compressor is not strong
enough?

the gauge reads 26-27" max, never goes higher even when i dont use the
jar and just stop the hole up.

This is not strong enough by commercial standards, but it will still be a lot better
than nothing. If the investment is not boiling, you miss much of the de-airing
activity, since if the investment boils, that acts to also agitate and mix the
investment, helping bubbles to rise to the surface and not adhere to the model. Also,
the added mechanical mixing of that boiling action increases the strength and univormity
of the investment.

But at 26 -27 inches of vacuum, you ARE still causing small air bubbles to expand
greatly, which both tries to then float them off the model, and makes them more likely
to be disloged by any vibration or agitation. So if you combine the vac level you get
with gently banging your fist on the investing table to create some vibrations, you may
well get decent results, if not quite as consistant and bubble free as with a commercial
unit.

You might try adding a bit more oil to the compressor. Not sure, but it may help
increase the internal seals. Use a proper vacuum pump oil, as some oils are volatile
enough that they contribute gasses to the area you would want to be evacuated.

Peter

Walt,
What is the altitude of your location. I am at 7000 feet and can never
pull more than about 27 inches of vacuum.

Mark


walt wrote:
just today i made a vacuum from a little .5hp lathe motor and an ac
compressor off a 78 cougar (thats whats painted on its side).

its about 50 degrees in the garage and i cant get it to 'boil' water
unless i heat it to about 90degrees (F) (takes about 10 sec)...bubbles
up then rolls till i break suction.

im using a small mouth quart jar and my sore hand to hold the suction
(just testing)

can anyone tell me if it sounds like the compressor is not strong
enough?

the gauge reads 26-27" max, never goes higher even when i dont use the
jar and just stop the hole up.

nah my altitude is no more than 500 ft

In automotive school (25yrs ago), to power up whole A/C systems we used 5.0+
HP 3 phase capacitor start motors to power up test systems. Automotive
compressors are very large compared to home sized units, if there anything
less than ideal cylinder conditionyou've got problems.

Also, you mentioned you're using .5HP, but not the RPM, as I recall, minimum
compressor shaft rpm needs to 2500 to 3000 rpm:
The following formula should help:
(x/m) y = z
x = motor rpm
m = motor pulley diameter
y = driven (compresor) pulley diameter
z = driven (compresor) rpm

Or, as a simple estimate, a 1725RPM, would need a pulley 1/2 the diameter of
the compressors (and that over drive situation could cause tremendous
slippage problems). with a 3450RPM motor you'd need a same size pulley to
get an adequate compressor speed.

Also, depending on the above situations, compressor clutch may be slipping
if it's old or if it's being driven too fast, also what are you using to
power the compressor clutch? If it's under powered that also will cause low
efficiency.

I personally use a compressor from a small "suitcase" type A/C Compressor to
vacuum my "dome".
The over sized and under powered or slipping components combinations maybe
problematical.

Ed

Back in the early 90's I worked for a company that dealt in the CFC
handling business. We used a shop made vacuum pump made from an auto AC
system. It was made for a Japanese car. I think the name on the label
statred with an 'S' ,do not recall the rest. We ran it from a 1 1/2 HP
motor @ either 1750 or 3500 RPM (dont recall which). We could pull 30+
inches of Vac. When we started to lose Vac. we would take it apart to
flatten the vanes of the valve plate. The plate was kinda star shaped
and had a vane to seal each cylinder as required. We had to flatten the
vane to completely seal the opening, then reassemble and it would work
fine until the next time. This may be your problem. YMMV.

i took some measurements and found the following

1,725 x = motor rpm
2.25" m = motor pulley diameter
6.25" y = driven (compresor) pulley diameter
4,791? z = driven (compresor) rpm

the compressor clutch isnt slipping, its been welded.

you can see a picture of it here
http://www.burdettecamping.com/vcmxprmnt.htm

i dont think i am having any slipping problems between the two pulleys
because while the pump runs, the 'frequency' of its compressions dont
seem to change even when the circuit is closed and it starts drawing
vacuum. (the 'sound' of the compression does change its pitch as the
mercury goes up which makes sense if it is reaching its full capacity)

i found that after adding a little oil to the suction side of the pump
it will draw 27.5" which is better.

peter, from my other post you said to read about halfway down on that
article from marc robinson i did then went back to read the whole
thing. very entertaining.

from that article and from all the posts on both my questions i feel i
have a little better understanding of how to proceed with some
experiments.

thanks very much

Congratulations. I thought that my home made vacuum system was on the ugly
side but yours is winner hands down . Your obviously an intellegent
experimenter and obviously know what your doing and probably have already
checked for leaks. If not, your gas pipe manifold may be a source of leakage as
well as the stem of your shut off valve. My day job involves some work with
vacuum systems and the valve in your photo looks suspiciously like one that I
have encounted problems with in the past with leakage around the stem. It is
also possible that your inexpensive vacuum gauge is just reading incorrectly.
You can check it by using it on a know good system to see what it reads if you
have access to one.
Mike in Arkansas