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Rebuilt vacuum pump blues

I have an old vacuum caster unit. I bought it in the late 70’s, had
it stored for 25 years, put it into service in my studio, but never
had it serviced. I finally broke down recently and sent for a
re-build kit for our Welch vacuum caster pump. My tech guy did a
terrific job replacing all the gaskets, scraping off the years of
gunk inside, and generally put it back together as good as new. We
checked all the hoses, connections, etc., and the bell jar and pad
seem to make a tight seal. We thought we were pump gods until we
turned the machine on.

Unfortunately, the gauge now shows that there is absolutely NO
improvement in suction pressure! The gauge shows just over 26 (
pounds, inches, or what?) pressure, not quite enough to evacuate all
the bubbles in our investment mixes.

We’re out of ideas here, short of bending the needle to show more
pressure than the machine really has.

Anybody got any ideas for us to get more vacuum pressure, short of
buying a brand new unit?

Jay Whaley


Could be a leak in your system, or the pump was not rebuilt
correctly. Give me a call, I just rebuilt 2 welch pumps last year
and have learned a few things to look for.

James Binnion
James Binnion Metal Arts

Jay, how far above sea level are you? This has something to do with
how much vacuum you can pull. I know that vacuum decreases as you get
to a higher elevation, but I’m not sure how much…Teddy

Don’t take life too seriously…you’ll never live through it.

Hi Jay,

The gauge shows just over 26 

Just make sure 26 is not the maximum at your altitude. If the gauge
is accurate you may not be able to do more that 26 or so above about
3800 feet.

The Jewelry Equipment Dr.

Hi Jay,

Those old Welch rotary pumps are great. (I have several) but they’re
really twitchy about the alignment of the rotor in the core when
you put the case back together. Welch has some sort of a jig that
they use when they do it, as I understand it. It’s got something to
do with the spacing between the rotor and the case walls. Which
particular pump to you have? (what’s the number?) The old casting
units tended to use 1405’s, IIRC.

The ‘26’ number you’re getting is in inches of mercury. (it’s just
what most non-scientific vacuum gages read in. For reasons that
would take half a book to explain.) You need 28-29 or so for
investing. (Anything beyond 29.5 is hard vacuum at sea level. (more
or less))

Some rebuilders coat the case seam with caulk or something to seal
it. You might try that. Set up your gage on a blind stub inlet hose,
to make sure you’re not measuring a leak from somewhere else in the
system. You also want to open the ballast valve. (the silver screw
knob on top of the oil case, opposite the inlet port.)

If you end up looking for a new pump, I’ve got a couple of huge old
ones I could bring down when I do the workshop. We can discuss it
offline if it turns out to be relevant.


Anybody got any ideas for us to get more vacuum pressure, short of
buying a brand new unit? 

Did you call Welsh or whomever you bought the rebuild kit from and
ask THEM for ideas? I’d bet they’ve got someone there who knows
something to check that isn’t covered in your rebuild kit…


Dear Jay,

I did the same thing a few years ago with no luck.

My conclusion was that the gaskets need to be factory installed.

The factory used a wrench that you can measure the torque on to
tighten the bolts. Just like an auto gasket wrench is used for
tightening valves in car motors. They use a certain pound of pressure
they can measure. This allows the seal to be uniform under vacuum

This was probably fifteen to twenty years ago, so I could be all
smoke here, but that was my conclusion. I bought a new pump and still
use it.

I’m not sure if Welch would give you the technique that
would confirm my comments.

If you are getting 26 inches you are very close. You might just try
tightening the bolts down a bit harder. You have nothing to lose.
Again, my opinion is the gasket or gaskets need to be tighter.

Best luck,

Todd Hawkinson
Southeast Technical College

Maybe the gauge itself isn’t working? (Just an idea from a totally
inexperienced caster…)


Can you run the pump without it being connected to anything other
than the gauge? If so let it run for about an hour and see what
pressure you have. If the pump has a ballast or outgas valve, open
that and run it a further half hour and then close it and see what
pressure you have again. IF OK, Then connect to your chamber and see
what you get after a few minutes. Any difference and you have to
replace or cleam any seals between te pump and the chamber. Look for
any backstreamed oil or moisture as this will outgas. You will
probaby have a problem with a small leak at a sealing point somewhere
and restripping the pipework, cleaning and trying again wil probably
cure if the pump purging worked OK

Nick Royall

Hi Jay,

What is your altitude?

I am at 5000 ft and 26 inches of vacuum is just about as good as I
can get. If you are at sea level then I think it should be closer to
32 inches of vacuum.

If your are near sea level then you may have leaks in the system.
You may need to vacuum for a longer time period, but if it is coming
to a full boil then it is working.

Good luck,
Ken Moore

Hi Jay,

My old pump shows only 26 lbs of vacuum too. But it works just fine.
Put a beaker of water in the chamber, apply the vacuum and if it
starts to boil in thirty to forty five seconds there’s no problem,
it’s working just fine, regardless of what the gauge shows. If not,
my guess is that the problem will likely be in the plumbing
somewhere. Check all of the fittings as well as the hoses, the
o-rings and seals in the valve might be allowing just enough air to
keep it from reaching full pull but not make any noise or show any
indication of a leak.

It doesn’t take much of a leak to really mess them up. I’m no vacuum
god, but my experience is that the pump will either work and pull a
good vacuum or it won’t pull anything at all. A partial or slow pull
usually means a leak and a machine that’s been stored since Disco was
cool probably has some hardening of the arteries.

Dave Phelps

Those old Welch rotary pumps are great. (I have several) but
they're *really* twitchy about the alignment of the rotor in the
core when you put the case back together. 

It is twitchy but a feeler gage and the the right info and it not
hard to do. It is just hard to understand the description without
looking at the dissasembled pump.

James Binnion
James Binnion Metal Arts


Take it apart and put it back together checking everything. Consider
your altitude, do a boil test, and if all else fails adjust your
attitude and bend the bloody needle. Gauges are often rather crude
devices :slight_smile:

Demand Designs
Analog/Digital Modelling & Goldsmithing

I don’t know if this has been mentioned before but check your line to
your table, it could be clogged or getting clogged with investment.

Good luck,

Thanks for your info., Jim, Brian, Peter and others!

From all the differing advice ( all excellent ) I have absolutely no
idea what my vacuum pump problem might be. My studio is in San Diego,
so I am maybe a few hundred feet above sea level. I can’t get a cup
of water to boil under my bell jar, and I think the machine parts and
all new gaskets went in right. Perhaps the rotating vanes were
miss-installed, who knows.

The strange thing is the performance is not better, not worse. The
hoses could be leaking, I understand, so maybe we will look there
next. Maybe all that sludge inside the pump was a feature of the
design, intended to make the pump airtight? If all else fails, perhaps
I will look to Brian for a used pump to replace my disco-era model.

So you’re saying Disco is NOT cool anymore? The SNAG Conference in
Seattle is featuring a 70’s theme dance in May, so perhaps then we
will see what is cool and what is not…

Jay Whaley

Hi Jay,

That sounds suspiciously like you’ve got a leak in your lines
somewhere. The crud in the case isn’t part of the operating system.
It’s just crud. (Investment dust+pump oil most likely, slowly lapping
your rotor vanes down to nothing…)

A good check would be to dismount the vacuum gage from where ever it
is, and then rig a stub line from your vacuum pump to nowhere but
the gage.

Just take some hose, put it on the inlet, (with hoseclamps, nach)
and then rig the vacuum gage onto a simple stub of whatever plumbing
pipe will fit your hose, and hoseclamp that into the end of the hose
from the pump. The idea is to see what the pump does with nothing
else involved. If it still won’t hit 29, you’ve got a problem with
the pump. If it does, then it’s time to start chasing leaks through
the plumbing of your investment chamber. One trick I’ve used with
success with bell jars that wouldn’t get a decent seal (usually due
to an old rubber mat) was to take a sponge, get it soaking wet, and
then sponge water around the outside bottom of the jar while it’s
first pumping down. The vacuum pulls the water under the jar edge,
and that helps seal it.


Since you are not seeing any significant difference in performance
from before the pump was rebuilt the most likely problem is that you
have a leak in the plumbing of the system not the pump. As for the
vanes and gaskets. The vanes if they were replaced are not
directional yet, they will wear in after a while but the original
sealing surface is not directional. Most of the gaskets in those
pumps are not for a vacuum seal but just to keep the oil in the case
so leaks in those would show oil coming out. I think you need to
look carefully at the plumbing between the pump and the investing
table. That is such a large leak you should be able to hear it with
the pump turned off. Borrow a stethoscope and pull as good a vacuum
as you can then shut the pump off and listen for the leak.

James Binnion
James Binnion Metal Arts