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MMD > Archives > February 2006 > 2006.02.15 > 11Prev  Next


Repairing Air Motor Leakage
By John A. Tuttle

Hi All,  I'm only writing this posting because I've encountered a few
seriously bad air motors in the past year and all of them had been
"rebuilt".  While the cloth was in excellent shape, they all leaked
like a sieve.

Also, the task of selling the repair job to the customer was
complicated by the fact that the motors 'seemed' to work pretty good.
In fact, I was initially somewhat misled by the performance of the
player, and thought that the main problem was the valves.  However,
I noted that when I was rewinding the roll, I still had to pump rather
quickly, which was unusual and prompted me to dig a little deeper.

In every case, the problem was both internal channel leakage and valve
leakage.  If I were to guess at percentages, I'd say that 40% of the
leakage was in the vacuum supply channel and 60% was in the sliding
valves.

Fixing the internal channel leaks was basically a snap.  I simply
sealed the holes in the block with duct tape, poured Phenoseal into the
channel until it was full, and poured out the excess.  Then I removed
the tape and allowed it to dry overnight.  The next morning, I tested
it again, and it was still leaking a small amount.  So, I taped it up
and sealed it again.

In one case, I had to re-seal the channel three times before it
was air-tight.  The block was severely damaged when the motor was
previously dismantled for rebuilding, and no effort was made to
ensure that it was air-tight when it was put back together.  Or, in
defense of the rebuilder, perhaps it was air-tight when the glue was
still pretty fresh, then, as the glue dried fully, it started leaking.
I try to give people the benefit of the doubt.  (See Note at end.)

By the way, here's a little tip that could save you some frustration
and/or embarrassment.  When you pour the Phenoseal in for the first
time, make sure the motor is facing 'bellows up'.  That way, if there
are cracks or leaks in and/or around the bellows, the sealer won't
get into or on the bellows.  Also, it's wise to pour in a relatively
small amount of sealer at first to see what happens.  In one case,
the leakage was so severe that the sealer seemed like it was leaking
out from everywhere, and I had a bit of a mess to clean up.  Live
and Learn...

In closing, the improvement in overall system performance that was
achieved by fixing the air motors was so marked that it was deemed
unnecessary to do any other repair work, the point being that it's
well worth the time spent to eliminate the motor as a possible problem
before performing other more involved tests.  Simply cap off or plug up
the hose leading to the air motor and check the pumping tension with
the motor out of the system.

Note: Something else came to mind while writing this posting which
might help explain the valve leakage.  I reviewed the pictures I took
while sanding the blocks and found one thing in common: all of them
were, for lack of a better term, 'warped'.  Looking from left to right
across the face of the block, they were all 'low' in the center.

This makes me wonder if the 'warpage' problem might have something to
do with re-surfacing the block before the bellows were mounted.  It was
fairly obvious that the blocks were all re-surfaced when the motor was
initially rebuilt, and it seemed odd that they were all warped the same
way.  Hmmm.  However, if the problems (leakage and warpage) resulted
from the glue fully drying, wouldn't the block be warped 'outward'
instead of 'inward'?

Musically,
John A Tuttle
Player-Care.com
Brick, New Jersey, USA


(Message sent Wed 15 Feb 2006, 13:57:42 GMT, from time zone GMT-0500.)

Key Words in Subject:  Air, Leakage, Motor, Repairing

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