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MMD > Archives > November 2000 > 2000.11.22 > 07Prev  Next

Adjusting Air Motor Valve Timing
By Craig Brougher

Gerry Ceccarini was asking if anyone has a method for timing a Standard
Air Motor, and Robbie wrote to me, asking if I would answer this one.
To date, my new address has not yet been posted back to the MMD mailing
list, so I was not aware that this newsletter was still operating.  At
any rate, I have a method, but it won't work if there is something
wrong, so let's first go through a checklist.

Let's say that everything in the air motor is perfect.  That means:

1.  When the crank runs, the center doesn't wobble even faintly (it's
not bent).

2.  The 5 bellows are each perfectly tight and open to a span slightly
greater than the crank throw so the cloth doesn't tighten when fully

3.  Each bellows is perfectly limber and isn't different from its
neighbor (not hinge-bound).

4.  The glue joint at the manifold body is perfectly tight.

5.  The slides and sliders are sanded true, flat, and without air
seepage (sometimes also caused by old cloth covering), and the slider
guides are close but don't bind.

6.  As the sliders move, they do not twist or lift off the slide
(as is sometimes caused by oversize leather nuts binding between the
pallet and the stem).

7.  All moving centers are free but not loose and sloppy.

These are the most likely problems, and of the list I would say #3 is
the most common.  To test for #3, remove the crank arm from the bellows
and very gently try to feel resistance.  The Standard Player Action
motor is so sensitive when properly restored that it can actually
operate slowly through a #70 brass bleed cup.

Now, to adjust such a motor, you gently suck on the supply, keeping
a constant friction on the crank with your fingers as it turns.  As
certain bellows just reach TDC (top dead center) ready to go over
center, you will notice that there is a weakness or hesitation,
compared to others.  That is caused by a pallet which has not quite
covered its exhaust hole.  So raise the pallet with its leather nuts
to cover the hole when it's at it's highest point.

You will notice that any air motor must be timed, and some motors use
just a little more advance than others do.  To make a smooth-running
reciprocating motor like this, each pallet must be advanced just a
hair.  Now if you really want to see what you're doing, don't cover the
pallets until last.  Use clear packing tape.  That way, you can see
when the inside edge of the pallet top is even with the bottom of the
exhaust hole.

So the first adjustment is just to set them all mechanically at this
point by eyeball.  Then put the motor on a constant source through a
restriction, at very low pressure -- like, say, 3" or less -- and at
some point it will start to lope.  The lope will be the open bellows
which is weakest to get over TDC.  Keep advancing its pallet upward
until you even out the rotation.

Craig Brougher

(Message sent Wed 22 Nov 2000, 14:46:21 GMT, from time zone GMT-0800.)

Key Words in Subject:  Adjusting, Air, Motor, Timing, Valve

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