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Air Motor Timing Part 1
How to time a player piano air motor

Hi All,

Adjusting a wind motor for optimum performance is actually pretty easy as long as you understand the theory of operation. Basically put, vacuum should be applied to each bellows just after it starts to close. If the vacuum is applied too soon, i.e. when a bellows is fully open, the motor will not run smoothly. If the vacuum is applied too late, i.e., when the bellows is too far collapsed, the motor won't have as much power (or torque). The 'trick' is finding a happy medium.

Unfortunately, due to the design of the sliding valves, it is impossible to 'see' the point (or line) that the valve must cross before vacuum enters a bellow. For that reason, and others, one of the easiest ways to 'time' (or adjust) each valve is to apply a relatively low level of vacuum (5-7 inches) and watch each bellows carefully as the crank shaft is allowed to rotate slowly.

NOTE: Many manufacturers state that the sliding valve is correctly adjusted when the vacuum supply hole that leads to the bellows is completely exposed and the lobe on the crank shaft is at its highest, or uppermost point. While this is a good general rule and a good starting point, it is not necessarily the optimum point for every air motor. In fact, in many cases, adjusting the sliding valve is this position will end up applying vacuum to the bellows too soon. So, try the following method.

After applying the vacuum, prevent the motor from turning by holding the crank shaft still. Then allow the crank shaft to rotate very slowly and keep an eye on the bellows. As each bellows enters its 'power stroke', or the point where it begins to pull closed, you will notice the cloth go from a relaxed state to a 'sucked inwards' state. This is evidence that vacuum has been applied. Note the relative position of the corresponding lobe on the crankshaft. It must be past the point where the bellows is open as far as it can open. Using whichever adjusting facility was provided by the manufacturer, adjust the sliding valves up or down until each of the bellows starts 'pulling closed' at the same relative point.

The above technique for timing an air motor does not rely on the every part of the motor being in "perfect" condition. In fact, it allows for minor errors in both construction and condition of the parts involved. In my opinion, it is a practical method which insures that the motor will work as good as it can.

As it relates to motor timing, a sliding valve is likely to 'stick' or 'hang' if vacuum is applied to a bellows too soon. I refer to the condition as 'chugging' or 'jerking'. What happens is that the power from one bellows is being robbed to force the next bellows over TDC. If the timing of one valve is far enough 'off', it could cause the motor to stop momentarily when the motor is under a heavy load. For that reason, I always test the motor under a heavy load after it's been timed. If the valves have been set too close to TDC, the motor will 'chug'.

It should also be mentioned that if leather nuts are used to adjust the location of the sliding valve, they should not compress the felt washers which are between the nuts and the wood. While they should be fairly snug, or tight enough to remove lost motion, compressing the felt washers will make it more difficult for the sliding valve to seat on the block of the motor.


John A Tuttle
Brick, New Jersey, USA

In closing, while there is, no doubt, some mathematical formula for timing an air motor perfectly, the practical approach takes into account the fact that each motor was hand made and therefore imperfect. It also takes into account the possibility that the sliding valves might not be perfectly seated to the block.

Here's a little Post Script:

It has always interested me that the four-bellows air motor works. What it indicates is that the power stroke for each bellows exceeds 90 degrees of rotation. If it didn't, there would be 'dead spots'. Since that is true, it seems only logical that the power strokes in a five-bellows motor overlap even more. However upon close inspection, it is found that the percentage of overlap depends on the length, or height of the crank shaft lobe. Therefore, owing to the fact that each manufacturer selected a lobe height that they felt was 'the best', there can be no one hard and fast rule concerning optimum air motor timing.

It's easy to say that the sliding valves should all be positioned such that the air motor runs smoothly under various loads, but that does not necessarily mean that the motor is timed for optimum performance. Given the variables, how can one determine if a motor is timed for optimum performance?

We start by understanding that the greatest amount of power a bellows can develop begins when vacuum is first applied to the (open) bellow. As the bellows collapses, power decreases, and it continues to decrease until the bellows is fully collapsed. However, in an air motor, the bellows should never open or close 100%. In fact, the bellows should be about 15% collapsed at the most open point and about 85% collapsed at their most closed point. It is within this 70% area that the bellows have the least amount of change in terms of their power. To even out the power that the bellows develops even more, the sliding valve opens the vacuum port gradually, not suddenly. And, at just about the same time as one bellows is reaching its maximum power, the next bellows in the sequence begins its power stroke. Because of these variables and the inability to actually measure the amount of torque that the motor is producing, the best way to determine that the motor is tuned for optimum performance is to continue to reduce the amount of air that's used to power the motor while keeping the vacuum level constant. This is accomplished by pinching off the supply hose with a C-clamp or flat clamp. Then, by trial-and-error, the sliding valves can be adjusted for optimum performance.

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This page was last revised February 5, 2013 by John A. Tuttle, who Assumes No Liability
For The Accuracy or Validity of the Statements and/or Opinions
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