Air Motor Timing

Below is my response to a posting in the Mechanical Music Digest which asked about setting the timing of an air motor.

Most men (and some women) understand why a spark plug is fired prior to top-dead-center (TDC). However, in an air motor the vacuum must be applied to each bellow shortly after the camshaft lobe for each bellow passes TDC.

The further past TDC that the vacuum is applied, the lower the torque will be. If the vacuum is applied before TDC, the air motor will have a noticable jerky nature.

The easiest way to insure that the vacuum is applied shortly after TDC is to apply a medium level of vacuum (15") to the motor and then hold the camshaft with your hand, preventing it from turning. Then, allow the camshaft to turn very slowly and watch each bellow as its associated slider valve moves over its associated vacuum port. As the cam lobe for each bellow passes TDC you can see the bellows cloth react to the applied vacuum. Then adjust the slider (up or down) so that the bellow 'pulls' the camshaft 'forward'. Then, allow the camshaft to move 180 degrees and check to make sure that the atmosphere is allowed to enter the bellow before the next bellow starts 'pulling' the camshaft 'forward'

Another thing to bear in mind is that if you get the 'start' point of each bellow too close to TDC, the felt bearings in the connecting rod will wear out much faster.

I realize that the term 'shortly' is not very precise. Frankly, I've never attempted to measure the actual number of degrees past TDC that produces optimum performance. I imagine there is such a number, but I've never seen it in print in any reference material. Most technical literature relates the optimum position of the slider to the physical location of the vacuum supply port, which varies from manufacturer to manufacturer. With a little bit of trial and error, you'll find the optimum timing.

I do know that a well timed (and well rebuilt) air motor will operate smoothly with the main vacuum hose moved away from the supply flange as much as 3/4", if the supply vacuum is about 20".

Musically,

John A. Tuttle