Which way should pump rotate on a 1938 Scaiffe compressor?
Bill: I've had conflicting responses to my question about which way a specific pump should be rotated by a (replacement) motor. The original post/question is at
...and the responses don't agree. I added my own comment in response to a question about whether or not the spokes of the pulley are airfoil-like in cross-section (they're not), but it didn't seem to clarify the situation. Any suggestions?
My original response indicated that, as long as no ancillary operations were impacted, the pumping process doesn't care what direction the crank rotates, as both directions impart a reciprocating motion to the piston, and that reciprocating motion pulls in air and then drives it into the tank.
Commenters have advised that often the motor has a fan to aid in cooling (TEFC type fans) and that in order for that fan to work, the motor shaft must turn in the direction that pulls, or pushes, air over the motor.
I don't believe that a compressor with no fan on the motor shaft, and no method of cooling the motor or pump with a fan attached to the motor shaft, will be negatively affected by a motor that rotates one way or the other.
If any other actions are created by the rotation of the motor and the subsequent operation of the pump, driven by that motor, then, yes, the direction is important.
In the photo in the original post, looking directly at it, I would say that this sheave is designed to rotate in a CCW direction, as the "blades" on the sheave appear to be oriented to drive air across the pump and help to cool it as the sheave rotates.
Therefore, I believe that the air compressor motor must be oriented so that the motor shaft, when turning the motor sheave, will drive the pump sheave in a CCW direction.