When balancing an overhung belt driven fan that runs at motor speed is there an issue that would make it hard to balance to a G2.5 tolerance? I've got a fan shop that said it would be hard to do.

Hi Pmech,
Are we talking field balancing or shop balancing? A proper balancing machine should be equipped to balance the fan rotor separately.

In the field, just make sure the sensor is not oriented in the direction of the belt tension. Two-plane balancing may be required for some overhung fans.

This fan is being built off site and i would assume that the fan wheel would be balanced separate on a balancing stand. We plan to visit this shop and check the final results. Can you give me the formula for oz.-in./lb to find the acceptable vibration levels?

Very sorry for the late reply.

It is not an easy task but it can be done with a proper approach. You need not to have a balancing machine if the imeller is not two long. ( D/L ratio is high)

First balance the fan on two knife edges to a maximum. Then fix the fan on the impeller and check the vibration with a known mass and a vibrometer. Selecting the correct mass is to be done on experience depending on the impeller weight. If it could not do completely in the first step you may have to repeat the trial two three times. If the vibration is very large( even difficult to measure the vibration) speed reduction for the test would be a better idea.