What other causes could there be for a broken motor shaft on a belt drive system that has only been in service for 20 hrs. Other than belt tension.

Torsional resonance

Walt

Residual stress due to NO curvature radius in the part of the shaft where there is change of diameter. Can you show a photo of the failure?
Miguel Kovac

I have seen shaft failure when the keyway was repaired by welding. The break showed an obvious change in the metal grain on one side of the key. The motor had recently been rebuilt by a local motor repair shop.

Was this a new or rebuilt motor? How far out on the shaft was the pulley mounted? A manufacturing flaw or inclusion in the material.
Was it rubbing on a guard?

Lee

This was a rebuilt motor I do not have any photos of the failure. The motor has now been sent back to the repair shop and he is supposed to inform me of the condition of the failure. Is there any way to detect if the shaft was cracked with vibration analysis.

When you stated that motor had been in service for only 20-hr, you implied that it was new. Obviously a lot more hours of fatigue stress could have accumulated as well as the other issues that were mentioned. Was heating involved for sheave installation/removal? Was shaft key of the correct size? Was the sheave loose on the shaft? Were the sheaves well aligned? Is there a history of bearing or Vee belt failures? Shaft cracks are difficult to detect with machine in service, and now it is too late.

Walt

There was no heat applied to install or remove the current sheave. This motor was currently running a timing belt type belt. It had recently came back from the repair shop where the bearings had been replaced. The bearing problem was detected and repaired before any Downtime occured. The key sizes and sheave alignment were correct and there was no looseness in sheave or bushing. In the past we ran a 15 groove power band on this motor and that sheave was always removed with heat and the belts were always over tensioned to prevent slippage.

WSEGRAV
So by your answers you have in fact given yourself some of the information that you asked for. The heating and cooling which is usually accompanied by a little hammering could be a cause, which may be amplified by the cogged belt which has its own built in vibration from the start could also have attributed to the failure. ALso as Walt stated by the factor of age alone and the overtensioning that took place over what ever time period is also a big contributor.Root Cause Failure Analysis is a lot of questions but like Walt said the information you supply in the first place must be as accurate and truthful as one can possibly be otherwise an assumption made on the existing info could lead one down the garden path and end up somewhere not related to the issue at hand.