If there is a noise coming from a freight door, and the sheave is moving but the chain is not, what is likely the problem?

The situation described indicates that the sheave (pulley) is in motion while the chain remains stationary. This scenario suggests a problem specifically related to the connection between the sheave and the chain.

If there is oil on the chain, it can lead to slippage between the chain and the toothed components that engage with it. Oil can cause a reduction in friction necessary for the chain to grip effectively on the sheave. When the sheave turns but the chain does not, it is a sign that the chain is not able to transfer the rotational motion effectively due to lubrication issues rather than mechanical failure or tightness.

While a broken sheave or a faulty motor could potentially lead to a similar situation, those issues would likely manifest differently, such as complete inoperability of both the sheave and chain or noticeable damage. A chain that is too tight could lead to other problems, but it would not typically result in the chain slipping in this manner. Thus, the presence of oil makes it the most likely cause of the problem where the sheave rotates without moving the chain.