Diagnosing an eccentric sheave with vibration analysis is easy to do. But you have to be careful not to mistake it for other common machine faults. For example, unbalance also shows up at 1x in the radial directions. Fortunately, you can always stop the machine, mount a dial indicator and measure the run out of the sheave directly.
Animations make learning easier!
As the eccentric sheave rotates around you can see how it wants to move in the radial directions (vertical and horizontal). It makes one complete cycle of movement with each rotation. The vibration forcing frequency is 1x, or the shaft rate, in the radial directions.
The belt tension changes things however…
…and what we measure in real life are higher levels of vibration in the direction of the belt tension. In each rotation of the eccentric sheave, it pulls the other sheave towards it. We will therefore also be able to measure the vibration on both the input and output bearings in the direction of belt tension.
When you look at the animation without the belt you would also expect the phase relationship between the vertical and the horizontal to be 90 degrees. The belt tension changes this too. This is one of the keys to diagnosing this fault correctly. The phase between the direction of belt tension and 90 degrees away from the belt tension will NOT be 90 degrees as expected but closer to 0 or 180 degrees. This is something that can be difficult to wrap your head around but if you take one of my courses I will try to explain why this happens!
As mentioned earlier, an eccentric sheave can be confused with unbalance. Both result in elevated 1x vibration. Although phase analysis might help you distinguish between the two, it is not always reliable. If the vibration levels are excessive and the problem needs to be resolved, the best approach might be to simply shut the machine down and measure the sheave runout with a dial indicator. You might also use a strobe light to “freeze” the sheave. You might actually see it moving in the direction of the belt tension.
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Here’s a link to a Youtube video ( https://www.youtube.com/watch?v=xrRZabcwFuY ) that describes these concepts in one short video. Please feel free to share it on social media and subscribe to my channel ( https://www.youtube.com/channel/UCSWbScnX0Bnb6iXbvtWAOSA ) so you’ll get notified when I add new content.
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