Diagnosing a misaligned sheave is easy to do with vibration analysis. Note how the sheave moves in the axial direction, one cycle of vibration per rotation of the shaft (1x). Note the phase relationships. As the top of the sheave moves to the right, the bottom moves to the left – meaning they are 180 degrees out of phase. The part of the sheave closest to the viewer moves out of phase with the part furthest away. This movement doesn’t happen at all when the sheave is aligned as you can see in the sheave on the right side of the video.
If the machine is not running, one could of course place a dial indicator on the axial face of the sheave and rotate it by hand to observe the movement in the axial direction. The benefit of using vibration analysis is that we can detect the defect while the machine is running.
Below you can see the axial movement and phase relationships for a misaligned sheave from a different angle.
If you have ever played with a strobe light to freeze a shaft, you might consider that the axial movement of the sheave might be visible if you “freeze” it with a strobe light. Simply adjust the strobe light to match the rotational rate of the sheave you want to look at.
Misaligned sheave – Why is it important?
Now that you can see how the sheave moves each time it goes around, think about what this is doing to the bearings! In each rotation of the shaft there are now new axial forces on the bearing. These repetitive forces cause fatigue and fatigue will cause your bearings to wear out faster. A 20% increase in load on a bearing cuts its life in half!
Why get certified with Zenco?
These videos / animations are just a small taste of what we offer in our courses. We have much more to tell you about misaligned sheaves and other common machinery malfunctions. Our certification is done in accordance with ISO 18436-2. Check out our course schedule and sign up for a course online or in person with Alan Friedman today! https://www.zencovibrations.com/events/
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