The control sheave bearing takes a lot of abuse. It is the smallest bearing in the headstock, and rotates faster than the quill and drive sleeve bearings. Depending upon the speed set, it may be rotating faster than the motor bearings as well. Since it is tiny, the ball bearings inside a very small, too. Any rust or debris that gets inside is bad news. Its life also depends upon the state of the idler shaft. If the shaft the control sheave slides on is not kept lubed, extra strain is placed on the bearing when the speed is changed. The bearing can be pulled out of its socket but more often the retaining clip is pulled out and the rollpin can drop down into the motor pan.

If the bearing seizes up, the button will rotate on the speed control quadrant and damage that part. I've seen 3 different quadrants completely melted off/sawn through by this situation. Why an owner continued to use a machine making that much noise for that long escapes me, but I've seen it 3 times already. If the bearing seizes up, the retaining clip will be twisted off and the pollpin lost as well.

Below is how to change just the bearing. If there's any other damage to the control sheave, replace the whole thing. If you have replaced your idler shaft, you may need to change the control sheave as well. I've found some brand new idler shafts that were slightly oversized. You would think a used control sheave would fit but sometimes not.

Note: some early control sheaves had built-in bearings. If you can't press the bearing out without destroying it, you have this type. You'll have to replace the whole control sheave in this case.