Then, the focus ring on the 100mm F2.8 macro began slipping after 15 years. The ultrasonic motor also began slipping. The problem is the manual focus ring & the ultrasonic motor transfer motion to the true focus ring through 3 roller bearings. The roller bearings gear down the motion, allowing more precise focusing than if the ultrasonic motor & manual focus ring were directly connected to the true focus ring.

The roller bearings have to grip the manual focus ring & ultrasonic motor to turn the true focus ring. The true focus ring has to have enough friction to make the manual focus ring sticky but not too much friction to keep from getting turned by the manual focus ring.

Either the manual focus ring & ultrasonic motor lost their friction over 15 years or the true focus ring gained friction. After cleaning the ultrasonic motor & manual focus ring & relubricating the true focus ring, the manual focus ring still slipped. The ultrasonic motor was slightly more reliable. The true focus ring still takes a lot of force to turn.

That leaves trying different greases to try loosening the true focus ring. The true focus ring has 2 races comprised of plastic rubbing on plastic. It's a very hard part to reach. Designing for manufacturability was the exact opposite of what Japanese strove for, 30 years ago.

Canon is shifting all production to R lenses while converting all its EF lenses to boutique L models. The 100mm F2.8 macro is no longer made. The lion kingdom recommends the 100mm F2 with an extension tube for macros, but it too is no longer made. The extension tube is a lot more practical than the full macro lens.

Managed to get it focusing at least better than it was. The key was cleaning the roller bearings themselves, the roller bearing shafts, then lubricating a race attached to the roller bearings. Lubrication got all over a slip ring, but it still worked. Stripped a few screws & tweeked a few flat flex cables.

It'll probably go another 15 years before failing again. Then, it'll be 30 years old. It'll require buying a used lens as a parts donor or just have to be retired.