I’ve talked about them in the past. I’ve talked about special tools for adjusting them, but what is the big deal about end shakes?
End shake refers to the amount of play that a wheel has to move laterally along its axis between the jewels which support it. In modern watches this is typically supposed to be somewhere between 0.005mm and 0.06mm depending on the part and the specifications of the watch. As a general rule the end shake needs to be large enough to allow the wheel to turn freely and small enough to keep it from coming out of the jewel, disengaging with the adjacent pinions and wheels, or contacting anything it should not.
The higher quality the watch, or the thinner the watch the more important end shakes are. Too much end shake will bring a watch to a halt (should the gear contact something or disengage) but the end shakes also affect the accuracy of the watch. In the case of the wheel pictured before (those of you familiar with the inside of a Rolex will recognize it instantly) the more end shake there is the more the red wheel is free to tilt. In this case the end shake was large enough to allow the teeth of the wheel to be ground down quite severely.
At this point the power reserve had been decreased to 36 hours but of course there were a lot of aluminum shavings inside the watch. The watch wasn’t going to run much longer at its rate of decline. (This represents about 8 months of damage).