Re: I thought friction was friction. Are there varieties?

Yes Roger, friction is friction, yet it does take several forms. However, your book on clocks refers to a frictional effect, one that results from geometry. This frictional effect is called positive mechanical feedback. Your book calls it engaging friction.

If you look at my sketch, a block (with an angled beam) is supported on rollers. As this block moves, as indicated, the beam produces a frictional drag that tries to incline it more. Such bending increases the load on the beam, so the drag force increases. Eventually, the end of the beam may slip. If so, elastic energy will cause it to overshoot the neutral position—then start another cycle. A non-linear vibration results. It is sometimes called ‘stick-slip,’ but is more properly called sprag-slip.

If the direction of motion is reversed, friction tends to bend the beam away from the rubbing surface, thereby decreasing its load. This negative mechanical feedback (what your book called disengaging friction) stabilizes the frictional force.

One-way clutches use mechanical feedback, positive to lock in one direction, negative for free motion in the other. Some squeaky door hinges behave the same way, but are noisy in one direction and quiet in the other.

Clock escapements use negative mechanical feedback to avoid chatter and to minimize the effects of friction on the accuracy of the timepiece. This design concept, and a good lubricant, helps make clocks both accurate and durable.