[1] In a rotating machine, such as motor, engine, gear train, etc., precession can occur when too much clearance exists between a shaft and a bushing, or between the races and rolling elements in roller and ball bearings.
Often a result of wear, inadequate lubrication (too little or too thin), or lack of precision engineering, such precession is usually accompanied by excess vibration and an audible rubbing or buzzing noise.
In stationary parts on a rotating object, such as a bolt threaded into a hole, because the sideways, or radial, load constantly shifts position during use, this lateral force translates into a rolling force that moves opposite to the direction of rotation.
[1] This precession is a process purely due to contact forces and does not depend on inertia and is not inversely proportional to spin rate.
Shimano uses a lockring with detents to hold cassette sprockets in place, and this resists precession.
A bearing supported gear in a manual transmission rotates synchronously with its shaft due to the dog-gear engagement.
Mechanical precession is the process of a round part (in blue) in a round hole (in red) rolling in the direction opposite to the rotational direction of the applied radial force. (The applied radial force is depicted by the green arrow. The arrow's counterclockwise rotation depicts the direction of precession, while the direction of rotation is shown by the clockwise rotation of the blue square. The center of the blue square is traversing counterclockwise along a small circle, the orbit, of diameter equal to the difference of the diameters of the red circle and the blue circle, even though the blue square rotates clockwise). If the blue circle has a diameter
d
and the red circle a diameter
d + δ.
The instant the green arrow is pointing downwards, the blue circle is pressed against the red circle at the bottom (point
A
on the blue circle). The force rotating counterclockwise causes the blue circle to roll around the red circle clockwise. When it has rolled a distance
πd
, the circumference of the blue circle, point
A
again touches the red circle. Since the circumference of the red circle is π(
d
+ δ), point
A
touches the red circle a distance πδ clockwise from the bottom.
