Precession (mechanical)

Precession is the process of a round part in a round hole, rotating with respect to each other, wherein the inner part begins rolling around the circumference of the outer bore, in a direction opposite of rotation. This is caused by too much clearance between them and a radial force on the part that constantly changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force.[1]

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.

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. This tends to accelerate the wear process, possibly leading to spalling, galling, or false brinelling (fretting wear) of the contact surfaces.

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. This can cause threaded parts to either tighten or loosen under a load, depending on the direction of rotation, typically with a force that can far exceed the typical torque of a wrench. For example, this is a common problem in bicycle pedals, thus on nearly all bikes built after the 1930s, the left-side pedal is equipped with left-hand (backwards) threads, to prevent it from unscrewing itself while riding.[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. It is completely unrelated to torque-free and torque-induced precession.