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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. 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 Bushing may refer to: *Bushing (bearing), a type of plain bearing * Bushing (electrical), an insulated device that allows a conductor to pass through a grounded conducting barrier *Bushing (isolator), a mechanical device used to reduce vibrationa ...
, or between the races and rolling elements in
roller Roller may refer to: Birds *Roller, a bird of the family Coraciidae * Roller (pigeon), a domesticated breed or variety of pigeon Devices * Roller (agricultural tool), a non-powered tool for flattening ground * Road roller, a vehicle for compa ...
and
ball bearing A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races. The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this ...
s. Often a result of wear, inadequate lubrication (too little or too thin), or lack of
precision engineering Precision engineering is a subdiscipline of electrical engineering, software engineering, electronics engineering, mechanical engineering, and optical engineering concerned with designing machines, fixtures, and other structures that have excep ...
, 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 Spall are fragments of a material that are broken off a larger solid body. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ball ...
,
galling Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is cau ...
, or
false brinelling False brinelling is a bearing damage caused by fretting, with or without corrosion, that causes imprints that look similar to brinelling, but are caused by a different mechanism. False brinelling may occur in bearings which act under small oscill ...
(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. This precession is a process purely due to
contact force A contact force is any force that occurs as a result of two objects making contact with each other. Contact forces are ubiquitous and are responsible for most visible interactions between macroscopic collections of matter. Pushing a car or kicking ...
s 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.


Examples

Precession caused by fretting can cause fastenings under large torque loads to unscrew themselves.


Automobile lug nuts

Automobiles have also used left-threaded
lug nut A lug nut or wheel nut is a fastener, specifically a nut, used to secure a wheel on a vehicle. Typically, lug nuts are found on automobiles, trucks (lorries), and other large vehicles using rubber tires. Design A lug nut is a nut fastener ...
s on left-side wheels, but now commonly use tapered lug nuts, which do not precess.


Bicycle pedals

Bicycle pedal The pedal is the part of a bicycle that the rider pushes with their foot to propel the vehicle. It provides the connection between the cyclist's foot or shoe and the crank allowing the leg to turn the bottom bracket spindle and propel the bicycle ...
s are left-threaded on the left-hand crank so that precession tightens the pedal rather than loosening it. This may seem counter-intuitive, but the torque exerted due to the precession is several orders of magnitude greater than that caused by a jammed pedal bearing. For a pedal, a rotating load arises from downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle, opposite to the rotation of the pedal. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces." Shimano SPD axle units, which can be unscrewed from the pedal body for servicing, have a left-hand thread where the axle unit screws into the right-hand pedal; the opposite case to the pedal-crank interface. Otherwise precession of the pedal body around the axle would tend to unscrew one from the other.


Bicycle bottom brackets

English threaded bicycle
bottom bracket The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. It contains a spindle to which the crankset attaches, and the bearings that allow the spindle and crankset to rotate. The ch ...
s are left-threaded on the right-hand (usually drive) side into the bottom bracket shell. This is the opposite of pedals into cranks because the sense of the relative motion between the parts is opposite. (Italian and French threaded bottom brackets have right-hand threading on both sides.)


Bicycle sprockets

Splined sprockets precess against any lockring which is screwed into the freehub. Shimano uses a lockring with detents to hold cassette sprockets in place, and this resists precession.
Sturmey-Archer Sturmey-Archer was a manufacturing company originally from Nottingham, England. It primarily produced bicycle hub gears, brakes and a great many other sundry bicycle components, most prominently during their heyday as a subsidiary of the Raleigh ...
once used 12-splined sprockets for 2- and 3-speed racing hubs, and these were secured with a left-threaded lockring for the same reason. (
Fixed gear bicycles A fixed-gear bicycle (or fixed-wheel bicycle, commonly known in some places as a fixie) is a bicycle that has a drivetrain with no freewheel mechanism. The freewheel was developed early in the history of bicycle design but the fixed-gear bicyc ...
also use a left-threaded lockring but this is not because of precession; it is merely to ensure that the lockring tends to tighten, should the sprocket begin to unscrew.)


Bearings in manual transmissions

A bearing supported gear in a manual transmission rotates synchronously with its shaft due to the dog-gear engagement. In this case, the small diametrical clearance in the bearing will induce precession of the roller group relative to the gear mitigating any fretting that occurs if the same bearing rollers always push against the same spot on the gear. Typically the 4th and 5th gears will have precession inducing features, while 1st through 3rd gears might not since cars spend less time in those gears. Transmission failure due to lack of precession is possible in gear boxes when low gears are engaged for long periods of time.


See also

*
Fretting Fretting refers to wear and sometimes corrosion damage of loaded surfaces in contact while they encounter small oscillatory movements tangential to the surface. Fretting is caused by adhesion of contact surface asperities, which are subsequent ...


References

{{Reflist Mechanics Precession