Precession (mechanical)
Encyclopedia
Precession is the process of a round part in a round hole rotating with respect to that hole because of clearance
between them and a radial force on the part that changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force. Fretting
between the part and the hole is often a result of this motion. "In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other."
"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. 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."
This precession is a process purely due to contact force
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
.
s on left-side wheels, but now commonly use tapered lug nuts, which do not precess.
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. Jobst Brandt, an engineer (author of "The Bicycle Wheel") and cyclist, has suggested a tapered seat for bicycle pedals similar to lug nuts to prevent the damage that fretting does to soft aluminium crankarms, but no major manufacturer has taken up the idea (probably because it requires compatible pedals and crankarms, and thus a new standard).
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.)
Clearance
A clearance can refer to:* in chess, A positional move, where a player moves a piece occupying a certain square away, replacing it with an allied piece that will strengthen the player's position....
between them and a radial force on the part that changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force. Fretting
Fretting
Fretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...
between the part and the hole is often a result of this motion. "In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other."
"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. 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."
This precession is a process purely due to contact force
Contact force
In physics, a contact force is a force that acts at the point of contact between two objects, in contrast to body forces. Contact forces are described by Newton's laws of motion, as with all other forces in dynamics....
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
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
.
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 nutLug nut
A lug nut is a fastener, specifically a nut, used to secure a wheel on a vehicle. Typically, lug nuts are found on automobiles, trucks , and other large vehicles utilizing rubber tires.-Design:...
s on left-side wheels, but now commonly use tapered lug nuts, which do not precess.
Bicycle pedals
Bicycle pedalBicycle pedal
A bicycle pedal is the part of a bicycle that the rider pushes with their foot to propel the bicycle. 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 wheels...
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. Jobst Brandt, an engineer (author of "The Bicycle Wheel") and cyclist, has suggested a tapered seat for bicycle pedals similar to lug nuts to prevent the damage that fretting does to soft aluminium crankarms, but no major manufacturer has taken up the idea (probably because it requires compatible pedals and crankarms, and thus a new standard).
Bicycle bottom brackets
English threaded bicycle bottom bracketBottom bracket
The bottom bracket on a bicycle connects the crankset to the bicycle and allows the crankset to rotate freely. It contains a spindle that the crankset attaches to, and the bearings that allow the spindle and cranks to rotate. The chainrings and pedals attach to the cranks...
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.)