Precession (mechanical)


Precession (mechanical)

Precession is the name given to the process of one part rotating with respect to another due to fretting between the two. It is described as "precession, in which a round object rolling in a circular ring in one direction will itself turn in the opposite direction." "In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other."cite web
url = http://www.faqs.org/faqs/bicycles-faq/part4/section-35.html
title = Fretting damage in Bicycle Mechanics
accessdate = 2006-12-26
]

"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."cite web
url = http://www.sheldonbrown.com/brandt/left.html
title = Left Hand Threads
last = Brandt
first = Jobst
year = 2004/5
accessdate = 2006-12-26
]

Fretting-induced precession is a purely mechanical process which 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 nuts on left-side wheels, but now commonly use tapered lug nuts, which do not fret.

Bicycle pedals

Bicycle pedals are left-threaded on the left-hand crank so that precession tightens the pedal rather than loosening it. 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, thus a new standard).

Bicycle bottom brackets

English threaded bicycle bottom brackets 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.)

See also

* Fretting

References


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Precession (disambiguation) — Precession refers to a change in the direction of the axis of a rotating object.Precession may specifically mean:*Precession (astronomy) mdash; the precession of the Earth s axis of rotation (also known as the precession of the equinoxes ), or… …   Wikipedia

  • Precession — For other uses, see Precession (disambiguation). Precession of a gyroscope 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… …   Wikipedia

  • Mechanical Animals — This article is about the album. For the tour, see Mechanical Animals Tour. Mechanical Animals Studio album by …   Wikipedia

  • The Mechanical Universe — Infobox Television show name = The Mechanical Universe caption = format = Physics camera = picture format = audio format = runtime = creator = Dr. James F. Blinn developer = producer = executive producer = starring = Dr. David Goodstein narrated …   Wikipedia

  • mechanics — /meuh kan iks/, n. 1. (used with a sing. v.) the branch of physics that deals with the action of forces on bodies and with motion, comprised of kinetics, statics, and kinematics. 2. (used with a sing. v.) the theoretical and practical application …   Universalium

  • History of Physics —     History of Physics     † Catholic Encyclopedia ► History of Physics     The subject will be treated under the following heads: I. A Glance at Ancient Physics; II. Science and Early Christian Scholars; III. A Glance at Arabian Physics; IV.… …   Catholic encyclopedia

  • Gyro monorail — The gyro monorail, gyroscopic monorail, gyro stabilized monorail, or gyrocar all denote a single rail land vehicle, road or rail, which uses the gyroscopic action of a spinning wheel, which is forced to precess, to overcome the inherent inverted… …   Wikipedia

  • Gyroscope — A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum [ [http://demonstrations.wolfram.com/Gyroscope/ Gyroscope] by Sándor Kabai, The Wolfram Demonstrations Project.] [… …   Wikipedia

  • Nutation — See also: Nutation in plants Rotation (green), Precession (blue) and Nutation in obliquity (red) of the Earth Nutation (from Latin: nūtāre, to nod) is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric… …   Wikipedia

  • General relativity — For a generally accessible and less technical introduction to the topic, see Introduction to general relativity. General relativity Introduction Mathematical formulation Resources …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.