Dictionary:
sprag (sprăg)  |
[Perhaps of Scandinavian origin.]
| US Military Dictionary: sprag |
n. a projection preventing the movement of platforms or pallets in the side guidance rails in an aircraft cabin.
See the Introduction, Abbreviations and Pronunciation for further details.
| WordNet: sprag |
The noun has 2 meanings:
Meaning #1:
a chock or bar wedged under a wheel or between the spokes to prevent a vehicle from rolling down an incline
Meaning #2:
a wooden prop used to support the roof of a mine
Synonym: pitprop
| Wikipedia: Sprag clutch |
A sprag clutch is a one-way freewheel clutch. It resembles a roller bearing, but instead of cylindrical rollers, non-revolving asymmetric figure-of-eight shaped sprags are used. When the unit rotates in one direction the rollers slip or free-wheel, but when a torque is applied in the opposite direction, the rollers tilt slightly, producing a wedging action and binding because of friction. The sprags are spring-loaded so that they lock without backlash.
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A sprag clutch is used in some automatic transmissions as a method of allowing the transmission to smoothly change gears under load. Various models of General Motors' turbo-hydramatic gearbox have used this system.
The process of changing up gears involves preparing for the change by releasing a clutch that prevents the transmission from turning faster than the gear that it is currently in and engaging the sprag such that it is freewheeling. The gearchange occurs by engaging the higher gear through the sprag to change from freewheeling to driving. Once the sprag has engaged drive in the higher gear, a clutch is engaged to place the transmission in that gear without the need for the sprag, which is then disengaged. By engaging and disengaging the various clutch packs within the transmission, one sprag can be used for all gearchanges.
This can also be used exclusively in first gear (transmission shift lever in D, but auto valve body or management selecting 1st) on some autos. that way it will automatically provide forward drive, but will _not_ provide any engine braking. This is done not so much to avoid engine braking per se, but rather to allow a low throttle opening 2-1 downshift as a car decelerates, to avoid a loud (and potentially abrupt) and unnerving jolt as a result of the downshift. On transmissions so equipped, manual selection of 1st gear typically engages an additional band that grips the same section as the one way clutch would engage, and thus allows for engine braking.
A sprag clutch is used in many helicopter designs to transfer power from the engine to the main rotor. In the event of an engine failure, the sprag clutch allows the main rotor to continue rotating faster than the engines so that the helicopter can enter autorotation.[1]
A sprag clutch is used in the electric starter motors of modern motorcycle engines to replace the Bendix gear common to automobile starters.
Many modern sport and racing bikes use sprag clutches in the primary drive between the engine and transmission. This prevents the rear wheel from losing traction during rapid downshifts when the engine speed is not matched to road speed. If a sprag clutch is not present, much greater care is required when downshifting because losing rear wheel traction can cause dangerous highside accidents.
On conveyor drives, a sprag clutch is used for indexing and anti-runback.
Overrunning occurs when two or more motors can be used to drive the same machine, including through the same shaft. A typical setup is as follows. The service drive, or prime mover, provides a combination of high speed and high power for normal operation. The secondary drive, or pony drive, can be run at low speed and low power, usually to position the conveyor for maintenance.
The overrunning clutch allows the secondary drive to remain stationary during normal high speed operation of the conveyor drive.
An indexing motion provides for accuracy in the drive system. A link is installed between the prime mover and one race of the sprag clutch. The reciprocating motion of the prime mover is converted to an indexing movement on the other race of the sprag clutch.
Backstopping prevents a loaded inclined conveyor from running back when the drive system is stopped. The outer race of the sprag clutch is fixed to the machine frame, usually via a torque arm. The inner race rotates freely in the forward direction of the conveyor drive. When the machine tries to run backward, the sprag clutch prevents this motion. In this application the sprag clutch is often called a "backstop" or "holdback".
Used as a secondary brake to prevent industrial hoists from catastrophically dropping their loads in the event of mechanical failure.
Sprag clutches may be oil or grease lubricated. Most sprag clutch manufacturers do not allow the use of lubricants containing extreme pressure additives.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
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Copyrights:
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| US Military Dictionary. The Oxford Essential Dictionary of the U.S. Military. Copyright © 2001, 2002 by Oxford University Press, Inc. All rights reserved. Read more |
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| WordNet. WordNet 1.7.1 Copyright © 2001 by Princeton University. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Sprag clutch". Read more |
