Sprag
From Wikipedia, the free encyclopedia
A sprag is a freewheel clutch used in a number of applications. It resembles a roller bearing with rollers shaped like a figure eight and cocked with a spring as it turns one way they stand up and bind because of oil friction and slip in the other when the shaft is running.
The word Sprag when used as a verb is a commonly used way of describing the way a tracked vehicle moves, i.e. to sprag left or to sprag across a construction site, (see also "to Track"). This use of Sprag is in common use in the UK Construction industry.
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[edit] Applications
[edit] Automatic transmissions
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.
[edit] Motorcycle electric starters
A sprag is used in the electric starters of modern motorcycle engines to replace the "bendex" common to auto starters.
[edit] Conveyors
On conveyor drives, a sprag clutch is used for indexing and anti-runback.
[edit] Overrunning
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.
[edit] Indexing
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.
[edit] Backstopping
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’’’.
[edit] Hoist Load Brake
Used as a secondary brake to prevent industrial hoists from catastrophically dropping their loads in the event of mechanical failure.
[edit] Lubrication
Sprag clutches may be oil or grease lubricated. Most sprag clutch manufacturers do not allow the use of lubricants containing extreme pressure additives.
[edit] See also
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