A ratchet (occasionally spelled rachet) is a mechanical device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Ratchets are widely used in machinery and tools. The word ''ratchet'' is also used informally to refer to a ratcheting
socket wrench.
__TOC__
Theory of operation
A ratchet consists of a round
gear or a linear
rack with teeth, and a pivoting, spring-loaded finger called a ''
pawl'' (or ''click'', in clocks and watches
) that engages the teeth. The teeth are uniform but are usually
asymmetrical, with each tooth having a moderate slope on one edge and a much steeper slope on the other edge.
When the teeth are moving in the unrestricted (i.e. forward) direction, the pawl easily slides up and over the gently sloped edges of the teeth, with a spring forcing it (often with an audible 'click') into the depression between the teeth as it passes the tip of each tooth. When the teeth move in the opposite (backward) direction, however, the pawl will catch against the steeply sloped edge of the first tooth it encounters, thereby locking it against the tooth and preventing any further motion in that direction.
Backlash
Because the ratchet can only stop backward motion at discrete points (i.e., at tooth boundaries), a ratchet does allow a limited amount of backward motion. This backward motion—which is limited to a maximum distance equal to the spacing between the teeth—is called
backlash. In cases where backlash must be minimized, a smooth, toothless ratchet with a high friction surface such as
rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds.
Types of polyisoprene ...
is sometimes used. The pawl bears against the surface at an angle so that any backward motion will cause the pawl to jam against the surface and thus prevent any further backward motion. Since the backward travel distance is primarily a function of the compressibility of the high friction surface, this mechanism can result in significantly reduced backlash.
Uses
Ratchet mechanisms are used in a wide variety of applications, including these:
*
Cable ties
*
Capstans
*
Caulking guns
*
Clock
A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, a ...
s
*
Computer keyboard
A computer keyboard is a built-in or peripheral input device modeled after the typewriter keyboard which uses an arrangement of buttons or Push-button, keys to act as Mechanical keyboard, mechanical levers or Electronic switching system, electro ...
s
*
Freewheel
image:Freewheel en.svg, Freewheel mechanism
In mechanical engineering, mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission (mechanics), transmission that disengages the driveshaft from the driv ...
(overrunning clutch)
*
Grease guns
*
Handcuffs
Handcuffs are Physical restraint, restraint devices designed to secure an individual's wrists in proximity to each other. They comprise two parts, linked together by a Link chain, chain, a hinge, or rigid bar. Each cuff has a rotating arm whi ...
*
Jacks
*
Anti-rollback devices used in
roller coasters
*
Loom
A loom is a device used to weaving, weave cloth and tapestry. The basic purpose of any loom is to hold the Warp (weaving), warp threads under tension (mechanics), tension to facilitate the interweaving of the weft threads. The precise shape of ...
s
*
Seatbelts
*
Select-fire switch
*
Slacklines
*
Socket wrenches
*
Tie down straps
*
Turnstiles
*
Typewriter
A typewriter is a Machine, mechanical or electromechanical machine for typing characters. Typically, a typewriter has an array of Button (control), keys, and each one causes a different single character to be produced on paper by striking an i ...
s
Gallery
File:First socket wrench.png, Ratcheting socket wrench or spanner.
File:Comealong.jpg, A ratchet lever hoist, also known as a 'come-along.'
File:ZipTieCrossSection.JPG, The ratchet mechanism of a cable tie.
File:Turning the bezel on a diving watch to mark a point in time.webm, Setting the bezel of a diving watch to the start time of a dive. For safety reasons, the ring has a ratchet so it cannot be turned or knocked in the opposite direction.
See also
*
Brownian ratchet
*
Sprag clutch
*
Check valve, a device that allows fluids to flow in only one direction
*
Escapement
An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to t ...
, a similar device used in clocks
*
Diode
A diode is a two-Terminal (electronics), terminal electronic component that conducts electric current primarily in One-way traffic, one direction (asymmetric electrical conductance, conductance). It has low (ideally zero) Electrical resistance ...
, a device that allows electric current to flow in only one direction
References
{{Commons category, Ratchets
Mechanisms (engineering)