A
ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. The term is also commonly used to refer to a
socket wrenchA socket wrench, more commonly referred to as a ratchet, is a type of wrench, or tightening tool, that uses separate, removable sockets to fit many different sizes of fittings and fasteners, most commonly nuts and bolts.-Description:...
, which employs an internal ratchet mechanism.
Ratchet mechanisms are used in a variety of devices, including:
- Cable tie
A cable tie is a type of fastener, especially for binding several electronic cables or wires together and to organize cables and wires.- Design and use :In its most popular form, a cable tie consists of a sturdy Nylon tape...
s
- Capstan
A capstan is a rotating machine used to apply force to ropes, cables, and hawsers, commonly, though not exclusively, in a maritime setting.- History :...
s
- Clock
A clock is an instrument used to indicate, measure, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell"...
s
- Hoist
A hoist is a device used for lifting or lowering a load by means of a drum or lift-wheel around which rope or chain wraps. It may be manually operated, electrically or pneumatically driven and may use chain, fiber or wire rope as its lifting medium. The load is attached to the hoist by means of a...
s
- Jacks
- Roller coaster
The roller coaster is a popular amusement ride developed for amusement parks and modern theme parks. LaMarcus Adna Thompson patented the first coasters on January 20, 1885...
s
- Spanner
Spanner may refer to one of the following.*Spanner, a synonym of wrench*The SpannerTM by AbbeyMoor Medical, Inc. FDA approved prostatic stent for treatment of urethral blockages.*Spanner, a type of screw drive head...
s
- Turnstile
A turnstile, also called a baffle gate, is a form of gate which allows one person to pass at a time. It can also be made so as to enforce one-way traffic of people, and in addition, it can restrict passage only to people who insert a coin, a ticket, a pass, or similar...
s
- Winder
Winder may refer to:* winder , a device for transferring energy into a mechanical storage such as mainsprings or the weights of a longcase clock. Its supply may be the hand—most familiarly so—or an electric motor....
s
A ratchet consists of a round
gearA gear is a component within a transmission device that transmits rotational torque by applying a force to the teeth of another gear or device. A gear is different from a pulley in that a gear is a round wheel that has linkages that mesh with other gear teeth, allowing force to be fully...
(see Figure 1) or linear
rackA rack and pinion is a pair of gears which convert rotational motion into linear motion. The circular pinion engages teeth on a flat bar - the rack...
with teeth, and a pivoting, springloaded finger called a
pawl (or
click) that engages the teeth.
Discussion
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A
ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. The term is also commonly used to refer to a
socket wrenchA socket wrench, more commonly referred to as a ratchet, is a type of wrench, or tightening tool, that uses separate, removable sockets to fit many different sizes of fittings and fasteners, most commonly nuts and bolts.-Description:...
, which employs an internal ratchet mechanism.
Ratchet mechanisms are used in a variety of devices, including:
- Cable tie
A cable tie is a type of fastener, especially for binding several electronic cables or wires together and to organize cables and wires.- Design and use :In its most popular form, a cable tie consists of a sturdy Nylon tape...
s
- Capstan
A capstan is a rotating machine used to apply force to ropes, cables, and hawsers, commonly, though not exclusively, in a maritime setting.- History :...
s
- Clock
A clock is an instrument used to indicate, measure, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell"...
s
- Hoist
A hoist is a device used for lifting or lowering a load by means of a drum or lift-wheel around which rope or chain wraps. It may be manually operated, electrically or pneumatically driven and may use chain, fiber or wire rope as its lifting medium. The load is attached to the hoist by means of a...
s
- Jacks
- Roller coaster
The roller coaster is a popular amusement ride developed for amusement parks and modern theme parks. LaMarcus Adna Thompson patented the first coasters on January 20, 1885...
s
- Spanner
Spanner may refer to one of the following.*Spanner, a synonym of wrench*The SpannerTM by AbbeyMoor Medical, Inc. FDA approved prostatic stent for treatment of urethral blockages.*Spanner, a type of screw drive head...
s
- Turnstile
A turnstile, also called a baffle gate, is a form of gate which allows one person to pass at a time. It can also be made so as to enforce one-way traffic of people, and in addition, it can restrict passage only to people who insert a coin, a ticket, a pass, or similar...
s
- Winder
Winder may refer to:* winder , a device for transferring energy into a mechanical storage such as mainsprings or the weights of a longcase clock. Its supply may be the hand—most familiarly so—or an electric motor....
s
Theory of operation
A ratchet consists of a round
gearA gear is a component within a transmission device that transmits rotational torque by applying a force to the teeth of another gear or device. A gear is different from a pulley in that a gear is a round wheel that has linkages that mesh with other gear teeth, allowing force to be fully...
(see Figure 1) or linear
rackA rack and pinion is a pair of gears which convert rotational motion into linear motion. The circular pinion engages teeth on a flat bar - the rack...
with teeth, and a pivoting, springloaded finger called a
pawl (or
click) that engages the teeth. The teeth are uniform but 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 (see Figure 2), 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
backlashIn mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established. For example, in a pair of gears, backlash is the amount...
. In cases where backlash must be minimized, a smooth, toothless ratchet with a high friction surface such as
rubberNatural rubber is an elastomer that was originally derived from a milky colloidal suspension, or latex, found in the sap of some plants. The purified form of natural rubber is the chemical polyisoprene, which can also be produced synthetically...
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.
External links
- Silent Ratchet patent (a toothless ratchet) on the USPTO. The patent includes a full description of ratchet technologies, advantages and drawbacks.