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 wrench A 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. It generally includes a ratcheting mechanism that allows the nut to be tightened or loosened with a reciprocating motion,, which employs an internal ratchet mechanism.
Ratchet mechanisms are used in a variety of devices, including:
- Cable ties A cable tie is a type of fastener, especially for binding several electronic cables or wires together and to organize cables and wires
- Capstans The word, connected with the Old French capestan or cabestan, from Old Provençal cabestan, from capestre "pulley cord,", from Latin capistrum, -a halter, from capere, to take hold of, seems to have come into English (14th century) from Portuguese or Spanish shipmen at the time of the Crusades. Both device and word are considered Spanish
- Clocks A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". For horologists and other specialists the term clock continues to mean exclusively a device with a striking mechanism for announcing intervals of time acoustically, by ringing
- Hoists 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 lifting hook
- Jacks A jack is mechanical device used to lift heavy loads or apply great forces. Jacks employ a screw thread or hydraulic cylinder to apply very high linear forces
- Roller coasters 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. In essence a specialized railroad system, a roller coaster consists of a track that rises in designed patterns, sometimes with one or more inversions that turn the rider
- Spanners
- Turnstiles 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. Thus a turnstile can be used in the case of paid access , for example public
- Winders
Theory of operation
Figure 1: A ratchet featuring a pawl (a) and gear (b). Figure 2: A ratchet moving in its "forward" direction.A ratchet consists of a round gear A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, magnitude, and (see Figure 1) or linear rack A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. The circular pinion engages teeth on a linear "gear" bar – the rack. Rotational motion applied to the pinion will cause the rack to move to the side, up to the limit of its travel. For example, in a rack with teeth, and a pivoting, springloaded finger called a pawl (or click[1]) 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 backlash In 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 of clearance between mated gear teeth. In other. In cases where backlash must be minimized, a smooth, toothless ratchet with a high friction surface such as rubber Natural rubber is an elastomer that was originally derived from latex, a milky colloid found in the sap of some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the latex sap collected and refined into a usable rubber. The purified form of natural rubber is the chemical polyisoprene, which can also 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.
See also
- Freewheel In mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive is sometimes mistakenly called a freewheel, but is otherwise unrelated
- Sprag A sprag is a one-way 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. When the unit rotates in one direction, the rollers stand up and bind because of friction, and when the unit is rotated in the opposite direction, the rollers slip or freewheel
References
- ^ "Know your terminology - Clocks". Hints and Tips. British Horological Institute. http://www.bhi.co.uk/hintsandtips.html. Retrieved 2008-05-15.
External links
- Silent Ratchet patent (a toothless ratchet) on the USPTO The United States Patent and Trademark Office is an agency in the United States Department of Commerce that issues patents to inventors and businesses for their inventions, and trademark registration for product and intellectual property identification. The patent includes a full description of ratchet technologies, advantages and drawbacks.
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