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1. a mechanical device that connects two things
2. a device for connecting railway cars or trucks together
3. the part of the body of a horse, dog, or other quadruped that lies between the forequarters and the hindquarters
4. Electronics the act or process of linking two or more circuits so that power can be transferred between them usually by mutual induction, as in a transformer, or by means of a capacitor or inductor common to both circuits
5. Physics an interaction between different properties of a system, such as a group of atoms or nuclei, or between two or more systems
6. Genetics the occurrence of two specified nonallelic genes from the same parent on the same chromosome
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(for clutch), a device for permanent or temporary joining of shafts, pipes, and steel-wire ropes and cables.

In terms of function, a distinction is made between joint couplings, which provide both strength and airtightness of a connection and protection from corrosion, and drive couplings for machines, which transmit rotary motion and torque from one shaft to another (which is usually coaxial with the first) or from a shaft to a part that fits freely on it (a pulley or gear) without changing the torque. In addition, drive couplings perform other important functions, such as compensation for small assembly errors, disengagement of shafts, automatic control, stepless variation of a transmission ratio, and protection of machines from damage under emergency conditions. Couplings are used to transmit both very small and substantial torques and powers (up to several thousand kilowatts). The variety of methods for transmitting torque and the diversity of the functions performed by couplings have resulted in many types of designs for modern couplings. The most common types have been standardized.

Torque can be transmitted in a coupling by a mechanical link between the parts, which may be provided by fixed joints or kinematic pairs (couplings with geometric locking), by frictional forces or magnetic attraction (couplings with force locking), or by inertial forces or inductive interaction of electromagnetic fields (couplings with dynamic locking). Couplings are divided into the following types on the basis of their purpose and type of operation: permanent couplings; controlled couplings, which make possible engagement and disengagement of shafts by means of a control system; automatic couplings, which engage and disengage shafts automatically during operation according to changes in the operating conditions; safety clutches, which disengage shafts upon serious disruption of the normal operating conditions of a machine; and slip couplings, which transmit torque only when the rate of rotation of the driven shaft is less than that of the driving shaft.

Permanent couplings. Permanent couplings have geometric locking and are made in several types. Jointed, or fixed, couplings (Figure l,a) engage shafts without providing for their relative displacement. Jointed couplings allow small deviations of the shafts from coaxiality. The most common couplings of this type are toothed couplings (Figure 1,b). Rigid movable couplings permit considerable departure from coaxiality. For example, asynchronous articulated couplings that permit axial misalignments up to 45° but not transverse or longitudinal displacements of the axes (Cardan mechanisms) are widely used; also popular are double Cardan mechanisms—that is, combinations of two single Cardan mechanisms (Figure l,c). Synchronous articulated couplings, which transmit the motion by means of ball-and-socket joints, provide a constant gear ratio at any angle between the axes of the connected shafts. Such couplings are used in motor vehicles with front-wheel drive. Synchronous couplings also include the full-floating, or cross-sliding, couplings (Figure l,d), which are designed to permit some transverse displacements of the shafts and to compensate for small angular deflections and axial displacements. Elastic and elastic-damping

Figure 1. Permanent couplings: (a) rigid sleeve coupling, (b) jointed toothed coupling, (c) combination of two Cardan mechanisms with an intermediate shaft, (d) full-floating claw-disk coupling, (e) boxpin coupling, (f) coupling with toroidal jacket; (1) shafts being connected, (2) coupling sleeve, (3) sleeves with external teeth, (4) housing with internal teeth, (5) coupling halves, (6) intermediate shaft, (7) intermediate disk, (8) toroidal elastic jacket; (Δ) transverse displacements of shafts, (δ) angular displacement, 1) and (δ2,) shaft angles of the Cardan mechanisms

Figure 2. Controlled couplings: (a) dog clutch, (b) toothed clutch with synchronizer, (c) friction coupling with electromagnetic control, (d) friction coupling with a ferromagnetic mixture, (e) synchronized electroinduction coupling; (1) coupling halves, (2) external teeth, (3) conical friction surfaces, (4) movable ring with internal teeth, (5) disks of coupling halves, (6) ferromagnetic mixture, (7) excitation coil, (8) and (9) magnetic cores of coupling halves, with split poles

couplings are also used as sliding couplings. This group includes box-pin couplings (Figure l,e), which are used extensively to connect the shafts of electric motors to the shafts of driven machines, and also the more modern design of coupling with a toroidal jacket (Figure 1,f)

Controlled couplings. Controlled couplings, or clutches, which have geometric or force locking, are also distinguished by their great variety. The group of couplings with geometric locking includes the dog clutches (Figure 2,a) and toothed clutches, which are noted for their compactness of design but cannot be engaged during high-speed operation if the difference between the angular velocities of the meshing halves is great. This disadvantage is eliminated in toothed couplings with synchronizers (Figure 2,b) that ensure shockless engagement when idling, since the friction surfaces are the first to come into contact, equalizing the rotational velocities of the clutch halves through slippage before the teeth are meshed. This type of clutch is used in motor-vehicle transmissions.

Controlled couplings with mechanical connection by means of force locking include friction couplings, which can engage during operation and under load. They are made with one or more disks, with cylindrical or conical friction surfaces, and with mechanical, pneumatic, hydraulic, or electromagnetic control (Figure 2,c). This type of coupling is used in automatic systems since it can be remotely controlled.

A group of couplings with force locking by electromagnetic coupling is formed by clutches with a fluid or powdered ferromagnetic mixture (Figure 2,d) in which a magnetic flux is produced when an electric current passes through an excitation coil. As a result, the ferromagnetic mixture that fills the gap between the halves of the coupling becomes magnetized and adheres to the surfaces of the halves. Such couplings are used extensively in copying lathes and other processing machinery. Force locking with electromagnetic coupling is achieved in synchronized electroinduction clutches, which have magnetic cores with split poles on both halves (Figure 2,e). Torque is transmitted between the shafts when a current passes through the excitation coil, as a result of which a magnetic attractive force is developed between the poles of the clutch halves.

Automatic couplings. Automatic couplings engage and disengage according to changes in the operating conditions of a machine. They include (1) single-revolution couplings, which

Figure 3. Automatic couplings: (a) freewheeling clutch, (b) centrifugal clutch; (1) drive sprocket, (2) rollers, (3) driven band, (4) housing, (5) friction lining

operate in a certain position for one revolution or several revolutions of the shaft (they are used in presses and hammers to stop the slider in the top position); (2) freewheeling clutches (Figure 3,a), which transmit torque only for one direction of rotation of the driving half of the clutch relative to the driven half and disengage when the direction of rotation is reversed (they are used in bicycles and in automatic transmissions for motor vehicles and machine tools); (3) centrifugal clutches (Figure 3,b), which engage and disengage according to the speed of rotation of the driving half (they are used as starter clutches in drives and also as safety clutches to limit the speed of rotation of a driven machine); and (4) torque-limiting couplings, which are used most often as safety clutches to disengage a machine upon a dangerous increase in torque. The safety function is also performed by other types of couplings that permit slippage and have design features suitable for the purpose.

Sliding couplings. Sliding couplings have dynamic locking by means of a mechanical link (hydrodynamic couplings) or an electrical link (electroinduction asynchronous couplings). Such couplings transmit torque only when the driven half lags behind the driving half of the coupling—that is, when there is slippage. The hydrodynamic type has a locking system with a fluid working medium. Such couplings are used as starting, control, and safety couplings in hydrodynamic transmissions. Electroinduction asynchronous couplings are operated by the forces of magnetic interaction that arise when slippage occurs between the driving half of the coupling, which has an excitation coil and a magnetic core with split poles, and the driven half, which has a solid magnetic core. Such couplings are used for control and starting functions and, sometimes, as speed regulators.


Reshetov, D. N. Detail mashin, 3rd ed. Moscow, 1974.
Kratkii spravochnik mashinostroitelia. Moscow, 1966.
Detali mashin, 7th ed. Moscow, 1972.
Poliakov, V. S. , and I. D. Barbash. Mufty, 4th ed. Leningrad, 1973.
Detali mashin: Spravochnik, vol. 1, 3rd ed. Edited by N. S. Acherkan. Moscow, 1968.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A mutual relation between two circuits that permits energy transfer from one to another, through a wire, resistor, transformer, capacitor, or other device.
A hardware device used to make a temporary connection between two wires.
Any device that serves to connect the ends of adjacent parts, as railroad cars.
A metal collar with internal threads used to connect two sections of threaded pipe.
(mechanical engineering)
The mechanical fastening that connects shafts together for power transmission. Also known as shaft coupling.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


A short internally threaded section of pipe, used to join two pipes or conduits.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


(programming, hardware)
The degree to which components depend on one another. There are two types of coupling, "tight" and "loose". Loose coupling is desirable for good software engineering but tight coupling may be necessary for maximum performance. Coupling is increased when the data exchanged between components becomes larger or more complex.
This article is provided by FOLDOC - Free Online Dictionary of Computing (
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