Fluid coupling


Fluid coupling

A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch. It also has widespread application in marine and industrial machine drives, where variable speed operation and/or controlled start-up without shock loading of the power transmission system is essential.

Overview

A fluid coupling consists of a sealed chamber containing two toroidal-shaped, vaned components, the pump and turbine, immersed in fluid (usually oil). The pump or "driving torus" (the latter a General Motors automotive term) is rotated by the , which is typically an internal combustion engine or electric motor. The pump's motion imparts a relatively complex centripetal motion to the fluid. Simplified, this is a centrifugal force that throws the oil outwards against the coupling's housing, whose shape forces the flow in the direction of the "turbine" or "driven torus" (the latter also a General Motors term). Here, Coriolis force reaction transfers the angular fluid momentum outward and across, applying torque to the turbine, thus causing it to rotate in the same direction as the pump. The fluid leaving the center of the turbine returns to the pump, where the cycle endlessly repeats.

Automotive applications

In automotive applications, the pump typically is connected to the flywheel of the engine—in fact, the coupling's enclosure may be part of the flywheel proper, and thus is turned by the engine's crankshaft. The turbine is connected to the input shaft of the transmission. As engine speed increases while the transmission is in gear, torque is transferred from the engine to the input shaft by the motion of the fluid, propelling the vehicle. In this regard, the behavior of the fluid coupling strongly resembles that of a mechanical clutch driving a manual transmission.

Aviation applications

The most prominent use of fluid couplings in aero applications was in the Wright turbo-compound reciprocating engine, in which three power recovery turbines extracted approximately 20 percent of the energy (about 500 HP or 375 kW) from the engine's exhaust gases and then, using three fluid couplings and gearing, converted low torque high-speed turbine rotation to low-speed, high-torque output to drive the propeller.

tall speed

An important characteristic of a fluid coupling is its stall speed. The stall speed is defined as the highest speed at which the pump can turn when the turbine is locked and maximum input power is applied, a condition which could occur in an automobile if the driver were to fully open the throttle while applying the brakes with a force sufficient to keeping the vehicle from moving. Under stall conditions, all of the engine's power would be dissipated in the fluid coupling as heat, possibly leading to damage.

lip

A fluid coupling cannot achieve 100 percent power transmission efficiency, as some of the energy transferred to the fluid by the pump will be lost to friction (transformed to heat). As a result, the turbine will always spin slower than the pump, this difference increasing with an increase in load on the coupling and/or a decrease in speed. This speed difference is called "slip" or "slippage".

Turbulence

Also affecting the fluid coupling's efficiency is the fact that the fluid returning from the turbine to the pump when there is a large difference in speed between the pump and turbine is moving counter to the direction of the pump's rotation, resulting in some braking effect and a good deal of turbulence. This effect substantially increases as the difference between pump and turbine speed increases, causing efficiency to rapidly deteriorate with increasing load or at reduced rotational speed.

Calculations

Generally speaking, the power transmitting capability of a given fluid coupling is exponentially related to pump speed, a characteristic that generally works well with applications where the applied load doesn't fluctuate to a great degree. The torque transmitting capacity of any hydrodynamic coupling can be described by the expression r(N^2)(D^5), where r is the mass density of the fluid, N is the impeller speed, and D is the impeller diameter. In the case of automotive applications, where loading can vary to considerable extremes, r(N^2)(D^5) is only an approximation. Stop-and-go driving will tend to operate the coupling in its least efficient range, causing an adverse effect on fuel economy.

Usage

Fluid couplings were used in a variety of early semi-automatic transmissions and automatic transmissions. Since the late 1940s, the more versatile hydrodynamic torque converter has replaced the fluid coupling in automotive applications. Fluid couplings are still widely used in industrial applications, especially in machine drives that involve high inertia starts or constant cyclic loading.

ee also

* Water brake


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • fluid coupling — noun also fluid flywheel : a coupling (as a fluid drive) in which fluid (as oil) intervenes between moving members to transmit pressure and torque * * * Mach. an apparatus in which a fluid, usually oil, transmits torque from one shaft to another …   Useful english dictionary

  • fluid coupling — On a manual transmission, there is a mechanical connection between the engine and transmission through the clutch. On an automatic transmission a fluid coupling provides a viscous fluid to connect the engine output and the transmission. It… …   Dictionary of automotive terms

  • fluid coupling — hidraulinė mova statusas T sritis automatika atitikmenys: angl. fluid coupling; hydraulic clutch; hydraulic coupling vok. Flüssigkeitskupplung, f; hydrodynamische Kupplung, f; Hydrokupplung, f; Strömungskupplung, f rus. гидравлическая муфта, f… …   Automatikos terminų žodynas

  • fluid coupling — Mach. an apparatus in which a fluid, usually oil, transmits torque from one shaft to another, producing an equal torque in the other shaft. Also called hydraulic coupling. Cf. hydraulic torque converter. [1935 40] * * * …   Universalium

  • Fluid Drive — is the trademarked name that Chrysler Corporation assigned to a transmission driveline combination offered from 1939 through 1953 in Chryslers and Desotos, and from 1939 through 1954 in Dodge models. The fluid drive element was a hydraulic… …   Wikipedia

  • Coupling — This article is about a mechanical connection between two objects. For other uses, see Coupling (disambiguation). Rotating coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power.… …   Wikipedia

  • fluid drive — noun an automotive power coupling • Hypernyms: ↑coupling, ↑coupler • Part Holonyms: ↑transmission, ↑transmission system * * * noun or fluid clutch …   Useful english dictionary

  • fluid flywheel — noun a kind of fluid coupling in which the flywheel is the driving rotor • Hypernyms: ↑flywheel * * * noun see fluid coupling * * * fluid flywheel, a type of fluid coupling used in certain automobiles in connection with an automatic or… …   Useful english dictionary

  • fluid clutch — noun see fluid drive * * * fluid clutch, = fluid coupling. (Cf. ↑fluid coupling) …   Useful english dictionary

  • coupling — A connecting device used between two objects so motion of one will be imparted to the other; it may be mechanical, hydraulic, or electrical. Also see doughnut coupling föttinger coupling flexible coupling fluid coupling foettinger coupling guibo… …   Dictionary of automotive terms


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.