Fluid coupling

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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.

A fluid coupling is a sealed chamber containing two toroid shaped impellers immersed in fluid (usually oil). The driving impeller, often referred to as the pump or driving torus (the latter a General Motors automotive term), is rotated by the prime mover, which is typically an internal combustion engine or electric motor. The motion of the pump's radial chambers imparts a relatively complex centripetal motion to the fluid. The moving fluid reaches the center of the driven impeller, referred to as the turbine or driven torus (the latter also a General Motors term), where Coriolis force reaction transfers the angular fluid momentum outward, applying torque to the turbine, thus causing it to rotate in the same direction as the pump. The fluid leaving the outer edges of the turbine returns to the pump, where the cycle repeats.

In automotive applications, the pump 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.

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.

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 prime mover speed. This speed difference is called slip or slippage.

Also affecting the fluid coupling's efficiency is the fact that the fluid returning from the turbine to the pump is moving in the opposite 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.

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.

Fluid couplings were used in a variety of early semi-automatic transmissions and automatic transmissions, the largest such application being in the General Motors single, dual range and Jetaway Hydramatic models. Since the late 1940's, 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.