Hydraulic Clutch

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 is a sealed chamber containing two similarly shaped toroids facing components,impeller and turbine, 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. Simplified, this is a centrifugal force that throws the oil outwards. The moving fluid reaches the casing and the enclosed shape forces the flow in the direction of the driven impeller, referred to as the turbine or driven torus (the latter also a General Motors term). Here, the 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 repeats.

Automotive Applications

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 in this case is often referred to as a fluid flywheel), 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. Hydrodynamic clutch used in an automobile is also called Foettinger clutch.

Slip

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.

Turbulence

Also affecting the fluid coupling's efficiency is the fact that the fluid returning from the turbine to the pump is sometimes 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 or at reduced rotational speed. As the braking effect turbulence is mainly at the inner volume of the unit, an additional "fan shaped" component can be used to re-direct the fluid towards a corrected flow path that greatly reduces the drag that is evident at slow speed. The added component, (placed between or around the toroids), is often called a "stator" or "reactor" and is also often fitted with a one-way clutch to allow unhindered fluid flow at higher speeds. Once the stator is in place the device effectively becomes what is commonly known as a torque converter.

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(N2)(D5), 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(N2)(D5) 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.