What is a Fluid Coupling?

Lori Kilchermann

A fluid coupling is a device that converts or transmits rotating mechanical energy or power to be used in other applications. While finding a home in automobile transmissions as well as marine drive units, this device has also is part of many manufacturing plants where hydraulic machines are being used. It allows a machine to start operation with less shock than a typical transmission would generate.

In a typical automobile, the fluid coupling is most commonly called a torque converter.
In a typical automobile, the fluid coupling is most commonly called a torque converter.

The fluid coupling consists of only three basic components. It contains an outer shell or housing, and two turbines, an input and output. Both of the turbines are contained within the housing via oil-tight seals. The input turbine is connected to the power supply, typically an electric or internal combustion piston engine. The output turbine is connected to the drive train of the vehicle or the drive system of a machine.

In a typical automobile, the fluid coupling is most commonly called a torque converter. The torque converter bolts onto the engine via the flywheel or starter ring and attaches to the transmission's input shaft. From there, it directly powers and moves the vehicle. Many variables are factored into the design of this fluid coupling, including the vehicle's weight, engine torque, horsepower, optimal operating speed and usage. All factors are carefully considered when choosing the coupling's proper stall speed and lock-up speed.

Stall speed is the maximum speed at which the coupling can turn at full power when the output turbine is locked in place. In an automobile, this is the maximum speed at which the engine can turn without spinning or turning the drive tires. Engine output is measured in heat with a fluid coupling. Excessive heat can quickly ruin the component. This is why racing transmissions do not work on street driven vehicles. The heat from high-stall converters burn up the transmission prematurely.

A fluid coupling operates with a minimum amount of slip engineered into its design. A coupling cannot develop torque if both the input and output turbines are spinning at the same speed. A well-tuned coupling can maintain 94 percent efficiency with the remaining 6 percent being expressed as heat.

The coupling can also be used as a braking system. The fluid absorbs the rotational forces and expresses them as heat while the engine is slowing down. This is due to friction within the coupling as the fluid rushes through the turbines.

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Discussion Comments


@anon14851 - I assume this is to keep all the oil from coming out? I can see why that would be useful, especially with larger machinery where you might have gallons of fluid in there.

Of course, when you check your transmission fluid, you aren't tilting the tranny. Unless it's tilted already?


while checking the oil in a fluid coupling, it is generally checked on 120 degree from first quadrant.

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