Fluid drives, fluid couplings or hydraulic couplings are hydrodynamic devices that transmits rotational power. There are 4 main components of a fluid drive: The housing, the input element (including the impeller), the output element (including the runner) and the oil. The housing contains the oil and the rotating mechanical components. The input element connects to the driving equipment like a motor or a turbine. Inside the coupling or fluid drive is an impeller. The impeller drives. The output element connects to the driven equipment such as a pump or fan. Inside the coupling is a runner. Between the input and output elements is oil. Kinetic energy is added to the oil when input element rotates. This energy is smoothly transferred to the output element and the driven equipment.

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TRI fluid couplings are used for soft starts, vibration isolation and equipment protection. They are run with full oil or with just cooling oil.

TRI fluid drives are similar to fluid couplings with the added variable speed functions. The output speed of a fluid drive can be reduced by removing some of the oil form the inside of the coupling. This creates a slip condition causing the output shaft to slow down. A typical application for the fluid drive is to control the speed of a pump or compressor.

TRI Services and upgrades fluid drives by other suppliers including the Gyrol^® fluid drives originally produced by American Blower.