Principles of Operation
Introduction
 There are three essential parts to a fluid coupling: the driving (input) section known as the
impeller the driven (output) section known as the runner and the casing which bolts to the impeller
enclosing the runner providing an oil tight reservoir. Both impeller and runner each represents half
of a hollow torus with flat radial vanes. At the inner circumference a conical baffle is attached to
the impeller and a flat baffle is attached to the runner. These components comprise the working
circuit.
The operation of the fluid coupling requires mechanical input energy, normally provided by a
standard NEMA B electric motor which is connected to the impeller and casing. The runner, which has
no mechanical connection with the impeller, is directly connected to the driven load. A variety of
mechanical connections; couplings, sheaves, and hollow shaft mountings, are available to provide the
mounting configuration best suited to the application. Finally the fluid coupling must be initially
charged by removing the fill (fusible) plug and adding the recommended amount of oil based on the
required torque.
Starting
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Standard NEMA B motors are recommended when using fluid couplings and will start virtually
unloaded. Since the motor is mechanically connected to the impeller and casing, the low inertia of
these components and the oil are the only loads imposed. As the electric motor accelerates to
running speed, the impeller begins to centrifugally pump oil to the stationary runner. Transmission
of oil is diffused by the conical impeller baffle, producing a gradual increase in torque, allowing
the motor to accelerate rapidly to full running speed. When all the oil is pumped into the working
circuit, continuous circulation of oil will occur between the impeller and runner forming a flow
path like a helical spring formed in a ring.
As soon as the transmitted torque reaches the value of the resisting torque, the runner starts
rotating and accelerates the driven load. The time required to reach full speed is dependent on the
inertia of the driven load, the resistive torque, and the torque being transmitted by the fluid
coupling.
Running
 The operation of a fluid coupling is based on the hydrokinetic principles and requires that the
output speed be less than the input. This difference in speed is called slip. Further this principle
provides that the output torque is equivalent to the input torque, since windage and oil circulation
losses are negligible. Therefore, efficiency equals 100% minus the percent of slip.
At full running speed fluid couplings will normally slip between 1% and 4%. The oil circulation
between the impeller and runner has formed a vortex at the outside circumference of the working
circuit and is no longer deflected by the conical baffle.
Overload - Stall
 Should the load torque increase, the slip will increase, which causes the runner to drop in speed.
The vortex of oil circulating between the impeller and runner will expand to provide additional
torque. The extent to which this vortex can expand is limited by the flat baffle on the runner.
Consequently fluid couplings provide inherent overload protection.
If the increase in torque causes the oil in the working circuit to expand to the point of
contacting the baffle, the coupling will stall and slip will be 100%. This continuous high slip
generates heat and the oil temperature will rise unless the overload is removed.
 When the temperature
rises to the temperature limit of the fusible plug, the core of the plug will melt, release oil from
the coupling and effectively disconnect power to the output shaft. To prevent the loss of oil the use
of a proximity cutout switch or thermal trip plug and limit switch is recommended.
Coupling guards must be designed to permit air circulation for cooling and to protect when oil is
released from fusible plug due to overload.
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