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FLUID COUPLING
LUID COUPLINGS HAVE been used for almost one hundred years for the transmission of torque through fluid flow. The constant fill coupling is a hydrodynamic fluid coupling working on the Fottinger principle. The torque developed by the electric motor is converted by the primary side of the coupling that is directly connected to the motor shaft, into kinetic energy. It is then converted back into mechanical energy by the secondary side of the coupling. No wear occurs; no mechanical contact of the power transmitting parts occurs. The fluid coupling behaves according to the propeller law. The torque transmitted increases with the square and the power transmitted with the cube of the input speed. The starting conditions of the coupling allow the electric motor to start-up under virtually no load because the load characteristic at start-up is parabolic. The motor is only loaded with the full coupling torque after reaching its pullout speed. After run-up of the machine, the motor is just loaded with the required torque of the driven machine at a low coupling slip.
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.
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.
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.
Advantages:-
The use of a Fluid Coupling offers the following advantages over the direct coupling of a drive system:
1. Motor soft start: standard motors of any voltage or characteristic can be started with very little load. This allows the time of the current inrush to be reduced and also for the motor to have the maximum speed and torque available for accelerating the load.
2. Shock dampening and overload protection: there is no mechanical connection between motor and driven machine. The torque is only transmitted by a fluid flow. The level of fluid can be adjusted to limit starting and overload torque. Fluid couplings also provide excellent shock dampening.
3. Maximum torque transmission: the fluid coupling oil filling can be tailored to suit the motor characteristic. It is possible for a fluid coupling to transmit the full motor breakdown torque for up to 20 seconds.
APPLICATIONS:
1) Torque Limitation - Belt Conveyors
2) Soft Start for High Inertia Machines: Crushers, Chippers, Centrifuges, Fans
3) Maximum Torque Transmission: Chain Conveyors, Sizers, Double Roll Crushers, Bucket
wheels
4) Load Sharing- Equipment with Multiple Drive Motors