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Fans are used in copper ore processing facilities to exhaust hot air generated by equipment. Fans dealing with high temperature gases need to rotate continuously at low speed when turned off during cooling periods. When the fan's main drive stops, the overrunning clutch engages, secondary motor/gear reducer drivetrain that rotates the fan very slowly. This cooling period is necessary to reduce the temperature of the large squirrel cage fans and 4" diameter shafts, protecting them from sagging and warping damage due to their weight and exposure to elevated temperatures.
The overrunning clutch will automatically disengage the low-speed drive when the main high-speed motor starts, preventing possible damage to the low-speed drive.