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Coupling Renewed For Half Cost Of Replacement

15 January 2013

When the bore on the rigid half of a high-torque steel mill coupling was badly galled during a maintenance teardown, the East Chicago, Indiana, mill put the damaged part in the plant’s bone yard.

However, engineers at Kop-Flex – part of the Power Transmission Solutions business of Emerson Industrial Automation – developed a clever solution that saved the customer almost half the cost to replace the $112,000 part, which transmits approximately 10,000,000 pound inches (1,130,000Nm) of torque to a roughing stand.

The CM-30 cast steel gear coupling connects the pinion on a 10,000hp bull gear motor to a roughing stand on an 84-inch (21 m) hot strip mill. To transmit the required torque, the coupling’s rigid half attaches to the pinion with a heavy interference fit of 0.028 inch (0.7mm), plus two massive split- taper keys 4 x 5.5 x 47 inches (100 x 140 x 1,194mm), each weighing about 340lbs (154kg).

To create a new bore in the part without using a welded plug, Kop-Flex engineers proposed a new solution using a shrink-fitted, splined- in bushing, because splined connections can transmit high torque loads. Engineers designed the splines to withstand the bending and compressive forces with an adequate safety factor. They also designed the splines to achieve an interference fit (negative backlash) of 0.015 to 0.020 inch (0.4 to 0.5mm), along with the bushing’s diametral interference.