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Wrecked steel mill coupling renewed for half the cost of replacement

Bore ‘replaced’ with splined-in insert — correct diameter restored, original rating maintained

February 11, 2013
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By Dan Kocel

was badly galled during a maintenance teardown, an East Chicago, Ind., mill put the damaged part in the plant’s boneyard. However, engineers at Kop-Flex — part of the Power Transmission Solutions business of Emerson Industrial Automation — developed a clever solution that saved almost half the cost it would have taken to replace the $112,000-part, which transmits approximately 10,000,000 pound inches of torque to a roughing stand.

The CM-30 cast-steel gear coupling connects the pinion on a 10,000 horsepower bull gear motor to a roughing stand on an 84-inch 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, plus two massive split-taper keys 4 x 5.5 x 47 inches, each weighing about 340 lbs.

A hopeless case

According to the manager of maintenance/spares at the East Chicago mill, the rigid half of the coupling, which weighs about 31,000 lbs. and has a bolt-hole diameter of almost six feet, had to be removed from a damaged bull gear shaft after the gearbox lost a tooth. During removal, the coupling’s bore was badly gouged.

“We considered the damaged part hopeless,” the manager says. “We were going to replace the bull-gear shaft, so we could not simply bore-out the coupling to a larger size which would then not fit. A standard coupling repair procedure would involve enlarging the bore, shrink fitting and welding a plug in the bore, and then re-boring the center diameter to correct size.”

However, the manager goes on to point out, the rigid half is ASTM A27 cast steel, and the diametral force of a heavy interference fit for the plug would create internal stresses exceeding the limits for this part, and welding would not provide the strength to transmit the torque.

“Our solution was to scrap the rigid half and replace it,” he says. “Months later when assessing our mill’s inventory of critical spare parts, we realized we had no spares for this part, so we asked Kop-Flex for a quote. The Kop-Flex manager suggested we consider repairing the damaged part, so we agreed to have them assess the engineering options.”

Expertise pays off

To create a new bore in the part without using a welded plug, Kop-Flex engineers proposed using a shrink-fitted, splined-in bushing, because splined connections can transmit high torque loads. The splines were designed to withstand the bending and compressive forces with an adequate safety factor. The design achieved an interference fit (negative backlash) of 0.015 to 0.020 inch, along with the bushing’s diametral interference. The repair bushing itself is 39 inches in diameter, with an approximate weight of 8,000 lbs.

The bore of the salvaged rigid half was enlarged and cut with mating splines, as well as locating surfaces for the bushing. After shrink-fitting the two parts, the faces were machined flush with the existing flange surface.

The repaired part was installed and tested on a herringbone bull gear at a vendor’s plant and determined to meet specifications. The repair saved the customer $54,000 over the cost of a new rigid half, and the repaired coupling will be reinstalled at the mill during a planned August shutdown.

A splined repair is also an excellent way to replace worn gears on large gear coupling hubs. The existing gears are cut off the outside diameter and replaced with a spline section. A repair ring, with gear teeth on the outside diameter and splines on the inside diameter, is then installed on the modified hub. This is an economical and proven way to repair large gear couplings with excessive tooth wear.

For more information, an interactive page-flip version of Emerson’s Industrial Coupling Catalog is online at Printed copies can be ordered at

Dan Kocel is global metals industry manager, Emerson Power Transmission Solutions.

About Emerson

Emerson, based in St. Louis, Mo., is a global leader in bringing technology and engineering together to provide innovative solutions for customers in industrial, commercial and consumer markets around the world. The company is comprised of five business segments: Process Management, Industrial Automation, Network Power, Climate Technologies, and Commercial & Residential Solutions. Sales in fiscal 2012 were $24.4 billion. For more information, visit

Emerson Industrial Automation is a business of Emerson. Its products include alternators, electric motors and drives, electrical distribution devices and mechanical power transmission, fluid automation and ultrasonic joining solutions. Emerson brands include Appleton, ASCO, Branson Ultrasonics, Browning, Control Techniques, Kop-Flex, Leroy-Somer, McGill, Morse, Numatics, O-Z/Gedney, Rollway, Sealmaster, SSB Wind Systems and System Plast. For more information, visit



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