June 2010 / Volume 62 / Issue 6|
Restoring a massive machine
By CTE Staff
Look up “gigantic” in a dictionary and you might find Hannon Electric Co.’s main facility pictured next to the entry. The company’s 165,000-sq.-ft. Canton, Ohio, machine shop is one of the nation’s largest serving the electromechanical industry. It’s also one of the few with a specialized department for balancing large motor shafts, both in-house and as a contract service for other companies.
Courtesy of Riten Industries
The plant is equipped with high-capacity CNC engine lathes, the largest being a Craven lathe with a 130 " swing and 29 ' between centers. After initially seeking a 120 "×25 ' lathe, Hannon purchased the even-larger Craven used in 2000.
Hannon, which specializes in repairing and rebuilding electric motors up to 60,000 hp, was beginning to quote on new business for motor shafts weighing up to 180,000 lbs. These shafts do not lend themselves to being run with steady rests because they support the middle of the workpiece. That’s where the windings are, so the steady rest would damage a motor shaft. That meant the capacity of the lathe between centers was critical.
The lathe also needed to be exceptionally rigid to meet the required accuracy of 0.005 " TIR for size, roundness and taper—a difficult standard for large workpieces held between centers, according to Hannon.
The Craven lathe came with a conventional dead tailstock that utilized live centers with a 4.7 " gage line diameter. Hannon soon realized the tailstock was undersized and being overpowered when loads exceeded 70,000 lbs. That damaged the live centers, leading to excessive and time-consuming rework of out-of-tolerance parts and a growing concern for operator safety because a shaft might become loose, drop onto the machine and pull itself free of the chuck. Although the largest shafts might only be rotating at 10 to 50 rpm, they have a great deal of rolling inertia, which makes them difficult to stop and potentially dangerous, according to Mitchell Kirby, vice president of manufacturing for Riten Industries Inc., Washington Court House, Ohio, a manufacturer of workholding devices. “Large parts can kill people,” he said.
For weeks, Hannon was sending malfunctioning live centers to Riten for repair before it became apparent that the problem required a more comprehensive and permanent solution. Riten engineers were called in for a complete functional evaluation of the lathe. The review included workpiece weight, speeds and feeds, static and dynamic deflection, spindle shaft stiffness and rigidity requirements, tolerance requirements, tooling packages, bearing fits and capacity, thermal displacement, heat dissipation and maintenance procedures.
According to Riten, it uncovered numerous factors that were contributing to the problem. The live center, despite its extra heavy-duty design, lacked the capacity to handle the increasingly heavy loads. Riten traced these premature failures to the lack of rigidity in the tailstock, which was deflecting 0.010 " per side under the heavier loads. Inspection of the tailstock revealed it was from a smaller machine and the previous owner modified it for use on the Craven.
Although the proposed solution was to locate an original Craven maximum-capacity tailstock housing and couple it with a heavy-duty live quill from Riten, a global search for the housing was unsuccessful. The alternative was to convert the dead tailstock to a higher-capacity live tailstock and quill assembly. The new design would eliminate the approximate 12 " overhang associated with a live center by eliminating the extra bearing assembly and address the rigidity and capacity issues.
Riten designed and manufactured a 16 "-dia. live quill rated for a maximum workpiece weight of 200,000 lbs. During the manufacturing process, Riten provided engineering oversight, coordinating the efforts of a design firm and a fabricator responsible for the tailstock. Riten then installed the quill in the completed tailstock and supervised the installation of the entire assembly.
Concurrent with the tailstock, Riten reviewed the headstock to ensure it was compatible with the tailstock’s increased capacity and could support the load. The existing bearings were determined to have sufficient capacity.
After returning the lathe to service, final part sizes are within 0.002 "—less than half the maximum tolerance of 0.005 ". “We were impressed by Riten’s technical knowledge, not only on workholding issues but also on the total operating dynamics of our machinery,” said Steve Harper, Hannon’s COO.
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