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Shaking removes stress from racing cranks.

A simple sub-harmonic vibration process has cut stress-relieving time and eliminated conventional furnace stress relieving for a special line of crankshafts. Callies Performance Inc, Fostoria, OH, has chosen the 90-min nonthermal process to replace 13-hr thermal stress relieving for all its crankshafts used in its high-performance automotive and marine racing engines.

The process, called Meta-Lax, was developed by Bonal Technologies, Southfield, MI. It is based on the difference in harmonic peak between a stressed part and that same part after stress relief (thermal or otherwise). It evolved from using this dynamic characteristic to nondestructively test parts to confirm proper stress relief.

To initially determine how much vibration is required for a given part, a force inducer and sensing transducer are connected to it and the harmonic frequency of the part measured. Then, the part is sub-harmonically vibrated for 20 min and allowed to "relax" for an additional half hour. It is then retested, and any shift in harmonic frequency indicates stresses have been relieved. This process is repeated until no frequency shift occurs (indicating no additional benefit) to determine the length of processing required. Parts up to 15,000 lb take 90 min or less.

The process removes thermally induced residual stresses in most ferrous and nonferrous metals, including hot-rolled, forged, and cast parts without altering their mechanical properties. Unlike thermal stress relieving, the sub-harmonic process can relieve residual stress in metals previously hardened by heat treatment without affecting hardness.

Other vibratory processes operating at resonant frequencies, Bonal Technologies' Tom Hebel points out, produce inconsistent results, and if done for extended periods, can fatigue the workpiece. In contrast, Meta-Lax subharmonic stress relief is both verifiable and predictable, and it consistently matches or exceeds furnace-stress-relief properties.

Beyond the major time and energy savings over furnace stress relieving, the process leaves no furnace scale nor causes warpage or distortion. It is also a relatively portable process that can be applied anywhere on the shop floor.
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Publication:Tooling & Production
Date:Jun 1, 1991
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