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Various induction hardening metallurgical variables for several induction hardening experiments in hardening gears are outlined and related to heat energy inputs and their effect on the bending fatigue strength of gears. The experiments included residual stress measurements by X-ray diffraction and by sectioning and etching techniques; bending fatigue tests; metallurgical examination comprising micro-hardness traverse tests and microstructure-macrostructure evaluations; and dimensional analysis. Based on the results obtained, it was concluded that detrimental residual tensile stresses in the fillet surface of the tooth and the presence of dark etching transformation products near the surface can affect the performance of induction hardened gears in bending fatigue, and that metallurgical requirements can be specified to improve the reliability of induction hardened gears.

IN the first part of this paper, Mr. Halgren discusses modified 4-square testing of gears in relation to service performance, field tests, and other accelerated laboratory test methods. It contains a short description and photographs of nine different designs of modified 4-square gear testing machines now in use. Description of machines includes comments on mounting rigidity, mass-elastic system, load application, measurement and control, shut-off devices, and lubrication. Mr. Wulpi discusses single-tooth fatigue testing which is used to measure the bending strength of gear teeth. One tooth at a time is tested in the manner of a cantilever beam. By successive testing, many teeth of the same gear may be tested, and endurance limits obtained if desired. Results of these tests show that it is possible to determine clearly the effect of various metallurgical factors, such as material, heat-treatment, or surface-finishing treatment upon the bending-fatigue strength of gear teeth.

INDUCTION heating, the authors show, has been successfully applied to the hardening of many types of gears. Over a million transmission gears have been produced with its aid and thousands of induction-hardened final-drive gears and pinions are giving satisfactory field service. It has resulted in lower costs (due to the substitution of carbon for alloy steel, fewer machining operations, and lower heat-treating costs) and improved quality (due to a lack of distortion and better stress distribution). The first part of this paper, by Mr. Kincaid, covers the equipment and methods used in handling various gear jobs. Then Mr. Knowlton covers the engineering tests and service performance of various types of induction-hardened gears.