H4-HH-7-A gs made of spheroidal graphite ENGJS or welded ve Helical gear reducers H4

Flender/Flender Gear Units/Helical gear reducers H4

heat treatment. Fundamentally, small distortions can be expected only if the work piece has low, homogeneous residual stress level. High residual stresses also increase the risk of crack formation and provide the worst possible preconditions for the hardening process. Avoidable distortions are caused chiey by thermal stresses and by residual and processing stresses. In order to control these, requirements must be imposed on the material, on the bevel gear design and on the heat treatment itself. Minimal residual stresses will occur only in an original material which is as homogeneous as possible. Non-homogeneity, for example segregation, leads to stresses which can be released during cutting or heat treatment. Residual stresses can also be induced in the work piece through unsuitable cutting parameters or blunt tools, and always lead to distortions if they are released by heat treatment. Apart from the stress state of the bevel gear, the blank form has signicant effect. design optimized for low distortion will envisage an even mass distribu- tion and gradual variations in cross section. When work pieces are installed in rack for heat treatment and hardening, force may under certain circumstances be exerted on them. Since the high temperature yield strength of case hardening steel is low at the necessary process temperatures, deformations of the support rack may be reproduced irreversibly on the work piece. Other factors which increase distortion are steep temperature gradients inside the work piece which lead to time-shifted and locally-shifted thermal effects if heating rates are too high. Also of importance are the packing density and mass distribution on the rack, which inuence the ow of the process and quenching media around the work pieces. This is an especially critical factor where bevel gears are concerned, as there are widely different geometrical conditions for ows around work pieces at tooth tip and root. Fig. 6.2 Classication of hardening distortion