Machinery's Handbook, 31st Edition
2320 GEAR MATERIALS tempered to a hardness that will permit the cutting of the teeth. This treatment gives a highly refined structure, great toughness, and, in spite of the low hardness, excellent wear- ing qualities. The lower strength is somewhat compensated for by the elimination of the increment loads due to the impacts which are caused by inaccuracies. When steels that have a low degree of hardness penetration from surface to core are treated in this manner, the design cannot be based on the physical properties corresponding to the hardness at the surface. Since the physical properties are determined by the hardness, the drop in hard- ness from surface to core will give lower physical properties at the root of the tooth, where the stress is greatest. The quenching medium may be either oil, water, or brine, depending on the steel used and hardness penetration desired. The amount of distortion, of course, is immaterial, because the machining is done after heat treating. Making Pinion Harder than Gear to Equalize Wear.— Beneficial results from a wear standpoint are obtained by making the pinion harder than the gear. The pinion, having a lesser number of teeth than the gear, naturally does more work per tooth, and the differen tial in hardness between the pinion and the gear (the amount being dependent on the ratio) serves to equalize the rate of wear. The harder pinion teeth correct the errors in the gear teeth to some extent by the initial wear and then seem to burnish the teeth of the gear and increase its ability to withstand wear by the greater hardness due to the cold-working of the surface. In applications where the gear ratio is high and there are no severe shock loads, a case-hardened pinion running with an oil-treated gear, treated to a Brinell Hard- ness at which the teeth may be cut after treating, is an excellent combination. The pinion, being relatively small, is distorted but little, and distortion in the gear is circumvented by cutting the teeth after treatment. Forged and Rolled Carbon Steels for Gears.— These compositions cover steel for gears in three groups, according to heat treatment, as follows: a) case-hardened gears b) unhardened gears, not heat treated after machining c) hardened and tempered gears Forged and rolled carbon gear steels are purchased on the basis of the requirements as to chemical composition specified in Table 1. Class N steel will normally be ordered in ten point carbon ranges within these limits. Requirements as to physical properties have been omitted, but when they are called for the requirements as to carbon shall be omitted. The steels may be made by either or both the open hearth and electric furnace processes. Table 1. Compositions of Forged and Rolled Carbon Steels for Gears Heat Treatment Class Carbon Manganese Phosphorus Sulfur Case-hardened C 0.15–0.25 0.40–0.70 0.045 max 0.055 max Untreated N 0.25–0.50 0.50–0.80 0.045 max 0.055 max Hardened (or untreated) H 0.40–0.50 0.40–0.70 0.045 max 0.055 max Forged and Rolled Alloy Steels for Gears.— These compositions cover alloy steel for gears, in two classes according to heat treatment, as follows: a) case-hardened gears b) hardened and tempered gears Forged and rolled alloy gear steels are purchased on the basis of the requirements as to chemical composition specified in Table 2. Requirements as to physical properties have been omitted. The steel shall be made by either or both the open hearth and electric furnace process.
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