The Cadillac Company has used S.A.E. 3250 steel for at least 8 years. This is medium nickel-chromium steel. Many other kinds have also been tried. Experience has shown that transmission gears made of carburized steel are not within 30 per cent as accurate as those made of oil-treated steel. This may be because of the fact that more attention has been paid to oil-hardened than to carburized steel gears. Efforts to control the distortion of carburized gears were unsuccessful. The hardening was done in salt pots, lead pots and open furnaces, heated by gas, oil and electricity. The same thing applies to spur gears. Oil-treated steel for rear axles has not been tried. When transmission gears were made from drop-forged blanks made by the conventional pegged-out process from flat stock they became oval. Upset gear forgings are used as fast as the forging suppliers can become equipped for the work. When a slug is cut from the end of a long bar, then up-ended and drop-hammered, the flow of metal is such that the distortion is much less and the tooth strength greater. When the flow of metal in rolling is in a certain direction the gears will contain flow-lines. Some teeth will be strong because the flow is lengthwise of the teeth; others weak because the flow is across them.
Special impact tests were made. One tooth was knocked out by a drop-hammer impact; then one on the opposite side was broken off. Teeth quartering to each other were found to be of vastly different strength. Teeth drop-forged by the pegged-out method vary in strength depending upon the flow-lines. Upset gear teeth are not so strong as the strongest of the teeth made from oil-treated steel forgings but are much stronger than the average. Upset gears have no weak teeth, for the flow is through the teeth, not across them; the teeth are uniformly strong all the way round.
Ring gears warp. They follow the same rule as regards flow-lines. When they are pegged out flat they try to get back to the flat-rolled bar form. If the ring gear is upset so that the flow is through the teeth it will become slightly distorted but will not tend to become oval. The strength will increase from 20 to 40 per cent, depending upon the skill with which the two methods of forging are executed. Upset gears absorb about 1300 ft-lb. of impact before cracking as against 260 ft-lb. for gears in which the flow is crosswise.
Originally the process used in making transmission gears was to rough-cut them on gear-hobbing machines, then finish to size, harden, wire-brush and match. Through automatic control and electric furnaces the warpage in heat-treating has been decreased and the uniform expansion can be allowed for. But the greatest improvement has been in the grinding of finished gears. The teeth of gears need be shaped only to 0.0050 in. oversize, leaving 0.0025 in. on a side for grinding. Two gears are shaped at a time, doubling the output; one man runs three machines where he used to run two. Three gear-shaping machines and one man produce the same number of gears that six machines and three men did formerly. Single-purpose hobbing machines cut the time still farther. New hobbing machines not only run faster than the old ones but they do away with gear-shaping machines entirely because the grinding machines will finish hobbed teeth as well as they will finish shaped teeth. Backlash, tolerance and eccentricity are also greatly reduced.
In assembling, the advantage is with ground gears as compared with cut gears. Tear-downs are fewer than 6 per cent and the standard of quietness is noticeably better. Scrap has been reduced from 3 to ⅛ of 1 per cent. When the first gear-grinding machines were installed, noises in the bearings and other parts appeared that had not been noticeable before. Ground gears save trouble, labor and time and have almost eliminated tear-downs and rematching.


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