COORDINATING GEAR DESIGN AND PRODUCTION METHODS
Periodically recurring problems of gear noise and wear which seem to arise from no specific cause frequently affect the manufacturing side of the automotive industry and especially the gear-manufacturers. While much has been written and discussed about the mathematics and geometry of gears, which should overcome all of these problems, the trouble unfortunately still persists. The paper outlines the experience of the organization with which the author is connected in solving a rather difficult problem that offered an opportunity for a more thorough analysis than did its predecessors. Laboratory and dynamometer analyses of the product showed that it compared favorably with the output, of other factories. Throughout the entire preliminary investigation evidence was found that the coordination of the high points found in each steel as furnished by the mill, its heat-treatment, the details of the tooth form, the mathematics of gear action and the manufacturing processes would enable great strides to be made in both increasing the performance and reducing the cost of the product.
The first point attacked was the steel, which was a No. 5150 S.A.E. Steel that had been substituted some years before for the chrome-vanadium No. 6145 steel originally used. A series of tests was made on practically the entire group of chrome-carbon, chromevanadium, chrome-manganese, manganese-molybdenum and chrome-nickel steels, using the lead-pot, salt-bath, cyanide and electric-furnace methods of heat-treatment. While the conclusion reached was that very little difference in the performance of any of these steels with normal treatment could be detected, the high wearvalue of cyanide-treated chrome-steel arid the even better value of chrome-vanadium steel when cyanide treated were the two outstanding features. In every case a metallurgical representative from the steel mill worked with the organization's metallurgist so as to take advantage of all the knowledge available for getting the best results from each type of steel. These tests furnished the basis for probably the most vital information required for motor-car-transmission design, the ratio of dynamometer hours to miles of field operation of the car in intermediate gear. This figure varied from 1:1000 to 1:8000, a safe average being 1:3000.
A study of the characteristics of the wear of the first series of dynamometer tests revived interest in some calculations and tests that had previously been made on modified-addendum gears, it being apparent that the wear on the gear tooth bore a definite relation to the square inches of active tooth surface rather than to merely the pressure per lineal inch on the tooth. This resulted in an effort to make the number of square inches of active tooth surface on each pair of gears as nearly equal as possible, with the result that a reduction of 20 per cent in the weight of a transmission of a given capacity was effected.
Another example of coordinated activity was the grinding of gear teeth. About 3 years ago the decision was made that the gear teeth should be ground. After installing a battery of gear-tooth grinding-machines operating on the generating principle, it was found that a consistently good production could not be obtained and the form-wheel type of machine was substituted, the reason for the change being that the principles of approved grinding-machine construction and grinding-wheel practice were not violated and that the degree of accuracy required is merely a matter of machine maintenance and proper attention to the forming device.
As the result of the fullest cooperation from the steel mill, the forge shop, the heat-treating and metallurgical departments, the machine-tool builder, the grinding-wheel manufacturer and the engineer, the immediate problems of the factory and its customer have been solved, the gear-noise problem has not recurred to any great extent and the likelihood of its recurring in the future has been reduced. A greatly improved product, with in most instances a lowered cost, has followed, but the most valuable accomplishment in the author's opinion is the bringing about of a spirit of cooperation in the shop and with the customer.