After a brief historical review of the development of worm-gears, the author deals with worms and worm-wheels in detail, presenting the subjects of proper choice of materials, tooth-shapes, worm-gear efficiency, the stresses imposed on worm-gearing and worm-gear axles. Usually, he says, the worm is made of case-hardened steel of S.A.E. No. 1020 grade; however, when the worm-diameter is smaller and the stresses are greater, nickel-steels such as S.A.E. Nos. 2315 and 2320 grades are utilized. The worm should be properly heat-treated and carbonized to produce a glass-hard surface. Grinding of the worm-thread is necessary to remove distortions. Bronze is the only material of which the author knows that will enable the worm-wheel to withstand the high stresses imposed by motor-vehicle axles, and three typical bronze alloys are in common use. The degree of hardness of the bronze is very important
Duralumin, forged and heat-treated and used for worm-gears, costs approximately the same as bronze and reduces the weight two-thirds; such worm-wheels have withstood severe service.
As to tooth-shape, the common pressure-angle is 30 deg. This angle produces an included axial-angle of 60 deg. and a normal included-angle of 40 or 50 deg., depending upon the lead, and also secures proper reversibility. A properly made worm-gear is as efficient as any other form of gearing, according to the author, and he mentions efficiencies of from 97 to 99 per cent attained by hour-glass-shaped worms under ideal loads and conditions. Tooth-pressures and rubbing velocities are the two important considerations affecting stresses on worm-gears; the relative stresses vary with the lead angle. In conclusion, it is stated that if a worm-gear axle is unsuccessful, this is due to imperfections of design and of manufacture and not because the principle of the worm-gear drive is not practicable.