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worm gear[′wərm ‚gir]
a mechanical device for transmitting rotation between shafts whose axes cross (usually at right angles) by means of a worm and a mating gear (the worm wheel or worm gear). The worm is a screw having a trapezoidal or nearly trapezoidal thread, and the worm wheel is a gear wheel with special curved teeth. The worm wheel subtends the worm over a specific arc (usually up to 100°). The worm is usually the driving member, and the worm wheel is driven; in rare cases (as in step-up gearing) the worm may be the driven member. Cylindrical worms are used in the most common (single enveloping) worm gearing; worms having threads on a toroidal surface are used in double enveloping worm gearing.
Depending on the number of thread starts, worms may be classified as single-, double-, or quadruple-threaded; depending on the profile of the thread helix, they may be classified as Archimedean, concave, or other type. The helix of an Archimedean worm in an axial cross section is described by a trapezoid and is easily machined on a lathe. Worms with a concave helix profile show promise for offering lower contact stresses and better conditions for the formation of an oil wedge. Involute worms are preferred for power transmissions; the helix profile in a cross section is described by an involute. The gear ratio of a worm gear is i = z2/z1, where z1 is the number of starts on the worm and z2 is the number of teeth on the worm wheel.
Gear ratios for worm gears are usually between 8 and 100 (in a number of cases, for example, the drives for machine-tool tables of large diameter, it may be as high as 1,000). Worm gears are used in machine drives and control devices because they afford high gear ratios in a transmission of relatively small size. The advantages of worm gears include smooth and noiseless operation. They may be irreversible, or self-locking; that is, they may transmit rotation only from the worm to the worm wheel, so that some mechanisms can be made without a locking device. A disadvantage is the substantial slip between the mating members—the worm threads and gear teeth. This results in intense evolution of heat, increased wear, and a tendency to seize; it also accounts for the relatively low efficiency of worm gearing (averaging 0.7–0.75 in single-threaded, 0.8–0.85 in double-threaded, and 0.86–0.92 in quadruple-threaded worm gears).
In order to reduce wear and decrease the chances of seizing, gear wheels are made from materials having excellent antifriction properties—mostly bronzes, such as true bronze (for slip velocities of 5–35 m/sec) and aluminum-iron bronze (for slip velocities up to 10 m/sec). For economy of use of nonferrous metals, worms are made up as composites with a bronze rim mounted on a hub of steel or pig iron. For slip velocities up to 2 m/sec and large diameters, the worm wheels can be made entirely of pig iron. The worm wheels used in instruments and small transmissions can be made of a resin-impregnated plastic laminate or nylon. The worms are usually made of high-grade carbon or alloy steels that are heat-treated to a high degree of hardness. The surfaces are ground and polished to improve the finish.
The low efficiency and substantial heat evolution of worm gears have limited them to low- and medium-power applications (usually up to 50 kilowatts, rarely up to 200 kilowatts). They are used in hoisting and conveying equipment, metal-cutting machine tools, motor vehicles, trolleybuses, metallurgical machinery, and hydraulic engineering installations. Well-lubricated reduction worm gears enclosed in a housing are the most widely used type.
REFERENCESDetail mashin: Raschet i konstruirovanie: Spravochnik, 3rd ed., vol. 3. Moscow, 1969.
Chasovnikov, L. D. Peredachi zatsepleniem, 2nd ed. Moscow, 1969.
Reshetov, D. N. Detail mashin, 3rd ed. Moscow, 1974.
Mukha, T. I., B. V. Ianush, and A. P. Tsupikov. Privody mashin: Spravochnik. Leningrad, 1975.
A. A. PARKHOMENKO