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A worm drive is a gear train in which a worm (which is a gear in the form of a Screw thread) meshes with a worm wheel (which is similar in appearance to a spur gear). Its main purpose is to translate the motion of two perpendicular axes or to translate circular motion to linear motion (example: band type hose clamp).The two machine element are also called the worm screw and worm gear. The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm wheel, or the worm drive as a unit.
The worm drive or "endless screw" was invented by either Archytas of Tarentum, Apollonius of Perga, or Archimedes, the last one being the most probable author.Witold Rybczynski, . London, 2000. Page 139. The worm drive later appeared in the Indian subcontinent, for use in roller , during the Delhi Sultanate in the thirteenth or fourteenth centuries.Irfan Habib, Economic History of Medieval India, 1200–1500, page 53, Pearson Education
Worm drive configurations in which the wheel cannot drive the worm are called self-locking. Whether a worm drive is self-locking depends on the lead angle, the pressure angle, and the coefficient of friction.
It was recognized since the invention of the worm drive that it was most effective when a large gear ratio was to be used; up until the 1900s, it continued to be used for this purpose, though it found limited applications in the early development of electric motors as the drives would overheat at high shaft speeds. The modern applications of the worm drive began shortly after the introduction of more effective lubrication methods through closed gear housings.
Worm drives are a compact means of substantially decreasing speed and increasing torque. Small are generally high-speed and low-torque; the addition of a worm drive increases the range of applications that it may be suitable for, especially when the worm drive's compactness is considered.
Worm drives are used in Machine press, rolling mills, conveyor system, mining industry machines, on , and circular saws. In addition, milling heads and are positioned using high-precision duplex worm drives with adjustable backlash. Worm drives are used on many lift/elevator and escalator drive applications, due to their compact size and their non-reversibility.
In the era of sailing ships, the introduction of a worm drive to control the rudder was a significant advance. Prior to its introduction, a rope drum drive controlled the rudder. Rough seas could apply substantial force to the rudder, often requiring several men to steer the vessel—some drives had two large-diameter wheels so that up to four crewmen could operate the rudder.
Worm drives have been used in a few automotive rear-axle final drives (though not the differential itself). They took advantage of the location of the worm being at either the very top or very bottom of the differential crown wheel. In the 1910s, they were common on trucks; to gain the most clearance on muddy roads, the worm was placed on top. In the 1920s, the Stutz firm used them on its cars; to have a lower floor than its competitors, the worm was located on the bottom. An example circa 1960 was the Peugeot 404. The worm drive protects the vehicle against rollback. This ability has largely fallen from favour, due to the higher-than-necessary reduction ratios.
A more recent exception to this is the Torsen differential, which uses worm wheels and planetary worms, in place of the bevel gearing of conventional open differentials. Torsen differentials are most prominently featured in the Humvee and some commercial Hummer vehicles, and as a centre differential in some all-wheel drive systems, such as Audi's quattro. Very heavy trucks, such as those used to carry aggregates, often use a worm drive differential for strength. The worm drive is not as efficient as a hypoid gear, and such trucks invariably have a very large differential housing, with a correspondingly large volume of gear oil, to absorb and dissipate the heat created.
Worm drives are used as the tuning mechanism for many musical instruments, including , , , , and many (although most high-end banjos use planetary gears or friction pegs). A worm drive tuning device is called a machine head.
Plastic worm drives are often used on small battery-operated electric motors, to provide an output with a lower angular velocity (fewer revolutions per minute) than that of the motor, which operates best at a fairly high speed, in addition to being quieter than a drive with metal gears. This motor-worm-drive system is often used in toys and other small electrical devices.
A worm drive is used on Jubilee-type or Jubilee Clip. The tightening screw's worm thread engages with the slots on the clamp band.
Occasionally a worm drive is designed to run in reverse, resulting in the worm shaft turning much faster than the input. Examples of this may be seen in some hand-cranked , blacksmithing forge Centrifugal fan, or the wind governor in a music box.
A left-hand helical gear or left-hand worm is one in which the teeth twist anticlockwise as they recede from an observer looking along the axis.
Manufacture
Worm wheels are first gashing to rough out the teeth then further refined closer to the final shape of the gear often within . If the gashing is done accurately enough special tools will not be required for the hobbing process. After a few passes of gashing, they are hobbed to their final shape
See also
External links
Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes an e-book library of classic texts on mechanical design and engineering.
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