A worm drive is a
gear arrangement in which a worm (which is a gear in the form of a
screw
A screw is an externally helical threaded fastener capable of being tightened or released by a twisting force (torque) to the screw head, head. The most common uses of screws are to hold objects together and there are many forms for a variety ...
) meshes with a worm wheel (which is similar in appearance to a
spur gear
Spur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk with teeth projecting radially. Viewing the gear at 90 degrees from the shaft length (side on) the tooth faces are straight and aligned parallel to ...
). 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
A hose clamp is a device used to attach and seal a hose (tubing), hose onto a fitting such as a barb (hose), barb or nipple (plumbing), nipple.
Materials
The core range of hose clamps on the open market are made from materials like stainles ...
).The two
elements 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
Archytas (; ; 435/410–360/350 BC) was an Ancient Greek mathematician, music theorist, statesman, and strategist from the ancient city of Taras (Tarentum) in Southern Italy. He was a scientist and philosopher affiliated with the Pythagorean ...
of Tarentum,
Apollonius of Perga
Apollonius of Perga ( ; ) was an ancient Greek geometer and astronomer known for his work on conic sections. Beginning from the earlier contributions of Euclid and Archimedes on the topic, he brought them to the state prior to the invention o ...
, or
Archimedes
Archimedes of Syracuse ( ; ) was an Ancient Greece, Ancient Greek Greek mathematics, mathematician, physicist, engineer, astronomer, and Invention, inventor from the ancient city of Syracuse, Sicily, Syracuse in History of Greek and Hellenis ...
, the last one being the most probable author.
Witold Rybczynski
Witold Rybczynski (born 1 March 1943) is a Canadian American architect, professor and writer. He is currently the Martin and Margy Meyerson Professor Emeritus of Urbanism at the University of Pennsylvania.
Early life
Rybczynski was born in E ...
, '' One good turn : a natural history of the screwdriver and the screw''. London, 2000. Page 139. The worm drive later appeared in the
Indian subcontinent
The Indian subcontinent is a physiographic region of Asia below the Himalayas which projects into the Indian Ocean between the Bay of Bengal to the east and the Arabian Sea to the west. It is now divided between Bangladesh, India, and Pakista ...
, for use in roller
cotton gin
A cotton gin—meaning "cotton engine"—is a machine that quickly and easily separates cotton fibers from their seeds, enabling much greater productivity than manual cotton separation.. Reprinted by McGraw-Hill, New York and London, 1926 (); ...
s, during the
Delhi Sultanate
The Delhi Sultanate or the Sultanate of Delhi was a Medieval India, late medieval empire primarily based in Delhi that stretched over large parts of the Indian subcontinent for more than three centuries. in the thirteenth or fourteenth centuries.
Irfan Habib
Irfan Habib (born 10 August 1931) is an Indian historian of ancient and medieval India, following the methodology of Marxist historiography in his contributions to economic history. He is known for his strong stance against Hindutva. He has au ...
''Economic History of Medieval India, 1200–1500'', page 53
Pearson Education
Pearson Education, known since 2011 as simply Pearson, is the educational publishing and services subsidiary of the international corporation Pearson plc. The subsidiary was formed in 1998, when Pearson plc acquired Simon & Schuster's educatio ...
Explanation

A
gearbox
A transmission (also called a gearbox) is a mechanical device invented by Louis Renault (who founded Renault) which uses a gear set—two or more gears working together—to change the speed, direction of rotation, or torque multiplication/r ...
designed using a worm and worm wheel is considerably smaller than one made from plain spur gears, and has its drive axes at 90° to each other.
With a ''single-start'' worm, for each 360° turn of the worm, the worm wheel advances by only one tooth. Therefore, regardless of the worm's size (sensible engineering limits notwithstanding), the gear ratio is the ''"size of the worm wheel - to - 1"''.
Given a single-start worm, a 20-tooth worm wheel reduces the speed by the ratio of 20:1. With spur gears, a gear of 12 teeth must match with a 240-tooth gear to achieve the same 20:1 ratio. Therefore, if the diametrical pitch (DP) of each gear is the same, then, in terms of the physical size of the 240 tooth gear to that of the 20 tooth gear, the worm arrangement is considerably smaller in volume.
Types
Types of worm drives
The entire drive (both worm and wheel) can be classified as follows:
;Non-throated worm drives
: These don't have a ''throat'', or groove, machined around the circumference of either the worm or worm wheel.
;Single-throated worm drives
: The worm wheel is throated.
;Double-throated worm drives
: Both gears are throated. This type of gearing can support the highest loading.
Types of worms
These classifications refer to the worm itself:
;''Enveloping worm'' (hourglass worm)
: The worm has one or more teeth, and increases in diameter from its middle portion toward both ends.
;''Double-enveloping worm''
: The worm’s gearing comprises enveloping worms mated with fully enveloping worm wheels. It is also known as globoidal worm gearing.
[
]
Direction of transmission
Unlike with ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible when using large reduction ratios. This is due to the greater friction involved between the worm and worm wheel, and is especially prevalent when a single-start (one spiral) worm is used. This can be an advantage when it is desired to eliminate any possibility of the output driving the input. If a multi-start worm (multiple spirals) is used, then the ratio reduces accordingly, and the ''braking effect'' of a worm and worm wheel may need to be discounted, as the wheel may be able to drive the worm.
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.
History
The invention of the worm drive is attributed by some to Archimedes
Archimedes of Syracuse ( ; ) was an Ancient Greece, Ancient Greek Greek mathematics, mathematician, physicist, engineer, astronomer, and Invention, inventor from the ancient city of Syracuse, Sicily, Syracuse in History of Greek and Hellenis ...
during the First Punic War
The First Punic War (264–241 BC) was the first of three wars fought between Rome and Carthage, the two main powers of the western Mediterranean in the early 3rd century BC. For 23 years, in the longest continuous conflict and grea ...
, wherein the size of the ships being built necessitated a much larger crane than was available at the time. The crane developed for this purpose utilised a worm drive and several magnifying gears and was named the ''barulkon''. The description of this crane was recorded in the Library of Alexandria
The Great Library of Alexandria in Alexandria, Egypt, was one of the largest and most significant libraries of the ancient world. The library was part of a larger research institution called the Mouseion, which was dedicated to the Muses, ...
, and subsequent engineers would draw upon Archimedes' ideas until the first technical drawings of a worm drive were developed by Leonardo da Vinci
Leonardo di ser Piero da Vinci (15 April 1452 - 2 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially rested o ...
; the design was limited by the fact that metallic gears had not been invented by the advent of the 15th century, and the drive was never built in his lifetime.
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.
Applications
In early 20th century automobiles prior to the introduction of power steering, the effect of a flat or blowout on one of the front wheels tended to pull the steering mechanism toward the side with the flat tire. The use of a worm drive reduced this effect. Further worm drive development led to recirculating ball bearings to reduce frictional forces, which transmitted some steering force to the wheel. This aids vehicle control, and reduces wear that could cause difficulties in steering precisely.
Worm drives are a compact means of substantially decreasing speed and increasing torque. Small electric motor
An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...
s 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 presses, rolling mill
In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is simi ...
s, conveying engineering, mining industry machines, on rudder
A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, airship, or other vehicle that moves through a fluid medium (usually air or water). On an airplane, the rudder is used primarily to counter adverse yaw ...
s, and circular saw
A circular saw or a buzz saw, is a power-saw using a toothed or Abrasive saw, abrasive disk (mathematics), disc or blade to cut different materials using a rotary motion spinning around an Arbor (tool), arbor. A hole saw and ring saw also use ...
s. In addition, milling
Milling may refer to:
* Milling (minting), forming narrow ridges around the edge of a coin
* Milling (grinding), breaking solid materials into smaller pieces by grinding, crushing, or cutting in a mill
* Milling (machining), a process of using ro ...
heads and rotary table
A rotary table is a precision work positioning device used in metalworking. It enables the operator to drill or cut work at exact intervals around a fixed (usually horizontal or vertical) axis. Some rotary tables allow the use of index plates ...
s are positioned using high-precision duplex worm drives with adjustable backlash
Backlash may refer to:
Literature
* '' Backlash: The Undeclared War Against American Women'', a 1991 book by Susan Faludi
* ''Backlash'' (Star Wars novel), a 2010 novel by Aaron Allston
* Backlash (Marc Slayton), a comic book character from ...
. 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
A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, airship, or other vehicle that moves through a fluid medium (usually air or water). On an airplane, the rudder is used primarily to counter adverse yaw ...
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
The Stutz Motor Car Company was an American automobile Automotive industry, manufacturer based in Indianapolis, Indiana that produced high-end Sports cars, sports and Luxury vehicle, luxury cars. The company was founded in 1911 as the Idea ...
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
Torsen Torque-Sensing (full name Torsen traction) is a type of limited-slip differential used in automobiles.
It was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. Torsen is a portmanteau of Torque-Sensing. '' ...
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
The High Mobility Multipurpose Wheeled Vehicle (HMMWV; colloquial: Humvee) is a family of Military light utility vehicle, light, four-wheel drive Military vehicle#Military trucks, military trucks and utility vehicles produced by AM General. It ...
and some commercial Hummer
Hummer (stylized in all caps) is an American brand of Pickup truck, pickups launched in 1992 when AM General began selling a civilian version of the M998 Humvee. Although discontinued in 2010, Hummer returned as a model under GMC (automob ...
vehicles, and as a centre differential in some all-wheel drive
An all-wheel drive vehicle (AWD vehicle) is one with a powertrain capable of providing power to all its wheels, whether full-time or on-demand.
Types
The most common forms of all-wheel drive are:
;1x1 : All unicycles Reflects one axle with ...
systems, such as Audi
Audi AG () is a German automotive manufacturer of luxury vehicles headquartered in Ingolstadt, Bavaria, Germany. A subsidiary of the Volkswagen Group, Audi produces vehicles in nine production facilities worldwide.
The origins of the compa ...
'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
Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It has high viscosity and usually contains organosulfur compounds. Some modern automatic transaxles ...
, to absorb and dissipate the heat created.
Worm drives are used as the tuning mechanism for many musical instruments, including guitar
The guitar is a stringed musical instrument that is usually fretted (with Fretless guitar, some exceptions) and typically has six or Twelve-string guitar, twelve strings. It is usually held flat against the player's body and played by strumming ...
s, double bass
The double bass (), also known as the upright bass, the acoustic bass, the bull fiddle, or simply the bass, is the largest and lowest-pitched string instrument, chordophone in the modern orchestra, symphony orchestra (excluding rare additions ...
es, mandolin
A mandolin (, ; literally "small mandola") is a Chordophone, stringed musical instrument in the lute family and is generally Plucked string instrument, plucked with a plectrum, pick. It most commonly has four Course (music), courses of doubled St ...
s, bouzouki
The bouzouki (, also ; ; alt. pl. ''bouzoukia'', , from Greek , from Turkish ) is a musical instrument popular in West Asia (Syria, Iraq), Europe and Balkans (Greece, North Macedonia, Bulgaria, Turkey). It is a member of the long-necked lute fam ...
s, and many banjo
The banjo is a stringed instrument with a thin membrane stretched over a frame or cavity to form a resonator. The membrane is typically circular, and in modern forms is usually made of plastic, where early membranes were made of animal skin.
...
s (although most high-end banjos use planetary gears or friction pegs). A worm drive tuning device is called a machine head
A machine head (also referred to as a tuning machine, tuner, or gear head) is a geared apparatus for tuning stringed musical instruments by adjusting string tension. Machine heads are used on mandolins, guitars, double basses, and others, and ...
.
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 hose clamp
A hose clamp is a device used to attach and seal a hose (tubing), hose onto a fitting such as a barb (hose), barb or nipple (plumbing), nipple.
Materials
The core range of hose clamps on the open market are made from materials like stainles ...
s or Jubilee clamps. 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 centrifuge
A centrifuge is a device that uses centrifugal force to subject a specimen to a specified constant force - for example, to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby ...
s, blacksmithing forge blower, or the wind governor
A governor is an politician, administrative leader and head of a polity or Region#Political regions, political region, in some cases, such as governor-general, governors-general, as the head of a state's official representative. Depending on the ...
in a musical box
A music box (American English) or musical box (British English) is an automatic musical instrument in a box that produces Musical note, musical notes by using a set of pins placed on a revolving cylinder (geometry), cylinder or disc to pluck ...
.
Left-hand and right-hand worm
A right-hand helical gear or right-hand worm is one in which the teeth twist clockwise as they recede from an observer looking along the axis. The designations, right-hand and left-hand, are the same as in the long-established practice for screw threads, both external and internal. Two external helical gears operating on parallel axes must be of opposite hand. An internal helical gear and its pinion must be of the same hand.
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 gashed 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
* List of gear nomenclature
This page lists the standard US nomenclature used in the description of mechanical gear construction and function, together with definitions of the terms. The terminology was established by the American Gear Manufacturers Association (AGMA), und ...
* Gear
A gear or gearwheel is a rotating machine part typically used to transmit rotational motion and/or torque by means of a series of teeth that engage with compatible teeth of another gear or other part. The teeth can be integral saliences or ...
* Linear actuator
A linear actuator is an actuator that creates linear motion (i.e., in a straight line), in contrast to the circular motion of a conventional electric motor. Linear actuators are used in machine tools and industrial machinery, in computer perip ...
, some forms occasionally also referred to as worm gear or worm drive
* Rack and pinion
rack and pinion is a type of linear actuator that comprises a circular gear (the '' pinion'') engaging a linear gear (the ''rack''). Together, they convert between rotational motion and linear motion: rotating the pinion causes the rack to be d ...
* Slewing drive
References
External links
Kinematic Models for Design Digital Library (KMODDL)
br /> Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes a
e-book library
of classic texts on mechanical design and engineering.
Various Metric Gears downloadable design specifications, 2D-3D models and catalogues
Various Worm Gearboxes, 3D models
Machining of Worm Shaft and Worm Gears
{{Authority control
Gears
Greek inventions
Archimedes