A worm drive is a gear arrangement in which a worm meshes with a worm gear.
The two elements are also called the worm screw and worm wheel. The
terminology is often confused by imprecise use of the term worm gear to
refer to the worm, the worm gear, or the worm drive as a unit.
Like other gear arrangements, a worm drive can reduce rotational speed or
transmit higher torque. The image shows a section of a gear box with a worm gear
driven by a worm. A worm is an example of a screw, one of the six simple
machines. Explanation
A gearbox 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-gear advances
only one tooth of the gear. Therefore, regardless of the worm's size, the gear
ratio is the "size of the worm gear - to - 1". Given a single start worm, a 20
tooth worm gear 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 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 There are three different types of gears
that can be used in a worm drive. The first are non-throated worm gears.
These don't have a throat, or groove, machined around the circumference of
either the worm or worm wheel. The second are single-throated worm gears,
in which the worm wheel is throated. The final type are double-throated worm
gears, which have both gears throated. This type of gearing can support the
highest loading. An enveloping worm has one or more teeth
and increases in diameter from its middle portion toward both ends.
Double-enveloping wormgearing comprises enveloping worms mated with fully
enveloping wormgears. It is also known as globoidal wormgearing.
Direction of transmission Unlike with ordinary gear trains, the
direction of transmission is not reversible when using large reduction
ratios, due to the greater friction involved between the worm and
worm-wheel, when usually a single start worm is used. This can be an advantage
when it is desired to eliminate any possibility of the output driving the
input. If a multistart worm is used then the ratio reduces accordingly and the
braking effect of a worm and worm-gear may need to be discounted, as the gear
may be able to drive the worm. Worm gear configurations in which the
gear cannot drive the worm are called self-locking. Whether a worm and gear is
self-locking depends on the lead angle, the pressure angle, and the coefficient
of friction; however, it is roughly correct to say that a worm and gear are
self-locking if the tangent of the lead angle is less than the coefficient of
friction. 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 screw
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 aides 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 motors
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
mills, conveying engineering, mining industry machines, on rudders, and worm
drive saws. In addition, milling heads and rotary tables are positioned using
high-precision duplex worm drives with adjustable backlash. Worm gears are used
on many lift/elevator and escalator-drive applications due to
their compact size and the non-reversibility of the gear.
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 the
rudder, often requiring several men to steer the vessel—some drives had two
large-diameter wheels so up to four crewmen could operate the rudder.
Worm drives have been used in a few automotive rear-axle final drives. They
took advantage of the location of the gear 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 gear was placed on
top. In the 1920s the Stutz firm used them on its cars; to have a lower floor
than its competitors, the gear was located on the bottom. An example from
around 1960 was the Peugeot 404. The worm gear carries the differential
gearing, which 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 worms
and planetary worm gears in place of the bevel gearing of conventional open
differentials. Torsen differentials are most prominently featured in the HMMWV
and some commercial Hummer vehicles, and as a center 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 gear 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 guitars, double-basses,
mandolins, bouzoukis, and many banjos. 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 than that of the motor, which operates best at a fairly high
speed. This motor-worm-gear drive system is often used in toys and other small
electrical devices. A worm drive is used on jubilee-type
hose clamps or jubilee clamps. The tightening screw's worm thread engages
with the slots on the clamp band. Occasionally a worm gear is designed to
run in reverse, resulting in the output shaft turning much faster than the
input. Examples of this may be seen in some hand-cranked centrifuges or the
wind governor in a musical box. 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
counterclockwise as they recede from an observer looking along the axis.
Manufacture Worm wheels are first gashed to rough
out the teeth and then hobbed to the final dimensions.
See also List of gear nomenclature
Gear Slewing drive
References = Bibliography=
Oberg, Erik. "Spiral and worm gearing". The Industrial Press.
External links Kinematic Models for Design Digital
Library 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.
Formulae & Calculations for Worm Drive Various Metric Gears downloadable design
specifications, 2D-3D models and catalogues
Various Worm Gearboxes, 3D models Machining of Worm Shaft and Worm Gears