Gears

1. A gear or cogwheel is a rotating machine part
having cut teeth, or cogs, which mesh with another
toothed part to transmit torque.

2. Geared devices can change the speed, torque, and
direction of a power source. Gears almost always
produce a change in torque, creating a mechanical
advantage, through their gear ratio, and thus may
be considered a simple machine.

3. Early examples of gears date from the 4th century BCE in China.(Zhan
Goo times - Late East Zhou dynasty), which have been preserved at
the Luoyang Museum of Henan Province, China. The earliest gears in
Europe were circa CE 50 by Hero of Alexandria,[4] but they can be traced
back to the Greek mechanics of the Alexandrian school in the 3rd
century BCE and were greatly developed by the
Greek polymath Archimedes (287–212 BCE).

5. The definite velocity ratio that teeth give
gears provides an advantage over other
drives (such as traction drives and V-belts) in
precision machines such as watches that
depend upon an exact velocity ratio. In cases
where driver and follower are proximal, gears
also have an advantage over other drives in
the reduced number of parts required; the
downside is that gears are more expensive to
manufacture and their lubrication
requirements may impose a higher operating
cost per hour.

6. An external gear is one with the teeth
formed on the outer surface of a cylinder
or cone. Conversely, an internal gear is
one with the teeth formed on the inner
surface of a cylinder or cone.
EXTERNAL VS INTERNAL GEARS

7. Spur gears or straight-cut gears are the simplest type of
gear. They consist of a cylinder or disk with teeth projecting
radically.

8. Helical or "dry fixed" gears offer a refinement over spur
gears. The leading edges of the teeth are not parallel to the
axis of rotation, but are set at an angle.

9. For a 'crossed' or 'skew' configuration, the gears
must have the same pressure angle and normal
pitch; however, the helix angle and handedness
can be different. The relationship between the two
shafts is actually defined by the helix angle(s) of
the two shafts and the handedness, as defined.
{displaystyle E=beta _{1}+beta _{2}} for gears of
the same handedness{displaystyle E=beta _{1}-
beta _{2}} for gears of opposite handedness

10. Double helical gears and herringbone gears are
similar but the difference is that herringbone
gears don't have a groove in the middle like
double helical gears do.

11. A bevel gear is shaped like a right circular cone with most of
its tip cut off. When two bevel gears mesh, their imaginary
vertices must occupy the same point. Their shaft axes also
intersect at this point, forming an arbitrary non-straight
angle between the shafts.

12. Main article: Spiral bevel gear
Spiral bevel gears can be manufactured as Gleason
types (circular arc with non-constant tooth depth),
Oerlikon and Curvex types (circular arc with constant
tooth depth), Klingelnberg Cyclo-Palloid (Epicycloide
with constant tooth depth) or Klingelnberg Palloid.

13. Hypoid gears resemble spiral bevel gears except the shaft
axes do not intersect. The pitch surfaces appear conical but,
to compensate for the offset shaft, are in
fact hyperboloids of revolution.

14. Main article: Crown gear
Crown gears or contrite gears are a particular form of
bevel gear whose teeth project at right angles to the
plane of the wheel; in their orientation the teeth
resemble the points on a crown.

15. Main articles: Worm driveand Slewing drive
Worms resemble screws. A worm is meshed with
a worm wheel, which looks similar to a spur gear.

16. Main article: Non-circular gear
Non-circular gears are designed for special purposes. While
a regular gear is optimized to transmit torque to another
engaged member with minimum noise and wear and
maximum efficiency, a non-circular gear's main objective
might be ratio variations, axle displacement oscillations and
more.

17. Main article: Rack and pinion
A rack is a toothed bar or rod that can be thought of as a
sector gear with an infinitely large radius of curvature.
Torque can be converted to linear force by meshing a rack
with a pinion: the pinion turns; the rack moves in a straight
line.

18. Main article: Epicyclic gearing
In epicyclical gearing one or more of the gear axes moves.
Examples are sun and planet gearing (see below), cyclical
drive, and mechanical differentials.

19. Main article: Harmonic Drive
A harmonic gear is a specialized gearing mechanism often
used in industrial motion control, robotics
and aerospace for its advantages over traditional gearing
systems, including lack of backlash, compactness and high
gear ratios.

20. Main article: magnetic coupling
Main article: magnetic Gear
All cogs of each gear component of magnetic gears act as a
constant magnet with periodic alternation of opposite
magnetic poles on mating surfaces. Gear components are
mounted with a backlash capability similar to other
mechanical gearings. Although they cannot exert as much
force as a traditional gear, such gears work without
touching and so are immune to wear, have very low noise
and can slip without damage making them very reliable

22. Since it is impractical to calculate circular pitch
with irrational numbers, mechanical engineers
usually use a scaling factor that replaces it with a
regular value instead. This is known as
the module or modulus of the wheel and is simply
defined as{displaystyle m=p/pi }where m is the
module and p the circular pitch. The units of
module are customarily millimeters; an English
Module is sometimes used with the units of inches.
When the diametric pitch, DP, is in English units.

27. Numerous nonferrous alloys, cast irons, powder-
metallurgy and plastics are used in the manufacture of
gears. However, steels are most commonly used
because of their high strength-to-weight ratio and low
cost.

28. As of 2014, an estimated 80% of all gearing produced
worldwMolded gearing is usually either powder
metallurgy or plastic.[30] Many gears are done when
they leave the mold (including injection
molded plastic and die cast metal gears), but
powdered metal gears require sintering and sand
castings or investment castings require gear
cutting or other machining to finish them. ide is
produced by net shape molding.

29. Overall gear geometry can be inspected
and verified using various methods such
as industrial CT scanning, coordinate-
measuring machines, white light
scanner or laser scanning. Particularly
useful for plastic gears, industrial CT
scanning can inspect internal geometry
and imperfections such as porosity.

30. The gear mechanism was previously
considered exclusively artificial—but in
2013, scientists from the University of
Cambridge announced their discovery that
the juvenile form of a common
insect Issus (species Issus coleoptratus),
found in many European gardens, has a
gear-like mechanism in its hind legs.