1.2 Types of Gear
The gear has been classified through a lot of methods. The classification that is most popular is through the different
position of the gear axis. Another way is through method of manufacturing, material and Tooth profile.
This time we introduce the methods through different position of gear axis and manufacturing.

Classifications by the position of gear axis
(1) Parallel axis gear (The teeth are parallel to axis)

Spur geara) Spur gear
This is Cylindrical gear. Tooth trace is parallel to axis.
This is a highly demanded gear, which is easy to manufacture and to assemble.

b) Helical gear
This is a Cylindrical gear. Tooth trace has helix curve.
Helical gear provides more strength, less oscillationand lower noise level compared with Spur gears.
However Helical gear provides a thrust load to axis
direction. If changed rotating direction, thrust load isreversed.

c) Internal gear
This is a cylindrical gear ring with teeth formed at theinner diameter. The most popular demand for Internal
gears is used for mechanism of planetary gear train.
There are two types of Tooth trace, one is parallel andthe other is helix to axis. However gear with parallelaxis has higher demand.

d) Straight Rack
It is thought that radius of spur gear grew infinite to
become a straight line. It can be matched with Spurgear to convert between the rectilinear motion andthe rotary motion.

Helical gear

Internal gear

Straight Rack
Helical Racke) Helical Rack
It is thought that radius of Helical gear grew infinite to
become a straight line and Tooth trace is also straightline. It can be matched with Helical gear to convert
between the rectilinear motion and the rotary motion.

3f) Double helical gear
The shape looks like two Helical gear joined together.
Therefore this type of gear does not have thrust load
to axis during operation.

Double helical gear∗Other types of gear designs used for parallel axis
such as non-circular gear and Eccentricity gear areavailable but omitted this time.

(2) Intersecting axis gearg) Straight bevel (Miter) gear
This is gear with Tooth trace, which is a straight line in
the same direction as surface element of Pitch cone.
Miter gear has shaft angle of 90° and gear ratio of 1:1.

h) Angular straight bevel gear
Angular straight bevel gear which does not have shaft
angle of 90°.

i) Spiral bevel gear
Tooth trace is described as a curve with spiral angle.
Spiral bevel gear has advantage over Straight bevel
gear for gear strength, oscillation and noise level.
Disadvantage of Spiral bevel gear is axial thrust load.
Therefore proper bearing location and firm support
are needed.

j) Angular spiral bevel gear
Angular spiral bevel gear does not have shaft angle of90°.

k) Zerol® bevel gear
This is Zerol® bevel gear, similar to Spiral bevel gearwith zero spiral angle. The Tooth trace is described asa curve with spiral angle of zero degree. (Occasionally,spiral angle 10° or below are also called Zerol® bevelgear.)
The force of tooth action is the same as Straight bevelgear.
( Ⓡ mark is Gleason Works trademark)4

Straight bevel (Miter) gear

Angular straight bevel gear

Spiral bevel gear

Angular spiral bevel gear

Zerol® bevel gearl) Face gear
is Face gear. A Toothed disk gear, can be matchedwith Spur or Helical gear. There are two types of Facegear with shaft angle 90°, intersecting axis and Nonintersecting axis.

Face gear(3) Skew gear (Non Intersected Gear)m) Cylindrical worm gear
This is a Worm gear pair consisting of Cylindricalworm gear and Worm wheel.
Meaning of Cylindrical worm gear is that the thread
has one or more starts.
This Worm gear pair provides high speed reducing
ratio and low noise level. Disadvantages are low efficiency and generation of heat.

n) Crossed helical gear (Screw gear)
This is a Gear pair for transmission between Nonparallel and Non-intersecting axes of Cylindrical gears
as pair of Helical or Spur gear. Due to spot contact intheory, design of transmission should be light load.

o) Hypoid® gear
This is a gear for transmission between Non-paralleland Non-intersecting axis of conical gear. This gear issimilar to Spiral bevel gear. Most popular usage is forDeferential gear for automotive.
( Ⓡ mark is Gleason Works trademark)Cylindrical worm
Cylindrical worm

Cylindrical worm wheel
Crossed helical gear (Screw gear)

Hypoid® gear∗O thers including Enveloping worm, Spiroid® andHelicon gear types of gear designs are available for
Non-parallel and Non-intersecting axis but omitted
this time. ( Ⓡ mark is ITW trademark)5

(4)	Machine elements compared with gear for similar shape and purpose of usage.

Sprocketsp) Sprockets
This is Sprocket wheel used for matching with bushed
chain and Ladder chains. Usage is for transmitting apower over long distances between axes.
Ratchet gearr) Ratchet gear
This is Ratchet gear, which looks like the teeth of saw
formed at external wheel used for positioning (indexing) and preventing inversion.

s) Timing PulleyThis is Timing pulley using matching timing belt (beltwith teeth). Usage is for transmitting power over long
distance between axes.

6Timing PulleyClassifications by different manufacturingmethods.
(1) Machined gear
Tooth of Spur and Helical gears are machined,
method of manufacturing are hobbing machine, gear
shaper, gear plainer and other machining. For mass
production, method of broaching is used. As for Bevel
gear process, dedicated cutting machine is usuallyused. However hobbing machine is rarely used.

(2) Shaving gear
This is a shaving gear that shaves out the minute finishing layer on the Tooth flank by shaving machineand its cutter.

(3) Ground gear
Grinding machine is used for making ground toothflank. Classified as two methods of Form grinding andGeneral grinding. Both processes use grinding wheel(Diamond and CBN). To obtain high quality gear, accurate lathing of the gear is necessary. The electrolytic
ground gear has been developed recently but explanation will be omitted this time.

(4) Precision cold rolled processed gear
This is a gear done using plastic working (cold rolling) to form tooth by compression. KG Worm gearsof module 0.5 to 2.0 are manufactured by using theCold rolling forming method. The fabrication of Coldrolling-Worm gear is formed by rotating the hydrauliccompression which causes the teeth to rise on bothsides of the rolling tooth machine. The Tooth flank ofWorm gear has glassy finish like mirror.

(5) Injection molded gear.
This is a gear formed by injecting molten plastic intoa mold and applying pressure for a fixed duration.

(6) Sintering Gear
This is Sintering gear. Metallic powder is put into themold before applying pressure and heat to gear moldto harden. Occasionally there are re-pressuring andre-sintering. Impregnation of oil, heat treatment andsurface treatment can also be applied after the 1st
sintering process. Sintering gear is suitable for massproduction.
∗ On the other hand, we have the gears with pressed,
forged or EMS (Electric Spark Machine and Wire Cut
Sparking Machine) but omitted this time.
Refer to Table 1 for an estimate of System of accuracy
depending on difference in gear processing methods
used for economical production.
Table 1.
Comparison table of System of accuracy between
different gear manufacturing methods(Confirmed by JIS B 1702-1995)
Fabrication and Distinctive
heat treatment
System of Accuracy
of JIS B1702 (old)Non-Quenchinggear

HobbingShaving

Quenched gear

HobbingShaving

012345678

Grinding7