Gear? Where it is used and why? We know that gear is power transmitting element but there are many any elements like - rope, chain drives which can also be used. There are many benefits of using gear over belt, rope, chain drive, that's why we used gear. Here we provide a complete description about gear, types of gear. Spur gear, Helical gear, Worm gear, Bevel gear, these are some important types of gear which are described briefly with their terminology. A complete difference is clarified between all types of gear relating their shapes and position of shaft.

1. Gear
Gear

2. What is Gear?
• A gear or cogwheel is a
rotating machine part
having cut teeth, or
cogs, which mesh with
another toothed part in
order to transmit
torque.
• In most cases with
teeth on the one gear
being of identical
shape, and often also
with that shape on the
other gear.

3. Types of
Gears1. According to the position of
axis of the shafts -
(a.) Parallel
(b.) Intersecting
(c.) Non-Intersecting and non-parallel

4. (a.) Parallel and co-planar shafts
(i) Spur Gear -They
consist of a cylinder
or disk with the teeth
projecting radially.
(ii) Helical Gear-
The leading edges of
the teeth are not
parallel to the axis of
rotation, but are set
at an angle.
Spur
Gear
Helical
Gear

5. (b.) Intersecting/ non-parallel and Co-planer
Shaft
• Bevel Gear - Two
non-parallel and
intersecting, but co-
planer shafts are
connected by the bevel
gear and the arrangement
is called bevel gearing.
• Bevel gears, like spur
gears may also have their
teeth inclined to the face
of the bevel, in this case
they are known as helical
bevel gears.

6. (c) Non-Intersecting and non-
parallel and Non Co-planar
• This types of
shafts are
connected by
gears known as
"Skew Bevel
Gear“ or "Spiral
Gear" and the
arrangement is
known as "Skew
Bevel Gearing".

7. 2. According to the
Peripheral Velocity of the
Gears
(a.) Low Velocity
( Less than 3 m / s )
(b.) Medium Velocity
( 3-15 m / s )
(c.) High Velocity
( Greater than 15 m / s )

8. 3. According to Type of Gearing
(a.) External Gearing
(b.) Internal Gearing
(c.) Both Internal and External

9. (a.) External Gearing
• In external Gearing,
the two shafts mesh
externally with each
other.
• The larger of two
wheels is wheel or
Gear and the smaller
wheel is called pinion.
• In an external gearing,
the motion of the two
wheels is always
unlike or opposite.

10. (b.) Internal
Gearing
• In internal gearing, the
two gears mesh
internally with each
other.
• The larger of these two
wheels is called
Annular Wheel and the
smaller wheel or gear
is called Pinion.
• In internal gearing the
motion of the wheels is
always same or like.

11. (c.) Both Internal and
External• The gear of two shaft
meshes externally and
internally with the
gears in a straight line.
Such type of gear is
called Rack and Pinion.
• The straight line gear
is called Rack and the
circular wheel is called
Pinion.
• With the help of Rack
and Pinion we can
convert Linear motion
into rotary motion and
vice-versa.

12. 4. According to the
Position of Teeth on the
Gear Surface
• (a.) Straight
• (b.) Inclined
• (c.) Curved

13. Some Important Terminology of Spur
Gear

14. • 1. Pitch Circle : - It is an imaginary circle which by pure rolling
action, would give the same motion as the actual gear.
• 2. Pitch Circle Diameter : - It is the diameter of the pitch circle.
• 3. Pitch Point : - It is a common point of contact between two
pitch circles.
• 4. Pitch surface : - The surface of the imaginary rolling cylinder
(cone, etc.) that the toothed gear may be considered to replace.
• 5. Addendum circle : - A circle bounding the ends of the teeth,
in a right section of the gear.
• 6. Root (or dedendum ) circle : - The circle bounding the spaces
between the teeth, in a right section of the gear.
• 7. Addendum : - The radial distance between the pitch circle
and the addendum circle.
• 8. Dedendum : - The radial distance between the pitch circle
and the root circle.

15. 11. Flank of a tooth : - The part of the tooth surface lying inside the pitch surface.
12. Circular thickness ( also called the tooth thickness ) : The thickness of the tooth
measured on the pitch circle. It is the length of an arc and not the length of a
straight line.
13. Tooth space : - The distance between adjacent teeth measured on the pitch circle.
14. Backlash : - The difference between the circle thickness of one gear and the tooth
space of the mating gear.
Backlash = Space width – Tooth thickness
15. Circular pitch (Pc) : - The width of a tooth and a space, measured on the pitch circle
or it is the distance measured on the circumference of the pitch circle from a point
of one tooth to the corresponding point on the next tooth.
16. Diametral pitch (Pd) : - The number of teeth of a gear per inch of its pitch diameter.
A toothed gear must have an integral number of teeth. The circular pitch, therefore,
equals the pitch circumference divided by the number of teeth. The diametral pitch
is, by definition, the number of teeth divided by the pitch diameter.
17. Module (m) : - Pitch diameter divided by number of teeth. The pitch diameter is
usually specified in inches or millimetres; in the former case the module is the
inverse of diametral pitch.

16. Helical Gear
Terminology
1. Helix Angle : - Angle between a tangent to the
helix and the gear axis. It is zero in the limiting
case of a spur gear, albeit it can considered as the
hypotenuse angle as well.
2. Axial Pitch : - It is the distance, parallel to the axis,
between similar face of adjacent teeth. It is the
same as circular pitch and denoted by Pc.
3. Normal Pitch : - It is the distance between similar
faces of adjacent teeth along a helix on the pitch
cylinders normal to teeth. It is denoted by Pn.

17. Worm Gear
• Worm gears
resemble screws. A
worm gear is
usually meshed
with a spur gear or
a helical gear,
which is called the
gear, wheel, or
worm wheel.
• To achieve a high
torque, low speed
gear ratio.

18. Worm Gear
Terminology

19. 1. Lead : - Distance from any point on a thread to the corresponding point
on the next turn of the same thread, measured parallel to
the axis.
2. Linear Pitch ( p ) : - Distance from any point on a thread to the
corresponding point on the adjacent thread, measured parallel to
the axis. For a single-thread worm, lead and linear pitch are the same.
3. Lead Angle : - Angle between a tangent to the helix and a plane
perpendicular to the axis. Note that it is the complement of the
helix angle which is usually given for helical gears.
4. Tooth Pressure Angle : - It is the measured in a plane containing the
axis of the worm gear and is equal to one-half the thread profile.
5. Normal Pitch : - It is the distance measured along the normal to the
threads between two corresponding points on two adjacent threads of
the worm.

20. Bevel Gear
• A bevel gear is shaped
like a right circular
cone with most of its
tip cut off.
• The angle between the
shafts can be anything
except zero or 180
degrees.
• When two bevel gears
mesh, their imaginary
vertices must occupy
the same point.

21. Spiral Bevels
• Spiral bevel gears have
the same advantages
and disadvantages
relative to their
straight-cut cousins as
helical gears do to spur
gears.
• Straight bevel gears
are generally used only
at speeds below 5 m / s
(1000 ft/min), or, for
small gears, 1000
r.p.m.

22. Bevel Gear Terminology1. Pitch Cone : - It is the cone containing the pitch elements of the teeth.
2. Cone Centre : - It is the apex of pitch cone. It may be defined as that point where the
axis of two mating gears intersect each-other.
3. Pitch Angle : - It is the angle made by the pitch line with the axis of shaft.
4. Cone Distance : - It is the length of pitch cone element. It is also called as a Pitch
Cone Radius.
5. Addendum Angle : - It is the angle subtended by the addendum of the tooth at the cone
centre.
6. Dedendum Angle : - It is the angle subtended by the dedendum of the tooth at the cone
centre.
7. Face Angle : - It is the angle subtended by the face of tooth at the cone centre. It is
equal to pitch angle plus addendum angle.
8. Root Angle : - It is the angle subtended by the root of the tooth at the cone centre. It is
equal to the pitch angle minus dedendum angle.
9. Back Cone : - It is the imaginary cone, perpendicular to the pitch cone at the end of
the tooth.
10. Back Cone Distance : - It is the length of the back cone.

23. For more visit
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A power point presentation by
Yogendra Singh Shekhawat
B. Tech, Mechanical Engineering