1. Gear and Gear Terminology
By: Gp Cap Dr Hamid Ullah Khan Niazi

2. Objectives After completing this unit you should be able to learn:
Identify and state the purposes of six types of gears used in the industry
Apply various formulas for calculating gear-tooth dimensions.

3. What are Gears??
Gears are a means of changing the rate of rotation of a machinery shaft. They can also change the direction of the axis of rotation and can change rotary motion to linear motion.
Unfortunately, mechanical engineers sometimes shy away from the use of gears and rely on the advent of electronic controls and the availability of toothed belts, since robust gears for high-speed and/or high-power machinery are often very complex to design. However, for dedicated, high-speed machinery such as an automobile transmission, gears are the optimal medium for low energy loss, high accuracy and low play.

4. Gears and Gearing
Gears are used to transmit power positively from one shaft to another by means of successively engaging teeth (in two gears).
Gears may also may be used to increase or decrease the speed of the driven shaft, thus decreasing or increasing the torque of the driven member.
Shafts in a gear drive or train are generally parallel. They may, however, be driven at any angle by means of suitably designed gears.
They are used in place of belt drives and other forms of friction drives when exact speed ratios and power transmission must be maintained.

5. Types of Gears Miter Gears: when the shafts are at
right angles and the gears are of the
same size, they are called miter
gears.
Angular Bevel Gears: if the axis of the
shafts intersect at any other angle
apart from 90O, the gears are known
as angular bevel gears.
Hypoid Gears: Modified bevel gears
having helical teeth are known as
hypoid gears. Hypoid gears are used
in automobile drives.
Worm and Worm Gear: When shafts
are at right angles and considerable
reduction in speed is required, worm
and worm gear are used.
Rack and Pinion: When it is
necessary to convert rotary motion
into linear motion, this may be used.
Spur Gear: generally used to transmit
power between two parallel shafts to
which they are attached.
Internal Gears: are used where the
shafts are parallel and the centers
must be closer together than could be
achieved with a spur or helical
gearing.
Helical gears : may be used to
connect parallel shafts or shafts
which are at an angle.
Herringbone Gears: on most
installations where it is necessary to
overcome end thrust, such gears are
used.
Bevel Gears: When two shafts are
located at an angle with their axial
lines intersecting at 90O, power is
generally transmitted by means of
bevel gears. .

6. Types of Gears

9. A) Bevel Gear Transmit power at 90o B) The Miter gear C) Angular bevel gears D) Hypoid Gears

11. Gear Terminology Addendum: is the radial distance between
the pitch and circle and the outside
diameter or height of the tooth above the
pitch circle.
Center Distance: shortest distance between
the axes of two mating gears or the
distance equal to one half the sum of the
pitch diameters.
Chordal Addendum: is the radial distance
measured from the top of the tooth where
the Chordal thickness and the pitch circle
intersect the edge of the tooth.
Chordal Thickness: is the thickness of the
tooth measured at the pitch circle.
Circular Pitch: distance from a point on one
tooth to a corresponding point on the next
tooth.
Circular Thickness: is the thickness of the
tooth measured on the pitch circle. It is also
measured on the arc thickness.
Clearance: is the radial distance between the
top of one tooth and the bottom of the mating
tooth space.
Dedendum: radial distance from the pitch circle to the
bottom of the tooth space.
Diametral pitch: is the ratio of the number of teeth for
each inch of pitch diameter of the gear.
Working Depth: is the distance that a gear tooth
extends into the tooth, space of a mating gear which is
equal to the two addendums
Involute : is the curved line produced by a point of a
stretched string when it is unwrapped from a given
Cylinder.
Linear Pitch: is the distance from a point on one
tooth to a corresponding point on the next tooth of
a gear rack.
Module: is the pitch diameter of a gear divided by
the number of teeth( for metric gears).
Outside diameter: is the overall diameter of the
gear, which is the pitch circle plus the two
addendums.
Pitch circle: is the circle which has the radius of half
the pitch diameter with its center at the axis of the
gear.
Pitch circumference : is the circumference of the
Pitch circle.
Pitch diameter: is the diameter of the pitch circle
which is equal to the outside diameter minus two
addendums
Pressure angle: is the angle formed by the line
through a point of contact of mating teeth and
tangent to the two base circles and a line at right
angles to the center line of the gears.
Root circle: is the circle formed by the bottoms of
the tooth space.
Root diameter: is the diameter of the root circle.
Tooth Thickness: is the thickness of the tooth
measured on the pitch circle.
Whole depth: is the full depth of the tooth or
distance equal to the addendum plus the
Dedendum.

12. Parts Of a Gear

13. Meshing of Gears In Multitude Ways
Meshing Circular Spur Gears

14. Rack and Pinion Spur Gears

15. Circular Worm Gear and Mating Cylindrical Worm

16. Inside Spur Gear with Mating Pinion Spur Gear

17. Rules and Formulas for Spur Gears

19. Gear Cutting
Most gears cut on a milling machine are generally used to repair or replace a gear that has been broken or lost or is no longer carried in inventory. Industry generally mass-produce gears on special machines designed for this purpose. The most common types of gear–generating machines are the gear-shaping and gear-hobbing machines.. It is more economical to buy a gear from a gear manufacturing company than to produce gears on a machine in order to carry out faster production and to save time.

20. Involute Gear Cutters

21. Comparative Gear Tooth sizes 4 to 16DP

22. To Cut A Spur Gear

23. Gear Tooth Measurement

25. Reading Assignment
Do problems and examples on Gear and Gear Cutting

26. Summary
In order to eliminate slippage and provide positive drive gears are used.

27. Questions?