This document provides an overview of spur gears, including their classification, terminology, and characteristics. It begins by defining gears and classifying them into different types based on how their shafts connect. It then focuses on spur gears, describing their key features such as teeth being parallel to the axis and imposing radial loads. The document outlines important gear tooth terminology like pitch circle, pressure angle, addendum, and dedendum. It also defines concepts like path of contact, length of contact, and arc of contact that are important for understanding gear engagement and operation.

1. Pimpri Chinchwad Education Trust’s
Pimpri Chinchwad College of Engineering and
Research, Pune
Course- Theory of Machines-II
Topic -SPUR GEAR
Online Lecture 1
By
Prof.Fodase G.M.

2. Unit-I: SPUR GEAR
• Classification of gears
• Gear tooth terminology –
• Fundamental Law of toothed
gearing and involute gearing
• Length of path of contact and
contact ratio
• Interference and undercutting
• Minimum number of teeth on gear
and pinion

3. Power Transmission
3

4. Definition….
Gears: Gears are machine
elements that transmit motion by
means of successively engaging
teeth. The gear teeth act like
small levers.
Discs with teeth on the periphery
are known as gears.

5. 5
Gear Classification
 Connecting Parallel Shafts
• Spur Gears
• Helical Gears / Herringbone Gears
• Rack & Pinion Gears
 Connecting Intersecting Shafts
• Bevel Gears
• Helical Bevel Gears
 Connecting Non-parallel Non-intersecting Shafts
• Spiral Gears
• Worm & Worm Gear

6. Types of gears
Spur gears
• Teeth are parallel to axis of shaft
• Imposes radial load on the shaft
• Have straight teeth Suitable for
low to medium speed application
• Noise in high speed application
due to sudden contact over the
entire face width
• Relatively high ratios can be
achieved
6

7. 7
Helical Gears
Helical gears have angled teeth which form a
curve that resembles a segment of a helix.
Helical gears are meshed in parallel or
crossed orientations and are used because
they offer a more refined operation and run
much smoother and quieter than spur gears
for example. Helical gears can operate at
high speeds and transmit large amounts of
torque.
A disadvantage of helical gears is the thrust
generated by the curved teeth when under
load, this is usually handled by a suitable
thrust bearing to help take this load.
Double helical gears A double helical gear is similar to 2 separate
helical gears joined together but mirrored,
this helps eliminate the thrust that a single
helical gear would create as in effect there is
equal thrust in each direction cancelling

8. 8
Worm gears The worm resembles the thread of a screw,
and are usually meshed with a worm wheel
which looks similar to a typical spur gear.
Worm gears are an excellent way to increase
torque output while reducing rotational
speed.
Used for Non-parallel & Non-intersecting
shafts.
Large speed reduction upto 100:1
Worm can easily turn the gear but…
Gear cannot turn worm.
This locking feature acts as a brake.
Used in conveyor systems.
Rack and pinion gears
Rack: Straight Gear with infinite diameter
Used to convert rotational motion to
translational motion by means of a gear
mesh
Application: Rack and pinion steering
system used on many cars in the past.
Used in machine tools

9. 9
Spiral Gears Skew Gears / Crossed Helical Gears
Used for Non-parallel & Non-intersecting shafts
Point contact between mating teeth
Low load transmission
Bevel Bevel gears have a cone shape which
enables them to mesh at various angles
except 0 and 180 degrees.
The teeth of a bevel gear can be straight
cut, similar to that of a spur gears teeth, or
they can be curved along their length with
each tooth sitting at an angle (Spiral bevel
gear).

10. 10
Terminology For Spur Gears

11. 11
Gear Tooth Terminology

12. 12
• Pitch Circle : An imaginary circle which by pure rolling action would give the
same motion as actual gear.
• Pitch Circle Diameter : Diameter of pitch circle
• Pitch Point : Common point of contact between two pitch circles
• Pressure Angle: Angle between common normal to two gear teeth at the point of
contact & common tangent at pitch point
• Addendum: The radial distance between the PC and the top of the teeth
• Addendum Circle: Circle drawn from top of tooth & concentric with PC
• Dedendum: The radial distance between the bottom of the tooth to PC
• Dedendum Circle: Circle drawn from Bottom of tooth & concentric with PC
• Circular pitch (pc): Distance measured on circumference of PC from a point of
one tooth to the corresponding point on next tooth
D= Diameter of PC
T= No. of teethT
D
pc



13. 13
OR
• Diametral pitch (Pd): The ratio of the number of teeth to PCD.
• Module (m): Ratio of Pitch circle Diameter (mm) to No. of Teeth
• Clearance: Difference between the dedendum of one gear and the addendum of the
mating gear
• Total depth : Radial distance equal to sum of addendum & dedendum ( working depth
plus clearance )
• Working depth : Depth of engagement of two gears i.e., the sum of addenda of two
mating gears
• Tooth Thickness : Width of teeth measured along PC
• Tooth Space: Distance between adjacent teeth measured along PC
• Backlash: Difference between tooth thickness and tooth space on PC
pcD
T
pd


T
D
m 
2
2
1
1
T
D
T
D
pc


2
1
2
1
T
T
D
D


14. 14
• Face of a tooth: Surface of gear tooth above PC
• Flank of a tooth: Surface of gear tooth below PC
• Top Land : Surface of top of tooth
• Face width : Width of gear tooth measured parallel to its axis
• Fillet Radius : connects root circle to profile of tooth
• Path of contact : path traced by a point of contact of two teeth from beginning
to end of engagement
• Length of path of contact : Length of common normal cut-off by addendum
circles of wheel & pinion
• Arc of contact : Path traced by a point on pitch circle frm beginning to end of
engagement
• Arc of Approach : path of contact from beginning of engagement to pitch point
• Arc of Recess : path of contact from pitch point to end of engagement

15. 15