Unit 2

1. Design of Spur Gears

2. Gear Nomenclature

3. Pinion
Gear
Force Analysis of Spur Gears
F
Ft
Fr
Ft
Fr
F

4. Tangential Component (Ft)
• It is the useful component – It transmits
power
• Magnitude of torque and power can be
determined

5. Radial Component (Fr)
• Radial component is always directed towards
the centre of gear.
• It does not do work
• It causes bending of shaft
• Also called as transverse or bending force

6. Resultant force
•

7. Lewis Equation – Tooth Bending Stress
In Lewis analysis the gear tooth is considered as
a cantilever beam with static normal force F
applied at the tip of the gear tooth.
Ft
Fr
h
F

8. Assumptions
1. The full load is applied to the tip of a single tooth
in static condition.
2. The radial component is negligible.
3. The load is distributed uniformly across the full
face width.
4. Forces due to tooth sliding friction are negligible.
5. Stress concentration in the tooth fillet is
negligible.

9. •

10. Dynamic Effects
When the gear drives at moderate or high
speeds, there will be dynamic effects on the
gear.
To account the dynamic effect, the velocity
factor or dynamic factor is introduced in the
dynamic loading conditions.
It depends on the type of gear manufacturing.

11. Dynamic Tooth Loads (Buckingham’s
Equation)
Dynamic loads are due to the following reasons
• Inaccuracies of tooth spacing
• Irregularity in tooth profiles
• Elastic parts
• Unbalance of rotating masses, etc.
Dynamic Load = Transmitted load + Incremental Load
Fd=Ft+FI

12. Fatigue strength or Wear strength or
Wear tooth load
The primary reason for gear failure is wear between
the tooth. To avoid this failure, gear properties has
to be selected properly.
The maximum wear load value depends on tooth
profiles and fatigue properties of materials
Fw=d1 b Q Kw
Q- Ratio factor
Kw-Load stress factor or material combination
factor