Gears are toothed cylindrical wheels used for transmitting mechanical power from one rotating shaft to another.

1. Gear
• Gears are toothed cylindrical wheels used for transmitting mechanical
power from one rotating shaft to another.
• Gears are most often used in transmissions to convert an electric motor’s
high speed and low torque to a shaft’s requirements for low speed high
torque:
• Speed is easy to generate, because voltage is easy to generate
• Torque is difficult to generate because it requires large amounts of current
• Gears essentially allow positive engagement between teeth so high forces
can be transmitted while still undergoing essentially rolling contact
• Gears do not depend on friction and do best when friction is minimized

2. Spur Gear
• The effect of slipping is to reduce the velocity ratio of the system. In
precision machines, in which a definite velocity ratio is of importance
• A gear drive is also provided, when the distance between the driver
and the follower is very small

3. Principle of transmission
• Provide means of connection and disconnection of engine with rest of
• power train without shock and smoothly.
• Provide means to transfer power in opposite direction
• Enable power transmission at varied angles and varied lengths.
• Enable diversion of power flow at right angles.
• Provide means to transfer power in opposite direction.

4. conjugate action Spur Gears Tooth forms
• The gears must be designed such that the ratio of rotational speeds of
driven and driver gear is always constant. When the tooth profiles of
two meshing gears produce a constant angular velocity during
meshing, they are said to be executing conjugate action.
• Gears are mostly designed to produce conjugate action. Theoretically,
it is possible to select an arbitrary profile for one tooth and then to find
a profile for the meshing tooth, which will give conjugate action.

5. System of gear teeth
• Systems of Gear Teeth
The following four systems of gear teeth are commonly used in
practice.
1. 14 (1/2)° Composite system,
2. 14(1 /2)° Full depth involute system,
3. 20° Full depth involute system,
4. 20° Stub involute system.

6. contact ratio
• Contact Ratio is the ratio of the sum of the arc of action and the face
advance to the circular pitch.
• To assure continuous smooth tooth action, as one pair of teeth ceases
action a succeeding pair of teeth must already have come into
engagement. It is desirable to have as much overlap as is possible. A
measure of this overlap action is the contact ratio.

7. Standard proportions of gear systems
S. No. Particulars 14 (1/2) composite
or full depth
involute system
20° full depth
involute system
20° stub involute
system
1 Addendum 1m 1m 0.8 m
2 Dedendum 1.25 m 1.25 m 1 m
3 Working depth 2 m 2 m 1.60 m

8. Interference in involute gears
• The tip of tooth on the pinion will then undercut the tooth on the
wheel at the root and remove part of the involute profile of tooth on
the wheel. This effect is known as interference and occurs when the
teeth are being cut .
• the phenomenon when the tip of a tooth undercuts the root on its
mating gear is known as interference.
• Interference may only be prevented, if the addendum circles of the two
mating gears cut the common tangent to the base circles between the
points of tangency.

9. Backlash
Backlash. It is the difference between the tooth space and the tooth
thickness, as measured on the pitch circle.
Backlash is defined as the amount by which the with of tooth space
excceds the thickness of the engaging tooth measured along the pitch
circle
Backlash compensates for machining errors
Backlash compensates for thermal expansion of teeth

10. Selection of gear materials
• The gear material should have sufficient strength to resist failure due
to breakage of the tooth.
• The gear material should have sufficient surface endurance strength to
avoid failure due to destructive pitting.
• The material should have low coefficient of friction to avoid failure
due to scoring .
• Alloy steels are superior to plain carbon steels in this respect ,due to
consistent thermal distortion.

11. Gear manufacturing methods
• Manufacture of gears needs several processing operations isequential
stages depending upon the material and type of the gears and quality
desired.

12. Design considerations
• The accuracy of the output of a gear depends on the accuracy of its
design and manufacturing.
• Strength of the gear in order to avoid failure at starting torques or
under dynamic loading during running conditions.
• Gear teeth must have good wear characteristics.
• Selection of material combination.
• Proper alignment and compactness of drive
• Provision of adequate and proper lubrication arrangement.

13. Beam strength of gear tooth
• WEAR STRENGTH
• Due to rolling and sliding actions of the gear teeth, the following types
of surface destructions (wear) may occur:
• Abrasive wear. Scratching of the tooth surface due to the presence of
foreign materials in the lubricant is called abrasive wear.
Corrosive wear. Chemical reactions on the surface of a gear cause
corrosive wear.
Pitting. Repeated application of the stress cycle, known as pitting,
cause fatigue failure.
Scoring. Inadequate lubrication between metal-to metal contact cause
scoring.

14. Dynamic tooth load
The dynamic loads are due to the following reasons :
1. Inaccuracies of tooth spacing,
2. Irregularities in tooth profiles,
3. Deflections of teeth under load.
WD = WT + WI
where WD = Total dynamic load,
WT = Steady load due to transmitted torque, and
WI = Increment load due to dynamic action.

15. Wear strength of gear tooth
• The maximum load that gear teeth can carry, without premature
wear, depends upon the radii of curvature of the tooth profiles and
on the elasticity and surface fatigue limits of the materials.
Ww = DP.b.Q.K
Ww = Maximum or limiting load for wear in newton's,
DP = Pitch circle diameter of the pinion in mm,
b = Face width of the pinion in mm,
Q = Ratio factor=2xV R/( V R +1) ,for external gears
Q = Ratio factor=2xV R/( V R -1) for internal gears

16. Failure of gear tooth
The different modes of failure of gear teeth
Bending failure.
1. Bending failure.
2. Scoring.
3. Abrasive wear.
4. Corrosive wear.

17. Design of spur gears
The following requirements must be met in the design of a gear drive :
(a) The gear teeth should have sufficient strength so that they will not
fail under static loading or dynamic loading during normal running
conditions.
(b) The gear teeth should have wear characteristics so that their life is
satisfactory.
(c) The use of space and material should be economical.
(d) The alignment of the gears and deflections of the shafts must be
considered because they effect on the performance of the gears.
(e) The lubrication of the gears must be satisfactory.

18. AGMA and Indian standards
• AGMA has helped to set national gearing standards since 1916.
• The association also serves the focal point within the United States
for the development