Gears mom2 2

Mehran University College of Engineering
and Technology Khairpur

Mechanics of machine 2
TOOTHED GEARING
Abdul Ahad Noohani (MUCET KHAIRPUR)

Why do we use belt and rope
drive and where the gear drive?
Slipping of belt or rope is the common phenomenon in
the transmission of motion or power between two
shafts.

The effect of slipping is to reduce the velocity ratio of
the system

2

Gears or toothed wheels
In Precision machines where constant velocity ratio is
of importance gears or toothed wheels are used.

A gear drive is also provided when the distance
between driver and follower is very small

3

Law of gearing:
Condition for constant velocity ratio
Let
Q : is the point of contact b/w two teeth

P : is the pitch point

T T : is the common tangent

MN: is the common normal to the curves at Q

when considered on wheel 1
Point Q moves in the direction QC with velocity
V1

when considered on wheel 2
Point Q moves in the direction QD with velocity
V2
4

If the teeth are to remain in contact then the components of these
velocities along the common normal MN
must be equal

5

From this equation we see that velocity ratio is inversely proportional
to the ratio of distances of the point p from O1 and O2

Therefore for keeping the velocity ratio constant

The common normal at the point of contact between a pair of teeth
must always pass through the pitch point

6

Velocity of Sliding of Teeth
The velocity of sliding is the velocity of one
tooth relative to its mating tooth along the
common tangent at the point of contact.

7

From similar triangles QEC and O1MQ

From similar triangles QCD and O2 NQ

Putting values

8

Lec # 02

Involute Teeth
An involute of a circle is a plane curve generated by a point on a
taut string which in unwrapped from a reel as shown in Fig.

normal at any point of an involute is a tangent to the circle.

9

We see that the common normal MN intersects the line of centres O1O2 at
the fixed point P (called pitch point). Therefore the involute teeth satisfy the
fundamental condition of constant velocity ratio.

10

The pressure angle (φ):
It is the angle which the common normal to the
base circles (i.e. MN) makes with the common
tangent to the pitch circles.

Force due to power transmitted:
When the power is being transmitted, the
maximum force is exerted along the common
normal through the pitch point

This force may be resolved into tangential
and radial or normal component

Tangential component FT = F cos φ

Radial or normal component FR = F sin φ.

11

Torque exerted on the gear shaft:

= FT × r
Where, r is the pitch circle radius of the gear

The tangential force :provides the driving torque or
transmission of power.

The radial or normal force: produces radial deflection
of the rim and bending of the shafts.

Expression for Power : P=T.ω

12

EXAMPLE : 1
DATA:
Calculate the total load (F) due to the power transmitted
Given :

T

13

Length of Path of Contact:
The length of path of contact is the length of common normal cutoff
by the addendum circles of the wheel and the pinion.

the length of path of contact is KL which is the sum of the parts of the
path of contacts KP and PL.

KL = KP + PL

KP = Path of Approach

PL = Path of recess

14

Length of path of approach:

………..(i)
Considering the triangles KO2N and PO2N

From triangle KO2N

O2K = RA
O2N = R Cos φ
From triangle PO2N
PN = R Sin φ

Put values in equation (i), we get

………..(ii)

15

Length of path of recess:

In the same way by considering the triangles M1OL and O1MP
We derive the expression for Path of recess

………..(iii)

Length of path of contact:
We get the total length of path of contact by taking the sum of (ii) and (iii)

16

Length of Arc of contact:
• Arc of contact is the path traced by a point on the pitch circle
from the beginning to the end of engagement of a given pair of
teeth.
• the arc of contact is EPF or GPH

Length of Arc of contact :

17

Contact Ratio (or Number of Pairs of Teeth in Contact)

The contact ratio or the number of pairs of teeth in contact is
defined as the ratio of the length of the arc of contact to the circular
pitch .

18

Circular pitch: 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.

Module: It is the ratio of the pitch circle diameter in millimeters to the number of
teeth. It is usually denoted by m.

19

Example 12.4
Given :
φ = 20° t = 20 G = T/t = 2 m = 5 mm v = 1.2 m/s
addendum = 1 module= 5 mm

find
1. The angle turned through by pinion when one pair of
teeth is in mesh
2. The maximum velocity of sliding

1. The angle turned through by pinion

20

2,The maximum velocity of sliding

Given :
φ = 20° t = 20 G = T/t = 2 m = 5 mm v = 1.2 m/s
addendum = 1 module= 5 mm
21

Lecture # 03

Interference in Involute Gears

“The phenomenon when the tip of tooth
undercuts the root on its mating gear is
known as interference.”

A little consideration will show, that if
the radius of the addendum circle of
pinion is increased to O1N, the point of
contact L will move from L to N.

When this radius is further increased,
the point of contact L will be on the
inside of base circle of wheel and not on
the involute profile of tooth on wheel

22

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.

From triangle O1MP

From triangle O2NP

23

In case the addenda on pinion and wheel is such that the path of
approach and path of recess are half of their maximum possible
values, then

24

Lecture # 04

Minimum Number of Teeth on the Pinion in Order to Avoid Interference

In order to avoid interference, the addendum circles for the two
mating gears must cut the common tangent to the base circles
between the points of tangency.

The limiting condition reaches, when the addendum circles of
pinion and wheel pass through points N and M (see Fig)

25

Let,

t = Number of teeth on the pinion,

T = Number of teeth on the wheel,

m = Module of the teeth,

r = Pitch circle radius of pinion = m . t / 2

G = Gear ratio = T / t = R / r

φ = Pressure angle or angle of obliquity.

26

AP.m = Addendum of the pinion, where AP is a fraction by which the standard
addendum of one module for the pinion should be multiplied in order to avoid
interference.

We know that the addendum of the pinion
AP.m = O1N – O1P …………………..(1)
From triangle O1NP r

Cos(90 +φ) = Sin φ

27

Putting values in equation (1) we get

28

If the pinion and wheel have equal teeth,

then G = 1

Therefore the above equation reduces to

29

Minimum Number of Teeth on the Wheel in Order to Avoid Interference

If the pinion and wheel have equal teeth,

then G = 1

Therefore the above equation reduces to

30

Example 12.13
Two gear wheels mesh externally and are to give a velocity
ratio of 3 to 1. The teeth are of involute form ; module = 6
mm, addendum = one module, pressure angle = 20°. The
pinion rotates at 90 r.p.m.
Determine :

1. The number of teeth on the pinion to avoid interference
on it and the corresponding number of teeth on the
wheel.

2. The length of path and arc of contact

3.The number of pairs of teeth in contact, and

4. The maximum velocity of sliding.
31

Given :
G = T / t = 3 ; m = 6 mm ; A P = A W = 1 module = 6 mm ;
φ = 20° ; N 1 = 90 r.p.m. or ω1 = 2π × 90 / 60 = 9.43 rad/s

1. Number of teeth on the pinion to avoid interference on it and the
corresponding number of teeth on the wheel.

We know that number of teeth on the pinion to avoid interference,

32

2. The length of path and arc of contact

path of approach path of recess

Length of path of contact

33

3. Number of pairs of teeth in contact

Number of pairs of teeth in contact

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4. The maximum velocity of sliding.

35

Mehran University College of Engineering
and Technology Khairpur

MECHANICS OF MACHINE ii
TOOTHED GEARING
Abdul Ahad Noohani (MUCET KHAIRPUR)

Gears mom2 2

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