Gears mom2 2

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Mehran University College of Engineering and Technology Khairpur

Mechanics of machine 2Mechanics of machine 2

TOOTHED GEARINGTOOTHED GEARING

Abdul Ahad Noohani (MUCET KHAIRPUR)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

� 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

Gears or toothed wheels

between driver and follower is very small

Law of gearing:

Condition for constant velocity ratio

LetQ : is the point of contact b/w two teeth

P : is the pitch point

T T : is the common tangentT T : is the common tangent

MN: is the common normal to the curves at Q

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

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

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

� 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

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.

From similar triangles QEC and O1MQ

From similar triangles QCD and O2 NQ

Putting values

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.

Lec # 02

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

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

When the power is being transmitted, themaximum force is exerted along the commonnormal through the pitch point

Force due to power transmitted:

FT = F cos φ

normal through the pitch point

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

Tangential component

Radial or normal component FR = F sin φ.

� 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.transmission of power.

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

Expression for Power : P=T.ω

EXAMPLE : 1

Calculate the total load (F) due to the power transmitted

Given :

�Length of Path of Contact:

� The length of path of contact is the length of common normal cutoffby 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 thepath of contacts KP and PL.

KL = KP + PL

KP = Path of Approach

PL = Path of recess

Length of path of approach:

Considering the triangles KO2N and PO2N

From triangle KO2N

………..(i)

From triangle PO2NPN = R Sin φ

O2K = RA

O2N = R Cos φ

Put values in equation (i), we get

………..(ii)

Length of path of recess:

In the same way by considering the triangles M1OL and O1MPWe 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)

Length of Arc of contact:

• Arc of contact is the path traced by a point on the pitch circlefrom 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 :

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

The contact ratio or the number of pairs of teeth in contact isdefined as the ratio of the length of the arc of contact to the circularpitch .

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.

Example 12.4

Given :

φ = 20° t = 20 G = T/t = 2 m = 5 mm v = 1.2 m/saddendum = 1 module= 5 mm

find1. The angle turned through by pinion when one pair of teeth is in mesh2. The maximum velocity of sliding

2. The maximum velocity of sliding

1. The angle turned through by pinion

2,The maximum velocity of sliding

Given :

φ = 20° t = 20 G = T/t = 2 m = 5 mm v = 1.2 m/saddendum = 1 module= 5 mm

2,The maximum velocity of sliding

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 ifthe radius of the addendum circle of

Lecture # 03

the radius of the addendum circle ofpinion is increased to O1N, the point ofcontact L will move from L to N.

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

� interference may only be prevented, if the addendum circles ofthe two mating gears cut the common tangent to the base circlesbetween the points of tangency.

From triangle O1MP

From triangle O2NP

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

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

� In order to avoid interference, the addendum circles for the twomating gears must cut the common tangent to the base circlesbetween the points of tangency.

Lecture # 04

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

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.

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

We know that the addendum of the pinion

AP.m = O1N – O1P

rFrom triangle O1NP

…………………..(1)

Cos(90 +φ) = Sin φ

Putting values in equation (1) we get

If the pinion and wheel have equal teeth,

then G = 1

Therefore the above equation reduces to

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

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

1. The number of teeth on the pinion to avoid interference

1. The number of teeth on the pinion to avoid interferenceon it and the corresponding number of teeth on thewheel.

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.

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

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

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

2. The length of path and arc of contact

path of approach path of recess

Length of path of contact

3. Number of pairs of teeth in contact

Number of pairs of teeth in contact

4. The maximum velocity of sliding.

Mehran University College of Engineering and Technology Khairpur

MECHANICS OF MACHINE iiMECHANICS OF MACHINE ii

TOOTHED GEARINGTOOTHED GEARING

Abdul Ahad Noohani (MUCET KHAIRPUR)Abdul Ahad Noohani (MUCET KHAIRPUR)

Gears mom2 2

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