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Course : B.Tech Mechanical

Subject : Elements of Mechanical Engineering

Unit-5

Contents:

Introduction

Methods of drive,

Power transmission elements

shaft Spindle and axle

Belt-drive

Pulleys

Power transmitted by a belt, Chain

drive, Friction drive, Gear drive

Introduction

Need:

◦ Every actuation or system need power or

motion to drive itself from source.

◦ Thus transmission of power from a

source(such as an engine or motor)

through a machine to an output actuation

is needed to do all machine tasks.

◦ This task can be achieved by different

methods of drives.

Different Terms of TPM

Driving member

Driven member

Idler member

Pulley

Belt

Chain

Rope

Gear

Methods of drive

Belts

Chains

Gears

Used when the distance between

the shaft centers is LARGE

Used when the distance between

the shaft centers is LARGE and no

slip is permitted

Used when the distance between

the shaft centers is adequately

less

Belts and Belt Drive

What is Belt?

A belt is continuous bond of flexible

material passing over pulleys to

transmit motion from one shaft to

another.

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Belt and Belt Drive…

Flat Belt:

◦ It has narrow rectangular cross-section

◦ Used for their simplicity due to minimum

bending stress on pulleys

◦ Load capacity depends on its width

◦ Only one belt is used in whole drive.

◦ Materials used for Belts are Leather,

Rubber, Textile, Balata, steel.

Belt and Belt Drive…

V Belt:

◦ It has trapezoidal section running on

pulleys with grooves cut to match belt.

◦ Normal angle between the sides of the

groove is 40 deg.

◦ Used when distance between two shafts

are too small for flat belt drive.

◦ More efficient than flat belt due to wedge

action in groove.

◦ Made up of fabric coated with rubber.

Belt and Belt Drive…

Round Belt:

◦ It has round section.

◦ Diameter range is 3 to12 mm but usually

from 4 to 8 mm.

◦ Used to transmit low power, mainly in

instruments, table-type machine tools, in

clothing industry and household

appliances.

◦ Made up of leather. Canvas and rubber.

Belt and Belt Drive…

Belt Drive:

◦ It consist of of driver and driven pulleys

and the belt which is mounted on pulleys

with certain amount of tension and

transmits peripheral force by friction.

◦ Two types of drives:

Open belt drive

Cross belt drive

Belt and Belt Drive…◦ Open belt drive

The drive in which Parallel shafts rotate in

same direction is called open belt drive.

Belt is subject to tension and bending.

Belt and Belt Drive…◦ Cross belt drive

The drive in which parallel shafts rotate in

opposite direction.

More wear and tear of belt in this drive, but it

transmit more power than open belt drive.

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Open belt drive Vs. Close belt drive

Open Belt Drive Closed Belt Drive

Both driver and the driven rotates in

the same direction

Driver and driven rotates in opposite

direction

When the shafts are horizontal,

inclined it is effective to transmit the

power

Even if the shafts are vertical it is

effective to transmit the power

As there is no rubbing point, the life of

the belt is more

Due to the rubbing point, the life of

the belt reduces.

Require less length of the belt

compared to crossed belt drive for

same centre distance, pulley

diameters.

Require more length of belt compared

to open belt drive for the same centre

distance, pulley diameters.

Belt and Belt Drive…

◦ Application of belt drives:

Transmit power from low or medium capacity

electric motors to operative machines

To transmit power from small prime movers

Belt and Belt Drive…

◦ Transmission of power in belt drive: Driving pulley L and driven pulley M is

connected my belt. The driving pulley pulls the belt from one side and delivers the same to the other.

Hence tension is in tight side will be more than slack side.

T1 = tension in the tight side, N

T2 = tension in the slack side, N

v = velocity of the belt, m/s

Now effective turning force = T1 - T2

Hence, work = Force x Distance = (T1 - T2 ) v Nm/s

Thus. Power = (T1 - T2 ) v

Pulleys and Pulley Drive…

Types of Pulley Drive:

◦ Idler pulley

◦ Stepped pulley

◦ Fast-Loose pulley

Chains and Chain Drive

◦ What is chain?

A chain consists of links connected by

joints which provide for flexibility for chain.

Chains and Chain Drive…

◦ Chain drive:

A chain drive consists of two sprockets

and chain

Most often, the power is conveyed by a

roller chain, known as the drive chain,

passing over a sprocket gear, with the

teeth of the gear meshing with the holes

in the links of the chain.

The gear is turned, and this pulls the

chain putting mechanical force into the

system

Advantages of Chain Drives

◦ Do not slip

◦ Maintain constant and precise speed.

◦ Good service life

◦ Easy to install and repair

Disadvantages of Chain Drives

◦ Noisy

◦ Need lubrication

◦ Weight of the chain

Chains and Chain Drive…

Roller Chain Drive:

◦ The hollow rollers are held inside two flat link plates

which are joined together by sleeves or bushing

passing inside the rollers.

Chains and Chain Drive…

Roller Chain Drive:

◦ Consecutive sets of such assemblies are connected

together with another pair of plates called pin link

plates, which in turn are held by central pins passing

through the sleeves. Rivets are used to join the link

plates. Sometimes the roller link plates are joined

together by rollers themselves and no sleeves are

used.

Chains and Chain Drive…

Silent Chain Drive:

It consist of a number offlat links, tooth shaped atends and joined togetherby long cross pins. Thesprockets are usuallywider than those of theroller chain and have acentral groove whichholds retained platesprovided in the centrallinks for keeping thechain on the sprocketsecurely.

Chains and Chain Drive…

Use of chain drive:

◦ Motorcycles

◦ Bicycles

◦ Automobiles

◦ Conveyers

◦ Agricultural machinery

◦ Oil-well drilling machines

◦ Machine tools

Chains and Chain Drive…

What is Gear?

◦ A gear is wheel provided with teeth which mesh

with the teethe on another wheel, or on to a

rack, so as to give a positive transmission of

motion from one component to another.

Gears and Gear Drive…

What is Gear?

◦ A gear is wheel provided with teeth which mesh

with the teethe on another wheel, or on to a

rack, so as to give a positive transmission of

motion from one component to another.

Gears and Gear Drive…

Types of Gears:

According to the position of axes of the shafts.

a. Parallel

1.Spur Gear

2.Helical Gear

3.Rack and Pinion

b. Intersecting

Bevel Gear

c. Non-intersecting and Non-parallel

worm and worm gears

Gears and Gear Drive…

Gears and Gear Drive…

TYPES OF

GEARS

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Spur gears

Teeth is parallel to axis of

rotation

Transmit power from one shaft

to another parallel shaft

USES

Generally it is used in Electric

screwdriver, oscillating

sprinkler, windup alarm clock,

washing machine and clothes

dryer

Gears and Gear Drive…

Spur gear types:

◦ External type

◦ Internal type

Gears and Gear Drive…

Helical gears

The teeth of the gear are helix around the gear. The helical gear run more smoothly and more quietly at high speed and curvilinear contact of gear teeth giving gradual engagement.

USES

Generally it is used in automobile power transmission.

Gears and Gear Drive…

Bevel gearsBevel gear teeth are varying in

cross section along the tooth width. The axis of two moving gears are inclined in the bevel gear.

USES

When the axis of the two shafts are inclined and intersect each other

Gears and Gear Drive…

Worm and worm wheel

A worm has one or more number of helical

threads of trapezoidal shape cut on it.

Gears and Gear Drive…

USES

To transmit power

between two shafts

having their axis at

right angles and non-

coplaner. Like in

drilling machine.

Rack and pinion

Rack is a spur gear of infinite diameter. The

rack is mesh with another small gear known as

pinion

Gears and Gear Drive…

USES

It is used to convert

rotary motion into

linear motion.

Such as lathe , drilling ,

planning machines.

NOMENCLATURE OF SPUR GEARS

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Pitch circle. It is an imaginary circle which by pure rolling action would give the same motion as the actual gear.

Pitch circle diameter. It is the diameter of the pitch circle. The size of the gear is usually specified by the pitch circle diameter. It is also known as pitch diameter.

Pitch point. It is a common point of contact between two pitch circles.

Pitch surface. It is the surface of the rolling discs which the meshing gears have replaced at the pitch circle.

Pressure angle or angle of obliquity. It is the angle between the common normal to two gear teeth at the point of contact and the common tangent at the pitch point. It is usually denoted by φ. The standard pressure angles are 14 1/2 ° and 20°.

Addendum. It is the radial distance of a tooth from the pitch

circle to the top of the tooth.

Dedendum. It is the radial distance of a tooth from the pitch

circle to the bottom of the tooth.

Addendum circle. It is the circle drawn through the top of the

teeth and is concentric with the pitch circle.

Dedendum circle. It is the circle drawn through the bottom of

the teeth. It is also called root circle.

Note : Root circle diameter =

Pitch circle diameter × cosφ ,

where φ is the pressure angle.

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. It is usually denoted by Pc

,Mathematically,

A little consideration will show that the two gears will mesh

together correctly, if the two wheels have the same circular

pitch.

Note : If D1 and D2 are the diameters of the two meshing gears

having the teeth T1 and T2 respectively, then for them to mesh

correctly,

Diametral pitch. It is the ratio of number of teeth to the pitch

circle diameter in millimetres. It is denoted by pd.

Mathematically,

Module. It is the ratio of the pitch circle diameter in millimeters to

the number of teeth. It is usually denoted by m. Mathematically,

Clearance. It is the radial distance from the top of the tooth to the

bottom of the tooth, in a meshing gear. A circle passing through

the top of the meshing gear is known as clearance circle.

Total depth. It is the radial distance between the addendum and

the dedendum circles of a gear. It is equal to the sum of the

addendum and dedendum.

Working depth. It is the radial distance from the addendum circle to

the clearance circle. It is equal to the sum of the addendum of the

two meshing gears.

Tooth thickness. It is the width of the tooth measured along the pitch

circle.

Tooth space . It is the width of space between the two adjacent teeth

measured along the pitch circle.

Backlash. It is the difference between the tooth space and the tooth

thickness, as measured along the pitch circle. Theoretically, the

backlash should be zero, but in actual practice some backlash must

be allowed to prevent jamming of the teeth due to tooth errors and

thermal expansion.

Face of tooth. It is the surface of the gear tooth above the pitch

surface.

Flank of tooth. It is the surface of the gear tooth below the pitch

surface.

Top land. It is the surface of the top of the tooth.

Face width. It is the width of the gear tooth measured parallel to its

axis.

Profile. It is the curve formed by the face and flank of the tooth.

Fillet radius. It is the radius that connects the root circle to the profile

of the tooth.

Path of contact. It is the path traced by the

point of contact of two teeth from the

beginning to the end of engagement.

Length of the path of contact. It is the

length of the common normal cut-off by the

addendum circles of the wheel and pinion.

Arc of contact. It 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 consists of two

parts, i.e.

(a) Arc of approach. It is the portion of the

path of contact from the beginning of the

engagement to the pitch point.

(b) Arc of recess. It is the portion of the

path of contact from the pitch point to the

end of the engagement of a pair of teeth.

Gear Trains

When two or more gears are made to mesh

with each other to transmit power from one

shaft to other. Such an arrangement is called

gear train.

Types:

Simple gear train

Compound gear train

Epicyclic gear train

Simple gear train

When there is only one gear on each

shaft, then it is known as simple gear

train.

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Simple gear train…

When the distance between the two gears is large and we need Constant velocity ratio:

The motion from one gear to another, in such a case, may be transmitted by either of the following two methods

1. By providing the large sized gear

2. By providing one or more intermediate gears.

The first method (i.e. providing large sized gears) is very inconvenient and uneconomical method

whereas the latter method (i.e. providing one or more intermediate gear) is very convenient and economical.

Simple gear train…

Intermediate gears:

Intermediate gears are called

idle gears, as they do not

effect the speed ratio or train

value of the system.

1. To connect gears where a

large centre distance is

required

2. To obtain the desired

direction of motion of the

driven gear(i.e. clockwise or

anticlockwise)

Simple gear train… Intermediate gears:

when the number of intermediate gears are odd,

the motion of both the gears (i.e. driver and driven

or follower) is like.

If the number of intermediate gears are even, the

motion of the driven or follower will be in the

opposite direction of the driver.

ADVANTAGES of Simple Gear Train

to connect gears where a large center distance is

required

to obtain desired direction of motion of the driven

gear ( CW or CCW)

to obtain high speed ratio

Compound gear train

When there are more than one gear on a

shaft , then the gear train is called a

compound train of gear.

ADVANTAGES of Compound Gear Train

A much larger speed reduction from the first

shaft to the last shaft can be obtained with

small gear.

If a simple gear trains used to give a large

speed reduction, the last gear has to be very

large.

Epicyclic Gear Train

When there is relative motion between two or more of the axes of wheels, such arrangement is called epicyclic gear train.

A small gear at the center called the sun, several medium sized gears called the planets and a large external gear called the ring gear.

ADVANTAGES of Epicyclic Gear

Train

•They have higher gear ratios.

•They are popular for automatic transmissions in

automobiles.

•They are also used in bicycles for controlling power

of pedaling automatically or manually.

•They are also used for power transmission

between internal combustion engine and an electric

motor.

Review Questions

Short Answer Questions1. Differentiate between open belt and crossed belt

drive.

2. What are the commonly used materials for flat belts?

3. List out the applications of belt drives.

4. What do you mean by slip in a belt drive?

5. Differentiate between belt drive and chain drive.

Long Answer Questions1. Explain with neat sketch, the types of various drives.

2. List out the advantages and disadvantages of all

drives.

3. Compare flat belt and V belts.

4. Briefly explain chain drives.

5. Briefly explain pulley drives.

Sources/Links1. Image references

2. http://mechteacher.com/mt/wp-content/uploads/2013/05/Flat-belt-drive.png

3. http://www.globalspec.com/ImageRepository/LearnMore/20135/crossed%20belt%20drive0cb067c5f9ea41978ae3d270411d8715.png

4. http://www.thecartech.com/subjects/auto_eng/Gear_Types_files/image001.jpg

5. http://file1.cucdc.com/cwfiles/11/66011/picture/F5.4%20Nomenclature%20of%20the%20spur%20gear%20teeth.jpg

6. http://nptel.ac.in/courses/Webcourse-contents/IIT-Delhi/Kinematics%20of%20Machine/site/gear/pics/image012b.png

Content References

– Elements of Mechanical Engineering by H.G. Katariya, J.P Hadiya, S.M.Bhatt , Books India Publication.

-Elements of Mechanical Engineering by V.K.Manglik, PHI

-Elements of Mechanical Engineering by R.K Rajput.

-Elements of Mechanical Engineering by P.S.Desai & S.B.Soni

Any Question ?

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