MAURBHANJ SCHOOL OF ENGINEERING�BARIPADA : 757107 , ODISHA�

Branch : Mechanical Engineering

Semester : 4^th Sem

Subject : THEORY OF MACHINE

Topic : POWER TRANSMISSION

Faculty : Er. A DAS

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OBJECTIVE:

• The objective of the subject is to gain knowledge on the principles and procedure for the design of mechanical power transmission component.
• To understand the standard procedure available for design of transmission elements.
• To learn to use standard data and catalogues.
• Normally we are using PSG design data book for designing the mechanical components.

OUTCOME:

• Upon completion of this course, the students can able to successfully design transmission components used in Engine and machines.

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UNIT-I Introduction

• To transmit power from flexible elements such as belts, chains and ropes are frequently used.

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• Pulleys are mounted on a shaft and a continuous belt or rope is passed over them.

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• In belt and ropes, power is transmitted due to friction between them and pulleys. In case of chain drives, sprocket wheels are used.

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BELT DRIVES

• Belt drive is a mechanical drive in which the driving and driven shaft are connected by a loop of flexible material called as belt through pulleys mounted on the shafts

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• The distance between the shaft is large, then belts (or) ropes (or) chains are used

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• It can absorb a good amount of shock and vibration
• It can take care of some degree of misalignment between the driven and the driver machine shafts

Design

• Material - Leather, Rubber, Plastics, Fabric
• No. of ply and Thickness
• Maximum belt stress per unit width
• Density of Belt material
• Coefficient of friction of the belt material

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Types of Belts�

• Flat Belts
• V – Belts
• Ribbed Belts
• Toothed or timing belts

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FLAT BELTS

V - BELT

V-belt drive

RIBBED BELTS�

TOOTHED OR TIMING BELTS�

Types of Flat Belt Drives�

Open Belt Drive

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shafts arranged parallel and rotating in same directions

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Open belt drive

Open Belt Drive with One Idler Pulley

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when an open belt drive can be used due to small angle of contact on the smaller pulley.

Idler pulleys also known as jockey pulleys. It are used to obtain high velocity ratio and required belt tension

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Flat belt drive with idler pulley

Open belt drive with many idler pulley

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Used when it is desired to transmit motion from one shaft to several parallel shafts

Crossed or twisted belt drive�

Used with shafts arranged parallel and rotating in the opposite directions

Crossed belt drive

Quarted twist or Quarter turn drive�

Used with shafts arranged at right angles and rotating in one definite directions

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Stepped or Cone pulley drive�

Used for changing the speed of the driven shaft while the main or driving shaft runs at constant speed

Fast and loose pulley�

Used when the driven shaft is to be started or stopped whenever desired without interfering with the driving shaft

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Compound drive�

Used when several units are to be driven from one central shaft

Selection of Flat Belt Drive�

It depends on

• Power to be Transmitted
• Speed of Driver and Driven Shafts
• Shaft relationship
• Service conditions
• Speed reduction ratio
• Centre distance
• Space available

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Belt Materials�

The desirable properties of belt materials are

• High C.O.F
• Flexibility
• Durability
• Strength

Leather Belts�

• made of animal hides
• Leathers for belting may be tanned with oak, or chrome salts.
• Oak tanned belt is fairly stiff
• Chrome tanned leather is soft and pliable
• Belts specified according to number of layers as single ply, double ply or triple ply belts. Double belts or triple belts are made by cementing two or three strips together with hair sides outside

Fabric & Cotton Belts�

• Obtained by stitching two or more plies of canvas or cotton duck.
• Treated with linseed oil to make it water proof.
• These belts are cheap and most suitable for farmwork, quarry and saw mills

Rubber Belts�

• These belts are made up of plies of fabric impregnated with vulcanized rubber or synthetic rubber
• Easily made endless
• Saw mills, chemical plants and paper mills largely use rubber belts

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Balata Belts�

• Balata is gum similar to rubber. Balata belts are made in the same manner as the rubber belts made.
• They are acid proof and water proof. These belts cannot be used at temperature above 40°C, because at this temperature it softens and became sticky.
• Nylon Core Belts
• Camel’s Hair belts

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Velocity ratio of belt drive�

The ratio between the speeds of the driver and driven respectively.

• Velocity ratio = N₂/N₁ = ω₂/ω₁ = D/d
• Where D & d = diameter of driver and driven respectively
• N_{2 &} N₁ = Speed of driven & driver respectively
• ω₂ & ω₁ = Angular velocities of driven & driver respectively

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Effect of belt thickness on velocity ratio:

Considering the thickness of belt (t)

N₂/N₁ = (D + t)/(d + t)

Effect of slip on velocity ratio:

• Slip is defined as the relative motion between the belt and pulley.
• The difference between the linear speed of the pulley rim and belt is the measure of slip.
• The reason is, there is a tendency for the belt to carry with it on the underside between the pulley and the belt. The frictional grip between the pulley and the grip is insufficient in lower side.
• The slip reduces the velocity ratio of the drive.

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Slip can be reduced BY

• Roughening the belt by dressing
• By crowning the pulley

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• Let S₁ = Percentage slip between the driver and the belt.
• S₂ = Percentage slip between the driven and the belt.
• S = Total percentage slip = S₁ + S₂

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• Velocity ratio = N₂/N₁ = D/d [1 – ((S_{1 +} S₂)/100)] = D/d [1 – (S/100)]
• If thickness of the belt (t) is considered, then

• Velocity ratio = N₂/N₁ = (D+t)/(d+t) [1 – ((S_{1 +} S₂)/100)]

= (D+t)/(d+t) [1 – (S/100)]

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Effect of creep on belt�

Let

σ₁ = stresses in the belt on tight side

σ₂ = stresses in the slack side

E = Young’s modulus of belt material

• Velocity ratio = N₂/N₁ = (D)/(d) [[E + √ σ₁ ]/[E + √ σ₂ ]]

Law of Belting

• Law of belting states that, the centre line of the belt, as it approaches the pulley, must lie in a plane perpendicular to the axis of that pulley or must lie in the plane of the pulley. Otherwise the belt will run off the pulley

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Power Transmitted by the belt�

Power Transmitted by the belt

• P = (T₁ – T₂ ) v watts

Where T₁ = tension in the tight side.

T₂ = tension in the slack side.

V = linear velocity of the belt in m/s

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Centrifugal Tension (T_c) �

• T_c = waste load, increases the tension without increasing power capacity.
• T_c = mV²
• m = mass / unit length (Kg/m)
• V = linear velocity (m/s)
• Initial tension
• T_o = [T₁ + T₂]/2 [neglecting centrifugal tension]
• T_o = [T₁ + T₂ + 2T_c]/2 [considering centrifugal tension]

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Continue…

Maximum tension when the belt subjected to centrifugal tension

• T = T₁ + T_c
• T = maximum stress X cross sectional area of the belt
• = σ b t
• σ = maximum stress in N/m²
• b = width in m
• t = thickness in m

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Continue…

• When Centrifugal tension taken for consideration
• Tension in tight side is T_t1 = T₁ + T_c
• Tension in tight side is T_t2 = T₂ + T_c
• Then Power Transmitted by the belt
• P = (T_t1 – T_t2 ) V watt
• After simplification P = (T₁ – T₂ ) V watt
• It shows centrifugal tension doesn’t have any effect on power transmission

V – BELTS AND PULLEYS

• v-belts are used with electric motors to drive blowers, compressors, appliances (like mixer, grinder, etc., machine tools (like lathe, drilling machine, etc), farm and industrial machinery, and so on. V-belts are endless and run in grooved pulleys.
• V-belts are made in trapezoidal section. The power is transmitted by the wedging action between the belt and the V-groove in the pulley or sheave.

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MATERIALS OF V-BELTS�

• V-belts are made of cotton fabric and cords moulded in rubber and covered with fabric and rubber.

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• ADVANTAGES
• Power transmitted is more due to wedging action in the grooved pulley.
• V-belt is more compact, quiet and shock absorbing.
• Higher velocity ratio(upto 10)can be obtained.
• DISADVANTAGES
• It cannot be used with large centre distances.
• It cannot be used for large power.
• The efficiency of the V-belt is lower than that of the flat belt.

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Continue…

TYPES OF V-BELTS

According to Bureau of Indian standards (BIS :2494-1974), the V-belts are classified as A,B,C,D and E type (based on the cross-section of V-belts).

SPECIFICATIONS OF V-BELTS

V-belts are designated by its type and nominal inside length. For example, a C2845 belts has a cross-section of type C and has a nominal inside length of 2845mm.

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ADVANTAGES OF V – BELT DRIVE OVER FLAT BELT DRIVE

• Power transmitted is more due to wedging action in the grooved pulley.
• V – belt is more compact, quiet & shock absorbing.
• The drive is positive because, the slip is negligible due to wedge action.
• Higher velocity ratio (upto 10)
• V – belt drive can operate in any position (i.e., horizontal, vertical, inclined)
• Multiple V – belts can be used, thus enabling more power transmission.

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DISADVANTAGES OF V – BELT DRIVE OVER FLAT BELT DRIVE

• It cannot be used with large centre distances.
• It is not durable because bending stress is more.
• V – belt is subjected to certain amount of creep, therefore it is not suitable for constant speed application & timing devices.
• It cannot be used for large power
• V – belt efficiency less than flat belt efficiency
• The construction of V – grooved pulleys is complicated and costlier compared with flat belt pulley.

V – FLAT DRIVES

• In a V – belt drive, if the large grooved pulley is replaced by a Flat – faced pulley & smaller pulley remains V – grooved), then the drive is known as V – flat drive.

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• V – flat drives are used in domestic piston pumps, domestic clothes drier and larger punch press.

DESIGN WIRE ROPES AND PULLEYS�

• Wire ropes are used whenever large power is to be transmitted over long distances (upto 150 m).

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• The wire ropes are extensively used in elevators, oil well drilling, mine hoists, cranes, conveyors, suspension bridges and other material handling equipments.

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ADVANTAGES OF WIRE ROPES

• Lighter weight and high strength to weight ratio.
• More reliable in operation.
• Silent operation even at high working speeds.
• Less danger for damage due to jerks.

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Continue…

Materials of wire ropes:

Wrought iron, cast steel, plow steel and alloy steel.

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Classification of wire ropes:

Cross lay ropes:

• In these ropes, the strands are twisted into a rope in the opposite direction to that of the wires in the strands.

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Continue…

Parallel lay ropes:

• In these ropes, the direction of twist of the wires in the strand is the same as that of the strands in the rope .

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Composite laid ropes:

• In these ropes, the wires in two adjacent strands are twisted in the opposite direction.

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Continue…

Specification of wire ropes:

For example, a 6 × 7 rope means a rope made from six strands with seven wires in each strand.

Design procedure for a wire rope :

1.Selection of suitable wire rope:

First select the suitable type of wire rope for the given application

CHAIN DRIVES

• Chain drive is intermediate between belt and gear drives.
• Chain drives are used for velocity ratios less than 10m/s, power ratings upto 125kW.

Application:

Transportation industry such as bicycles, motor cycles and automobile vehicles, agricultural machinery, metal and wood working machine, textile machinery, building construction, material handling machinery.

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CHAIN DRIVE

ADVANTAGES

• They can be used for long as well as short centre distances.
• More compact than belt
• No slip between chain & sprocket. So they provide positive drive.
• One chain can be arranged to drive several sprockets.
• Higher efficiency (upto 98%) of the drive
• Transmit more power than belt drives
• Smaller load on the shafts than in belt drives.

Disadvantages:

• Precise alignment of shafts than the belt drives
• They require proper maintenance
• Noisy operation
• More complicated design

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Types of chain drives:

1. Link chains (or) welded chains
2. Transmission chains (or) roller chains
3. Silent chains (or) inverted tooth chains

LINK CHAINS

• Low capacity hoisting machines such as hoists and hand operated cranes, lifting.

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Advantages:

• Good flexibility in all directions
• Smaller than pulley diameters and drums
• Simple design and manufacture

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Disadvantages:

• Heavy weight
• Sudden jerk
• Sudden failure
• Intensive wear

TRANSMISSION (OR) ROLLER CHAINS

• Readily available, transmitting power between parallel shafts.
• Roller chain consists of an endless chain running over two sprockets.

Construction of roller chains:

• It consists of alternate links made of inner and outer link plates.
• Outer plates are known as pin link (or) coupling link.
• Inner plate… roller link, pin, bushing & roller.

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SILENT CHAINS (OR) INVERTED TOOTH CHAINS

CHAIN MATERIALS

• Link plates are made of cold-rolled, medium-carbon (or) alloy steels such as C45, C50 & 40crl.
• Pin, bushing, rollers are made of carburizing steel such as C15, C20, 30Ni4crl.

THANK YOU