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UNIT I A Introduction to Mechanical Engineering— Mechanical Elements Power transmission shafts, Axles, Keys, Bush and Ball Bearings, Flywheels and Governors

Unit 1 Machine Elements

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UNIT I AIntroduction to Mechanical EngineeringMechanical Elements Power transmission shafts, Axles, Keys, Bush and Ball Bearings, Flywheels and GovernorsIntroductionMachine element is an individual component or a group of components of a machine which performs a specific function.

Its function may be of holding the components together, to transmit power or to give supports.

Depending upon these functions only, the machine elements are following types.1) Machine elements used for holding the components.2) for transmitting the power.3) for support of other components.Machine ElementsMachine elements used for holding the components :-These type of machine elements include nuts and bolts, screw, cotters, keys, couplings, pins, revettes.Machine elements used for transmitting the power :-Machine elements like gears, shafts, clutches and brakes, pulleys, belts, chain, sprocket, are used to transmit power from one place to another.Machine elements used for support of other components:-Machine elements like bearings, axles, brackets, hangers etc. are used to give support to rotating or stationary members.

Basic RequirementsSafetyReliabilityMinimum WeightManufacturabilityMachinabilityDurabilityEfficiencyShaftIntroductionA shaft is a rotating member/machine element, which is used to transmit power from one place to another. In order to transfer the power from one shaft to another the various members such as pulleys, gears, crank etc. are mounted on it. These members are mounted on the shaft by means of keys or splines.Types of ShaftThe following two types of shafts are important from subject point of view.1) Machine shaft: 2) Transmission shaft:

Machine shaft:These shafts form an integral part of the machine itself.e.g. crank shaft of an I.C. engine.Transmission shaft:These shafts transmit power from the source of power to the machine which absorbs that power.e.g. the power is transmitted from motor to compressor by means of a transmission shaft.The transmission shaft is usually circular in cross section. The shaft is always stepped (having different cross sections of shafts) for positioning transmission elements like gears, pulleys and bearings. Transmission shafts may be solid or hollow.

Types of Shaft

A shaft which is used to transmit the power between the source and the machine absorbing the power is called transmission shaft

The transmission shaft supports transmission elements like : gears, pulleys, sprockets, etc

The transmission shafts are subjected to torque, bending moment, and axial force.

Main shaft between air compressor and gas turbine in air craft engine.Transmission Shaft

AXLEAn axle is a non-rotating machine element which is used to support rotating machine elements like wheels, pulleys, etc. In appearance, an axle looks like a shaft.An axle does not transmit torque. It is subjected to a bending moment and / or axial force.e.g. Rear axle of railway wagon, Axle of motor car, Supporting wheels.

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KeysIntroductionKey is defined as a demountable machine element which, when assembled into key seats, provides a positive means for transmitting torque between the shaft arid hub.It is used to transmit torque from shaft to pulleys, gears, sprockets, fly wheel etc.18

A key performs following two basic functions.1) The primary function of key is to transmit the torque from the shaft to the hub of machine element and vice-versa.

2) The second function of the key is to prevent relative rotational motion between the shaft and mounted machine element like gear or pulley.

A keyed joint consisting of shaft, hub and keyA recess or slot machined on the shaft and or in the hub to accommodate the key is called as key way.Keys are designed in order to withstand shear and compressive stresses resulting from transmission of torque.Generally, material of the key is selected, which has less strength than the shaft material.

Types of KeysKeys are broadly classified asa) Saddle keysb) Sunk keysc) Round keyd) Splines

Saddle KeysSaddle keys are fitted only in the key way of one member of the mating surface i.e. either shaft or hub.Saddle keys are only of uniform width, and tapered in thickness along the length.Power transmission of the saddle key is due to the frictional forces set up between the keys and the shaft.Saddle keys are of two types -i) Flat saddle keyii) Hollow saddle key

Flat Saddle KeyThe flat saddle key is a taper key which fits in a key way in the hub and is flat on the shaft as shown in Fig.

Hollow Saddle KeyA hollow saddle key is a taper key which fits in a key way in the hub and its lower surface of the key is hollow or curved to fit on the curved surface of the shaft as shown in Fig.

Advantages of Saddle Keys1) Construction is simple.2) Mounting cost is less as compared to other keys as there is key way on the hub only.3) No weakening of shaft as there is no key way.

Disadvantages of Saddle Keys1) Used for transmission of lighter loads only.2) No positive transmission of power as means of power transmission in friction.

Applications Used for light loads only like temporary fastening in fixing and setting of eccentric cams etc.

Sunk KeysSunk keys are inserted half in the key way of hub and half in the key way of the shaft. Sunk keys are of uniform width and tapered in thickness along the length.Power transmission of the sunk key is due to the tangential force between key and mating surfaces. No relative motion between the shaft and hub no slip between them Hence it is a positive drive.

Sunk keys are of following types.i) Parallel sunk keyii) Taper sunk keyiii) Feather keyiv) Woodruff key

Parallel Sunk Key

A parallel key is square or rectangular in cross section and of uniform thickness and width over its length.

BearingsIntroductionBearing is a mechanical element, which supports the rotating element. Thus it permits the relative rotating motion between two parts.Due to relative motion, a moving part comes in contact causing friction with the stationary part, resulting wear of mating surface. To reduce the frictional resistance and wear, a lubricant is provided between the mating surface.A lubricant is a substance placed between the two surfaces which reduces friction, wear and takes away heat.Generally mineral oil, vegetable oil or grease are used as lubricant in bearings.Bearings28

29Classification of BearingsRolling contact bearings

Ball bearingsRoller bearingsSingle row deep groove bearingsDouble row deep groove bearingsAngular contact bearingsCylindrical Roller bearingsNeedle Roller bearingsTaper Roller bearingsTaper roller thrust Roller bearings

Sliding contact bearings Guide bearingsJournal bearings Thrust bearings Collar bearingsFootstep or pivot bearings

Solid journal bearingBush bearingSplit bearing or plummer blockSliding Contact BearingsThe motion between the shaft and the bearing is purely sliding.

Because of the surface contact, the friction between the rotating shaft and bearing surface is high and hence lubrication is necessary.

They are used for crankshaft in I.C. engine, ball mills, centrifugal machines in sugar industry, turbo-generators centrifugal pumps. etc.

Rolling Contact Bearing Rolling motion takes place along the surfaces of contact between the parts. Steel balls or rollers are interposed between the moving and fixed elements, which offer rolling friction. They are also known as antifriction bearings. They have point or line contact with moving element.Rolling Contact Bearing (Antifriction Bearing)This type of bearing have rolling contact instead of sliding contact. This results in low frictional resistance than the sliding contact hence name as antifriction bearings.A rolling contact bearing, irrespective of any type has four major parts.1) Outer race2) Inner race3) Rolling elements like ball, roller or needle4) Cage which holds the rolling elements together with equal space.A rolling contact bearing is as shown in Fig.

Classification of Rolling Contact Bearing1) Depending upon the type of rolling element:i) Ball bearingii) Roller bearing2) Depending upon the direction of load:i) Radial bearingii) Thrust bearingiii) Combination of radial and thrust load Ball bearing36

The Ball BearingIt is as shown in Fig. It also consists of four major parts.(i) Outer race (ii) Inner race (iii) Balls and (iv) Cage It consist of number of balls placed between the races. The balls are placed properly by the cage, which acts like a separator. All balls are free to rotate within the cage and are able to roll inside the outer race and inner race. A position clearance is kept between the races to accomplish slight misalignment of balls due to load. Ball bearings are able to take only radial loads. It is used in high speed applications like grinders, cutters etc. Deep groove ball bearings are able to sustain radial load as well as axial (thrust) load.

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Difference between Rolling Contact and Sliding Contact BearingSr. NoFactorsRolling Contact BearingSliding Contact Bearing1Starting TorqueLowHigh2Running FrictionLowHigh3LoadCan carry combined radial and axial loadCan take load only in one direction4Axial Space requiredLessMoreDifference between Rolling Contact and Sliding Contact BearingSr. NoFactorsRolling Contact BearingSliding Contact Bearing5Radial Space requiredMoreLess6LubricationSimpleContinuous and adequate7Initial CostLowMore8Maintenance CostLowMore9Noise at higher speedsMoreLessDifference between Rolling Contact and Sliding Contact BearingSr. NoFactorsRolling ContactSliding Contact10Load carrying capacityLowMore11Working LifeLessMore12Shock absorbing capacityPoorGood

13SuitabilityFor frequent starting of machineContinuous working of machine.UNIT I BPower Transmission Devices Belt and Belt derives, Chain drives, Types of Gears, Types of Couplings, Friction Clutch (Cone and Single Plate), Brakes

DrivesDrives are the mechanism used to transmit power & speed from prime mover to machine.

The commonly used prime mover is an electric motor. (I. C. engines, steam engines)46Belt DrivesIntroductionBelts are used to transmit power between two parallel shafts by means of friction using pulleys.A belt drive consists of three elements viz, driving and driven pulleys and a belt, which envelops them. The power is transmitted from driving pulley to driven pulley because of frictional grip between the belt and pulley surface.

Speed ratio of Belt drive (Velocity ratio):

= [ speed of driver shaft ] / [ speed of driven shaft ]

= n / N = D / d

= n / N = (D+t) / (d+t) [ if belt thickness is considered] Types of Belts (According to Material Used)The material used for belts must be strong, flexible and durable. It must have a high coefficient of friction. Belts are classified according to material as.

1) Leather belt 2) Cotton or fabric belt

3) Rubber belt

4) Balata belt : layers of fabric impregnated with balata gun..Types of Belts (According to Shape)1) Flat belt2) Vee belt3) Circular or rope belt4) Timing belt

Difference between Flat Belt and Vee BeltFACTORSFLAT BELTV-BELT1. CostLess, easy to produceHigh, material is costly2. MaintenanceLowHigh3. Simplicity of designSimple to design & manuf.Complex to design & manuf.4. Center distanceCan be used for long center distance shaftsSuitable for short to medium center distance shafts5. Power transmitting capacityLess (less frictional contact)High (more frictional contact)6. Overall size of beltBigger sizeCompact size7. OperationNoisySmooth & quiet8. Speed ratioUp-to 4:1Up-to 7:1Types of Belt Drive (According to its Arrangement)The power from one shaft to another can be transmitted by different arrangements of the belts. According to its arrangement, Belt drives are classified as1) Open belt drive2) Crossed belt drive3) Drive with Idler Pulley4) Drive with Jockey or Rider pulley.Open belt driveIt is used, when shafts are arranged parallel and rotating in the same direction, as shown in Fig. In this case, the driver A pulls the belt from one side (i.e. lower side) and delivers it to the other side (i.e. upper side).

Thus the tension in the lower side belt will be more than that in the upper side belt. Hence lower side belt is known as tight side, whereas the upper side belt is known as slack side.

Crossed belt driveThis belt drive is used when shaft are arranged parallel and rotating in the opposite direction as shown in Fig. A crossed belt drive can transmit more power than open belt drive, as the angle of wrap is more. However, the belt has to bend in two different planes and it wears out more.

Drive with Idler PulleyWith constant use, the belt is permanently stretched a little in length. This reduces the initial tension in the belt leading to lower power transmission capacity. However, the tension in the belt can be restored to the original value, by using idler pulley as shown in above Fig.

Drive with Idler PulleyA bell crank lever, hinged on the axis of the smaller pulley, supports adjustable weights on its one arm and the axis of a pulley on the other. The pulley is free to rotate on its axis and is known as idler pulley. Owing to the weights on one arm of the lever, the pulley exerts pressure on the belt increasing the tension and increasing angle of contact. Thus life of the belt is increased and power capacity is restored to the previous value.

Advantages of Belt drive:Can be used for long distances.Lubrication is not required.Not affected by dust & dirt. (flat)Cheap & easy to maintain.

Disadvantages:Low power transmission capacityCant use for extremely high speedsLife span is less as compared to gear drives.Occupy more space as compared to gear drives.

57Chain Drive

Chain Drive

A belt or rope drive does not give constant velocity ratio due to slip, which may occur due to overload or due to greasy contact between belt and pulley. A belt or rope can be replaced by a chain, whenever there is a need to have a constant velocity ratio or a positive drive.The chain drive is intermediate between belt and gear drives. It has some features of belt drives and some of gear drives. Like gears, chains are made up of metal and therefore they are stronger and occupy lesser space and give constant velocity ratio. Like belts, they are flexible and are used for longer centre distance.The chains are made up of number of rigid links, which are hinged together by pin joints in order to provide the necessary flexibility to chain. It is an endless chain. A wheel, which has projected teeth to fit into the recesses in the chain, is known as sprocket wheel or simply sprocket. The sprockets and chain are thus constrained to move together without slipping and ensures perfect velocity ratio