Transmission Elements

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The Transmission elements of Gears, Belts, Ropes, Coupling, Cams

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Calibri:

Power Transmission Systems Power transmission systems are used to transmit motion from the prime mover to the end equipment. They are also used for motion conversion like rotary to linear and vice versa. Power transmission systems widely used in robotics are Gears Belts Chains 1

Times New Roman:

Gears Gears are devices that have teeth cut on them at regular intervals for transmitting motion without slipping. Gears are used for transmitting motion when distance between two shafts are small. Gears have higher transmission efficiency and higher load carrying capacity. Cast iron and high strength alloy steels are used for making gears. Nylon is also used. 2

Century Gothic:

Classification of Gears Based on arrangement of teeth for engaging with another gear External gears Internal gears Based on teeth design Spur gears Helical gears Bevel gears Worm gears Racks 3

Aharoni:

External Gear 4 Animation

Courier New:

Internal Gear 5 Animation

Wingdings:

Spur Gear 6 Straight Teeth Parallel to Gear axis Simplest Gear Easy to Maintain & Manufacture Only one teeth engages with another in a time. So Load carrying capacity is lesser than others. Noisy operation, Because tooth of the driver collides with the driven at the meshing point

Office Theme:

Helical Gear 7 teeth cut with an inclination to the Gear axis. This angle is called Helix angle. The teeth may be either right hand helix or Left hand helix. Mating Gears should have Opposite Helix Two or more tooth mesh with mating gear at a given time. So load carrying capacity is high. Continuous contact between the teeth of two meshing gears, they runs smooth, and get heated up quickly. Due to helix side thrust develops, So thrust bearing or Double helical gears are used

Power Transmission Systems:

Bevel Gear 8 To transmit motion between shafts with axis intersecting with each other at certain angle It has a conical form with teeth cut over the cone. Bevel gears with equal numbers of teeth and shaft axes at 90 degrees are called miter gears

Gears:

Types of Bevel Gear 9 Straight teeth arranged radials with respect to gear axis over the cone. Teeth spiraling around the cone. Higher load carrying Capacity. Due to the Intersecting angle of the Shafts, proper bearings with adequate strength should be provided for smoother O/p.

Classification of Gears:

Worm Gear Animation 10 Motion transmitted between the shafts with axis perpendicular to each other Same Tooth pitch between mating & large gear reduction ratio is possible in a single stage. The gear outer surface is either straight or concave face with the radius of curvature equal to the inner radius of the worm.

External Gear:

Rack and Pinion Animation 11 Rack has series of teeth cut parallel to each other along a straight blank. Same tooth pitch for rack and pinion.

Internal Gear:

Simple Gear Train

Spur Gear:

Compound Gear Train

Helical Gear:

Reverted Gear Train The reverted gear trains are used in automotive transmissions, lathe back gears, industrial speed reducers, and in clocks

Bevel Gear:

Epicyclic Gear Train The epicyclic gear trains are used in the back gear of lathe, differential gears of the automobiles, hoists, pulley blocks, wrist watches etc

Types of Bevel Gear:

Backlash in Gears Backlash is the total distance of excessive play produced at the meshing point between the teeth of two meshing gears. Backlash reduces positioning accuracy. Backlash is eliminated by using anti backlash gears. 16

Worm Gear:

Belt, Rope and Chain Drives Introduction The belts or ropes are used to transmit power from one shaft to another by means of pulleys which rotate at the same speed or at different speeds. The amount of power transmitted depends upon the following factors : 1. The velocity of the belt. 2. The tension under which the belt is placed on the pulleys. 3. The arc of contact between the belt and the smaller pulley. 4. The conditions under which the belt is used. (a) The shafts should be properly in line to insure uniform tension (b) The pulleys should not be too close together, in order that the arc of contact on the smaller pulley may be as large as possible.

Rack and Pinion:

Selection of a Belt Drive Following are the various important factors upon which the selection of a belt drive depends: 1. Speed of the driving and driven shafts, 2. Speed reduction ratio, 3. Power to be transmitted, 4 . Centre distance between the shafts, 5. Positive drive requirements, 6 . Shafts layout, 7. Space available, and 8 . Service conditions.

Simple Gear Train:

Types of Belt Drives The belt drives are usually classified into the following three groups : Light drives: These are used to transmit small powers at belt speeds up to about 10 m/s , as in agricultural machines and small machine tools . Medium drives: These are used to transmit medium power at belt speeds over 10 m/s but up to 22 m/s , as in Machine tools . Heavy drives: These are used to transmit large powers at belt speeds above 22 m/s , as in Compressors and Generators.

Compound Gear Train:

Types of Belts Types of Belts

Reverted Gear Train:

Types of Belts Flat belt: The flat belt, as shown in Fig. is mostly used in the Factories and workshops , where a moderate amount of power is to be transmitted, from one pulley to another when the two pulleys are not more than 8 meters apart. V-belt: The V-belt, as shown in Fig. is mostly used in the Factories and work­shops , where a moderate amount of power is to be transmitted, from one pulley to another, when the two pulleys are very near to each other. Circular belt or rope: The circular belt or rope, as shown in Fig. is mostly used in the Factories and workshops , where a great amount of power is to be transmitted, from one pulley to another, when the two pulleys are More than 8 meters apart.

Epicyclic Gear Train:

Material used for Belts The material used for belts and ropes must be Strong, Flexible, Durable & Must have a high coefficient of friction. The belts, according to the material used, are classified as follows : Leather belts: The most important material for the belt is leather . The best leather belts are made from 1.2 meters to 1.5 meters long strips cut from either side of the back bone of the top grade steer hides. The hair side of the leather is smoother and harder than the flesh side, but the flesh side is stronger.

Backlash in Gears:

Cotton or Fabric belts: Most of the fabric belts are made by Folding Canvass or Cotton duck to three or more layers (depending upon the thickness desired) and stitching together. These belts are Woven also into a strip of the desired width and thickness . Rubber belt: The rubber belts are made of layers of Fabric impregnate with Rubber com ­ position and have a thin layer of rubber on the faces. These belts are very flexible but are quickly destroyed if allowed to come into contact with heat, oil or grease. Balata belts: These belts are similar to rubber belts except that Balata gum is used in place of Rubber. These belts are Acid proof and Water proof and it is Not effected by Animal oils or Alkalis. Note: The balata belts should not be at temperatures above 40° C because at this temperature the balata begins to soften and becomes sticky. The strength of balata belts is 25 per cent higher than rubber belts.

Belt, Rope and Chain Drives:

Belt Drive Terms

Selection of a Belt Drive:

Types of Flat Belt Drives Open Belt Drive . The open belt drive, as shown in Fig. is used with shafts Arranged Parallel and rotating in the Same Direction. In this case, the driver A pulls the belt from one side (i.e. lower side RQ) and delivers it to the other side (i.e. upper side LM). Thus the tension in the lower side belt will be more than that in the upper side belt.

Types of Belt Drives :

Crossed or twist belt drive: The crossed or twist belt drive, as shown in Fig. is used with shafts Arranged Parallel and Rotating in the Opposite Directions.

Types of Belts:

Quarter turn belt drive: The quarter turn belt drive also known as right angle belt drive , as shown in Fig. is used with shafts arranged at right angles and rotating in one definite direction . In order to prevent the belt from leaving the pulley, the width of the face of the pulley should be greater or equal to 1.4 b, where b is the width of belt .

Types of Belts :

Belt drive with idler pulleys: A belt drive with an idler pulley, as shown in Fig. is used with shafts arranged parallel and when an open belt drive cannot be used due to small angle of contact on the smaller pulley. This type of drive is provided to Obtain high Velocity ratio and when the required belt tension cannot be obtained by other means.

Material used for Belts :

Compound belt drive: A compound belt drive, as shown in Fig. is used when power is transmitted from one shaft to another through a number of pulleys.

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Stepped or cone pulley drive: A stepped or cone pulley drive, as shown in Fig. is used for Changing the speed of the driven shaft while the main or driving shaft runs at constant speed. This is accomplished by shifting the belt from one part of the steps to the other.

Belt Drive Terms:

Fast and loose pulley drive: A fast and loose pulley drive, as shown in Fig. is used when the driven (or) machine shaft is to be started or stopped when ever desired Without Interfering with the driving shaft.

Types of Flat Belt Drives:

Velocity Ratio of Belt Drive It is the ratio between the velocities of the driver and the follower or driven. It may be expressed, mathematically, as discussed below : Let d1 = Diameter of the driver, d2 = Diameter of the follower, N1 = Speed of the driver in r.p.m., and N2 = Speed of the follower in r.p.m.

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Slip of Belt The motion of belts and shafts assuming a firm Frictional Grip between the belts and the shafts. But sometimes, the frictional Grip becomes insufficient . This may cause some forward motion of the driver without carrying the belt with it. This may also cause some forward motion of the belt without carrying the driven pulley with it. This is called slip of the belt and is expressed as a percentage.

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Creep of Belt When the belt passes from the slack side to the tight side, a certain portion of the belt extends and it contracts again when the belt passes from the tight side to slack side. Due to these changes of length, there is a relative motion between the belt and the pulley surfaces. This relative motion is termed as creep.

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Power Transmitted by a Belt Fig. 11.14 shows the driving pulley (or driver) A and the driven pulley (or follower) B. We have already discussed that the driving pulley pulls the belt from one side and delivers the same to the other side. It is thus obvious that the tension on the former side (i.e. tight side) will be greater than the latter side (i.e. slack side) Let T1 and T2 = Tensions in the tight and slack side of the belt respectively in Newton's, V = Velocity of the Belt drive ( m/s ) P = ( T1 – T2 ) v Watts

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V-belt drive We have already discussed that a V-belt is mostly used in Factories and Workshops where a Great Amount of power is to be transmitted from one pulley to another when the two pulleys are very near to each other. The V-belts are made of fabric and cords molded in rubber and covered with fabric and rubber, as shown in Fig. These belts are molded to a Trapezoidal Shape and are made endless. These are particularly suitable for Short drives i.e. when the shafts are at a short distance apart. The included angle for the V-belt is usually from 30° – 40°.

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Rubber

Velocity Ratio of Belt Drive:

Timing belts Timing belts , (also known as Toothed, Notch, Cog, or Synchronous belts) are a positive transfer belt and can track relative movement. These belts have teeth that fit into a matching toothed pulley.

Creep of Belt:

Crowning of pulleys

Power Transmitted by a Belt:

Advantages of V-belt Drive Following are the advantages and disadvantages of the V-belt drive over flat belt drive. Advantages The V-belt drive gives compactness due to the small distance between the centers of pulleys. The drive is positive, because the slip between the belt and the pulley groove is negligible. Since the V-belts are made endless and there is no joint trouble, therefore the drive is smooth . It provides longer life, 3 to 5 years . It can be easily installed and removed. The operation of the belt and pulley is quiet. The belts have the ability to cushion the shock when machines are started. The high velocity ratio may be obtained.

V-belt drive:

Rope Drive The rope drives are widely used where a large amount of power is to be transmitted, from one pulley to another, over a considerable distance Fibre ropes, and Wire ropes. The fibre ropes operate successfully when the pulleys are about 60 meters apart The wire ropes are used when the pulleys are up to 150 meters apart . It may be noted that frictional grip in case of rope drives is more than that in V-drive. One of the main advantage of rope drives is that a number of separate drives may be taken from the one driving pulley. For example, in many spinning mills, the line shaft on each floor is driven by ropes passing directly from the main engine pulley on the ground floor.

PowerPoint Presentation:

Fibre Ropes The ropes for transmitting power are usually made from Fibrous materials such as Hemp, Manila and Cotton. The Hemp and Manila fibres are rough , therefore the ropes made from these fibres are not very flexible and possesses poor mechanical properties . The hemp ropes have less strength as compared to manila ropes. When the hemp and manila ropes are bent over the sheave (or pulley), there is some sliding of fibres, causing the rope to wear and chafe internally. The rope fibres are lubricated with a tar, tallow or graphite . The lubrication also makes the rope moisture proof. The hemp ropes are suitable only for hand operated hoisting machinery as tie ropes for lifting tackle

PowerPoint Presentation:

Fibre Ropes The cotton ropes are very Soft and Smooth . The lubrication of cotton ropes is not necessary . But if it is done, it reduces the external wear between the rope and the grooves of its sheaves. It may be noted that manila ropes are more durable and stronger than cotton ropes. The cotton ropes are costlier than manila ropes

Timing belts:

Advantages of Fibre Rope Drives The fibre rope drives have the following advantages : 1. They give smooth, steady and quiet service. 2. They are little affected by out door conditions. 3. The shafts may be out of strict alignment. 4. The power may be taken off in any direction and in fractional parts of the whole amount. 5. They give high mechanical efficiency.

PowerPoint Presentation:

Sheave for Fibre Ropes The fibre ropes are usually circular in cross-section . The sheave for the fibre ropes is shown in Fig. The groove angle of the pulley for rope drives is usually 45° . The grooves in the pulleys are made narrow at the bottom and the rope is pinched between the edges of the V-groove to increase the holding power of the rope on the pulley

Crowning of pulleys:

Wire Ropes When a large amount of power is to be transmitted over long distances from one pulley to another (i.e. when the pulleys are upto 150 metres apart), then wire ropes are used. The wire ropes are widely used in elevators, mine hoists, cranes, conveyors, hauling devices and suspension bridges . The wire ropes run on grooved pulleys but they rest on the bottom of the grooves and are not wedged between the sides of the grooves.

Advantages of V-belt Drive :


Rope Drive :

Chain Drives We have seen in belt and rope drives that slipping may occur. In order to Avoid Slipping , steel chains are used. The chains are made up of rigid links which are hinged together in order to provide the necessary flexibility for warping around the driving and driven wheels .

Fibre Ropes :

Chain and Sprocket

Fibre Ropes :

Advantages of Chain Drive 1. As no slip takes place during chain drive, hence perfect velocity ratio is obtained. 2. Since the chains are made of metal, therefore they occupy less space in width than a belt or rope drive. 3. The chain drives may be used when the distance between the shafts is less. 4. The chain drive gives a high transmission efficiency (upto 98 per cent). 5. The chain drive gives less load on the shafts. 6. The chain drive has the ability of transmitting motion to several shafts by one chain only.

Advantages of Fibre Rope Drives:

Disadvantages 1. The production cost of chains is relatively high. 2. The chain drive needs accurate mounting and careful maintenance. 3. The chain drive has velocity fluctuations especially when unduly stretched.

Sheave for Fibre Ropes :

Terms Used in Chain Drive Pitch of the chain : It is the distance between the hinge centre of a link and the corresponding hinge centre of the adjacent link as shown in Fig. It is usually denoted by p .

Wire Ropes :

Pitch circle diameter of the chain sprocket It is the diameter of the circle on which the hinge centers of the chain lie, when the chain is wrapped round a sprocket as shown in Fig. The points A, B, C, and D are the hinge centers of the chain and the circle drawn through these centers is called pitch circle and its diameter (d) is known as pitch circle diameter .

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Classification of Chains The chains, on the basis of their use, are classified into the following three groups : Hoisting and hauling (or crane) chains, Conveyor (or tractive ) chains, and Power transmitting (or driving) chains

Chain Drives :

Hoisting and Hauling Chains These chains are used for hoisting and hauling purposes. The hoisting and hauling chains are of the following two types : 1. Chain with oval links. The links of this type of chain are of oval shape , as shown in Fig. 11.29 (a). The joint of each link is welded . The sprockets which are used for this type of chain have receptacles to receive the links . Such type of chains are used only at low speeds such as in chain hoists and in anchors for marine works.

Chain and Sprocket:

Chain with square links. The links of this type of chain are of square shape , as shown in Fig. 11.29 (b). Such type of chains are used in hoists, cranes, dredges . The manufacturing cost of this type of chain is less than that of chain with oval links, but in these chains, the kinking occurs easily on overloading.

Advantages of Chain Drive :

Conveyor Chains These chains are used for elevating and conveying the materials continuously. The conveyor chains are of the following two types Detachable or hook joint type chain, as shown in Fig. 11.30 (a), and Closed joint type chain, as shown in Fig The conveyor chains are usually made of malleable cast iron. These chains do not have smooth running qualities. The conveyor chains run at slow speeds of about 3 to 12 km.p.h.

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Block chain A block chain, as shown in Fig. 11.31, is also known as bush chain . This type of chain was used in the early stages of development in the power transmission. Because of rubbing between the teeth and the links, It produces noise. Such type of chains are used to some extent as conveyor chain at small speed.

Terms Used in Chain Drive :

Bush roller chain A bush roller chain, as shown in Fig. consists of outer plates or pin link plates, inner plates or roller link plates, pins, bushes and rollers. A pin passes through the bush which is secured in the holes of the roller between the two sides of the chain Strong and simple in construction. It gives good service Little noise. Little lubrication.

Pitch circle diameter of the chain sprocket :

Roller Chain Construction

Classification of Chains :

Couplings Couplings are used to connect two different shafts. Couplings are used to connect prime mover to the actuator and other intermediate transmission systems. A coupling should transmit power without loss. They should be easily assembled and disassembled. 63

Hoisting and Hauling Chains :

Types of couplings Couplings are of two types Rigid coupling Connect shafts which is perfectly aligned to their axis. Higher Transmission efficiency. Flange coupling Split muff coupling Sleeve coupling Flexible coupling Connect such misaligned shafts Universal joint coupling Oldham coupling 64

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Flange Coupling 65 To Connect motors with the end equipment. Shaft end is attached with Locking key and Flange. Locking key is used to connect shafts to transmit power without slipping. Suitable Provisions

Conveyor Chains :

Sleeve coupling A Sleeve coupling is a basic type of coupling . This consists of a pipe whose bore is finished to the required tolerance based on the shaft size. Based on the usage of the coupling a keyway is made in the bore in order to transmit the torque by means of the key. Two threaded holes are provided in order to lock the coupling in position

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Jaw Coupling Curved jaw couplings are well suited to servo applications where accuracy of torque transmission is required. The three-piece design, consisting of curved jaw hubs which are press fit into an elastomeric spider, allows the coupling to operate with zero-backlash

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Universal Joint Coupling 68 Transmit power at certain angle between the shafts.

Roller Chain Construction:

Universal Joint Coupling 69 This tolerates the variations of inclination angle while running. C shaped forks of two shafts are displaced 90° Mechanically – to yield in any direction.

Couplings:

Universal Joint Coupling Animation 70

Types of couplings:

Universal Joint Coupling Animation 71

Flange Coupling:

Oldham Coupling 72 Flanges with grooves on them attached to the shaft ends. Floating plate projections fit into the Flange grooves. Projections are 90° apart, it provides self locking.

Sleeve coupling:

Oldham Coupling 73

Jaw Coupling:

Oldham Coupling Animation 74

Universal Joint Coupling:

Cam and Follower Mechanism Cam is a specialized structure that has a profile corresponding to the path that the follower should take. When the cam is moved the follower moves according to the cam profile. Types of Cam and follower mechanism Radial cam Linear cam Closed track radial cam Closed track linear cam Drum type cam 75

Universal Joint Coupling:

TYPES OF FOLLOWER

Universal Joint Coupling:

Cam and Follower Mechanisms 77

Universal Joint Coupling:

Cam and Follower Mechanisms Animation 78 Cam – Rotary motion; Follower – Linear Motion, Perpendicular to Cam axis. Follower moves radially with respect to Cam axis. Radial Cam

Oldham Coupling:

Cam and Follower Mechanisms Animation 79 Cam – Linear Motion; Follower – Linear Motion But direction is perpendicular to each other.

Oldham Coupling:

Cam and Follower Mechanisms Animation 80 This is for Specific Applications Cam – Rotary motion; Follower – Linear Motion, radically to the Cam axis.

Oldham Coupling:

Cam and Follower Mechanisms Animation 81

Cam and Follower Mechanism:

Cam and Follower Mechanisms Animation 82 Cam and follower moves in a linear fashion, but perpendicular to each other.

TYPES OF FOLLOWER:

Cam and Follower Mechanisms Animation 83 Cam is a Cylindrical Structure on which grooves cut with the required profile. Follower moves along the groove It moves linearly in a direction parallel to the drum axis when the drum rotated.

Cam and Follower Mechanisms:

Thank you Prepared by M.Ganesh Murugan.

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