Presentation on theme: "Objectives of the chapter:"— Presentation transcript:
1 Belt drives and chain drives are the major types of flexible power transmission elements Objectives of the chapter:Describe the basic features of a belt drive systemDescribe of several types of belt drivesSpecify suitable types and sizes of belts and sheavesSpecify the primary installation variables for belt drivesDescribe the basic features of a chain drive systemDescribe several types of chain drives
2 Area application of the belt drives Belt drives are applied where the rotationalspeeds are relatively high .The linear speed of a belt is m/min, which results in relatively low tensile forces in the belt.The high speed of the electromotor makes belt drives somewhat ideal for that first stage of reduction.Belts operate on sheaves or pulleys, whereas chains operate on toothed wheels called sprockets.
3 Disadvantages of the belt drives At lower speeds, the tension in the belt becomes too largefor typical belt cross sections, and slipping up tp 3%mayoccur between the sides of the belt and the pulley thatcarries it.At higher speeds, dynamic effects such as centrifugalforces, belt whip, catch of air, and vibration reduce theeffectiveness of the drive and its life.Improvement of the belt drivesSome belt designs employ high-strength, reinforcingstrands and a cogged design that engages matchinggrooves in the pulleys to enhance their ability to transmitthe high forces at low speeds.Disadvantages of the belt drives
4 Basic belt drive geometry The belt is placing around the two sheaves while the centerdistance between them is reduced, then sheaves are moved apartFriction causes the belt to grip the driving sheave, increasing thetension in one side, called the "tight side," of the driveThe opposite side of the belt is still under tension (at a smaller value) that is called the "slack side."
5 TYPES OF BELT DRIVESTypes of belts: flat belts, grooved or cogged belts, standard V-belt, double-angle V-belts, and others.WrappedconstructionDie cut, cog typeSynchronous beltVee – bandDouble angle V-beltPoly-rib belt
6 The belt drives are classified on the basis of peripheral speed 1. Light drives: the transmission of small powers at beltspeeds up to 12m/s (agriculture machines, smallmachine tools, etc)2. Medium drives: for medium powers at 12 – 24 m/s(machine tools, cars, etc).3. Heavy drives: for large powers and speed > 24m/s(generators, compressors, main drives)1.
7 The flat belt is the simplest type, and made from leather, fabric or rubber-coated fabric.
8 Crossed belt ortwist belt driveBelt drive with idlepulley or Jockey pulleyThe coefficient of friction between the belt material andthe pulley surface: μ = 0.15 – 0.5Tension for the belt material f = 0.27 – 1.7 N/mm2
9 Stress in the belt b – width of a belt Tension ft = F/bt 1. Initial tension fi = f1 + f22. Stress due to bending of the belt over the pulleyfb = E(t/D)E – module of elasticity of the belt material
10 There are standard belt width (25 – 600)mm, and thickness (5 – 12)mm 3. Stress due to the effect of centrifugal forcefc = Fc/bt = ρV2,fc= 0, if V < 10 m/sρ – density of the belt material (1000 – 1400)kg/m3V = 15-20m/s – recommended4. The maximum stress fmax = fi + fb + fc5. Ratio of driving and driven forces F1/F2 = eμθThere are standard belt width (25 – 600)mm, andthickness (5 – 12)mm
11 Design of the belt drives The belt drive is designed for the power to be transmittedthat depends upon:difference in belt tension,coefficient of friction,area of contact andcenter distanceDesign power P = (F1 – F2)Vs = TtV (w)F1 – tension on the tight side (N)F2 – tension on the slack side (N)V – belt speed (m/s)Ft – net belt pull (N)s = (1.2 – 1.4) service factor (oily, jerky loads,shock, reversed load)
12 bt =F1/(ft – ρV2)C1 for V > 10 m/s Belt sectionbt = F1/ftC1 for V < 10 m/sbt =F1/(ft – ρV2)C1 for V > 10 m/sft – allowable stress in a belt (N/mm2)C1 = 0.6 – a corrector factor depending uponthe angle of center line of drive with horizontal,type of drivesGeneral belt equationF1 – F2 = bt(ft - ρV2)[(eμθ – 1)/ eμθ]
13 Pulley size D > 50t D = 1.2(P/nmax)1/3 B = 1.2b d = 0.005D + 3 Number of spokes n = 4 (D = ), n = 6 D > 450,H = 0.8do, h = H/2
14 V-belt drive is a widely used type of belt in industrial drives and vehicular application.The V-shape causes the belt to wedge tightlyinto the groove, increasing friction and allowinghigh torques to be transmittedThe belts have high-strength cords positionedat the pitch diameter of the belt cross sectionto increase the tensile strength of the belt.The cords, made from natural fibers, syntheticstrands, or steel. are embedded in a firm rubbercompound
15 Advantages of the V – belts drive Higher velocity ratio up to 7 – 10Provide long life, 3 –5 yearsPossibility of using small center distanceTransmit higher torsional moment at less width and tensionsDisadvantagesCannot be used with large center distancesSubjected to a certain amount of creepMore complex designBelt life above 82oC and below – 15o is shortenedCentrifugal force prevents the use at speed above 50m/s and at speed below 5m/s are not economical
16 Typical V-belt section and groove geometry The pulley has a circumferential groove 2. The size of a sheave is indicated by its pitch diameter, slightly smaller than the outside diameter of the sheave.
17 2. The linear speed of the pitch line of both sheaves is The speed ratio between the driving and the driven sheaves is inversely proportional to the ratio of the sheave pitch diameters (if no slipping).V-BELT DRIVES2. The linear speed of the pitch line of both sheaves isthe same as and equal to the belt speed, vb. Thenvb = R1w1 = R2w2 = DIw1/2 = D2w2/2The angular velocity ratio is3. The maximum total stress occurs where the belt enters thesmaller sheave. The design value of the ratio of the tight side tension to the slack side tension is 5.0 for V-belt drives.
18 The angles of contact of the belt on each sheaves is The relationships between pitch length, L, center distance, C, and the sheave diametersL = 2C (D2 + D1) + (D2 - D1)2/4CB = 4L – 6.28(D2 + D1)The angles of contact of the belt on each sheaves isThe length of the span between the two sheaves, over whichthe belt is unsupported, is
19 Cog belts are applied to standard sheaves. The cogs give the belt Synchronous beltsSynchronous belts, (timing belts) ride on sprockets having mating grooves into which the teeth on the belt seat.Cog belts are applied to standardsheaves. The cogs give the beltgreater flexibility and higherefficiency compared with standardbelts.Synchronous belts are constructed with ribs or teeth across the underside of the belt. The teeth providing a positive drive without slippage.
20 CHAINSChain drives are used to transmit rotational motion and torque from one shaft to another, smoothly, quietly, and inexpensively. Chain drives provide the flexibility of a belt drive with the positive engagement feature of a gear drive. Chain drives are suited for applications with large distances (8m) between the respective shafts, slow speed, and high torque. More complex design than a belt drive
21 Chains are made from a series of interconnected links. Types of ChainsChains are made from a series of interconnected links.Roller chain. A roller chain is the most common type of chain used for power transmission.Large roller chains are rated to 450 kW. The roller chain design provides quiet and efficient operation but must be lubricated.
22 Multiple-strand roller chain Multiple-strand chains used to increase the amount ofpower transmitted by the chain drive.Equation is used to calculate the power transmitted througheach chain.A multi-strand factor has been experimentally determined.Power per chain =total power transmitted/multi-strand factor
23 Construction of bush-roller chain Standard sizes: p = (6.35 – 76.0) mm,d = (7.772 – )mm, l = (6.35 – )mm,t = – 9.525)mm,Tensile strength F = – kN
24 Offset sidebar roller chain An offset sidebar roller chain is less expensive than a roller chain but has slightly less power capability.It has an open construction that allows it to withstand dirt and contaminants, which can wear out other chains.Silent chainAn inverted tooth, silent chain is the expensive chain to manufacture. It efficiently used in applications that require high-speed, smooth, and quiet power transmission (machine tools). Lubrication is required to keep these in reliable operation. .
27 Dimensions of the various parts of the chain Roller diameter d = (5/8)*pitchPin diameter dp = 5/16)*pitchChain width bi = (5/8)*pitchThickness of link plates t = (1/8)*pitchWidth between outer plates b0 = bi + 2tMaximal height of roller link h = 0.82*pitchLength of roller l = 0.9bi – 0.15
28 Chains are classified by a pitch, p, Chain PitchChains are classified by a pitch, p,which is the distance between the pinsthat connect the adjacent links.Roller chains have a size designationaccording to the power transmissionrequirements.Sprocketsteeth.Sprockets are the toothed wheels that connect to the shaft and mate with the chain. The teeth on the sprocket are designed with geometry to conform to the chain pin and link.
30 The number of teeth, N, in the sprocket is a commonly referenced property. Sprockets should have at least 17 teeth, unless they operate at very low speeds, under 100 rpm.The larger sprocket should normally have no more than 120 teeth.It is preferable to have an odd number of teeth on the driving sprocket (17, ) and an even number of pitches (links) in the chain to avoid a special link.
31 The pitch diameter, D, of the sprocket is measured to the point on the teeth where the center of the chain rides.The pitch diameter of a sprocket with N teeth for a chain with a pitch of p is determined byThe chain length, L, is the total length of the chain expressed in number of links, or pitches, computed as
32 The center distance for a given chain length computed as The angle of contact, θ, is a measure of the angularengagement of the chain on each sprocket.Θ ≥ 1200 – recommendation of manufacturers.In operation, chain drives should be designed so thatthe slack side is on the bottom or lower side.
33 CHAIN DRIVE KINEMATICS The velocity ratio VR is defined as the angular speed of the driver sprocket divided by the angular speed of the driven sprocket.VR = ωdr/ ωdn = ω1/ ω2 = D2/D1 = N2/N1.VR > 1 is typically ratioChains speed computed by Vc = D1/2ω1 = D2/2ω2Lubrication for the chain is important for the drive and there are recommendations:1. Low speed Vc < 100 m/min – manual lubrication;2. Moderate speed Vc < 500 m/min – bath lubrication;3. High speed Vc > 500 m/min – oil stream lubrication.
34 Selection of a chain For a given application: life expectance, space, speed, and costChoice of a drive will depend upon:pitch, numberof chains, and sprocket sizeThe following factors should be analyzed:Type of chain (speed recommendations)Number of teeth in the wheel (min. and max.)Chordal action Vmin=2πnRcos(π/z), Vmax=2πnRn – angular velocity,z – number of teeth,R – pitch radius of thesprocket
35 4. Chain velocity and drive ratio 6:1 and no more then <9:1 (pitch, number of teeth in pinion) 5. Wheel centers and length of chain6. Chain pitch (6.25; 8; 9.525; 12.7; …76.2)For bushed and roller chain, p = n(P*k/zωpbx)1/3,coefficient n = 28, pb = 35N/mm2 at n < 50 rev/minpb = 13.7 N/mm2 at n = 2800 rev/minFor silent chain, p = n(P*k/zωpbb)1/3, n = 60,pb = 20 N/mm2 at n < 50 rev/minpb = 7.8 N/mm2 at n = 2800 rev/minP - power(kW), k = – load factor, z – number of teeth, ω – an angular velocity of the driving sprocket, x – number of chain strands, b = (1.5-8)– the width of the chain, pb – the allowable bearing pressure
36 8. Chain designation and specification 7. Sprockets8. Chain designation and specificationSprocketDp = p/sin(π/z) – pitch diameter,α = [1400-(900/z)] to [1200-(900/z)] – roller setting angleDo = Dp + d – outer diameter
37 Dr = Dp – d – root diameter R = p – tooth flank radiust = 0.93b for p < 12.7 simple chain wheelst = 0.91b for duplex and triplex chain wheelst = 0.88b for quardplex chain wheels and abovet1 = (number of chain strands –1)*pt + tpt – transverse pitch of strands (strands spacing) – standard – 5.64 – 91.27
38 Design of the chain drive Design calculation are carried out to find the best proportions of chain and sprockets in the next steps:Select the type of chain (given speed)Find the number of teeth of smaller sprocket (table)Select the chain pitch (formula or table)Calculate the total load Fo= F + Fc + Ff,F(N) = P(kW)/V(m/s)– driving force on tight side. (force on slack side is considered 0). Fc = wV2/g (w – weight) – centrifugal force of inertia. Ff = kfwC – sagging force, coefficient of arrangement of chain kf = 1 for vertical position; kf = 2 centerline inclined at angle up to 45o; kf = 4 – horizontal drives
39 . Safety factor Fs = Fu/Fo (compare with table) Fu = 106p2(N) breaking strength of roller chain ,p – pitchFu = 106p(N) for silent chain5. The chain is checked for wear by unit pressure in joints pb = Fk/A (N/mm2),k – load factor,A – projection of the joint pivot,A = (dp +lb)*z – for roller chain, dp – diameter of joint pivot, lb =(bi+2tp) - bush length, z – number of chain strandsA = 0.76dpb for silent chain, b – chain width,