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PART- II: Advanced Agricultural Machinery Design -Belt Drives.ppt

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PART- II: Advanced Agricultural Machinery Design -Belt Drives.ppt

The main sources of power for agricultural machines, i.e., the diesel engine for self-propelled machines and the electrical motor for many stationary machines used on the farmstead.
Pull-type machines must receive propulsion and rotary power from the tractor.
Power is transmitted from the tractor to the machine by means of traction, power-take-off drives (PTO), and/or by fluid power.
Rotary power is also transmitted by means of belts and chains.
Topics related to rotary power transmission are presented here.

The main sources of power for agricultural machines, i.e., the diesel engine for self-propelled machines and the electrical motor for many stationary machines used on the farmstead.
Pull-type machines must receive propulsion and rotary power from the tractor.
Power is transmitted from the tractor to the machine by means of traction, power-take-off drives (PTO), and/or by fluid power.
Rotary power is also transmitted by means of belts and chains.
Topics related to rotary power transmission are presented here.

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PART- II: Advanced Agricultural Machinery Design -Belt Drives.ppt

  1. 1. Adama, Ethiopia Biniam Zewdie G/Kidan * •Haramaya Institute of University P.O.Box:138; Dire Dawa, Ethiopia •Mobile: +251910408218/+25191582832 •E-mail: nzg2001nzg@gmail.com/zewdienico@gmail.com
  2. 2. The main sources of power for agricultural machines, i.e., the diesel engine for self-propelled machines and the electrical motor for many stationary machines used on the farmstead.  Pull-type machines must receive propulsion and rotary power from the tractor.  Power is transmitted from the tractor to the machine by means of traction, power-take-off drives (PTO), and/or by fluid power.  Rotary power is also transmitted by means of belts and chains.  Topics related to rotary power transmission are presented here.
  3. 3. Figure: Drive system for an industrial application employing a belt drive, a gear
  4. 4. • Belts, ropes, chains, and other similar elastic or flexible machine elements are used in conveying systems and in the transmission of power over comparatively long distances. • It often happens that these elements can be used as a replacement for gears, shafts, bearings, and other relatively rigid power- transmission devices. • In many cases their use simplifies the design of
  5. 5. • In addition, since these elements are elastic and usually quite long, they play an important part in absorbing shock loads and in damping out and isolating the effects of vibration. • This is an important advantage as far as machine life is concerned.
  6. 6. • Crowned pulleys are used for flat belts, and grooved pulleys, or sheaves, for round and V belts. • Timing belts require toothed wheels, or sprockets. The timing belt does not stretch or slip and consequently transmits power at a constant angular-velocity ratio. • In all cases, the pulley axes must be separated by a certain minimum distance, depending upon the belt type and size, to
  7. 7. Other characteristics of belts are: They may be used for long center distances. • Except for timing belts, there is some slip and creep, and so the angular velocity ratio between the driving and driven shafts is neither constant nor exactly equal to the ratio of the pulley diameters.
  8. 8. Types of Belt: -Light, medium and Heavy drives: • Light drives transmit small powers at belt speeds up to 10 m/s like in agricultural machines and small machine tools. • Medium drives are used to transmit medium power at belt speeds over 10m/s and up to 22 m/s in machine tools. • Heavy drives are used to transmit power above 22m/s like in compressors and generators
  9. 9. CHARACTERISTICS OF FLAT BELT MATERIALS
  10. 10. The flat belts connecting pulleys can be arranged in a number of ways depending upon the purpose of the drive. The arrangements are described below : • Open Belt Drive An endless belt goes around two pulleys one of which is driving and another driven. Tight side of belt, as a rule, is kept lower so that the catenary action of the top portion is utilised in increasing angle of lap of the pulleys. In this arrangement, as shown in Figures (a) and (b), the two pulleys rotate in the same direction. • The centre line of the pulleys could be horizontal, vertical and inclined to horizontal.
  11. 11. Compound belt drive is used when power is transmitted from one shaft to another through number of pulleys
  12. 12. Cross or Twist Belt Drive • Belt running on two parallel shafts but crossing itself, as shown in Figure (c) would cause the pulleys to rotate in the direction opposite to each other. • In such an arrangement excessive belt wear would, occur because the belt will rub on itself. • This wear is minimised if centre distance rip larger than twenty times the width of beIt. • The speed of this type of drive must not exceed 15 m/s and for belts wider than 200 mm the idler pulley must be used to reduce rubbing. • Capacity of such belt drive is only 75% of an equivalent open belt drive if the ratio of pulley diameters is less than 3:1. If, however, this ratio is higher the capacity of drive is
  13. 13. Similarities between open belt drive and closed belt drive • In both the cases, driving and driven shafts must be parallel. • Both can transmit power and motion for substantially large distances, as compared to gear or chain drive. • Slip can occur in both the cases. So they are non-positive drive.
  14. 14. Open Belt Drive Cross Belt Drive belt proceeds from top of one pulley to the top of other pulley without crossing. belt proceeds from top of one pulley to the bottom of other pulley and thus crosses itself. Driving shaft and driven shaft rotate in same direction. Driving shaft and driven shaft rotate in opposite direction. Contact angle (or wrap angle) between the belt and pulley is comparatively small (always below 180º). Contact angle between the belt and pulley is comparatively large (always above 180º). Length of the open belt is smaller as compared to cross belt. For the same pulley diameter and same centre distance between driver and driven shaft, longer belt is required in cross belt drive. Here belt remains in same plane in every rotation during its operation. Here belt bends in two different planes in every rotation during its operation. Here belt does not rub with itself. So belt life increases. Here belt rubs with itself and thus life of the belt reduces. Open belt drive is suitable when driving and driven shafts are in horizontal or little bit inclined. Cross belt drive can be advantageously applied for horizontal, inclined and vertical positions of driving and driven
  15. 15. Quarter-Twist Drive • In this arrangements two shafts are at right angle to each other. The chance of slipping can be avoided by providing larger face width of pulley. • For this arrangement, illustrated in Figures (d) and (e), pulley face width exceeds the belt width by at least 40% Belt Drive with an Idler Pulley • When centre distance between the shafts is short, the angle of lap on smaller pulley becomes small. 'This smaller angle of lap does not permit high belt tension. • If it is not possible to increase belt tension by any other means to the desired value, then an idler pulley, as shown in Figure (f) is used to increase the belt tension. • This drive has a disadvantages in the sense that belt
  16. 16. Belt Drives with many Pulleys • Figures (g) and (h) show an arrangement in which a number of pulleys are driven by a single belt. • In such an arrangement either all the pulleys could rotate in the same direction [its in Figure (g)] or some may rotate in the direction opposite to that of others [as in Figure (h)].
  17. 17. The advantages of V-belt drives may be summed up below : a) Compact layout. The minimum centre distance is 1/2(D, + D2) + 3h, where h is the height of belt section. A V-belt drive is accommodated in a machine. b) High velocity ratios up to 10 are permissible. c) Noiseless operation. d) The drive can take up shocks, especially at starting. e) The reversal of direction whereby the tight side can be on top does not change power. f) The angle of inclination of centre line does not affect power. g) The V-belt is not affected by heat, moisture and dust. h) Though power capacity of one belt is not very high, several of them run on the same set of pulleys and increase power of drive. The drives can be made to have variable speed drives.
  18. 18. The V-belt drive has a few distinct drawbacks as compared to flat belt drive. These are stated as under : a) V-belts have lower life than flat belts mainly because of high bending stress that is caused due to higher ratio of belt depth to diameter of pulley than that in case of flat belt drive and also because of high frequency of stress variation. b) The design of pulleys for V-belt drives is more complicated than for flat belt drives. c) V-belt drives cannot be used for larger power. d) The centre distance in case of V-belts may not be large. The V-belt drives are used quite widely and within their power range (i.e. up to 1100 kW), employment of V-belt drives is second
  19. 19. THE DESIGNER MUST DO THE FOLLOWING:  Choose the type of power transmission elements to be used: gears, belt drives, chain drives, or other types. in fact, some power transmission systems use two or more types in series to optimize the performance of each.  Specify how the rotating elements are arranged and how the power transmission elements are mounted on shafts.  Design the shafts to be safe under the expected torques and bending loads and properly locate the power transmission elements and the bearings. it is likely that the shafts will have several diameters and special features to accommodate keys, couplings, retaining rings, and other details. the dimensions of all features must be specified, along with the tolerances on the dimensions and surface finishes. DESIGN CONSIDERATION
  20. 20.  Specify suitable bearings to support the shafts and determine how they will be mounted on the shafts and how they will be held in housing.  Specify keys to connect the shaft to the power transmission elements; couplings to connect the shaft from the driver to the input shaft of the transmission or to connect the output shaft to the driven machine; seals to effectively exclude contaminants from entering the transmission; and other accessories.  Place all of the elements in a suitable housing that provides for the mounting of all elements and for their protection from the environment and their DESIGN CONSIDERATION
  21. 21. Design Consideration of Belt • Speed of the driving and driven shaft • Speed reduction ratio • Power to be transmitted • Positive drive requirement • Space available • Service condition • Shaft layout • Center distance between shaft
  22. 22. Design Consideration of Belt Terminology • Belt Forces: Main forces acting in the belt drive are given by α = Arc of belt contact, rad θ = angle of v-belt
  23. 23. Drawbacks of belt drives are: 1. Endless belts usually cannot be repaired when they break. They must be replaced. 2. Slippage can occur, particularly if belt tension is not properly set and checked frequently. Also, wear of belts, sheaves, and bearings can reduce tension, which makes re-tensioning necessary. 3. Adverse service environments (extreme temperature ranges, high moisture, oily or chemically filled atmospheres, etc.) can damage belts or cause severe slipping. Advantages of belt drives include: 1.No lubrication is required, or desired. 2.Maintenance is minimal and infrequent. 3.Belts dampen sudden shocks or changes in loading. 4.Quiet, smooth operation. 5.Sheaves (pulleys) are usually less expensive than chain drive sprockets and exhibit little wear over long
  24. 24. THE END THANKS

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