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Note for Basics of Mechanical Engineering - BME By Suman Kumar Acharya

  • Basics of Mechanical Engineering - BME
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BELT DRIVE 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. It may be noted that (a) The shafts should be properly in line to insure uniform tension across the belt section. (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. (c) The pulleys should not be so far apart as to cause the belt to weigh heavily on the shafts, thus increasing the friction load on the bearings. (d) A long belt tends to swing from side to side, causing the belt to run out of the pulleys, which in turn develops crooked spots in the belt. (e) The tight side of the belt should be at the bottom, so that whatever sag is present on the loose side will increase the arc of contact at the pulleys. ( f ) In order to obtain good results with flat belts, the maximum distance between the shafts should not exceed 10 metres and the minimum should not be less than 3.5 times the diameter of the larger pulley. 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 8. Service conditions.

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Types of Belt Drives The belt drives are usually classified into the following three groups : 1. Light drives. These are used to transmit small powers at belt speeds upto about 10 m/s, as in agricultural machines and small machine tools. 2. 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. 3. Heavy drives. These are used to transmit large powers at belt speeds above 22 m/s, as in compressors and generators Types of Belts 1. Flat belt. The flat belt 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 metres apart. 2. V-belt. The V-belt 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. 3. Circular belt or rope. The circular belt or rope 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. Material used for Belts 1. 2. 3. 4. Leather belts Cotton or fabric belts Rubber belt Balata belts. Advantages of Flat Belt Drive 1. 2. 3. 4. Different velocity ratios can be obtained by using a stepped one pulley. A belt drive can be used as a clutch, by shifting the belt from fast pulley to loose pulley. Design of flat belt drive is simple. Flat belt drive is relatively cheap and easy to maintain.

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5. Flat belt drives are flexible, which gives protection 6. Flat belt drives can be used for long center distance.(Upto 15 m) Advantages of Flat Belt Drive 1. Since Velocity ration is not constant, flat belt drive is not a positive drive. 2. Flat belt drives have larger dimensions and occupy more space. 3. Flat belt drive is not suitable for smaller center Distance. (Less than 1 m) Types of Flat Belt Drives 1. 2. 3. 4. 5. 6. 7. Open belt drive Crossed or twist belt drive Quarter turn belt drive Belt drive with idler pulleys Compound belt drive Stepped or cone pulley drive Fast and loose pulley drive Open belt drive The open belt drive 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. The lower side belt (because of more tension) is known as tight side whereas the upper side belt (because of less tension) is known as slack side. Crossed or twist belt drive The crossed or twist belt drive is used with shafts arranged parallel and rotating in the opposite directions. In this case, the driver pulls the belt from one side (i.e. RQ) and delivers it to the other side (i.e. LM). Thus the tension in the belt RQ will be more than that in the belt LM.

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The belt RQ (because of more tension) is known as tight side, whereas the belt LM (because of less tension) is known as slack side. A little consideration will show that at a point where the belt crosses, it rubs against each other and there will be excessive wear and tear. In order to avoid this, the shafts should be placed at a maximum distance of 20 b, where b is the width of belt and the speed of the belt should be less than 15 m/s. Quarter turn belt drive. The quarter turn belt drive also known as right angle belt drive 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. In case the pulleys cannot be arranged, as shown in Fig.(a), or when the reversible motion is desired, then a quarter turn belt drive with guide pulley, as shown in Fig. (b), may be used. Belt drive with idler pulleys. A belt drive with an idler pulley, as shown in Fig. (a), 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. When it is desired to transmit motion from one shaft to several shafts, all arranged in parallel, a belt drive with many idler pulleys, as shown in Fig. (b), may be employed.

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