EXPERIMENT NO.6


                                       
                                   EXPERIMENT NO.6
11.0    Questions for confirmation of learning:
1.      Belts are used to transmit power from one shaft to another by means of pulleys.
2.      Flat belts and V belts
3.      Open belt drive and Cross belt drive
4.             π (r1 +r2 ) + 2x + (r1 -  r2)2
                                                    x 
      
   5           P = ( T1 – T2) v


     6.          N2 /N1         =             d1 /d2

            

12.0    . Conclusion:      1. should not               2.decrease                  3. is not
             a)  No joints, made endless, smooth drive, noiseless transmission of power.
             b)  Positive drive as slip is negligible.
                       When the belt is wound around the two pulleys ( driver and driven ) it’s two ends are joined together  so that the belt may continuously move over the pulleys. The motion of the belt from the driver & the follower is governed by a firm grip, due to friction between belt and pulleys. In order to increase this grip the belt is tightened up. At this stage even when the pulleys are stationary the belt is subjected to the tension that is initial tension.
13.0     Questions:
1. Whenever the power is to be transmitted between  two centers within the range of 8 to 10 meters belt drives are used. Gear drives are used when distance between driver & driven is short.
2. Slip : Sometimes the frictional grip becomes insufficient. This may cause some forward        motion of the driver without carrying belt with it. This may also cause some forward motion  of  belt without carrying the driven pulley with it. This is called slip.
3.      Idler pulley : A belt with an idler pulley 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 Parallel, a belt drive with many idler pulleys may be employed.
4.      Materials for belt: 1.Leather           2. cotton or fabric   3. Rubber        4. Balata
            5.      Flat belts are recommended when the distance between the shafts is max. 8 m. and min.                      should not be less than 3.5 times the diameter of larger pulley. And speed up to 22 m /s.
            6.      V belts  are used when two pulleys are very near to each other. Used where need of  
                   increased  frictional grip is necessary, which reduces tendency of slipping. Operation of belt  
                    and pulley is required to be quiet , where high velocity ratio is required.        
            7.      The open belt drive is used with shafts arranged parallel and rotating in same direction  
            8.      The crossed belt drive is used with shafts arranged parallel and rotating in opposite direction.
            9.       Flat belt drives running over cylindrical pulleys quickly wander off the pulleys. To prevent                this the pulley is crowned, machined to slightly spindle shaped profile. When flat belt on                     such      a pulley is off center and the pulley rotating, the belt quickly moves up to the largest               radius and stays there. The crown is important to keep the belt tracking stable, preventing the               belt from walking off the edge of the pulley.
          10.     Fast and loose pulley: Fast and loose pulley drive is used when the driven or machine shaft              is to be started or stopped whenever desired without interfering with the driving shaft. A                     pulley which is keyed to the machine shaft is called as fast pulley. A loose pulley runs freely              over the machine shaft and incapable of transmitting any power. When the driven shaft is                    required to be stopped, the belt is pushed on to the loose pulley by means of sliding bar having            belt forks.
          11.  A stepped or cone pulley 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.
     12.     a) Compactness due to small distance between the centers. b) Positive drive as slip is  
     negligible. c) Operation of belt and pulley is quiet. d) V belts may be used in either    
    directions.  e) High velocity ratio may be obtained. f) Frictional grip is more. g) Walking off  
    of the belt is avoided.
          13.  Velocity ratio is reduced due to slip of the belt and power transmitted is less.
          14.  Flour mills factories and workshops are the places where flat belt  are used.
          15.  Power transmission in motor cars and in lorries.
          16.  The motion of the belt from the driver and follower is governed by a firm grip, due to friction              between belt and pulley. In order to increase this grip, the belt is tightened up. At this stage,               even when the pulleys are stationary the belt is subjected to some tension, called initial                        tension.  

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