By Norm Christopherson

      On occasion new and correctly sized and adjusted belts on new pulleys slip no matter what the technician does to correct the problem.  The problem is sort of a mystery and often goes unsolved.  The problem brings with it belts that wear out quickly, pulleys that wear out quickly, reduced blower rpm and cfms, and increased noise.

       The mystery is easily solved with knowledge of arc of contact.  A belt correctly wrapped around a section of pulley is a matter of friction if it is not going to slip.  Consider some extreme examples of poor arc of contact.  The two pulleys in figure 13-4 are very different in size.

Figure 13-4

      There are two considerations here.  First, the size difference between the two pulleys is considerable.  This extreme difference means that the belt will go around a large portion of the circumference of the large pulley, and the belt will contact only a small portion of the smaller circumference on the small pulley.  The small pulley has so little pulley to belt contact that not enough friction is available to keep it from slipping.

       Secondly, the two pulleys are so close together that this contributes to the lack of contact at the smaller pulley.  If the pulleys were moved away from one another by sufficient distance, the belt would increase its contact around the smaller pulley.  Usually that distance increase is so large that it is not practical.  Another solution is to reduce the diameter of both pulleys by the same ratio and increase the distance to some extent as is practical.  Another solution is the one utilized on the drive system of clothes dryers.  On a clothes dryer the large drum that holds the clothes is the large pulley, with the long flat belt going around the outside of the drum.

  The motor and motor pulley are very close to the drum as space is not available to move them very far away from each other.  The belt will slip on the motor pulley unless something is done.  The solution is an idler pulley. The idler pulley rides on the backside of a belt and pulls the belt in towards the two pulleys forcing additional contact between the motor pulley and the belt. (See the diagram in figure 13-5)

Figure 13-5

 The major fault with using idler pulleys is that they wear out belts sooner.  Belts are not made to have a pulley on the backside.

       So, what is arc of contact?  There are 360 degrees in a circle and a pulley is a circle.  There are two pulleys involved and the belt wraps part way around each.  If the two pulleys were of the same size, the belt would ride 180 degrees on each to add up to 360 degrees.  Whatever number of degrees the belt rides on one pulley, the remainder to make up exactly 360 degrees is what the belt is riding on the other pulley.  In order to ensure that a pulley and belt will not slip the minimum number of degrees of arc of contact is 120 on the smaller pulley.  The formula to calculate the arc of contact for the smaller pulley is as follows:

Using the two pulley diameters of  8" on the smaller and  16" on the larger, (See figure 13-6) with a center-to-center distance between the shafts of  18",  the arc of contact is as follows:

Figure 13-6

      The arc of contact on the smaller pulley is 153.3 degrees. This is greater than the recommended minimum of 120 degrees to prevent belt slippage.  Notched v belts are another method of helping prevent slippage if the minimum arc of contact is not possible to achieve.

Norm is a technical writer, seminar speaker and test proctor for EPA, 410A and ESCO & NATE certifications.

He can be contacted at nchristo@juno.com

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