Belt Length Changes

By Norm Christopherson  

      In the process of making a motor pulley size change the old belt may no longer be the correct length.  A new belt of the correct length will need to be obtained.  But, how can the new belt length be determined without resorting to the trial and error method?

      The belt length formula solves the problem.  First of all, some basic measurements of the pulleys and shaft center-to-center distances must be made.  See figure 13-3 for an illustration of the pulley sizes for this example.   

 

Figure 13-3

       The original 8" pulley, which was used with the old 3000 cfm, has been replaced with the new 10.7" motor pulley to give the corrected 4000 cfm, matching the 10-ton system.  Now the belt length is different and needs to be calculated.  The dimensions in the diagram above were taken and will be used to make the calculation. 

    

       When solving math problems, which contain brackets and parentheses, remember to work from the inside of the problem to the outside.  That is, solve the parentheses first, then the brackets next and finally the remainder of the math as this example problem does. The capital letter D stands for the larger diameter pulley and the small letter d stands for the smaller pulley.  The two shafts are measured center to center and are 20 inches apart in this example.  The letter C stands for this center-to-center measurement.  Prior to taking the center-to-center measurements, the motor hold down bolts should be loosened and the motor moved to asit in the center of the bolt adjustment slots.  This ensures that after the new belt is installed, there is some adjustment available in both directions for belt tension. 

 

Purchasing an 82-inch belt will fit the requirements for the new pulley and there will be sufficient space on the motor base to adjust the belt tension. Norm is a technical writer, seminar speaker and test proctor for EPA, 410A and ESCO & NATE certifications.