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 sit 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.
He can be contacted at nchristo@juno.com