Belt Sizer Instructions

A. To simulate the force needed to rotate the driven pulley, disconnect the belt from the drive pulley. Use a fish weighing scale or load cell and tie a string to its hook. Tape the other end of the string to the driven pulley (or roller) and wrap it around several times. Then pull on the scale as hard and fast as the motor would ever pull. Read the Minimum Tangential Force Needed off the fish scale or load cell and jump to C below.

B. If you are moving boxes on a conveyor, select the Cross Section Calculator. and use the Force needed to Rotate Conveyor Roller calculator to determine the belt's Minimum Tangential Force Needed. Beware that this calculation is for an ideal environment where a conveyor is moving hard bottomed boxes. If your conveyor is old, in a dirty environment, poorly maintained, has heavy or long rollers, and/or moves boxes that have soft bottoms or that rub against the frame, etc., then set the coefficient of friction to 0.1 or larger as you may need a thicker belt. When in doubt, use Method A above with the maximum weight box on the rollers.

C. Select the Cross Section Calculator and scroll down the table below the Force Needed to Rotate calculator until you find a row with a number in the Belt Loading Tension column that is greater than your belt's Minimum Tangential Force Needed. Click on that row and read the belt cross section profile that should do the job.

BEWARE that extruded urethane belts are not precision belts because they have fairly wide tolerance ranges on several different variables, including dimensions, durometer, melt index, and modulus of elasticity. Therefore, we recommend overkill by a factor of 3, if possible, but at least 1.5. For example, if your minimum tangential force is 1 pound, then we recommend a belt loading tension of 3 pounds (1.5 lbs minimum). (The lifespan of a belt with a 1.5 factor probably will be shorter than a belt with a 3 factor).

Note: Belts with 83A, 85A or 88A durometer (hardness) are preferred because they have the longest flex life. 95% of the belts we sell are those durometers. Only use 92A durometer when there is no other option.

2. Determine the Minimum Pulley Diameter (MPD) for your selected belt cross section profile by using the Minimum Pulley Diameter Calculator found on the right side of the Cross Section Calculator. (Footnotes at the bottom of the page show that for durometers other than 83A and 85A, you need to multiply the MPD by various factors. (For example, for 92A belts multiply the MPD by 1.6; for 70A durometer belts multiply the MPD by 0.7). If the diameter of your smallest pulley is substantially smaller then the calculated MPD, then you need a larger pulley.

However, our MPD numbers are conservative so you can violate them by 10%, but certainly not more than 20%. Still, the more you violate MPD, the more the belt lifespan will be reduced.

Alternately, you can choose a smaller cross section profile as long as you use more than one belt running in parallel, but realize that the sum of all the belts must add up to the minimum tangential force needed.

3. Select our Length Calculator and scroll down to Method 2 or Method 3 to determine the belt's CUT LENGTH, which is the Industry Standard for designating urethane belt length. Note that a table of recommended belt stretch factors is found in Method Two. The final belt size should be described like this*:

3/16" x 13.50" 83A

belt cross section x cut length  durometer

(Flat belts use:  thickness x width x cut length  durometer)

4. Select our Tension Calculator and determine the maximum tension that is needed to install the belt. If this tension is so large that it will bend a shaft, then you need to change the percent stretch in 3 above and re-do 4. Alternately, you can use larger shafts on your pulleys.

If there is any doubt about your belt size, please ask DuraBelt to check your calculations and send you a free sample. (Exception: There may be a charge for expensive belts like cleated belts or large flat belts). Make sure you install it to triple verify that the size is correct.

BEWARE: Your motto should be

"Measure twice, cut once"

because if you order an incorrect size, it may not be returnable, or there may be a hefty 50% restocking charge. We do not normally charge for returns if the belt is new, unused and is a common size, but if it is old, used or uncommon, then we cannot resell it, nor can we recycle it because we do not use regrind; so you may end up owning an expensive paper weight.

*In order to prevent confusion and eliminate errors, we require that purchase orders either describe the belt (as shown above) or use our part number.  POs that specify only a customer’s part numbers and/or drawings do so at their own risk, and we shall not be responsible for our misinterpretation or errors resulting therefrom.  In the event that a part number and/or drawing conflict with the description, the description shall prevail.  POs that specify a length by anything other than cut length (also called pitch circumference or net endless length) and do not indicate that it is a diameter or circumference, ID, OD, IC,or OC, shall be deemed to have specified the cut length.