Measuring Triangular Belts Profiles

  • Real-life examples of measurements – all values below are measured with light touch of the caliper:

      height of v-belts

      nominal 10 mm; measured 9.3 – 9.7 mm (in convex curves, 9.8 mm)

      width of v-belts

      nominal 12.5 mm; measured 12.3 – 12.4 mm

      Measuring triangular belts  

    Measuring v-belt length Lw. It is usually proposed to measure a v-belt by cutting it and nailing it flat to a board [source]. However, this is not what you want if you need to determine the size of a new v-belt that has lost its labelling. So here is a different procedure that I developed and tested successfully:

      Mark a line on your flat, hard floor by taping 2 measuring sticks to it, and also tape an end stop marker like a flat wood piece to the start of the first stick.

      Add a small cable tie around the v-belt to be measured and use that for marking the start and aligning it to the start marker you taped to the ground.

      Roll your v-belt on the ground along the measuring sticks until you went one full circle.

      Take the measure and interpret it as effective length Lw of the v-belt (which it is, approximately).

      You could do two or three measurements and take the average, but this seems not necessary as this kind of measuring, properly executed, has repeatable results that are up to 1 mm exact. In contrast, measuring with a flexible measuring tape while holding the v-belt in your hands is not recommended, because the results are less exact (my experience: repeated measurement of a single belt resulted in 3505 mm Lw and 3520 mm Lw, while the result with the roll-on-floor technique was 3522 mm Lw). When measuring while holding the belt in your hand you have to take special care not to bend it during measuring, as bending will increase the measurement more towards La; for the above example, measuring with light bending increased the measurement to 3533 mm Lw while it should have been 3522 mm Lw).

      It is said that the measure taken by this procedure is the “median length” of the v-belt [source], though “median” here it is probably not meant in the mathematical sense of “length at a profile height where half of the profile area is above and half is below that height”. In practice, these measurements are said to be a good enough approximation of the effective length Lw. While it is a good enough approximate, my tests suggest that the length measured this way also depends on the properties of the belt build-up, so is not always the average, median or whatever length that could be generically specified. The measurement is for “something between inner and outer length”; exactly which mostly depends on how the belt is built, namely, how far to the outside the pull-resistant strings are located. Because, these seem to work like a hinge when bending or unbending the belt, affecting the depth of the area on each side that gets either compressed (so, shortened) or pulled on (so, lengthened). These fibres are normally located right below the upper edge of the belt, so the measure is normally more towards the outer length than the Lw measure is.

      Measuring v-belt length La. It is said that one can do that by placing the belt in circular shape on a flat surface and placing a flexible measuring tape around it [source]. However in practice, this is hard to do as you need some tension on the tape to not measure too much. It works well when the velt is still mounted, though.

      Determining Lw without a belt. In case you have a device needing a belt but don’t know which one, place a rope around the belt path that has approx. the thickness of the pulley profile grooves. Mark the length of rope you need, take it out again and measure that length when laying it straight on flat ground. This is a good approximate for the effective length Lw of a fitting belt. (As an alternative, there is a formula to calculate this from La or Li, and online calculators for that.)

      Various experiences with measuring v-belts

      At times, some v-belts seem to use the wrong signing schema. One belt had the classic profile, so the signing of “20 x 3550” was to be interpreted as Li = 3550 mm. However, all measurements turned out with Lw = 3522 mm, so an even larger measure of 3550 mm can only be La, not Li. The formula produces a result coherent with this: La = Lw + 31 mm = 3522 mm + 31 mm = 3553 mm. Another, quite old belt from Continental was SPA profile so should be labelled with Lw, yet the label said “12,5 x 1200 La“.

      Amount of difference between measured flat length and Lw. In one case, a classic 20 x 2000 mm Li belt was measured with 2060 mm flat length. Lw according to the formula is Lw = Li + 48 mm = 2048 mm Lw. The difference of 12 mm is quite small and probably comes from the fact that the pull-resistant fibres are not exactly located at the diameter corresponding to Lw; see above.

      Different formulas for converting to / from Lw. Interestingly, there seems to be at times a slight divergence of what formula is to be used for converting from effective length Lw to Li and La. For example, this v-belt conversion table and basically all other such conversion tables on the web state for a 17 mm wide classical v-belt: Lw = Li + 40. However, one 17 mm classical v-belt belt was found with an inscription saying “PETER-BTR 17 x 1320 Li / 1363 Lw”, corresponding to Lw = Li + 43.

      Storing v-belts correctly

      It is said that v-belts are normally built by manufacturers to reach a lab runtime of 25 000 hours [source]. If this is reached in practice also depends on proper storage conditions.

      If properly stored, hexagonal belts properties do not change for several years. However, most rubber-based products will deteriorate if improperly stored or handled (like being exposed to oxygen, ozone, extreme temperatures, light, humidity or various solvent agents).

      Therefore, the storage location should be dry and dust-free and must not contain chemicals, oils or solvents at the same time. V-belts should be stored without any force on them (pressure or pulling force) to avoid any permanent shape changes and other damages. This also means that they should only be stored in hanging condition if the hanger pin is at least ten times the height of the belt profile.