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Belt Tolerances

(Specification, Spec Sheet, Data Sheet)

Below are our standard belt tolerances. On special request, we can often do better on some of the dimensions, but please note that elastic urethane belts are not precision belts. The key tolerance is the Modulus of Elasticity for each resin lot, which can vary by +/-10%. In other words, the belt tension can vary from one order to another by up to +/-10%. Since modulus data fall on a bell shaped curve, the chances that resin lots will vary by this much is slim, but it is still possible. Therefore, you should design your application so it will work throughout that range.

This also means that being excessively tight on other dimensions may give you a false sense of precision. For example, a customer specified a 60" belt with a cut length tolerance of +/-1/16", which is a miniscule +/-0.1%. The modulus tolerance is 100 times greater. Anything gained by such a precise length would be totally invisible -- completely dwarfed by the modulus -- so why pay extra for precision hand cutting.

Dimension Name Standard Tolerances
Modulus of Elasticity +/-10%  Be sure to allow for this tension range in your application's design.
Cross-Section Diameter of round cord +/- 0.005" (+/-0.13mm) or +/-3%, whichever is greater.
Exception: 1/4" (6.3mm) is usually +/-0.005" (+/-.13mm)
Thickness of flat belts 0.030" to 0.039" (0.75mm to 1mm):    +/-0.003" (+/-0.08mm)
0.055" to 1/16" (1.4mm to 1.6mm):    +/-0.005" (+/-0.13mm)
3/32" (2.4mm) and over:                     +/-0.010" (+/- 0.25mm)
Width of flat belts +/-.015" (+/-0.3mm) or 1.5%, whichever is greater
V-Belt dimensions +/-3% on width, +/-3.5% on height, +/-3° on angle
Cut Length
(same as pitch circumference)
Standard cut length tolerance is +/-1/8" (+/-3.2mm) or +/-1%, whichever is greater, but on 83A durometer or harder belts we can usually do better on special request, i.e. +/- 3/32" (+/-2.4mm) on short belts 15" or less. On softer belts like 70A tolerance is +/- 5/32" (+/-4mm) or +/-1.5%, whichever is greater We also make belts for other companies whose tolerance range is -1/4"(-6.3mm) +zero, but other belt variables vary so much that there is no practical difference in performance.
Shore A and D Durometers Most are +/-3. Different resin manufactures have different tolerances, e.g. static dissipative cord is +/-5. Since durometer is a surface feature, it often does not correlate well with performance. (Note it is not possible to accurately measure round belt durometers for belts that have a cross-section diameter of less than 1/4". You would have to melt them down and make a slab, but that usually changes the durometer, giving a false reading. We have found that expensive O-ring fixtures are not accurate for belts that vary in thickness by +/-0.005" or more. Instead, you need to cut the belt so as to make a 1/4" thick cylinder, which should be placed flat side up, so the needle impinges on a flat surface. If you try to take a reading on the round surface of a cord, the durometer needle will deflect and give a false reading. We believe the best way to measure a belt without destroying it is by feel, using a person experienced with working with urethane belts.)
Flash created by butt welding +/- 0.010" (+/-0.26mm) per side, but we can do better on request. Our new robot grinder reduces the flash to +/-0.005" (+/-0.13mm)  For ultra smooth, thin grinds with no dust particles, please specify "robot grinds".
Length of ground area Not to exceed 1/2", but almost always less than this --Typically 3/8" or 1/4". Our new robot grinder is typically 1/16" (1.6mm) wide. For ultra thin grinds, please specify "robot grinds".
Joint Alignment, measured at edges of ground section Axial tolerance is the same as the absolute range of the cross section diameter tolerances above. Angular is 5° and is also a function of how they are packaged. Sometimes the belts get bent when they are packaged and take a set by the time they have reached the end user, but that bend vanishes immediately after they are installed. Therefore, this in not a tolerance that is normally specified.
Number of weld joints per belt In almost all cases there will be only one weld joint per belt. However, Dura-Belt reserves the right to elect to use multiple weld joints for any belt. Normally this only happens with belts that are made from very expensive material and/or with belts that are very long in length. Otherwise, there may be an excessive amount of wasted material that would increase the cost of the belt. Customers, who require only one joint per belt, must specify this when ordering and must mandate it on their purchase order.
Belts are often weigh counted and packaged in wire-tied bunches of what appears to be 50 or 100 pieces Because of the fairly wide tolerances above, weigh counts of bunches may not be exact. See What causes shortages (under-counts) in wire-tied bunches?
Color variations Thermoplastic ester-based polyurethane, used in 97% of our belts, is manufactured by companies like BASF and Dow. It is not made in a continuous process, but rather each lot is made in separate vats. Therefore, each lot can have properties that vary in durometer, modulus of elasticity, melt index and color. Our un-colored urethane that we call "clear" is formally called "natural color" by the manufacturers. It is translucent and can vary in tint from water clear to milky white to tan to golden yellow to amber color. Unfortunately, we cannot order specific colors from manufacturers. They will not accept such requests because they have no control over natural color. Manufactures assure us that there is no difference in performance based on "natural color".
Surface texture variations Surface Texture can vary slightly from smooth to fine matte to chalky white. The latter is called "bloom". It occurs on ester-based urethane when small amounts of oligomers slowly migrate over time (e.g. a few months) to the belt's surface. Bloom can be easily rubbed off with your fingers, rubbing alcohol or hot water and does not effect the belt's performance or its rating as food grade. Urethane is made from long chain polymers. Oligomers are short chain polymers that are a by-product of long chain polymerization..

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