Synchronous belts
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Frequently asked questions
How do I read synchronous belt codes such as 450L100 or HTD 1200-8M-30?
For inch belts to ISO 5296, 450L100 means a 45.0 inch pitch length, L pitch (9.525 mm) and 1.00 inch width. For metric HTD belts, 1200-8M-30 means 1200 mm pitch length, 8 mm pitch and 30 mm width. Pitch length divided by pitch gives the number of teeth, which is the quickest check.
Are trapezoidal (XL, L, H) and curvilinear (HTD, STD) belts interchangeable?
No, the tooth forms differ, so the belt must run on pulleys of the same profile. Trapezoidal teeth appear on older and lighter duty machines, while curvilinear HTD or STD teeth spread stress better and carry more load. Switching profiles means replacing both pulleys.
How do I measure the pitch when the belt code is worn off?
Count a number of teeth, for example 10, measure from the centre of the first to the last tooth, and divide by the number of gaps. Common pitches are 5.08 mm (XL), 9.525 mm (L), 12.7 mm (H) and 3, 5, 8 or 14 mm (HTD). Look closely at whether teeth are round or trapezoidal, since a 5 mm pitch exists as both T5 and HTD 5M.
Glass fibre or steel cords: when should I choose which?
Glass fibre cords are common, stretch little and run well on small pulleys in most machines. Steel or aramid cords are used where minimum stretch is needed for precise positioning or for heavy shock loads. Glass cords do not tolerate crimping, so the belt must not be folded sharply in storage or during installation.
Does a synchronous belt need as much tension as a V-belt?
No, a synchronous belt transmits power through tooth engagement, so it only needs enough tension to stop teeth jumping at peak load. Overtensioning wears the tooth roots, raises noise and shortens bearing life. Recommended tension is best checked with a sonic meter against the belt maker's value.
Why does a synchronous belt track to one side of the pulley?
Some drift to one side is normal because of the helical lay of the tensile cords, which is why at least one pulley usually has flanges. If the belt rubs hard on a flange or climbs off, the pulleys are usually not parallel or the shafts are angularly misaligned. Correct the alignment instead of simply adding tension.
Which failure signs on a synchronous belt point to a cause?
Sheared teeth usually come from shock overload or too few teeth in mesh on the small pulley. Worn tooth flanks and lots of rubber dust indicate misalignment, while a clean transverse break usually means the cords were kinked before fitting. Wear in the land between teeth often goes with excessive tension.
Can synchronous belts handle oil, heat and chemicals?
Chloroprene rubber belts tolerate light oil and moderate temperatures, while polyurethane belts resist oil, grease and abrasion better but are sensitive to hot water and steam. Continuous oil splash or high temperature calls for guarding or a specialised belt material. When ordering, please state the operating temperature and the substances the belt will contact.
