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Power rating values

Power Ratings for synchronous belt drives - still a relevant basis for a drive design?

This article on power transmission values for synchronous belt drives has been published in the February 2004 edition of the French magazine “Entraînements et Systèmes” as well as in the December 2003 issue of „Antriebstechnik“ a leading German industrial trade magazine. Author of the article is Matthias Farrenkopf, Technical Director Power Transmission of Gates Industrial Business Unit.

A large number of industrial synchronous belt drives are being designed and built by users, using generally available product information. Proper catalogues are helpful tools through which manufacturers can release information about existing product ranges, new products and Power Ratings into the market. The user will rely on such product information; power ratings and safety factors to avoid any risk for the application and subsequent operation.

This results in many applications being overdesigned, or conversely operating with a very high safety factor. This over-designing of applications provides a certain freedom to use power ratings as a marketing tool and as an initial and obvious sales argument, whilst real operating lifetimes are very rarely examined.

With the development of HTD® synchronous belts, Gates carried out an extensive range of tests, demonstrating the relationships between belt life, belt effective tension and the influence of the pulley diameter. This was established as the basis for the power rating values, which are based on a unique service life. This systematic approach has also been applied to all other subsequent Gates synchronous belt developments.

Comparing the catalogue power rating data with the test results of various different products as available in the market, carried out in line with Gates internal test standards, it has become clear that the different products are following a different basis for the service life. The differences that have been found cannot be explained otherwise (Fig. 1) and it is apparent that the products are not directly comparable with each other. Transmission of the quoted power rating values is possible, but a different service life may be expected. For over-designed drives, this may possibly have no effect, but for borderline applications it can be critical.

In the long-term this can lead to uncertainty on the market, because there is no way of determining service life. This uncertainty can also affect the overall image of timing belt drives - a harmful consequence may be that versatile machine components are replaced by alternative drive components.

Standardising the ways of working would be quite an improvement. Uniform, standardised test rigs with clearly defined test conditions would create a comparable basis. Established failure modes and criteria, specific belt characteristics before and after test and defined test targets would be a comprehensible aid for the market and its users. In this case, parallels can be drawn with the automotive industry, where this methodology was initiated years ago.
It would also bring belts into line with other transmission elements such as couplings and bearings.

Fig. 1: Comparison of catalogue power rating values and test results

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