You can't blame engineers for using breaking load as a criterion for selecting chain. Breaking load was one of the earliest chain standards, so engineers got used to specifying it that way and many of them still do. The task now is to instil an understanding among specifiers that breaking load is no guide to chain performance, and that chain strength is not the best measure of chain life. in fact, high tensile strength can actually lead to a shortened working life if component strength has been achieved by making parts hard but brittle.
It was back in 1925 when the first chain standard was introduced: BS 228 for steel roller chain and chain sprockets. BS 228 initially contained only minimal requirements for chain and sprocket dimensions but it was broadened in 1934 when minimum breaking loads were included.
Chain design and manufacture has come a long way since then, and a correctly selected modern chain will now last years on applications that, back in the mid twentieth century, might have worn out in a matter of months. Modern chain is a much more reliable, high-tech product than its earlier cousins, but to get the full benefit of all the recent innovations engineers need to select more carefully from all the new options that are available to them.
The first thing is to put considerations of breaking load to one side. Chain should be operated well below any load at which it could break. In fact, permanent damage will occur if the load on a chain is such that it exceeds the elastic limit of the steel parts. Chain should be operated below its endurance limit to ensure that wear is the mode of failure, rather than fatigue, or even complete failure because of overload.
Of great importance to achieving longer working life is the overall ability of the selected chain to manage the applied loads and provide fatigue resistance well above the application's anticipated working loads. Most good chain manufacturers will know the fatigue limits of their products and it is this that engineers should be taking into consideration rather than breaking loads.
To help make chain selection easier and to ensure that engineers get the optimum value and working life out of chain, Renold has produced a chain selector program that takes the hit-and-miss out of increasingly complex choices. The program can be downloaded free of charge from the company's web site and it is capable of selecting the best chain for any given application. That is, the smallest chain available that will last a minimum of 15,000 hours (or 30,000 hours for Renold Synergy) based on factors such as speed, load and the power ratings of the user's application.
The program selects the best chain from eleven different product types and 400 chain variants. It is easy to use and, according to Mr Turner, users will be able to make their first selections within minutes of installation. The detailed search results can be printed out and show the optimum chain to meet all criteria, including corrosion resistance, resistance to abrasive debris, lack of lubrication or the need for high performance.