why does some chain measure under pitch?

Leaf chain is manufactured to International Standard ISO 4347:2004 and it allows for ‘LL’ (EL) series leaf chains to be constructed from linkplates that are also used for short pitch transmission roller chain.The actual pitch of the leaf chain not necessarily being equal to its nominal pitch but it does depend upon the manufacturer.

To enable short pitch transmission roller chain to consistently interact and mesh with sprockets it must be manufactured with as near perfect pitch as possible. To achieve this scenario it is necessary for the outer and centre linkplate components to actually be manufactured short of nominal pitch.

When ‘LL’ (EL) series forklift truck leaf chains are manufactured from these roller chain components the linkplates that are used, in the construction of the leaf chain, are the outer linkplates and the centre (intermediate) linkplates found in multi-strand roller chains.  Since these elements are manufactured short of pitch it will be found that the actual pitch of the resulting leaf chain will always be shorter than the quoted ‘nominal’ pitch.

The diagram shows how this under pitching affects the actual length of 21 pitches of typical ¾” ‘nominal’ pitch ‘LL’ series leaf chain.  In this example the difference between a chain manufactured using leaf chain specific components and one manufactured using short pitch transmission roller chain components will results in a length differential of 0.75%.

When measuring the above leaf chains for wear, compared to nominal pitch, it is important to note that the shorter chain, constructed using roller chain components, will be 0.75% more worn than the chain made with leaf chain specific elements. This differential becomes increasingly more important as the forklift truck leaf chain progresses towards its end of life.  For example a leaf chain made from roller chain parts will be 3.75% worn when only appearing to be 3% worn against nominal pitch.  This would adversely affect both manually calculated and all chain wear gauge readings.

If doubts exist about the original chain pitch when measuring forklift truck chain for wear then it will be necessary to determine the leaf chains actual pitch so that allowance can be made in order to determine the true amount of wear in the leaf chain.  This can usually be accomplished by firstly measuring the pitch of the leaf chain across an outer link close to the leaf chain anchor bolt.  Fortunately this section normally has little or no chain wear.  Compare this with the most worn segment of chain, normally found in the section of leaf chain that regularly passes over the leaf chain pulley.

To eliminate this potential problem all FB forklift truck leaf chain (including ‘LL’ series) is specifically designed and manufactured from components that are solely used for leaf chain.  Therefore the ‘actual’ pitch of FB leaf chain will equal the quoted ‘nominal’ pitch.

By specially selecting purpose made leaf chain components FB ensure that there is correct seating of the leaf chain linkplate onto the leaf chain rivet pin which increases breaking and fatigue strength as well as improving all the various leaf chain elements