Leaf chain system design – 6 things to consider
A leaf chain system design for safety-critical forms of materials handling equipment such as forklift trucks or telehandler booms needs to incorporate some essential design elements that will ensure there is no compromise in the functionality, compliance and longevity of the chain.
In this blog post we explore six features to consider in leaf chain system design:
1. Tensile load
You will firstly need to decide if you are going to use the tensile load in one of the international chain standards or if you will work from the figures in your chosen chain manufacturer’s catalogue.
All chain should meet the minimum tensile requirements of the standard, however each manufacturer will have their own data which can be significantly different. Most manufacturers will exceed any international standard by 20%.
It is recommended that the chain standards are used as a dimensional reference, particularly for chain anchors bolts slots, as this will ensure interchangeability as chain is replaced during its working life.
2. Bearing pressure
While a significant amount of focus is placed on meeting the minimum requirements of the machinery standard, this can sometimes lead to less than ideal choices.
If you are comparing chains with equal tensile strength, then it is recommended to select the one with the largest bearing area – which can be calculated by multiplying the plate thickness in millimetres by the total articulating plates by the pin diameter in millimetres.
Let’s consider the differences between the BL646 and the BL644 leaf chains, for example. Both chains have a minimum tensile strength of 127kN so are well above the ISO standard of 97.5kN, however the 4×6 lacing of the BL646 has a 50% greater bearing area than the BL644.
As the width and number of links increases, the fatigue limit will also reduce, which will require the need for more frequent replacement of chain.
The fatigue strength of a leaf chain is impacted by the number of articulating links, with the risk of an increasingly uneven dispersal of load as more links are added.
Like any stamped components, chain components have tolerances and a leaf chain with more links and longer pins will be more susceptible to bending under load.
Higher quality chains will have close tolerances on the chain pitch and high precision bores in the link plates. The application of a high pre-load to a leaf chain at the end the manufacturing process can also bed-in the components and increase fatigue strength.
While the general process for leaf chain is the same for most manufacturers, each will have their own opinions as to what they believe offers the best performance. They should also be able to supply you with fatigue data for each size of leaf chain that they produce.
4. Pulley design
When designing a leaf chain pulley it is important to consider the ISO4347 international standards.
To achieve normal wear life rates, for example, it is recommended that the minimum running diameter of a pulley is equal to five times the nominal pitch of the chain.
The minimum width between the flanges should also be 1.05 times the pin length (or pin width if using riveted bearing pins.) The minimum outside diameter (or flange diameter) can be calculated by combining the running diameter with the link plate height.
5. Chain termination
The strongest possible connection will be achieved when you combine the maximum number of sheer faces with the greatest tensile strength – so the ideal chain termination is when leaf chain ends on inner links and fits internally to the chain anchor.
If space is restricted then it is also possible to connect the chain anchor on the outer links – but with the chain rivet pin being the widest part and the chain being joined to the anchor with a riveting or cotter pin unit.
If the termination or last links of the chain are both on the inner links or outer links the chain length must be an odd number of pitches i.e 137 and can only be adjusted by x 2 the chain pitch. If the ends are odd (i.e one inner and one outer) the chain length can be moved by x 1 chain pitch.
6. Chain anchor thread length
Over time, chain will gradually increase in length as a direct result of articulation around the chain sheave or pulley. When designing a leaf chain chain system it’s important to consider adjustment due to wear – and that the chain is no longer used once its elongation limit has been reached.
It is recommended that the amount of adjustment be set to allow no more than 3% of the chain length which articulates the pulley.
In the case of a 5000mm chain, for example, where 3800mm wraps round the pulley, there should be no more than 114mm of adjustment.
What’s also important that the tensile strength of the thread is greater than the strength of the chain that’s been selected.
It’s best to calculate the strength of the thread in the shear and the tensile of the head and in the tensile strength of the threads. Chain anchors should also be either constructed or heat-treated, to achieve a strength of 850Kn mm².