When selecting the right leaf chain for a particular application, the most important consideration is that the minimum tensile or breaking strength of the chain is safe and legal for the load and type of machinery.
While all chain manufacturers publish their minimum tensile strength requirements, it is often up to the design engineer to make a decision as to which tensile figure on which to base their design. Most good quality leaf chain manufacturers will exceed any international standard by 20%.
It should be noted that it has been common in the industrial chain industry to focus on tensile strength as an indication of leaf chain product quality.
Making leaf chain stronger may seem easy - ie by just increasing the hardness of links and pins. But this method can result in the chain becoming brittle and less resistant to shock loads and can reduce its fatigue limit.
The graphs below show tests on two chains.
The chain above is from a low-cost manufacturer who hardened the pins and plates to increase tensile strength.
In this example, the chain failed suddenly as a brittle failure - and with no prior evidence of ductility or plastic degradation.
The example above is a chain using higher grade material, but with lower hardness, which has resulted in a higher tensile load.
Most chain company catalogues contain a formula for working out a chain’s recommended working load which can be used to give you an indication or starting point for chain selection.
However, many companies are wary of unsupported design, as this method can often result in over specification.
Many countries issue their own standards which normally mirror the two international standards - ISO4347 or BS29.B.
The International ISO standard contains LL & LH (BL) and the American standard contains BL (LH). The key difference in the standards is that the ISO standard contains a minimum fatigue requirement.
Conversion of an inch in the American standard to European metric also throws up some small dimensional differences.
It is recommended that the chain standards are used as a dimensional reference only, particularly for chain anchors bolts slots, as this will ensure interchangeability.
While all chains should meet the minimum tensile requirements of the standard, each manufacturer will have their own data which may be significantly different.
There are currently three levels of safety requirements that a chain must fulfil. The first is the Machinery Directive 2006/42/EC, implemented in the UK as the Supply of Machinery (safety) Regulations 2008, which requires a minimum safety factor of 4:1.
This means that the minimum tensile strength of a leaf chain must be four times the maximum load it is used to support.
However, most leaf chain manufacturers recommend a greater safety factor.
For example, ISO 4347:2015 requires a factor of 5:1. Different legal requirements also apply to different types of machines.
Standard forklifts and telescopic handlers, for example, require a factor of 5:1; while man-up forklifts and passenger lifts require a factor of 10:1 and 20:1 respectively.
Other factors that need to be considered when selecting a leaf chain are:
All machinery supplied in the European economic area must comply with the machinery directive 2006/42/EC (with specific reference to 184.108.40.206. Pulleys, drums, wheels, ropes and chains) which sets out the design requirements for chain.
It’s important to bear in mind though that this directive is generic and covers all types of chain used in lifting.
For leaf and roller chain, most manufacturers would neither approve or recommend a design which uses the 4:1 safety factor contained in the machinery directive as it is a minimum standard only.
Design engineers should note however that the Mobile Elevated Work Platform (MEWP) standard refers to the chain system and a decision needs to be made as to which parts of the design are included in the chain system.
Many machine manufacturers take this to mean chain and chain anchor bolts, supplied together, as a chain assembly.