Using the minimum safety factor in the Machinery Standards can be dangerous
When designing lifting or handling equipment such as forklifts and telehandlers, most design engineers would look to comply with the specific standard for the machine they are designing.
For any machine sold in Europe, that’s the Machinery Directive – Directive 2006/42/EC – a standard that is intended to ensure common levels of safety in machinery that is used throughout Europe.
In the section covering leaf chain, the Machinery Directive states that the minimum safety factor when lifting a weight should be 4:1. In other words, the leaf chain should be able to lift four times the maximum weight it will be lifting in its working life. This would result in a leaf chain that operates at 25% of its ultimate tensile strength.
If we look to Industrial trucks standards ISO 3691 and BS EN 16307, they state that the minimum safety factor when lifting a weight should be 5:1. This would give you a leaf chain that operates at 20% of its ultimate tensile strength.
The main standard for telehandlers, BS EN 1459 and boomed access platforms BS EN 280, also allows (under certain conditions) 5:1, or 20% of a leaf chain’s ultimate tensile strength.
Some designers adopt a default position of working just to the minimum standard however a lot of the focus of the standards is on meeting minimum static safety factors. In the real world, this can lead to some poor choices.
In this post, we explain some of the reasons why following the Machinery standards and Directive blindly can be dangerous.
The fatigue strength of a leaf chain is related to the number of articulating links in the leaf chain, with an increasingly uneven load dispersal occurring as more links are added. Leaf chain components, like any stamped components, have tolerances and a leaf chain with more links has longer pins which bend more under load.
A good quality leaf chain with a 2 x 3 lacing pattern has a fatigue limit of about 20% of its tensile strength, whereas a 6 x 6 leaf chain will have a fatigue limit of about 14% of its tensile strength. For maximum fatigue strength, you should be selecting a leaf chain with the smallest number of articulating links.
Just taking machinery standards as a guide means you may end up with a safe leaf chain that doesn’t last very long.
Reduced working life is almost always connected with insufficient lubrication or lubrication not reaching the area between pin and plate bores. When a leaf chain operates under high load lubrication is forced away from the load-bearing areas. To ensure a good service life we advise keeping bearing pressure between 0.15–0.18 kN/mm2.
One of the most common leaf chain types in the world is BL834 – when working at 25% of its UTS, the bearing pressure is higher than we would recommend at 0.20 kN/mm2. Only considering bearing pressure this leaf chain should operate between 13% to 18% of UTS.
Once more, just following the safety factor in machinery standards could result in specifying a leaf chain that does not retain lubricant. Without sufficient lubricant, the leaf chain will have a greatly reduced operating life. To overcome this issue, you would need more frequent re-lubrication with potentially more expensive lubricants designed to handle higher bearing pressure.
Leaf chain works because the pin and the outer links are an interference fit. Internal excessive friction caused by high loading, combined with articulation around the pulley and made worse by inadequate lubrication, results in turned pins. The likelihood of turned pins occurring increases for most leaf chains above 15% of a leaf chain’s tensile strength and when a leaf chain operates at 25% tensile strength, they can even occur the very first time that the leaf chain is used.
Once a pin is turned, the whole leaf chain has to be scrapped.
Another aspect of leaf chain selection is the cost of making a leaf chain and a leaf chain’s associated components such as anchor bolts and pulleys.
If you opt for a leaf chain that is in common use, you should find standard parts available. But if your design requires custom parts, then selecting a leaf chain type can have a significant impact on material sizes and the manufacturing cost of those parts.
On the other hand, if you specify a leaf chain that is not in common use, then replacing it may become an issue with the leaf chain being difficult to source. Replacement parts for running repairs may not be available and/or demand a premium price. And if the associated parts such as anchor pins are less common – you will have the same issues with those – making your choice of leaf chain ultimately expensive.
Making your leaf chain selection based on machinery standards alone could ultimately be more expensive. It may be that a leaf chain that is stronger than you need, but that is more commonly available, or for which the associated parts are cheaper to source, is actually a better choice for overall economy.
Other factors that may have an impact on leaf chain choice includes the industrial environment that the leaf chain is to be used in, it’s interaction with other parts on the machine and the way it will be treated when in use – ie, Is it likely to be maintained and lubricated regularly or not so often? How many times will it be used each day and how much of its working life will it spend at maximum load?
Look at the bigger picture
Choosing a leaf chain to use in a lifting system design must, therefore, take account of many factors – not just the safety factor. While it may be easy to pick a leaf chain out of a manufacturer’s catalogue based on projected strength requirements and a safety factor, this is unlikely to result in a leaf chain that is the optimum leaf chain for your design.
Working with a leaf chain specialist and benefitting from their advice and expertise will have long term benefits – they will be able to guide you to the right leaf chain for your design and can help with other aspects of your design and manufacture saving you money and future headaches.
Note – The figures used in this article are based on our own leaf chain’s performance. All chain companies publish tensile strength information. There has been a tendency within the chain industry to compare quality based on strength alone, which can lead to leaf chains that do not perform well in other areas. Data on fatigue is generally not published but all good manufacturers should be able to supply it.