Leaf chain size, construction and materials have a direct impact on a chain’s strength and lifting capacity and are crucial in ensuring optimum performance, avoiding equipment damage and maintaining safety.
If the chain that controls a telehandler boom should fail or break during use, the consequences will be severe - with the potential for:
So what are the factors a design engineer should consider when deciding on the right leaf chain, and leaf chain products, for a telehandler application?
On this page, we:
Leaf chain is a form of rollerless chain commonly used in a wide variety of materials handling applications and lifting apparatus including:
Leaf chain is a simple construction of link plates and pins, with sheaves (not sprockets) being used to change the direction of the chains.
The component parts of leaf chain are dimensionally similar to standard roller chains which means that the economic production of leaf chain is largely dependent on factories producing large quantities of roller chain.
Evidence of the use of chain dates back thousands of years:
Leaf chain parts are identified by a system which uses different combinations of letters and numbers to describe the following characteristics:
The first two letters of a leaf chain part number refer to the leaf chain series.
The first one or two digits of a leaf chain part number (after the letters) refer to the pitch of the chain and vary according to the series.
AL series chain is measured in 8ths of an inch - but only the numerator of the fraction is used. For example, 4 represents 4/8" which can be reduced to ½" and 10 represents 10/8" which can be reduced to 1 ¼".
EL-LL and BL-LL series chain is measured in both 8ths and 16ths of an inch. For example, BL12 represents 12/8" and LL24 represents 24/16" (both of which can be reduced to 1 ½")
The metric pitch can be found by dividing the numerator by the denominator and multiplying by 25.4.
The final two digits in the part number describe the chain lacing, with the last number referring to the number of inner plates and the second to last number referring to the number of outer and intermediate plates.
When selecting the right leaf chain for a particular application, a crucial consideration is that the minimum tensile strength of the chain is safe and legal for the type of machinery and its load.
Most chain catalogues contain a formula for working out a chain’s recommended working load which will provide a starting point for chain selection. Many companies, however, are wary of unsupported design, as it can often result in over-specification. This means that the calculations are often very conservative.
While all chain manufacturers publish their minimum tensile strength requirements, it is often up to the design engineer to determine which tensile figure on which to base their design. Most high-quality leaf chain manufacturers will exceed any international standard by 20%.
It is common in the industrial chain industry to focus on tensile strength as an indication of leaf chain product quality. But while making leaf chain stronger (by increasing the hardness of links and pins) may seem an easy fix, it can often lead to brittleness, less shock resistance and reduced fatigue limit.
Many countries issue their own standards which normally mirror the two international standards - ISO4347 or BS29.B.
The European ISO standard contains LL & LH (BL) and the American standard contains BL (LH).
A key difference is that the ISO standard contains a minimum fatigue requirement.
Conversion of an inch in the American standard to European metric can also throw up some small dimensional differences.
It is recommended that the chain standards are used as a dimensional reference only –particularly for chain anchor bolts slots – to 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.
The Machinery Directive 2006/42/EC, implemented in the UK as the Supply of Machinery (Safety) Regulations 2008, requires a minimum safety factor of 4:1 which means that the minimum tensile strength of a leaf chain must be four times the maximum load it is used to support.
Most leaf chain manufacturers, however, recommend a greater safety margin. Based on a number of factors, you can end up with a safety ratio of between 7:1 and 12:1.
Different legal requirements also apply to different types of machines. Standard forklifts and telescopic handlers require a factor of 5:1; while man-up forklifts require a factor of 10:1 and passenger lifts a factor of 20:1.
Other features 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 E 2006/42/EC which sets out the design requirements for chain – with specific reference to 18.104.22.168. Pulleys, Drums, Wheels, Ropes and Chains.
However, these directives are generic and cover all types of chain used in lifting.
For leaf and roller chain, most manufacturers would neither approve, nor recommend, a design which uses the 4:1 safety factor contained in the Machinery Directive.
Design engineers should note however that the Mobile Elevated Work Platform (MEWP) standard refers to the chain system. Many machine manufacturers take this to mean chain and chain anchor bolts, supplied together, as a chain assembly.
When designing a chain system, it’s important to allow for the adjustment that will be required as a result of chain elongation due to wear. Over time, chain will increase in length due to articulation around the chain sheave or pulley.
However, to ensure that chain is not used once its elongation limit has been reached, it is recommended that the amount of adjustment should be limited to no more than 3% of the chain length which articulates the pulley.
The thread must also have a tensile strength greater than the tensile strength of the chain that has been selected.
As a starting point, you should design the chain anchor to be as strong as the tensile strength of the chain and the final design must have a strength equal to or greater than the working load plus the safety factor for the machine.
We recommend calculating the strength in three places: the shear and tensile of the head and the tensile strength of the threads (as per the diagram).
Our experience is that chain anchors will need to be made from a material with a strength of around 850kN mm2 or a material that has been heat-treated to reach that strength.
The strongest connection is achieved when a leaf chain ends on inner links and fits internally to the chain anchor. This is the preferred choice as it ensures the maximum number of sheer faces along with the greatest tensile strength.
If space is limited, the chain can connect to the chain anchor on the outer links with the chain rivet pin being the widest part. However, when using this method, the chain should be joined to the anchor with a riveting or cotter pin unit.
A chain anchor pin must never be pushed through the chain’s outer links, as this can result in the load being supported by just the two outer links on their own.
“The strongest connection is achieved when the leaf chain ends on inner links and fits internally to the chain anchor”
If the termination, or last links, of the chain are both on either the inner links or the outer links, the chain length can be adjusted by x 2 the chain pitch.
If the ends are odd (ie one inner and one outer) the chain length can be moved by x 1 the chain pitch.
Typically, each stage of a boom has a set of chains for extension and retraction. These act against each other, so great care must be taken when adjusting each chain as it will have the opposite effect on the opposing chain.
Over-tensioning can be a problem, both during the set-up of a new machine and in the carrying out of machine maintenance, as it adds load to the chain which can result in it operating above its tensile capacity.
Some leaf chain manufacturers supply an equation for calculating the required torque on chain anchor nuts to remove chain sag.
While this is practical when the machine is being built in the factory, doing this in the field would be difficult.
Setting a minimum and maximum distance the chain should sit from the boom and or the position of nuts on the chain anchor can help ensure chain tension is constantly applied in service and without the need for specialised equipment.
A lot of the focus of the machinery standard is on meeting minimum safety factors. But this can sometimes lead to poor choices.
When comparing chains with equal tensile strength, for example, it is recommended to select the one with the largest bearing area.
The bearing area calculation in mm2 is as follows:
Plate Thickness (mm) x Total Articulating Plates x Pin Diameter (mm)
In the example below, we have two chains with a minimum tensile strength of 127kN (ISO standard 97.5kN) - however, the BL646 (4x6 lacing) has 50% greater bearing area than the BL644.
Ideally, it’s advisable to keep bearing pressure between 0.15–0.18 kN/mm2
The bearing pressure calculation is as follows:
(kN/mm2) total load on chain (kN) ÷ bearing area (mm2)
With booms, there is often more space availability in width than there is in height as the booms pass over each other. This can result in the decision to select a wide small pitch chain to help keep the pulley diameter small.
But as the width and number of links increases, the fatigue limit reduces. Turning forces of more links acting on the pin also increase the chance of breaking the press-fit between the outer plate and rivet pins – which can result in the chain needing to be replaced after even a short time of increased load or poor lubrication.
Although 8 x 8 and 10 x 10 chains are still shown in many chain catalogues, for example, they are not in the international standards and should only be selected as a last resort.
Below is the recommended dimension of a leaf chain pulley/sheave.
The running diameter should be a minimum of 5x the pitch of the chain.
The larger the diameter, the longer the life of the chain. So a chain pulley with only x 4 pitch will reduce chain life by 25% while a pulley with 7 x pitch will increase chain life by 33%.
The running surface of the pulley needs to be hard, and slightly harder than the chain, so it is the chain plates that wear and not the chain pulley. It is also recommended that the working faces of the pulley are hardened to 50 HRC (Rockwell C Hardness) minimum. If space is an issue, adjustments can be made to the flange diameter and the flange angle – however, it is recommended that the height of the flange is always above the centre line of chain.
When designing a leaf chain pulley (also known as a leaf chain roller or leaf chain sheave), it is important to consider the following international standards (ISO4347).
For normal wear life rates, it is recommended that:
“The speed and frequency of operation must also be considered when designing pulleys, rollers or sheaves for leaf chain.”
The fatigue strength of a leaf chain is related to the number of articulating links, with an increasingly uneven load dispersal occurring as more links are added.
Chain components, like any stamped components, have tolerances and a leaf chain with more links has longer pins which will bend more under load.
High-quality chains will have closer tolerances on the chain pitch and the bores in the link plates will be of a higher precision. The application of a high pre-load to a leaf chain at the end of the manufacturing process can also bed the components in and increase fatigue strength.
While the general process for leaf chain is the same for each manufacturer, they will each have their own ideas on the features that they believe give the best performance.
ISO 4347 added a minimum fatigue requirement in the 2015 revision. All good manufacturers should be able to supply fatigue data for each size of leaf chain.
Care is required when making comparisons as the test length can be 3 million to 10 Million cycles and the stress range impacts on the results.
The table below gives the fatigue limit approximation as the number of plates increases, as a percentage of tensile strength:
|Plate combination||Good quality brands||Average or medium quality brands||Defective or poor-quality brands|
|2x2 and 2x3||20%||13%||6.7%|
|3x4 and 4x4||14%||9%||4.5%|
|4x6 and 6x6||12%||7.7%||4%|
Leaf chain assemblies are packaged solutions that can ease scheduling and supply-related issues by providing a uniquely specified and coordinated assembly of parts that are tailored to the customer’s needs. They can address a particular application and can be issued directly onto the production line.
Leaf chain assemblies are suitable both for new production and for ongoing repairs and maintenance.
A complete component package can bring together the whole family of parts, including items as diverse as composite bushings, nylon stops, mast pulleys, chain rollers and mast pivot pins.
Leaf chain can be supplied pre-cut to length to eliminate chain stock loss from unusable offcuts. The chain can also be assembled to individually designed leaf chain anchor bolts and blocks.
Coatings, lubricants and surface finishes can be applied to all parts according to application.
Leaf chain and anchors can also be delivered in temporary protective plastic tubing to reduce chain contamination and assist clean assembly.
Specific heat treatments, integral bushings or bearings can all be supplied to specification. For difficult environments, permanent protection can also be provided in the form of heat-shrunk Neoprene that is applied to specially lubricated chain.
Order entry systems and production control routines ensure efficient control of an OEM’s scheduled requirements. Working directly from production build programmes enables deliveries to be made to the line as needed.
The inevitable fluctuations in customer schedule demands can also be smoothed by buffer stocking policies, which minimise the risk of interruptions to production.
Daily requests from parts and service departments for spares stock or VOR emergencies can be handled equally quickly and efficiently.
And comprehensive stocks, late afternoon order processing and overnight deliveries combine to provide a reassuringly effective service.
Experienced design and production engineers are available to offer technical help and advice on application design and related topics. A comprehensive in-house design facility and an extensive library of information can also ensure the selection of the most cost-effective solutions.
Selecting the right leaf chain for a specific materials handling application relies on an understanding of crucial design considerations – from the implications of minimum tensile strength to the effects of shock load, internal bearing pressure, fatigue and environmental factors.
Opting for a leaf chain assembly, over individual component parts, can offer some measurable advantages in terms of ease of assembly, enhanced product quality, improved safety standards and efficiency of production.