When selecting the right leaf chain for a particular application, the most important consideration is that the minimum tensile or breaking strength of the leaf chain is safe and legal for the load and type of machinery.
While all leaf 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 leaf chain industry to focus on tensile strength as an indication of leaf chain product quality.
Making leaf chain stronger may seem easy – i.e. by just increasing the hardness of links and pins. But this method can result in the leaf chain becoming brittle and less resistant to shock loads and can reduce its fatigue limit.
The graphs below show tests on two leaf chains.
The leaf chain above is from a low-cost manufacturer who hardened the pins and plates to increase tensile strength.
In this example, the leaf chain failed suddenly as a brittle failure – and with no prior evidence of ductility or plastic degradation.
The example above is a leaf chain using higher grade material, but with lower hardness, which has resulted in a higher tensile load.
Traditional calculation of working load
Most leaf chain company catalogues contain a formula for working out a leaf chain’s recommended working load which can be used to give you an indication or starting point for leaf chain selection.
However, many companies are wary of unsupported design, as this method can often result in over specification.
Leaf chain anchor thread selection
When designing a leaf chain system, it’s important to consider adjustment as a result of elongation due to wear. Over time, leaf chain will increase in length due to articulation around the leaf chain sheave or pulley.
To ensure that leaf chain is not used once its elongation limit has been reached, it is recommended that the amount of adjustment should be set to allow no more than 3% of the leaf chain length which articulates the pulley.
For example, a 5000mm long leaf chain (of which 3800mm wraps the pulley) should have no more than 114mm of adjustment.
As a starting point, you should design the leaf chain anchor to be as strong as the tensile strength of the leaf chain and the final design must have a strength equal to or greater than the working load plus the safety factor 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 below.)
Our experience is that leaf chain anchors will need to be made from a material or heat treated to have a strength around 850kN mm².
Leaf chain termination
The preferred choice, and the strongest connection is achieved when leaf chain ends on inner links and fits internally to the chain anchor to ensure the maximum number of sheer faces and the greatest tensile strength.
If space is limited, the leaf chain can connect to the leaf chain anchor on the outer links with the leaf chain rivet pin being the widest part.
However, when using this method, the leaf chain should be joined to the anchor with a riveting or cotter pin unit.
It is vital that a leaf chain anchor pin is never pushed through the leaf chain outer links, as this can result in the load being taken by only the two outer links.
Leaf chain length
If the termination, or last links, of the leaf chain are both on either the inner links or the outer links, the leaf chain length can be adjusted by x 2 the chain pitch.
If the ends are odd (i.e. one inner and one outer) the leaf chain length can be moved by x 1 the leaf chain pitch.
Leaf chain tension and adjustment
Typically each stage of the boom has a set of leaf chains for extension and retraction. These act against each other, so great care must be taken when adjusting each leaf chain as it will have the opposite effect on the opposing leaf 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 leaf chain anchor nuts to remove leaf chain sag.
While this is practical while the machine is being built in the factory, doing this in the field is difficult.
Setting a minimum and maximum distance the leaf chain should sit from the boom and/or the position of nuts on the leaf chain anchor can help ensure leaf chain tension is more constantly applied in service and without the need for 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 leaf 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 above we have two leaf chains with a minimum tensile strength of 127kN (ISO standard 97.5kN) – however, the BL646 (4×6 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 leaf 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 leaf chain needing to be replaced after even a short time of increased load or poor lubrication.
Although 8 x 8 and 10 x 10 leaf chains are still shown in many leaf chain catalogues, for example, they are not in the international standards and should be selected only as a last resort.
The fatigue strength of a leaf chain is linked to the number of articulating links, 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.
High quality leaf chains will have close tolerances on the leaf chain pitch and the bores in the link plates will be high precision. The application of a high pre-load to a leaf chain at the end 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 version. 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 and to 10 million cycles and the stress range impact on the results.
The table below gives the fatigue limit approximation as the number of plates increase:
|Plate combination||Good quality brands||Average or medium quality brands||Defective or poor quality brands|
|2 x 2 and 2 x 3||20%||13%||6.70%|
|3 x 4 and 4 x 4||14%||9%||4.50%|
|4 x 6 and 6 x 6||12%||7.70%||4%|
|8 x 8||10%||6.30%||3%|
Below is the recommended dimension of a leaf chain pulley/sheave.
The running diameter should be a minimum of 5x the pitch of the leaf chain.
The larger the diameter, the longer life of the leaf chain. So a leaf chain pulley with only x 4 pitch will reduce leaf chain life by 25% while a pulley with 7 x pitch will increase leaf chain life by 33%.
The running surface of the pulley needs to be hard, and slightly more than the leaf chain hardness, so it is the leaf chain plates that wear and not the leaf chain pulley. If space is an issue, adjustments can be made to the flange diameter and the flange angle – however, it is recommended that the flange is always above the centre line of leaf 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 standard: ISO4347 – Leaf Chains, Clevises and Sheaves. Dimensions, measuring forces, tensile strengths and dynamic strengths.
For normal wear life rates, it is recommended that:
- The minimum running diameter of the pulley is equal to 5 x the nominal pitch of the leaf chain
- The minimum width between the flanges is 1.05 times the pin length (or width over riveted bearing pins)
- The minimum outside diameter (or flange diameter) is the running diameter plus the link plate height
The above dimensions can vary according to space and application requirements, but it should be noted that any deviation from the recommended standard will result in either an increase (or decrease) in leaf chain wear.
By increasing the diameter of the pulley, for example, the angle of articulation will decrease, leading to less wear.
By decreasing the diameter of the pulley, the angle of articulation will increase, leading to more wear.
The speed and frequency of operation must also be considered when designing pulleys, rollers or sheaves for leaf chain.
It is also recommended that the working faces of the pulley are hardened to 50 HRC (Rockwell C Hardness) minimum.
The pulley must be designed to be harder than the leaf chain – so that it is the easily-replaceable leaf chain and not the pulley that is subject to wear.