Why an anchor bolt is so important for leaf chain system design
To some people, a leaf chain anchor bolt is simply a piece of steel comprised of a thread and some slots.
But when it comes to the design and the choice of materials that go into the creation of that bolt, there are a great number of details to consider.
A safety-critical component
The main job of a leaf chain anchor bolt is to effectively join the leaf chain to the mast of the forklift.
Any part of a leaf chain that is taking any load is deemed to be a safety critical part, so it is vital that it is constructed from high-tensile alloy steel according to individual application stresses and the required breaking strength of each particular leaf chain.
Whilst many applications are simple and may only require a standard product, there will occasionally be unusual stresses, such as tension, compression, shear and bending effect that will require a special design.
In the majority of materials handling applications, two leaf chains will sit side by side. Even the smallest of differences in the dimension of the leaf chains can create tolerance issues, so a leaf chain anchor can be used to balance or share the load, especially when using multiple lengths of leaf chain.
Misalignment can also be an issue in longer lengths of leaf chain. A pivoting leaf chain anchor allows the head of the anchor to swivel which enables the leaf chain to move slightly.
Leaf chain termination
There are two main types of anchor bolt, which are differentiated according to the termination of the leaf chain.
- An ‘ends inner’ – in which a male leaf chain end is joined to an equivalent female bolt
- An ‘ends outer’ – in which a female leaf chain end is connected to a male bolt
As production methods get leaner there is a growing trend to move from ends inner termination to ends outer anchor bolts.
Using an ends outer termination enables you to trim down the amount of material on the anchor bolt as you are effectively fitting the bolt into the middle of the outer plates.
But while this method allows you to use less material, it can also mean that there will be some differences when it comes to calculation of strength.
The less material you have around the pin, the lower the ultimate tensile strength (or UTS) of the bolt, so this will need to be matched to the strength of the leaf chain.
Leaf chain elongation
As a leaf chain is used, it wears and elongates over time.
How quickly a leaf chain will elongate depends on the frequency of use. If you’re using a leaf chain just once a year, for example, then it is likely to be some considerable time before you see any signs of wear.
But if you’re using the leaf chain 24hrs a day, and if it’s always under tension and is at its maximum load, then it is going to wear significantly more quickly.
Leaf chain is considered to have reached the end of its life at 2% elongation and is deemed dangerous at 3% elongation. By calculating the maximum elongation in mm on the section of leaf chain which will wear, you can design in a limit to the adjustment so the leaf chain can’t be used once it reaches the end of its life.
Threaded anchor bolts
To ensure the leaf chain remains in tension and at the correct operating length throughout its use, a threaded anchor bolt is used at either or both ends of the leaf chain. That thread is then used to take the adjustment out after the leaf chain has elongated.
Most leaf chain anchors have coarse threads which have a larger pitch than fine threads. In work environments where there is a tendency for rough handling, coarse threads are more tolerant of abuse and will assemble and disassemble more easily and quickly than fine threads.
A fine thread can be specified when thread adjustment is crucial for the application or when a stronger thread is required but lack of space precludes increasing the thread diameter to the next size.
By having a threaded anchor bolt at both ends of a leaf chain you can divide the adjustment between the two to avoid overly long threads which can be prone to bending and expensive to replace.
Sometimes space constraints mean that you can only have a threaded anchor bolt at one end and then a fixed anchor bolt (ie a shear face or an anchor block) at the other.
This is especially common within a boom section where there may be an area that is less easily accessible by the technician.
By placing the threaded part at the end that you have easier access to, you can then carry out any adjustments without having to worry about the other end of the leaf chain.
It is also possible to design leaf chain anchor blocks which enable some adjustment. While these can be costly and complex to manufacture, they may be the best option when designing multi-stage systems where leaf chains counteract each other and leaf chain adjustment needs to be simultaneous
Bespoke VMC equipment
The centre of the bolt is arguably the most important part as it takes the majority of the load.
Traditional forging or gang mill methods of anchor bolt manufacture can create bolts that have off-centre or non-parallel head flats which increases the risk of early fatigue failure of the thread or shaft.
Using a combination of the latest VMC equipment and bespoke work holding methods, all flats can be centralised on the thread of the bolt to ensure the load on the pin is pulled in line with the anchor bolt, rather than away from the line of pull. The flats will also be perfectly parallel and perpendicular, so the head section has same tensile strength throughout.
Traditional methods of slotting can also result in anchor bolt slots that are off-centre, non-parallel or that have misaligned pin holes.
The use of VMC production equipment will ensure that the slots are manufactured exactly in line with the bolt centre line, that all slots are parallel to the line of pull of the leaf chain and that the forces on the anchor pin are perfectly square.
The provision of a test certificate ensures that every leaf chain and every anchor bolt is completely traceable.
In the case of FB Chain’s leaf chain anchor bolts, a unique number is etched onto each part which directly relates to the works order number or test certificate number.
This simple detail provides a wealth of information – from who made the part, to the specific machines the part went on, the batch of steel that was used and where that steel originated from.
From this same number, it’s also possible to show any additional costs that may have been incurred in the process of manufacturing the part, how many parts were lost, who checked the part and what the dimensions were.
All documentation related to the individual part documents is then saved on a computer system which means any potential queries or issues can be quickly identified and resolved.