Key considerations when designing leaf chain anchor bolts
Leaf chain anchor bolts are a crucial part of a leaf chain assembly – if they are not designed without considering all the relevant factors then they may not conform to industry standards or they may even fail. Here are the main considerations when designing an anchor bolt for use with a leaf chain.
While each manufacturer’s leaf chain can have slightly different dimensions, we would always recommend making the slots and holes to the sizes shown in ISO4347. Using this standard will ensure interchangeability should a purchasing team decide to change suppliers, or another brand of leaf chain is fitted as a replacement.
Don’t design to fail
Leaf chain anchor bolts see varying forces, so when designing new leaf chain anchor bolts you need to look at two key areas, the head strength and the thread strength.
The surface area of the head and slots affect tensile and shear strength. This surface area needs to be sufficient to provide a strength equal to, or greater than, the Ultimate Tensile Strength (UTS) of the leaf chain.
The thread strength also needs to be equal to, or greater than, the strength of the leaf chain.
It is accepted practice and a part of most machinery standards that leaf chain anchors should be equal in strength to the leaf chain that is attached to them.
But for various reasons you may have over specified the leaf chain – using a leaf chain that is well over the safety factor you are working to in your design. At which point you may then decide to specify an anchor bolt that matches the leaf chain’s strength – but this is not strictly necessary – the anchor bolt strength only needs to exceed your required strength.
As the slot and hole dimensions are of fixed, standard sizes, there is limited scope for varying the head dimensions. A choice between ends outers or ends inner finish will determine the vast majority of anchor head dimensions.
This means that the surface area in tensile and shear are fairly consistent from our experience. Using that area to calculate strength we find a material between 800 to 900 N/mm2 is normally required – we start our calculations using 850 N/mm2. Using this material will provide enough strength for the bolt so that you do not need to complete any strengthening operations after production.
If you are designing a threaded leaf chain anchor the table below gives you an idea of the variation in strength achieved by different threads. (We have a full listing available).
A strong connection
The strongest connection is achieved when the leaf chain ends on inner links and fits internally to the chain anchor, ensuring the maximum number of sheer faces and the greatest tensile strength. This is the preferred method of connection.
If space is limited however, the leaf chain can connect to the anchor on the outer links with the chain rivet pin being the widest part.
Along with space-saving, the main benefit from using an ends outer finish is that you can save cost by using a smaller bar size for your anchor bolts. The problem with this approach is that you now have less material when calculating shear and tensile areas which will result in a weaker anchor bolt head. If the strength is lower than the required strength, then you may want to look at specifying a higher strength material or increasing strength of the part by through hardening the part. This will add cost to the part and may reduce the savings from selecting a smaller bar size.
You should be careful with how you connect the leaf chain to the anchor bolt. A connecting link is the recommended method of connecting the chain to the anchor for an ends outer finish. If normal anchor pins are used, the leaf chain outer plates need to be reamed out, this is often a manual process completed on a pillar drill or something similar. This will never leave the same precise, concentric, circular finish as tooling would do – meaning that the pin will not sit evenly, causing inconsistent wear and increasing the potential for fatigue-related issues.
Preventing unsafe use of elongated leaf chain
Leaf chain should always be replaced when elongated as a result of wear. To prevent prolonged unsafe use, you can limit the range of adjustment. Calculate the section of the chain which will articulate and therefore wear and keep the available adjustment length below this figure. For example, a 5000 mm long leaf chain (of which 3800 mm wraps the pulley) should have no more than 114 mm of adjustment.
Size of thread
Bolts with coarse threads have a larger pitch and a coarse threaded bolt is generally specified for most applications unless the application requires finer adjustments or a small increase in strength.
In this case, a fine thread may be a good option with an additional advantage that a fine thread is less likely to loosen with vibration. However, fine threads are more susceptible to damage and are not always great in high-speed assembly where they are more likely to seize.
In some cases, you may choose to heat treat raw materials to achieve the desired mechanical properties. But non-uniform heating and quench can cause problems such as residual stress, quench cracks, and deformation and/or distortion. To check for this, you may want to add some method of non-destructive magnetic flux crack detection to ensure treated parts have no hidden flaws.
You will also need to consider any subsequent surface treatments like zinc plating, as some coatings contain elements that can affect the bolt. In particular, some will add hydrogen to the carbon in the steel, which can cause it to become more brittle. It is considered that the risk of hydrogen embrittlement significantly increases in high strength steel that has a tensile strength 1000 N/mm2 or above.
Designing leaf chain anchor bolts is perhaps a little more complicated than you might imagine, but if you take these considerations into account, a good, safe design can be achieved.
If you are in any doubt as to the best solution for your design, or just need some help, then you can always talk to our experts who will be happy to help you with advice based on our many years of experience.