A Guide to Pulley Design
Pulleys, also known as sheaves, are an essential item in a leaf chain system. It is a crucial moving part as an incorrectly designed pulley will have a big impact on performance and the working life of the equipment or the chain, or both. Pulleys should be designed to allow the chain free movement as the ram moves up and down. Ideally, they should be designed to last the lifetime of the machine they are being installed on (unless design criteria dictate otherwise).
Design Rules of Thumb:
Just like a leaf chain anchor, the basic dimensions of your selected chain will dictate the dimensions of the pulley. You can find recommendations in ISO 4347 – Leaf chains, clevises and sheaves — Dimensions, measuring forces, tensile strengths and dynamic strengths.
When designing a pulley, I start with the running diameter which is the surface that the chain wraps around. The recommendation is 5x pitch of the chain, so if you had a ¾” (19.05mm) pitch chain, then the running diameter of the pulley should be 95.25mm (5 x 19.05mm).
Chain articulation and movement is a principal factor in chain wear. The smaller the diameter of the running area the more articulations of the links. As can be seen from the image below. Decreasing the running diameter size will increase chain wear by a significant amount.
It’s possible that your application may not have space for a large pulley, or the planned chain alignment may dictate your pulley size – as you will want to ensure you have a linear pull on the chain. This may also affect your chain choice as you may want to decrease pitch so that you still maintain a good wear rate. Please see our e-book related to telehandler design.
If this is the case, you will need to take that into account and allow for more maintenance when you write your planned maintenance schedule.
Once you know the running diameter (RD) you should then calculate its overall diameter. Add the plate height of the chain to the RD, this will leave half a plate height of chain on either side of the running circumference which will be enough material to stop the chain from leaving the pulley if any type of bump or collision causes the chain to move.
The rivet pin is the widest part of the chain and by applying this calculation to its dimension it will ensure there is enough room either side of the pin so that any movement of the chain from side to side will mean that the pin does not hit the pulley flanges. The dimension between the pulley flanges should be 1.05 x rivet pin length of your chain. The flanges are recommended to be angled by 15 degrees – see illustration below.
Depending on your space constraints the example given may be appropriate, but you could vary these dimensions to slim the pulley width down if necessary.
Material:
Chain links are hardened to between 40-48 HRC. Any material or surface that is below these values will wear as the chain moves across it. You could choose to use a hard material already within the ranges stated. But this will be difficult to machine and may not be cost-effective. A forged component could work, but you will still need to machine the bore accurately afterwards.
We choose to use low carbon steel and heat treat the component up to the required values. This allows for easy machining before heat treatment. Pulleys are made from a larger material bar and by reducing material spec you can lower cost.
If you follow the heat treatment recommendations, then you will need to choose a material that can be heat treated to the required range. If you choose not to harden your pulley, then you could choose a material that is already the required hardness, or you could choose to use plastic or Nylon. If you choose to use a hard material, then this will increase the amount of machining. Nylon or plastic is very soft and tends to wear quickly with plastic swarf becoming an issue. It comes off in spoils and gets caught in the chain.
Our recommended materials for heat treated components would be EN3B or EN1A. These materials are ideal for heat treatment and the required range.
Heat treatment:
Dependant on the material you have selected you may need to heat treat your pulley. We would recommend that the surface hardness of the pulley be greater than that of the chain. So that the chain wears over time and not the pulley. The pulley being the more expensive item to replace.
I would recommend carburizing the pulley between 50-55HRC. There is no need to through harden these parts as it would add to the cost and isn’t necessary.
Bushes:
Bush selection is important. With the majority of leaf chain applications, the speed of the pulley is slow, the load is high and consistently smooth rotation is required. If the pulley does not turn freely and sticks this will induce shock loading conditions into the chain whilst also making the load bounce. The travel of pulley depends on each application and this will have an impact on your selection. You will also want to consider the environment that the machine is to be used in. Dirt or foreign intrusive material could cause sticking issues or could interfere with the lubrication.
We manufacture a simple solution, a turned part with a sintered bronze bush. We find from experience that this suits the vast majority of machines but if your machine’s lifecycle is quite hardy or you are going to experience high loads you may want to look at other solutions. During this selection process you will need to consider what is known as the PV factor. This is a calculation that takes into account, speed, load and bearing surface area. As this whole process is quite technical and requires specialist knowledge, we can offer free advice to help you with the calculations or in sourcing an appropriate solution.
Sealed For Life:
Sealed-for-life leaf chain rollers increase service intervals and minimise machine downtime due to maintenance. The design also eliminates the need for the costly cross-drilled pivot pins with grease nipples on standard pulleys.
Conclusion
Getting the design of the pulley right is vital for the correct operation and longevity of leaf chain assemblies. This post should guide you through the process and provide useful working recommendations. Always remember that if you need any help or advice with pulley design or any other aspect of leaf chain assemblies, you can always contact us and we will endeavour to help.
