We recently had two cases of food producers experiencing problems with POM (also known as Acetal or Polyacetal) plastic modular belts fitted with POM rods that were making a squeaking sound. Both belts were in use for applications in dry environments. So I thought it would be good to look at the issue of material combinations and explain our recommendations.
Understanding the rules
Engineers know that as a general rule, using the same material for parts in moving contact with each other should be avoided. The main reason for this is that it can lead to excessive wear. This rule is also valid for the rod module combination in a modular belt.
Why squeaking occurs
In addition to wear, there’s also another issue: if POM slides on POM, it makes a squeaking noise. But how does this happen?
Looking at friction measurements at very low speed, it is clear that the friction curve goes up and down like a saw tooth. This is accompanied by a ticking noise at every step. This noise comes from the slip-stick phenomenon. Slip-stick means that the movement of two surfaces alternates between sticking to each other and sliding over each other. Slip-stick usually happens when the static friction is higher than the sliding friction, and it mostly occurs at slow sliding speeds with high contact pressure.
When not to use a POM belt with POM rods
Even if movement between the parts is only in the area of the drive and along the return section, in general it’s not a good idea to combine POM with POM. As described above, not only wear but also squeaking can occur. Both issues are often present if the belt is running under dry conditions – as in our problem cases. It’s a situation that quite often arises in bakery applications, for example.
What if the belt is always wet and/or the product releases fat? In this case, the problems disappear – and you can use a POM belt with POM rods. There are many examples of this good solution for wet or fatty applications in meat processing plants.
Why a POM belt and PA rods work well together
To avoid problems in dry conditions, the best combination is a POM belt with PA rods. This offers very good wear resistance, which in turn delivers a very long belt life, with reduced maintenance and replacement costs. But while this is an excellent combination for dry applications, it is not the preferred solution when the belt is running wet.
When POM belts with PBT rods are preferred
Even in applications that are not very wet, PA rods could lead to rod elongation, because PA is hygroscopic. To avoid this, we recommend PBT rods as the best choice for these applications, although there are also price considerations involved.
Can POM sprockets on a POM belt cause squeaking?
When a sprocket tooth touches the belt engagement position at low speed and with high pressure. The movement is stopped because of high static friction. Due to the sprocket rotation, the forces on the belt change direction, causing the sprocket tooth to slide further onto the sprocket position on the belt. This slip-stick happens several times until the sprocket tooth arrives at a position where the static friction is higher than the forces trying to move the tooth further. This would normally create noise as well, which starts at a certain load and increases with a higher load. However Habasit uses pre-lubricated POM that prevents the squeaking due to the lubricating effect, a solution that would be too expensive to use for POM rods.
If you’d like to read more about plastic modular belt materials and characteristics, see my previous blog on this topic: http://blog.habasit.com/2017/08/material-selection-when-using-a-plastic-modular-belt/
And if you want to be sure you’ve chosen the correct rod or belt material, you can always contact your Habasit specialist to discuss the optimal materials for your application. Don’t forget to supply all the details about the environment: it’s only with a total overview that we can give you the best advice.