Using a polyurethane wheel has many benefits. However, using poly wheels outside their rated limits can cause them to fail for several reasons.
Overloading and excess speed are the two main reasons why polyurethane can fail. There is another type of failure (which rarely happens) called bond failure. Below I will cover these types of failures and their causes. Knowing why a wheel has failed gives you a better understanding of the situation. So, when it is time to replace it, using the correct version can eliminate future failures.
When polyurethane wheels carry more weight or are used to create shock loads greater than the polyurethane rating, they begin to fail. This failure usually doesn’t happen right away. When the wheel is overloaded, the internal stresses in the poly cause small tears. Continually using the overloaded wheel means the breaks or tears of poly no longer help disperse the load and overstress the surrounding poly.
This overstressing creates more tears and breaks. This situation continues until the tears work their way through the poly and cause a failure. (Shown in Figures A & B). Different polyurethane types have different physical properties, so that overloading failures can look different. The material in Figure A shows internal breakage within the poly, while Figure B shows sidewall cracking.
The next type of polyurethane failure happens when the material becomes too hot and melts. This occurs when the wheel runs at speeds faster than it’s rated for or is used in an environment where surrounding temperatures and speed cause it to become overheated.
Once the material becomes overheated, it begins to melt from the inside. Once the material starts to melt, and the wheel continues to be used, more poly melts until a path is made to the surface. Once the melted poly reaches the surface, the weight on the wheel forces the material out of the rolling surface or side wall. Figure C shows a wheel that became hot enough to melt the polyurethane, come out of the rolling surface, and cool back down.
The last failure covered is not a failure of the polyurethane material itself but a failure when trying to adhere polyurethane to another material. This type of failure is less common, but understanding it will help determine why a wheel might have failed.
During the manufacturing process of adding polyurethane to a wheel core, an adhesive adheres the two materials together. This connection is called the bond, which can fail for various reasons. When a bond failure occurs, the poly material becomes detached from the wheel core.
Figure D shows an extreme case where the poly material did not adhere to the wheel core in a large area. This is noticeable due to no poly material on the wheel core when the poly broke free. There are varying degrees of bond failure — from the entire poly material coming off the wheel core leaving no poly material behind — to a wheel core with poly still attached everywhere besides a couple of areas. Another type of bond failure can occur due to excess heat degrading the bond strength over time. The most significant factor in identifying a bond failure is looking for areas on the wheel core with no poly attached. Looking back to Figure A shows what a good bond should look like when a wheel fails. As you can see, varying degrees of material thickness remain on the wheel core.
Knowing what caused a wheel to fail will help you understand what is happening in your application. This will also help our sales department determine what wheel size and material should be used to replace a failed wheel. In addition, it’s also important to choose a manufacturer like Caster Concepts that is ISO 9001-2015 certified and selects and processes your polyurethane in-house to mitigate these types of failures.
If you have any other questions regarding poly failure, please get in touch with your Caster Concepts Representative.