There are many advantages of using polyurethane wheels. However, using poly wheels beyond the specified limits can cause them to fail due to several reasons. The next time you have casters and aren’t sure what the bearings are in buttermilk, I would check the grease shock. If the axle or wheel shocks, the industrial castors do not have ball bearings. The next step is to put the wheel on it without weight. If the wheel spins freely then you have a roller bearing. If the wheel is stretchy and hard to spin, you have a taper bearing. I hope this helps you determine what bearing your caster has on your wheel.
The main culprits of polyurethane wheel failure are overloading, overheating and bond failure. Overloading and excessive speed are the two main reasons polyurethane fails. There is another type of failure (which rarely occurs) called bond failure. Below I will explain about these types of failures and their causes. Knowing why a wheel has failed gives you a better understanding of the situation. So, when the time comes to replace it, future failures can be eliminated by using the correct version.
When polyurethane wheels are overloaded or used to create shock loads greater than the polyurethane rating, they fail. This failure usually does not occur immediately. When the wheel is overloaded, the internal stress in the poly causes small tears. Constantly using an overloaded wheel means that a break or tear of the poly will not help spread the load and put more stress on the surrounding poly.
This excessive stress creates more tears and breaks. This condition continues until the tears work their way through the poly and cause failure. Different polyurethane types have different physical properties, so overloading failures may appear differently. The material in Figure A shows internal cracking within the poly, while Figure B shows sidewall cracking.
Heating the polyurethane wheel
Another type of polyurethane failure occurs when the material gets too hot and melts. This occurs when the wheel is driven faster than its rated speed or is used in an environment where it overheats due to the ambient temperature and speed.
Once the material overheats, it starts to melt from the inside. Once the material begins to melt, and the cycle continues, more poly is melted until a path is formed on the surface. Once the molten poly reaches the surface, the load on the wheel pushes the material off the rolling surface or sidewall. Figure C shows a wheel that has become hot enough to melt the polyurethane, come off the rolling surface, and cool back down.
The ultimate failure to be covered is not failure of the polyurethane material itself, but failure when trying to adhere the polyurethane to another material. This type of failure is less common, but understanding it will help determine why the wheel may fail.During the manufacturing process of adding polyurethane to the wheel core, an adhesive sticks the two materials together. This connection is called a bond, which can fail for various reasons. When the bond fails, the poly material separates from the wheel core.
It shows an extreme case where the poly material does not adhere to the wheel core over a large area. This is notable because there is no poly material on the wheel core when the poly is released. There are varying degrees of bond failure – from the entire poly material leaving the wheel core to no poly material remaining – to a wheel core in which the poly is still attached everywhere except in certain areas. Another type of bond failure can be caused by heat increasing the strength of the bond over time.