This is a replacement wheel bearing for the Pro-Cart AT1-2014 wheels. It is made from machined steel and hardened so it can handle extreme use.
The wheel bearings may need to be replaced if they are damaged by overloading or shock loads (such as a piece of stone dropped on it, or in the event the cart is dropped or falls from a truck).
We don't see this very often, but when we do, it is usually caused by a cart that was overloaded at one time (causing the initial damage) and then continued use overtime causes further damage.
Please be sure not to overload the Pro-Cart, and always inspect it for issues prior to use.