O-ring for Ball Winder Flange
This is the black o-ring that goes around the large circular section of the spindle. It needs to be replace whenever it shows sins of cracking or other damage or wear and tear. It acts like a tire on the unit so it sees more wear than any other component. However, it lasts and lasts as well. Typical replacement rate for a heavy user such as a yarn shop would be yearly. Knitter at home - every 2-4 years. A lot also depends on the dryness in your location. Dryness dries the rubber out more quickly. The other thing that causes wear and cracking is use how tightly you have it pressing against the steel shaft. The tighter, the more stress on the rubber, the more cracking.
Just keep an eye on it and replace it every so often (never more often that yearly unless you are experiencing strnage wonky balls that could be caused by the o-ring.
Sold in pairs (2 o-rings).