Ball bearing hinges may have two, three or five knuckles. They differ from plain bearing hinges in that they have a steel ball bearing, or buffer, in between the knuckles. Standard weight ball bearing hinges have bearings only at the top and bottom knuckles, so a five-knuckle hinge would have two bearings. Heavy-duty ball bearing hinges have bearings between every knuckle. These units tend to be durable, better performing and less noisy than standard hinges.