The funny thing about what everyone knows is that everyone can be wrong. For instance, everybody thinks that the America's Cup is an ocean-sailing race, and yet the Swiss managed to win the Cup. For those readers who are geographically-challenged, Switzerland is a land-locked country comprised entirely of mountains.
What does this have to do with ball bearings. Very little, I suspect, but balls have very little to do with ball bearings, either. Ball bearings look more like hula hoops. But don't try using them for that – you will find them inconveniently heavy and painfully small.
You can view a picture of a ball bearing in the middle of this ball bearing supplier's page.
So what are those stunted metal tube donuts called ball bearings for anyway? Are they used as a spare wheel? Do they hold in evil shop-floor spirits? No, they help things move more efficiently. In an early demonstration of bearing usage, three ladies pulled a locomotive (It was just a demonstration, not a career development).
Many bearings look very similar, whether they are ball bearings, roller bearings or other bearings. What?! Other bearings?
What is a ball bearing, anyway?
Ball bearings are formed with an outer ring, an inner ring, a cage or a retainer inside, and a rolling element inside, typically a ball (which is why they are called ball bearings). Roller bearings are formed using a roller instead of a ball, which is why they are called roller bearings (Yes, finally something that makes sense!). Other bearings look just like metal tubes, called plain bearings or bush bearings. They look like sawed off pipe or tube (something my metal tube bending client would be turning into architecturally glamorous structural supports).
The principle of bearings is the same principle behind the wheel: things move better by rolling than by sliding. They are called "bearings" because they bear the weight of the object, such as an inline skate or the head of dentist's drill, allowing the object to glide over them with incredible ease and speed. Unlike wheels, they don't turn on an axel; they turn on themselves.
You can see this in action with some great cut-away pictures of bearings.
The balls or rollers spin on themselves inside the bearing, reducing friction for the machine parts attached to them. It's much neater than using a bucket of oil, especially in dental equipment, and significantly more reliable than hamsters on a wheel.
Once upon a time, all bearings were metal – like a metal tube or pipe with metal balls stuck inside. These days, more and more are made of ceramic or even plastic (like everything else in this world!).
If you are still confused about why ball bearings are not shaped like balls, just remember that you drive on a parkway and park on a driveway. And you can even try sailing your sea-craft through the Swiss Alps. But don't try playing a game of one-on one basketbearing.