Jon Steiner, age 10, of Esko, Minnesota, for his question: .
How do they make ball bearings?
By the age of 10 you have graduated from beginner's roller skates to the real thing. You have learned the basic skills and can cope safely with the super roller skates that roll so much faster and farther. These rollers get their super roll from sets of ball bearings in their wheels.
Ball bearings do their smooth, hard work behind the scenes. We see them only when time comes to remove them from the heart of a wheel and replace them with new ones. They look for all the world like perfect little round metal marbles coated with greasy film. And so they are. Your bicycle wheels must make smooth, even turns or the ride will be as bumpy as a bronco. The bike may have six sets of ball bearings that keep things running smoothly. These greasy little metal marbles also prevent the moving parts of the machine from chafing and becoming hot.
The making of ball bearings is a special job because they have to be as near per¬fect as possible. They must be strong enough to support the heavier axles and other machine parts that move around them, but they must not be strong enough to press these moving parts out of shn;c wear them out. They must be perfectly round or the circles that move around them will be bumpy. The ball bearings made for auto wheels must be stronger than those made to support the pressure you put on your bike or roller skates.
Many ball bearings are made for different jobs, but the basic process is more or less the same. The outside skin is made of the finest steel, especially refined in a super electric furnace. Steel is slightly elastic. Under pressure, it gives and springs back into shape. The steel for ball bearings gets extra pounding, kneading and rolling to make sure that its ingredients are evenly mixed. And the mixture must be molded in an even thickness. A steel tube is molded over a circular furnace and cut into perfectly exact lengths to make the ball bearing skins. Then other machinery molds and presses the sections into perfect shapes, grinds and polishes them smooth and shiny.
The stuffing for ball bearings is not as hard as their springy steel skins and much softer than the axles and other machine parts. An alloy mixture called Babbitt metal is used for this job. There are several recipes for making Babbitt metal, de¬pending on the pressure that the ball bearings will have to take. As a rule, about 90% of the alloy is tin. Other ingredients may be lead, copper and antimony. The finished alloy is firm and durable, but soft enough to yield under pressure. The finished ball bearings will be arranged in perfectly matched sets and placed in metal frames. They are separated but each will roll around in its slot. As it rolls, it touches an axle or other machine part and moves it smoothly around.
Shoving a heavy box is hard work because friction with the floor exerts a drag. But if you strew a few marbles on the floor, the box will roll merrily along, maybe with no help at all. A wheel turns around a central axle but friction between the moving parts creates a drag and also heat. Ball bearings around the axle act like little marbles. As they roll merrily around, the wheel rolls smoothly with them. Even specks of dust would upset the operation, so ball bearings are demagnetized to shoo them away. They are also coated with slippery smooth lubricating oil.