Sunday, November 21, 2010

Know How: Ball Bearings

One of my favorite science fiction books when I was a kid was Jules Verne’s The Mysterious Island. It’s been a few decades, but I seem to recall a party of intrepid adventurers being stranded on a volcanic island, where they managed to reconstruct modern (1860) technology more or less from scratch. There are a few reasons why this wouldn’t work, starting with the fact that volcanic islands don’t have iron ore, but the problem is still interesting. The modern world is utterly dependent on thousands or millions of technological devices, so I thought I might start finding out how a few of them were made.

One pretty important invention that we probably don’t think much about is the roller bearing, but it plays a critical role in all sorts of machinery. Marble players and others have long admired the shiny and seemingly perfect spherical roller bearing. So how do you make one?

If you have played with wax or clay, you probably have a clue. If you roll that piece of wax between your palms back and forth, it assumes a cylindrical shape, and if you throw in some crosswise motion, it will become roughly spherical. It’s pretty obvious what’s going on: the pressure of your hand pushes down the bumps and tends to fill in the holes.

How stuff works has an excellent description of how ball bearings are made, but it leaves out explanation of one key detail. After rough shaping:

Next the balls go into a machine that removes the flash. This machine rolls the ball between two very heavy hardened steel plates called rill plates.

[See link for pictures of the rill plates and machine]

One rill plate is stationary and the other one spins. The plates have grooves machined into them that guide the balls around in a circular path. You can see that one of the plates has a section cut out of it; this is where the balls enter and exit the grooves. When the machine is running, the grooves are completely filled with balls. Once a ball has traveled through a groove, it falls into the open section in the plate and tumbles around for a little while before entering a different groove. By making sure the balls travel through many different grooves, all the balls will come out of the machine the same size even if there are differences between the grooves.
As the ball travels through the groove, it spins and tumbles, the rough edges get broken off, and the ball gets squeezed into a spherical shape, a little like rolling a ball of dough between your hands. This squeezing of the balls compresses the metal, giving the balls a very hard surface. Because the balls are metal, this operation generates a lot of heat, so water pours over the balls and plates to cool them.

Why spirals, we might ask? This detail is crucial. On the spiral path, the inside contact point of the ball rolls along a shorter path than that on the outside. Consequently, it is continually slipping and changing its rotational axis as it rolls and the lines of contact wander over the whole ball rather than tracing out a circle. Thus the smoothing action affects the entire surface of the sphere. Fine grinding and polishing phases use the same sort of rill plate plus some grinding and polishing compounds.