Ball Bearings: Battling Friction

6/16/2017

There are few things in this world that are as simple as they are crucial to our civilisation; and these things exist as a solution to an old problem. To fight gravity, we have pulleys; for multiplying force we use levers; and so on...

But in our modern times, the vexing problem of friction are the things that keeps engineers awake at night. And a very simple tool that have not essentially changed in centuries are the best things we have to keep it at bay...

"Friction is necessary for motion, labor necessary for birth."
Mimi Kennedy; actress, author, and activist

The idea of using a rolling element to move heavy items dates back to ancient Egypt. The Egyptians used logs to roll their large stone pieces closer to the construction areas when building the pyramids.

Da Vinci's prototype wooden bearings

At first bearings were manufactured of Lignum Vitae, a very heavy, hard, naturally oily wood native to Central America and the West Indies. The natural oils in this wood assisted in the manufacturing process by acting as a cutting fluid. These bearings are best known for “wet” applications such as propeller driven vessels, water wheels, and pumps. Wooden bearings were known to be long wearing, strong, readily available and easy to replace. They were lubricated with tallow or other animal fats. In many turbine type applications you will still find wooden bearings, however Lignum Vitae is not as readily available as it once was. Many companies are using maplewood impregnated with petroleum wax as a suitable substitute.

Leonardo da Vinci, famous for his painting and drawings, also had many ideas for mechanical engineering projects. Many of his drawings were of pumps, hoists, cranes, and various weapons of war. During his employ as a hydraulic engineer serving the Duke of Milan, he spent much of his time analyzing bearings, linkages, gears and various other mechanical transmission modes. Many of Da Vinci’s ideas are still celebrated in the engineering world today.

With the 1700s the changes in manufacturing processes were changing the way people lived and worked. Iron was becoming more popular and was replacing wood in many factories. With new progress in manufacturing there was also a need for more precise machine tools. The wood turning lathe is known to be the oldest machine tool, and in the mid 1700s, innovations in iron allowed for the production of more precise machine tools. With new inventions, there came a need for more sources of energy to power these machines. The steam engine became a practical source of power with the invention of the boring machine. This enabled the growth of industry and the greater need for machinery to be built, which led to new styles of bearings required in the building of these new machines.

With the development of new styles of bearings, came the need for new materials to make bearings from.

In 1839 Isaac Babbit invented an antifriction alloy with a low melt temperature. This alloy could be formed and molded to produce an ideal surface for bearings. With the introduction of this “Babbit metal”, the use of wooden bearings diminished slightly. In the latter half of the 1800s, new steel making processes were created by Henry Bessemer (1813-1898). His new process allowed steel bearings to be made much more economically.

With the inventions of the 1900s including motorcars, robotics, computers, and the newer, faster machine tools, bearings have become more significant to production lines. Newer materials have enabled us to produce bearings at a lesser cost to the consumer.

Materials used in bearings are also used in common everyday living. One example is polytetrafluoroethylene, commonly known as PTFE, the nonstick coating in many pots and pans. Bearings are now made with a variety of metals, plastics, and, in some cases, wood is still in use.

How They Are Made

If you have ever rolled a couple of those little metal balls found in a ball bearing around in your hand, you have noticed how perfectly round and incredibly smooth they feel. You might have wondered how anything could be made that perfect. It's actually a pretty neat process that starts with a metal wire and ends with a perfect shiny ball.

The first stage in the process is a cold or hot forming operation. A wire of metal approximately the diameter of the finished ball is fed through a heading machine. This machine has a metal cavity the shape of a hemisphere on each side. It slams shut on the wire forcing the piece of metal into the shape of a ball. The process leaves a ring of metal (called flash) around the ball, so the balls coming out of this machine look something like the planet Saturn.

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.

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.

The variables in this process are the pressure that squeezes the plates together, the speed the plates spin and the duration the balls are left in the machine. Properly setting these variables will consistently produce balls of the correct size.

After this operation the balls may be heat-treated. This hardens the balls, but it also changes their size. The size of bearing balls has to be perfect, sometimes within millionths of a centimeter, so a few more operations are needed after heat-treating.

The balls next go through a grinding operation. The same kind of machine is used, but this time the coolant contains an abrasive. The balls travel through the grooves again and get ground down and compressed to their final dimensions.

Finally the balls go through a lapping operation. Again, the same kind of machine is used, but this time the plates are made of a softer metal, and the machine uses much less pressure to squeeze the plates together. Also, the machine uses a polishing paste rather than an abrasive. This process gives the balls their perfect smooth shiny surface, without removing any more material.

The last step in the process is inspection. The balls are measured with very accurate machinery to determine if they meet the required tolerances using laser scanning micrometers which accurate to within millionths of a centimeter.

Manufacturers use a very similar process to make metal pellets for air guns, plastic balls for bearings and even the plastic balls used in roll-on deodorant.

Ponder this

Why are there gaps between the ball bearings? Would they work better and bear less friction if they are in full contact with each other? Why or why not?

Although history has shown the use of rollers, what prevented the development and use of spherical bearings until the 16th century?

Discuss

Friction is both essential and unwanted in mechanics. As the quote above mentions, without friction there can be no motion; however in mechanics, friction will lead to heat as well as mechanical wear and tear. Are there any emerging technologies that would accommodate both the need for friction as well as the presence of its undesirable traits?