Printer-friendly version

08 August 2003

Tribology: What is it, and what does it do for us?

By Marty Carlson

Anichkov Bridge, St. Petersburg, Russia

A Brief History

Tribology comes from two Greek roots; tribos, friction, and the ubiquitous suffix -ology, the study of. The study of friction. This was very important back in the old days. For one thing, their chariot wheels regularly got hot and occasionally even fell off in the heat of battle.

It was known that slippery things, such as fat from animals, eased this problem and a profession grew out of that: the tribologist. For generations master tribologists passed their knowledge and secret recipes down to their students. Those recipes might include certain ground plants, for instance, to keep the oil stuck to the axle or to stop the animal fat from smelling badly so soon.

Fundamentals

There are many characteristics of a lubricating oil that are important. The first consideration of a lubricating oil is its viscosity, or thickness. This determines how well it flows. A small pump with a highly viscous oil wouldn’t last very long.

Stability is important too. As an oil is degraded through heat and chemical interactions with air, water, suspended metal, metal surfaces, and such-like, its viscosity can change. It can get thicker if the molecules are joining into bigger ones, through polymerization for example. It can get thinner if they are breaking apart into smaller molecules, through hydrolysis for example. Additionally, the acidity of the oil can change, causing corrosion to metal surfaces. These conditions must be controlled, and this is often done with the use of chemical additives.

Response to temperature is important, as anyone starting their car on a cold winter morning in the North can attest to. If your car’s oil were a solid on those minus thirty degree mornings in Northern Minnesota, you would be quite regularly late to work. This can be estimated by a characteristic called the "pour point". Additionally, as an oil heats up it gets thinner, or less viscous. If an oil becomes too thin or too thick it will result in turbulence and inefficiencies in the flow, which will result in unprotected surfaces, and damage. So again, these must be controlled, often with the use of additives.

The wear characteristics, or how well the oil protects the contact surfaces are extremely important of course, since this is the reason for using them in the first place. More on this next.

Technology Today

1993 Spartan II

Today’s tribologists face a more difficult task than that of keeping two loose metal bearings from fusing together. In a piston hydraulic pump, for example, tolerances may be as tight as a hundred thousandth of an inch. Metal surfaces must be very smooth and predictable indeed at these tolerances.

To combat the wear that can occur under these circumstance we can add chemical additives to oils. A small amount of zinc or phosphorus, for example, may have the effect of placing small beads on the surface of the metal, which "roll" along another metal surface like nano-sized ball bearings. Alternatively, a molecule with a long chain, such as sulfurized vegetable oil, might be "attached" to the surface of the metal to be protected, where it will act like the smooth "fuzz" of a peach.

Molecular ball bearings, and molecular fuzz

The complexity, which the early practitioners of tribology could not have foreseen, is demonstrated by a question that might occur to the attentive reader. What lubricates the ‘molecular ball bearings’? This is a truly excellent question and in fact we do see synergy when using some combinations of these types of additives.

Conclusion

This is just sliding along the surface of all that is happening in that can of oil you bought at the store last week. At first glance tribology may seem as simple as wiping animal fat on chariot wheels. However, as one approaches this field of study from the point of view of understanding these interactions, a puzzle worthy of the greatest of minds unfolds. Yet because the results of basic physical tests can illuminate much of the chemistry that is occurring, it is a good fit for the imaginative Amateur Scientist.