Physics: Principles and Problems

Chapter 13: States of Matter

In the News

Lubrication

January 2005

Why things (including pieces of candy in your pocket) grab each other

Solid objects are solid because their molecules exhibit very strong molecular forces and thus stick together in a rigid structure. These intermolecular forces do not stop at the surface of the solid, but tend to attract other molecules which come into contact with that surface. Depending on the chemical nature of the two substances, a solid will grab onto the molecules of a liquid or another solid. The strength of the bond is very dependent upon the areas of the surfaces in contact.

Now, consider the situation of a surface molecule that is suddenly contacted by and surrounded by molecules identical to itself. It loses its status as a surface molecule and attracts all surrounding molecules with equal, strong force. Thus do identical materials which come into close contact tend to weld into a single piece. Two caramels will do this because they easily distort and so fit together closely over a large area.

Metal parts grab each other like caramels (but they're harder to chew):

Metal parts in an automobile engine act the same way. Two pieces of metal, if they are quite smooth and placed in a vacuum, will weld into a single piece when pressed together. The vacuum is usually necessary to exclude the air molecules which cling to all surfaces, though sufficient pressure will squeeze out air and weld two metal parts together.

Screech, screech, squeak:

Such 'pressure welds' are responsible for wear in an automobile engine. A microscopic examination of the metal parts shows why: the surfaces of even smooth parts are actually quite rough, and when pressed together only peaks on the surfaces touch. Because their area is very small, these peaks which touch will find themselves under enough pressure to squeeze out the air between them, distort into intimate contact, and weld together. As the parts are moved over one another, these microscopic welds are continually made and then broken. Some force is required to break these welds: we see this force as friction.

How an engine winds up in the junkyard:

As these tiny pressure welds are made and broken, small pieces of metal break off of the surface, causing the surface to become rough. Some combinations of metals, notably bronze and steel, are relatively immune to this pressure welding and are thus frequently used for surfaces that will come into rubbing contact. The heavily-loaded steel crankshaft in an automobile engine rotates in bearing shells made of an alloy of copper, lead and silver.

Check your oil whenever you buy gas. Here's why:

But even these bearings must be lubricated. A lubricant is a substance, generally liquid oil, which will wet and thus cling to the metal surfaces. The oil molecules are large and thus tend to prevent pressure welds between the parts that they separate. As long as the oil film is maintained and the oil itself is changed occasionally so the chemical action of metals and fuel residue does not contaminate it, the parts can move over each other for years without wear.

EP grease is in your CV joints

Under very high pressures the oil film will squeeze out, allowing the parts to touch, weld together and be damaged. If such high pressures are anticipated, as in a race car's transmission, or the axles of your own car, we use special 'extreme pressure' or “EP” lubricants. Chemically-active ingredients in these (molybdenum disulfide is one) react with the surface of the steel to form a tough layer of iron-molybdenum compound on the surfaces of the parts. This layer will not permit pressure welds to form.

Why would you polish an airplane?

Even gases tend to stick to solids. This is an important consideration in the design of aircraft and the fastest race cars because gas molecules which stick to the vehicle skin tend to pull neighboring gas molecules along and thus cause drag. For this reason, aircraft and racecar finishes can significantly affect the performance of these vehicles.

Activity:

Turn a bicycle upside-down. Crank the pedals so that the rear wheel rotates quickly. Feel the wind generated by the tire. The tire is relatively smooth. How can it produce wind?

Ask a mechanic for an automobile brake disk that was damaged because it came into contact with the metal portion of the brake pad. See if you can find the places where the pad and the disk welded together and then broke apart.

References:

http://www.molyslip.co.uk/theory.html

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-1424/c-2.pdf

http://www.batterystuff.com/lubrication/lubrication_tutorial.htm