Automotive Lubricants Part I - Oil

Many hundreds of years ago someone figured out that fluids are not compressible. Therefore, fluid can be used to form a film between two moving metal parts, preventing them from touching together. That's pretty handy, since automobile engines have moving parts and we don't want them touching.

Keeping parts from touching doesn't mean we won't get heat from friction, but we can remove the heat as long as we keep the fluid (oil, of course) moving. A lubricating oil with the necessary properties and characteristics will serve six functions, among which is the removal of heat. Here they are:

1. Controls friction between load-bearing surfaces
2. Reduces wear by preventing metal-to-metal contact between moving parts
3. Limits the temperature by carrying away heat from fluid friction and combustion of fuel
4. Reduces corrosion by coating metal parts and by flushing debris from between moving parts
5. Dampens mechanical shock in gears
6. Forms a seal on the walls of the cylinders

That, in a nutshell, is what oil and other lubricants do in engines. This is how they do it...

First, The Theory

It was Irving Langmuir (1881-1957), American chemist, who figured out why oil lubricates. In his research on surface tension and surface chemistry he developed a new technique (employing monolayers, i.e., layers of molecules one molecule thick) for the study of molecules. According to his theoretical work, at least three layers of oil exist between two lubricated bearing surfaces. Two of the oil films are boundary films, a condition in which the oil film is neither so thin as to cause seizure nor so thick as to create a full film of oil between the shaft and the bearing. One of the boundary films clings to the surface of the rotating journal. The other boundary film clings to the stationary lining of the bearing. However, boundary film lubrication alone is not sufficient to protect metal surfaces from friction, so between the two boundary films are one or more fluid films that slide layer upon layer. This combination lubricates the parts and prevents seizure.
For his contributions in surface chemistry he received the 1932 Nobel Prize in Chemistry. (It was Langmuir, by the way, who discovered that the introduction of particles of dry ice and iodide into a cloud of low temperature containing sufficient moisture in tiny droplets triggered a chain reaction producing rain or snow, depending on the condition of the weather.)

Anyway, back to the lubrication process. In a bearing journal, for instance, as the journal begins to turn, the oil adhering to the surface of the journal is carried into a space between it and the bearing. The oil film increases in thickness and tends to lift the journal away from the bearing. (Remember, a liquid is incompressible.) As the shaft speed increases, the journal takes a position somewhere between the bearing surfaces and rides on the sliding layers of oil. This means that, in properly-spaced components where oil of the right viscosity is pumped at correct pressure, no mechanical wear takes place during operaton.

So What Affects Lubrication?

A number of factors determine the effectiveness of oil film lubrication. They include load, temperature, viscosity, flow rate, speed, alignment, condition of the bearing surfaces, running clearances, and purity of the lubricant. Many of these factors, of course, are interrelated and interdependent. All are taken into account by the engine designer before specifying a particular grade or type of oil, but there are fundamental properties that lubricants must possess, such as adhesiveness and cohesiveness.

A lubricant must stick (adhesiveness) to the bearing surfaces and support the load at operating speeds. More adhesiveness is required to make a lubricant stick to bearing surfaces at high speeds than at low speeds.

At low speeds, resistance from being squeezed out from between the bearing surfaces (cohesiveness) is required on the part of the lubricant. Generally, the greater the viscosity, the greater the cohesiveness. Large clearances between bearing surfaces require the use of a lubricant with a high viscosity and cohesiveness that will provide an adequate lubricating oil film. The larger the clearances, the greater resistance the lubricant must have so it will not be pounded out.

If the lubricant is pounded out, the lubricating oil film will be destroyed. High unit loading of a bearing will also require the use of a lubricant with a high viscosity. A lubricant that is subjected to high loading must be sufficiently cohesive to hold together and maintain the oil film.