FRICTION, ENGINES AND OILS

Press, Volume CVI, Issue 31123, 28 July 1966, Page 14

FRICTION, ENGINES AND OILS

VRICTTON can be both: A friend and enemy to thej motorist Friction between the; tyres and the ground keeps’ the car on the road; friction! in clutch linings, pressure! plate and flywheel feeds power to the gearbox, and friction between the brake linings and the drum stops the car. But friction can be undesirable. It can burn out bearings, cause rapid wear, and even cause engine seizure. Lubrication and friction are opposite sides of the same coin, say Caltex Oil engineers in a technical handbook. Friction is resistance to motion, and lubrication is the over-; coming of this resistance. A demonstration of friction Is rubbing your hands together to warm them. A silver coin rubbed vigorously on fabric will become too hot to touch. These examples show how friction can overheat a bearing if it is not properly) lubricated. THREE TYPES There are three types of frcition, dry, boundary and: fluid. Dry friction occurs when there is relative motion between two surfaces in intimate contact, with no third material between them, for instance when two metal surfaces are not lubricated. The molecules of the metals are so close together that they tend to adhere to each other. Molecules of the softer metal are torn away, either sticking to the other metal or working free. The ultimate result is the familiar damage found in cases of lubrication failure. The metal is scored or scuffed, and the bearing surface is ruined. Metal surfaces always have microscopic uneveness “hills and valleys”—even when apparently polished to a mirror finish. When two surfaces slide against each other, with no lubrication, these hills and valleys catch on one another.

LUBRICATION The aim of lubrication is to provide a film between the two surfaces that will prevent them coming into contact The amount of friction between two surfaces is expressed in terms of the “coefficient of friction.” This is the ratio between the force required to maintain movement between the surfaces and the total weight on, or pressure between, the surfaces.

For example, if a push of 101 b was required to maintain motion, and the total pressure between the surfaces was 20lb, the coefficient of friction is 10 divided by 23, or 0.5.

The load on the wheel bearings of the weight of a vehicle requires a force to start the vehicle moving. The ratio of this force to the total weight on the bearings is the co-efficient of friction., With really dry friction, the co-efficient can be anything from five to 10. Boundary lubrication can drop this to 0.1 to 0.3. Full lubrication drops the value below ,0L BOUNDARY Boundary friction occurs when the valleys in the metal surfaces are filled up with lubricant This occurs on the cylinder walls and piston rings of an engine started from cold. At first this thin film of boundary lubrication is all that prevents dry friction and damage. As the engine warms

i up, oil from the crankcase : flows and gives thick-film lub- ■ rication. j FLUID i Fluid friction exists within a lubricant itself. It is the I friction between the mole- ! cules of the lubricant as they I rub against one another. It I is the resistance that one part of the fluid offers to the flow lof another part of the same i fluid. Even though it occurs in the lubricant itself it is still ■ friction, and thus it generates heat. Thicker (higher i viscosity 1 fluids generate jmore fluid friction and heat than thinner fluids. Thus, the most important single property of an oil, from the point of view of fluid friction, is its viscosity. The same applies to a grease. It is possible for fluid friction in a thin film of lubrijcant to become sufficiently 'high to cause a significant temperature rise. This, in I turn, reduces the lubricant j viscosity so much that bound.ary lubrication occurs. | Under boundary lubrication conditions, the moving parts are in partial contact, causing further heating. Again, this reduces viscosity still further, increasing metal-to-me tai contact. The vicious circle continues until the bearing fails. OILS DIFFER Oils of the same viscosity can vary greatly in their ability to lubricate under boundary conditions. This is because it is not the bulk of the oil which overcomes friction in these conditions, but the molecules of oil that are so constructed that they have an affinity, or attraction, for the metal surfaces. They cling to the surfaces while the rest of the oil flows around them. When the engine is stopped, the polar molecules, as they are called, remain to form a protective film when the rest of the oil drains away. The more such polar molecules an oil has, the more “oiliness” it possesses. Modern engines operate under more severe conditions than ever before, and thus call for better lubricants. A 1965 engine run on the oils of 20 or 30 years ago would soon fail.

To produce satisfactory lub-j ricants, it has become neces- | sary to fortify straight mineral oils with “oiliness” additives. These additives greatly reduce wear in boundary conditions, and other additives are used to prevent sludge formation, foaming, oxidation, and other undesirable tendencies. EP OILS Some operating conditions are so severe that the usual additives are not adequate. Such conditions occur in modern hypoid differentials, the gears of which have a com-1 bined rolling and sliding ac-l ,'tion, under high pressure. Temperatures become so high that the usual “oiliness” additives are melted off the i gear teeth. For this reason special “extreme pressure” (EP) additives are used. This explains why differen- ; tials must never be filled [ with any oil but the stipui lated EP type. . EP additives are special i chemical compounds, usually I containing sulphur or chlorine, which react with metal f surfaces at very high temI peratures. Thus the temperal tunes that would break down > normal additives actually help

by activating the EP additives. The additives, when they react, form a protective film of iron sulphide or chloride which protects the metal surfaces, and prevents dry friction. Enough additive is put in the oil to replenish the EP “coating” when it is worn away. A good EP oil is continually conditioning the gears. MOLY Certain solids may be used for lubrication in cases where extremely high temperatures would burn away organic materials. These include graphite and molybdenum disulphide. Both are very heat resistant, and "moly” offers a low coefficient of friction when baked on to metal surfaces. When these solids are of the correct particle size, and suitably applied, they help in preventing metal seizure.