Rolling Friction. When one body rolls upon another, the friction is much less than when one body slids upon another. The resistance in this case is called the rolling resistance or friction. This can readily be demonstrated by attempting to carry as much upon a sled which has no wheels as upon a wagon or other vehicle mounted on wheels. Many farm implements are now using some type of antifriction bearing in the form of balls or rollers to diminish the amount of friction, which materially increases the efficiency of the machine. Friction in moving parts of machinery causes wasted energy and it is, therefore, desirable to reduce it to the smallest possible amount. However, in clutches, or to prevent slippage of belts on pulleys, friction is necessary and useful.
Lubrication as a Remedy for Friction. Lubrication tends to reduce friction. The theory of the action of lubrication is that a thin film of the lubricant adheres to the bearing and another to the shaft, completely separating the metal surfaces. Then, these films slip one on the other, which reduces the amount of friction. This is because the friction between the films of lubricants is much less than that between the metal parts. A lubricant may act in different ways in reducing the amount of friction: first, by changing the greater resistance of metal to metal to the relatively small resistance of oil over oil; second, by filling up the small depressions in the two frictional surfaces and in this way preventing the so-called interlocking (Fig. 5–1).
Forms of Lubricants. Lubricants are available in three forms: fluid oils, semisolids, and solids. Fluid oils are those that flow freely, such as gas-engine cylinder oils and oils used for lubricating various bearings by means of oil holes or oil cups. Semisolids include the soft greases, such as transmission and differential grease. Solid lubricants consist of graphite and mica. Of these forms, soft greases and oils are most generally used to lubricate farm implements.
FIG. 5–1. How a lubricant keeps two pieces of metal apart. (John Deere.)
Kinds and Sources of Lubricants. All lubricants have three general sources: animal, vegetable, and mineral. Animal oils are lard, tallow, and fish oils. Vegetable oils are cottonseed oil, castor oil, olive oil, and linseed oil. Mineral oils are oils obtained by refining crude petroleum. Of all these, mineral oils are the most universally used on the farm, because they can withstand higher temperatures without breaking down.
Manufacture of Lubricants. The diagram in Fig. 5–2 shows the steps in the refining process and the points of extraction where fuels, light, medium, and heavy oils, and the extra-heavy lube stock are obtained.
SERVICE CLASSIFICATIONS OF AUTOMOTIVE ENGINE LUBRICATING OILS
Prior to 1952, automotive types of engine oils were classed as regular, premium, and heavy-duty. The American Petroleum Institute has set up a new system of service classification, designations, and definitions for automotive types of engine lubricating oils. The three basic factors involved are:
1. The type of engine
2. The kind of service
3. The nature of the fuel
The two basic engine types are the gasoline or other spark-ignition engines and the diesel engines. Oils for the gasoline and spark-ignition engines are given the designation of M, while the oils for the diesels are designated as D oils.
FIG. 5–2. Diagram showing refining process and the points where the oils and greases are obtained. (Standard Oil Company.)
Three kinds of service are set up for the gasoline or other spark engines. These are:
1. Service ML—light and favorable
2. Service MM—moderate to severe
3. Service MS—unfavorable or severe
The American Petroleum Institute (API) defines and explains the three classes of services as follows:
Service ML. “Service typical of gasoline or other spark-ignition engines operating under light and favorable service conditions, the engines having no special lubricating requirements and having no design characteristics sensitive to deposit formation.” Thus, ML service can be summarized as:
1. Light and favorable conditions
2. No special lubrication problems to control
3. No design characteristics sensitive to deposit formation
4. Mostly moderate-speed driving
5. No severe low-temperature or high-temperature service
Service MM. “Service typical of gasoline and other spark-ignition engines operating under moderate to severe service conditions, but presenting problems of deposit or bearing corrosion control when crankcase oil temperatures are high.” The MM service is summarized as:
1. Moderate to severe conditions
2. Some high-temperature problems of deposit or bearing corrosion to control
3. No design characteristics sensitive to deposit formation
4. Fuel of suitable characteristics
5. No severe low-temperature service or prolonged idling
Service MS. “Service typical of gasoline or other spark-ignition engines operating under unfavorable or severe types of service conditions, and where there are special lubrication requirements for deposit or bearing corrosion control, due to operating conditions or to fuel or to engine design characteristics.”
Under the MS service, there are two types of severe service or operating conditions, namely, (1) start-and-stop service and (2) high-temperature service. The MS service is summarized as:
1. Unfavorable or severe conditions
2. The most severe service encountered
3. Definite special lubrication problems to control
4. Low-temperature severe service
a. Much stop and go
b. Much idling
c. Much crankcase condensation
d. Much unburned fuel
5. High-temperature severe service
a. Prolonged high speeds
b. Inadequate cooling
c. Adverse fuel impurities
Diesel Engine Services. Service DG—“Service typical of diesel engines in any operation where there are no exceptionally severe requirements for wear or deposit control due to fuel or engine design characteristics.” The DG service is summarized as:
1. General or moderate
2. No exceptionally severe lubrication problems to control
3. No design characteristics adversely influencing wear and deposits
4. No adversely low or adversely high engine-operating-temperature conditions
5.