Carelessness in lubrication is the principal cause of tractor trouble. There is nothing complicated or difficult about keeping a tractor properly oiled; yet more tractors break down from careless lubrication than from any other cause. Every tractor-maker issues an oiling diagram and oiling instructions, and there is no excuse for an operator whose machine does not get the right kind of lubricant in the right quantity at each place where lubrication is necessary.
The cause of wear is friction; oil reduces friction and so reduces wear. No matter how smooth and highly polished two pieces of steel may be, there will be friction between them if they are rubbed together, and they will wear each other. If they are oiled, the particles of oil will keep the pieces from touching each other, and there will be no wear.
Other substances than oil can be used; there are some kinds of machinery that are lubricated with water, for instance. For general use, however, oil and grease are the best, and are practically always used.
The object of a lubricant is to keep two pieces of metal from touching; it must therefore be able to get between them, and must stay there. If the pieces are large and heavy, there will be much greater pressure on the oil than if they are small and light, and the oil must be able to withstand this pressure and resist being squeezed out. The oil that would keep the small, light pieces apart might not be able to stand the pressure of a greater weight, and might be squeezed out from between two heavy pieces.
Oil has a tendency to cling to whatever it touches, and thick oil or grease has more of this tendency than a thin, or “runny” oil. If a thick oil or grease is used on light machinery, such as a sewing machine, this clinging tendency would make the machine run hard, and might even prevent its operation.
When oil is heated, it becomes thinner, or more “runny.” Through this, an oil used in a hot place might get so thin that it would not lubricate; and on the other hand, an oil that works all right in the heat of summer might get so thick on a cold winter day as to be useless.
A slow-moving part of a machine uses a thick oil or a grease; a thin oil must be used for a part that moves at high speed.
Some of the parts of a tractor move slowly and some at high speed; some are cool and some are hot. Different kinds of lubricants are therefore required, and it is a grave mistake to use a lubricant that is not suitable to the work that it is required to do.
The engine is the most difficult part of a tractor to lubricate, and the part that suffers most if the supply fails or if the wrong kind of lubricant is used. In the first place, it is so hot that any oil will burn, being turned to carbon; the best that can be expected of an oil is that it will resist burning until it has done its work of lubricating the piston and cylinder.
A tractor engine is more difficult to oil than an automobile or truck engine for the reason that it works harder and more steadily. An automobile engine is rarely driven to the limit of its power; it has frequent opportunities to cool when running down hill. A tractor engine, on the other hand, works at its full power all day long with no opportunities to cool off. An oil that gives good satisfaction on an automobile might ruin a tractor engine through its inability to withstand the greater heat.