(116) General Notes on Lubrication.

No matter how carefully the surface of a shaft or bearing may be finished, there always remains a slight roughness or burr of metal, which although of microscopic proportions is productive of friction or wear. Each minute projection of metal on a dry shaft acts exactly as a lathe tool, when the shaft revolves in cutting a groove in the stationary bearing. Since there are a multitude of these projections in a journal, the wear would be very rapid, and would in a short time completely destroy either the shaft or bearing, no matter how highly finished in the beginning.

When lubricating oil is introduced into a bearing it immediately covers the rubbing surface, and as the oil has a considerable resistance to being deformed, or is “stiff,” it separates the surface of the shaft from that of the bearing for a distance equal to the thickness of the oil film. With ordinary lubricants this distance is more than enough to raise the irregularities of the shaft out of engagement with those of the bearing. This property of “stiffness” in the oil is known as “viscosity.” The value of viscosity varies greatly with different grades of oil, and also with the temperature with the result that the allowable pressure on the oil per square inch also varies. With oils of low viscosity a small pressure per square inch on the bearing will squeeze it out, and allow the two metallic surfaces to come against into contact, causing wear and friction, while an oil of greater viscosity will successfully resist the pressure.

The life and satisfactory operation of the engine depends almost entirely upon the lubricant and the devices that apply it to the bearings. Excessive wear and change in the adjustments are nearly always the result of defective lubricating devices or a poor lubricant. The principal lubricants are:

(1) Solid lubricants such as graphite, soapstone, or mica.

(2) Semi-solid lubricants such as vaseline, tallow, and soap emulsions, or greases compounded of animal fats, vegetable and mineral oils; and

(3) Liquid lubricants, such as sperm oil, or one of the products of petroleum, the latter medium being the class of lubricant most suitable for internal combustion engines, owing to its combining the qualities of a high flash-point with a comparative freedom from either acidity or causticity.

Oils of animal or vegetable origin should never be used with gas engine as the high temperatures encountered will char and render them useless. Tallow and lard oil are especially to be avoided, at least in a pure state.

In the cylinder only the best grade of GAS ENGINE cylinder oil should be used, which according to different makers has a flash point ranging from 500 to 700 degrees. Using cheap oil in the cylinder is an expensive luxury. In general, the oils having the highest flash points have also the objectionable tendency of causing carbon deposits in the combustion chamber and rings which is productive of preignition and compression leakage. The lower flash oils have a tendency to vaporize and to carry off with the exhaust which will leave the walls insufficiently lubricated unless an excessive amount is fed to the cylinder. By starting with samples of well known brands recommended by the builder of the engine it will be an easy matter to find which is the cheapest and gives the best results. In figuring the cost of oil do not take the cost per gallon as a basis, but the cost for so many hours of running, or better yet the number of horse-power hours. Unless you are fond of buying replacements and new parts do not stint on the oil supply.

On the other hand, an excess of oil should be avoided as this means not only a waste of oil through the exhaust pipe, but trouble with carbon deposits and ignition troubles as well. Foul igniters, misfiring, and stuck piston rings are the inevitable result of a flood of lubricating oil. When a whitish yellow cloud of smoke appears at the end of the exhaust pipe, cut down the oil feed. The exhaust should be colorless and practically odorless.

Too much oil cannot be fed to the main bearings of the crank shaft if the waste oil is caught, filtered and returned to the bearings by a circulating system, for the flood of oil not only insures ample lubrication but removes the heat generated as well. The bearings require a much lighter oil, of a lower fire test than the cylinder oil. It is evident that its viscosity is a most important element, as it determines the allowable pressure on the shaft. The viscosity of an oil varies with the temperature and is greatly reduced at cylinder heat. A comparative test of the viscosity or load bearing qualities of an oil may be made by making bubbles with it by means of a clay pipe; the larger the bubble, the higher the viscosity of the oil.

Different sizes of bearings, and bearing pressures, call for oils of different viscosities, and consequently an oil that would be suitable for one engine would not answer for another; heavy bodied oils being used for heavy bearing pressures, and light thin oil for small high speed bearings. The best way to determine the value of an oil for a particular shaft bearing is by experiment, attention being paid to its adaptability for the feeding devices used.

The compression attained in a gas engine cylinder depends to a certain extent upon the body of the cylinder oil, for many engines that leak compression past the rings with thin oil will work satisfactorily with a heavy viscous oil that clings tightly to the surfaces. An engine will often lose compression when an oil of poor quality is used.

Air cooled engine cylinders require an oil of heavier body than water cooled because of the higher temperature of the cylinder walls. Gum and sticky residue are usually formed by animal oils or adulterants added to the numeral oil base. Oils containing free acids should be avoided as they not only corrode and etch the bearing, but also clog the oil pipes or feeds with the products of the corrosion.

Free acid is left from the refining process, and may be determined by means of litmus paper inserted into the oil. If the litmus paper turns red after coming into contact with the oil, acid is present, and the oil should be rejected.

The following are the characteristics of an oil suitable for use on an engine:

(a) The oil must be viscous enough to properly support the bearings or to prevent leakage past the piston rings.

(b) It should be thin enough so that it can be properly handled by the oil pumps, or drip freely in the oil cups.

(d) It should contain no free acid.

Ordinarily a good grade of fairly heavy machine oil will be suitable for use on the bearings of the average engine, such as the cam-shaft and crank-shaft bearings.

Only very light clean oil, or vaseline should be used on ball-bearings, as heavy greases and solid lubricants pack in the races and cause binding or breakages.

Flake graphite is much used as lubricant, and too much cannot be said in its favor, as it furnishes a smooth, even coat over the shaft, fills up small scores and depressions, and makes the use of light oil possible under heavy bearing pressures. With graphite, less oil is used, as the graphite is practically permanent, and should the oil fail for a time, the graphite coat will provide the necessary lubrication until the feed is resumed without danger of a scoring or cutting. In fact, when graphite is used, the oil simply acts as medium by which the graphite is carried to the bearings.

If graphite is injected into the cylinder in small quantities it greatly improves the compression, as it fills up all small cuts and abrasions in the cylinder walls.

A good mixture to use for bearings is about 1½ teaspoonsful of graphite, to a pint of light machine oil, thoroughly mixed.

Graphite can be placed in the crank chamber of a splash feed engine, by means of an insect powder gun.

Trouble with oil cups is always in evidence during cold weather, as the oil congeals, and does not drip properly into the bearings. The fluidity of the oil can be increased in cold weather by the addition of about ten per cent of kerosene to the oil.

If too much oil is fed to the cylinders, the piston rings will be clogged with gum, and a loss of compression, or a tight piston will be the result. An excess of oil will short-circuit the igniter or sharp plugs, and will form a thick deposit in the combustion chamber that will eventually result in preignition or back-firing. Deposits and gum formed in the cylinder will cause leaky valves and a loss of compression. Feed enough oil to insure perfect lubrication, but not enough to cause light colored smoke at the exhaust.

Lubricating systems may be divided into three principal classes: Sight-feed, splash system, and the force feed system. Sight feeding by means of dripping oil cups is too common to require description, and is used on many stationary engines, both large and small.

The splash system is in general use on small high speed engines both stationary, and of the automobile type.

The force feed system in which oil is fed under pressure by a pump is by far the most desirable as the amount of oil fed is given in positive quantities proportional to the engine speed, and with sufficient pressure to force it past any ordinary obstructions that may exist in the oil pipe.

Another system that is half splash, and half force feed, is the pump circulated system much used in automobiles.

THE SPLASH FEED SYSTEM is the simplest of all, as the bearings are lubricated by the oil spray caused by the connecting rod end splashing through an oil puddle located in the bottom of the closed crank case. The piston and cylinder are lubricated by the spray, as well as the bearings, as the lower end of the piston projects into the crank chamber at the moment that the connecting rod end strikes the oil puddle.

To maintain constant lubrication, it is necessary that the oil in the puddle be kept at a constant height, or as in some cases be varied in such a way that the surface of the puddle is raised and lowered in proportion to the load on the engine. In the average engine the oil level is maintained by overflow pipes or openings that allow any excess of oil over the fixed level to flow back to the pump. In the Knight engine the puddles are formed in movable cups which are connected with the throttle in such a way that the opening of the throttle raises the oil level and supplies more oil to the engine at the greater load, or speed.

Oil in splash systems is supplied by a low pressure pump, usually of the rotary type, in the base of the engine. Oil from the pump passes to the bearings, drops into the puddle, overflows through the overflow opening, and returns to the pump through a filter, the same oil being used over and over again until exhausted. This strainer should be removed occasionally and the dirt removed, for should it be allowed to collect it is likely to obstruct the oil supply. The oil should be replaced before it becomes too black or foul, the crank case and bearings thoroughly cleaned with kerosene, and new oil replaced. The supply may be interrupted by the failure of the pump, caused by sheared keys or leakage of air in the suction line due to cracks. It would be well to run the engine for a few minutes with the kerosene in the crank case, in order that all of the oil may be removed. See that the drain cock is closed at the bottom of the cylinder or all of the oil will be lost. Lock the valve handle carefully so that it cannot jar open. If light colored smoke appears in intermittent puffs with a multiple cylinder engine, it indicates that one cylinder is receiving too much oil.