Зображення сторінки
PDF
ePub

LESSON TWELVE

METHODS OF LUBRICATING THE AUTOMOBILE
POWER PLANT

Q. What is lubrication?

A. Lubrication consists of interposing between two surfaces that have relatively great friction, a cushioning and almost frictionless medium that will separate these surfaces and enable the parts to move with expenditure of less power.

Q. Why is lubrication necessary?

A. If the moving parts of machines which work on or by each other and which are in frictional contact were not separated by a film of lubricating oil or grease the metallic contact would result in abrading the surfaces and producing rapid depreciation. In some parts enough friction might exist to cause the pieces in contact to become so hot that they would seize together. This would be very liable to occur in high speed bearings or in bearings subjected to heavy pressure.

Q. What is friction?

A. Friction is the retarding force that tends to prevent movement of all mechanism which is in motion. The friction may result from internal or external forces and it is exerted in such a way as to bring all moving parts to rest. Friction is generally caused by the interlocking of minute projecting particles which extend from the surfaces of all substances regardless of their nature or the accuracy of surface finish. Friction is greater between soft materials than it is between hard substances and the rougher the surfaces in contact the greater will be the amount of friction between them. It requires power to overcome friction in all cases and the greater the amount of friction present the more power is needed to overcome it.

Q. Should friction be eliminated in all parts of automobiles? A. Some parts of motor cars perform work where friction is necessary to enable them to function properly while other parts will work more efficiently as the amount of friction is reduced. For instance, it is necessary to have friction between the parts of the clutch used to transmit power from the engine to the gearset and it is necessary to have friction between the brakes and the brake drums when these members are applied to stop motion of the car. If there was no friction between the tires of the driving wheels or traction members and the ground it would be impossible for the power plant to propel the car. At the other hand if there was too much friction at the bearing points of the motor, gearset, power transmission parts and wheels, it would be practically impossible to propel the car without using large quantities of power. It is important that friction be eliminated or reduced to as low a point as possible at all bearing points in order to obtain efficient power generation and transmission. The more friction there is in clutches and brakes he more efficient these members are.

Q. Name the principal lubricants and their derivation.

A. The principal lubricating mediums commonly used to separate the parts of machinery in motion are fluid oils and semifluid and solid greases derived from mineral and animal sources. Solids, such as graphite, which is a form of amorphous carbon, are also used for lubricating purposes.

Q. Are all lubricants suitable for the same parts?

A. The lubricating mediums best adapted differ with the nature of the work the parts are to perform. An oil that is suitable for one portion of the automobile mechanism may prove actually injurious to other parts.

Q. Name the conditions where oil can be used?

A. Oils are the only lubricants that can be successfully employed in lubricating automobile power plants. Oil is necessary wherever the amount of space for the film of lubricant is small or where considerable heat is present.

Q. Name conditions where only greases can be utilized.

A. In certain parts of the power transmission system such as in

the differential gears and driving gearing at the rear axle it is imperative that only semi-fluid greases be used, because the rear construction as ordinarily built is not oil-tight and would not retain fluid lubricants. The more viscous or heavy bodied greases are also necessary in sliding gear transmissions and gear driving systems, because oil would not have sufficient cushioning properties and would soon be forced from between the working surfaces of the gear teeth. Greases reduce noise and wear where pressures are heavy, while oils are better adapted on rapidly moving, close fitting parts.

Q. What are the best lubricants for automobile use and why? A. Automobile lubricants should be derived from a crude petroleum base because oils of this nature are inorganic and are not apt to decompose by exposure to the air or by heat as are the organic lubricants derived from animal fats or vegetable sources.

Q. What is the most difficult part of the automobile to oil properly?

A. The cylinders of the automobile power plant are the most difficult portions to lubricate satisfactorily on account of the heat generated and great care is necessary in selecting oils for this purpose.

Q. What kind of lubricant should be used in the cylinder?

A. Gas engine cylinder oil should have sufficient body to maintain a positive film between the piston and cylinder walls and yet should not be so heavy that it will gum up the piston rings. It should have a high fire test which means that it should not decompose readily by heat and it should have a good cold test in order that it will not thicken too much at low temperatures. Its viscosity should be such that it will flow readily through pipes and so it can be pumped without difficulty by the various forms of gear and plunger pumps used to circulate oil. It should be free from acid and should be a pure mineral oil without adulterants or animal or vegetable fillers. It should be thoroughly filtered to remove the free carbon and other impurities which would be deposited in the combustion chamber when the oil is vaporized by the intense heat of the explosion.

Q. How can a lubricant be tested for acid?

A. A simple test for acid in oil can be made by dissolving a small quantity of the lubricant in alcohol which may be slightly warmed by immersing the test tube or flask in which the alcohol is contained in warm water. This will facilitate dissolving the lubricant, especially if heavy greases are tested. When the oil has been dissolved the solution may be tested with litmus paper which may be procured at any drug store. A piece of blue litmus paper will turn red when placed in the solution if any acid is present.

Another test which requires more time but which gives positive results can be made by soaking several strands of candle wicking or waste in the suspected lubricant and then wrapping these around a brightly polished steel shaft, placing this in the direct sunlight but not exposed to the elements. If pronounced etching or rusty spots are present under the strands of oil soaked material on the steel after the shaft has been exposed a few days it may be inferred that the oil contains acid and is not suitable for use on machines.

Q. What is the effect of acid in oil?

A. The presence of acid in oil is not desired because this will produce rusting or etching of the brightly finished bearing surfaces and will roughen them so they have considerable friction. Oils that contain acid are much quicker to act injuriously when used in the cylinder because they are rapidly decomposed by the heat present at that point. Obviously, rusty, roughened surfaces will have more friction than smooth, bright, well-polished ones.

Q. What is viscosity and how is it determined?

A. Viscosity is a term used in speaking of the body of oils and is measured by allowing a given quantity of the oil to be tested to escape from a container, a drop at a time. When two oils are compared for viscosity the one that flows from the container quickest is the most fluid, and therefore has less viscosity than one of heavier body which cannot flow as fast. In some parts of automobiles it is necessary that oils have considerable body, these points being where shock and heavy pressures exist, as in gear boxes and axles. Tight fitting bearings that should be free running usually require thinner oils on account of the small space existing between the journals and the brasses.

Q.

What is flash test and how is it made?

A. The flash test is an indication of the amount of heating am oil will stand before the vapors given off will flash or catch fire. The method of performing the simple test is outlined at Fig. 113. A quantity of the oil to be tested is placed in a porcelain casserole which is heated by a Bunsen burner placed under the oil container which is carried on a tripod. A thermometer is placed in the oil to indicate the rise in temperature and when the vapors rising from

Lighted Taper

Casserole

Thermometer

-Bunsen Burner

GosTube

Fig. 113.-Defining Method of Testing Cylinder Oil for Flashing

Point.

the surface can be flashed by a lighted wax taper the reading on the thermometer will indicate the flash point. In this connection it will be well to state that the thermometer is a special form which will indicate temperatures up to 650 degrees F. The bulb should be supported so that it will be kept from contact with the sides or bottom of the dish which are somewhat hotter than the oil and which would give a false reading. The flashpoint is made chiefly in connection with tests of cylinder oils, and it is desirable that these have

« НазадПродовжити »