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Q. Why are tires usually made the same size on automobiles?

A. It is common practice to make the front and rear wheels the same size on most American cars because there is very little difference possible between front and rear shoes with the average weight distribution.

If two different sizes of tires are used, one must carry two spare casings and two different sizes of inner tubes to make satisfactory repairs. Then again, there is no way of equalizing the wear on the tires. If the casings are the same size, the front tires may be used to replace the weaker rear shoes and these members in turn transferred to the front wheels after a car has been in operation for a time. Rear tires invariably wear faster than front ones, so that after they have worn to a point where they are considered weak for the rear wheels they may have ample strength for the lighter duties at the front end of the car and the front tires, which are in better condition, may be placed on the rear wheels.

Q. How is air introduced in inner tubes?

A. Air is usually pumped into the inner tubes by hand or power operated air pumps of a portable pattern that is carried as part of the equipment of the car. A simple form of check valve is used at the tube that permits one to pump air into the tire but which seats automatically by pressure from within the tube to prevent air from escaping.

Q. Are there any substitutes for air?

A. Various compounds have been devised that are intended to be introduced into inner tubes or outer casings to take the place of air but none of these are practical because they are not sufficiently resilient and many of them are compounds that are actually injurious to the rubber of which the tire is composed. Some of these are affected by heat while others will flatten out at the point of contact between the tire and the ground if the car is left standing for any length of time.

Q. What are puncture proof pneumatic tires and how are they made?

A. Various attempts have been made to devise puncture proof pneumatic tires by using leather or leather and metal treads applied to the tires of the conventional pattern or to imbed a series of metal discs overlapping each other like scales on a fish in the tread of a tire before this is cured. Most of these tires have a grave defect of generating considerable heat while in use and owing to the stiffness of the composite metal and leather or metal and rubber tread much resiliency is sacrificed.

LESSON TWENTY-FIVE

AUTOMOBILE BEARINGS AND THEIR CARE

Q. What are the functions of a bearing?

A. Bearings are used to support the various revolving shafts that are called upon to transmit power from one part of the motor car to the other and are needed to support rotating parts of all machines.

Q. Name principal types of bearings.

A. Bearings may be divided into two general classes, plain and anti-friction types.

Q. Where are plain bearings used?

A. At the present time the use of plain bearings is confined principally to the power plant and various unimportant points on the motor car chassis.

Q. Why are anti-friction bearings utilized ?

A. Considerable power is absorbed at the bearing points when a shaft runs in a plain journal or bushing because considerable friction is present due to the sliding of the surfaces over each other. With bearings of the anti-friction types rolling friction is substituted for sliding friction and much less power is absorbed.

Q. Where are anti-friction bearings used in preference to plain journals?

A. Anti-friction bearings are used at practically all points of the mechanism where it is desirable to employ bearings that will not need continual attention and lubrication and that will absorb but very little power when subjected to heavy loads. Anti-friction bearings will carry stresses without depreciating that would soon produce deterioration of the most efficient plain journals.

Q. What materials are used in making plain bushings?

A. The shafts used in motor car construction are generally of steel and the surrounding boxes or bushings of parts that have severe duties to perform are generally of bronze, Babbit metal, or other anti-friction metals of this nature.

Q. What precaution is essential to insure continued service from plain bearings?

A. The bushings or boxes of a plain bearing must be accurately fitted to the shafts they are to bear against, yet must havesufficient space to provide a film of oil between the working surfaces. Positive means of supplying lubricant must be used because it is really the film of oil between the two surfaces that acts as a friction reducer and makes the plain bearing practical.

Q. What causes overheating of plain bearings?

A. Overheating of plain bearings is generally due to lack of lubrication or the use of poor lubricant. If the bearings have just been refitted the overheating may be caused by a binding due to too intimate contact between the bushings and the shaft they encircle, which does not permit the introduction of an oil film to absorb the heat generated by friction.

Q. Why are soft metal bearings more suited than hard bronze bushing for general use?

A. The use of soft metals, such as Babbit or white brass is preferred by many designers because these not only have superior friction reducing qualities in themselves but have a low enough melting point so that if lubrication fails and friction heat is developed the metal lining of the bearing will run and will not injure the shaft. With hard metal such as bronze any impurities or abrasive matter in the oil will produce rapid cutting of the surfaces and if a bearing heats up it will not only score the bushings but will cut deep grooves in the shaft journals which are usually costly to repair. New white metal linings may be easily inserted because the shaft is not damaged. Soft metal boxes are also easier to fit to a shaft and will adapt themselves better to irregularities on the journal surface than will the hard material. Hard bronzes or hardened steel bush

A.

ings must be used at points subjected to considerable heat or where much stress is to be supported by a bearing having relatively small size. Q. Where are soft metal bearings commonly used?

In most automobile power plants the main bearings of the crankshaft and the lower connecting rod bearings are generally of soft metal while the upper end of the connecting rod, camshaft bearings, and various small bearings supporting timing gears, magneto and pump drive gears, and other minor shafts of this nature utilize bronze or hardened steel bushings. A soft metal can only be used where ample bearing surface is provided because it is easier to deform it under load than hard metals.

Q. How are soft metal bearings inserted in boxes?

A. On a number of the cheaper engines the Babbit metal is poured into the shell cast integral with trankcase to receive it and a mandrel of steel smaller than the shaft diameter is employed as a core piece around which the melted metal is poured. Soft metal boxes are sometimes die cast in the form of bushings which may be inserted in machined housings or boxes provided to receive them.

Q. What are “die-cast” bearings?

A. “Die-cast" bushings are components formed by pouring molten metal in a mould composed of two pieces of steel machined very accurately so that the surface of the casting is smooth and true to the mould. In order to insure positive filling of the mould with hot metal it may be forced in under pressure which insures that every little corner will be reached. Die-castings are well suited as bearings because they may be moulded true enough to size, so practically no finish will be necessary.

Q. How are plain bearings fitted to shafts?

A. Plain bearings may be fitted to shafts in two ways. If the bushing is a solid form it is usually smoothed out after boring by a tool known as a reamer which enlarges the bore so it is a few thousandths of an inch larger than the shaft diameter. Boxes of large size are usually of the divided pattern and are fitted by a "scraping in” process.

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