Зображення сторінки
PDF
ePub

and lighter tires. The following table shows the proper air pressures for various sizes of tires. A simple way to remember these pressures is to bear in mind that there should be twenty pounds air pressure for each inch of tire diameter.

[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][ocr errors][merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small]

Q. What happens if tires are not sufficiently inflated?

A. If tires are not filled with the proper amount of air they will be distorted unduly under load and the fabric and rubber plies of which the casing is composed will be subjected to serious bending which may cause the various layers to separate from each other, this weakening the casing.

Q. What is the result if tires are too highly inflated?

A. If tires are pumped up too much the construction is subjected to stress that it was not designed to withstand and the inner tube will "blow out" through any weak point in the casing. Hard tires also subject the mechanism to considerable vibration because they are not sufficiently resilient.

Q. Which tire should be larger, the front or rear?

A. As a rule, the greater strain is taken by the rear tires because these members carry the larger portion of the car weight and in addition are depended upon to furnish traction and propel the vehicle. The front tires on most cars can be made smaller than the rear ones if desired, because they are called upon to support less weight and are not subjected to the wear resulting from skidding, driving stresses and braking reaction.

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?

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 have sufficient 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

« НазадПродовжити »