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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

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?

A. 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 crankcase 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.

Q. Describe "scraping in" bearings.

A. The process of scraping bearings is a simple though tedious one that requires some degree of mechanical skill. The shaft to be fitted in the boxes is covered with a light film or coating of Prussian blue or other pigment and is allowed to rest in the box and then revolved. Any high spots in the bearing will scrape the blue from the shaft and the color indicates the points on the bushing that contact with the shaft. When the shaft is first inserted in the box it may bear

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Fig. 231A.-Engine Main Bearing of Plain Bushing Type.

at only one or two points. These are eliminated by sharp hardened steel instruments having cutting edges that scrape away the high spots. The bearing is tested after every scraping operation with the piece of shaft it is to encircle and it has been scraped to a suitable fitted surface when the shaft bears practically in the entire bushing, which condition is reached when the blue color is distributed uniformly over the surface of the bushing. The white metals are easier

to scrape than the bronzes and for this reason are widely used where bearings must be fitted by this process.

Q. Name principal types of anti-friction bearings.

A. Anti-friction bearings are those in which rolling friction is substituted for sliding friction and are shown clearly at Fig. 231.

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Roller

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Fig. 231.-Representative Types of Anti-Friction Bearings. All anti-friction bearings may be divided into two general classes, those employing balls, as shown at A and B, and the other types which use rollers to support the load as shown at C and D.

Q. Name principal forms of roller bearings.
A. The principal types of roller bearings that have received

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