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Q. Are plain bearings the only practical kind?
A. While plain bushings are more generally used on the connecting rod big ends and main journals of the crankshaft, anti-friction bearings of the ball or roller type have been used with some degree of success.
Q. What are the advantages of anti-friction bearings?
A. Anti-friction bearings, especially those of the ball type, turn much more easily than plain bushings and less power is consumed by bearing friction when they are employed. They are also more enduring than the plain journals, and when properly selected show absolutely no wear after extended periods of operation. (Fig. 50.)
Q. What is the accepted relation between cylinder bore and piston stroke?
A. The piston stroke of modern engines is longer than the diameter of the bore. Usually the piston movement is equal to about 1.25 times the diameter of the piston. The piston stroke in extreme cases may be 1.75 or 2 times the cylinder bore.
Q. What is a long stroke motor?
A. Any motor where the ratio of stroke to bore is greater than 1.5 to 1 is termed a “long stroke motor.”
Q. What limits length of stroke?
A. The permissible length of stroke of the piston is limited by an arbitrary piston speed which it is not desirable to exceed because difficulties in lubrication and cooling are apparent at greater piston speeds than that established by common practice.
Q. Define “piston speed” and give commonly accepted limits.
A. The stroke of the piston in inches multiplied by 2 and this product multiplied by the number of revolutions per minute and this in turn divided by 12 will give the piston speed in feet per minute. An engine with a six inch stroke going at 1,000 R. P. M. will have a piston speed of 1000 feet per minute, which is the commonly accepted limit.
Q. Of two engines going at the same crankshaft speed will the long stroke or short stroke engine have the highest piston speed?
A. The piston of the long stroke motor will obviously have the greatest piston speed. The shorter the stroke of an engine, the greater the number of revolutions its crankshaft can attain.
Q. What is the crankshaft?
A. The crankshaft is the part of the motor which converts the reciprocating motion of the piston to a continuous rotary movement suitable for driving the wheels of the automobile.
Fig. 50.—Showing Ball-Bearing Crankshaft of the Two-Bearing Four
Throw Type used in White Motors. Q. What are crankshafts made of?
A. Crankshafts are invariably made of steel and are usually formed by the drop forging process which forges them out approximately to the correct form. Crankshafts employed on moderate priced engines are usually of carbon steel, while those utilized on high powered engines are made of chrome-nickel and other high tensile strength alloy steels.
Q. What is the relation between crankshaft throw and piston stroke?
A. The crankshaft throw, which is the distance between the center line of the crank pin and that of the crankshaft proper, is always half the piston stroke.
Q. Do all crankshafts have the same number of throws?
A. Crankshafts intended for multiple cylinder engines have more throws than simple forms intended for one or two cylinder engines.
Q. Are crarkshafts always in one piece?
A. Crankshafts are not always made from one forging or piece of metal. They are sometimes composed of a number of distinct machined pieces fastened together by mechanical means. (Fig. 51.)
Q. Why are built-up crankshafts necessary?
A. On small single cylinder and two cylinder V engines, such as used for motorcycle propulsion, the connecting rod is usually formed
in one piece and the crankshaft assembly must be completed after the connecting rod is in place on the crank pin. With a split connecting rod construction one piece crankshafts are employed, whereas when one-piece connecting
rods are used it is necesCONNECTING
sary to use a built-up crankshaft.
Q. What are counter weights?
A. Counter weights
are fan shaped masses of LIRIER.
cast iron attached to the crankshaft of single cylinder and some forms of
two cylinder motors to -FLYWHEEL
balance the reciprocating weight of the
pistons and a part of the Fig. 51. --Sectional View of Typical Air- connecting rods. (Fig.
Cooled Motorcycle Engine Showing
Q. How are they attached to the crankshaft? A. The conventional method of attaching counter weights is to bolt them to the crankshaft webs, though on built-up crankshafts used on motor cycle engines they are sometimes cast or forged integral with the flywheel members that correspond to the webs of the usual crankshaft and to which the crank pin is attached.
Fig. 52.-Showing Various Forms of One and Two Cylinder Crank
shafts. A-Single Throw Crankshaft With Balance Weights. B-One-Throw Two-Cylinder Crankshaft With Balance Weights. C—Crankshaft for Two-Cylinder Opposed Motor.
Q. How are crankpins spaced in two cylinder engines?
A. Crank pins of two cylinder engines may be arranged in two different ways. In vertical engines both connecting rods may be attached to a common crank pin, as shown at Fig. 52 B, or they may be placed at 180 degrees or opposite each other, as shown at Fig. 52 C and Fig. 53 A. When both crank pins are on the same plane in a two cylinder vertical engine it is possible to have the explosions follow in regular sequence, but the engine is not in correct mechanical balance. If the crank pins are spaced as outlined at Fig. 53 A, the engine is in better mechanical balance, but the explosions are not separated by regular intervals. The best method of utilizing two cylinders is to use a double opposed motor. In this case the crank pin arrangement is as shown at Fig. 52 C. Counter weights are necessary when both connecting rods act on the same crank pin, just as in a one cylinder engine.
Fig. 53.—Unconventional Two-Cylinder Crankshaft. A–For Two
Cylinder Vertical Engine. B-For Two-Cylinder V Engine. Q. How are the crankpins spaced in three cylinder motor?
A. Crank pins of a three cylinder motor are arranged on thirds of a circle or 120 degrees apart. (Fig. 54 A.)
Q. How are crankpins arranged on four cylinder crankshafts ?
A. The conventional arrangement of crank pins on a four cylinder crankshaft is outlined at Fig. 54 B. Four separate crank throws are provided on two planes, spaced 180 degrees apart. The crank pins for connecting rods 1 and 4 are on a line and the pistons move up and down together. The crank throws of pistons 2 and 3 are on the same line.