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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 crankshafts 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 LIPER
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.
Q. How are crankpins spaced on six cylinder engines?
A. The crank pins of six cylinder crankshafts may be arranged in two ways. The simplest arrangement is outlined at Fig. 54 C. In this three crank pins are provided, spaced 120 degrees apart, just the same as in a three cylinder crankshaft, each crank pin serving for two cylinders. The other method outlined at Fig. 54 D may be said to be composed of two three cylinder crank shafts joined together. The crank pins for cylinders 1 and 6 are on the same plane, the pistons of cylinders 2 and 5 move together, as is also true of the pistons of cylinders 3 and 4.
Fig. 54.-Outlining Construction and Arrangement of Throws of
Crankshaft for Three, Four and Six-Cylinder Motors. Q. How many main bearings on crankshaft?
A. The crankshafts of one, two, and some four cylinder motors have but two main bearings, one at each end. A three cylinder crankshaft usually has four main bearings. Four cylinder crankshafts used when motor cylinders are cast in pairs usually have three bearings, and when individual cylinder castings are used a four cylinder crankshaft will have five main bearings. Six cylinder crankshafts have either five or seven main bearings as a rule, though
on some very compact six cylinder engines a three main bearing crankshaft may be used.
Q. What is the influence of cylinder design on crankshaft length?
A. Motors employing block casings can have shorter crankshafts than those in which cylinders are cast in pairs or individually.
Q. What are crankshaft main bearings made of?
A. The crankshaft main bearings are generally made of the same material as employed for the bushings for the connecting rod big ends. Ball bearings have been used as main bearings and their use is more general in this application than it is in connecting rods. (Fig. 50.) Q.
What is the function of the crankcase? A. The crank case is utilized to support the crankshaft and to act as a bed for the engine cylinders. It keeps the working part of the cylinder in perfect alinement with the crankshaft and camshaft carried and protected by the crank case and at the same time it serves as a carrying or supporting member by which the power plant is attached to the chassis.
Q. What are crankcases made of?
A. Automobile engine crankcases may be made of cast aluminum, cast iron, or bronze castings. The first named material is most generally used on account of its lightness. It has about the same strength as cast iron and weighs but one-third as much. On engines that are manufactured in large quantities, stamped sheet metal, such as steel and aluminum, have been utilized as the lower portion of the crank case.
Q. Name three common types of crankcase.
Crank cases may be divided into three types, the barrel form, the horizontally divided, and the vertically divided.
Q. What is the barrel type crankcase?
A. The barrel type is a form in which practically the entire crankcase is cast in one piece, end plates being provided for holding the main journals and also to cover the large openings in the ends of the crank case through which the crank shaft assembly is introduced.