Hand book of calculations for engineers and firemen : Relating to the steam engine, the steam boiler, pumps, shafting, etc. . k pin ; B - web;D, D = bosses; E = key Fig. 118. crank arm, which is, of course, equal to half the stroke of the Ion. B is the web of the crank, D, D the bosses. Cranks of this form are generally of cast iron, and are attached to the main shaft by means of a key K. The fastening of a movable 260 Hand Book of Calculations. CRANKS AND ECCENTRICS. crank on a shaft requires the greatest care, because all thestresses thrown on the crank are liable to reversion during eachstr
Hand book of calculations for engineers and firemen : Relating to the steam engine, the steam boiler, pumps, shafting, etc. . k pin ; B - web;D, D = bosses; E = key Fig. 118. crank arm, which is, of course, equal to half the stroke of the Ion. B is the web of the crank, D, D the bosses. Cranks of this form are generally of cast iron, and are attached to the main shaft by means of a key K. The fastening of a movable 260 Hand Book of Calculations. CRANKS AND ECCENTRICS. crank on a shaft requires the greatest care, because all thestresses thrown on the crank are liable to reversion during eachstroke, especially in the case of a slow-running engine workingwith a considerable cushion of steam. Very severe reversals ofpressure also occur if water is allowed to accumulate in thecylinder. In such cases the piston is brought up dead beforethe end of the stroke is reached, while the crank endeavors topull on, thus throwing a heavy strain on all the connections,and amongst others on the key. Cranks of this type in addi-tion to being keyed are generally shrunk on to the shaft, orelse are forced on by hydraulic Fig. 119. Pig. 119 shows an end elevation and cross-section of anotherform of cast-iron crank, called a disc crank. It is, as its nameimplies, formed of a disc of cast iron, attached to the shaft bythe methods just described, and provided with a wrought iron orsteel crank-pin. The portion of the disc opposite to the pin isusually much thicker and heavier than the remainder of thedisc, this extra weight being used as a balance to the weightsof the reciprocating parts. The crank-pin is the portion of theengine which receives thegreatest stress, and special care must therefore be given to itsdesign and lubrication. See Cut 122. Hand Book of Calculations. 261 The Eccentric. There is a species of crank called the eccentric, in which thepin is so large that it completely envelopes the shaft. Such acrank is shown at Fig. 121 (B). The distance ac is the same
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