A practical treatise on the steam engine

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1878 Excerpt: ...on the piston. These strains may be conveniently divided into two: one considered as acting directly upon the crank, omitting all notice of the pin, and another which simply tends to twist the crank. A rectangular section, like Fig. 1, Plate XLII., distributes the metal in the best manner for resisting the former, but it is not equally efficient in resisting a twisting movement; and therefore another section of crank shown by _ best for resisting the strains which a crank has to endure. In calculating the strength of cranks, the pressures expected from steam in the cylinder may not be so great as those arising from the reciprocating parts when high speeds are employed, nor are they to be compared with the effects of accidental lodgments of water in the cylinder. Therefore a very considerable margin of strength is requisite, and for wrought-iron cranks the outer radius of the boss may be equal to the diameter of the wrought-iron shaft on which it is keyed, and the same proportion applies to crank pins, while with cast-iron cranks the radii of the two bosses may be 1 1 to 1-25 times the diameters of shaft and crank pin respectively. Cranks, especially those of wrought iron, are often made so that it is necessary to heat and expand them, and when set upon the shaft, a very firm grip is secured, and a satisfactory resistance given to the irregular, complicated strains to which they are subjected. Balanced Cranks.--The circular or disc form of crank is eminently well suited for cast iron, and its smooth surface very much improves the appearance of a steam engine. Advantage is generally taken of the circular form to provide a mass of metal opposite to the crank, for the purpose of balancing it. In vertical engines this balance weight suffices when made equal to t...