The Crank-Shaft.—The crank-shaft should be made of the best mild steel. Those shafts are to be preferred the cranks of which are not forged on (Fig. 30), but cut out of the mass of metal; furthermore, the brackets or supports should be planed and shaped so that they are square in cross-section.
Fig. 31.—Correct design of crank-shaft.
Such a design involves fine workmanship and speaks well for the construction of the whole engine. Moreover, it enables the bearings to be brought nearer each other, reduces to a minimum that part of the crank-shaft which may be considered the weakest, and permits a rational and exact counterbalancing of the moving parts, such as the crank and the end of the connecting-rod. The best manufacturers have adopted the method of fastening to the cranks balancing weights secured to the brackets, especially for high-speed engines or for engines of high power. The projecting surface of the crank-pin should, as a rule, be calculated for a pressure of 1,400 pounds per square inch.
Fig. 32.—Crank-shaft with balancing weight.
Cams, Rollers, etc.—The cams, rollers, thrust-bearings, as well as the piston-pin in particular, should be made of good steel, case-hardened to a depth of at least .08 of an inch. Their hardness and the degree of cementation may be tested by means of a file. This is the method followed by the best manufacturers.
Bearings.—All the bearings and all guides should be adjustable to take up the wear. They are usually made of bronze or of the best anti-friction metal.