Exercise 36: Wrought-iron Overhung Crank.—Draw the two elevations shown in fig. 37, also a plan. Scale 1½ inches to a foot.
Proportions of Overhung Cranks.
| D = diameter of shaft. | ||
| d = ” crank-pin. | ||
| Length of large boss | = ·9 D. | |
| Diameter ” | = 1·8 D. | |
| Length of small boss | = 1·1 d. | |
| Diameter ” | = 1·8 d. | |
| Width of crank arm at centre of shaft | = 1·3 D. | |
| ” ” crank-pin | = 1·5 d. | |
The thickness of the crank arm may be roughly taken as = ·7 D.
Exercise 37.—Design a wrought-iron crank for an engine having a stroke of 4 feet. The crank-shaft is 9 inches in diameter, and the crank-pin is 4¾ inches in diameter and 6½ inches long.
Fig. 38.
Locomotive Cranked Axle.—As an example of a cranked shaft we take the cranked axle for a locomotive with inside cylinders shown in fig. 38; here the crank and shaft or axle are forged in one piece. A is the wheel seat, B the journal, C the crank-pin, and D and E the crank arms. Only one half of the axle is shown in fig. 38, but the other half is exactly the same. The cranks on the two halves are, however, at right angles to one another. The ends of the crank arms are turned in the lathe, the crank-pin ends being turned at the same time as the axle, and the other ends at the same time as the crank-pin. This consideration determines the centres for the arcs shown in the end view.
Exercise 38.—Draw to a scale of 2 inches to a foot the side and end elevations of the locomotive cranked axle partly shown in fig. 38. The distance between the centre lines of the cylinders is 2 feet.
