Exercise 48: Locomotive Piston.—A part elevation and part section of a locomotive piston, for a cylinder having a bore 18 inches in diameter, is shown in fig. 48. Draw this, and also a view looking on the nut in the direction of the axis of the piston rod. Scale 6 inches to a foot.
Note.—The reason why the part of the piston rod within the piston has such a quick taper is that the piston has to be taken off the rod while it is in the cylinder. The cross-head being forged on the end of the piston rod prevents the piston and piston rod being withdrawn together.
Large Pistons.—Pistons of large diameter are generally provided with two cast-iron packing rings placed within the same groove. These rings are pressed outwards against the cylinder, and also against the sides of the groove by one or more springs. One form of this packing (Lancaster's) is shown in fig. 49. Here one spring only is used, and it is first made a straight spiral spring, and then bent round and its ends united. The action of the spring will be clearly understood from the illustration. For the purpose of admitting the packing rings the piston is divided into two parts, one the piston proper, and the other the junk ring. In fig. 49, A is the junk ring, which is secured to the piston by means of bolts as shown.
Exercise 49: Marine Engine Piston.—The piston illustrated by fig. 49 is for the high-pressure cylinder of a marine engine. The piston, junk ring, and packing rings are of cast iron. The piston rod and nut are of wrought iron, so also are the junk ring bolts. The nuts for the latter are of brass. The spiral spring is made from steel wire 3⁄8 inch diameter. An enlarged section of one of the packing rings is shown at (a). A front elevation of the locking arrangement for the piston rod nut is shown at (b). A sectional plan of one of the nuts for the junk ring bolts is shown at (c).
First draw the vertical section of this piston, next draw a plan, one-third of which is to show the piston complete, one-third to show the junk ring removed, and the remaining third to be a horizontal section through between the packing rings. The details (a) and (c) need not be drawn separately. Scale 3 inches to a foot.
Fig. 49.
Proportions of Marine Engine Pistons.—Mr. Seaton, in his 'Manual of Marine Engineering,' gives the following rules for designing marine engine pistons:—
| D | = | diameter of piston in inches. | |
| p | = | effective pressure in lbs. per square inch. | |
| x | = | D — 50 | × √p + 1. |