| Thickness of | front of piston near boss | 0·2 × x. |
| ” | ” ” rim | 0·17 × x. |
| ” | back of piston | 0·18 × x. |
| ” | boss around rod | 0·3 × x. |
| ” | flange inside packing ring | 0·23 × x. |
| ” | ” at edge | 0·25 × x. |
| ” | junk ring at edge | 0·23 × x. |
| ” | ” inside packing ring. | 0·21 × x. |
| ” | ” at bolt-holes | 0·35 × x. |
| ” | metal around piston edge | 0·25 × x. |
| Breadth of packing ring | 0·63 × x. | |
| Depth of piston at centre | 1·4 × x. | |
| Lap of junk ring on piston | 0·45 × x. | |
| Space between piston body and packing ring | 0·3 × x. | |
| Diameter of junk-ring bolts | 0·1 × x + ·25 inch. | |
| Pitch of junk-ring bolts | 10 diameters. | |
| Number of webs in piston | D + 20 ———. 12 | |
| Thickness | ” | 0·18 × x. |
Exercise 50: Design for Marine Engine Piston.—Calculate by Seaton's rules the dimensions for a marine engine piston 40 inches in diameter, and subjected to an effective pressure of 36 lbs. per square inch. Then make the necessary working drawings for this piston to a scale of, say, 3 inches to a foot.
Note.—Take the dimensions got by calculation to the nearest 1-16th of an inch.
XV. STUFFING-BOXES.
Fig. 50.
In fig. 50 is shown a gland and stuffing-box for the piston rod of a vertical engine. A B is the piston rod, C D a portion of the cylinder cover, and E F the stuffing-box. Fitting into the bottom of the stuffing-box is a brass bush H. The space K around the rod A B is filled with packing, of which there is a variety of kinds, the simplest being greased hempen rope. The packing is compressed by screwing down the cast-iron gland L M, which is lined with a brass bush N. In this case the gland is screwed down by means of three stud-bolts P, which are screwed into a flange cast on the stuffing-box. Surrounding the rod on the top of the gland there is a recess R for holding the lubricant.
