400

MODERN STEAM PRACTICE.

1 c r>a

1 :{• i J

Fig. 285.—Piston Rings.

aa., Pistons, b b, Packing rings, cc, Junk rings.d d, Bolts with recessed nuts. e, Bolt with hole tappedin the body of the piston.

gain more surface in the glands, and by the increased weight ofthe rod and its adjuncts to relieve the piston rubbing surface. Thetotal rubbing surface or depthof the piston, for double trunks,may be taken at one-eleventhof the diameter of the cylinder;for double piston-rod engineson the return connecting prin-ciple, one-sixth; for singlepiston-rod and single-trunkarrangements, one-fifth: thisdepth being the breadth overthe surface in contact with thecylinder. Most pistons haveprojecting rings cast alongwith the junk ring, and some

have also projecting rings cast along with the main body of thepiston—both having in view the equalization of the junk ring andend surface in Connec-tion with the mainpacking ring; this isnecessary, as the rub-bing surfaces on thejunk and back ring havethe weight of the piston,and in some instancesthe thrust of the con-necting rod, to sustain,while the packing ringhas not so much dutyto perform. In Orderto make the junk ringand other surfaces tobear more equally, andto keep the piston cen-tral with the cylinder,two pieces of cast ironare placed on the underside, between the packing ring and the main body of the piston,pitched about one-fifth of the diameter of the piston apart; in this

Fig. 286. —Piston.

1, Piston, b, Packing ring, ccc, Springs, dd, Cast-iron pieces.e e, Bosses for piston rods. f, Boss for air-pump rod. G G, Ribsto strengthen the body of the piston. h, Tongue at division.