358

MODERN STEAM PRACTICE.

The use of this guard piece is to allow a wedge to be driven betweenit and the brass block, which contracts the spring ring; the pistonis then placed in the cylinder, and when thewedge is removed the ring expands and fitsthe cylinder exactly. Curved Steel springs arethen inserted between the piston and the pack-ing ring all round; thus with its own elasticity,and with the aid of these springs, the surfacesbetween the piston and cylinder are made quitesteam-tight. All the parts should be carefullyturned, and the surfaces between the junk ringand the piston scraped; no grinding materialshould be used to make this joint tight; in factthe use of emery for making steam joints, orfor getting up journals, has long been discarded,as the small particles of grit are sometimesimbedded in the metal, and soon play havocwith rubbing surfaces.

The piston rod is secured by a nut, sometimesplaced at the top, in other cases at the bottom,square threads for which are cut on therod. The nut is sometimes flush with the top of the piston, and isscrewed into the recess by a Spanner , fitted with a pin which takes

— Piston RBlock Piece.

A, Piston. b, Packing ring.C, Junk ring. D, Bolt andrecessed nut. e, Block andguard.

Fig. 236.—Top Nut for Piston Rod. Fig. 237.—Bottom Nut for Piston Rod.

a, Piston, b, Cone on piston rod. c, Nut. a, Piston, b, Cone . c, Nut. d, Pin.

the holes drilled in the top surface of the nut; a better plan, how-ever, is to recess the nut partly, leaving sufficient projection so thatit can be tightened up with an ordinary key. Of course the end of