40 6

MODERN STEAM PRACTICE.

than the main part of the rod, receives a large nut, by which thepiston rod is secured to the piston. The part of the rod passingthrough the piston is of increased diameter and cone-shaped, thiscone is drawn through a corresponding hole bored in the piston,and held firmly by the large nut at the end, which is kept fromturning backwards by a split pin bearing on it, and passing througha hole bored in the end of the rod. This cone in some examplesis no larger than the rod at the one end, and is reduced at the endnearest the nut, in which case no raised part is required to be forgedon the rod; a collar, however, is sometimes left to screw the pistonagainst. Some makers prefer having that part of the rod which

Fig. 288.—Tapered Rod and Nut for Piston. Fig. 289.—Parallel Rod and Nut for Piston.

a, Piston, b, Taper on rod. c, Nut. a, Piston, b, Parallel part on the piston rod. c, Nut.

D, Split pin. d, Split pin.

passes through the piston quite parallel, or nearly so, with a shoulderformed by the reduction of diameter, against which the piston isscrewed. The screw is generally of a V form, rounded at the topand bottom of the thread; others are cut square, but the V threadis preferable. The nut in most examples projects from the bodyof the piston, and bears on a turned raised part formed on thecasting; in others it is recessed into the piston, and in others againit is flush with it, or a small projection of the nut is left, so that itcan be turned round with an ordinary Spanner . When the rodpasses through a crosshead, its end is turned down quite paralleland secured to the crosshead with a nut, in the same way as for thepiston. Some engineers place a jam nut on the back of the mainone, with split pins passing through both nuts, thus making a verysecure fastening. These lock nuts should be placed on all theparts which are liable to shake loose, more especially for direct-action engines, as the speed at which these are driven is liable to