Several varieties of slide-rods are used; a few of which are represented by the Figures in Plates 1 and 2. Steel is useful for slide-rods, because it is less liable to wear by the friction of the packing in the packing-box. When slide-rods are of iron, they require the fibres of the metal to be well closed by angular-gap rounding tools; when this is well done, the durability of the rod is much greater than with iron of open texture, which collects grit and other foreign matters. Iron of close texture is also necessary to ensure a smooth surface to the rod after being hardened, which is sometimes done.

When steel is used for slide-rods, it must be of very fibrous character, and be thoroughly softened by heating and gradual cooling in coke or charcoal. Such treatment will tend to prevent sudden breaks down, which will occur with hard steel of all kinds.

When the old-fashioned D slide-valve is employed, a rod similar to that shown by Fig. 27 is made use of to connect the two D portions; and two valves are thus formed, connected by one rod. The intermediate portion of this rod is therefore in the steam space of the slide-box; and, to protect the iron from the ravages of the steam, the rod is coated with gun-metal throughout the length of the intermediate part, including the two flanges or collars denoted by C C.

The diameter of the iron hidden by the gun-metal should not exceed the diameter of the screw at each end of the rod ; and, being of this dimension, the cheapest method of forging the rod is by welding a collar to each end, at a proper distance from the extremity. By this plan, the two portions intended for the screws require upsetting or thickening to admit of a good weld, and also hammering to obtain the requisite solidity for the screws.

The lower D valve is fastened to the rod by one or two nuts; the upper D valve is secured by a joint-nut, which is denoted by Figs. 28 and 29. Into the joint-gap of the nut is placed the square boss of the upper or gland slide-rod shown by Fig. 30. The connexion is effected by a square pin or bolt being fastened in the joint-nut after the boss of the upper slide-rod is put into the gap.

The forging of the joint-nut consists in thoroughly welding and hammering a square bar of iron, and cutting off a solid piece which is long enough to make the nut; the gap in which is afterwards made by drilling and slotting.

The upper slide-rod may be made of soft fibrous steel, for the advantage of having a good sliding surface for contact with the packing in the packing-box. If the rod is made of steel that cannot be wielded, it is sometimes necessary to make the whole of the sliding part and screw portion by reducing it from a bar whose width and thickness are about two-thirds the width and thickness of the intended square boss. The sliding part may be produced at the end of the bar ; or may be reduced while a lump is allowed to remain at the extremity, for the convenience, if necessary, of upsetting to form the boss. By upsetting, a part of the drawing may be avoided. In either case, the length of bar required is ascertained by applying the rule in p. 8, and substituting slide-rod stem for key stem.

These upper slide-rods are also made of iron which is well hammered by a steam-hammer, and also well closed by a pair of angular-gap tools. Very little reducing is sufficient when iron is employed. The square boss is formed by doubling and welding a lump at the end of a bar or rod, which is equal in diameter to the diameter of the required sliding part. The diameter of the iron made use of is only sufficient to admit a good welding and closing, to reduce it to the desired diameter of the slide-rod when forged.