EXAMPLES.

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EX. A 3.-Make working dimensioned drawings full size, showing two views of a piston slide valve for Ex. A, as in Fig. 184. Outside exhaust lap, "; inside steam lap, "; diameter of valve at small part, 1"; rod, " through valve; "beyond valve. Diameter of valve and size of ports from. cylinder drawing, Fig. 180.

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(Find distance between centre of steam ports from Fig. 180, and draw outline of ports, then width of piston part of valve = width of port + out. side lap + inside lap. It is better to screw the collar on the rod where it enters the valve against a wrought-iron or brass bush, rather than against the cast-iron valve, therefore show such a bush, and the way the rod is fixed; show a keep pin to prevent the nut working loose.)

EX. B 3.—Make working dimensioned drawings full size, showing three views of the expansion and main slide valves for Ex. B, as in Fig. 185. Expansion valve faces, 61" long, 17" wide. Main valve-total length, 7"; total width, 61"; total depth, 7"; steam lap, "; exhaust lap, "; width of openings through valve, "; rod through main valve, " diameter; expansion valve rod, " diameter; 1" diameter square thread screw for length of, 7". Length of gunmetal nuts in expansion valves, 13". Show the expansion arrangement of Fig. 186 on the same drawing.

(Make outline drawing of port face and back end of steam chest, with the flange for the expansion gear, and showing centre lines of valve-rods, sizes from Fig 151 and Ex. B1. Notice that passages through main valve are widened out so that only the metal near the top and bottom face needs machining, also that top face of valve is recessed to save machining; show projecting faces on ends of main valve for nuts to bed against, and turn valve-rod down between the screwed ports so that it may be easily passed through the valve. Lock nuts at each end. The gunmetal nuts for the expansion valves are dropped in place from above, and bear against the flanges, one of which is shown at each side.)

SECTION XXVIII.

CROSSHEADS-GUIDES-CONNECTING-RODS.

THE crosshead, guide blocks, and the crosshead or small end of the connecting-rod all connect at the same part, and as the form of each depends so much upon the arrangement of the whole, it will be convenient to consider them together before dealing with the details of the separate parts.

(179) Crosshead.-The crosshead is the name given to the piece fixed to the outside end of the piston-rod, and moving with it. The "guide block" or "slide block" is that part which is fixed to the crosshead, or forms a part of it, and moves with it, between parallel parts called "guides." The "crosshead pin or gudgeon" is the part attached to the "connecting-rod," which rod joins the crosshead to the crank pin.

(180) Forms of Crossheads.-The crosshead may be in the form of a block rigidly fixed to the piston-rod, or forming a part of it, containing brasses for the crosshead pin bearing, and having one or two guide plates connected to it, according to whether the engine has single or double guides. Such a crosshead for a single slipper guide is seen in Fig. 187, the guide plate or

"slipper" of cast iron or gunmetal being fixed by set screws to the under side as shown; the brasses BB are held in position by two bolts, b b, and a steel cap, C, the crosshead and cap being cut away at an angle of 60° around the bolts to save weight. In this example, which is for the vertical engine of Ex. A, the crosshead and the piston-rod, PR, are forged in one piece of mild steel. For such a crosshead the small end of the connectingrod is simply forked like an ordinary pin or knuckle joint as shown in Fig. 178 (see § 59), having the crosshead pin fixed to it. The slipper plate, S P, moves between guides formed by bolting two plates, G P, to a raised part of the engine framing, the cross-section being as shown in the end elevation of Fig. 187 (see also Fig. 178, D). This arrangement is often spoken of as a single slipper guide.

In the above design it will be noticed that the brasses are part of the crosshead, and the crosshead pin part of the connectingrod, but in other patterns now to be described the brasses are a part of the connecting-rod, and the crosshead pin of the crosshead.

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In the design of Fig. 188 the crosshead is a cast-iron box, CB, having the top and bottom faces circular and turned to an arc, having its centre at the centre line of the piston-rod, P R. The

piston-rod is connected by a cottered joint, and the crosshead pin, CP, passes through bosses cast on the inside of the crosshead, the distance between them being equal to the length of the brasses in the small end of the connecting-rod. The guide G is then a cast-iron trunk having part of the sides removed, and bored inside to receive the crosshead, and with flanges attached

for bolting to the engine bed. The diameter of the guide inside, and, therefore, of the crosshead, is so arranged that the connectingrod shall clear the guide when in its highest and lowest positions. The crosshead and guides as dimensioned in the figure are for a horizontal engine, cylinder 8" diameter, and 10" stroke.

The chief advantage of this form of trunk guides is that the machining can all be done in the lathe or boring machine, and is thus much cheaper than the planing and shaping required for the slipper guide, and for the guides of Fig. 190. This type of trunk guide is, therefore, very common in ordinary commercial engines, both vertical and horizontal.

The box form of crosshead with its inside pin necessitates a special arrangement of the small end of the connecting-rod, as

it is evident the brasses could not be adjusted from inside the pin. The design to meet this difficulty is shown in Fig. 189, end C; the rod is forged with an enlarged end, which is then slotted out to receive the brasses, B B', and the wedge block, W, which is moved up and down against the tapered face of the front brass, B, by turning round the screw shown, thus adjusting the brasses. The back brass, B', is a semicircular bush often provided with a pin to prevent movement, but this is really

unnecessary, as there is no tendency for the brass to move sideways when in position.

In the third design, shown in Fig. 190a, the crosshead C is a wrought-iron fork or knuckle joint, fastened to the piston-rod PR by a cotter, and holding the crosshead pin C P, thus making an ordinary pin joint (see § 59). The crosshead pin is extended on each side, and fits into cast-iron guide blocks, G B, which move between double guide bars, G (Fig. 1906) of T section, bolted at one end to a projection on the cylinder cover (Fig. 181), and at the other to a vertical frame, G F (Fig. 1906). This is also a common arrangement for horizontal engines, but is more expensive, although no more convenient than the slipper guides of Fig. 187. It possesses one advantage in common with the slipper guide as compared with the trunk guides, in allowing freer access to the piston-rod gland and to the crosshead brasses.

The design of the small end of the connecting-rod for this type of crosshead will be explained under the heading of connectingrods. The crosshead and guides of Fig. 190a, b are for the horizontal engine of Ex. B.

(181) Connecting-Rod Ends.-The ends of connecting-rods which join the crank pin are provided with brasses surrounding the pin, held in position in two different ways. In one design shown in Fig. 189, the connecting-rod end, CP, is enlarged, making what is called a T end (marked T), and the brasses B B are fixed between it by bolts passing through them and through a cap C, the nuts being on the rod end, as in the figure, or, as frequently seen, on the outside of the cap. The brasses are adjusted for wear by filing away the common face of contact at a, or by thinning a packing piece which is often fitted between the brasses. In a similar and very economical design, the rod is forged with a large solid end, which is first bored for the brasses and bolts, and then slotted across the centre to