The pistons in engines smaller than 10-in. cylinder are solid, with rings sprung in, but for 10 in. and over they are as shown in Fig. 13. The main characteristic is that the rings are made much too large for the cylinder, sprung in with considerable force and pinned in that position. and the outside turned to a perfect fit to the cylinder. After this, the pin-holes in the rings are filled to admit of the rings being compressed, while not allowed to expand. Only a part of the thickness of the piston is used to secure it to the rod, this being done to give adoitional length to the piston-rod bush. The castings are very thin and light, and are thoroughly ribbed for strength. The only part that can wear is the bull-ring, which is packed down to keep the piston in the center of the cylinder by liners made of narrow strips of sheet metal. Flanges cast on the spider and follower inside of the piston-rings make them so stiff that only four studs are used. The pistons are secured to the rods by two taper fits, a parallel thread, and shrink fit.
The piston packings are simply Babbitt-metal bushings, with reamed holes slightly larger than the rods, so as to be a free sliding fit. One form is shown in Fig. 14. They rest in spherical seats, which are free to move in any direction.
The cross-head is shown in Fig. 15. Tt is of steel or malleable iron casting, and is threaded on the piston-rod and secured by being split and clamped by the binding-bolts. The cross head pin is a hollow steel casting made fast to the connecting-rod, and turns in two adjustable Babbitt-lined boxes in the cross-head. The object of this is to secure lightness, extra wearing surface, to prevent side swinging of the connecting-rod at the fly-wheel end, and to give ready means of oiling. The cross-head is what is known as the slipper-guide sort, the lower guide being adjustable in the vertical direction. It rests upon and is bolted upon two inclined planes, and may be readily raised or lowered to bring the piston-rod in perfect alignment.
The crank-shaft and wheels are shown in Fig. 16. The steel crank-pin and shafts forced into the large bosses of the two wheels form a solid structure, dividing the strain equally between the bearings, and give an opportunity to balance the reciprocating parts properly, furnish a support for the governor, and relieve the main bearings of a good part of the thrust of the piston.
The main journal-boxes are shown in Fig. 17. These sleeves, A, are made eccentric and lined with Babbitt-metal cheek-pieces B, which bring the shaft concentric with the outside of the shell. The cheek-pieces are retained in place by Babbitt-metal feather Cat the bottom, and a brass wedge D at the top. This furnishes a complete bearing at the bottom and sides,
and one in which the wear can be compensated for. Narrow metal liners are introduced at the bottom, which can be removed and placed by the side of the wedge at the top. By this change the cheek-pieces are shifted down, and, being wedge-shaped, the opening is closed.
The governor, shown in Fig. 18, consists of a single ball linked to the eccentric and con nected to a spring by a metal strap, and so located and weighted as to counterbalance the eccentric and its attachments. When the speed of the engine reaches the point where the centrifugal force of the governor-ball overcomes the resistance of the spring, the ball moves away from the center of rotation, and in doing so it carries the eccentric nearer the shaft, shortens its throw and the travel of the valve, and reduces the steam admitted to the cylinder.
The eccentric is cast upon a swinging plate, which is pivoted to the boss of the fly-wheel. The eccentric-plate is subject to a twisting strain, to resist which, in addition to the long stud and journal, there are two links connecting it to the governor-ball.
The valve motion has two peculiarities: the position in which the eccentric-plate is pivoted to the wheel, which gives a variable lead to the steam admission, and the direct con nection between eccentric and valve. The method of securing the slide to the valve-rod is shown in Fig. 19. The method of securing the rod to the valve admits of the valve being removed and returned without disturbing the adjustment.
The valve controls the distribution of steam very much as is done by the common D valve, but having a variable travel controlled by the governor, it varies the amount of steam admitted as the work imposed on the engine varies. The valve, as will be seen in Fig. 21, is a rectangular plate, quite thin, and with five openings through it. It is made flat on its two sides, and of uniform thickness. The valve works within an opening formed by the valve seat and a pressure-plate and two distance-pieces placed above and below it (see Fig. 20). The pressure-plate has recesses in it opposite the ports in the valve-seat, and the distance pieces are made about in. thicker than the valve. The pressure-plate resting against the distance-pieces relieves the valve of all pressure, and it works within its opening the same as a piston-valve. By the recesses in the pressure-plate and the small openings through the valve double ports are opened both for steam admission and exhaust.