The engine or the mechanism for applying the pressure of the steam generated in the boiler to is controlled from the cab by means of a lever known as the reversing lever. Inside the cylinder works the piston. This is a disk-shaped structure of metal, and is made in various ways, but al ways so as to fit the cylinder steam-tight, and so as to have ample strength to convey the push and pull of the steam to the piston-rod. The piston-rod is eirculsr in section and is made of forged steel, It passes through the back cylinder bead by means of a circular orifice packed so as the rotation of the driving wheels, con sists of the cylinders, pistons a nil piston rods, connecting-rods and parallel rods, with their attachments and appurtenances. The cylinders are placed one on each side of the locomotive near the front end, as shown in Fig. 8. There are three parts to each cylinder, viz. the cylinder, the steam-chest, and saddle. The saddles of the two cylinders, being bolted together, afford a support for the boiler, and in turn rest on the frame, as shown in Fig. S. The steam-chest on each cylinder re ceives the steam from the boiler and contains the valve which admits it to the cylinder. Fig,. 12 is a longitudinal section showing the inside of the cylinder and steam-chest of a simple locomo tive. As will be seen, there is a steam-port from the chest to each end of the cylinder and between them an exhaust port, b: the motion of the valve alternately opens and closes the steam-ports so as to admit steam first at one end and then at the other end of the cylinder. In the drawing, Fig. 12, steam is shown entering the forward end of the cylinder and exhausting from the rear end. The ralre most commonly used is a slide valve of D shape, but piston valves are also em ployed. A piston valve is shown in the suc ceeding section on compound locomotives. The sliding motion of the valve is controlled by a device commonly called the valve-gear.
Various forms of valve-gear are employed, but the one in most extensive use is the Stephenson link, the operation of which is explained in the article on STEAM-ENGINES. The valve motion to be steam-tight and yet allow for movement to the rod. At its rear end the piston-rod connects with the cross-head, NI, working between the guides G(Fig. 12). In Fig. 13 are shown another form of cross-head and guides and also the ar rangement of the connecting-rod and the side rods for an engine with six drivers. Connecting-rods and side rods are made of forged steel and have 1 sections.
The frame of a locomotive is the platform by which the boiler and engine are supported on the wheels. In America bar frames are nearly al
ways employed. Fig. 14 shows the frame for a heavy freight locomotive. It will be seen that the top and bottom rods of the frame are con nected by vertical struts known as pedestal jaws, between which come the boxes of the driv ing wheels. On the bottoms of the pedestal-jaws are bars or binders known as pedestal-caps. The number of jaws and style of frame are governed mostly by the number of driving wheels. In order to make an engine ride easily and to reduce the shock and concussion on the run ning gear, the engine is provided with springs placed over each driving box and carried on a saddle which straddles the frame and rests on the driving box, as shown by Fig. 14. In order to equalize the weight between drivers there are provided bars between the drivers which are called equalizing bars.
Driving wheels are made with cast iron or cast steel centres with a forged steel tire. The usual method of attaching the tire to the cen tre is to bore the tire slightly smaller than the centre and then expand it by heat until it slips over the centre onto which it shrinks. Driving wheels always have counterbalance weights at tached on the side opposite the crank-pin to neutralize the shock which an unbalanced wheel gives to the track. As an indication of the quan tities and quality of the different materials which enter into the construction of a modern heavy locomotive, as described above, the following figures given by W. H. Marshall, superin tendent of motive power. Lake Shore and Michi gan Southern Railway, are of interest. The en gine selected as an example is a passenger en gine, weighing about 175,000 pounds, and having three pairs of SO-inch drivers. Some of the small details have not been estimated. and others have been omitted from these figures because of their miscellaneous character; such, for in stance, as the brasswork about the engine, cab fittings. the piping, boiler-lagging, boiler-jackets, etc. Exclusive of these the amount of wrought iron in the engine is 16,950 pounds; cast iron, 19.550 pounds, of which 11,350 pounds are in the cylinders and 1760 pounds in the grates; cast steel. 27.660 pounds, of which about 16.000 pounds are in the driving and truck wheels; pressed steel, 1650 pounds; rolled-steel plates and shapes. 3770 pounds; forged steel, 11,270 pounds; tire-steel, 10.500 pounds; malleable iron, 1300 pounds; and wood, 4100 pounds. The boiler weighs 47.S50 pounds, and the water in the boiler 22,500 pounds, making a total weight of 167.100 pounds.