The Locomotive Of Today

The Engine : Valve Gear. 87 valve closes, and then the piston still moves along-, reaching- the point of release as shown at O, when the port is opened to exhaust, and the port at the other end closed, causing’ “compression.” . The point at which the steam is cut off can be vaned by means of the reversing gear, as we shall fully explain läter. The point of “cut-off” is usually measured in terms of the percentage of the entire stroke of the piston ; thus, cutting off at 25 per cent, means that the valve closes to steam when the piston has reached 25 per cent, of its entire stroke, 5° Per cent, when it has travelled half its stroke, and so on. All the other functions of the slide valve then take place earlier. or later in the stroke, according to whether the engine is cutting off early or late. The necessary movement is imparted to the valves by means of the “motion’ or valve gear, as already mentioned. This may be of a great variety of types. On the first loco- motives, which only ran at some four or five miles per hour, a collar was fitted at each end of the rod, actuating the three- way cock which did duty for a valve, and a projecting' arm upon the crosshead caught the collar when the piston reached the end of its stroke and reversed the direction of the steam , similarly, when the other end was reached, the arm struck the other collar, and again reversed the cock. For starting the engine it was necessary to observe the position of the cranks and. move th© valve rod by band, so that tlie steam coulcl be admitted to the required side of the piston, after which the engine took up the movement automatically. No advantage could be taken of the expansive properties of the steam, as the full steam pressure was maintained throughout the stroke. When the success of the locomotive was assured, and its speed inereased, the need for improving- the valve gear became ap- parent, and amongst the advances leading up to the present motion was the employment of a single eccentric for each valve and cylinder, loose upon the axle, and having- catches at the side to engage studs upon the axle when moved laterally. A lever and gear worked by a treadle 011 the footplate controlled the position of the eccentrics. When starting the engine, the driver put the eccentrics out of gear by the treadle ; then by means of a lever he raised the small ends of the eccentric rods, and notin g the position of the cranks, or if more convenient the balance weights in the wheels, he, by means of another handle, moved the valves to open the necessary ports to steam, and workeel them by hånd until the engine was moving, then, with the treadle, he threw the eccentrics over to engage the studs, at the same time dropping the small ends of the rods to