The Locomotive Of Today

84 The Engine Piston Valves. themselves to any slight irregularity better than more rigid rjieans can. This device is peculiarly suitable for American engines, where outside cylinders with steam chests above are invariably used, but it would be difficult if not absolutely impossible to fit it to our engines with steam chests between the cylinders, though it could be used on those engines having their steam chests above or below. As an engine, when running with steam shut off, and the motion left in gear—that is fore gear running forward and back gear when moving backwards—has a continual tendency to withdraw air from the steam chest and discharge it up the chimney at each stroke of the piston, thus forming a partial vacuum within the steam chest, with a retarding effect on the engine, a small relief valve is fitted to the chest, as will be seen at the left hand of D, where a circular vertical valve takes its seat upon a bush, and whilst its under side is in free communication with the atmosphere its upper side is acted upon by the pressure within the steam chest, so that when the pressure above is less than that below the valve lifts and air can enter and destroy the partial vacuum. Another remedy for the excessive friction referred to, and one applicable to the class of engines precluded from the use of the Richardson and similar balanced slides, is found in the employment of piston valves. A successful and favourite form of these for locomotives is shown at F, and is known as Smith’s patent. It has no large surfaces exposed to steam pressure, and consequently no great power is required to move it. The ports are made with a cylindrical face, and a bush is inserted, having holes through it opposite the port, and being bridged over at intervals, as otherwise the rings would be likely to catch in it and cause damage. A stuffing box is provided at each end to ensure the piston working fair. The rod or spindle has a collar forged upon it, and against this a cap of cast Steel with three radial grooves is threaded, and is prevented from altering its position by a key let into the rod; then the piston ring itself, in three pieces, is placed on, each of the three pieces having a radial feather projecting inward, and engaging into the grooves of the cap. The ring is made of hard gunmetal in one bush, and turned to size and shape, then cut across into three pieces, the joints being kept by means of the grooves and feathers against blank places in the bush, so that no steam may escape through them into the ports. Next a ring of softer metal is placed in position, and effectually covers the side of the cuts, preventing steam from passing into the exhaust chamber. This ring is split, the joint in it being made by a feather half in each side of the