The Locomotive Of Today

The Engine : Piston Valves. 85 cut, and it is kept from turning by means of a set screw in the cast iron distance socket, which is placed upon the spindle next; then a wrought iron liner, of the required thickness to keep the pistons the right distance apart, is put on ; then another distance socket, the soft metal ring; then the gun metal ring in three segments, and another Steel cap, the whole being held by a nut screwed on to the spindle and tightened up to the steel cap. It will be seen that the three segments are held to the valve face by the steam pressure, which acts inside exactly as in the ordinary flat valves, but here the sur- faces exposed to pressure are much smaller in comparison. Another great advantage of the Smith valve, is its provision for the escape of any water which may accumulate in the cylinders. When an engine is fitted with ordinary slide valves, and trapped water has collected in such a quantity as to more than fill the clearance spaces, it can force the valve from the face, and escape into the steam chest; to do this however the pressure must be very great when the regulator is open, as the area of the port under water pressure, tending to force the valve off the face is small compared with the area of the back, upon which the steam exerts pressure. If ordinary piston valves are employed with solid rings, similar to those used in the cylinder, trapped water would be absolutely prevented from leaving the cylinders, and as water is incompressible, damage would be almost certain to ensue, but with the valve under consideration the three segments are always free to leave the face and fali inwards, and when the pressure outside overcomes that upon an equal area inside they collapse and allow water to escape; thus, this piston valve performs two separate and important functions—reducing the power required to move it, and also acting as a release valve for water trapped in the cylinders. It has been estimated that the resistance due to friction is one-ninth that of the ordinary slide valve, and this has to some extent been verified in practice by noting the relative wear of the two forms of valve. This reduction of friction prevents heating of eccentric straps, etc., and minimises the risk of failure of working parts of a locomotive valve gear. In the event of breakage of a cylinder cover or connecting rod, the valve on the disabled side of the engine may be fixed in mid-position as usual, and the engine brought home with the remaining cylinder. There is no fear of broken segments of the valve entering the ports, as all broken portions are held in place, whereas with an ordinary slide valve, when broken, damage is often caused by fragments getting through to the cylinders.