The Locomotive Of Today

86 The Engine : Lap, Lead, Action of Slide Valnes. At G is shown a cross section of the valve where the radial feathers upon the rings and the sockets of the cap may be plainly seen. It now becomes necessary to explain a few terms used when speaking of slide valve motions. The lap or over-lap is the amount that the valve projects over and beyond the port opening when the valve is in its central position. It is shown in black at H to be more easily recognised. It is provided to give a varying point of cut-off, so that the expansion of the steam may be utilised in the cylinder. Inside lap is a projection inside the valve and over the inside of the steam port, and is used to delay the release of the steam in the cylinder; it is similarly shown at J. Inside lead, or negative lap, shown at K, is provided to hasten the release of the steam, and is the reverse of inside lap, as the width of the cavity is greater than the distance over the ports. Inside lap is seldom used on locomotives, but inside lead is met with on some engines, especially compounds. As a rule, however, locomotives have the inside width of the cavity equal to the distance between the outside edges of the bridges or port bars which separate the ports. The cycle of operations for each revolution of the crank is as follows :—First steam is admitted to the cylinder, called “admission, then the valve closes, termed “cut-off”; the steam shut in the cylinders then exerts its pressure by “ expansion.” Next the valve opens to exhaust, called “release”; the distance between the cut-off and release, through which the piston is propelled by the expansion of the steam, is known as the “expansion period." The valve closes the port to the exhaust at the end towards which the piston is moving before the piston reaches the end of its stroke, and the steam left in the cylinder is compressed, forming a “ cushion ” to assist in bringing the piston to rest without shock, and as the valve here opens a little to live steam, called the “lead,” the piston is ready to perform its return stroke as soon as the crank has passed its “dead centre.” Admission, cut-off, expansion, and release then take place from the other end, bringing the piston back to the point of starting. All the above operations are repeated for each revolution of the driving wheels. At L the piston is shown at the extreme end of its stroke, and the slide valve is open to “lead.” At M the piston has travelled along some distance—about half the distance to the point of cut-off—and the valve has opened its maximum width to steam ; it then commences to close, and when the piston has reached the point of “cut-off”—shown at N—the