FIG. 61.—STEPHENSON’S LINK MOTION
Stephenson’s son Robert is commonly credited with the invention of an ingenious link motion, although the invention is also claimed for W. T. James of New York. We must pause a moment for a description of this ingenious link motion, because it became a standard in locomotive construction that is still in service. As explained in Chapter IX steam should be used expansively in order to obtain a maximum of efficiency. After a certain amount of steam has been admitted into the cylinder it is cut off from the boiler and it pushes the piston by its own expansion. The speed of the locomotive must be varied by varying the point of cut-off, and the direction of motion of the locomotive is reversed by reversing the motion of the valve. In the Stephenson link motion this is all effected by the operation of a single lever. Fig. 61 shows the arrangement. The valve is moved back and forth by the rod A which is connected to the arm B suspended from the frame of the locomotive. The drive shaft is shown at C and it carries two eccentrics projecting on opposite sides of the shaft which move the rods D and E back and forth. The outer ends of these rods are connected to opposite ends of a curved link F and a pin G on the arm B engages a slot in the link. The link is held up by a rod H. As the drive shaft rotates the slotted link is oscillated back and forth on its own center. When the link is lowered so that the pin G is at the center of the slot there is no motion of the valve rod A, but when the pin is at the top or at the bottom of the slot the valve rod has its greatest motion. As the top of the link is moving in one direction while the bottom is moving in the other direction, it will be plain that the action of the valve when G is at the top of the link will be the reverse of the action when G is at the bottom. The link is raised and lowered by means of a lever I which is connected to one arm of a T-shaped lever J which on one side of its fulcrum is connected to the rod H and on the other to the counterweight K.
There are certain defects in the Stephenson link motion that we cannot discuss here. These became serious as engines grew larger and more powerful so that to-day it has been largely superseded by other valve gears. The most important of these is the Walschaerts gear, invented by a Belgian engineer. This gear, unlike Stephenson’s, is conspicuously placed outside the drivers and is particularly noticeable because of its peculiar grasshopper motion. The action of the gear is so complicated that a description of it would be out of place in this book.
MODERN LOCOMOTIVES
We cannot enter minutely into the development of the locomotive from the crude machine of Trevithic’s time to monsters of to-day. There has been a progressive growth of locomotives in power and in speed. Our biggest freight engines are so powerful that they cannot be used for pulling alone because they can pull a greater load than the draw-bars of the cars can stand. If placed at the head of a long train they would yank the forward cars loose from the rest. Hence they are placed at the rear of the train to act as pushers or in the middle of the train where half their energy is expended in pushing the cars ahead of them and the other half in pulling the rest of the train.
The most powerful steam locomotive of to-day (1921) weighs 342 tons and its tender 107 making a total of 449 tons. Its length is 105 feet and its boiler 8 feet 7⅛ inches in diameter. Its low pressure cylinders (4 feet in diameter) are larger than the locomotive boilers of 50 years ago. Its high pressure cylinders are 30 inches in diameter and the stroke is 32 inches. It may be operated either compound or simple, i. e., the smaller cylinders may exhaust into the larger ones or they may take steam direct from the boiler. The tractive effort compound is 147,200 pounds and simple 176,600 pounds and the total horsepower developed is 5,040. Each cylinder drives five coupled drivers, in other words there are twenty power-driven wheels with a pair of trailers and a pair of pilot wheels. Six and a half tons of coal are consumed per hour.
In the matter of speed a mile per minute has become common and regular scheduled runs over long distances at an average well above sixty miles per hour have been maintained, but the present tendency is to reduce speed somewhat in favor of safety.