DESCRIPTION OF MOTION

This motion has been designed by A.J. Stevens, General Master Mechanic, Central Pacific Railroad, to overcome the well-known objections to the link-motion,—viz., letting the steam escape early in the stroke when the engine is running hooked up; also, the closing of the exhaust-port early, leading to excessive compression. The motion is developed from the Walschaert motion, well known in Europe, and applied to some extent to narrow-gauge engines in this country.

ARRANGEMENT OF THE MOTION

In the Stevens motion two valves are employed, one at each end of the cylinder, as can be seen in Fig.27. The valves have supplementary passages for steam and exhaust, being an improvement on the Allen valve. Valves without the supplementary exhaust-port have been designed by Mr. Stevens for his engines, and they are shown in two positions in the supplementary figures 2 and 3 [p. 288]. These valves closely resemble the Allen valve; but, in operating, steam is admitted to, and exhausted from, the cylinder through the same passage. This arrangement provides double area of admission and double area for exhaust steam,—an important consideration, especially with high piston speed.

VALVE MOVEMENT

The valves are actuated by two motions,—one taken from a single eccentric, the other derived from a connection with the cross-head. The single eccentric is used to give both forward and backward motion, and is set in the proper angle to the crank to produce motion in either direction. For reversing the motion of the valve, a curved rocker-arm R (Fig.27) is used, on which moves a sliding-block r. Attached to the sliding-block is the link H, which connects with the lap-and-lead lever D. The lower end of this lever is attached by the link d to the cross-head, from which the lever receives an oscillating motion. This lever is suspended by a hanger from any convenient part of the engine. Projecting from one side of the lap-and-lead lever are two pins, to which are connected the valve-stem rods m and n. These pins are set upon the lever at points between 90 and 180 degrees apart, so as to give a differential movement to the valve, which can not be obtained when the pins are set opposite on the lever.

VALVE-STEMS AND STUFFING-BOXES.

Each valve has its own rod, and separate connection with the lever. In most instances the stem of the back valve is made hollow, and the stem of the forward valve is passed through it, so as to avoid the use of more than one stuffing-box upon the steam-chest; but in several instances they have been fitted up with two stuffing-boxes, the separate valve-stems working side by side.

HOW MOVEMENT OF VALVE IS GOVERNED.

The valve being coupled up by independent rods to the lap-and-lead lever, they will move together, and in the same general direction, from the action of the eccentric. But besides this movement, common to both, each valve is controlled and acted on by the angular position of the two pins, which results from the rotary or oscillating movement of the lap-and-lead lever upon its center; and each valve, instead of having a uniform rate of travel, has a variable movement. The degree of this variable movement is governed, both by the distance of the pins from the center upon which the lever works and the length of the lever, and can be increased or diminished by changing the position of one or the length of the other in construction. The horizontal movement of the valve received from the lap-and-lead lever while the same is at mid travel, is very slight, while the piston or the cross-head is traveling very rapidly. By this slow movement of the valve, steam is retained in the cylinder until the piston has nearly completed its stroke; and, on the other hand, the exhaust is kept open. As the cross-head approaches either end of the stroke, the action of the valves is very much accelerated, receiving, as they do, their motion from the combined action of the lap-and-lead lever and the eccentric.

HOW EXHAUST LEAD IS CONTROLLED.

The exhaust lead is controlled by the pin in the lap-and-lead lever, which is on the center, or horizontal, when the cross-head is at either end of the cylinder. The steam lead is controlled by the pin that is vertical, or above the center of the lap-and-lead lever, when the cross-head is at either end of the stroke. Both steam and exhaust lead are uniform at all points of cut-off. By this arrangement of the valve-gear and valves, steam can be cut off at any point of the stroke (by moving the sliding-block toward the center of the rocker-arm), and is retained, if desired, to the last inch of the stroke. In receiving, the steam is evenly distributed at all points of the cut-off.

There are about as many pieces to this gearing as to the ordinary link-gear; but it is considerably cheaper in construction, and much more durable; while it does away with two eccentrics, and their straps and connections.