CHAPTER XXI.
THE STEVENS VALVE-GEAR.
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
Fig. 27.
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.