Movement No. 150 demonstrates a variable expansion valve motion for steam engines, using a series of cams of varying throw mounted on a sliding shaft. Multiple cams — each with a different lobe height (throw) — are fixed along a shaft that can slide lengthwise. A single lever, connected to the valve-rod, is positioned to be acted upon by whichever cam is slid into alignment with it. By shifting the shaft axially, the operator selects a different cam: a cam with a larger throw produces a greater valve movement, admitting steam for a longer portion of the piston stroke (less expansion); a cam with a smaller throw produces less valve movement, cutting off steam earlier and allowing it to expand more in the cylinder (greater expansion and fuel economy). This selective cam mechanism allows the engine operator to precisely control the steam cut-off point and adjust engine power and efficiency on the fly — an important advancement in 19th century steam engine control technology.
150. A valve motion for working steam expansively. The series of cams of varying throw are movable lengthwise of the shaft so that either may be made to act upon the lever to which the valve-rod is connected. A greater or less movement of the valve is produced, according as a cam of greater or less throw is opposite the lever.