Corliss Valve Gear

US 6,162 is "Cut-off and working the valves of steam-engines" — a valve gear, not an engine. The whole patent is about when the valves move.

On a plywood base, mark four points around a central pivot: two admission (top), two exhaust (bottom). This is the layout Corliss uses.

Mount a disc that can rock a few degrees each way on a central pin. This is the wrist-plate — one part that will drive all four valves.

Make an offset crank on a shaft and connect it by a rod to the wrist-plate. Turn the shaft: the wrist-plate rocks back and forth from one input.

Run four link-rods from the wrist-plate to four pivoting "valves". One rocking plate now opens and closes all four in sequence. Turn the shaft and watch the cycle.

Note the timing: an admission valve opens as its exhaust closes, on each side alternately. The single plate enforces the correct order automatically.

Instead of linking the admission valve rigidly, let the wrist-plate pull it open through a catch, with a spring trying to close it. This is the trip mechanism.

Set the catch to release partway through the opening. The valve is yanked shut by the spring almost instantly — far faster than a linked valve. Fast cutoff is the whole point.

Shift where the catch releases and mark the shaft angle at closure each time with the stopwatch cycle. Earlier release = steam admitted for less of the stroke = earlier cutoff.

Connect the catch position to a weight that stands in for the governor. Move the weight "out" and the cutoff comes earlier. Now speed controls fuel admission.

Run one side rigid, one side tripped. The rigid valve opens and closes slowly and symmetrically; the tripped valve opens gently and closes like a trap. That sharp closure is what saves steam.

With a protractor, measure how few degrees the wrist-plate actually rocks. A small, precise oscillation runs the whole valve set — elegance through geometry.

The patent. US 6,162, "Cut-off and working the valves of steam-engines", granted 10 March 1849 to George H. Corliss of Providence, Rhode Island, and reissued 13 May 1851 as No. 206. The reissue matters: a granted patent can be re-examined and re-granted with amended claims, and this one was.

What the claim actually covers. Corliss claims the use of a wrist-plate to carry the valve motion from a single eccentric to four separate valves, and the use of releasing (trip) valves with variable cutoff under governor control. Both halves matter. The wrist-plate is the mechanical economy — one moving part times four valves, in the right order, for free. The trip-and-drop cutoff is the thermodynamic economy — steam is admitted for only part of the stroke and then expands, and the exact cutoff point is set by the governor, so the engine trims its own fuel to the load.

Why the trip is cleverer than a linked valve. A valve linked rigidly to the drive opens and closes at the same slow, symmetric rate. Corliss instead has the wrist-plate latch the admission valve open and then release it, letting a spring (or a falling weight, the "dashpot") slam it shut in a fraction of the time. A sharp cutoff wastes almost no steam in a half-open valve. Steps 7 to 11 build and measure exactly that difference.

Regulation as a side effect of the same mechanism. Because the governor sets the release point, a Corliss engine holds its speed remarkably steady as the load changes — an early, elegant example of feedback control built into linkage rather than electronics. The governor does not throttle the steam (which wastes it); it changes how long steam is admitted.

What it became. The Corliss engine dominated heavy stationary power for half a century — mills, waterworks, factories — precisely because it burned perhaps a third less coal than the engines it replaced. Its most famous moment came at the 1876 Centennial Exhibition in Philadelphia, where a single gigantic Corliss engine in Machinery Hall drove the shafting for the entire building's exhibits through a forest of belts. When it was ceremonially started, it became the symbol of American industrial power. The valve gear you modelled here is what made it sip coal while doing it.