Patent Page: Callahan’s Cam-Stopper Engine

By Staff
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P.T. Coffield’s 1897 patent for the “cam stopper” as used on Callahan engines. The mechanism for coupling and uncoupling the sideshaft is at the crankshaft end of the sideshaft. The vertical flyball governor acts directly on the mechanism.

From the beginning, engine manufacturers were faced with the issue of wear. By the dawn of the 4-stroke engine, machining capacity had reached almost artistic levels, yet metallurgy was, relatively speaking, still in its infancy. Unless constantly bathed in lubricating oil, cast iron parts, no matter how well machined, were prone to wear. This led to a number of interesting innovations, including the famous Callahan “Cam Stopper” engine.

Awarded patent no. 579,789 on March 30, 1897, the design was the work of engineer Peter T. Coffield, who had hired on with W.P. Callahan & Co., Dayton, Ohio, following a stint at Springfield Gas Engine Co. in nearby Springfield, Ohio. According to Coffield’s patent description, a primary goal of the design was to “reduce the wear upon the essential parts by allowing them to remain at rest when the speed of the engine rises above that required.” Coffield’s approach to achieving this goal was a unique and clever scheme that involved coupling and uncoupling a sideshaft for valve control, the entire sideshaft brought in and out of motion depending upon engine speed and demand.

To achieve this, Coffield designed a pawl-and-dog clutch, which he incorporated into the crankshaft end of the sideshaft. Following established practice, a spiral gear on the crankshaft drove a corresponding gear on a sideshaft to turn the shaft, which was equipped with cams for valve actuation. Any similarity with normal practice ended there, however, because instead of being fixed, the spiral gear on the sideshaft could be coupled or uncoupled by a pawl and dog incorporated on a disc at the end of the sideshaft.

A detail drawing from Coffield’s patent showing the cam stopper mechanism. The governor moved a pawl (j) that engaged or disengaged a dog (i) to the sideshaft (A). When disengaged, the cam gear (A1) spun freely and the sideshaft quit rotating, timed to hold the exhaust valve open.

Briefly, a vertical flyball governor controlled engine speed by acting upon a pawl. At slow speed the relaxed pawl allowed the dog to contact a lug on the sideshaft, thereby connecting the sideshaft gear to the sideshaft for rotation. As engine speed increased the flyball governor acted on the pawl. This caused the pawl to swing out, allowing the dog to swing free of the lug on the sideshaft, disengaging the sideshaft gear from the sideshaft, causing the sideshaft to quit turning. Importantly, the process was orchestrated so that sideshaft disengagement occurred only when the exhaust cam on the sideshaft was in position to hold the exhaust valve open, to allow the engine to continue rotating without compression. The cylinder head-mounted igniter, operated by its own adjustable cam at the forward end of the sideshaft, was naturally taken out of action at the same time, saving battery life. Likewise, when the governor quit pressing against the pawl, the dog would re-engage to lock the sideshaft gear to the sideshaft, and only at the point necessary to continue normal 4-stroke operation.

With power-operated intake and exhaust valves, Coffield’s design would have been particularly advantageous for engines employed in industrial, continuous-operation applications such as running a line shaft, where smooth and consistent engine cycles were more important than for a farm engine running an ensilage or a buzz saw.

Detail of the intake (Fig 2) and exhaust (Fig 3) cams. Note the hairpin springs for holding the valves shut. The intake cam opened two valves, one for fuel (c) and the other for air (a).

Built in sizes ranging from 4hp to 100hp, Coffield’s design proved itself well in practice, and apparently inspired similar schemes by at least four other companies. Columbus Machine Co., Columbus, Ohio, Field-Brundage Co., Jackson, Michigan, One Minute Mfg. Co., Newton, Iowa, and Orr & Sembower, Reading, Pennsylvania, built engines employing variations on the Callahan cam stopper theme, all of them appearing some years after the first Callahan cam stopper hit the market in 1896.

Renowned for being extremely well made and long lasting, Callahan engines are today amongst the most collectible of vintage gas engines.

Know of an interesting patent? Contact Gas Engine Magazine at 1503 SW 42nd St., Topeka, KS 66609-1265, or email

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