Back at the eccentrics

Now that Sam Thompson completed shaping the valves on the Shay's three steam engines last June, he has time to turn his attention to fabricating the replacement for the middle eccentric on the drive shaft. In time, Sam will machine the billet of cast iron into one of two eccentric halves.

The eccentric os "a mechanical device with an off-center axis of revolution that converts the rotary motion of one component of a mechanism to reciprocating motion in another."

Much like the valve lifters in an internal combustion engine, the Shay's eccentrics lift the valves that let steam into the cylinder.