Pneumatic Stepper Motor by Mark J E Bellis, 14th November 2009 The original pneumatic steam engine was the second model of set 8868, a refuse truck, from 1991. The grab and arm raising functions of the truck formed a logical loop that repeated its sequence as long as air was supplied. My first steam engine in the early 1990s, based on this system, used two cylinders and two valve switches to turn a shaft. Reversing this engine pneumatically would take quite a few valve switches because there are two pairs of pipes to swap over. Using two 4-switch reversers would need another 8 valves, making it large and expensive. A gear system could reverse the direction of rotation with no additional valves, but there is a large dead-band in the valve positions. A 3-cylinder stepper motor is much easier to reverse pneumatically. No additional switch valves are required. The sequence of operation determines the direction of rotation. Fixing the valve levers to cranks on a shaft allows rotary motion to be transmitted from one place to another. Both ends of the cylinders are used here but the system can work with just one end used. Small cylinders are used because they fit better in the loading gauge of an L-gauge train. Operation is open-loop, with no feedback. The motor and valves would be inside the boiler of a steam loco, with the battery in the tender. The speed of rotation would be transmitted in proportion to motor speed, as long as sufficient air were supplied. The rotation is jumpy, in 60-degree intervals, but might be smoothed with development of the system. The crank length is slightly too long for the small cylinders because their extension is not quite 2M. A working pneumatic L-gauge loco is surely one of the holy grails of LEGO trains. This system cheats a bit by not using the pneumatic parts as a steam engine. Mark J E Bellis