The distinguishing feature of the Melville Clark player action was the "spring wind" mechanism which ran the music roll and became an integral part of the transmission. The winding of the main-spring was done by linkage to the pumping pedal movement. When fully wound, a slipping clutch would prevent overwinding. This was a nice feature for two reasons: it eliminated the terrific drain on the vacuum system otherwise required to run an air motor, and no pumping was required on rewind, due to the energy stored in the spring.
Earlier player stacks pushed directly on the keys with poppet wires and guide rail immediately behind the cover in back of the keyboard. The folding fall board normally found on pianos, was eliminated to make room for this player action. The stack was four rows of pneumatics facing forward. The valve chest was above and to the rear: Two rows of horizontal valves which screwed directly into the moveable leaf of a pouch-pneumatic through the inside valve seat. In order to have access to the pouch chamber, all of the valves had to be un-screwed first. Also there was a three-tier stack with same valve arrangement, but with smaller pneumatics which engaged with whippen of piano action. A later type of pneumatic stack followed; still with the spring wind, but with the more conventional three tier stack with circle pouches in individual chests above the pneumatics which engaged with the whippen through poppet wires at rear.
Unfortunately, there is no technical data on the spring-driven air motor mentioned above. And although Art Reblitz's book mentions Melville Clark numerous times, even his book doesn't contain any definitive information about this particular action.
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