Movement No. 89 demonstrates the eccentric mechanism, one of the most important devices in steam engine history. An eccentric disk is mounted off-center on a rotating crankshaft. As the shaft turns, the eccentric disk revolves within a close-fitting eccentric strap, which is connected to a rod. Because the disk’s center does not align with the shaft’s center, the strap is pushed back and forth, converting the continuous rotary motion of the crankshaft into reciprocating rectilinear motion. This motion was primarily used to operate the slide valves of steam engines, precisely timing the admission and exhaust of steam. The same mechanism was also adapted for driving pumps and other auxiliary machinery.
89. An eccentric generally used on the crank-shaft for communicating the reciprocating rectilinear motion to the valves of steam engines, and sometimes used for pumping.