Movement No. 93 presents the Scotch yoke mechanism — a elegant variation of the ordinary crank that eliminates the need for a connecting rod entirely. A crank pin (crank-wrist) rotates with the crankshaft and rides inside a slot cut into a yoke. As the crank rotates, the pin slides back and forth within the slot while simultaneously pushing the yoke sideways, converting rotary motion directly into pure sinusoidal reciprocating motion. Unlike the ordinary crank-and-rod arrangement, the Scotch yoke produces a perfectly sinusoidal output with no angular deviation, and the yoke always moves in a perfectly straight line. This makes it ideal for applications requiring smooth, pure harmonic motion, such as in testing equipment, fluid pumps, and certain engine designs.
93. Crank motion, with the crank-wrist working in a slotted yoke, thereby dispensing with the oscillating connecting-rod or pitman.