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The Scotch yoke is a mechanical device that converts the horizontal motion of a back-and-forth sliding bar into a rotational motion, or converts rotational motion to horizontal motion, also called linear motion. The parts of this device include a sliding bar, a yoke on that bar with a slot cut out, and a smaller bar connected to the yoke and affixed by a pin through the yoke slot to the sliding bar. As the bar slides back and forth, or reciprocates, the smaller bar is forced to slide up and down in the yoke slot, creating a rotational movement.
In many internal combustion engines, linear motion is converted into rotational motion by means of a crankshaft, a piston and a rod that connects them. The Scotch yoke is considered to be a more efficient means of producing the rotational motion as it spends more time at the high point of its rotation than a piston and it has fewer parts. As a disadvantage, the device suffers more wear and tear and has a shorter useful life, which reduces its effectiveness for constant use.
The Scotch yoke was used as an energy conversion device for the Bourke engine, a two-stroke engine developed early in the 20th century by engineer and inventor Russell Bourke. With far fewer moving parts because of the Scotch yoke, it was considered more efficient than other engines, produced greater power for its weight and produced lower emissions. Critics challenged its benefits saying the engine would run too hot and lose efficiency and need extra casing to compensate for the double motion required by the Scotch yoke, which would offset the weight to power advantage. Bourke developed working prototypes prior to World War II, but the engine was never widely produced. This device continues to be used in some engines, particularly engines driven by steam or hot air.
Non-combustion uses of the Scotch yoke focus on its benefits in construction of actuators for high pressure pipelines. An actuator is used to convert the flow of liquid or steam into energy or to slow or stop the movement of the substance in the pipeline. A Scotch yoke actuator converts the linear motion of the substance through a pipeline to a rotational motion which can be used to produce energy. Similarly, by adjusting the transfer from linear movement to rotational, the Scotch yoke actuator can stop or regulate the flow of a substance through a pipeline.