The scotch yoke converts horizontal motion to rotational motion and is more efficient than a piston in producing rotational motion. It was used in the Bourke engine but had drawbacks. It is used in actuators for high-pressure pipelines to convert linear motion to rotational motion.
The scotch yoke is a mechanical device that converts the horizontal motion of a slider from side to side into rotational motion, or converts rotational motion to horizontal motion, also called linear motion. Parts of this device include a slide bar, a yoke on that bar with a slot cut out, and a smaller bar connected to the yoke and pinned through the yoke slot to the slide 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 motion.
In many internal combustion engines, linear motion is converted to rotary motion by means of a crankshaft, a piston, and a connecting rod. The scotch yoke is considered a more efficient means of producing rotational motion, since it spends more time at the top of its rotation than a piston and has fewer parts. On the downside, the device suffers from more wear and tear and has a shorter lifespan, reducing its effectiveness for constant use.
The scotch yoke was used as the power conversion device for the Bourke engine, a two-stroke engine developed in the early 20th century by engineer and inventor Russell Bourke. With far fewer moving parts due to the scotch yoke, it was considered more efficient than other engines, producing more power for its weight, and producing fewer emissions. Critics challenged its benefits saying that the engine would run too hot and lose efficiency and would need an extra case to compensate for the double movement required by the scotch yoke, thus offsetting the weight with the power advantage. Bourke developed working prototypes before World War II, but the engine was never widely produced. This device continues to be used on some engines, particularly engines powered by steam or hot air.
The non-combustion uses of the scotch yoke focus on its benefits in the construction of actuators for high-pressure pipelines. An actuator is used to convert the flow of liquid or vapor 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 pipe into a rotational motion that can be used to produce power. Similarly, by adjusting the transfer from linear to rotational motion, the scotch yoke actuator can stop or regulate the flow of a substance through a pipeline.
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