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What is the Scotch Yoke?

By Paul Woods
Updated Feb 22, 2024
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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.

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Discussion Comments

By nony — On Aug 10, 2011

@MrMoody - I’ve always loved simplicity in design as an approach to solving problems. I had a computer mentor who gave me a bit of advice when I was writing algorithms: “Remember, always find the easiest way to get there.”

What I like about the Scotch Yoke is that it seems to be quite simple in design, so much so that I bet you could probably build your own contraption at home.

You might use the wheels of a bike for your rotational part, and perhaps some PVC pikes for the horizontal bars, but I am sure it would be easy to do. You could probably rig it into a pump or a generator of some sort too.

By MrMoody — On Aug 09, 2011

@Charred - I’m open to all sorts of experimentation myself as well. I like the fact that the Scotch Yoke can function two ways.

It can convert horizontal motion to rotational motion, and vice versa. I saw an animation of a Scotch Yoke mechanism once and the setup does seem to be somewhat intuitive.

As to the critics charge that there would be more wear and tear with this kind of engine, I think scientists could experiment with a variety of materials – even titanium, for that matter, to make it last longer.

By Charred — On Aug 08, 2011

I wouldn’t mind seeing the Scotch Yoke being used again at least in a proof of concept automobile, especially if it’s supposed to be more efficient and has fewer moving parts.

Nowadays everyone is looking for a more efficient way of building automobiles, whether it’s through the use of electric motors or hybrid cars. I wouldn’t mind seeing some variation from the standard internal combustion engine as part of that process; after all, the internal combustion engine in its current form has been in the mass market for over a hundred years.

The article says that the Scotch Yoke is not as effective, but I’m sure that inventors could tweak it in some way to make it more efficient. In the end, let the buyers decide what kind of engine they want in their car.

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