The Scotch yoke (also known as slotted link mechanism) is a

reciprocating motion Reciprocating motion, also called reciprocation, is a repetitive up-and-down or back-and-forth linear motion. It is found in a wide range of mechanisms, including reciprocating engines and pumps. The two opposite motions that comprise a single ...

mechanism, converting the linear motion of a slider into

rotational motion Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis interse ...

, or vice versa. The

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder (engine), cylinder a ...

or other reciprocating part is directly coupled to a sliding

yoke A yoke is a wooden beam used between a pair of oxen or other animals to enable them to pull together on a load when working in pairs, as oxen usually do; some yokes are fitted to individual animals. There are several types of yoke, used in dif ...

with a slot that engages a pin on the rotating part. The location of the piston versus time is simple harmonic motion, i.e., a

sine wave A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic function, periodic wave whose waveform (shape) is the trigonometric function, trigonometric sine, sine function. In mechanics, as a linear motion over time, this is ''simple ...

having constant amplitude and constant frequency, given a constant

rotational speed Rotational frequency, also known as rotational speed or rate of rotation (symbols ''ν'', lowercase Greek nu, and also ''n''), is the frequency of rotation of an object around an axis. Its SI unit is the reciprocal seconds (s−1); other com ...

Applications

This setup is most commonly used in control valve actuators in high-pressure oil and gas pipelines. Although not a common metalworking machine nowadays, crude

shaper In machining, a shaper is a type of machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is (archetypally) line ...

s can use Scotch yokes. Almost all those use a Whitworth linkage, which gives a slow speed forward cutting stroke and a faster return. It has been used in various internal combustion engines, such as the

Bourke engine The Bourke engine was an attempt by Russell Bourke in the 1920s to improve the two-stroke internal combustion engine. Despite finishing his design and building several working engines, the onset of World War II, lack of test results, and the ...

, SyTech engine, and many

hot air engine A hot air engine (historically called an air engine or caloric theory, caloric engine) is any heat engine that uses the expansion and contraction of air under the influence of a temperature change to convert thermal energy into mechanical work. ...

s and

steam engine A steam engine is a heat engine that performs Work (physics), mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a Cylinder (locomotive), cyl ...

s. The term ''scotch yoke'' continues to be used when the slot in the yoke is shorter than the diameter of the circle made by the crank pin. For example, the

side rod A coupling rod or side rod connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some Diesel locomotive, diesel and Electric locomotive, electric locomotives, especially older ones and switcher locom ...

s of a locomotive may have scotch yokes to permit vertical motion of intermediate driving axles. What is essentially a Scotch yoke is used in the

Tide-Predicting Machine No. 2 Tide-Predicting Machine No. 2, also known as Old Brass Brains, was a special-purpose mechanical computer that uses gears, pulleys, chains, and other mechanical components to compute the height and time of high and low tides for specific location ...

to generate a sinusoidal motion (sine functions).

Internal combustion engine uses

Under ideal engineering conditions, force is applied directly in the line of travel of the assembly. The sinusoidal motion, cosinusoidal velocity, and sinusoidal acceleration (assuming constant angular velocity) result in smoother operation. The higher percentage of time spent at top dead centre (dwell) improves theoretical engine efficiency of constant volume combustion cycles. It allows the elimination of joints typically served by a wrist pin, and near elimination of piston skirts and cylinder scuffing, as side loading of piston due to sine of

connecting rod A connecting rod, also called a 'con rod', is the part of a reciprocating engine, piston engine which connects the piston to the crankshaft. Together with the crank (mechanism), crank, the connecting rod converts the reciprocating motion of the p ...

angle is mitigated. The longer the distance between the piston and the yoke, the less wear that occurs, but greater the inertia, making such increases in the piston rod length realistically only suitable for lower RPM (but higher torque) applications. The Scotch yoke is not used in most internal combustion engines because of the rapid wear of the slot in the yoke caused by sliding friction and high contact pressures. This is mitigated by a sliding block between the crank and the slot in the piston rod. Also, increased heat loss during combustion due to extended dwell at top dead centre offsets any constant volume combustion improvements in real engines. In an engine application, less percent of the time is spent at bottom dead centre when compared to a conventional piston and crankshaft mechanism, which reduces blowdown time for

two-stroke engine A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a Thermodynamic power cycle, power cycle with two strokes of the piston, one up and one down, in one revolution of the crankshaft in contrast to a f ...

s. Experiments have shown that extended dwell time does not work well with constant volume combustion Otto cycle engines. Gains might be more apparent in internal combustion engines using a stratified direct injection (Diesel or similar) cycle to reduce heat losses.

Modifications

An improved Scotch yoke, with a means of absorbing sideways thrust, was patented in 1978 by William L. Carlson, Jr., .

References

External links

Brock Institute for Advanced Studies: Scotch Yoke

Comparing Simple Crank/Slider and Scotch Yoke Mechanisms
by Fred Klingener,

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; Active demo. {{Piston engine configurations Engine technology Linkages (mechanical)