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 r ...

mechanism, converting the linear motion of a slider into

rotational motion Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

, 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 and is made gas-tig ...

or other reciprocating part is directly coupled to a sliding

yoke A yoke is a wooden beam sometimes 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, us ...

with a slot that engages a pin on the rotating part. The location of the piston versus time is

simple harmonic motion In mechanics and physics, simple harmonic motion (sometimes abbreviated ) is a special type of periodic motion of a body resulting from a dynamic equilibrium between an inertial force, proportional to the acceleration of the body away from the ...

, i.e., a

sine wave A sine wave, sinusoidal wave, or just sinusoid is a curve, mathematical curve defined in terms of the ''sine'' trigonometric function, of which it is the graph of a function, graph. It is a type of continuous wave and also a Smoothness, smooth p ...

having constant amplitude and constant frequency, given a constant

rotational speed Rotational frequency (also known as rotational speed or rate of rotation) of an object Rotation around a fixed axis, rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second ...

Applications

This setup is most commonly used in control

valve actuator A valve actuator is the mechanism for opening and closing a valve. Manually operated valves require someone in attendance to adjust them using a direct or geared mechanism attached to the valve stem. Power-operated actuators, using gas pressure, ...

s in high-pressure oil and gas pipelines. Although not a common metalworking machine nowadays, crude

shaper 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) linear instead of ...

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 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. These engines m ...

s and

steam engine A steam engine is a heat engine that performs 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. This pushing force can be trans ...

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 A crankpin or crank pin, also known as a rod bearing journal, is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to t ...

. 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 and electric locomotives, especially older ones and shunters, also have them. The coupling rods transfer t ...

s of a locomotive may have scotch yokes to permit vertical motion of intermediate

driving axle On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled ...

s. What is essentially a Scotch yoke is used in the Tide-Predicting Machine No. 2 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 In a reciprocating engine, the dead centre is the position of a piston in which it is either farthest from, or nearest to, the crankshaft. The former is known as Top Dead Centre (TDC) while the latter is known as Bottom Dead Centre (BDC). ...

(dwell) improves theoretical engine efficiency of constant volume combustion cycles. It allows the elimination of joints typically served by a

wrist pin In internal combustion engines, the gudgeon pin (UK, wrist pin or piston pin US) connects the piston to the connecting rod, and provides a bearing for the connecting rod to pivot upon as the piston moves.Nunney, Malcolm James (2007) "The Recipr ...

, 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 piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...

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

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 power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of t ...

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 Otto cycle 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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