Construction of Scotch Yoke Mechanism

Scotch Yoke Mechanism:

Scotch Yoke Mechanism

Scotch Yoke Mechanism is the reciprocating mechanism that converts rotational motions to reciprocating motion or vice-versa. It is also knowns as a slotted link mechanism. This mechanism is an inversion of the double slider crankshaft.

It can be used to either convert the linear motion of a slider to the rotational motion of a crank, or it can be used to convert the rotational motion of a crank to the linear motion of a slider. This mechanism is commonly used in control valve actuators in high-pressure gas and oil pipelines.

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Working Principle of Scotch Yoke Mechanism:

This is a simple mechanism; the rotary motion of the pin is converted to linear motion. Firstly, the power is supplied to the DC motor to be connected; when the shaft starts at the moment of rotation, now the crankpin rotates the slider inside the yoke part and also moves in the forward direction.

When the crank will be rotated in a counterclockwise direction, and the yoke will be a forward-facing displacement moment. The maximum displacement of the yoke depends on the length of the crank.

The crank has completed the revolution of the clock at the same time as yoke sliding completely moved forward. When this position takes longer to initiate the return stork, after spending time, the crank will be rotated continuously to return to its initial state of rotation. Therefore the yoke moves in a backward direction and returns to the starting position.

Therefore the crank has full revolution; At the same time, the yoke will complete the forward and backward movement of the sliding. Through full revolution of the crank, the yoke will be sliding through equal to the double length of the crank. Yoke displacement can be controlled by varying the crank length.

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Construction of Scotch Yoke Mechanism:

The main parts of the scotch yoke mechanism are the crank, sliding pin, sliding yoke with slot, fixed LInk, connecting rod or piston that achieves the output reprocessing motion of the scotch yoke mechanism.

The piston or reciprocating part is added to a sliding yoke. This sliding yoke has a slot that attaches a sliding pin that slides inside the sliding yoke, and the double slider is a non-fixed slider of the crankshaft.

This sliding pin is also attached to a crank which is a fixed link to the double slider crankshaft on the other side. This crank rotates around the fixed link as its center point and sliding pin as its endpoint.

When the crank rotates around the fixed link, the sliding pin slides into the slot of the yoke, and the yoke slides forward and backward.

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Working of Scotch Yoke Mechanism:

In the Scotch yoke mechanism, the rotary motions of the crank are converted to the linear motion of the alternating part. Power is supplied from the DC motor shafts, & the crank attached to the shaft rotates.

As the cranks rotate, the pin or the slider attached to the crank begins to slide into the yoke and also moves the yoke. When the pins of the crank are at the top & the crank rotates in the clockwise direction, the yolk will get displacement in the forward direction.

The maximum displacements of the yoke will be equal to the lengths of the crank. When the crank completes one-quarter of its rotation, the yoke reverses its movement and begins to move in the backward direction and continues in the backward direction until the crank is at its most left.

It does not reach the position and then moves in a forward direction. In this way, the rotational speed of the crank is converted to the reciprocating motion of the yolk. This rotating motion of the yolk is transferred to the piston or connecting rod as needed.

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Example Application of Scotch Yoke Mechanism:

Scotch Yoke Pneumatic Actuator

Scotch Yoke Mechanism of pneumatics valve (linear to rotary motion)

When the pneumatic actuator, which mechanism converts rotary motion into linear motion. The spring to be attached to the piston because it will return in the initial position. That provides great torque, so it is used in some industrial valves.

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Application of Scotch Yoke Mechanism:

  • That mechanism is commonly used in control valves, catalysts that involve high pressures of oil and gas pipelines.
  • It also helped to create a double hack jaw.
  • Reciprocating pump (the rotational motion converted into reciprocating motion, it require Piston movement).
  • It is used in beam engines when the rotational speed is converted to linear sliding speed.
  • Using this mechanism to make toys and more.

Advantages of Scotch Yoke Mechanism:

  • Easy construction of assembly and operation.
  • They are used to perform cutting, sliding and etc.
  • It is a direct conversion method of rotary into Linear.
  • High torque to be made with the small size of cylinder.
  • It spent high time in top dead center, so improve engine efficiency.

Also, Read: Inversion of Double Slider Crank Mechanism

Disadvantages of Scotch Yoke Mechanism:

  • The high wear is produced due to sliding friction and high contact pressure.
  • List percentage of time spent in the bottom center position to reduce the below time in a two-stroke engine.
  • The Piston motions are a pure sine wave that will occur over time give a constant rotation speed.
  • It requires a proper guideway for reciprocating motion of the arm.


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