Scotch Yoke Mechanism – Construction and Working

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

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


The main parts of Scotch Yoke Mechanism are Crank, Sliding Pin, Sliding Yoke with Slot, Fixed LInk, Connecting Rod or Piston which receives the output reciprocating motion of Scotch Yoke Mechanism.

The piston or the reciprocating part is coupled to a sliding yoke.
This sliding Yoke has a slot that engages a sliding pin which slides inside the sliding yoke and is a non-fixed slider of double slider crankshaft.
This sliding pin also attached to a crank which on the other side has a fixed link of double slider crankshaft. This crank rotates around the fixed link as its center point and the sliding pin as its endpoint.

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


In the Scotch Yoke Mechanism, the rotary motion of crank is converted into linear motion of the reciprocating part. The power is supplied from the DC motor shaft and the crank attached to the shaft starts rotating. As the crank rotates the pin or the slider connected to the crank starts sliding in the yoke and also moves the yoke forward. When the pin of the crank is at the top and the crank rotates in a clockwise direction the yolk will get a displacement in the forward direction. The maximum displacement of the yoke will be equal to the length of the crank. When the crank completes one-fourth of its rotation, the yoke reverses its movement and start moving in the backward direction and continue moving in the backward direction until crank reaches its leftmost position and then again move in forward direction. Like this, the rotational motion of crank is converted to reciprocating motion of yolk. This reciprocating motion of yolk is transferred to the piston or connecting rod as needed.

Advantages of Scotch Yoke Mechanism:-
1) Higher Torque Output with small cylinder size.
2) Fewer Moving parts and hence a smoother operation.
3) Higher percentage of time spent at the top dead center improving engine efficiency.
4) It can be used to perform various operations like cutting, slotting, etc.
5) Process can be automated.
6) Direct conversion of rotary motion into reciprocating motion.

Disadvantages of Scotch Yoke Mechanism:-
1) Rapid wear of the slot in the yoke caused by sliding friction and high contact pressures.
2) Lesser percentage of the time spent at the bottom dead center reducing the blow-down time for two-stroke engines.
3) Guideways are necessary for proper reciprocating of arms.

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