Scotch Yoke Mechanism Working Principle

Basic of Scotch Yoke Mechanism

It is  one of reciprocating motion mechanism, it should be convert an rotary motion into sliding of linear motion. When the Piston or reciprocating part to directly coupled together for the sliding of yoke  including a slot. That engages a pin at reciprocating part. When the internal combustion engine to convert rotary motion into linear motion by means of crankshaft rotation.Where the Piston and connecting rod should be connected together.  The Scotch Yoke Mechanism to be consider for more efficient because of the rotational motion to spent more time at high point of it rotate than Piston part.

Scotch yoke Mechanism working priciple
Scotch yoke Mechanism Diagram

Working principle of Scotch Yoke Mechanism

It is a simple mechanism, the rotary motion of pin convert into linear motion. First, the power supplied to be connected in DC motor, when the shaft to start in rotation moment, now the crank rotate the pin slider inside of yoke part and also move in forward direction. When the Crank will be rotate in clockwise direction and yoke will be getting displacement moment at forward. The maximum displacement of yoke depends upon the length of Crank.

The crank is completed the clockwise Revolution at the same time the Yoke sliding completely moved in forward. When this position take more time to start return stork. After spent time, the crank will be rotate in continuously it to be come back in initial position of rotation. So the Yoke move in backward direction and come back for initial position. Therefore the crank has full Revolution to be completed,At the same time the Yoke will be complete the forward and backward movement of Sliding. By means of the full revolution of Crank, the Yoke will be sliding through equal of double length of Crank. The Yoke displacement can be controlled by varying of crank length.

Beam Engine using Scotch yoke Mechanism

Example Of Scotch yoke Mechanism
Example Of Scotch yoke Mechanism diagram

At Position A

  • It indicates with beginning position of pin and initial position of connecting rod(#1 and #2)

At position B

  •  Now the disc rotate together, the slot with reciprocate motion and position will be changed, when the connecting rod(#1) travel upward direction and also connecting rod(#2) at upward direction.

At position C

  • The Disc rotates further both connecting rod #1 & #2 to get an Extreme upper position.

At position D

  •  The  disc rotate continuously, the position of C of connecting rod #1 and #2 Start to come back with downward for its initial position.


  •  That mechanism commonly used in control valve, activator including high pressure of oil and Gas pipeline.
  •  It also used to help of making double hack jaw.
  •  Reciprocating pump (the rotational motion converted into reciprocating motion, it require Piston movement).
  • It is used in beam engine when the rotational motion converted into linear sliding motion.
  • By using this mechanism to make toys and more.

Example application of Scotch Yoke Mechanism

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

Scotch yoke Pneumatic Actuator
Scotch yoke Pneumatic Actuator

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

 Advantages of Scotch Yoke Mechanism

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

Disadvantages of Scotch Yoke Mechanism

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


Please enter your comment!
Please enter your name here