Scotch yoke Mechanism

Scotch yoke Mechanism

 

Introduction:

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 Crank Chain .
        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.

 Concept of mechanism :

        The   Scotch yoke is a reciprocating motion mechanism, converting the linear motion of a slider into rotational motion or vice-versa.   The piston or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a pin on the rotating part. The shape of the motion of the piston is a pure sine wave over time given a constant rotational speed.

 

 

Scotch yoke Mechanism ( Fig 1.1 )

 

 

Explanation Of  Diagram :

As we know this is inversion of   Double Slider Crank Chain.  So the inversion is obtained by fixing either the link 1 or link 3.  In Fig, link 1 is fixed. In this mechanism, when the link 2 (which corresponds to crank) rotates about B as centre , the link 4 (which corresponds to a frame) reciprocates. The fixed link 1 guides the frame.

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.

 

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.