Lecture - 14 ( double slider crank mechanism)

DOUBLE SLIDER CRANK MECHANISM 

Double slider crank mechanism is a combination of four links having two turning pairs and two sliding pairs. Diagram of this mechanism is shown below-

This mechanism consists of three inversions as listed below-
  • First inversion (fixed slotted plate)
  • Second inversion (fixed slider)
  • Third inversion (fixed connecting rod)

First inversion  

When slotted plate is fixed then it become the first inversion of double slider crank mechanism. Its example is elliptical trammel. Diagram of elliptical trammel is shown below- 

An electrical trammel is a simple mechanism which can trace exact elliptical path. It consists of two shuttles which are are confined to perpendicular channels or Rails, and a rod which is attached to shuttles by pivots at fixed position along the rod. As shuttle moves back and forth, each along its channel, the end of the rod moves in an elliptical path. Elliptical trammel is used in automatic tool changer in a machining.
Consider the following diagram of elliptical trammel, here point P lies between point A and B on a straight line. The straight line forms angle 'θ' with respect to x-axis. The point P is X distance away from y-axis and Y distance away from x-axis. 
From the above derivation we get the equation of ellipse. Therefore it can be concluded that each locus on line AB will draw an ellipse.
Different sizes of ellipse can be generated with the help of elliptical trammel just by adjusting AP and BP.

Always remember that each locus on line AB  will draw ellipse accept midpoint, midpoint will always generate a circle.
Let us consider point P lies in middle of line AB such that AP and PB are equal.

Second inversion 

When anyone of the slider is fixed in the mechanism then it become second inversion of double slider crank. Example of second inversion is scotch yoke mechanism. In this mechanism rotation is converted into reciprocation motion or vice-versa. Diagram of scotch yoke mechanism is shown below-
Scotch yoke mechanism is used in power hexa machine.

Third inversion

When link connecting slider is fixed then it become the third inversion of double slider crank mechanism. 


It's example is Oldham's Coupling. Oldhams coupling is used to connect the shaft having lateral misalignment. Its diagram is shown below-

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