Lecture - 12 (Single slider crank mechanism (part -1))

SINGLE-SLIDER CRANK MECHANISM 

Single slider crank mechanism consist of four links, three turning pairs and one sliding pair. It was developed by sir James Watt. 
It has four inversions which are listed below - 
  • First inversion (fixed cylinder)
  • Second inversion (fixed crank)
  • Third inversion (fixed connecting rod)
  • Fourth inversion (fixed slider)

First inversion -  

In first inversion cylinder will be fixed. It is also called basic inversion.
Examples - 
In the figure shown below mechanism of internal combustion engine is there. Here link 1 (cylinder) is fixed, link 2 (crank) is in rotation, link 3 (connecting rod) is in general motion and link 4 (piston) is in sliding motion.


In this case piston is our input and Crank is our output, which means we are giving reciprocation as input and getting rotation as output.
This mechanism can be used in opposite direction also and in that case this mechanism become a compressor. In compressor we are giving input as rotation and getting output as reciprocation motion.

Other example of single slider crank mechanism is external combustion engine as shown in the figure below-

Here link 1 (cylinder) is fixed, link 2 (crank) is in rotation, link 3 (connecting rod) is in general motion and link 4 is in reciprocation motion. Here our link 4 is piston, Piston Rod and crosshead because they are manufacturer as single body. There is no joint between piston, piston Rod and crosshead.
In this case also reciprocation is input and rotation is output.

Second inversion 

When Crank of single slider crank mechanism is fixed then it is called second inversion.
Practical examples of second inversion are whitworth quick return motion mechanism and rotating internal combustion engine mechanism.

  • WHITWORTH QUICK RETURN MOTION MECHANISM 
In this mechanism time taken in cutting stroke is more as compared to time taken in in return stroke and that's why it is called as quickreturn motion mechanism. Whitworth is the scientist who discovered this mechanism.

In the mechanism link 1 is slotted bar, link 2 is crank which is fixed, link 3 is driving crank and link 4 is slider which is sliding inside slotted bar. R1 & R2 is the distance travelled by Ram linearly. Distance between R1 and R2 is equal to distance between P1 and P2.
STROKE (P1 P2) = 2 (DP) 
Here 'β' is return stroke angle and 'α' is cutting stroke angle. Return stroke angle will always be less than cutting stroke angle because of quick return motion.
Quick return ratio is defined as the ratio of time taken in cutting stroke to the time taken in return stroke. Quick return ratio will always be greater than 1.


  • ROTARY INTERNAL COMBUSTION ENGINE MECHANISM

In this mechanism when combustion takes place inside the cylinder, first of all input comes on piston in the form of force. Now this force is transmitted to connecting rod and then to crank but crank is fixed which makes connecting rod and Piston to rotate due to which cylinder block rotates and we get output in the form of rotation. It is also called the gnome engine. This mechanism is used in aeroplanes, Where propellers are mounted on cylinder blocks.

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