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.

Comments

Post a Comment

Popular posts from this blog

Lecture - 8 ( problems on degrees of freedom)

Image
PROBLEM 1  - For the mechanism shown in figure, calculate it's degrees of freedom (F) SOLUTION 1  -  number of links (L) = 8 Number of binary joints (J) = 10 Number of higher pair (H) = 0 Number of those motions which are not the part of mechanism (N) = 0 KUTZBACH equation: F=3(L-1) - 2(J) - H - N F = 3(8-1) - 2(10) - 0 - 0 = 1 F = 1  PROBLEM 2 -  Calculate degrees of freedom for given mechanism SOLUTION 2 -   L = 4 ; J = 3 ; H = 1 ; N = 1 F = 3(L-1) - 2J - H - N = 3(4-1) -2(3) -1 -1 = 1 F = 1    PROBLEM 3 -  Calculate degrees of freedom for given mechanism.  SOLUTION 3 -   L = 5 ; H = 1 ; J = 5 ; N = 0 F = 3(5-1) -2(5) - 1 - 0 = 1  F = 1 PROBLEM 4 -  Calculate degrees of freedom for given mechanism SOLUTION 4 -    Here one link is spring. Spring is the link of flexible length. Spring can be replaced by 2 links connected with turning pair. L = 5 ; J = 5 ; H = 1 ; N = 0 F = 3(5-1) -2(5) - 1 = 1  F = 1
Read more

Lecture - 11 (Transmission angle in four bar mechanism)

Image
The angle between coupler link and output link in four bar mechanism is known as transmission angle. It is represented as ' μ'.   In the given figure Let us consider angle between link AO and link OC (fixed link) is represented by ' θ'.   Let us consider length of link OC is 'a', length of link OA is 'b', length of AB is 'c' and length of BC is 'd'.  According to law of cosine we get -  Now  μ will be different for different values of  θ. μ will be maximum for  θ = 180 degrees  & minimum for  θ = 0 degrees.   Problem 1-  Calculate maximum and minimum value of transmission angle for the following mechanism. Length of link OC is 5cm, length of link AO is 1cm, length of link AB is 4cm and length of link BC is 3cm. Solution 1 -   To calculate minimum value of transmission angle observe the mechanism when  θ = 0 degrees   To calculate maximum value of transmission angle observe the mechanism when  θ = 180 degr
Read more

Lecture - 15 (Mechanical advantage, mechanism efficiency, toggle mechanism)

Image
Mechanical advantage   It is defined as the ratio of the force produced by a machine to the force applied to it, used in assessing the performance of machine.  The combination of pulley system is shown in the below figure-  Here for lifting download F force is required but the effort given to the system is 8 times less, which is the good example of mechanical advantage. For the above system mechanical advantage is 8 Mechanical advantage = load / effort  Mechanical advantage for linear motion is -  Mechanical advantage for angular motion is -  Efficiency of system or mechanism Efficiency of the system is defined as the ratio of output power to the input power. PROBLEM - Find out mechanical advantage of the given mechanism having 100% efficiency. Solution -  In the sense of angular velocity It means that above mechanism is toogle mechanism.  Toggle mechanism   Those mechanism whose mechanical advantage is so high that it approaches
Read more