Inversions of four bar mechanism

Prerequisite : Classification of Four bar mechanism and Harding's notation

Inversions of the mechanism is obtained when the the fixed link is changed to a free link and another link is fixed. A single four bar mechanism can give different type of motion based on the link which is fixed.

Harding's notation for four bar mechanism

Inversions of class 1 four bar mechanism

1. When link 'b' is fixed : Crank Rocker or Crank Lever mechanism, in the the shortest link rotates 360 deg where as the other link oscillates
2. When link 'a' is fixed : Crank Rocker or Crank Lever mechanism, in the the shortest link rotates 360 deg where as the other link oscillates
3. When link 'd' is fixed : Drag link or Double crank mechanism in which the links 'a' and 'b' undergoes complete 360 deg motion
4. When link 'c' is fixed : Double rocker or Double lever mechanism in which no link makes a complete rotation about its joints. In such case it is similar to class 2 four bar mechanisms

Inversions of class 2 four bar mechanism

Since in class 2 mechanisms, no link can complete complete complete rotation, all the mechanism are double rocker or double lever mechanisms.

Please apologize for not adding the pictures of these inversions. you can find these pictures on "Kinematics and Linkage Design" by Allen S Hall (Fig 3.4)