FOUR BAR MECHANISM
The four bar linkage, as shown in figure below, is a basic mechanism which is quite common. Further, the vast majority of planar one degree-of-freedom (DOF) mechanisms have "equivalent" four bar mechanisms. The four bar has two rotating links (2 and 4)) which have fixed pivots. One of the levers would be an input rotation, while the other would be the output rotation. The two levers have their fixed pivots with the ground link (1) and are connected by the coupler link (3).
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| FOUR BAR MECHANISM |
Crank (2) - a ground pivoted link which is continuously rotatable.
Rocker (4) - a ground pivoted link that is only capable of oscillating between two limit positions and cannot rotate continuously.
Coupler (3) - a link opposite to the fixed link.
1. If the length of any link is greater than the sum of lengths of other three links then it cannot act as four bar linkage.
2. If the sum of the lengths of the largest and the shortest links is less than the sum of the lengths of the other two links, then the linkage is known as a class-1 four bar linkage
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| crank rocker |
In fig the links adjacent to the shortest link b is fixed. The mechanism such obtained is known as crank-lever or crank rocker mechanism.
If the shortest link b is fixed, the mechanism obtained is crank-crank or double crank mechanism.
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| double crank |
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| double rocker |