Position Analysis of a Grashofian Double Rocker RRRR Mechanism
Schematic Diagram of a 4 Bar RRRR Linkage
The 4 bar RRRR mechanism is forms the basis of any study on mechanism. It is widely used in various forms because of the relative simplicity of design and manufacture as well as durability. Grashof's criteria is used to distinguish between 4 bar RRRR linkages depending on the rotatability of the individual links of a 4 bar mechanism. In brief, if l is the longest link, s the longest link and the other two links have length p and q then the following cases arise
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l + s < p + q : Grashofian linkage
- l + s < p + q , shortest link s is the ground link : Double Crank
- l + s < p + q , shortest link is the coupler : Double Rocker
- l + s < p + q , shortest link is neither coupler nor ground : Crank Rocker
- l + s > p + q : Non Grashofian Double Rocker
For a more detailed introduction to Grashof criteria see the animated guide that follows. You can either click on the animation itself to move from step to step as per your convenience. Alternatively you can use the controls at the bottom of the animation to see it at your own pace.
In a Crank Rocker, unlike a Double Crank, only the input link (link 2, which is the crank) rotates througha full circle about the ground (link 1, which is the frame), while the oputput link (link4, which is the rocker) does not. The output link oscillates instead between two limiting positions. Thus the position analysis of a Crank Rocker involves, apart from finding out the coupler curve and the relation between the input and output orientations, finding the limiting posiitons of the link 4 and the corresponding positions of link2. Since the input link rotates through a full circle it is still possible to drive this mechanism using a simple continuous rotary drive, that is we may connect it directly to a simple motor. However we cannot connect the output to a continuous rotary drive directly since the output link moves back and forth.