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Four Bar Linkage

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Four bar linkage consists of four pin connected links and is widely used for generating irregular motion. The length of each link is defined by the distance between the axes of its pin joints. The length of the different links determines the rotational behavior of the four bar linkage. This behavior is predicted by using the Grashof condition which is a relationship based on the link lengths.

The Grashof criterion is divided into three classes. All three classes of linkages can be obtained using the SpectraQuest device. Within each class different inversions can be obtained by grounding different links. In class I, when the any one of the link adjacent to the shortest link is grounded, the shortest link will rotate fully and the link pivoted to the ground oscillates. When the shortest link is grounded, the motion is double crank. The links pivoted to the ground makes complete revolutions.

The figure above shows the four bar linkage apparatus. The nuts can be loosened and slid along the links. Different four bar linkage inversions can be obtained by changing the link length (by installing the nuts at different positions of the link).