four-bar linkage
A four-bar linkage [or] simply a 4-bar or four-bar is the simplest movable linkage . It consists of four rigid bodies (called bars or links), each attached to two others by single joints or pivots to form a closed loop. Four-bars are simple mechanisms common in mechanical engineering machine design and fall under the study of kinematics . If each joint has one rotational degree of freedom (i.e., it is a pivot), then the mechanism is usually planar , and the 4-bar is determinate if the positions of any two bodies are known (although there may be two solutions). One body typically does not move (called the ground link , fixed link , or the frame ), so the position of only one other body is needed to find all positions. The two links connected to the ground link are called grounded links . The remaining link, not directly connected to the ground link, is called the coupler link . In terms of mechanical action, one of the grounded links is selected to be the input link , i.e., th