A four bar linkage or simply a 4-bar or four-bar is the simplest movable linkage. It consists of 4 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.

Planar four-bar linkages perform a wide variety of motions with a few simple parts. They were also popular in the past due to the ease of calculations, prior to computers, compared to more complicated mechanisms.

Grashof's law is applied to pinned linkages and states; "The sum of the shortest and longest link of a planar four bar linkage cannot be greater than the sum of remaining two links if there is to be continuous relative motion between the links." Below are the possible types of pinned, four-bar linkages;

*If the input link may rotate full 360 degrees, it is called a crank. The linkage is called a crank-rocker if the input link is a crank and the opposite link is a rocker. If the opposite link is also a crank the linkage is called a double-crank.
*Pantograph (four-bar, two degrees of freedom, i.e., only one pivot joint is fixed.)
* Crank-slider, (four bar, one degree of freedom)
*Double wishbone suspension

ee also

* [http://www.mechanisms101.com/fourbar01.html mechanisms101.com] - Flash Four-bar Linkages simulator

* [http://www.softintegration.com/chhtml/toolkit/mechanism/fourbar/ softintegration.com] - Animated GIF Four-bar linkage with triangular coupler link

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