Four-Bar Linkage and Coupler Curve

The four-bar mechanism, in its simplest form, has four bar-shaped links and four turning pairs, as shown in the animation. The fixed link (the black bar) may actually be bar-shaped, but more frequently it represents the frame of a machine and in that case is usually a massive casting of irregular shape. This interesting linkage system is the building block of more complicated mechanical linkage systems. Therefore it is a fundamental concept to be learned by mechanical engineering students.

In the four-bar mechanism, one of the rotating members usually is the driver and called as the crank or the driver ( the red bar). The other usually is called as rocker or follower (the green bar). The floating link ( the blue bar) that connects the crank and the rocker is called as connecting rod, and the fixed link ( the black bar ) is called as frame.

Many mechanism can be broken down into equivalent four-bar linkages. These mechanism has many aplication in mecanical operations. They can be considered to be one of the fundemental mechanisms.

In this animation You can move red or green ball for different arrangement. Now you can also change length of linkages. Note that when ball is moved to a point where this mechanism can't handle, rocker and connecting rod will be separated from crank. However end of connecting rod will try to reach to tip of the crank. This is because connecting rod and follower use forward kinematics to reach to tip of the crank.

Grashof's law is applied to four-bar mechanisms.

How the components on the animation works?

You can hide and show grids by clicking on Grid button
You can clear drawings by clicking Clear button. This is useful when ewer you move or change length of components to clean the unwanted curves
Curve drawing will stop after two complete revolution of Crank and animation will get faster. This is will eliminate load on your CPU.
You can resume drawing any time by clicking Clear button
You can drag green and red circles to move four-bar mechanism or change the length of frame (black bar)
You can also change location of green and red circles by changing their x and y coordinates. Coordinate entry text box background color correspond the circle color.
You MUST press ENTER button after you entered a value into coordinate text input boxes.
You can change rotation speed of the crank by changing the value in the rotation speed text box.
Setting rotation speed to zero will stop the crank.
Negative speed values will turn crank in apposite direction
The blue circle which draws the curve is on the same plane with blue bar. The curve drawn by blue circle is called as Coupler Curve.
The coordinate points of blue circle is displayed in blue input text boxes and you can change these values any time
X coordinate axis of this point start from where blue and green bar joins and extent toward where blue and red bar joins (positive direction)
Y coordinate extends upward from the corner where blue and green bar joins
You can also drag the blue circle when the crank stopped, or during its motion if you can catch it.