1:22
Double-crank
Four-bar linkage series, part 2: if the shortest link is the frame, both links pivoted on the frame turn fully.
Four-bar linkage series · Part 1 · ASOMmini
The crank-rocker
One crank turns fully while the rocker swings back and forth: a widely used type of four-bar linkage. The Grashof condition tells you when it occurs.
1:17
The principle
When does a link turn fully?
The answer comes from the Grashof condition: the shortest link of a four-bar linkage turns fully relative to its neighboring links if the shortest and longest links together are no longer than the other two together. Which type results then depends on the choice of frame.
The usual names are: the shortest link is called a, the others b, c and d in clockwise order. The fixed link is the frame, the one opposite it the coupler. In a crank-rocker, a neighbor of a is the frame: a is pivoted on the frame and turns fully as a crank, while the opposite link swings as a rocker.
What the video shows
- Link lengths a = 29 (shortest link), b = 55, c = 61 and d = 81 (longest link).
- The Grashof condition is met: a + d = 110 is less than b + c = 116.
- d is the frame. The trajectories show it: a turns on a full circle, while c swings back and forth on a circular arc.
In ASOMmini
From the textbook straight into the model
In ASOMmini, you build every type in this series with just a few clicks and immediately see how it moves.
Change lengths, see the effect
Move a joint, and ASOMmini recalculates the motion in real time – so you can see when a link no longer turns fully.
Paths and diagrams
Trajectories show the paths of the joints, and diagrams show angles, velocities and accelerations over the entire motion sequence. More on analysis
Forces and torques as well
With masses, springs and friction, the basic type becomes a concept whose forces and torques you know. More on force calculation
Four-bar linkage series
All example videos
More parts of the series
1:22
1:21
Double-rocker
Four-bar linkage series, part 3: if the frame is opposite the shortest link, both rockers swing and only the coupler turns fully.
1:01
Parallelogram and antiparallelogram linkage
Four-bar linkage series, part 4: links of equal length are opposite each other, as a parallelogram or crossed – the limit case of the Grashof condition.
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