1:17
Crank-rocker
Four-bar linkage series, part 1: the crank turns fully while the rocker swings. The Grashof condition and the choice of frame tell you when this is the case.
Four-bar linkage series · Part 5 · ASOMmini
Isosceles four-bar linkages
If links of equal length are next to each other, the frame decides: isosceles double-crank or isosceles crank-rocker. If all links are of equal length, the result is the rhombus linkage.
1:11
The principle
Neighbors of equal length
These linkages meet the Grashof condition as an equation, just like the parallelogram and antiparallelogram linkages, and they too have change points at which they can flip. Here, however, the links of equal length are next to each other.
If a short link is the frame, both neighbors turn fully: the isosceles double-crank. If a long link is the frame, the short link pivoted on the frame becomes a crank and the long one a rocker: the isosceles crank-rocker. Both are used only in special cases because, unlike their namesakes, they can flip.
Special case: rhombus linkage
If all links are of equal length, not only the links but also the joints lie on top of each other in pairs at the change points. Even small manufacturing deviations can then noticeably change the motion, forces and torques – something to keep in mind when simulating.
The videos
Every type in motion
1:11Isosceles double-crank
a = d = 50, b = c = 146, so a + b = 196 = c + d. Both neighbors of the frame turn fully.
1:24Isosceles crank-rocker
a = b = 100, c = d = 158, so a + c = 258 = b + d. The short link turns fully as a crank, while the long one swings as a rocker.
0:59Rhombus linkage
a = b = c = d = 150, so a + b = 300 = c + d. Also called a rhombus crank.
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 – and ASOMmini calculates its forces and torques. More on force calculation
Four-bar linkage series
All example videos
More parts of the series
1:17
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.
1:01
Parallelogram and antiparallelogram linkages
Four-bar linkage series, part 4: links of equal length are opposite each other, as a parallelogram or crossed – a limit case of the Grashof condition.
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