Input: pink crank rotating regularly.
Output: orange crank rotating regularly.
The cranks rotate in the same direction with the same velocity
Lengths of cranks : a
Lengths of blue bars : 2.5a
Lengths of green bars: 2.5a + 2.5a
Distances between stationary bearings: b + b.
Green curve: locus of center of joint between two green bars.
Because here b = 4a, unstable positions happen when all the bars are in line. One working cycle corresponds 2 revolutions of the input.