These linkages are four-bar Grashof crank-rockers where the output link moves more quickly on the return stroke (decreasing theta4) than it does on the forward stroke (increasing theta4).
The time ratio (Tr) of a quick return linkage is defined as:
Tr = Rt/Ft
Rt = Time spent on return stroke
Ft = Time spent on forward stroke
Four bar quick return linkages work well down to time ratios of about 2/3. Beneath this the transmission angles become poor.
In this example:
r1 = 2.5 , r2 = 1 , r3 = 3.2 , r4 = 3 , rp = 5
theta1 = -20 , beta = 30 deg
and Tr = ~0.69
To see animations of this linkage select the links below: