Figure 2-10b shows a fourbar slider-crank transformed via rule #4 by the substitution
of a half joint for the coupler. The first version shown retains the same motion of
the slider as the original linkage by use of a curved slot in link 4. The effective coupler
. is always perpendicular to the tangent of the slot and falls on the line of the original coupler.
The second version shown has the slot made straight and perpendicular to the slider
axis. The effective coupler now is "pivoted" at infinity. This is called a Scotch yoke
and gives exact simple harmonic motion of the slider in response to a constant speed input
to the crank.