This is a example from Robert L. Norton's books on machinery: "Design a fourbar Grashof crank-rocker to give 45° of rocker rotation with equal time forward and back, from a constant speed motor input. Solution 1)Draw the output link O4B in both extreme positions, B1 and B2 in any convenient location, such that desired angle of motion O4 is subtended 2)Draw the chord B1B2 and extend it in either direction. 3)Select a convenient point O2 on line B1B2 extended. 4)Bisect line segment B1B2, and draw a circle of that radius about O2 5)Label two intersection of the circle and B1B2 extended, A1 and A2 6)Measure the lenght of the coupler as A1 to B1 or A2 to B2 7)Measure ground length 1, crank length 2, and rocker lenght 4. 8)Find the Grashof condition. If non-Grashof, redo steps 3 to 8 with O2 farther from O4 9)make a model of the linkage and check its function and transmission angles. Here is what the mechanism seems I understand the steps but I can't understand by what step does the mechanism have equal time forward back and forth and how can we understand this mechanism has equal time forward and back. Is there any criteria or formula which show mechanims' back and forth time.