Mechanical Design Handbook

In the previous post [ 3-Position Motion Generation Four-Bar Linkage Synthesis using Unigraphics NX4 Sketch ], the locations of fixed pivots O 2 and O 4 are fixed due to the fixed locations of moving pivots A and B. Sometimes, the location of O 2 and O 4 are undesirable with respect to your design constraints. More flexible method to get desirable locations of O 2 and O 4 will be shown in this post. 1) Draw link AB in its three design positions A 1 B 1 , A 2 B 2 , and A 3 B 3 as shown above. 2) Draw new attachment points C 1 and D 1 and other lines to form the rigid link ABDC. Do the same for position 2 and 3. Use "Constraint" command in Unigraphics NX4 sketch to set the equal length constraint to all relevant lines e.g. A 1 C 1 , A 2 C 2 , and A 3 C 3 have the same length, but don't need to specify the fixed value for it. Once we complete these settings at all positions, it means we have set the fixed relationship between our desired line AB and ot

In the real design problem, it will be more practical with 3 specified positions of a line in sequential order than 2-position synthesis. We can use a logical extension of the linkage synthesis technique described in [ Four-bar linkage Synthesis using Unigraphics NX4 Sketch ] to do the linkage synthesis with 3 positions. Assume that we have to design the mechanism that move the link AB through 3 positions as shown below. The rectangle represents the limit of link AB, so link AB cannot interfere with the rectangle. 1) Draw link AB in its three design positions A 1 B 1 , A 2 B 2 , and A 3 B 3  as shown above. 2) Draw construction lines from point A 1  to A 2  and A 2  to A 3 3) Bisect line A 1 A 2  and line A 2 A 3  and extend their perpendicular bisectors until they intersect with each other. Label the intersection O 2 . 4) Draw line O 2 A 1 . It's link 2. 5) Repeat the same for another end of the link. Draw construction lines from point B 1  to B 2  and B 2  to B 3

In most design problems, we need to design the mechanism that moves between required positions. In this post, we will show the motion generation synthesis method to control a line in the plane to move in sequential positions. Any CAD software can be used to do this. But here we use Unigraphics NX4 sketch to draw and use "Animate Dimension" command to simulate the movement of all concerned links. Assume that we need to design the mechanism to move line AB to A'B'. A simple and most commonly used four-bar linkage will be used here. Let's see how to find the fixed pivoting point at the ground. 1) Draw the link AB and A'B' in its desired positions in sketch of Unigraphics NX4 sketch (or any other CAD software) as shown in the above figure. 2) Draw construction lines from point A to A' and bisect line AA' and extend the perpendicular bisectors in convenience direction . (The choice is yours). 3) Select any convenient point on bisector as