Four-bar linkage Synthesis using Unigraphics NX4 Sketch

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 the fixed pivot O₂. For this example, we use length O₂A = 500 mm.
4) Draw line O₂A as link 1.
5) Do the same for point B and B'. Draw construction lines from point B to B' and bisect line BB' and extend the perpendicular bisectors in convenient direction.

6) Select any convenient point on bisector as the fixed pivot O₄. For this example, we select 60^o angle between O₄B and O₄B'
7) Draw line O₄B as link 2.
8) Draw line O₂M and set the same length as O₂A.
9) Draw line O₄N and set the same length as O₄B.
10) Draw line MN and set the same length as AB.
11) Set angular dimension between O₄B and O₄N to any desired value e.g. 45 degrees.

12) Select "Animate Dimension" Command in Unigraphics NX4 sketch and set lower limt to 0 and Upper limit to 60 and Steps/Cycle to 150 (the more steps, the smoother simulation).

See the results of four-bar linkage synthesis in the following video clip.



Further reading:

• Type Synthesis of Parallel Mechanisms
• Design of Machinery: An Introduction to the Synthesis and Analysis of Mechanisms and Machines
• Kinematic Synthesis of Linkages (Mechanical Engineering Series)
• Applied Linkage Synthesis
• Analysis of the Four-Bar Linkage: Its Application to the Synthesis of Mechanisms
• Kinematic synthesis of linkages by approximate stretch-rotation techniques (Massachusetts Institute of Technology. Dept. of Mechanical Engineering. Thesis. 1977. M.S)

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