3-Position Motion Generation Synthesis with Alternate Moving Pivots using Unigraphics NX4 Sketch

In the previous post [3-Position Motion Generation Four-Bar Linkage Synthesis using Unigraphics NX4 Sketch], the locations of fixed pivots O₂ and O₄ are fixed due to the fixed locations of moving pivots A and B. Sometimes, the location of O₂ and O₄ are undesirable with respect to your design constraints. More flexible method to get desirable locations of O₂ and O₄ will be shown in this post.

1) Draw link AB in its three design positions A₁B₁, A₂B₂, and A₃B₃ as shown above.

2) Draw new attachment points C₁ and D₁ 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. A₁C₁, A₂C₂, and A₃C₃ 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 other moving pivots CD.

3) Draw construction lines from point C₁ to C₂ and C₂ to C₃

4) Bisect line C₁C₂ and line C₂C₂ and extend their perpendicular bisectors until they intersect with each other. Label the intersection O₂.

5) Draw line O₂C₁. It's link 2.

6) Repeat the same for another end of the link. Draw construction lines from point D₁ to D₂ and D₂ to D₃

7) Bisect line D₁D₂ and line D₂D₃ and extend their perpendicular bisectors until they intersect with each other. Label the intersection O₄.
8) Draw line O₄D₁. It's link 4.

9) Draw line O₂M and set the same length as O₂C₁.
10) Draw line O₄N and set the same length as O₄D₁.
11) Draw line MN and set the same length as C₁D₁. Do the same for remaining lines, set constraint so that it form the same rigid link as ABDC.
12) Set angular dimension between O₄D₁ and O₄N to any desired value e.g. 20 degrees.

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

See the results of four-bar linkage from three-position motion generation synthesis with alternate moving pivots in the following video clip.

Further reading:

• Design of Machinery with Student Resource DVD
• Design of Machinery (Mcgraw-Hill Series in Mechanical Engineering)
• Design of Machinery: An Introduction to the Synthesis and Analysis of Mechanisms and Machines
• Machine Design: An Integrated Approach (3rd Edition)
• Kinematics, Dynamics, and Design of Machinery
• The Mechanical Design Process (Mcgraw-Hill Series in Mechanical Engineering)

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