four bar linkage 3 position synthesis (4min video)

I’ve had occasion to do some four bar linkages lately, and it got me thinking that it might be worthwhile to do some short screencasts on some mechanisms synthesis techniques.

This video shows how to define a four bar linkage based using 3 different positions of the coupler bar (the middle bar in a four-bar linkage) as an input.

Please note that this procedure does not guarantee that your four-bar will go from position to position smoothly (read: without bars going over center). It does however provide a good starting point (especially if done parametrically in CAD) for exploring mechanism feasibility.

four bar mechanism synthesis recap

1. Draw the three positions of the coupler bar. For the purposes of the next few steps, we will refer to one end of the bar as “A”, and the other as “B”
2. Create a line segment between the “A” points at each position of the bar (this will result in four line segments, two for each end of the coupler).
3. Create a perpendicular line through the midpoint of each of your newly sketched line segments (this will result in two perpendicular lines).
4. The intersection of this point is an anchor point ,we’ll call it Anchor Point A
5. Repeat steps 2-4 using point “B”. This new intersection is your other anchor point, Anchor Point B.
6. Create a line segment between Anchor Point A and “A” on one of the positions of the coupler bar
7. Create a line segment between Anchor Point B and “B” on the same position of the coupler bar
8. The two segments from Steps 6 + 7 plus the coupler bar define your four-bar linkage.

Done.

Related posts:

1. video: inverted peaucellier mechanism
2. video: flexible shaft couplings
3. video: mechanisms, courtesy of Ralph Steiner

Tagged as: four bar linkage, mechanism design, mechanism synthesis

Article by chris loughnane

Chris is Design Engineer + Sustainability Advocate at a Boston area design consultancy and has written 134 sweet posts for the Product Development Notebook.

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