Sunday, September 26, 2010

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:

Saturday, September 25, 2010

3-Position Motion Generation Four-Bar Linkage Synthesis using Unigraphics NX4 Sketch

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 A1B1, A2B2, and A3Bas shown above.
2) Draw construction lines from point A1 to A2 and Ato A3

3) Bisect line A1A2 and line A2A3 and extend their perpendicular bisectors until they intersect with each other. Label the intersection O2.
4) Draw line O2A1. It's link 2.
5) Repeat the same for another end of the link. Draw construction lines from point B1 to B2 and Bto B3


6) Bisect line B1B2 and line B2B3 and extend their perpendicular bisectors until they intersect with each other. Label the intersection O4.
7) Draw line O4B1. It's link 4.

8) Draw line O2M and set the same length as O2A1.
9) Draw line O4N and set the same length as O4B1.
10) Draw line MN and set the same length as A1B1.
11) Set angular dimension between O4B1 and O4N to any desired value e.g. 15 degrees.

12) Select "Animate Dimension" Command in Unigraphics NX4 sketch and set lower limt to 0 and Upper limit to 56.355 (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 in the following video clip.

Friday, September 24, 2010

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 60o 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: