Four-Bar-Lifter Dynamics

[leorcc]

Nice job Adam, very usefull indeed, I like it a lot.

[adam]

I got an e-mail from John Reid expressing some concern that my calculations were incorrect. He noticed that there are spikes at the end of the torque graph for most 4 bar systems. This is caused because the force applied begins to act perpendicular to the path of motion. Its similar to this situation:

http://www.totalinsanity.net/situation.JPGhttp://www.totalinsanity.net/situation.JPG

Sorry for the bad mspaint drawing, but Im just trying to get the point
across. Imagine the blue slots are immovable. The purple arrow is some
weight, and the black lines are perfect links. As the green forces are
pulling, once they get to where the black links are almost horizontal, the
green forces will need to be infinity, to hold the purple force steady. The
forces start acting perpendicular to the path of motion they are trying to
create. The same thing starts to happen at the end of some 4 bar motions.
This is why they never reach full extension if they are powered by the front
bar (they might get really close, but they never get all the way out. This
is good, because it keeps the rear link from binding on the retract).

I went back anyway to make sure my calculations were correct. I found a different error in my calculations (a sign error) which throws off the graph a little. I suggest anyone who downloaded the program to re download it, as the previous version is giving out incorrect results.

http://www.totalinsanity.net/tut/mechanical/4barfrontbar.phphttp://www.totalinsanity.net/tut/mec...arfrontbar.php

I am sincerely sorry for the error. I should have checked over my results more thoroughly before.

[adam]

Ba! I found some other stuff wrong. The programs code matches that of my spreadsheet which I was using to test out the concept, but the results arent matching up. I need to look it over some more. I hate programing...

Sorry again to drag up an old thread, but I updated the program.

First off, I did this a while ago but never posted here. I changed the method by which it calculates the torque. Instead of using a bunch of statics calculations, it just does a work balance. So basically, it uses trig to iterate through the motion of the arm by changing the powered arms angle slightly each iteration. This generates the trajectory of the arm. Over each iteration, it looks at how far the tip of the arm moved vertically, and how far the powered arm moved in radians. It knows the force the tip lifted, so it just does F*D=T*A where A is the angle change in the arm. The iterations are very small, so it generates a smooth graph. The results it gives are the same as the spreadsheet I had made earlier which relied on statics, but this method is much simpler, and less error prone. It also allows me to add in a rear lift without changing much of the programming, see below.

Other than this, I added in a variable for rear lift, so you can raise up the lower pivot point of the rear bar. This is in a lot of designs, so I thought it was an important addition.

Also, I added in support for rear bar powered lifters. Its still at the same url:

http://www.totalinsanity.net/tut/mechanical/4barfrontbar.phphttp://www.totalinsanity.net/tut/mec...arfrontbar.php

The next thing I want to do is make a dynamic calculator for pneumatic powered systems, and a static calculator for systems powered with electric linear actuators. But dont hold your breath...

(Message edited by Adam on January 13, 2007)

[woody]

Adam ........your download link is duff.

This link should work.

http://www.totalinsanity.net/tut/mechanical/FourBar.ziphttp://www.totalinsanity.net/tut/mechanical/FourBar.zip

[adam]

Whoops, brain fart... thanks for the tip.