# Four bar calculation help

If the height of the base frame is 12’‘, and we have a 38’’ vertical structure from which to put our four bar structure, how long should the four bars be if we want to reach the top rack? here’s a diagram of what i am thinking of in the attachment. ive been trying to crunch the numbers but i can’t seem to come up with it…

Neat idea, I like four-bar linkages a lot (as my students will attest!)

I suggest you use Inventor, and learn how to link dimension parameters to an Excel file. This make it easy to change, and to create formulas for dimensions.

With an parallel arrangement like yours, the numbers are pretty easy, but don’t have the time right now to do the math for you. Take a look at it and see if you can spot the symmetry and work it out.

This is very rough but…

top peg= 110ish inches
tilting the tube virtical 110-15= 95 in
pivoting on claw= 95-15= 80 in
four bar actuation 80/2= 40 in
assuming four bar set back 25"= sq.rt. (40^2+ 25^2)= aprox 47 in
but now the four bar is right on the outside of the robot, to compinsate for the pivoting claw minus part extruding from frame must move tower back.
15-8=9 25+9= 34 in
add bumped height 40+7=47 in

In summery: with a 47 inch four bar linkage 34 inches into the robot and 47 inches up plus a 9 inch adjustable arm plus an 8 inch claw sticking out of that when the four bar is fully up and the adjustable arm is up holding the tube virtically you will be able to score on the middle top row. (the highest)

These are just rough though, I suggest that you redo the calculations for yourself.

thanks. we probably will have to add some length to compensate for the fact that the peg only comes 16 inches out from the alliance wall, which means that the bars will have to be angled instead of straight vertical…

This is a good design, very do-able in the time frame with the resources (and issues) you have.
If the numbers aren’t quite working for you, keep in mind that it’s okay to forego reaching the upper tier pegs. Yes, you lose some scoring potenial, but if it’s the difference between that and not finishing the robot, it’s a fair tradeoff.

Good luck, and keep us informed on your progress.

We have found that the problems with geometry are about the same for putting tubes on the top row and picking them up off the floor. With a typical 4-bar setup you have to come up with a clever way for the arm to fold up to fit in the original envelope (28x38x60).

Best of luck!