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Originally Posted by Sachiel7
Hello All,
I would like to hear any input from those who have tried either 4 bar, or something similar. I'd like to hear some of the pros and cons from experience.
Also, I would like to get some ideas from teams from last year who had successful capping lifts for the large platform goal.
I just want to see some designs from a different perspective, and get some first hand input on what works well (Since our arm last year wasn't that complicated, but cant achieve this years task)
So, any input you can offer would be helpful!
Thanks!
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In 2003 we use a 4bar for or "stacking" arm. I put stacking in quotes because we never used it for that in competition. It worked just fine though we did have a few problems in development.
4bars can be used to lift things reasonably efficiently, but you should be careful to remember about large torques and separation distances. Let's say that you are using a 4bar to lift 3 stacked tetras. To maximize the amount of lift you have mounted the fixed end of the 4bar at the top of the 60 inch envelope and let the arm hang down to the floor to fit in the box. We'll neglect loads from the gripper etc and focus on the tetras.
The highest load on the arm will be when the arm is straight out with the three tetras. Using the diagonal of the box your arm length is going to be at least 5' 6". Put 30lbs of tetras at the end of it (in dealing with loads always round up, with capabilities round down) you now have almost 2000 in lbs of moment around the fixed end of the arm that need to be reacted.
You could do that with a motor, but that would require a pretty substantial gearbox. The one we built in 2003 was a 1000:1 reduction on a CIM motor. We could lift a stack of 7 boxes 3 feet in about three seconds. Of course the stacks weren't stable enough to take being lifted like that, but that is beside the point here. To lift we applied a torque to the upper links (we had two in parallel for stability. I think you would need something similar here.
Another approach is the use a cable diagonally across the parallelogram formed by the 4 bar. In this case, assuming your 4bar was 6" deep, you would need a cable with something like 4000lbs of tension on it. The nice part is you don't have to pull it very far. But you do have to resist all of that load and transmit it into the structure somehow.
These problems aren't all that different from those of other arm systems and they are all solvable, you just have to think about them for a bit. In our case the 4bar was a little on the heavy side, but thinking about it later I could have probably reduced it substantially if I had tried a little harder. Our 4bar was originally designed to use the cable method. That puts all of the solid members in compression. Beams are rather unstable in compression and that limits the amount of speed holes I'm willing to put in. But when we went to install the cable, we couldn't get the motors to fit. So we built the gearbox. At that point the lower bars had hardly any load on them and I could have switched from 1x1x1/8 square to L-angle and saved weight, but that was no clear until after we had already made weight, so who cared?
One thing to note if you use a cable system. We needed cables running across both diagonals to ensure the lift would move in both directions. We discovered that the two cables move at different speeds at different parts of the travel. So at one end the pulling cable will be moving comparatively fast and the slack cable will be moving much slower. At the other end of the travel they will be reversed. this was another reason we went to the gearbox.
After all that, a 4bar is still a possible concept for this year. Having given it a fair amount of thought over the intervening years, I think we can deal with the minor issues.
One more thing, I wanted to precision drill the holes for the pins on our 4bar so the arms would be interchangeable. While I was out one day some people just went ahead and drilled them by hand. They did a good enough job that I didn't notice that they were hand drilled until we were trying to reassemble the 4bar after installing the gearbox. All of a sudden we couldn't figure out which arm went where. With four bars, four positions and two possible orientations we just couldn't figure out how to get it back together again. So we made the holes really big and sloppy. It didn't seem to hurt the operation. So a 4bar will work just fine with a lot of "play".
Sorry to take so long. I missed this post the day it came out and I've been off-line for a couple of days since then.
ChrisH