pic: Team 3735 frame

Our frame is starting to look like something. Winching system powered by two window motors and at the bottom you can see the beginnings of our electronics protection box. Yes i know it’s wood i wanted aluminum but the team needed to save some money and wood is easier to work with. Not bad for rookie team 3735 so far I think.

looks fantastic, one thing i would be sure to do is to put a physical stop in the arm somewhere. our 2008 robots arm went over backwards and that is not a pretty sight. even if there is no programing error, things can still push that over, and i always advocate for safety redundancy. for a rookie team i am very impressed though.

330’s arm went over backwards in 2005, 2007, and 2008, or had that capability. Don’t put that physical stop in unless you need it; if you tip over backwards, it could save you a match.

If going over backwards will disable or destroy the arm, on the other hand, then yes, you want a hard stop.

my concern is that once it gets over the top

A. you wont be able to retract it back to forward safely.
B. you will tip over.
C. it may destroy the window motors.
D. if it goes over you probably wont be able to move.

the window motors in a winch makes it impossible to actuate once it goes to the other side.

A: Quite possible, with the pictured setup.
B: I highly doubt it. Just because you guys may have does not mean that that is necessarily true. CG plays a role. I know 330 was able to reach back in 2005, 2007, and 2008. We didn’t do it–except to self-right–but if we had reached back, especially in 2007, with a light load, I doubt we’d have tipped (2008 and 2005, we’d simply have self-righted if we had).
C: If you seize up the gearbox, maybe. I’m not sure there’d be enough force left in the cables to do that after they stretched.
D: Almost correct, except that maybe you mean “move backwards” or “turn”. Forwards probably wouldn’t be too bad of an issue.

A number of things would depend on how the winch system worked–if it was set up as shown in this picture, then yes, there could be a problem near the top. If, however, it was run to a pulley on the side of the arm, and wrapped such that it was a continuous pull on either side, or had some other form of pull on the main arm side, then I don’t think there would be a problem.

With this setup, I’m pretty sure that the worst thing would be the arm not being retractable if it went over the top of the robot.

For the team, I’d suggest running an extra cable off the front side of the winch to the arm, at about the same distance from the pivot as the one shown, but running out when the arm goes up. That should take care of the arm going over backwards on you.

Don’t be ashamed of using wood. Wood is a wonderful material that more teams should use. It’s cheap, easy to use, and requires no fancy tools.

This is a completely different design. Note the use of string instead of a sprocket. Winch-down arms like this going over backwards may be unable to go back over forwards.

Thanks for all the concerns. At first we did plan to run the winch to both sides of the arm but for time and complexities sake we decided to just do the back. We will be using a hard stop eventually but we haven’t gotten the arm built exactly to standard. (In the picture it’s just a piece of aluminum tubing) Of course we are concerned with it flipping over/ falling over so right now we have the winching system geared to pull the arm up and down in about 3 to 4 seconds. While that is a little on the slow side compared to other teams are I believe it’s best to start out slow and then we can speed it up easily later if it’s safe.

If you’ll notice, I did see that. It just took a post or two.

Regarding the complexity, it’s not that complex. If you’re already going to wrap around the winch drum, you could drill a small hole all the way through, run the string through that, wrap a few more times in the same direction, and
then attach it at the same distance from the pivot (or a shorter distance, but you’ll get some slack that way). If you’re unsure, you can cut the string extra long and adjust as needed.