We built an overpass from steel pipe rather than PVC, but as a result, took some liberties in assembling it so as to avoid a crazy number of beveled cuts and welding. The finished product is a bit, uh, bouncy. We’ve gone as far as inserting a second steel pipe that sits inside those called out by FIRST to add strength to the midspan joint.
Has anyone else with a field built to spec. found this to be true? I’m curious how much of the deflection we’re seeing might be present in the FIRST fields or if it’s something that’s unique to ours that we should try to fix.
I wasn’t at the Manchester kickoff, nor have we built an overpass to the official specs, so I can’t make a direct comment on the field. However, the real field is aluminum pipe, not steel as called out in the “simplified” drawings. I think the welded joints on the aluminum field should be a bit stronger (and stiffer) than the NPT connections on the steel pipes. The extra stiffness of the connections should carry over to a stiffer overpass in general. However, the long span of the overpass is still a long span, and the aluminum (as you know) is 1/3rd the stiffness of steel. So, I can only imagine that, yes, it’ll probably be as bouncy as you’re presently experiencing.
We’ve already taken some good shots on the overpass with our robot, and the ball certainly makes the thing shake like jello. We’ve also knocked our overpass over. I don’t recommend doing that, ever.
I can’t give you the exact specs, but during development two of us were able to do pull-ups on the Overpass without significantly deflecting it. Note that this is NOT a recommended practice! (and no one should attempt this on a competition field!). But it gives an indication of the stiffness of the competition field structure. We did notice that while the structure was not terribly bouncy, the little bit of deflection that we did cause seemed to stress the pipes through deflection and into deformation.
Hmmm… This seems a bit of a concern considering the possibility of robot-to-overpass interaction during hybrid mode.
Are they going to have replacement overpass parts available should the overpass be damaged or should teams just prepare for the overpass to not conform to the specs once the compitition starts?
Hopefully, no robots should ever be hanging on the Overpass, even during Hybrid Mode. If they do, and the field is damaged as a result, then they are going to run into Rule <G34> and <G35>. But the likelihood of that is small. As noted, it took two of us hanging on the Overpass to deform it. Without getting into specifics, I think we can accept that I outweigh a legal FIRST robot (at least this year). So unless there are multiple robots hanging on the Overpass, I don’t think this is going to be a problem.
I don’t know if the trucks that transport the fields around will be carrying any extra field parts. Based on past practice, I would doubt it. But all they will really need is one of these: http://www.waynebesen.com/Sledge.gif
Ours was built with black pipe with pvc outside it. (Not having looked at the specs carefully, nor looked at the Kickoff structure, I have no idea what it was built to.) It is bouncy, especially during Jumpball, but not too worrisome. We’ve got enough students that know enough to tell others not to hang from it.
I would think, Dave, that a hanging load is not very likely during a game, but in our limited experience so far I’d be more worried that a robot would push or pull the overpass sideways (such as in hybrid mode when the robot doesn’t know better), or even worse one gets tangled in it while driving forward with another pushing along behind. Would the plates holding up the overpass be held down by the carpet enough to resist a wayward robot?
We actually made our full track completely out of PVC (trying to save some money & weight), but when we stood it up it sagged, so we bought some cheaper and lighter electrical conduit 1" diameter to brace the insides. Before putting it in the overpass (with all the pieces glued & or pinned) it seems to hold together pretty well. We ran the robot into it a few times, bounced the trackball against it and several other abusive actions, and it seemed to stay together, so it seems to be a reasonable modification to the design.
We built an overpass out of PVC to test designs. As Kim said, PVC is cheaper and easier to assemble. We found out that the design isn’t as stable or rigid as aluminum or steel, but we mad due. All you would have to do is have two people hold the “legs” of the overpass. When thrown or placed on the overpass, the trackball does cause it to move slightly, which won’t happen during an actual competition. That’s just something that we will have to take into account.
Ahh…my favorite tool…if it wont fit, bring out the hammer. Generally not good practice when you are putting a gearbox together, but that hasn’t stopped me
We built an overpass out of steel pipe, pretty much exactly to spec. Don’t know about bounciness, but it does sag a couple of inches in the centre of the span.
Dave,
They had one at kickoff but were using it to separate the center plates with the velcro on them from the carpet.
I think it is safe to say, plan on some sag when you are setting/adjusting your max robot height to go under the bar.
We did the pvc with 1" black pipe in the overpass only. Just PVC on the ends as per the basic team plan (no black pipe). We used 1/8" acft cable criss/crossed on the end supports for more firmness. QUESTION: 78inches, is that from the carpet to the top of the over pass pipe, or the bottom of the overpass pipe?. How much “sag” is in the center of the official game overpass?
