Over the past 6 years, our team has done well using a welded square tubing frame. In addition, we really haven’t built a successful (or what I consider successful) non-tank-tread driven robot since '03. However, I wanted to get our students learning about alternative drivetrains & frame construction methods and push them to expand their mindset beyond welded tubing frames and tank treads. With this photo submission, I’m hoping to get some feedback/suggestions from those teams that are well-versed in successful riveted frame construction. If anyone else has questions/comments, I’d be happy to answer them as well.
This 6-wheeled drivetrain designed this summer uses 1" x 3" x 1/8" aluminum U-channel riveted together at the four primary assembly points. A piece of 7/8" x 7/8" x 1/8" 90-degree angle aluminum is located in both interior corners where the frame rail butts against the end rail. Rivets go through all three pieces (frame rail & both 90-degree angles), with the 90-degree angle pieces providing mounting tabs to rivet to the end rail. T-shaped plates are riveted to both the top and bottom for additional joint strength and stability.
Total weight including motors, wheels, chains, and fasteners (minus battery and bumpers) is just under 43 lbs.
Questions:
I was planning on using general purpose, 5/32" stainless steel blind rivets. Are there alternative suggestions? 3/16"?
Will rivets work for this high-impact application, or would it be better to accept the small amount of additional weight and use bolts/nuts?
If rivets will work, are there enough rivets to do the job?
As long as you use enough of them, rivets work just fine for holding frames together, especially since you also have the top and bottom gusset plates. For some of the more high strength areas, you may want to stick with 3/16" steel rivets (such as where the front/rear frame members intersect with the longitudinal drive train members), but for things like the top and bottom gusset plates, 1/8" steel rivets work just fine.
Unless you need something to be removable, use rivets if at all possible. It makes it so much easier when you don’t have to figure out how to get a wrench into the chassis to tighten nuts.
Otherwise, this definitely looks like a good chassis and drive train design so far.
Is that U-channel or Rectangle Tubing where the wheels are mounted? Now someone with more experience with cantilevered wheels correct me, but don’t you need a place for 2 bearings to support the axle? If it is indeed tubing then never mind.
1618 did rivets in the frame this year, 1/4" aluminum ones to match the kitbot. We ran them rivet-for-bolt, 72 in all, and had zero problems in testing and spirited running at Chesapeake. We had bumpers on three sides (the rear bumper was dropped after we realized how much of a pain in the butt it was to attach when we were already compliant), which seems to help by all indications.
Last year, we used 3/16" rivets to hold together the two members of our arm’s tower. We used two on each side–you can see them on one side here, just beside the radio antenna. Even with questionable mounting of our tower, particularly in the rear, the rivets held together fine through all of Palmetto and our off-season demonstrations. The rivets sheared on one side following a particularly nasty tip at Brunswick Eruption and would’ve been easily fixed had I not left the team’s rivet tool 700 miles away in South Carolina. (oops.) At that point, we just enlarged one hole to 1/4", stuck in a kitbot nut/bolt, and ran off to the next match.
Just two points of data–your mileage may vary, but I will note that rivets are the win.
Team Pink this year riveted their whole frame together, we are planning to rivet the frame and then weld. Ask why?
This frame here:
If you rivet the whole frame together with 90 degree angled aluminum. We took about three weeks to weld this whole thing. If you rivet it first, the welding is a lot easier for our welders. Meaning that you can have a frame welded in about 4 days.
From this picture it looks to me like they are cantilevered, which worries me. Also it looks like the wheels are height adjustable in some way, I would love to have a closer look at how you are approaching it. Hopefully there is more support on those cantilevered then it looks, but overall it should be a solid frame that can take the beating your drivers will certainly put it through.
1726 did a riveted chassis this year, made from 1 x 4 x 1/4" pultruded fiberglass channel, and .060" sheet 3003 aluminum for the full belly pan and top gussets. We used 3/16" rivets from the hardware store. It worked just fine. Also we used cantilevered axles, mounted directly to the fiberglass, and had no problem with them. There was quite a bit of design work in this chassis, so fabrication and assembly was straightforward.
The cantilevered axles worked because they were mounted with flange nuts with a 1" diameter bearing surface, and the fiberglass is thick enough that it would not flex or be damaged by the load. There were no lightening holes in any of it. Fiberglass is about half the density of aluminum, so the weight of 1/4" thick fiberglass is about the same as 1/8" thick aluminum of the same size.
The bearing block is the same for all six wheels. We intended on making several sets of blocks with the axle hole located at different heights, both with and without bearings. With this, we could conduct experiments with wheel height, demonstrating what happens when all six wheels are at the same height, or when the center wheel is lowered by a 1/16" or 1/8", etc. One frame, many options.
I took a look at the pictures and noticed you notched the U-channel to insert the end into the end rail similar to this design. However, there appeared to be nothing joining the four fiberglass frame members together except for the bellypan and the four sheetmetal gusset plates on top of the four corners, with nothing actually inside the corners. Is that correct?
Correct. It was plenty strong enough with just attachments on the top and bottom, any more brackets would have been unnecessary and too much work to put in. Thin aluminum channel may require more bracing if it’s constructed in the same way, but the fiberglass is nice and stiff.
In the design M. Mellott posted, it appears that the attachment at the web of the channel is probably not necessary, since most of the web between the wheel and the end of the frame has been cut away. Also the bending load is highest at the flange of the channel.
I’d suggest not cutting out the side frame members for lightening…leave them strong, and you should be able to eliminate the corner pieces that connect the webs of the channels, while enlarging the top and bottom gussets so that they make good-sized triangles.
Our frames are typically assembled with a sort-of combination of press-fit construction held with rivets. In 2008, we inserted 1.5" square tube into channel with a 1.5" inside width; a tight fit. We then fixed it in place with four 1/4" rivets – two on top and two below.
It holds very well.
Are you concerned about the torsional stiffness of the channel that contains the axle mounts? Rectangular tubing in the same application would be quite a bit stiffer.
The open channel seems to me to be easier to work with than tubing…and any twisting of the main side members could also be reduced by adding a lightweight channel crossmember across the middle of the chassis. This could be integrated into the structure that holds the electronics, or placed right next to the transmissions.
You make a good point. I’m always concerned about the overall strength of the frame. I believe that the base frame is the foundation of the robot, and a team should never have to worry about it as far as maintenance–too many other things need attention and, barring a catastrophy, should be rock solid. We’ve always had solid frames, but often “solid” translated to “heavy”.
I suppose we were trying to be a little different (as far as our past designs go), not only with the use of U-channel, but also the added weight savers. This is why we were hoping to develope the design over the summer, then construct and test/abuse during the fall once school starts up.
We have been doing a folded sheet metal riveted frame for 3 years now. While in general I really like it, this years game was particularly rough on it. The high-speed nature of overdrive combined with the less than compliant barrier wall made for some interesting bends in the frame this year. Our material is also a lot thinner as we use mostly 0.060 and 0.050 AL to make our frame.
If you do riveted joints, make sure that you check them periodically after major impacts and/or the end of the day. After a lot of wear and tear, then tend to loosen. Not that I would stop using them, but they do become a maintenance item. Also don’t use the largest rivet that will package because having an up size option at competition is handy should they become loose.
Fiberglass is a great material for this application. Just a few things to keep in mind while working with it:
Gloves. Fiberglass dust can itch and sting if it gets in your skin.
Goggles (a must no matter what you’re doing)
Dust mask/vacuum. Fiberglass dust likes to float around in the air if you don’t have a vacuum held to whatever you’re machining. Inhaling the stuff is not recommended…
Pay attention to what kind of stresses you’re putting on the fiber glass. We used fiberglass for our ball holding mechanism frame this year, and in the scheme of things, it worked great. The one issue we ran into with it (apart from the constant irritation of the fiberglass dust), was the fibrous nature of the material. Since fiberglass is a series of glass fibers held together by an epoxy, drilling holes in the material leaves exposed fiber ends, allowing the material to be pulled apart. This caused some failures in our design, resulting in tubing that was split down the middle. There were two fixes to the problem that we looked into. The first (and heavier solution) was to put reinforcing plastic inserts into the tubing to reduce the stress on the holes. The second (and less precise) solution was to epoxy any machined edges to reseal the fiber ends. Of course, this issue is only a problem under certain conditions. We had plenty of machined connections on the fiberglass that held up fine through the whole season. The key is predicting and observing the stresses that the parts will be under, and designing accordingly.
As for rivets…
The super structure for our ball gathering and shooting mechanism in 2006 was 1/8" aluminum with 1/8" rivets. Only after two regionals, championships, and two off seasons of constant abuse and the robot flipping over onto the super structure did the rivets start to come out. So riveting is definately a viable solution. As for maintenance, pop rivets are usually pretty easy to remove by simply drilling them out with a drill of the same diameter (i.e. 1/8" rivets removed with 1/8" drill bit) and using the hole from the shank as a center.
When using aluminum, I find that it’s good practice to use aluminum rivets. Since the steel is harder and generally stronger than aluminum (except for the rare cases like 7068), using a steel rivet can mar the aluminum. 3/16" aluminum rivets should be sufficient for this design, especially with the number that you appear to be using. I could be wrong though, as I’ve never actually constructed a riveted drive train. If you decide to use steel rivets, use washers between the expanding part of the rivet and the aluminum to distribute the force over a greater area.
I might also want to mention that we also used rivets on our mechanisms this year, and we did have several shake loose. These were mostly in the high stress points in our launching mechanism and lifting mechanism (which took a severe beating). They were all easy to replace, but it took us a couple of attempts to realize that if the plates aren’t clamped together very well when you rivet them, they will shake loose again in no time.
All the rivets in the frame stayed tight all season though!
(a mild escapade off topic)
This is interesting, where did you get your fiberglass from? The stuff we use drills and cuts very cleanly, and I’ve never experienced any irritation from it (though I think some team members found it to be itchy). We get ours from creative pultrusions, http://www.creativepultrusions.com/ )
I’ve looked into creative pultrusions before, and I attempted to convince the superiors to order from them, but it was easier to order from mcmaster just because of the account we’ve had established there for years now. It’s of notably lower quality, which might be the cause of the splitting effects we experienced. It still cut and machined quite cleanly and easily, but when we had too much radial force on some of the holes it basically destroyed itself. This was partially the material, and partially because of the location of the holes on the tube. That said, those problems would not have occurred with a metal of any sort. So really, the point I’m trying to make (in a rather roundabout way:p ) is to remember that fiberglass is just different from aluminum, and to design accordingly.
As for feeling the irritating effects of the fiberglass dust… Everybody is different. People with thicker skin don’t seem to have problems with it. I wish I could say the same for myself. Haha
