Bumper Attachment Design

[gwin99]

Hi everybody, this is my first thread! Anyways, my team had a lot of trouble with bumper attachments last year. They were constantly bending, breaking, being difficult to change, etctera. I am attempting to solidify our approach before build season starts. The plan for the sides is to use solid steel 1.5x1.5 in stock bolted to the robot with 1/4in thick u channel going over it and a 1/2 in diameter pin going through both. The front and back will be simply 1/4in thick u channel that goes over the box tubing of the frame, with the same 1/2in diameter pin going through. Please check out the attached renderings and let me know any feedback you might have. If it would help, let me know and I can also post the CAD and/or BOM!

[marshall]

I think it looks good but I would caution you about the attachment points. When you take impact, all of the force is going to be against those points so make sure you consider where they are located and that they have enough material backing them to handle the impacts.

Also, I can't recommend these enough for attaching bumpers: http://www.mcmaster.com/#90985a303/=u6jmdg

They are kinda expensive but you can re-use them year after year.

Our team was running drills the other night and those things can lead to some seriously short bumper changes (30 seconds and under). We're quite happy with them.

Good work on the design though and addressing bumpers before the 6th week of build season!

[gwin99]

I was planning to use these: http://www.mcmaster.com/#98480a562/=u6jsf0
Also pretty expensive, but 1/2 in diameter. I'll definitely take a closer look at yours though. Could you possibly clarify your point about the attachment points? We use a 6 wheel drivetrain, so for the sides the attachment points would be between the middle wheel and the wheel on either side, while the front and back attachment points would be as far to either side as the frame allows. Thanks for your feedback by the way! And I'll definitely force my teamates to run some drills on bumper changing

[cadandcookies]

Originally Posted by gwin99

Could you possibly clarify your point about the attachment points?

Not him, but the problem I see is that when you get hit, ALL of the impact from that hit is going to go through your two attachment bolts and into your frame. Last time 2220 used a bolted attachment system was 2012, and we noticed that after a season, the mounting holes in our frame were less holes than slots. And our bolts were often bent.

Basically, it'd be a good idea to distribute the impact over more than two attachment points.

[MrBasse]

I have no issues with changing bumpers, but have you considered a reversible set? After our first year bumper issues I will never go back to having more than one bumper to deal with. We use an elastic band that is looped to let us change bumpers in less than five seconds with no scraped knuckles or tools. We have loaded onto the field with the wrong color before and just flipped colors as we were setting the robot down with no issues.

The other benefit is that we can use a solid mount that might take ten minutes to put on, but we only have to do it once. Our current set from 2014 uses twelve bolts (2 front and rear and four on each side) and a piece of bar stock to distribute the load.

[gwin99]

I'll definitely look at reversible bumpers... Does anyone know of a good guide I could look at? I'n not particularly adept at sewing. Also To the issue of bolted attachment points into the frame, would you recommend bigger bolts, more bolts per attachment point (ie 4 vs 2), or something else?

[MrBasse]

Originally Posted by gwin99

I'll definitely look at reversible bumpers... Does anyone know of a good guide I could look at? I'n not particularly adept at sewing. Also To the issue of bolted attachment points into the frame, would you recommend bigger bolts, more bolts per attachment point (ie 4 vs 2), or something else?

I wrote what I will describe as a really bad white paper about ours. I sew ours for our team. I can't sew. They work every year just fine.

Here is a video from 2013 for how ours work: http://www.youtube.com/watch?v=XYTiRxLw7oA

We use 1/4-20 bolts for all attachments. We built a steel bar that ran the length of the robot last year and all bolts went through that into the frame. At the front and rear, we drilled and tapped a steel bar and ran bolts through the frame into that.

[gwin99]

Has anyone used these? I figure it might make the whole process easier, and they aren't all that much more expensive than buying the fabric for two sets of bumpers: http://www.robopromo.net/product_p/rp_fb.htm
Also, if we do end up going the reversible-route, what kind of attachments should we go with? Does anyone have some close up renders / CAD / pictures of a solid design they'd reccomend? If we went the reversible route I obviously wouldn't be using my current design with pins and such...

[MrBasse]

Here is a simple CAD view of our bumper mounts. It is just 1/8" steel that is screwed to the bumpers with 5/8" wood screws. The holes are 1/4" and the ends have a hole in the middle that is drilled and tapped to 1/4-20. On the sides the holes are drilled through the frame and bolted using a 1/2" 1/4-20 bolt and lock nut.

We used velcro our first year of reversible bumpers, and it drove me nuts. Between the velcro not holding together or pulling through the stitching that held it to the fabric, I swore never again. Elastic is so fast and so easy, I can't see any better way of doing it anymore. I have heard the robopromo fabric pieces are nice, and they do save a few headaches too.

[gwin99]

And are those rail things done on a cnc? If not, how'd you machine them?

[marshall]

Originally Posted by gwin99

I was planning to use these: http://www.mcmaster.com/#98480a562/=u6jsf0
Also pretty expensive, but 1/2 in diameter. I'll definitely take a closer look at yours though. Could you possibly clarify your point about the attachment points? We use a 6 wheel drivetrain, so for the sides the attachment points would be between the middle wheel and the wheel on either side, while the front and back attachment points would be as far to either side as the frame allows. Thanks for your feedback by the way! And I'll definitely force my teamates to run some drills on bumper changing

Our attachment points this past year were in the dead center of the front and back of our chassis. It was not a well thought out decision (we designed with usability in mind first) and we ended up bending the frame in both locations through rough contact and had to implement an emergency fix to get it resolved and then reinforce the frame.

It sounds like you have thought about it. Putting the forces into the corners is smart since there is reinforcement there already and you can take advantage of it.

[MrBasse]

Originally Posted by gwin99

And are those rail things done on a cnc? If not, how'd you machine them?

No CNC, I wish we had access. We had to go old school. We made them with a ruler, a piece of 3/4x1/8 steel strap, a marker, a drill, a grinder, and a MIG welder. It was tough to make them identical, but we got it close.

[Al Skierkiewicz]

I need to remind everyone here that while it is good to talk about designs, rules do change from year to year. I caution you to wait until the game is revealed before locking into a design.
That being said, I tell teams that you should never depend on the attachment hardware to be the mechanical basis for your bumper design. Many teams that use the attachments you show in the drawings above, have considered the forces involved in robot to robot interaction and designed the mounting such that bumper forces transfer to the frame through contact with the frame not through the hardware. In your first render for instance, if the brackets were such that the frame held the brackets in place and the hardware just kept it from falling off, that is a good design. In the third render, it appears that just gravity and the hardware keeps everything in place. If the other robot bumper system is lower than yours, the resulting force will force the bumper up and off the frame, leaving you with a bumper dragging behind you or left on the field.
As far as reversible bumpers go, I think they are a good idea when designed properly. I have said many time before, if you go to great lengths to make a beautiful robot, why would you put bumper covers that sag, drag on the floor, open and show the opposite color, etc. I am a proponent for bumper covers that do not have parts that sag outside of the bumper zone.