need help w/ bumper fastner designs

[lopz056]

Hey, I have been in charge of the design and build of the bumpers for the past few years and last year my design worked fairly well but had difficulty in the process of trying to take them off quickly in order for the pit crew to get to the insides of the robot. Im pretty sure i have a decent design this year but i know there are many more experienced people out there that have lots of good ideas and was hoping i could get some suggestions just so i have choices to go through. anything would be much appreciated

[Neil B]

Firstly, I'd suggest you only take the illustration of bumpers in the manual (Figure 8-1) as a recommendation, not an actual design. From what I can tell, it shows the bolt as the part that goes through the frame and using T-nuts in the plywood. I can say from experience that that is a very bad plan. When we used T-nuts on our bumpers in '06 they completely chewed up the holes they were originally set in after a few matches and lost their grip on the plywood. That meant that we had to spend a lot of time taking the cloth off our bumpers to search for t-nuts that had worked their way loose; making those particular fastening points unusable.

I would suggest that you place the bolts inside the bumper and sticking out the back. Just drill a few holes in the chassis, slip the bumper on and attach nuts; it was a 5 minute job for the whole robot and we could usually attach the bumpers completely while queuing for our next match.

[Scott Morgan]

Last year we used clevis pins to attach our bumpers. We attached clevis pins directly to the front and back bumpers by drilling holes in the bumper wood, and gluing the pin in. Then the pins were inserted through holes in the chassis rails and secured in place with cotter pins.

I'd recommend this system because it's simple, and you can change out bumpers very quickly.

[lopz056]

yeah, that was basically our desing for last year, except the threads were located inside the plywood. and we had bolts that would go throug pieces of square 1x1 in aluminum and into the thread. but same idea.it worked but then the square peices of metal causing the bolts not to fit. so by the time the second competition came around, we resorted to zip ties to keep the bolts from falling out.

[Tristan Lall]

In the past, the rule has been unspecific with regard to the precise design of allowable fastening methods. In the past, after some internal discussion, the inspectors have been instructed that since the illustration in the rule depicts mating threaded fasteners, and offers this as an example, threaded fasteners are (almost) universally allowable. As I recall, the discussion also covered the idea that standard nuts and washers could be reasonably considered as fasteners (which are included in the bumper weight), but that extra items like custom washers or brackets were to be regarded as parts of the robot. (That's a broad summary of a nuanced discussion; it is not an official ruling, nor an endorsement of any particular fastening method!)

Designs using clevis pins are sometimes seen, and were sometimes allowed (particularly in 2006), depending on the implementation details. However, in both this year's iteration of the rule (2008 <R08>), and last year's (2007 <R37>), it specifies a "bolt-and-fastener" method of attachment, which would seem to render a standard clevis pin illegal, but might arguably allow a clevis pin made from a bolt. (I don't know the details of 1138's clevis system, but I'd advise them to pay particular attention to this point for 2008, just to be safe.)

The written part of the rule also doesn't specify any required positions or orientations of the fasteners. As a result, the diagram (which depicts a bolt and a T-nut in the centre of the plywood) could be interpreted either as one possible configuration, or alternatively, as the only legal configuration. For the most part, this has been interpreted as the former by the officials (allowing more creativity and cutting the teams some slack). Under that interpretation, some teams have therefore installed bumpers with multiple rows of fasteners, fasteners inserted nonperpendicularly in the plywood, and fasteners inserted into holes drilled into the edges of the plywood. All of these things seem to be legal permutations.

There are also varying interpretations of whether you can design the bumper so that the fabric needs to be removed in order to unfasten the bumper. I would consider this to be antithetical to the statement (in the diagram) that bumpers be quickly removable. I've seen it both passed and failed at inspection—and sometimes passed once, then failed at the next event.

I believe that there was also a past Q&A response (likely in 2006, if my memory serves me correctly) specifying that it was not permissible to pierce the fabric of the bumper in order to install fasteners (even if the hole was patched, or reinforced). This is an issue when a team installs a regular hex nut on the inside of the bumper, and wants to tighten it with a nut driver or socket wrench. I'd avoid this design, in case a similar ruling is delivered this year.

Whatever you do, do not use zip-ties, hook and loop fasteners, or tape to secure your bumpers. Those will be rejected at inspection.

Note however, that with this rule in particular, you need to ask in the Q&A forum before proceeding. The official responses there will dictate how officials will interpret the ambiguity in the bumper rule for the 2008 season.

[chuckmerja]

Both one of my team members and I have tried to post to the FIRST forum re bumpers, as we are VERY concerned about what we are reading there and how it impacts our bumper attachment system.

The first bullet in R08 says "STANDARD BUMPERS must be designed as shown in figures 8-1 and 8-2. This is the only acceptable design for STANDARD BUMPERS." where the image shows a "T" nut attachment - and the rule says ONLY ACCEPTABLE.

Bullet 3 - "STANDARD BUMPERS must be attached to the ROBOT with a boltand-
fastener system to form a rigid, robust connection to the ROBOT structure (i.e. not attached with Velcro!)." does not say "THE nut and fastener system", which seems to leave open a little wiggle room for the attachment system.

Ours is definitely robust, quick, blind etc, but as we show in the pix we want have an aircraft quality pin as the attachment - of course we could use a bolt and nut here. However, a nit-picky inspector could cause us problems, so we are trying to get a ruling from the forum, but as I said earlyer, we can't post to that forum - could someone post this to the official FIRST forum for us - and of course discuss here.

Here's pic of our blind, easily detached, robust attachment (the bolt will go through holes in the aluminum backing plate, and the aluminum plate will be welded to the 1" square tube)
....

http://srvscience.org/FIRST/2008_bumper_attach.htm

[Jon Stratis]

From what i can see, your bumper will fulfill the definition of a standard bumper - You build the standard bumper, and attached it to that metal arm with a secure nut and bolt system. That arm, i believe, needs to also be securely attached to the rest of the robot with nuts and bolts - a pin probably wouldn't suffice. Another team posted a similar question to the GDC:

http://forums.usfirst.org/showthread.php?t=8551

Thank you for pointing out our ambiguous/erroneous language. We will amend the inspection check-list so that bolt and nut fasteners are accepted. In your case, we suggest that the "quick disconnect" pin be a bolt-nut combination. Please be sure that the Bumpers you use are compliant with the rest of Rule <R08>.