pic: 1726 week 4

Our robot as of 2/2/08. Everything you see there weighs ~61.3lbs, so we figure we’re doing pretty good on weight, considering how much the rest of the 'bot weighs. Our chassis team had a fun job with the single chain :wink:

The robot is projected to be finished within a day or two, but we’ll see how things go…

Very cool looking, I like the chain system on it.

You beat us on the chassis weight, we were 70-75!

Why not use two idlers rather than the curved plastic? It’d be a lot more efficient.

The green plastic is our tensioner. The hole is off center, so we just loosen it and rotate it to tension the chain.

I’m curious how we’d calculate the efficiency of it either way…any ideas?

The curved plastic is UHMW Polyethylene chain guide from McMaster. It is very light and has very little friction on the chain. Using idlers would have placed the top length of the chain a lot higher up from the frame, and would have been very difficult to mount.

I’d say the efficiency loss due to friction on the plastic is minimal and worth the benefit of not having to make very precise holes for idlers. As a general team moto the less machining the better.

Why the AL frame. You’re using poltrusion below. Why not continue with the uper frame?

The Al is for mounting the mechanism on it–it save weight (lots) and is easier to machine than pultrusion. Pultrusion must also be thicker to achieve the same strength as aluminum so it would be unwieldy to mount that way.

or, the short answer: we had the pultruded fiberglass channel for the frame already, but no thin angle, so we went with aluminum. Also the aluminum is available at Ace hardware, and easy to replace if it we have problems with it in competition.

Yes! As a pioneer of Pultruded Fiberglass in FIRST I would like to know why too!

our bot is so cool

We like to play around with different materials, finding the best material for each part, kind of as an engineering exercise. A lot of that has to do with the equipment we have available in the shop, time needed to order materials and build parts, as well as the loading on each part. I think fiberglass is really excellent for frames, but it is a bit more tricky to use it in the upper parts of the robot, and we haven’t gotten good enough at designing with it yet to feel confident using it there.

I’m sure the rest of the team is proud of it too, Stan.:slight_smile:

Ok we will let is slide this time, because it does look cool! We haven’t seen a 3 inch c channel before. It looks “mini”!

It’s actually 4" C channel on the frame. Gary did most of the chassis design on Inventor. Also our promobot we built in the fall used fiberglass for all the structural parts, so we are playing around with it a bit. Time to order more, though

Is the tensioner an off the shelf item or machined from a piece of round stock? If it is off the shelf where did you get it? If not what material is it, HMWPE or Nylon? We are looking for a new/easier tensioning system and we have been leaning toward the off-center circle. I would also like to know what materials work best for this method.



We made the green tensioners with oil impregnated nylon rod (2" diameter), turned on the lathe.

The white material (UHMW Polyethylene) that the chain guide is made of would work well too, I suppose…we had the nylon rod left over from last year, so we made the tensioners with it.

Also we added an internal/external tooth lockwasher between the tensioner and frame rail to make sure it would not slip.

Nevermind, Mr. Forbes beat me to it!