pic: 1726 week 4
 

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

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

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

Re: pic: 1726 week 4
 

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

Re: pic: 1726 week 4
 

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

Re: pic: 1726 week 4
 

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.

Re: pic: 1726 week 4
 

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.

Re: pic: 1726 week 4
 

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

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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.

Re: pic: 1726 week 4
 

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.

Re: pic: 1726 week 4
 

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

Re: pic: 1726 week 4
 

our bot is so cool

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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.

Re: pic: 1726 week 4
 

I'm sure the rest of the team is proud of it too, Stan.:)

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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"!

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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

Re: pic: 1726 week 4
 

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.

Thanks,

Amir

Re: pic: 1726 week 4
 

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.

Re: pic: 1726 week 4
 

Nevermind, Mr. Forbes beat me to it!

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The tensioner is nylon because it tends to self lubricate as it wears away. The piece was machined on a lathe--it was simply a matter of cutting away a little from each side of a thin piece leaving a small runner to guide the chain (you could also cut a groove)--I'll see if I can get some more detailed picture later.

We have been using nylon for tensioners since last year and have little visible wear on it even after two regionals.

Re: pic: 1726 week 4
 

We have used the nylon spring-loaded ones from McMaster and it is kind of clunky and hard to incorporate into the chassis.

Can you tell me which nylon round stock you used? I don't need a picture but others may benefit from one.

Thanks

Amir

Re: pic: 1726 week 4
 

I believe it was McMaster part # 8664K15

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Do a mcmaster search for oil-filled nylon rods I believe we used either the 1 1/2in or the 2 in rod.

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McMaster part number 8664K15 is enough to last for a few years :)

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i have to say i do like the pulltrusion shape that easily makes what would've taken multiple pieces of other materials to make. And it would've most likely ended up heavier.

looks like a great drive base, how much did you lower the center wheel?

Re: pic: 1726 week 4
 

Do you know if the oil-filled nylon is better than the oil-filled UHMWPE, part number 5243T23? The reason I ask is that the chain guides you are using are made from UHMWPE not nylon.

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Center wheel is lowered 1/8"

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We used the nylon because we had it on hand--Either should work well.

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When playing with the chain, the UHMWPE seemed to be a little more slippery. We used the nylon because we already had it, if I was ordering material for the tensioners i would go for the UHMWPE.