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  #16   Spotlight this post!  
Unread 01-04-2012, 04:39 PM
topgun topgun is offline
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Re: pic: Unobtainium 6wd - Cantilevered, Dead-Axle, Slot-Tensioned Drivetrain

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Originally Posted by Joe Johnson View Post
A lot that is cringe worthy as well, but overall, a nice effort.
For those of us that use the various drivetrain designs posted on CD as learning tools, what do you find cringe worthy? And why?

Thanks.
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Unread 01-04-2012, 06:47 PM
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Re: pic: Unobtainium 6wd - Cantilevered, Dead-Axle, Slot-Tensioned Drivetrain

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Originally Posted by topgun View Post
For those of us that use the various drivetrain designs posted on CD as learning tools, what do you find cringe worthy? And why?

Thanks.
Let me start by saying that I am home sick today and probably should not have used the term cringe worthy. I really don't want to pick on the design. There is a lot to like.

Here is the thing. For those who have been doing this for a long time, design is an emotional experience.

I believe that we humans utilize our emotional brains to sort through complex space of possible solutions. Our emotional brains are just really good at searching through complex system interactions.

So... ...When I see something obviously awesome, I often laugh (I recall the first time I looked up in the GA Dome I laughed and laughed as I realized how cleverly the designers used hoop strength and tension members together with compression columns to raise the higher and higher tent poles over that beautifully clear interior). When I see something not so awesome, I feel it in my stomach.

So... ...what do I think is less than awesome in the design?

I think it boils down to two conflicting functions of the 1/4 angle. When I first see it, it seems both not strong enough and too strong at the same time.

It doesn't seem strong enough to be the sole mount for the cantilever axles. While at the same time the 80-20 and 1/4 angle seem pretty heavy for what they are doing.

The thing about emotional brains is that they can be wrong. We educate our emotional brains the best we can but they sometimes point us in the wrong direction.

I would have to do the calculations to know (and I haven't).

So... ...that is probably the numb of what I was getting at (that and the connection between them draws my eye as well).

Let me close by saying I was too harsh. There is a lot to like and a lot that I would change. But a very interesting effort.

Joe J.
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Unread 01-04-2012, 10:26 PM
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Re: pic: Unobtainium 6wd - Cantilevered, Dead-Axle, Slot-Tensioned Drivetrain

Thank you Joe for your comments.

I agree with you about the 1/4" angle. I see teams driving down to thinner and thinner metals so to see something that thick makes me wonder.

Last year we accidentally ran our .090" 5052-H32 sheet metal frame into a cement column at full speed without bumpers. We ended up with a little ding on the flange, but the structural integrity remained. Made me think we could have gone to .080 or maybe even less.

I always like to know the tradeoffs on various drivetrain designs so I can help our students understand the tradeoffs and make decisions according to what we need.

I am also constantly looking for designs that use less precision machining because I see a lot of teams without that capability. That is one thing interesting about this design.
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Unread 01-04-2012, 11:28 PM
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Re: pic: Unobtainium 6wd - Cantilevered, Dead-Axle, Slot-Tensioned Drivetrain

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Originally Posted by squirrel View Post
Stover lock nuts, you mean?

I suggest you might want to find or figure out how to make a more substantial washer. Our design used large hex flanged nuts. You might be able to do this too, and use 1/2" fine thread threaded rod for the axle as we did. We used two jam nuts to retain the wheels, but this required that whoever was working on the robot needed to know not to tighten the nut too much!
I saw the CAD of 1726's 2008 robot way back in the day, but back then I didn't realize it was 1/8" C-channel for the side rails. I thought it was box, and the cantilevered shafts were supported at two points. Missed that one big time.

And here I thought this was an original idea .

The flanged Stover lock nuts look pretty nice, and I think they would go a long way to improving the design. Placing one on either side of the plate instead of the washers, upsizing to a 1/2" shaft of fine threaded rod and two jam nuts to keep the wheels from falling off seems to be the direction I'd like to go in... and then an FEA.

Questions:
Did your wheel bearings rest on the threads of your threaded rod? If so, any concerns with this? Excessive play? Still worth the trade-off IMO, but curious nonetheless.

Did you have frame flex issues at competition weight? Did you have to add more support to your C-channel side rails to prevent them from twisting, resulting in negative camber?

Was that a typo, or was your fibreglass C-channel really only 1/8" thick?

Thanks all who posted in this thread who shared their comments and concerns (yes, that includes you Dr. Joe!).

Really, as a programming and electronics guy, if I can't get the mechanical guys to cringe at least once or twice a day, I'm not doing my job right .

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I believe that we humans utilize our emotional brains to sort through complex space of possible solutions. Our emotional brains are just really good at searching through complex system interactions.
P.S. If you believe in the quote above as much as I do, you need to read Malcolm Gladwell's book "Blink." Not just a blatant plug for a fellow Canadian, but his work really expounds on why some people are so good at what they do. There is so much more behind the "emotion" and "gut feelings" we get when working in our fields of expertise. The vast amount of experience and expertise we subconsciously put into honing those "feelings" allows us to correctly decipher really complex problems in the blink of en eye. Often without even realizing the vast amount of information we actually considered in the process.
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Unread 01-05-2012, 08:45 AM
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Re: pic: Unobtainium 6wd - Cantilevered, Dead-Axle, Slot-Tensioned Drivetrain

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Originally Posted by Mr. Lim View Post
Did your wheel bearings rest on the threads of your threaded rod? If so, any concerns with this? Excessive play? Still worth the trade-off IMO, but curious nonetheless.
It's not a tight fit, there's a bit of slop, but it's only a few thousandths of an inch. And the reduced load bearing area of the threaded rod vs a solid shoulder bolt means that after decades of use, it might wear the threads enough that you'd want to think about replacing the axles. It's really not an issue at all. I've worked on enough old cars to have a pretty good feel for how long something like this will last.

Quote:
Did you have frame flex issues at competition weight? Did you have to add more support to your C-channel side rails to prevent them from twisting, resulting in negative camber?
The frame is surprisingly stout for how little there is to it. The 1/16" aluminum bellypan and top corner gussets really make a strong structure when combined with the channel. Did I mention it's light? Notice that there really is no material where it doesn't need to be.

Quote:
Was that a typo, or was your fibreglass C-channel really only 1/8" thick?
Yeah, only 1/8" thick, that's plenty strong. As far as weight, the fiberglass has roughly half the density of aluminum.

Again, this robot was designed for a specific game, and worked well for that game. The 2009 robot we made of wood was even more unconventional, and also worked very well for the game it had to play. The steel robot we made the following year didn't work as well, it had cantilevered axles, but because of the bumps it had to traverse, the axles bent significantly during the competitions.
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