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pic: Team 3572 Chassis Project
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Re: pic: Team 3572 Chassis Project
I really like most of the ideas behind this configuration. There is only one major modification I would make: Cantilever the middle wheels and remove the bent bracket on the outside altogether.
This bracket looks like it will receive all of the load that the wheels will put onto the frame. Its one major weakness is that it will most likely flex/bend in the direction parallel to the drive wheels when the robot is fully loaded at 150lbs or pushing/resisting another 150lb robot. This can be mitigated by adding angle supports along the outer frame, yet a steel cantilevered axle will handle it just as well without the extra parts. I'm still mulling over how other pieces would react to the various forces imparted upon them during a match, but nothing jumps out at me at the moment. |
Re: pic: Team 3572 Chassis Project
It's hard to tell from this angle, and I assume you checked this already, but it looks like the belts to the outer wheels might interfere with the CIMs.
Also, I can't really tell what's going on with the outer wheel axles, the hex looks like it just kinda merges with the 2x1 with no bearing, and on the oppisite side looks like it has been turned down to 1/4in or something. ...wait... Those are bolts aren't they lol, I have never seen someone cantilever bolts before, you might get some wobble if the threads are going through a bearing because the diameter won't be exactily that of the bearing or the tube they are running through... You might also consider adding a 3rd CIM in the center with a shorter belt and widening the pulley to accommodate. Those plates give me a bit of pause, you may want to brace them, but its worth a try in the off season. |
Re: pic: Team 3572 Chassis Project
Design isn't totally done yet. We were thinking of cantilevering the front and rear wheels and running dead axles with a support block inside the frame rail to make it solid. We would also use smooth shank fasteners.
We have never done this before, so our big fear is building something with weak points. That's why we plan on building it now and refining until 2016. The brackets around the center drive portion are to keep everyone happy, the big debate has started with us between cantilevering the whole thing or keeping those in to make sure the hex shafts stay aligned. The belts miss the CIMs by .100 gap. A major goal of this was to be able to build it with what we have in our shop currently. It's killing me that we have to send the rails off somewhere to get them machined for us as we don't have capacity to do it ourselves (a drill press just isn't that accurate, and drills don't make very good holes for bearings). |
Re: pic: Team 3572 Chassis Project
Can you explain the theory behind your locations of your 8 wheels?
I'm curious about your choice in spacing the two center wheels so close together, thanks. |
Re: pic: Team 3572 Chassis Project
What is your belt wrap for the stage off of your CIMs going to look like? If that is one belt tangent to the outside of all 3 pulleys, then I would question if you have enough belt wrap on the driven pulley in that arrangement.
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Re: pic: Team 3572 Chassis Project
I really don't trust the outside wheels. Our team made a minibot with cantilevered wheels on 3/8in bolts and despite it being substantially lighter than an FRC robot it could not handle the forces. Your wheels are also cantilevered out further than our minibot and are not on shock absorbing pneumatic tires leading me to trust this even less.
Also, you need blocks on the inside of the 2x1 to prevent the wheel bolt from just crushing it. |
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looks like the cims would interfere with the bumpers, and they could get damaged if hit to hart from another robot. Is there an advantage to having them pointing outside opposed to inside?
Other than that it seems really solid. btw, would the electronics board be mounted underneath? |
Re: pic: Team 3572 Chassis Project
Made a few updates. Looks pretty much the same but added in blocks for the front and rear axles. Also bumped the motors up and over to give .25" clearance for belt movement. Also I removed the braces on the outside of the frame to cantilever the wheels and added retaining bolts on the axles.
I'll try to add belts tomorrow so the clearances can be seen better. http://imgur.com/Xw2xC0l |
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We have always only used around 13-14 lbs of bumper, a more ridged attachment frame that eats up around 5 lbs of bumper weight could be all the protection the motors would need, I hope... |
Re: pic: Team 3572 Chassis Project
I like the layout, it is very creative.
It looks like the cim plates are 1/8" thick, if they are, I would suggest at least 1/4" thick with the Cims cantilevered off them. 1/8" works if both sides are supported, but a big bounce or hit will put a large load on that thin plate. The middle drive pulley to the cims looks cantilevered from the frame tube, I'm not sure if would would be an issue or not, but it would help if you can support it with a bearing on both sides. Some testing of the current layout will let you know, just something to watch out for. I would suggest the dead axles with blocks for the outside wheels like you suggested, we ran cantilevered dead axles last year and loved the performance over live axles. With your belt layout, it makes it easier to run dead axles versus live axles as well. |
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Just remember that the wires exist and have minimum bend radii. |
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The motor plates are 1/8", but they are also steel. This would allow us an easier time adding supports to them. A bar across might not be a problem depending on where the electronics end up. I would like to avoid it if possible though.
The majority of our components were made of 1/2" conduit last year as it weighs much less than the aluminum of equivalent strength. Plus, I hate TIG welding. Once I figure out what I'm doing, I'm usually done with the project. If we need support we will most likely go back to 1/2" conduit. $2.50 a stick can't be beat with proper welding ventilation. Plus when you are done you can paint it aluminum color and nobody ever knows. I'll keep an eye on the pulley for flex. If needed we can just extend the shaft and add another hole into the inner bearing plate. That was actually in the original design, but the shaft was cut short for prototyping and it seemed okay. Only building both sides and beating the snot out of it will tell if it will hold up though. |
Re: pic: Team 3572 Chassis Project
Looks pretty nice, but clean up the front/back axles in the cad. Either cantilevered live axle or dead axle will work, but decide on one pre-season.
Personally I recommend live axle, as it's easier to swap out wheels or gearboxes if something breaks. Also, 12fps is too slow. Gear for around 15fps at least. We ran around 16.5fps at CalGames on 4 cims with no problems; we were running 12fps during the first half of the season, and the extra speed was very exciting. |
Re: pic: Team 3572 Chassis Project
How do you adjust your belt tension and alignment?
Did you account for mechanical drivetrain loss when calculating your 12-13 fps? I'd be curious as to the drive belt engagement given your configuration. |
Re: pic: Team 3572 Chassis Project
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Speed is based on JVN's calculator set to 81%. and double checked by my awful math and an 80% efficiency. Belt runs can be seen here. After teaching drafting for ten years I realized I had never used the design accelerator in my life, so I had to play with it for a while to see what it could do. Ignore the fact that there is a 9mm belt on the wheel pulleys. That would be an 18mm belt there. |
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Looks interesting. I'd consider moving the motors lower and flipping them inside to lower the CG. Also, they look vulnerable where they are.
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I agree on practice, but without the funds for a practice robot and typically finishing up with hours to go, it is hard to come by. Also, a location big enough with carpet that would allow real practice has long been a dream of mine. |
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