pic: Chassis Idea
 

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Looks really good!


Nice Work!

A couple suggestions:

You could definitely get away with 1/16 inch wall tube on the inner parts of the frame to save some weight.

I'm assuming you're welding the extrusion. The other thing is that my team found is if you cut the edges of the square tube extrusion to 45 degree angles where they meet at the corners, it is more structally sound.

Are those wheels the ones you posted before?

Looks really nice, the rendering makes it look very appealing.

Re: pic: Chassis Idea
 

I see you have really been hitting up the CAD software recently! Awesome work!

Re: pic: Chassis Idea
 

My thoughts would be if you make it so that you can slide the battery in through the larger holes on the front (or back), and (this is something 190 did a while back) since you have a six wheel base I suggest moving the middle wheel down from the outer wheels. This is a common base that will tilt when you move allowing for a smaller turning radius and better maneuverability. This sometimes creates to much maneuverability or to little, depending on your needs. So in order to give yourself more, or less, maneuverability you can put high friction wheels in the middle and back sections and have the last two be omni wheels. This way if your driving one way you have high maneuver ability, while driving the opposite direction will allow you to stay more stable and go forward straighter for longer amounts of time without stray.
----------------------------------------
| High Friction High Friction |
| |
| High Friction High Friction |
| |
| Omni Omni |
-----------------------------------------

Re: pic: Chassis Idea
 

Very nice work! This is about as simple a box frame chassis as I've seen, and it is definitely a good start towards a top notch final design.

Some things to think about when designing a chassis (obviously some of these are dependent on the game and rules):

• How will we ensure that our chains will be properly tensioned?
• How we will adjust the drop of the center wheel (if any)?
• How will we mount our bumpers?
• How will we fix X when it fails? X = wheels, sprockets, chain, gearbox, motor, speed controller, cable, frame members, welds, etc. (and how long would such a repair take?)
• What sorts of COTS and custom made spare parts will we want to bring to competition with this drive base?
• Do we have easy access to all bolt heads, nuts, rivets, etc., for tightening or loosening?
• If something must go to make weight, where will we start?
• What is the center of gravity, and how can we move it to where we want it to be?
• How long will it take to build this once build season starts?
• How much does it cost? Can we lower the cost somehow?
• What are the wiring paths?
• Where can we put the battery?
• How will we lift this thing?

Hope this helps!

My only two questions/comments about your particular chassis are (1) if there is a ramp to climb, you will probably want to move your wheels further to the front and rear of the frame and (2) how do you plan on supporting the plywood base in the middle?

Re: pic: Chassis Idea
 

Thank you for the weight saving tip! I have been going through all of my options. I wasn't sure if that would be safe.

We are hoping to be able to weld it. Money and knowledge may hinder that though. If we can not then we will be creating brackets out of 1/16" Aluminum and riveting it together.

The wheels are a more recent version of what I posted. A picture of them can be found here. If you go over one you can see it with the gear. (I haven't put the bolts in yet. I am about to do that when I have finished this post.)

Thank you for all of your help. :D


I currently have a center drop of 1/16. I apologize for not mentioning that in my description.

Thank you for the tips on maneuverability. :D

Thank you for that list. I have only considered a few of them, and I will definitely be keeping all of that in mind as I perfect this design.

That is a good point, I had not thought about any ramps. The nice thing is that no design changes will need to be made; just the location of the wheels. :D

I am considering that at the moment. I have thought about mounting an aluminum cross beam in there. Do you have any suggestions?

-Rion

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Looks good.

I'm wondering...did you want the wheels on dead axles or live?

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(Starts editing the OP.)

The wheels are on live axles. Sorry about that.

-Rion

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Hopefully someone with more experience with 6wd can post here (I've never built a live axle system before) but what is the purpose of having a live axle system? To my understanding, the occasion when you use a live system is when the gearbox is directly driving either the center wheel (live) or a corner wheel but then the other two wheels are on dead axles.

Someone want to clarify this for me?

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As far as I can tell, there is no particularly compelling reason for this to use live axles.

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I meant to ask the question in general too. Is there less loss of power when using live axles or something? What is the benefit, if any?

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Live axles are great for small sprockets/wheels, cantilevered "west coast drive" or direct driven wheels. They are also great if you use 3/8" hex, 1/2" hex, or 1/2" keyed drive shafts with the ends stepped down to 3/8" with both ends of the shaft supported. This keeps the shaft in place without any need for shaft collars, roll pins, etc. Some wheels (like Colsons) are easy to broach (or press a knurled aluminum hub into), but nearly impossible to drill a bolt pattern into.

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The benefit of live axles comes from being able to use the axle as the transmission of power to the wheel. As opposed to bolting the sprocket directly to the wheel and placing bearings in the wheel, the live axle setup transmits power from the sprocket through the axle to the wheel.

Some setups may only function with a live axle setup. However given the choice between dead or live axle....I use dead axle wherever and whenever I can.

Art- If your supporting the shaft on both sides anyways (assuming a wheel your using can have a bolt pattern), then bolting each end of the shaft in place is even more simple IMO, instead of having to turn down the ends of the shafts.

-Brando

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Turning down the ends of shafts on a lathe takes one tool.

Drilling and tapping the ends takes three: a center drill, the drill bit, then the tap.

On a manual lathe, it's faster to turn the ends down.

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I will sum up my experiences with live and dead axles below.

Live axles are useful because:

• You can drive them from any point on the shaft (e.g. West Coast Drive where there is a bearing block between the driving sprocket and the wheel).
• The keyed/hex shaft becomes a "universal" interface for attaching wheels, bearings, sprockets, gears, brakes, encoders, etc. As long as your piece has an (ex.) 1/2" keyed bore, it will rotate along with everything else on the shaft. With a dead axle, you must attach pieces to each other directly, and not all wheel and sprocket combinations play nice in this regard.

Dead axles are useful because:

• You are minimizing the rotational mass of the wheel assembly and therefore maximizing efficiency since the shaft itself doesn't need to spin. Unless you have a truly massive axle or bad bearings/alignment, however, we're probably not talking a huge difference.
• In the cantilevered case, you are minimizing torsional loads on ball bearings with dead axles (since the bearings are still centered on the axis of rotation no matter what the deflection of the shaft).
• The closer together two bearings are, the easier it is to have them stay aligned. In a dead axle setup with two bearings in a single wheel, alignment is trivial. Live axle systems that support both ends of the shaft can get out of alignment more easily.

Did I miss anything?

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I'd be very weary of the weight you posted for a couple of reasons; You don't have everything modeled (chain, etc..), and even for what you have modeled, 30 lbs seems really light.

1/8" wall 1x1 is around .5 lbs a foot, and based on your 27x37, that puts you at around 17-18 lbs just for the 1x1. Add in two CIMs at 2.75 lbs each and the Gearboxes at 3.5 each and you see it starting to add up; this isn't even counting your mounting plates or wheels.

I'm not criticizing your design, it looks like a decent start to a decent drive. I just want people to realize that the weight a CAD program is the weight exactly as the model is modeled; So, if the model isn't accurate to real life in terms of included properties, applied materials, and custom mass properties (Setting the weight of parts like CIMs), the number won't be accurate either.

For the drive itself, I'd recommend you switch to a good deal of 1/16" wall, switch to dead axles and prioritize a light & robust tensioning system.

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I myself have been questioning the weight. I don't know exactly how much it will weigh. And honestly. There is no easy way to find out. (Without building it) The main thing that has me questioning is that is says this weighs 43.54 pounds...

The reason I went with live axles is simply because I have designed a live axle wheel. I will be making a dead axle of the same wheel and making a dead axle version of this.

Thank you for the notes. I will be taking all of them into account as I go over this design again, and again, and again. :yikes:

-Rion

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The AndyMark shifter in your CAD rendering looks just like the imported STEP file, which doesn't really have any mechanical properties. Open each individual part of the imported STEP assembly (they were all converted to .sldprt files), set the correct material, then manually set the CIM motor weight to 2.75 lbs.

Re: pic: Chassis Idea
 

We also use dead axles as structural members of our chassis (a round axle with tapped ends = chassis standoff.)

If you get creative with your axle placement and the structural design of your chassis this can eliminate the need for other structure and result in a lighter overall system.

-John

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I just noticed that when I opened it, there are no part files. I have probably done something wrong. If that is the case, how to I fix this?

Also. I have no idea what type of material to use for each part. I discovered the side plates are steel, past that I am lost. (RC, you need the GEN 1 in the CAD Library. :P )

If anyone could help me with this; please do not hesitate.

-Rion

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Okay, that is definitely not 43 lbs.

What you need to do is make sure every component is set correctly. If it's a machined part or metal piece that is a uniform material set it's material in SW, that will make apply the proper density and weight. For everything else go to tools > mass properties > check the box for assigned mass properties, and manually assign a mass. CIMs are 2.75 lbs, compressor ~5, etc... Someone posted a spreadsheet a while back that had most of the electronic's weights. You can do assigned mass properties for an assembly, and ignore the properties of it's components; for example, set the shifters to 3.4 lbs + 2.75 lbs (the weight of a CIM plus the gearbox).

As for trusting parts that have been imported or given to you from others; just don't. If a goal of yours is to use your CAD model to provide accurate weight estimates, get in the habit of making sure all parts are set correctly as they are made and/or inserted into the assembly. It is a horrible experience trying to figure out later on why the robot is 20 lbs lighter/heavier than you think it should be and you have 1000+ parts in the assembly.

Re: pic: Chassis Idea
 

A Box Drivetrain like this generally looks quite good on its own, but when you consider a real FIRST robot, some flaws pop up. From a structural standpoint, you need to ensure that you will be able to adapt this design to manipulators.

About 50% of the time my team had to take a chunk out of the front of the chassis to facilitate claws or collectors. Just keep this possibility in mind and adapt your design for potential geometry changes.

If you can't get it welded, this design should work quite well with rivets. Triangular corner gussets might be a good idea for some of the pieces, rather than just right angle aluminium.

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Thank you for this! I probably would have been killing myself come build season while working on CAD... Thank you very much. I'm going to get started on changing all the masses.


This is really just my first step in designing a working FRC chassis. I would except part to get chopped up. If they didn't, then the GDC decided not to be evil... (Wouldn't that be the day. :yikes: )

I have seen pictures of riveted Aluminum chassis so I figured I would be pretty safe in doing so. When I get far enough into the design process I will probably create a CAD with those in it. (Yes rivets and all. I may add wires... :rolleyes: )

-Rion

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That's pretty much a given when it comes to designing anything, you need to be able to adapt it to the circumstance. I'm sure Rion knows this already. It is the same with electrical layouts (hence why its funny when people complain about electronics placement in CAD).

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Consider contacting a local college. You could probably convince them to let you use their welding equipment (I would imagine most colleges have access to the equipment) Barring that, contact car repair places. Should be free and someone can teach you how/ do it themselves. Make the brackets anyway in case the welds break though.

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# of tools is kind of an unfair comparison here. Drilling the ends of a piece of stock is probably the simplest operation you can do on a manual lathe. No zeroing (assuming your going to just do a rough estimate of depth because your tapping the hole anyway) and no measuring of the piece to ensure correct diameter.

My guess is that the lathe operations would take close to the same amount of time for both kinds of axle, including the tool change for the drill. The tapping will cause the dead axle to take a longer amount of time, but we're talking about a matter of minutes which I do not feel is enough to justify changing a design from dead to live. There are a ton of other considerations obviously, but I just wanted to point that out.

-Brando

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Nice looking CAD.

We have done similar box chassis in the past and depending on the game we will more than likely do one again this year.. They are strong, simple and very reliable but heavy.

You can replace the shaft supports with simple aluminum blocks and make chain tensioning and changing wheel drop easier.

You can also loose the entire upper structure or wait and make one that is more game appropriate out of lighter stock.

We don't use welding anymore. Instead we glue everything together. Never had a failure.

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Will do Rion, Jeff and I have been working on getting more models up. We'll hopefully have that up before season.

-RC

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We have used the same method for our "dead axles" on a couple bots.

We have an inner and outer rail and simply use a shoulder bolt as the axle. 1/16 plastic tubing keeps the wheels centered between the rails.

One tool and a couple turns and the axle can be removed and the wheel drops out.

It's simple and it's worked well for us, and best of all it requires a minimum of machining since all of our machine work is off site and donated.

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Team 104 made a box frame like this the last 2 years. The frame was made out of 8020 Quick Frame 1/16" box tubing. We also used the associated plastic connectors. In a couple of hours we cut and whacked our frame together. We had no failures or structural issues. We did use polycarbonate covering and other structures to tie everything together which is critical. We used dead axles. The axle supports were made from 8020 10 S 8611 double retainer profile . The axle supports clamps onto the frame allowing the chain to be tensioned. For the axle span 3/8" grade 8 bolts work well and save weight. The bottom board also is a structural part of the frame. A piece of 1/4" Baltic birch plywood would be a good choice. To go one step further, laminate the board with a layer of 5.7 OZ carbon cloth on both sides. This type of frame is good for low resource Teams.

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I would love to hear more about this.

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We machine connectors (like 8020 quick frame) that slip inside the 1" square tube and then use Loctite Hysol to bond them together.

You can hit it with a sledge hammer and the aluminum will give way before the Hysol does.

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for dead axles we just use 1/2" bolts. We find them to be easy to take on and off because all they need to be attached is nut. Yes you do give up some weight and room with the added material but we feel that the simplicity makes up for it.

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Which Hysol formulation do you use? Epoxy, Urethane, or Polyurethane? Any other information about how you bond the aluminum frame together would be appreciated.

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IndySam I am with gburlison in that I would love to hear more about this. I never thought of gluing a frame together but that may be a good alternative to welding as we have a sponsor who could get us the 8020 parts easily enough (if we can't make our own) Do you have any examples of possibly a 2x1 tube connector? If you don't I would still love to see what your 1x1's look like.

Thanks,

Trent B

About the CAD Model, interesting choice of live axles but I suppose the wheel was the deciding factor.

I found too when I did some of my own CAD models, for things like batteries, cims and gearboxes its usually easiest just to set the weight and be done with it (no messing around with materials, but always use them on stuff you plan to build yourself (aluminum frame)).

I have yet to design a frame like this, might try to CAD one quick before the kickoff.

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Someone should really just start a new thread about this. I'm interested as well.

-RC

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Sorry Rion for somewhat taking over your thread.

Future discussion on frame gluing

http://www.chiefdelphi.com/forums/sh...176#post892176

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This isn't that uncommon of a practice guys, and its fairly easy to do even with 1x1 and 1x2 square tubing. We've done it for the past 3 years or so; however, we machine out nylon and UHMW (and even solid aluminum) blocks to insert into the ends of the tubing. Then simply rivet, screw, or bolt the block into place. The ends of the blocks (or wherever you need) are drilled and tapped. The screw or large rivets are easily replaceable with hard epoxy, loctite, or even gorilla glue (in some areas). And there you have a snug fitting frame connector! :D

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Here's a thread discussing live vs. dead axles.

http://www.chiefdelphi.com/forums/sh...ad.php?t=47117

-RC