pic: 6wd chassis - need feedback
 

Re: pic: 6wd chassis - need feedback
 

1/2" bolts for axels is way overkill, we've gotten away with 5/16", but usually we just use 3/8".

You also may need to add some standoffs to prevent the sides from crushing together. I might also suggest you make the flanges on the ends a bit longer to prevent it from buckiling in or twisting.

Lightening Holes? Delete them and see how much weight is actually saved. If it is on the order of 2-3 lbs. keep them, if it is 1-2 lbs.... is it really worth it? Personaly I'd only go after the dense stuff on the bot, like steel gears and sprockets, and leave the not-so-dense-stuff (like al) alone.

Re: pic: 6wd chassis - need feedback
 

What material are you looking at using for a bedpan?

Re: pic: 6wd chassis - need feedback
 

Even if it's 2-5 lbs, I would think with 1/8" aluminum you would want to keep that weight in the chassis anyway. More weight down low = lower CG, and with thin aluminum like that you may want the extra strength.

Take that with a grain of salt though, I'm one of the programmers :) . That's just what my dad likes to tell me about chassis.

Re: pic: 6wd chassis - need feedback
 

Very similar to our chassis last year, 1/8 worked well for us and the lightening holes look fine, I would highly suggest you keep them.

I would also switch the 1/2 bolts for 1/2 aluminum shafts. You will save a bunch of weight there. We have used 1/2 AI dead axles for three years and have never had a problem with them.

Re: pic: 6wd chassis - need feedback
 

Depending on what you are planning on cutting the lightening holes with, you might consider pocketing (only cutting half of the thickness out, rather than all the way through). This doesn't save as much weight, but will be stronger. Also, you might want to make the axles lower in the frame to improve ground clearance and make mounting manipulators easier.

Re: pic: 6wd chassis - need feedback
 

Since you are planning on welding all the joints, I would recommend notching the cross bracing 1" leg where it meets the perimeter of the frame so all the axle holes will be the same height from the base plane. Also be aware of needed clearance to chains or belts (if you use them) and the bottom of your frame for clearance when transitioning from a ramp to a horizontal surface.

Re: pic: 6wd chassis - need feedback
 

Looks pretty good. You're going to want some gussets on the corners though, especially on the front and back rails. If you run into a corner or a post or something head on with that, it's probably gonna bend.

Re: pic: 6wd chassis - need feedback
 

Agree with the gussets.

Also how are the cross bars in the center attached? those would be good to get welded if u can.

Re: pic: 6wd chassis - need feedback
 

Thx for the feedback everyone - keep it coming.

I'll take this opportunity to reply on some of the comments / suggestions so far.

I agree with the 1/2" bolts being way overkill - I'll check to see what bearings we have and how they fit in the wheels and sprockets we plan to use. Like I said before, we are very limited on machining capabilities so boring out holes in sprockets and gears is not an option.

for the 1/2" aluminum shafts - how do you attach them? I can see welding them to the inside rail but not the outside - or do you tap the ends and bolt them on that way?

the bracing would be welded as well but I'm not sure I understand the need to notch the aluminum - in fact, the reason for using angle is to have the added strength of the angle. notching would remove that (if I understand it correctly).

Clearance is definitely an open subject right now. if we don't have to climb anything then it's a moot point - but leving enough material at the top of the frame for manipulator attachments is a very good idea. Thx for the heads up!!

for the lightening holes - there is no guarantee that we will be able to get these cut out but I wanted some opinions just in case. I also have not figured out how to get Autodesk to calculate weight so I have not compared it with and without the holes to see the savings. I would think that 4-5lbs would be a good savings but if it weakens the frame too much then it's really not worth it. Again, I don't have any practical experience with this to know.

for the corner gussets - I thought about that as well. Not sure if it's worth the trouble but I'll leave that open as an option for improvement. I expect it to bend a little but should not actually cause any functional issues. There will be bumpers to help as well as the standoffs in the corners for the outer wheel plate that will help keep these in place (I think). if it's easy to do though, then I think we should.

By the way - no bedpan in the plans. At least not a structural one. we will obviously be using something to mount electronics and stuff like that but not an aluminum plate or something strong enough to help the frame any.

again - your feedback is greatly appreciated. having all of your experience at our fingertips is awesome!!

Re: pic: 6wd chassis - need feedback
 

From personal experience, you want those gussets. Team 971 had this starting the regional. That got completely bent up, was hammered out and a piece of 2"x2" 1/8" thick aluminum angle was bolted to the front....the next time it hit it still got bent
Keep in mind that that is 1/8" aluminum angle bolted over 1/8" aluminum sheet metal.

In 2007, Team 190 had to straighten their frame out with a car jack at BAE. Note that there are gussets (the little red things in the corners) but they weren't big enough. The entire frame is made of 1/8" thick 2x1 Aluminum C channel. We put better support in there and never had any more problems.

Bumpers will help, yes, but you want gussets too.

Re: pic: 6wd chassis - need feedback
 

A bedpan will strongly reinforce the frame in terms of hits on the corners. It can be very light, such as a 1/16th sheet of Aluminum, or even 3/8" plywood. We use 3/16" lexan or delrin, for the stiffness and ease of component mounting. It gives you a good surface for your electronics, and will also elimenate the need for gussets.

Re: pic: 6wd chassis - need feedback
 

Another method you could consider for axles is to use a 3/8" bolt that goes through a 1/2" OD tube. This lets you put the wheel in with its spacers already in place and also doubles as another standoff. We used this method after all of the trouble we went through adding spacers when changing a wheel on our rookie robot (using 1/2" bolts as axles).

I would also echo what Nuttyman said... the lack of diagonal bracing is a little worrysome. You could use corner gussets or perhaps a bellypan to reinforce the frame. I prefer bellypans since they provide a ton of diagonal bracing and also serve as an electronics mounting surface. In 2007, we had a 1/16" aluminum bellypan (weighed about 5 lbs) and it worked great.

The frame will probably be fine with those lightening holes if you're using bumpers. If you're using inventor, you can set the material and measure the weight of a piece by right-clicking on its title in parts tree and selecting "iproperties". Under the "physical" tab you can select a material and it will tell you the part's weight (or you can manually set a weight if you choose). In an assembly, you can click the update button on the "physical" tab and it will give you an estimate for the entire assembly.

Re: pic: 6wd chassis - need feedback
 

It looks like there are only a couple different sizes of lightening holes. And they're mostly right triangles. It's pretty easy to estimate the weight savings of all the right triangles...length by width times thickness times number of triangles divided by two, times 0.10 lbs/inch cubed (density of aluminum).

Re: pic: 6wd chassis - need feedback
 

An even faster way would be to simply look in inventor.

For a single part, set the material to aluminum under iproperties and it will give you a mass. Check the mass as is, then supress the lightening features and check again.

For an assembly, if all parts have an accurate material set, iproperties will give you a total mass. For parts such as motors, you simply override the mass value by typing your own.

I suggest you make this standard practice, good weight estimations are useful. Unless you're one of the great minds (which I'm far from) here, you can't really just guess the weight of a drivetrain (and I'm not even sure they would claim to be able to).

Re: pic: 6wd chassis - need feedback
 

Ok, got this. We have used cotter pins for the past 3 years and they work beautifully. Use 1/8 cotter pins and drill 1/8 inch holes on the mill toward the end of the shaft, both sides.

Re: pic: 6wd chassis - need feedback
 

(28 / 2) x 6 / 80 = a bit over a pound per half of the robot, for a little over two pounds overall. But you better double check!

Re: pic: 6wd chassis - need feedback
 

Well - it sounds like gussets or a bedpan are the way to go. I'll look at various options for material and weight considerations. if we are going to be adding something for the electronics and everything else anyway, then maybe the bedpan would be better - but it also can limit you to where you can mount everything. Plus, if we use Lexan or wood then it would be easy to cheeze hole it to cut some weight as well if needed.


I'll fool around with Inventor to get the weights - most of the pieces are duplicates of each other anyway so it could be fairly easy I'm thinking.

thx again.

Re: pic: 6wd chassis - need feedback
 

If you possess a means (such as a sheet metal bend/brake) to bend the edges of all of the 1/8" aluminum components into flanges, you'll drastically increase their strength.

Now if you do decide to go this route, you can't really use 6061, as that doesn't like being bent (you'll get stress fractures everywhere and the joint will be as good as useless).

A more preferable grade would be 5052, as you can usually bend this with a bend radius equal to the material thickness without any problems. However 5052 isn't real fun to machine, so everything would have to be punched or cut out with a laser/waterjet.

It's all about the design trade-offs.

Re: pic: 6wd chassis - need feedback
 

Keep in mind anywhere you weld the aluminum, it will become significantly weaker.

Re: pic: 6wd chassis - need feedback
 

What size wheels do you plan to use? At first look it seems like your wheels may hit your outer rails. If so, you will need to slide the dead axle mounts toward the center a bit. You can get an inventor wheel assy and add it to see how it fits. Also, I recommend dropping the center wheel 1/8-1/16. You can use chain tensioners like those on AndyMark's site. If you add sprockets to your wheels then you can also check your chain clearances. Make sure your sprocket size is such that you do not hit cross members. You should be able to cantilever your center wheel off of your direct drive axle, so no bearing in outer plate.

Re: pic: 6wd chassis - need feedback
 

The design is a refined copy of last years robot where we used Omni's on the 4 corners and AndyMarks for the center drive wheels (all 6" obviously). Really worked well for us with 0 drop on the center wheel. I have this particular design completed with shifters and wheels so there appears to be plenty of clearance for everything (although I need to double check the sprockets for interference with standoffs in the wheel modules. We will consider different design options depending on the game.

we don't have the option of bending the aluminum - at least not that I know of. The shop we were going to use for the welding may have something though.


thx again for the suggestions.

Re: pic: 6wd chassis - need feedback
 

Quick Question, are you planning to bias your center wheel? My team is notorious for our crazy tough 6 wheel drive chassis (sorry i dont know a huge part on the structure of it) and we always bias our center two wheels down just a little. this allows you to be able to turn much better than with all 6 wheels planted on the ground. another thing we prototyped in '07 was a 6wd, center biased. we cut part of the front section, re welded part of it in while installing some hinges, and made it so the front third of the robot, just the part holding the wheels, would articulate downward using some pneumatics. this way, with the push of a button, we could switch from having only 4 wheels on the ground for maneuverability to having 6 down for brute pushing power.

just an idea but you can look into it.

message me if you have any questions, i might be able to get you some schematics.:D