Originally Posted by Joe G. (Post 1392334)

Do you have plans to make use of the rather large dead space within the space created by the beveled sides? If this area was a little more open, it would seem to be an ideal location to tuck some electrical and pneumatic components.

I really like the one piece sheet metal battery mount design.

Originally Posted by AllenGregoryIV (Post 1392335)

Cooper this is looking really good, do you have pictures of the underside? Looks like the motors aren't directly driving the omni wheels shaft anymore.

One thing we did this year was make it so the front and back rails of the chassis were easily replaced. It worked out very well for maintenance.

Originally Posted by Cash4587 (Post 1392345)

Idk if I am doing this right but, It now looks like a block of Swiss cheese..

http://prntscr.com/406s5y

Originally Posted by Cash4587 (Post 1392345)

Idk if I am doing this right but, It now looks like a block of Swiss cheese..

http://prntscr.com/406s5y

Originally Posted by AllenGregoryIV (Post 1392350)

Just a thought, can you make the wheel wells thinner by moving the pulleys that connect the two modules to the same shaft as the new gear for the MiniCIMs?

Also make sure to look at access holes for the motor mounting screws and which tool you would use to replace them. Same with shaft collars, and retaining rings.

Overall its looking really nice, I'll probably be borrowing some elements of this if we are given another flatfish field.

Originally Posted by Cash4587 (Post 1392385)

I like that idea, however looking at it, it would make the module only about 1/4" smaller due to the fact that the versa wheel is so wide becasue of the versa keys. If I wanted to make the module smaller than the versa key on one side of the wheel would need to be milled off. Even though that is not hard to do, I would rather not make the versa wheel a "custom part" in order for it to fit in the butterfly module.

As for access holes, we will be using retaining rings and right-angle allen keys down there to be able to get in there. Not too big of a deal considering that I don't expect to ever need to do maintenance to a module while still on the robot. The whole module will come out if you remove 2 retaining rings. The only "hard to reach" item is the motors which really isn't too bad

Originally Posted by AllenGregoryIV (Post 1392402)

Also have you thought about a way to retain the bearing? Our press fits (which I'll admit weren't good to start with) came loose during the season and made maintenance even harder.

Originally Posted by AllenGregoryIV (Post 1392402)

Removing the module requires removing the hex shafts and from dealing with it this year I would try to avoid it at all costs. You don't have time during a competition to do that. During events we had motors come a little loose even with lock tight on the bolts. Someone mentioned thread locking bolts in another thread that I'm probably going to look at for mounting CIMs next year. The other idea that I might look into is doing something like 148 did with Raptor and other robots where they have plates that mount to the CIMs that have easy access fasteners that make swapping motors easy.

Also have you thought about a way to retain the bearing? Our press fits (which I'll admit weren't good to start with) came loose during the season and made maintenance even harder.

Originally Posted by AustinSchuh (Post 1392368)

Big squares save even more weight for less machining cost.

Originally Posted by AllenGregoryIV (Post 1392410)

I'm trying to think of other little annoyances from our drive train this year.

Are you going to spring up the modules or rely on the pneumatics to bring them up too? We really liked the springs because if we lost air or wanted to run our practice robot with out pneumatics it didn't really affect anything. If you go with springs make sure to fully CAD them we didn't and luckily had enough room to the side of the omni wheels to slide them over because in the intended configuration when the traction wheel was down the springs hit the omnis.

Think about the interface between the modules and the cylinders. We made small aluminum discs that just threaded on to the end of the piston. These pushed down on to VEXpro tube shaft. Over time we were able to dig pretty big groves into the tube shaft and would have to rotate them to get our ride height correct.

Also bumper mounts as always.

Originally Posted by Greg Woelki (Post 1392413)

This frame is quite beefy but keep in mind that the sharper the corners the higher the stress concentrations.

Originally Posted by Cash4587 (Post 1392415)

I've actually changed majority of the lightening holes to parallelograms. They look nicer to me, and I have used a pretty big fillet on all the corners (3/16 radius)I have not however put any Swiss cheese effect on the drive rails yet. They are not finalized, neither is the belly pan.
Seen here:
http://prntscr.com/40gv2q

Originally Posted by Greg Woelki (Post 1392416)

It is probably still strong enough, but taking out the same amount of material that way instead of with circles makes it significantly weaker.

Originally Posted by Greg Woelki (Post 1392416)

It is probably still strong enough, but taking out the same amount of material that way instead of with circles makes it significantly weaker.

Originally Posted by AdamHeard (Post 1392421)

Eh? I don't agree with this statement.

Triangular pocketing is far more optimal (unless too much is taken out) than circular. If an equal amount of weight is removed the triangular pattern is stronger unless it's laid out awful.

Originally Posted by Greg Woelki (Post 1392423)

I was under the impression that the smaller the radius, the higher the stress concentrations, like so: https://drive.google.com/file/d/0B4u...it?usp=sharing

I'll try doing the same thing with triangular cut outs though. Perhaps it also depends on how it is loaded.

Originally Posted by AdamHeard (Post 1392424)

Ah, yes. You make a good point but that isn't an argument against triangular pocketing, but rather an argument against too small of a radius.

Ideally triangular pocketing has webs sticking out in most directions of loading, so stress flows nicely and never has to take corners anyway. Isogrid is a really cool optimized pattern everyone should look up.

Originally Posted by AdamHeard (Post 1392424)

Ah, yes. You make a good point but that isn't an argument against triangular pocketing, but rather an argument against too small of a radius.

Ideally triangular pocketing has webs sticking out in most directions of loading, so stress flows nicely and never has to take corners anyway. Isogrid is a really cool optimized pattern everyone should look up.

Originally Posted by Greg Woelki (Post 1392429)

Am I approaching this incorrectly? Each side is the same length, is under the same load, and is the same weight. I started the triangles with 0.1" round radius and I worked up to 0.25" (in picture), adjusting their dimensions to keep the volumes the same as I went. There is still higher stress in the piece lightened with triangles and if I increase the radius of the round much more then they might as well be circles.

https://drive.google.com/file/d/0B4u...it?usp=sharing

Originally Posted by Adrian Clark (Post 1392430)

I really like this drivetrain, like a lot. Great job! It's been a lot of fun watching it get better and better. That being said, I think there's a few things left that could be optimized.

The first thing is space. I think electronic placement space is a very important aspect of a drivetrain. Without enough space you have to spend time coming up with ways to get everything to fit, and some of the solutions can make maintenance difficult. YMMV and I know a lot of teams are willing to make space sacrifices that make electronics placement difficult, and sometimes that trade off pays off. There's nothing wrong with finding sneaky ways to fit all your electronics on the bot, but I consider an ideal drivetrain one that keeps electronics placement simple. For that reason I suggest space as something to try to optimize.

The reason I think this drivetrains electronics space could be optimized is because of the large voids in between the modules and the voids created by the hexagonal frame perimeter. I think hex and octo frames are great, but I also think there's trade offs that should be considered. The first is space, is having such a wide hexagonal robot worth it if it limits space? And how will the frame shape effect superstructure and manipulation design? You may have accepted these trade offs, but what is brought into question is how hexagonal should you make your robot given those trade offs? Looking at your frame I notice that your sides are steeper than most octo and hex framed bots I've seen. Given that there's trade offs to having a non-rectangular frame, the key in designing this type of drivetrain is to balance those trade offs with the benefits of an octo or hex to get a shape that is effective in terms of space and interactions with other robots. Hex and octo bots are pretty new in frc, so not much is know as to how design one with the right shape. To determine how steep to make your corners I suggest building bumpers of different angles and testing it's effects on robot interaction to determine the best shape. Once you've done that you'll know how important frame shape is and then you can determine if having such a wide hexagon is worth the sacrifices when compared to a slightly smaller hexagon. There's teams that have done testing on this very subject, i'm hoping someone chimes in.

The first thing I thought of when I saw this design was that you could save quite a bit of space by putting the motors in the void between the modules. I read the previous thread and I noticed you had the same idea but didn't pursue it because it would be too complex. I don't know where you or your team draws the line for complexity, but I encourage you to not give up on that idea just yet. I think there's ways of getting a gearbox and motors in that space that are a little more complex than your current design but could be a great improvement and really take it to the next level. The most obvious reason to move the motors is so they don't take up valuable electronic space, but I think by moving the motors you can actually reduce the width of each side of your drivetrain pretty significantly. When I look at your power train I see two things that make each side wider than it needs to be: the gears for driving the wheels and the pulleys that connect the omni wheels together. My suggestion for narrowing your sides is to take the whole power train and put it in the gap between the outside of the hex frame and the inner yellow frame rail. In order to pull this off you would need to come up with a clever way to either replace each motor or remove the entire gearbox, which I think can be done. The advantage of this is that by putting a gearbox in between each module you eliminate the need for a belt to connect them, making the modules narrower. The main idea behind this is to take everything that's making the sides thick and put it in the void where there is space. There's a lot of ways you could do this without making it too complex, I've got some ideas but I'd like to see what you come up with :)

There's some other things, I ran out of time to post everything, I might post more later.

Oh, and just fyi: as for using loctite for holding cim screws in using the strongest loctite you have is not the way to go. Look up what strength thread locker you need for the fastener you're using and use that. If you put the wrong strength loctite on you might end up with an irremovable screw.




If you're seeing unusual stress risers in your results the first thing you should do is check your mesh. (assuming that's applicable to whatever program you're using). Also, for accurate results make sure you have at least three layers of mesh in the cross section.

-Adrian

Originally Posted by Cash4587 (Post 1392443)

How about this?
This isn't too bad for complication and it saves so much room. Although it does cost quite a bit for the bearing bore gears and for the bearings, It will leave plenty of room for electronics, at least I would say so.

http://prntscr.com/40ilna
http://prntscr.com/40ile3

Originally Posted by Adrian Clark (Post 1392446)

I like it, I think you've got the right idea. You could totally get away with this but here's some thoughts.

What made you choose a dead axle over live axle? And in this setup do you plan to cantilever the gears? I think you could save weight if you ran live axle gears as you could fit a much smaller gear in between the cims.

I think you could simplify it a bit by removing one of the gears between the gear on the module shaft and the closest cim.

I could be totally wrong here, but in those renders is the robot not as long?

-Adrian

Originally Posted by Cash4587 (Post 1392482)

I had planned to run these as dead axle cantilevered gears on shoulder bolts I chose dead so it could be ran on the shoulder bolts. If I did run it live axle, wouldn't it be more weight or close to the same, as I would have to run the axle all the way through and use more bearings?

I don't quite follow you on removing one of the gears. I put that many on there so I could remove the belt that attaches the two butterfly modules together.

In this configuration the robot would be the same length and width, but it could be easily modified and changed.

Sorry for any mistakes I typed this on mobile.

Originally Posted by tim-tim (Post 1392407)

Three thou is way too much interference for a press fit.
Around half a thou (.0005") is pretty standard in FRC. I suggest getting a 1.1245 (or 1.124) and .8745 (or .874) reamers to do the final finish.

Here is an old thread to reference: http://www.chiefdelphi.com/forums/ar...p/t-98825.html

Originally Posted by Adrian Clark (Post 1392486)

If I do run a simulation i'll also check the loading on the pinion gears. I'm not sure they'll like having power from four motors running through them. The pinion load is definitely something to worry about with a setup like this, and it could make things a lot more complicated if they can't handle the load.

Originally Posted by ekapalka (Post 1392491)

I don't mean to change the subject, but this information has highlighted a problem for me. I've been designing a gearbox for quite some time, and I've clearly been making a huge mistake with the hole sizes for the bearings I'm using. If an AndyMark flange bearing has an outer diameter is 1 inch, in accordance with this, the hole diameter for the bearing should be 1.0005 in? That seems really close. Could you just verify before I go back and change everything? Thanks

Originally Posted by Adrian Clark (Post 1392500)

However, there is no universal rule for bearing fits. What size you make the hole relative to the bearing OD depends on how tightly do want to retain the bearing if you want to retain it all, the size of the bearing, and what method you're using to make the bearing bore. There's a lot of info on this subject here on CD, the thread tim-tim linked to is a good place to start.

-Adrian

Originally Posted by Adrian Clark (Post 1392500)

Most teams use interference fits for their bearings. Doing so is an easy way to retain the bearing and accurately locate it. For a good fit, as tim-tim suggested, you want about .5 thou interference. This means you should make your bearing bore .5 thou smaller than the diameter of the bearing.

Originally Posted by AdamHeard (Post 1392492)

I agree. The pinions on the end could potentially see 8x stall torque under direction change, which is huge. I don't think they will hold up.

Originally Posted by AllenGregoryIV (Post 1392526)

Cooper, what if you took the pinions out of the gear train and just had the gears interact with each other. The pinions could mesh under each of the gears. Also you might be able to mount the gears with tube axle and a plate that is supported off the outer frame member (hex side) that would also act as your stand off for that frame member.

That setup would provide support to the outer rail, allow you to remove motors to save weight (if needed) but maintain the power from the remaining motors going to all wheels on the side.

Something like this
https://docs.google.com/file/d/0Bwvy...VCLVdvdlU/edit

That was extremely quick CAD just to show the idea. The MiniCims are 12:54 and the CIMs are 12:48 that should have the 48 and 54 tooth gears all spinning at around 1300rpm unless I did my math wrong, which is very possible.

Originally Posted by Cash4587 (Post 1392533)

I did some more playing around and came up with this. It could be a whole lot simpler to do and if I get the geometry right, It could be run in either this configuration OR in the 4 cim 4 mini cim configuration. It would only be different in where the motors are mounted. It is just a bit more tricky to figure out how to still have easy access to mounting holes with the six cims in this location.

http://prntscr.com/40u5lx

Originally Posted by AllenGregoryIV (Post 1392534)

How's power transferred to the wheels in that one?

Another version of the wheel well idea using belts in the power transmission. 2 90mm belts to the wheels and a 100mm belt connecting the two 60t gears.

https://docs.google.com/file/d/0Bwvy...RBZzYxZFE/edit

Originally Posted by AustinSchuh (Post 1392336)

Nice! Looks like a solid design, and like you got all the tricky bits right.

I'd put some material back in the front and back of the frame. Those parts take a beating... You can drop some of the material on the top of the frame, and on the rails holding the wheels. The frame itself will be strong enough, and the holes make it easy to inspect your robot for wear. Point impact loads from other robots and field elements should be your biggest loads. You can take the inner frame rails down to 060 as well. Reinforce the bearing holes when you do that.

Consider lightening the belly pan, and adding PEM nuts in for your electronics. Those small touches take a design to the next level.

Originally Posted by Cash4587 (Post 1392615)

I'm interested in using PEM nuts now for a specific feature on the chassis. Could you provide more information or an old thread that gives more info on these?