3771 Chassis

[klbarie]

Hey, so I am a mentor on team 3771. We are a rookie team this year, and would like to know what you guys think about our chassis design. It will have mecanum wheels and toughbox nanos. let us know! :D

http://i1182.photobucket.com/albums/x448/FRCTeam3771/2011RenderedFinalChassisDesign2.jpg

[tonystark_23]

Looks pretty good you guys, but add the electronic components and let us see and we will really be able to judge it! Keep up the good work!!

[klbarie]

Will do once we get an arm design settled so we know exactly where to put everything :D

[ttakashima]

Why not try and direct drive the mecanum wheels from the Toughbox-nano? It would save on weight and repair time

Otherwise it looks great. Might i ask how much that weighs approx.

[Mark Sheridan]

That looks pretty good. I assume that is 1x1" box tubing for the frame. I think you can save some time and weight by removing some of the tubing. I think the pieces that hold the wheels could be removed and you could mount directly to the frame below. Your robot won't be lower but it will save some time when building it. We use 1x1" aluminium box tubing and its pretty tough stuff.

Also, are you using the andy-mark toughbox gearboxes? You can mount two motors to one gearbox. You can shed some weight and time by reducing to two gearboxes for the four motors.

If you want to do 4 wheel drive on a frame that is longer than its width, it will be difficult to turn. The wheels have to have enough force to break its traction in order to slide. You must gear your drivetrain to have enough torque to skid your tires. Many teams use 6 wheel drive with a lowered center wheel to avoid this. You can search the forums about threads on the many benefits of going to 6 wheel drive.

[klbarie]

I have been hearing rumors that direct driving mecanums directly from nanos is bad because of the forces put in all those directions. and we are not sure how much it weighs. we are still waiting on our aluminum to come in the mail. But I will try and post here the weight of it once we have i assembled.

[ttakashima]

Also, are you using the andy-mark toughbox gearboxes? You can mount two motors to one gearbox. You can shed some weight and time by reducing to two gearboxes for the four motors.

If you want to do 4 wheel drive on a frame that is longer than its width, it will be difficult to turn. The wheels have to have enough force to break its traction in order to slide. You must gear your drivetrain to have enough torque to skid your tires. Many teams use 6 wheel drive with a lowered center wheel to avoid this. You can search the forums about threads on the many benefits of going to 6 wheel drive.

They stated that they will be using mecanum wheels, all 4 need to be driven independently to strafe.

Also the toughbox-nano only accepts 1 CIM motor, I believe those Toughboxes and wheels are only place holders.

[klbarie]

That looks pretty good. I assume that is 1x1" box tubing for the frame. I think you can save some time and weight by removing some of the tubing. I think the pieces that hold the wheels could be removed and you could mount directly to the frame below. Your robot won't be lower but it will save some time when building it. We use 1x1" aluminium box tubing and its pretty tough stuff.

Also, are you using the andy-mark toughbox gearboxes? You can mount two motors to one gearbox. You can shed some weight and time by reducing to two gearboxes for the four motors.

If you want to do 4 wheel drive on a frame that is longer than its width, it will be difficult to turn. The wheels have to have enough force to break its traction in order to slide. You must gear your drivetrain to have enough torque to skid your tires. Many teams use 6 wheel drive with a lowered center wheel to avoid this. You can search the forums about threads on the many benefits of going to 6 wheel drive.

Its going to be mecanum wheels and toughbox nanos, so we will need all the separate motors and gearboxes. and with the bumper zone being so low to the ground this year, we wanted to get the wheels as much into the frame as possible.

[klbarie]

They stated that they will be using mecanum wheels, all 4 need to be driven independently to strafe.

Also the toughbox-nano only accepts 1 CIM motor, I believe those Toughboxes and wheels are only place holders.

what he said XD

[Colin P]

I believe those Toughboxes and wheels are only place holders.

Precisely

[Mark Sheridan]

They stated that they will be using mecanum wheels, all 4 need to be driven independently to strafe.

Also the toughbox-nano only accepts 1 CIM motor, I believe those Toughboxes and wheels are only place holders.

Oh whoops, that makes a lot more sense.

[ttakashima]

Are you getting 2x1 box alum. in the mail as well? If you are you should look into using that. You could eliminate the top structure and reduce weight.

[klbarie]

Are you getting 2x1 box alum. in the mail as well? If you are you should look into using that. You could eliminate the top structure and reduce weight.

unfortunately, no. i had thought about that after we had ordered all of our stock. hope we can live with that extra pound or two, and we wont be drilling holes in it when it comes time to weigh in XD

[ttakashima]

i believe our 2009 robot had about 1000 holes in it, all of different sizes... took us 3 days to reduce our weight by 5 or 6 pounds... though we were using 80-20 material...

[klbarie]

i believe our 2009 robot had about 1000 holes in it, all of different sizes... took us 3 days to reduce our weight by 5 or 6 pounds... though we were using 80-20 material...

XD hopefully none of us have to go through that this year.

[Zachary Pilarsk]

its is a good design but you need to lose some weight and remove some parts of the frame

[Mark Sheridan]

As you build it, you can see how strong it is. The frame we have now kinda looks like your with all the top parts cut off. Its pretty stiff, i could jump in it with no worries. So we are not adding any more structure to it. When we add our arm, we will brace its structure to the frame to make the entire robot it more rigged. I think we are more concerned with the arm bending than the drivetrain.

[pfreivald]

Trim weight everywhere you can -- that looks like twice the chassis than is needed.

...and if you don't own a reliable, calibrated scale, buy one pronto. That way, as you modify and tweak from your original designs (which already have estimated weights, right?), you can re-weigh and re-estimate what final parts will be.

We've never been stuck having to drill lightening holes, but then, I'm awfully paranoid about it!

[SirTasty]

I'm not certain, but I suspect the much wider mecanum wheels might have trouble fitting in that housing. Also, your frame has a little bit too much unnecessary aluminum. It's important to be thinking about weight from kickoff day till robot inspection.

Best of luck!

[Lil' Lavery]

How are you attaching the frame elements together? Welding? Gussets?

[klbarie]

How are you attaching the frame elements together? Welding? Gussets?

our welder is going to practice on some scrap to make sure he would be comfortable doing it, but if he is not, gussets

[kmcclary]

A VERY important thing to remember about Mecanum wheels is that, UNLIKE standard wheels (or even Omniwheels!), for every forward or reverse pound of thrust you attempt to derive from a Mec Wheel, you ALSO get a pound of SIDE FORCE transmitted ALONG THE AXIS of the shaft, which MUST be transferred to the frame WITHOUT BINDING.

This is because the 45 degree rollers shafts "vector" the forces you apply equally to the side as well as forward & back..

To handle these extreme side forces, it means you need to add THRUST BEARINGS on both ends of each shaft. This insures the wheels "push" on the chassis and do not "bind" TO the chassis from friction whenever you drive them. (IOW, adding TBs GREATLY increase efficiency.)

We've had very good luck with VXB Bearings, for our Mecanum Drive thrust bearings:
... www.vxb.com (http://www.vxb.com)
Search for "Thrust", and find some that fit your shafts. (They ship same day, too... :D)

BTW... If anyone has other bearing sources they like, please also post them here! Thanks!

Thrust Bearings come as what looks like a triple pair stack... A pair of flat or grooved "washers", sandwiching a ring containing a circular set of either ball bearings or radially mounted needle bearings. Buy enough sets in your shaft size for each end of every wheel shaft (plus some spares...).

Simply add one tri-stack to each side of each wheel. Make sure if the "washers" are grooved that the grooves face the bearing ring.
(Personally I prefer stacking them immediately on either side of the wheel itself vs between the spacer and the frame, because this also eliminates all "spacer spin drag" on your shafts as well.)

...Easy Breezy...

BTW, TBs help with NON-mecanum drives too, by (a) removing your side force wheel-to-chassis friction whenever someone pushes you sideways! (At those times, every little advantage to help you escape is appreciated... ;)), and (b) to keep wheels from binding because someone went crazy in the pit tightening the shaft bolt down too much and the spacers pinched the wheel.

Note that you STILL need REGULAR bearings too, for allowing the wheel and sprocket assembly to spin on the shaft! The Thrust Bearings just "surround" your wheel-&-sprocket assemblies, to help them transmit the SIDE forces to the frame without binding the wheel.

If you forgot about them during initial assembly, no sweat. They're easy to add later in the pit when they come in simply by trimming the right amount of shaft spacer length and stacking them in around each Mecanum Wheel.


One OTHER thing... With Mecanum Drives, you also need to make sure all four wheels are on the ground at ALL times, or it gets wonky... With a flat field like this year's game (no "ramps"), "frame flex" is normally sufficient to eliminate the need for suspending a pair of wheels on a pivoting subframe,

BUT, if you make your frame TOO STIFF, you MAY encounter a touch of drive vectoring problems should one wheel lift up too much and loose traction. Watch for that.

IF it becomes a SERIOUS problem, there are methods to later add just a "wee bit" of spring action to one end's pair of wheels to cure it. The symptom to watch for will be the bot spinning or vectoring in directions you didn't intend it to go, especially with strong moves that tilt the bot slightly, removing pressure from at least one wheel.

Just let me know if this happens to you, and I can point you toward some solutions for it.

Good luck! I hope this helps...

- Keith
(Who also happens to be a List Dad for an Omni and Mecanum Drive elist...)

[David Dawson]

I guess I should post the finished product in this thread. The robot ended up with a finished weight of ~90 pounds. Overall I think we will field an excellent rookie bot this year.
https://s-hphotos-snc6.fbcdn.net/182781_10150418488685204_573265203_17454820_574358 4_n.jpg
https://s-hphotos-ash1.fbcdn.net/180106_10150425703000204_573265203_17545833_163312 _n.jpg
https://s-hphotos-ash1.fbcdn.net/180794_10150420898840204_573265203_17483102_188730 1_n.jpg

[Chris is me]

To handle these extreme side forces, it means you need to add THRUST BEARINGS on both ends of each shaft. This insures the wheels "push" on the chassis and do not "bind" TO the chassis from friction whenever you drive them. (IOW, adding TBs GREATLY increase efficiency.)

I've never heard of a team doing this... Is this true? Why can teams "get away" without thrust bearings?

[Garret]

Personally I love this type of frame (though I may be biased :rolleyes: ). It looks very similar to my team's frame.

http://www.chiefdelphi.com/media/photos/36277

I don't know if you said it already, what is the wall thickness on the tube you are using. My team always uses 1/16" wall thickness and never has any problems with strength, I will however just say that we do have a professional welder do our frame so that may help. but besides I like it, and if you are using 1/16" thickness it should be incredibly light.

[David Dawson]

I'm not certain but i believe its 1/8 inch aluminum but its still light due to welding vs bolts. this year we tried to keep it as simple as possible.

[Garret]

1/8 in will be very strong and still relatively light. We use 1/16" just because its lighter and gives us some flexibility when it comes to weight and room to "overbuild" the frame. Good luck and your robot looks really good.

[Akash Rastogi]

1/8 in will be very strong and still relatively light. We use 1/16" just because its lighter and gives us some flexibility when it comes to weight and room to "overbuild" the frame. Good luck and your robot looks really good.

Did you weld your frame with the 1/16"?

[Garret]

Yes, we have been lucky enough (despite in prior years building out of a garage) to always have a welding sponsor. We have never had problems with the 1/16" material being welded as far as I know.

[Grim Tuesday]

We use T-Slot for our lower frame, and 1/16 for the upper.

[Mark Sheridan]

That is a very nice frame. It will defiantly be durable. Much better then the frame done by my team during its rookie year.

[David Dawson]

Yea The mentors on this team know many of the *rookie mistakes* teams make and this year we did our best to avoid them.

[team 3311]

80/20 sucks for removing weight my first year we asked teams for 30 lbs of dead weight.
The next year we made a 1500+ 1/4" holes not fun