3771 Chassis

[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]

Originally Posted by Lil' Lavery

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
Search for "Thrust", and find some that fit your shafts. (They ship same day, too... )

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.

[Chris is me]

Originally Posted by kmcclary

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 ). 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]

Originally Posted by 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.

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.