Custom Frame

[Andrew Schreiber]

Originally Posted by squirrel

Mecanum wheels don't like a stiff frame, flexibility is good. They need all the wheels on the floor all the time, or they go off in strange directions.

Quoted for great truthiness.

Mecanum wheels rely on the fact the wheels produce forces in differing directions. These add up to get the desired motion. If you have different weight on each wheel (as you would with a stiff frame on a non-flat surface) you will get drastically changing behavior. It is the same reason you want to ensure that your CG is as balanced as possible as well.

There should be ways of compensating for different loading on your wheels, probably even doing so dynamically by placing load cells above wheel pods but it is hard math for me*. I find it simpler to just let it be sloppy and loose and be careful with my CG.

*I'm sure Ether will come in here and show me an elegant way of doing this but to me it is hard math.

[bardd]

I would use c-channels for the frame, but still a great design! Simple and works.

[baronep]

@ Bardd: The reason we aren't doing c-Chanel is simply because we have a sheetmetal sponsor lined up

@ MattC9: These are the mechanums :http://www.andymark.com/product-p/am-0083.htm They are 8"

@davepowers: The just the frame weighed 10 to 15 pounds w/o motors and gearboxes. The gear ratio is 12.75 to 1 (the same as the toughbhox)

[topgun]

We did something very similar last year. I would suggest doing .090 5052 AL. And orient your motors towards the front/back of the robot. Also, make sure you have the wheels almost as far forward/backward as you can go.

Do you really save much weight/cost by doing your own custom gearbox plates versus just using a Toughbox Nano?

[Garret]

When we did mecanums this year, we mounted the wheels using pillow blocks. When mounting the pillow blocks we used springs between the blocks and the frame to create a form of suspension.
http://www.chiefdelphi.com/media/photos/36399

[baronep]

Our sheetmetal guys say that they can produce .125, .1 and .080 thicknesses of 5052 alloy. Strength-wise, which thickness would be optimal? How would creating cutouts affect the weight/strength ratio?

[JohnSchneider]

.1 and .080....just slightly outside of that golden .090 ..... Id say probably .1

.125 is a bit overkill, but if you use a lot of lightening patters it works very well

Triangles, and fillet your edges...strength made easy!

[tim-tim]

Originally Posted by baronep

Our sheetmetal guys say that they can produce .125, .1 and .080 thicknesses of 5052 alloy. Strength-wise, which thickness would be optimal? How would creating cutouts affect the weight/strength ratio?

To answer your question, there is no 'correct' answer. The material should be selected based on application of the component being designed and what you (the designer) are asking the component to do.

Yes, typically teams do use .090 thk material for chassis construction. Is there a reason? I suppose that few (if any) have completed a structural analysis on the frame and all of the interaction associated. There are many types of 'quick calculations' that can be made, but these really won't be quick and will actually be a simplified representation of the actual frame.

I know I have not offered a lot of direct insight, so let me do that now:
5052 is a softer aluminum alloy, which is great for bending components because a tight radius can be held (making it almost a point). The con, it bends easy and does not have the strength of other alloys. In order from 'weaker' to 'stronger' alloys 5052, 6061, 2024, 7075. There are more alloys out there than this, but these are the most commonly used. The best part, they all have roughly the same density. Going with the 'stronger' materials will mean a larger bend radius is required for the part. ***IF you have mentors with an engineering background (or you want to learn yourself) you can design for thinner materials in places by switching materials.***

If you are welding it gets a bit more tricky, 5052 and 6061 are the alloys you want to stick with. Just another tidbit, be careful on the use of terms such as optimal.

Hope this helps, and let me know if you have any other questions, or need more clarification on certain items. It is early in the morning, and I apologize for the lost train of thoughts at times.