pic: Crab Module

[Akash Rastogi]

The module looks pretty weak at the bottom. Be careful how much you're pocketing that.

.

[Tom Line]

Agreed - that module is going to fail when it gets pushed sideways on carpet.

[Foster]

We built a 4 wheel pivot for last year. You can get to all of our drawings and calculations here DEWBOT VI Drive Train. One of our mentors (GDeaver) figured out a easy way to mount the tread on the wheel, you might want to look at that. (His posts are linked in the main article)

Our pivot has more material than yours for support. It's been through 2 regionals and 4 off season and is still in good shape. You might want to add some more support.

Good luck!

[Chris is me]

The module looks nice. You're probably only saving like a half pound with the side plate lightening, so I would just get rid of the lightening below the bearing.

The only other critique I have is that this looks exactly like the 221 module. While I don't suggest changing the design for the sake of being different, why not just buy modules from them?

[Clem1640]

I've just posted as a picture the basic drawings of 1640's Pivot. As Foster points out above, these pivots have gone through 6 competitions, 3 extended demos and a great deal of practice driving. They are sturdy and have held up to abuse extremely well.

2010 was our first year of using Pivot drive, but we started down this path last summer with prototyping, mathematical analysis, software development and (CAD) designing. By the time we actually built the working prototype on robot-scale, we had already gotten to version Mk 6c! That said, if you are looking to use pivot next year, you're starting your design process at the right time.

We used roller bearings for the co-axial drive shaft in lieu of ball bearing races. The smaller profile allowed the use of a 1" OD pivot tube.

For more info, see: http://wiki.team1640.com/index.php?t...VI_Drive_Train

[JesseK]

First of all, Welcome to ChiefDelphi!

I have a sample side plate from 118's crab module talk in 2009. The teardrop design is the strength of the side plate; to be honest most of the material that is not on the edge of the teardrop is excessive weight. The hole at the bottom of 118's plate is ~0.2", and all pocketing on the teardrop leaves a width of only 3/16" wide behind. The plate is 1/4" thick. I estimate the fillets at either a 1/8" or 3/16" radius as well.

The poster's particular teardrop design looks exactly like 118's except it looks like there's 1/4" worth of material left after pocketing instead of 3/16" (i.e. stronger if anything). The fillets on this design look somewhat small, which is the only thing I would suggest changing for a prototype.

I doubt the module will fail due to racking for the simple fact that the bottom axle is a dead axle. The axle also serves as a strut to complete the "box" support that keeps the module from racking (laterally failing, leaning, flexing, however you want to put it). The talk linked above gives more details.

Originally Posted by Chris is me

... While I don't suggest changing the design for the sake of being different, why not just buy modules from them?

Perhaps they want the experience in getting feedback on a proven design. Or, better yet, perhaps they'd like to try their hand at innovating the design? It's far from perfect since it's still quite expensive to produce. Perhaps I'll post an idea I've had for a few months on how to further progress the coaxial crab module design.