Octagonal WCD

[Jared]

Originally Posted by Ty Tremblay

It was quick and dirty, but here's what I did:

A worst case scenario of running into a robot at full speed with a high COG could lift the back 4 wheels off the ground. I chose this as starting point. Assuming a full weight robot with bumpers and a battery (~150lbs), that means we've got about 75lbs on each of the two wheels still contacting the ground.
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75 lbs is a very conservative worst case. If I were doing a quick simulation, I would put something like 300 pounds and aim for a factor of safety of around 2. Driving over a bump like 2012/2010 may see the entire weight of the robot drop on one wheel. You may see more than 150 lbs of load if your robot falls or roughly tips down onto that wheel. In 2013, we bent an axle when we smashed a wheel into the pyramid corner. I'd also be worried about the hex bearing's inner race in this situation.

SW simulation express isn't the right tool for this. IIRC, it doesn't support bearing fixtures, so you can't simulate torque on the shaft and the load of the robot. Also, it doesn't take into account the effect of strain rate (how quickly the part is being loaded), which will reduce the tensile strength of the material significantly.

If you're concerned that your cantilevered wheels are too weak, you can just add an additional plate on the inside with another bearing. It's likely worth the extra .5 pounds, rather than having the panic of replacing a wheel/bearing between elimination rounds.

[Ty Tremblay]

Originally Posted by Jared

75 lbs is a very conservative worst case.

SW simulation express isn't the right tool for this.

If you're concerned that your cantilevered wheels are too weak, you can just add an additional plate on the inside with another bearing. It's likely worth the extra .5 pounds, rather than having the panic of replacing a wheel/bearing between elimination rounds.

Thanks Jared. I agree with your points 100%. A second plate wouldn't certainly help.

[Jared]

Originally Posted by Ty Tremblay

Thanks Jared. I agree with your points 100%. A second plate wouldn't certainly help.

Sorry- I totally misread/spaced out when reading your last post-what you said earlier was fine. For some reason, I thought you said it had a FoS of 1 instead of 3, which is why I made the suggestions.

[asid61]

That's a neat way to incorporate the hexagonal design into a traditional WCD! All it would take is a plate and a piece of 2x1 for each corner.

[craigboez]

While not complete, I'm posting the CAD of this in case anybody would like to take a look. We'll likely develop this in to our 2015 drivetrain, but for now it remains a partially developed concept.

We have built 3 robots using the Vex 2-CIM ball shifters and overall we've been happy with them. We have used a method of attaching them to the rails very similar to what is in this CAD model and it has been rock solid. It requires some modifications to the way the gearboxes are assembled, in particular replacing a few of the hex standoffs and bolts with longer versions and adding a few nuts. This season we may migrate to a 3-CIM setup if we think it is worthwhile.

Caveats: the shafts are not included, the bumper attachment is still a work in progress, lots of fasteners are missing, and in general it lacks finish work, but I think it is a pretty decent start.



CAD here: 2811 Octagonal WCD.zip

[Joey Milia]

I would take a look at the 148 write up for their X009 base. Personally for a WCD hexagon or octagon base I like the wheels all on the outside. You wouldn't have to change as much and worry about the wheels on the inside.