This is a drivetrain we are currently building. It has a custom 3 CIM 2 speed gearbox designed for lots of acceleration with a reasonable top speed. It uses a pneumatic cylinder to shift one of the outside wheels, lifting the robot enough to only be on the shifted wheel and the other outside wheel, any of the wheels can be either traction wheels or omni wheels. We plan on first testing it with omni wheels on the outside and traction on the inside, however we have been warned that we may not be able to push anything in this configuration and have left omnis on the inside and tractions on the outside as an option. Overall, the entire thing only weighs 32 pounds, only slightly more than our 2014 robot’s 4 CIM standard tank drive. The cad is available at http://frcdesigns.com/designs-drive/
This has been strongly influenced by 624’s grasshopper drivetrain as well as other sheet metal robots like 67, 148 and 33, I’d like to thank those teams (and others) for making their CAD and/or help public. I don’t think this could exist without this.
Future improvements and tests: (I already have a list and it isn’t even built…)
I’d like to make the shifting modules out of bent, thin metal like 1114’s omni wheel modules.
Single speed gearbox, I’d like to try to get the shifting gear change by giving the different wheel’s different sprockets, however this may be an issue as the inside and outside wheels may touch the ground at the same time. We’ll try it out on this and possibly include it in a future iteration
Lighter gearbox, I’d like to try using thinner gearbox plates and using flanges for a better strength/weight ratio.
Upside down electrical board, while not in this CAD, the electrical board will be upside down as an experiment.
[*]3d printed encoder mount and spacers
Assuming that the chain shares the same axle that the module pivots on, In theory it should have the same tension in both positions. This is because it’s moving on an arc with a constant radius so the distance between the wheel and the pivot point never change.
Thats my understanding of it anyways. Please do correct me if I’m wrong.
Seems unlikely, since the chain runs between two wheels, neither of which is on the module pivot. I’m guessing the arc that the omni follows closely approximates a circular arc that maintains chain length. Since the omni doesn’t have to move very far, I’m thinking this can be close enough (though I haven’t run any numbers).
The chain run will lose .07 inches of length when in the up position, if this becomes an issue, we will add a spring tensioner to it. The gearboxes are a slightly modified version of the earlier posted one, the final version is at http://frcdesigns.com/designs-gearbox/
Way back when we ran an articulating drivetrain in 2011, we designed it so that the chain length was identical in the up and down position but it got a bit of slack while moving. We deemed this to be acceptable and never had any issues.
This is the same way my teams octocanum works. It rotates in the opposite direction as the way you have your module and only untensions for a brief amount of time. As long as the sprockets are aligned it won’t fail (unless you use it for 10 demos without maintenance over the summer and break the chain itself).