Having done two rather poor driving & underperforming non-tank schemes in the last two FTC seasons - one a 4-omniwheel drive with dual-wheel axles and one a Kiwi 3-wheel drive using 3-omniwheels per axle scheme (2nd bot was heavy), as head mentor, I was a little skeptical of doing anything but tank this season.
However, with the relaxed rules now allowing any 4" & under wheels to be used, combined with the low ~$50 cost of a (4-wheel) set of the VEX plastic hub mechanum wheels, this led our team captain to want to try one more time. I was still dubious, but allowed him to launch the experiment, as long as we went forward in parallel with a tank drive build too.
Well we just completed the DEC 7th Illinois regional in Chicago, and our team was the highest seed after qualifying rounds and captains of the winning alliance.
Though we had issues with our Block Party autonomous that kept having our bot become misdirected and hitting the wall, then driving under the balance beam and lifting/shifting the bridge, under threat of disqualification we reworked autonomous, abandoning our block dump route in favor of just a straight drive onto the bridge.
We were the best flag winders in the competition and by partnering up with one consistent autonomous I/R block scorer and two good end game hangers, our alliance went on to victory. The Mechanum drive was essential for the precise alignment of our flag winder shaft with the flag handle shaft, as many teams had adequate winders that only worked when they could properly position their tank drive bots, which most of the time they did not accomplish. Note how our flag winder scheme uses T-slotted aluminum framing to allow for quick and precise height adjustment of the winder shaft, as well as easy chain tensioning.
We used a non driven wheel axle made from a long 10-32 high strength bolt, that passes through a double brass sleeve bearing Tetrix channel overlap cube. This cube then mounts below our 20mm Bosch-Rexroth sloted aluminum frame base with a pair of 10-32 hardened screws and 10-32 T-nuts.
The tricky part was getting a 4-hole Tetrix hubs to integrate with the 7-spoke VEX plastic wheel hubs. Some strategically applied Shoe Goo between the two carries the main amount of the sprocket's torque over to the wheel.
Above the frame sits the motor with matching size sprocket sits located at a ~30º inboard offset from the wheel axle. The motors have adapter plates between them and the frame to allow for the needed lateral sprocket alignment offset. Motor adapter plates also mount to frame with 10-32 hardened steel bolts. This drive train is extremely durable and fairly precise (except when driving near 45º diagonally). We had good maneuverability on the bridge too, and stayed where we stopped. The power of the new Tetrix motors was very good at the 1:1 level.
This design allows for easy changes to wheelbase spread, as well as quick and precise chain tensioning. After competition, all four motor/wheel were still good enough to handle another day of competition with no tune up needed.
If you are tired of running with bent-axle tank drive schemes, I heartily recommend giving this directly driven mechanum wheels scheme a try.
NOTE: we did have to remove the illegal servo mount frames of the 1/4-scale servos on the morning of the competition, before inspection.
A few PICs of our drive train are below:
