we are thinking of doing a Mechanum drive train this year. Any recommendations for gearboxes, gear ratios?
If you’re just looking at simplifying the approach to goals, also consider mounting your manipulator(s) on a lateral slide - much less complicated.
That out of the way, gear ratios for Mecanum are calculated the same as for a skid steer, just recalling that you have four gearboxes rather than two. On wheel size, I wouldn’t want to go with smaller than 6" for any types of wheels this year because of the ramp up to the HAB. If using Mecanum on the KoP chassis, I’d make it 8" because the drive axles of the TB-micros are about an inch and a half above the base of the chassis.
Edit: see this topic for more info
We used the cheap clamping gear boxes from vex last year on our robot and it was pretty fast. I believe we used the 14:70 gears.
The 14:70 ratio included 4" wheels correct? We are looking at using the single stage clamping gearbox, but adding another stage of reduction through a bearing block for our 6" wheels.
We initially used 4" but switched to 6" because we wanted more clearance for getting up the ramp by the scale last year. Our robot only weighed like 80 pounds.
Even with the 6" wheels it was still very fast. It would spin the wheels on the carpet
Six inch wheels are faster than 4 inch (All else being equal). It looks like you would have a free speed of 27.91 ft/s, which is way to fast. Vex does not recommend using 6 inch wheels without another stage of reduction.
Yes it was fast.
We may have switched to 11 tooth pinions but I don’t remember the specifics.
I doubt it ever got to top speed on the field
We direct drove the wheels as in this picture
Did you manage to get encoders on those wheels? We are looking into that now.
No we just used a time based auto that worked decently enough, all our auton failures where we missed putting the cube in the switch we’re caused by mis alignment or our catapult flipper device not getting a good enough grip on the cubes, if I remember it correctly we scored the cubes with just our time based auto about 7 our of ten times or so, maybe 4 our of 7. If you check our blue alliance page you can see some pretty good examples
If you want encoders I think you should go with a drag wheel. I think 148 in 2911 shows theirs in their reveal video
Keep in mind that sometimes (often) slower is faster. If you’re not planning to play defense this year (and you’re thinking mecanum, so that’s probably a safe guess), your straight-line runs are going to be 25’ or less - often much less. I don’t know what specific weight you’re planning, but the past few times I’ve optimized sprints of a half court or less, the optimal free speed was rather lower than I was expecting, because acceleration at low speeds dominated the problem. Do the math.
In 2015 we (3974) used a drag wheel with an encoder. It was very effective going over the platform. We hit our Target distance every time.
And this is one of the major reasons why CD generally recommends against mecanums. Their static CoF is generally about 70% of an equivalent traction wheel, meaning they hit max acceleration (i.e. braking traction) sooner. This limits your robot’s acceleration, which can severely slow you down in games with short sprints (like this one).
Our Power Up machine used 6-inch mecanums with a 8.45:1 gear ratio. Same with the year before. Very possibly this year, too.