I’m looking at running 84/36/84/36/84 with 200RPM motors driving two of the 84T gears on each side (the wheels are of course mounted to the 84T gears as well).
Some benefits for us are that we can re-use all the green cartridges and 4" wheels we have on hand. It also creates a flexible width for the drivetrain as 35hole structure can be placed over the 36T gears with minimal interference from motors.
I’ve run several tests with the same motor/wheel spacing using 60/60/60/60/60, 72/48/72/48/72 and 84/36/84/36/84 gear combos and can’t find any discernable difference in performance.
My nagging concern is that going from 84/36 and then 36/84 has quite a large torque multiplier there so friction has a potential multiplier effect. So wanted to check if anyone has done this and run into issues (or not).
Assuming your drivetrain is well-built (everything is square, shafts have the proper amount of spacers, etc) you don’t need to worry about the friction from idler gears. If friction is a big concern, you can make some friction savings by using screw joints instead of shafts.
Side note: What is the application for this drivetrain? If it’s for competition use, 4" wheels at 200 RPM will be slower than many other teams.
9motorgang had a pretty good post recently discussing his recommendations for speed based on number of motors.
For 4 motors he recommends around 40-60 in/sec. (Your proposed DT is around 42) and I suspect that using his ranges assuming a decent build quality you won’t ever have any major issues with friction being an overwhelming force.
The reason that the speeds you are getting don’t match is because you didn’t build a gear ratio.
All of these ratios will have the exact same speed as each other, 43 in/sec. Think long and hard about what you are doing. You have a shaft connected to a 200 rpm motor and a wheel connected to that shaft. Why would attaching a gear to that same shaft change the speed at all?
With a spinning shaft, you have two points of contact where the shaft is running, ie the two bearings supporting the shaft. With screw joints, the screw itself is static (does not spin) and the wheel/gear spins around it. That is only one point of contact.
Additionally, screw joints usually have tighter tolerances than shafts (especially if the wheel/gear uses the brass round inserts), meaning less wiggling can occur to create extra friction.
Thanks @kevjar, we have rookie teams that we want to scale out a stable and easy to build drivetrain and manipulator. We need to build about 5 robots each year and need maximum parts reuse from year to year, so no cutting of c-channels etc. etc. Speed is not a concern.
Interesting videos on the screw joints and boxing. Thanks for sharing!
@9MotorGang all combinations I listed have the same input motor RPM (200) and same wheel output RPM (200) if the motors are driving the wheels directly. In these configurations the gear combinations will not impact the drivetrain RPM. What the gear ratios will impact is the sizing of the gears and the torque ratio from drive to idler. I already said in the original post that I had built and tested all three combinations, but value the feedback regardless.
Not being able to cut c-channels is not very good if you want a team to be competitive. It means that you are forced to build at max size, and greatly limits what you can build in the first place. I’d recommend rethinking this policy😊
@Koppel63310A, indeed, 100% agree. The non-rookie teams do this. We just need something basic beyond the hero- and kit- bots that give us maximum parts re-use.
I should add that on a limited basis we’ll drive the 36T gears in a 257RPM configuration, so this gives us a choice of 200 with green or 257 with blue, so great flexibility from that standpoint. Also allows 35 and 25 hole length or width. Hopefully this sets us up with some flexible starting options every year. That’s the idea anyway.