pic: 775pro Drive 1

Lol what are you going to do with all the CIM motors you saved? Maybe a cross field shooter?

775pros are typically utilized in the drivetrain often to save weight, and are versatile enough that they would suffice for a shooter-like application or anything else that may necessarily require a CIM (like an arm, climber, and so on).

I’m personally interested in seeing the reality of robots that purely use 775pros for their motors in the near future.

I love this design in particular as it leaves ample space in the bellypan for electronics and seems very lightweight for the potential power this drivetrain has. Joey, what’s the gearing of the drivetrain, and what prompted you to CAD this design?

So I’m not sure how familiar you are with older FIRST games, but 2013 was notable for having robots that really could make full-court shots. Of course I liked my own team’s design, but also look up some matches with 2169 (King TeC) for a very nicely executed full court shooter.

Your Kiwi drive was ridiculously cool. King TeC was extremely impressive as well.

Love the drivetrain. I’m very excited about 775pro drives for next season!

Interesting design. I like the idea. I would be skeptical of the ability for the pinion on those motors to hold up to the loads of a drivetrain in that way. I’ve heard of issues before using cim motors as idlers with 20dp gears, so my guess is the 32dp wouldn’t do great either (maybe even worse). It would be great to test and see the results of running something like this. This is a slick design and has some true potential.

How are you interfacing between the 3/8" hex 32dp gears and the 1/2" hex (or round) sprockets? Or are you not using only VexPro and WCP parts?

I sort of cheated and just opened up the 3/8" hex on the gear up to 1/2". One could theoretically do this in the shop if they have a 1/2" hex broach. My concern with this method is maintaining concentricity with the teeth of the gear after drilling a 1/2" hole in the center. If anyone has any ideas how to go about this, I’d love to hear them.

Thank you for the kind words. The overall gearing is 11.85:1, with a 12:80 reduction on the 32dp gears, and an 18:32 reduction on the sprockets. I wanted to see how much space a 775pro drive could really save, if the motors are kept as close to the frame as possible and no additional gearbox plates are used.

Actually after I posted that I remembered this from WCP. It should turn the 1/2" hex in the sprocket into 3/8" hex to match the gear.

My bigger question is what your final gear ratio is. I ran the numbers and I don’t see how you can get to a decent current per motor value with only a two-stage reduction using only one WCP 32dp spur gear reduction and one chain-and-sprocket reduction where the plate sprocket is smaller than the motor. I’m interested in how you managed to do it.

The driven wheel sprocket is larger than the driving motor sprocket. Final reduction is 11.85:1.

Sorry that was a typo. I meant to say smaller than the wheel.

With an 11.85:1 reduction and a 3" wheel (smallest drive wheel I can imagine), I get about 60 amps on a fully loaded (154lbs) robot. I think with that kind of current draw, you’ll end up burning motors, tripping breakers, and browning out your robot. Even with a 100lbs robot, you’re just below a 40A limit and still a good bit above a 30A limit.

Thanks for the compliment! Sir Kay is alive and well.

When it comes to drivetrain design, I am very specific. I see a few things here I’d like to point out. Starting with shaft collar usage. Looking closely, I see holes concentric with the hexagon for the shafts. A problem we addressed at North Star was how grease and generally bad stuff can be found on drivetrain axles after two regionals, causing shaft collars to slip off. I highly recommend tapping Thunderhex with a 1/4-20 and using a washer to keep everything together on all those hex axles.

Moving on to chain, I hope that’s #35, or you’re willing to be swapping #25 when it stretches. A few versa blocks would work well in this design.

Overall, I’d like to throw it out there that we haven’t been on the top of our game in previous years due to poor drivetrain selection in the past. 2014 and 2016 were stressful and rough build seasons because the base of our robot, the drivetrain, was excruciatingly troublesome. Remember that champions are made in the offseason and to test everything that you could in build season before next year. Good luck!

The strength of a gear is something you can calculate! Depending on how much math you feel like doing, that is.

Here’s a solid tutorial on how Lewis Bending Numbers work. As a heads up, LBN’s will only cover load, and aren’t really a great metric for sudden shock load. To avoid potential gear failures from sudden impact, you’ll want to be traction limited to prevent some pretty nasty forces.

Actually the thing i’m mostly concerned about here is Brownout (8 motor DT), and taking 8/16 ports on the PDP.

Any creative solutions for that?

I know gearing it down more would help with the current draw, but I wanted to see how stupid fast I could get this thing (22ft/sec!). All this belly pan space could afford you lots of tanks, so mechanisms could be pneumatic whereever possible without running the compressor too much.

This is honestly my biggest concern for an 8 motor drive like this, you’re either taking up ALL of the 40A breaker slots, or you’re powering 2+ of the motors with 30A breakers, neither seems preferable in my opinion.

By changing the gearing down to something reasonable, you can get each 775pro drawing no more than ~20A at stall. A setup like that wouldn’t brown, and would still have some serious advantages. A two speed makes the type of drivetrain where (on paper) you can have your cake AND eat it. Stupid acceleration and high speeds are totally possible in extremely lightweight packages, all by using 775pros responsibly.

While the 775pro and CIM have similar peak power numbers, small thermal mass, air cooled motors impose a *different *set of restrictions than CIMs. The 775pro drive trains that have been successfully used to date (at least to my awareness) have carefully been traction limited - that is, the wheels spin before the breakers trip. If you’re going to gear a 775pro drive train for a normal FRC weight robot above 20 ft/s, 8 motors may not be enough. On the other hand, the 775pro will smoke if it gets much more than 30A for an extended time period, so putting them on 30A breakers isn’t the liability that putting a CIM or miniCIM on a 30A breaker would be.

We have made a holding collar that steps to hold the gear or sprocket with a light press fit. Then slot the collar so you can clamp the collar/gear assembly in a lathe 3 jaw chuck or collet. “Stepping” the collar squares the gear so you can bore or drill and then ream the gear or sprocket to the correct diameter. :smiley::smiley: