First drive gearbox

Hi everyone ! I’ve just completed my first drive gearbox design and I would like to share it with you to have some feedbacks from more experience people. It’s design for a classic WCD. I’ve used 3 775 pros because of they’re lightweight and compact and I thought it coud be interesting to developp. It has an overall gear ratio of 16:1 which provide a free speed of 20.43ft/s whith 4" diameter wheels and an adjusted speed of 16.55 ft/s (according to JVN’s design calculator). I know it’s pretty fast but I love fast robots. I’ve mounted a SRX Magnetic encoder on the gearbox. It wheighs a little bit more than 1.7kg or 3.74lbs. Sorry for mistakes with writing English, I’m actually French.


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Congrats! This really looks nice for a first gearbox. I really like some of the placement and packaging you have. But there are a few issues that need addressing. Also, could you provide some more views or more pictures? There are a few details I can’t quite ascertain from the single view.

First, you have a snap ring between loads on your output shaft. This means you’ll have a pretty hefty stress concentration on that point on the output shaft, creating a possibility for failure. There are plenty of ways to constrain that shaft without putting a snap ring between loads.

Second, you’ll need to reconsider your gearing or motor choices. 775Pros are NOT drop in replacements for CIMs. Since you only have a single speed and it’s geared quite high, your robot will most likely not have enough torque to break traction. Also, you’ll almost certainly pop your breakers (40A and main breaker) within a second or two of stalling, not to mention the damage the stalling would do to your motors. If you want to keep your fast robot, you’ll either need to switch to CIMs and employ some voltage or current ramping to prevent brownouts, or you’ll need to go to 4 775Pro gearboxes with a high and low gear.

Third, I think you need to add back some support to your plates. I assume you’re using the three bolts that pass through both plates to secure the gearbox to the tubing, but you need to make sure you add those details. Also, the output bearing hole on the last plate probably needs some more reinforcing. It’s basically cantilevered out from the rest of the gearbox, and I feel like that introduces the possibility of that plate flexing a bit at that bearing hole and causing some serious binding.

All in all, still a great first gearbox. Good luck, and happy CADding!

I concur: 6 CIMs with an adjusted speed of 16.5 would be slightly aggressive; with 6 775 pros, you’re just asking for trouble. A CIM can support an 8V stall for nearly as long as a 775pro can support a 6V stall, and it gives more than 3x the torque doing so. 775s burn up when given 30A for less than an FRC match. While 775 pros have a theoretical maximum power a hair greater than a CIM, know that they can’t actually deliver more than 200W for half an FRC match - while a CIM can run at full power for longer than an FRC match.

Thanks for answering. Here are more pics:

http://i.imgur.com/D09gbln.png
http://i.imgur.com/e1Xf7JN.png
http://i.imgur.com/Dnh0hAX.png
http://i.imgur.com/Ju7ZH8u.png
http://i.imgur.com/nEKcbSp.png

For the snap ring issue, I think I will flip the bearings of the output shaft so that the flanges are on the other side of the plates and then put 2 clamping shaft collars. (I don’t have enough space to put one between the plates because of the gear from the first reduction stage).

Well, for the motor choice, I think a drivetrain with 6 775 pro on single speed gearboxes is possible, lots of team have succeeded with drivetrains powered by 775 pros, even with only 4 of them. A robot designed with that drivetrain will definitely not be a defensive robot and the pilot would have to train a lot to avoid stalling as much as possible. I will try to integrate a torque limiter or to find wheels that have a low coeff of friction to make the robot slip before the motors are stalled. I appreciate the weight of this gearbox, about 1.3 CIM all included (3.74 lbs). I’m not familiar enough whith 2 speed gearboxes yet to design one but I will try this summer.

Third, I think you need to add back some support to your plates. I assume you’re using the three bolts that pass through both plates to secure the gearbox to the tubing, but you need to make sure you add those details. Also, the output bearing hole on the last plate probably needs some more reinforcing. It’s basically cantilevered out from the rest of the gearbox, and I feel like that introduces the possibility of that plate flexing a bit at that bearing hole and causing some serious binding.

I’m not sure I’ve really understood what you mean here. We can’t see the fourth bolt on the picture I’ve posted but there is one, the plates are symmetrical. I don’t feel the output bearing hole as being a week point, the plates are 6.5mm thick aluminum.

6 775s will be fine as long as you watch your current. Breakers are less likely to be a problem compared to the motors themselves burning up or brownouts, but it’s best to monitor current just to prevent any bad accidents. Testing at an offseason is wise. Chances are you’ll get better performance than 4 CIMs but worse than 6; switching to 8 775s is a safer option for your first gearbox especially. Your speed is quite fast for a 6 775pro drive with no shifter, and I can almost guarantee you’ll burn things unless you add a lot of current limiting. ~16fps free speed is a lot safer if you are ok with the slightly lower top speed, although you’ll probably still want current monitoring.

The plates maybe be 6.5mm (~1/4"), but that doesn’t mean that they’ll be strong. Review your gearbox design against others posted on CD, especially the location and quantity of struts and the lack of “border” around your gearbox. 4 standoffs is the generally accepted norm, and I would stick with that until you have success with it.

Your output gear is definitely cantilevered. Whether or not that will effect things I can’t say. Why not spread the plates farther apart and just make sure all gears are between plates? The length of the motor means that it’s not going to take up more volume anyway, and you’ll be able to get away with something like what the WCP gearboxes do to constrain the output shaft. Make sure you can mount the gearbox to the chassis too - I don’t see any method of doing that at the moment! :stuck_out_tongue:

Really interesting design, especially for your first drive gearbox. It’s a lot better than my first gearboxes for sure.

Make sure you can mount the gearbox to the chassis too - I don’t see any method of doing that at the moment!

I wanted to mount it with the screws that currently holds the plates together

Really interesting design

Thanks !

My reflexion after your feedbacks led me to try to design a brand new gearbox, with 2 speeds and 3 775 pros. The programmers assure me they’re able to do an auto-shifting code to avoid stalling in high gear. I will make the low gear low enough to make sure no stalling is possible. Thanks a lot for your feedbacks, I would not have suspected some of the issues you found. If I’m proud enough of the new gearbox I may post photos on CD.