Geardrives for large scale
 

So I'm getting back to my old broken 21C 1:9 Abrams-in-a-box. Lots of broken plastic that I'm confident I can fix or upgrade, but those gearboxes, oy.

I thought I could fix them up to be decent runners, but upon disassembly I see there's little hope. I don't mind retrofitting better units but this big boy needs something suitable for large scale.
Question is, what are folks using in bigger builds? I've been dredging through some of the old threads on the 21C Abrams, but haven't found any help yet. There are some nice gearmotors at some of the robotics places, but I have no way of knowing if they have enough torque.


https://cimg9.ibsrv.net/gimg/www.rcu...549d99749c.jpg
Abrams-in-a-box. A big box.


https://cimg1.ibsrv.net/gimg/www.rcu...e201168e35.jpg
Opened up the gearbox... has some better features than the Stuart.


https://cimg5.ibsrv.net/gimg/www.rcu...068590fbc1.jpg
But just one of the bad things- that 6mm output shaft runs on stamped bracket. It's wearing a groove and galling at the same time.


https://cimg0.ibsrv.net/gimg/www.rcu...9877a98e82.jpg
Someone else's mod, but apparently the drive was too wimpy... or maybe mismatched, I dunno.

BTW, how do I get images to insert as thumbnails?

I'm thinking a couple of these from AliExpress could be the ticket:
https://www.aliexpress.com/item/3280...787644abe6SEKI

700 size motor, variety of ratios, might fit back-to-back in my 9.5" wide hull...

https://cimg8.ibsrv.net/gimg/www.rcu...f1f6627141.jpg
700 motor and planetary box

Hooben makes a larger gearbox that can mount a 550 motor or a 775 it says. Not sure about the shaft size as Hooben isn't great about posting specs.

https://www.aliexpress.com/item/4000...archweb201603_

I dont know how much an Abrams weighs in 1:9 but Ive used these type of motors in heavier, if not bigger, tanks.

If you seach for this on ebay (the .co.uk variant at least): high torque worm motor you end up with a huge selection of motors and drives.

The ones with the large black motors have worked well for me. They are rated at 12V but run fine on 7.2 or 9 V. You will lose rpm's but you will keep the torque (and I'm not getting into a discussion on that fact).
They are relatively small, with metal gears, loads of mouting options and pretty cheap relative to normal tank gearboxes.

You will have to figure out a shaft adapter I guess. And which rpm version you want (allowing for the lower voltage).

As an example, if your tank weights 20 kg and has a drive wheel radius of 5 cm, it will need about 2.7 Nm to get it up a 20 degree incline at 1m/s. Which is not really that much.

p

Hmm, something like this then:
https://www.aliexpress.com/item/4000005192663.html

I do like the form factor of these worm drives, fits nicely without resorting to chain drives and such.

About load calcs: Just driving, even going up an incline, may not be the worst case. The fellow who posted his failed attempt said that straight line was fine, but turn-in-place on smooth concrete was enough to stall the drives. Must take a lot to skid the tracks like that.


https://cimg1.ibsrv.net/gimg/www.rcu...3e84cea3c7.jpg

I personally use the TG gearboxes. Just be a bit creative on the sprocket and you can use it for a lot of stuff. This was for a project with an estimated 30-50lbs with motors. They have been sitting for a while so they aren't the prettiest, maybe this summer I will get around to reviving the project.
I asked a buddy to make me these and they are perfect fitment.
https://cimg3.ibsrv.net/gimg/www.rcu...073ed5365d.jpg
https://cimg4.ibsrv.net/gimg/www.rcu...7935af1ce7.jpg

Those Hooben boxes look great, and the TG unit is certainly serviceable... but I just ordered a pair of the right angle worm drives. Now to design and fab the rest of the needed hardware...