Drive gear size

[Daniel]

I've been using chain and sprocket reduction on my robot Vendetta for about 3 years. It has sometimes been giving me trouble but I have managed to get rid of most of the bugs. However at our Nationals on the weekend I destroyed all off my 250W scooter motors, so I think its time for a new idea.I built a beetleweight to test out-runner brushless motors in drive and a had some success. My plan was to scale it up into drive for Vendetta. The problem is I have never worked with gears before and am unsure what size MOD to choose. I'm thinking of using an 80 tooth MOD 1 bored out and welded onto my 250mm trolly wheel hubs with a 12 tooth pinon driving it from an interal gearbox made from ebay angle grinder bevel gear sets. Total reduction 16.7:1. I plan to use NTM 42-58 500kV / 1300W brushless motors with 6-cell LiPo. Does my choice of gears sound strong enough? Would I need an extra reduction stage?

[maddox10]

31 km/h. If the brushless have the torque to get it moving, it will be fast.
With the right speedo, you could have a suitable setup.

But with a Z90-12 secundary gearing (18.75 -1 ratio total) top speed will be 27.5km/h
The max advisable secundary gearratio is Z96-12 (20-1) and you still have 26 km/h

I know what I would choose.

[shakesc]

Daniel

interesting question line. Been looking at the same thing and there are alot of angle grinder gears on eBay, though I was looking at smaller wheels

The Australians seem to be into testing brushless motors as well for feathers

[Daniel]

Yep, I'm one of those Australians. My beetle weight brushless test drive worked, so its time to scale up. The problem is there has never been an Australian robot with a custom gear drive. We always use pre-built gearboxes or chains, so I have no experiance to draw from about what Mod gears can transfer that much power. I've seen a lot of european robots with custom gearboxes, so I guess this is the best place to draw experiance. Gears here are stupidly expensive, so much that it'll be cheaper to buy from Technobots, so I want to smallest gears that'll handle the drive. The current option that I've been modeling in Inventor has 12:30 spur gear first stage (ebay RC car pinons?) + 14:35 angle grinder set + 20:80 spur gear. Possible MOD 1 for the spur gears. Total reduction 25:1 and a no load speed of 5.8m/s. Sound good to you guys?

[Mouldy]

MOD1 will be fine on that ratio, try keep the gears above 8mm wide. The most important thing is to get the spacing correct between gears.

[maddox10]

Isn't a 3 stage gearbox not beating the purpose, having more power in a lighter, compacter size?

25-1 reduction. Final theoretical top speed 20.7km/h with 250mm wheels (huge wheels for a feather)

[shakesc]

The tricky bit is the gearbox for sure. I quite like the thought of the engineering challenge but then its sitting in a fighting robot that someone is going to smash into - simple and easily fixed/replaced comes to mind.
The alternative is to try and couple the motor to something like a Banebots gearbox http://banebots.com/c/P60K-S7

[Daniel]

Ok, 2 stage, MOD 1, wider then 8mm. Go for the 8.69m/s (31km/h). Thanks guys. I prefer to use m/s because our arena is 4.8m and I can gauge off that.

I can't drop in a pre-built gearbox because I'm using an old chassis. That chassis is made from 5mm wear plate and has taken a lot of hits from spinners with no damage, so I can't see any reason to chuck it in the bin. Its only as wide as 2 x 300W scooter motors, which makes it all a challenge.


Second question. In my beetle weight brushless gearbox I used Shapeways to print the gearbox housing which I then bolted into a biscuit tin. I was plaining to use the same thing in Vendetta, with a 3D printed housing that matched the scooter motor bolt pattern.
Here's a piccy of what I did with the beetle.