pic: Modular Gearbox

[Chris is me]

One more thing I want to remind you (and sorry to dominate the posts of this thread!) is that you want to make sure all of the tubing sizes you're using are readily commercially available. For example, .100 wall tubing is very uncommon outside of what Vex sells; most of it is 1/8" wall and 1/16" wall. Also, some of the odder sizes are only available in 6063 tubing, which, albeit weaker and crummier to machine, it is adequate for this application in 1/8" wall if there isn't a 6061 alternative.

[Ari423]

Originally Posted by Chris is me

One more thing I want to remind you (and sorry to dominate the posts of this thread!) is that you want to make sure all of the tubing sizes you're using are readily commercially available. For example, .100 wall tubing is very uncommon outside of what Vex sells; most of it is 1/8" wall and 1/16" wall. Also, some of the odder sizes are only available in 6063 tubing, which, albeit weaker and shittier to machine, it is adequate for this application in 1/8" wall if there isn't a 6061 alternative.

I don't mind your help at all! I thought 100 wall tubing was a common size but it seems I am mistaken. Also, I couldn't find any 3/4x6" tubing of any wall thickness so I changed the double block to 1x6x125". I can find that on McMaster, if not somewhere else cheaper. It makes the 2 CIM gearbox a little bit heavier and wider, but only slightly (.25" wider, .18 lbs heavier).

[nuclearnerd]

My idea was to have something similar to a VersaPlanetary in its versatility except a spur gearbox not planetary, so it can be used in higher torque situations like a drivetrain or a heavy arm

Having spent a season fixing exploding 3 stage versaplanetaries as they struggled to lift a heavy arm, I think the idea is great, especially if you can stack ratios to at least 200:1

[Ari423]

Originally Posted by nuclearnerd

Having spent a season fixing exploding 3 stage versaplanetaries as they struggled to lift a heavy arm, I think the idea is great, especially if you can stack ratios to at least 200:1

The big upside of planetary gearboxes is their huge reductions in small packages. Compared to the maximum 10:1 stage on a VP, the biggest reduction you can get from a spur gear stage with this system is 4.5:1 (9:1 over two stages using a combination of 3/8" and 1/2" gears). That would mean using only spur gear reductions to get up to 200:1 you would need 45 stages, or about a 4 ft wide gearbox.

Using a final 12:60 chain reduction (max for #35 chain w/ Vex sprockets) after the gearbox, it would take 9 stages. You could then further reduce that by putting the CIMs through a 10:1 VP reduction before it goes into this gearbox (which should be a lot more manageable than a 200:1 VP reduction). You would only need five stages direct driven with a 10:1 VP on the input side. With both the 10:1 VP reduction and the 12:60 chain reduction, you would only need two stages. If you want a 200:1 reduction without a final chain reduction, you can put a CIM through a 50:1 two-stage VP (the max it's rated for) and then into a two-stage spur gearbox. That will result in a max reduction of 672:1.

So in summary, yes you should be able to get a 200:1 reduction, it just takes more space than a VP. But, since it's (probably) rated for those high loads, it shouldn't explode every time you try to use it.

P.S. - All these huge reductions are theoretically possible, but are practically limited by the max torque capability of the shaft. I'm too lazy to look up the max torque rating but I'm sure it's online somewhere or other.

P.P.S. - VPs with a CIM input are rated for a max of 50:1, and only with a 1/2" hex output shaft. I'm not surprised a CIM through a 200:1 reduction exploded.

[asid61]

Originally Posted by Ari423

The big upside of planetary gearboxes is their huge reductions in small packages. Compared to the maximum 10:1 stage on a VP, the biggest reduction you can get from a spur gear stage with this system is 4.5:1 (9:1 over two stages using a combination of 3/8" and 1/2" gears). That would mean using only spur gear reductions to get up to 200:1 you would need 45 stages, or about a 4 ft wide gearbox.

You're not doing that calculation correctly. Gear ratios are multiplied, not added, so if you stacked 4 stages you would get a 4.5 * 4.5 * 4.5 * 4.5:1 ratio, which is about 400:1. Using a chain reduction for the last stage would give you even more.

[Ari423]

Originally Posted by asid61

You're not doing that calculation correctly. Gear ratios are multiplied, not added, so if you stacked 4 stages you would get a 4.5 * 4.5 * 4.5 * 4.5:1 ratio, which is about 400:1. Using a chain reduction for the last stage would give you even more.

Wow I don't know how I messed up so badly on that. I take back everything I said in that last post (except the thing about VPs not being rated for a CIM at 200:1). Yes, you should exceed a 200:1 ratio on the 4th stage. With the 60:12 chain reduction, you can do it in 3 stages. With the 50:1 VP reduction, you can do it in 2 stages. With both the 50:1 VP and 60:12 chain reduction, you can do it in one stage.

[nuclearnerd]

Originally Posted by Ari423

P.P.S. - VPs with a CIM input are rated for a max of 50:1, and only with a 1/2" hex output shaft. I'm not surprised a CIM through a 200:1 reduction exploded.

We used Bag motors, which vex rates up to 300:1 if you stack the stages in the right order. That said, we found the VP assembly pretty unreliable at the limit of the torque "rating", so your point stands.