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Huw
Brickset Towers, Hampshire, UKAdministrator Posts: **5,915**

I'm working on a GBC module and want to turn a wheel made from four of the yellow quarter-rack things from the Bucket Wheel Excavator (https://brickset.com/parts/6151167/1-4-circle-gear-rack-11x11) exactly one eighth of the speed of another wheel.

So, I need some gearing. Trouble is, the yellow wheel has 140 teeth so 1/8th is 17.5 and of course there are no 17.5 teeth gear wheels so it can't be done simply.

I suspect I'm out of luck but thought I'd ask here in case anyone better at maths than I can work out how to do it with a combination of 8, 12, 16, 20, 24, 36 and 40 tooth gears.

Driving it by a wheel or pulley is also possible. Its diameter is 166mm so circumference is 521.5mm which thus would need a wheel of 20.75mm diameter to drive it at 1/8th of the speed.

TIA!

So, I need some gearing. Trouble is, the yellow wheel has 140 teeth so 1/8th is 17.5 and of course there are no 17.5 teeth gear wheels so it can't be done simply.

I suspect I'm out of luck but thought I'd ask here in case anyone better at maths than I can work out how to do it with a combination of 8, 12, 16, 20, 24, 36 and 40 tooth gears.

Driving it by a wheel or pulley is also possible. Its diameter is 166mm so circumference is 521.5mm which thus would need a wheel of 20.75mm diameter to drive it at 1/8th of the speed.

TIA!

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## Comments

2761,419213So your key is to use the 8 and 16 tooth gears. 8:16 is 1/2, which is a multiple of 1/8th. In fact, 1/2 is 4 * 1/8.

Rig it up so you've got a 8:16 gear set up. Then rig that to another 8:16 gear set up. Then rig THAT to another 8:16 gear set up. So now you're running (1/2)*(1/2)*(1/2). You know what (1/2)*(1/2)*(1/2) equals? Why yes, 1/8th! Connect all that to your 140 tooth gear, and now the 140 is running at 1/8th of the input.

You don't have to use the 8 and 16 tooth gears. Those are just the smallest ones you can use. You can use any combination that is 1:2. So that would be the 12 and 24 tooth gears or the 20 and 40 tooth gears.

2765,915However having just read this article on the excellent website Brickset (!) there may be a way to drive it from the centre as you would a regular gear in which case either of those methods should work.

Back to the building table...

2,871I'm having a play with a spreadsheet to see if I can find some kind of way that normal gearing would work. There's certainly no easy way. The closest I've come so far is 8.10185 (going 8:40, then 24:8, 24:20 and 24:140. I'll keep trying...

Oops, just realised that 8.1 would make it that much

fasterthan the other one...15,419I think most successful long running GBC machines tend to allow for misses / misalignment, so that if a ball misses the slot it is meant to go in, it will wait for the next one. That way, if out of sync it doesn't matter too much.

2,871Closest I've got so far is 8:1.06 - going 8:20, 40:36, 40:140.

5,9152,8712,871Honestly, I'm quite surprised that Lego gave this gear 140 teeth. It just doesn't work as a multiple of any of the other gears. 4 more teeth on it would have worked beautifully.

2,871It just doesn't work. Nothing works, and all because it's 140-tooth. If it was 144-tooth it would be a piece of cake (8:16, 36:144).

I suspect this is a rare case of Lego not actually thinking a new part through properly.

2,871There is an important caveat though.The caveat is that I've used a gear which you don't list. Well actually, there's two different ones you could use, but I'm not sure if one of them would work or not.

You'd need either this old 14-tooth gear or this 28-tooth differential gear (which I'm not sure you could use, given it's not got an axle-hole)

For the 14-tooth, you'd go 40:16, 20:40, 14:140. For the 28-tooth, you'd go 20:16, 20:40, 28:140.

21315,419In fact, see Sariel's gear calculator http://gears.sariel.pl ... in the example there, red is 20 tooth and yellow is 140 tooth. Then instead of fixing the yellow ring and allowing the grey bar to be the rotator, fix the grey bar and the yellow ring will rotate at 1:8 speed of the red. I haven't got the parts with me to test what fits for the blue.

2,871Anyway, surely it'd be a 7:1 ratio? For every 7 times you turned the 20-tooth gear, the 140-tooth gear would turn once.

15,4195,9152,871