Technic: Zero backlash worm gear

[Imozeb]

I am designing a crank system for my 3D printed music box to turn the music cylinder. Since the music box cylinder is large about 30 cm diameter, the effects of backlash of only a degree or two is magnified.

How can I use lego gears to design a crank that increases torque and decreases speed with practically zero backlash?

I know I may receive some flack for this, but I am fine with a solution that requires some glue or 3D printed parts or springs, but I would prefer if it was all Technic.

 

Thanks!!!

[Erik Leppen]

The backlash is the smallest when the gear is very large. So, have you tried using the new 11x11 yellow quarter gear racks from the BWE 42055? Or the (old) Hailfire droid wheels? If you drive one of those using a 8t or so, there's so much reduction that the amount of play is almost zero.

Also, maybe you could play with friction. Play may be less of an issue if the drum has more friction. It will take more force to turn, but it will turn very slowly anyway, and with more friction, other parts of the instrument will also have less chance to push the drum a certain way.

Adding fritoin could be done by rolling the drum against one or more small tires on blue axle pins.

Another way is using exactly such tires to drive the drum (works best if the cylinder is exactly circular). Then you don't need gear teeth at all, but you use friction as a way to drive the cylinder. Use springs to push motor-driven tires against the outside of the drum. Use multiple for more torque/force.

[Zerobricks]

Use the new 3L worm gear. It has much lower backslash.

[aeh5040]

1 hour ago, Erik Leppen said:

The backlash is the smallest when the gear is very large. So, have you tried using the new 11x11 yellow quarter gear racks from the BWE 42055? Or the (old) Hailfire droid wheels? If you drive one of those using a 8t or so, there's so much reduction that the amount of play is almost zero.

I strongly agree with this suggestion. The BWE quarter gears are definitely the way to go.  I had a very similar constraint with this MOC, and the those gears were the only way to get close to the required accuracy.

I would also avoid worm gears in the gear train - spur gears have much less backlash.

[allanp]

I tend to think the double bevel gears have the least back lash. Also agree with the BWE quarter gears suggestion, however drive them with a large gear to minimize the small jump you get when the gear drives over the gap between the 1/4 gears, as I have found that the teeth are spaced just slightly too far apart which is partly the cause for the jerky movement of the wheel on the BWE. In the upcoming Lego creator expert carousel, it is driven by the BWE quarter gears which is driven by a 36t double bevel gear (as opposed to the 12t double bevel gears of the BWE) and the rotation is much smoother. It also has it's gearing down much closer to where it is needed avoiding twisting shafts which also causes jumpyness as the axle springs back. 

[mocbuild101]

11 hours ago, Erik Leppen said:

Also, maybe you could play with friction. Play may be less of an issue if the drum has more friction. It will take more force to turn, but it will turn very slowly anyway, and with more friction, other parts of the instrument will also have less chance to push the drum a certain way.

This would be a good solution. You could also use pulleys with a rubber drive belt, but that probably wouldn't have enough reduction.   

[Imozeb]

Thanks for all the helpful replies.

 

So for those of you who have used the BWE quarter gears, if you drive it with an 8t gear approximately how much backlash do you get at a point 15 cm from the center? 

 

I want to know that it will work before I buy them off the internet.

 

thanks again!

[Saberwing40k]

If you're making a 3D printed music box, why not 3D print the gears too? 

[Imozeb]

27 minutes ago, Saberwing40k said:

If you're making a 3D printed music box, why not 3D print the gears too? 

I tried first but the tolerances for the gear teeth were bad on my printer.

 

Also, isn't backlash related to the extra spacing between the teeth, not the angular velocity of the output shaft i.e. the BWE quarter gears would still have high backlash because of the crapy teeth spacing of the 8t gear?

[mocbuild101]

4 hours ago, Imozeb said:

Also, isn't backlash related to the extra spacing between the teeth, not the angular velocity of the output shaft?

Yes, the worst being 2 or more 8t gears meshed together.

[Saberwing40k]

I'd recommend looking into some Tamiya worm gearboxes, because Lego gears, by definition, have higher backlash. This Tamiya worm gearbox has multiple ratios you can build it with, and while it might not be cheaper, it's an all in one solution that would be quite precise and stable: https://www.tamiyausa.com/items/geniuseries-educational-kits-50/educational-construction-38000/4-speed-worm-gearbox-he-72008

 

Also, you've never specified why you need something with so little backlash. I can not see you needing that little backlash for just a crank on a music box.

[TheMindGarage]

I've found a solution with the BWE rings and 12-tooth gears. Using 8-tooth gears is pretty impractical because you'd need half-stud spacings.

If you position the 12-tooth gear 8 studs away from the center, you can drive the 140-tooth ring. You can also do this by doing a 7/4 offset - that's 7 studs horizontally away from the center and 4 vertically (total distance is 8.062 studs which might increase friction a little but it would actually reduce backlash further).

I don't think backlash is a problem anyway because it's only rotating in one direction. As long as the resistance/friction on the output remains constant, the backlash shouldn't be a problem. In fact, it might even be beneficial to have a little to prevent jerkiness or vibrations from affecting the speed. You could use a flywheel or a viscous coupling (one that winds up using rubber bands or springs) to smoothen the motion of the cylinder.

[doug72]

My BCE uses a 12t gear meshing with  BWE gear ring. The 12 T gear is driven by the new worm gear meshing with a 20T gear giving a very smooth rotation with very 'little backlash. Overall gear ratio = 233:1