Gearing Ratios

[SeagullOne]

I'm designing a gearbox that will be used on a 6V motor with a no-load speed of 30500 RPM and a stall torque of 8.3 oz-in. I'm planning on it being a 5-stage gearbox with compound gears. The maximum ratio I want to achieve is 768:1 for speed reduction and increased torque. The first four stages are an increase in a factor of 4:1, while the fifth stage is an increase in factor of 3:1. So we go 4*4*4*4*3.

I'd like to know if there are any dangers I'm not foreseeing here. I'm aware of loss of efficiency, but how much loss would I be looking at in theory. Is there a way I can calculate the loss of efficiency in my gear train? I've heard of some gear trains being ruined when too much torque is applied. How does this happen and is there a way to avoid it?

Also, I'll be reading the gears tutorial to see if that has any information. Thanks.

EDIT:

After reading the gears tutorial, I see that spur gears tend to have a 90% efficiency, depending on other factors of course. I wonder if that's 90% altogether or I lose about 10% efficiency each time (the latter makes more sense to me, though). I will be using spur gears, and I also intend to really smoother them in lubrication since I want their operation to be as quiet as possible (and increase efficiency).

[waltr]

I believe that is a 10% loss per stage (one set of spur gears) and this can vary depending on other factors.

I've heard of some gear trains being ruined when too much torque is applied.

Yes, this can happen as the reduction increases the torque that can be applied to the teeth on a gear increases. This means that the gears closer to the output need to be stronger. This can do done be eother making the teeth larger (courser pitch) or by making the gear thicker, teeth are wider.

Heres a link to some motor/gear box info that may be useful:
http://www.micromo.com/gearhead-tutorials.aspx
http://www.micromo.com/gearhead-construction-and-use.aspx

Also, if you have an R/C hobby servo open it and study the gear train. Commonly the gears get thicker on each reduction stage.

[futmacl]

I believe that is a 10% loss per stage (one set of spur gears) and this can vary depending on other factors.

No, that's extreme. Good spur gears have efficiency of 98-99%:

http://www.roymech.co.uk/Useful_Tables/Drive/Gear_Efficiency.html

Crappy ones may be lower, but in general, your entire gearbox should be over 80-85% if done well.

This means that the gears closer to the output need to be stronger. This can do done be eother making the teeth larger (courser pitch) or by making the gear thicker, teeth are wider.

One of the more important factors here is the pressure angle: using 14-15° for the initial and 20-22° for the final stages is more robust than a low angle all the way through. The material from which your gearbox is made also matters: Nylon is fairly flimsy but cheap, but there are other engineering plastics that can be used for this task.

Btw, here's my most recent geared transmission project:
http://www.flickr.com/photos/candy-bottoms/sets/72157625850569320/detail/

[waltr]

Thanks futmacl for the clarification and addition info.

[SeagullOne]

Thanks!
I'll see if I can widen the face width of the output gears. That should reduce the risk of ruining the teeth. Perhaps I'll even make the pitch courser as well.