1. Just found this to get some ideas Plastic Gears ...Clive

2. Originally Posted by irving2008
Trying to work out how to draw the teeth for this in my CAD program, but attached is an initial sketch of the epicyclic gears.

Current thinking is I basically want a mod 1.25 60 tooth internal ring with a couple of fixing lugs (the ring is static), a mod 1.25 30 tooth sun spur gear on a press fit 10 x 22 x 6mm bearing (this will be fixed to the rotor which will rotate freely on the 10mm axle using another 10 x 22 x 6 bearing), and 2 off mod 1.25 15mm planetary spur gears on press fit 3 x 9 x 5mm bearings (these will be fixed to a carrier thats connected to the 10mm axle ). Spinning the 10mm axle will spin the rotor at 1+60/30 = 3 times the rate or 180rpm. the torque in the gear train is around 6Nm
That looks ok for printing - probably best if I get the finer extruder going but I've made some gears about that size which work ok. Could the unit be thicker? I'm thinking that it's maybe a bit marginal on strength - probably easiest to just build it and test it than worry too much at this point though.

If you (or anyone else) can point me towards a gear profile calculator (can't find one that does the outer ring) I'll knock up something for a strength test.

3. So finally managed to draw tooth profile and rotate/replicate it round a circle... Inard CAD doesn't have an easy way to do this, but what do you expect for £6!

Last edited by irving2008; 27-06-2013 at 12:23 PM.

4. Irving Can't seem to open that link. ....Clive

5. Strength calcs: Calculate for input gear as worst case (Ref: Spur Gears)

Input torque = power/revs = 90/6 = 15Nm, add 50% for safety = 22.5Nm
Tangential force on tooth Ft = 2 * torque/PCD = 2 * 22.5/.01875 = 2400N

Bending stress = K * Ft/(b * m * Y)
K=velocity factor, estimated at 1.03
b = face width = 6mm
m = module = 1.25
Y = Lewis Form Factor = from tables, for 15T 20deg gear = .289 @ tip .49 @ centre of tooth, use .289

stress = 1.03 * 2400/( 6 * 1.25 * .289) = 1140MPa = ~165kPSI

PLA = 500kPSI so should be OK!
Last edited by irving2008; 27-06-2013 at 12:32 PM.

6. Originally Posted by Clive S
Irving Can't seem to open that link. ....Clive
should be fixed now...

7. Originally Posted by irving2008
So finally managed to draw tooth profile and rotate/replicate it round a circle... Inard CAD doesn't have an easy way to do this, but what do you expect for £6!
Can you post those as DXF files?

Must give the CAD program a try - looks good.

8. Originally Posted by FatFreddie
Can you post those as DXF files?

Must give the CAD program a try - looks good.
Yes its not bad, fairly intuitive, and seems to be the only one that works properly on an Android tablet. I tried a few others but not much good.
Last edited by irving2008; 27-06-2013 at 03:18 PM.

9. I'd go for a timing belt drive for three reasons:
• It's not unlikely that we might find the ratio needs changing and to change the ratio you only need to change one pulley compared to two gears.
• The torque ripple will also be less with a belt, unless maybe someone wants to have a go at making helical gears!
• Noise.
Last edited by Jonathan; 27-06-2013 at 06:54 PM. Reason: Formatting

10. Originally Posted by Jonathan
I'd go for a timing belt drive for three reasons:
• It's not unlikely that we might find the ratio needs changing and to change the ratio you only need to change one pulley compared to two gears.
• The torque ripple will also be less with a belt, unless maybe someone wants to have a go at making helical gears!
• Noise.
1. I suppose its possible it might need changing, but a 3:1 ratio means 180rpm on the rotor and thats more than enough, can even use smaller/fewer magnets as torque requirement reduced by same ratio.
2. Not sure what you mean by torque ripple in this context, its an artifact of a motor due to bad pole design aka cogging...
3. Maybe, but plastic gears shouldn't be that noisy at these low speeds...

Epicyclic is elegant solution to problem. Belt drive would need to find a way to route belt past magnetic pole piece each end of magnet carrier, or carrier would need to be vertical or at least at an angle making use of the U-channel mounting more difficult and making the mounting of the loadcell more complex. Also belt drive might increase the footprint.

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