Hand Ergometer

[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

[Clive S]

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

[FatFreddie]

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.

[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!

[Clive S]

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

[irving2008]

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!

[irving2008]

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

should be fixed now...

[FatFreddie]

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.

[irving2008]

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.

[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.