Hand Ergometer

[irving2008]

It could be 3D printed, in fact I was already thinking along those lines. Envelope would be around 80mm square x 6mm thick. I'd need the properties of PLA to check if its strong enough. What's the resolution of your printer?

[Clive S]

I have printed with both PLA and ABS and not sure they would be strong enough but laser printed I suppose could be done even if it had to be in two 3mm pieces and bonded back together. Do you have some sort of drawing! ...Clive

[FatFreddie]

It could be 3D printed, in fact I was already thinking along those lines. Envelope would be around 80mm square x 6mm thick. I'd need the properties of PLA to check if its strong enough. What's the resolution of your printer?

That size wouldn't be a problem, resolution is quoted at 0.0125mm and accuracy at 0.1mm (Mono Mendel - RepRapPro). The current nozzle is 0.5 mm (best used with a 0.4 mm or slightly smaller filament thickness) but I've got a 0.35 mm one that just needs putting together for finer detail.

Some material specs -

Interesting PDF on designing for FDM (http://ode11.com/publications/sme_rp_2001.pdf).

Because there is variable bond strength between the filaments, it's best to design so that the stresses are applied along the filaments but for things like gears that happens anyway since the slicer will trace round the perimeters.

We could do strength tests before you commit to the design...

[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...