Hand Ergometer

[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.

[irving2008]

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.

[Jonathan]

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.

1. It depends how well we need to control the torque, i.e. the resolution. It the ratio is wrong, so the magnets are never operated near saturation, then you've reduced the available range in which to control them. I guess the ratio would have to be a long way off for it to be significant in this application...
2. I wouldn't say it's due to bad design, more that it's a side effect of optimising a motor design for certain characteristics. In this context I meant simply that the handles will not feel as smooth to turn - there will be vibrations at frequencies dependent on the number of teeth on the gears.
3. True, I'd forgotten it was so low.

One of these would be fun to watch: