Hand Ergometer

[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:

[irving2008]

1. Surely you've got that backwards? The magnets are intended to be used at saturation in pick-up mode. Here we will run them well below saturation, hopefully somewhere on the linear-ish part of the B-I curve. I'm only using the saturated value of 1.6T to find the max possible retardation.

2. Ok I get you now. At the nominal design speed of 60rpm I guess there will be 'vibrations' at 90Hz from the planet/sun wheel interface and 60Hz from the ring/planet interface. I expect these will be small - I guess they'll depend on the quality of the tooth shape and the fit of the gears. We won't know til we try! :)

[FatFreddie]

We won't know til we try! :)

Current state of trying is an interference fit :-(

I printed some calibration parts and they came out fine so I need to look at it in more detail.

I suspect that the plastic will reduce vibration to some extent but as you say, trying it is the real test.

[irving2008]

Current state of trying is an interference fit :-(

I printed some calibration parts and they came out fine so I need to look at it in more detail.

I suspect that the plastic will reduce vibration to some extent but as you say, trying it is the real test.

Does that mean you've printed them? :)

I can't guarantee the accuracy of the drawings, I use the MOD1 profile found on the web then scaled it up 1.25 to get a MOD1.25 profile but the way I had to rotate it to get the final gear may be over/undersize. I suspect its the ring gear thats slightly under... maybe you could measure the OD and ID of the gears and let me know?

[FatFreddie]

I just printed some 1mm thick test pieces.

I loaded the 15 and 30 tooth gears into cambam and generated a profile with the built in generator and it was exactly the same outer and root diameters (just a bit more lateral clearance in the root) so I suspect you're right about the ring gear - unfortunately cambam doesn't do ring gears (I've bumped a thread asking for them).

Measured OD on the 30t gear is 40.0mm and 21.3mm on the 15t and the tooth tip diameter on the ring gear is 72.7mm and the root diameter is 76.4 (near as I can tell - it's a bit flimsy)

[irving2008]

I just printed some 1mm thick test pieces.

I loaded the 15 and 30 tooth gears into cambam and generated a profile with the built in generator and it was exactly the same outer and root diameters (just a bit more lateral clearance in the root) so I suspect you're right about the ring gear - unfortunately cambam doesn't do ring gears (I've bumped a thread asking for them).

Measured OD on the 30t gear is 40.0mm and 21.3mm on the 15t and the tooth tip diameter on the ring gear is 72.7mm and the root diameter is 76.4 (near as I can tell - it's a bit flimsy)

OD for MOD1.25 = (N+2)*1.25, so 30T = 40mm and 15T = 21.25mm.

tooth tip dia on ring gear should be (N-2)*MOD = 58 *1.25 = 72.5mm and root dia 78.125mm which is correct on drawing...

[FatFreddie]

Ok, looks like it's a problem with printing the internal features - let me have a look at it over the weekend - trying to learn Hadoop (if you don't know, you probably don't want to) at the moment and it's not conducive to concentrating on other stuff :-(

[FatFreddie]

Ok, some progress. I've now got a set of gears that mesh reasonably well but I'm not happy with the print quality so I'm going to try a different slicer - the one I'm using at the moment is Sli3er and it's fast and easy to use but it doesn't seem to be the best quality. I was using Skeinforge before which is better quality output but not very friendly - I'll have another go with that over the next couple of days.

[irving2008]

Ok, some progress. I've now got a set of gears that mesh reasonably well but I'm not happy with the print quality so I'm going to try a different slicer - the one I'm using at the moment is Sli3er and it's fast and easy to use but it doesn't seem to be the best quality. I was using Skeinforge before which is better quality output but not very friendly - I'll have another go with that over the next couple of days.

Thanks for your efforts on this. I'm in the process of drawing up slightly modified gears, the main difference being improvements to better support/locate the bearings and space the sun gear off the rotor boss - I may increase the thickness of the gears slightly. I'm also redrawing the supports, the rotor bosses and the planet carrier. I was thinking of 3D printing the carrier but I don't think the boss would be strong enough to take a grub screw to fix it to the axle (but what if the boss was thicker and had a slot for a nut to be inserted?). The other thought I had was 3D printing other components, the rotor bosses for example. I'm not sure about the cranks or the handles from a strength perspective, need to run the numbers. Also do you have any way to calculate the cost of a printed component?

[FatFreddie]

No problem, I'm enjoying having something constructive to do with the machine. Work is a bit binary at the moment so apologies if I go missing occasionally.

I had thought of printing some of the other parts - once I get something that can be strength tested we'll know better what's possible. I have had some unexpected breakages in the past but that was with mounting brackets where I think I underestimated the shock loads. One thing to bear in mind is that things like splines which would normally be difficult are relatively easy with 3D printing. The main problems are fine tolerances and overhangs so a spline would be best if it was tapered or with pinch bolt.

I haven't bought any filament for a while but I think it's around £30 a kilo. Partially hollow parts are the norm so material usage is quite efficient.