Hand Ergometer

[irving2008]

There's a bit more room, I can go wider and taller and thicker if need be. This new ring gear is 8mm thick and the new gears are also 8mm as I needed more space for bearings & bolt heads.

[irving2008]

I'm surprised it deflects that much, a quick calculation, crudely treating the side as a 6mm deep 'edge' x 6mm thick beam of length 90mm with fixed ends shows a deflection of .01mm per N of load... so deflecting a whole tooth is a 250N load, larger than expected but not impossible I suppose.

The v2 design (100mm long, 12mm deep 'edge' x 8mm thick) is .0013mm per N, so only 0.32mm deflection.

If need be I could add a surround of 8 x 8mm ally angle to constrain the PLA.

edit I was using the Youngs Modulus of PLA as 3450MPa. That article suggests a 50%+ reduction depending on how the filaments are laid down. The max strength requires cross-bonding, preferably at 45deg. So it could be that the forces are much lower, but the relative calculation above still holds.

[FatFreddie]

A couple of complicating factors - I forgot to say that the part is printed with 30% infill to save plastic so it's not as stiff as it could be (there are three solid layers on the top and the bottom (1.1mm) and on the perimeters (1.5mm - I'm extruding filament 0.5mm wide and 0.35mm thick)) and the concave bending of the sides away from the planet gear effectively doubles the deflection.

I tried the new ring gear (still 6mm and 30% fill) and it moved but didn't skip (at moderate loads - see below) so I don't think it will be an issue - more something to bear in mind for any future modifications - I'll know more when I've managed a proper load test.

Testing at 2KG/metre resulted in a broken gear carrier and splintering of the wooden batten attached to it so they need to be improved next - I might make an aluminium carrier as I've just converted the milling machine to a CNC-USB controller but haven't cut any metal with it yet :-)

I think I've pretty much tuned out the oversized first layer problem by putting a slight relief on the side of the teeth next to the bed - the 15t gears look good and the 30t isn't far off - same technique should work on the ring gear as well and has improved the feel of the gears quite a lot.

[irving2008]

Interesting...

I've designed a 2-gear planet carrier, see attached, but whether it'll be stiff enough in PLA to grip the 10mm shaft or not remains to be seen. The slot in the boss is for a nut as you can't put a thread in PLA.

I can go to a 3-gear solution, but a 4-gear will need 64/32/16 tooth gears else it can't be assembled.

[FatFreddie]

I think you're right - I can't see the PLA being strong enough to take the stress. One possibility might be to have a hole drilled in the shaft and a bolt through the hole so the torque is transferred to the outer skin via the bolt. As an aside, square nuts are best for small captive nuts in PLA - much more resistant to spinning in the slot. M3 are 5.5mm A/F and I've got some in stock, M4 and above are ok in hex.

I've put some proper bearings in and the gears are working much better. There is one tooth on each gear that is slightly malformed and that causes sticking but a couple of minutes sanding eased it considerably. Progress is a bit slow at the moment because I'm trying to get the printer working again after adding some hardware to allow me to print without it being connected to a PC (it keeps dropping the connection which is a pain) - all seems ok apart from it not heating the bed which, unfortunately, is a show stopper. If I can't fix it I'll revert to the old firmware but it's probably something fairly simple.

Your new ring gear design is much stiffer and I don't think skipping will be a problem. Engagement seems fine as long as it's within certain limits but when the engagement zone of the teeth moves to the more angled zone near the tip, the forces increase rapidly causing the skipping so a moderate increase in stiffness (and improved tolerances from the new slicer) has made a big difference.

[FatFreddie]

Hi Irving,

Sorry I've been quiet - took a while to get the printer going again and I've been busy - the new firmware needed all the accelerations and speeds sorting as well.

Where do we go from here? Two options I think -

1. I've got a very simple set of gears working which I can send you to have a look at (I gave up on the idea of destructively testing them, the originals weren't far off requirements and these are a lot better). PM me an address if you want to do this.

2. I can just build some gears to your spec - should have some time on Tuesday. If you want to do this, could you send me the latest drawings.

Mark

[irving2008]

Hi Irving,

Sorry I've been quiet - took a while to get the printer going again and I've been busy - the new firmware needed all the accelerations and speeds sorting as well.

Where do we go from here? Two options I think -

1. I've got a very simple set of gears working which I can send you to have a look at (I gave up on the idea of destructively testing them, the originals weren't far off requirements and these are a lot better). PM me an address if you want to do this.

2. I can just build some gears to your spec - should have some time on Tuesday. If you want to do this, could you send me the latest drawings.

Mark

Hi Mark,

Sorry not got back to you sooner, hasn't been a good few days given the heat (people with high-level spinal cord injury can't regulate body temperature easily).

I'm trying to finalise drawings for the gears and have beefed them up by increasing thickness as well as making the ring gear even more substantial to try avoid needing a seperate ally support, but will retain fixings for that in case. Overall assembly needed a rethink as can't get gearbox and magnets on same side of rotor without much more complex magnet carrier so going for magnets on one side only. Fortunately 3 x speed increase plus reduced torque requirement introduced by gearbox means can use smaller magnets anyway. I'll post that up shortly.

Also doing a 3-gear planet carrier to reduce gear tip loading. Going to make the carrier boss more substantial and have 2 grub screws at 90 deg so will give it a go in PLA. Worst case it'll do for testing and I'll get one made in ally later if need be.

Hope I'm not imposing too much.