Thread: Hand Ergometer
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.
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.
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.
No rush on any of this from my perspective but I'm away for the middle two weeks in August and will be busy in the run up and I don't want to become the bottleneck.
Not been a great week what with heat and other issues but trying to get back on track.
Been prevaricating over how to retain bearings in the planet gears The revised gears are 10mm thick and the bearings I'm going with are 3 x 9 x 4. The planet carrier has a 3mm boss and with a 3mm caphead screw as an axle the bearing will be centrally located in the gear against a recess. but that doesn't stop the gear sliding off the bearing. Normally you'd have some sort of bearing retainer but the only way I can see to do this with a 3D printed gear is either a groove 'not printed' to take a circlip which means an overhang (do-able but not ideal) or some sort of printed protrusion which will deform enough to allow the bearing to be pressed into place (is this possible?).
Just been having a look @ your project and although you have expended considerable man hours on the prototype, i was wondering if you had given any consideration to using a Shimano/sram geared rear hub ?
Just a thought.
Take care and i sincerely hope your circumstances improve in the near future.
Last edited by mekanik; 11-08-2013 at 11:20 AM.
Would the ring gears on the e-bay links not be better than plastic ?
those listed are cheap enough.
Could you use two spur gears with the correct ratio ?
Last edited by mekanik; 11-08-2013 at 01:30 PM.
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