Thread: Hand Ergometer
OK. Attached are image and DXF file for the 3D printable modded sun gear and rotor boss which now include a seat for a 15 x 10 x 4 magneto bearing. these go either side of the rotor and are joined by 25mm M3 cap headed bolts and captive nyloc nuts. Are these printable?
Crudely each contains 1.25e-5 cubic metres of material, which means 0.02kg of PLA
Last edited by irving2008; 01-07-2013 at 03:50 PM.
Shouldn't be a problem to print those.
The new slicer is better, both finish and sizing.
I'm trying to put something together for some destructive testing now :-)
Don't worry about the plastic usage, I've got loads in (as long as you're not too bothered about the colour).
Last edited by FatFreddie; 03-07-2013 at 08:45 PM.
Once again, thanks for your efforts & interest in this project.
Colour isn't an issue, tho I did read that different colours have slightly different mechanical properties. What colour(s) do you have?
Have new drawing nearly done for ring gear. Had to increase outer rectangle dimensions to make it mountable without machining a big chunk of ally to bolt it to.
Also been considering what else could be printed. Have drawn up planet carrier, the handle cranks and the main bearing supports but need to do some bending calcs to see if they're rigid enough. Will post new drawings tomorrow once completed and sanity checked. Reducing the components that need to be machined reduces the manufacturing costs dramatically.
I've got a variety of odds and ends - mostly silver, black and red but I've probably got enough fluoro pink to do the 15t gears if necessary ;-)
Can't say I've noticed much difference in strength between the colours but some manufacturers are decidedly worse for inter layer adhesion - I've got some that I only use for early prototypes.
Attached picture shows the (very crude) test rig - current limit is the stiffness of the ring gear - it bows out under pressure and skips. I'm printing one with a thicker outer now.
I've also got some issues with the first layer being oversized (the slicers tend to make the first layer slightly thinner to get good adhesion to the bed by squashing the plastic between the head and the base) which should tune out but which would be very easy to fix if Alibre was man enough to let me put a chamfer on the teeth :-(
The 'feel' of the gears is a bit rough (though this will improve), if it's important for it to feel very smooth, maybe some belt drive arrangement would be better - it needn't be any bigger. Alternatively we could use double helical gears which should print ok as long as the helix angle is fairly shallow. Probably best for me to get something working and send it to you for comments - I'm away this weekend but should be able to do some more next week.
Last edited by FatFreddie; 05-07-2013 at 11:15 AM.
The majority of the deflection is where the gear is closest to the outer edge of the piece - the straight edges by the gears become convex and the ones on the side away from the gears become concave - it moves enough for the teeth to slip over the tops. Moving to three or four planet gears may be a simple way to alleviate this without having to make the ring gear much stiffer. Your new design will also be somewhat stiffer - is there much room to play with?
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.
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.
Last edited by irving2008; 06-07-2013 at 01:16 PM.
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.
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.
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