Hand Ergometer

[Clive S]

Just found this to get some ideas Plastic Gears ...Clive

[FatFreddie]

Trying to work out how to draw the teeth for this in my CAD program, but attached is an initial sketch of the epicyclic gears.

Current thinking is I basically want a mod 1.25 60 tooth internal ring with a couple of fixing lugs (the ring is static), a mod 1.25 30 tooth sun spur gear on a press fit 10 x 22 x 6mm bearing (this will be fixed to the rotor which will rotate freely on the 10mm axle using another 10 x 22 x 6 bearing), and 2 off mod 1.25 15mm planetary spur gears on press fit 3 x 9 x 5mm bearings (these will be fixed to a carrier thats connected to the 10mm axle ). Spinning the 10mm axle will spin the rotor at 1+60/30 = 3 times the rate or 180rpm. the torque in the gear train is around 6Nm

That looks ok for printing - probably best if I get the finer extruder going but I've made some gears about that size which work ok. Could the unit be thicker? I'm thinking that it's maybe a bit marginal on strength - probably easiest to just build it and test it than worry too much at this point though.

If you (or anyone else) can point me towards a gear profile calculator (can't find one that does the outer ring) I'll knock up something for a strength test.

[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!

[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...

[irving2008]

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.

[FatFreddie]

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.