Another way to take drive through the normal chainline

[bobc]

OK I just spent £91 on bearings...... I had a couple of minor redesigns to do last night:
1) triplex chain loop must (obviously) have an even number of links.... DOH!!!!
2) means to grip the crank axle shaft considering it is far too hard material for me to drill it....
So [1] means I now have a 78 link loop - gear ratio 13.5:1
2) I will grind a couple of grooves in the axle & pop a 3mm ball bearing in, locating a matching bump in the sun sprocket. The whole will be squeezed on each side by tapered inserts locating the planet carrier sprocket bearings. phew. So there will be a couple of mechanical keys AND a couple of taperlock inserts - should be sufficient I reckon.

[bobc]

And... minutes before dropping the dxf's at the laser cutters, I've decided to change it all....
1) there's no need for triple planet with the binary planet system - I can use 2 and get a better reduction ratio
2) I don't like chains for very high speed work as they throw off all their lubricant....
So redesign (if clearances allow) will have dual binary planets, 76 tooth sun and 80 tooth ring, and a pair of 5mm HTD belts driving it (9mm wide)
I'll put a drawing up tonight ;^)
(note the harmonic chain loop is not moving fast so it should stay lubricated.....)

[Miles]

................., I've decided to change it all....

I usually do that just after I've ordered the stock....

[bobc]

OK here's the revised assembly - now with toothed belt drive...

The weird cutouts in the planet carriers are so I can grip it in my 3jaw chuck
Unfortunately the belt drive starts to make it a bit wide, but I should be able to make it work.... (I hope).
I'll get the order into the laser cutters tomorrow ;^)

[Miles]

[bobc]

Progress - a full set of bearings arrived today; I got the crank axle the other day & have made & rivetted the triplex chain loop.
The dxf files are at the laser cutters (should get the parts next week).
I went for stainless for the sun and ring gears, mild steel for the others: I'll get them zinc plate/gold passivated after machining for a bit of bling...
An unexpected expense was the cranks - cheapest I could find was £69 a pair - OUCH
Because this reacts back through the pedals, I'm going for a smaller motor/ESC setup (I had earmarked a rather large motor for it - but it actually lends itself more as a "performance improver" than an out and out electric bike drive) I think I've persuaded my mate Rich to be the guinea pig with his recumbent... watch this space ;^)

[Whiplash]

Awesome! So what kind of ratio can you get with this? I am anxious to see the finished product!

[bobc]

Hi Whiplash,
the present design is 19:1 down from the toothed pulleys to the big ring; it should be possible to get a further 10:1 or so from the motor. The pedals are geared down by 5% to the big ring, but that is a big ring with 62 teeth. So as a bike it is probably geared slightly higher because of the big ring. I should get the lasered parts on Wednesday; I do have some other bits to make on the lathe - I'll make a start tomorrow ;^)

[Whiplash]

Interesting! So what about pedal cadence? Will it be really high or low due to this setup? What about freewheeling? Will you need to pedal or can you coast while the motor is running?

[bobc]

The pedal cadence/gearing is an interesting compromise requirement. At the moment I'm thinking that the motor should supply only enough torque that can be comfortably held by the cyclist, so more motor power will be translated to higher speed (electrical) in a given gear. Gearing for the human is pretty much unchanged. The user can pedal at whatever cadence they want - their input is added to the electric motor input, by way of adding directly to the speed.
If the cyclist allows it, the electric motor will power the pedals backwards rather than the bike forwards.... Opportunity to fall over while frantically backpedalling, or repeatedly bash your shins.....
BTW I just got a very nice set of lasered parts for 40 notes - I forsee an evening on the lathe...... Oh and I've reduced the width overall by 6mm now I've looked at the real thing rather than the CAD model: - you can't beat hardware....
Stop press - I put the wrong number of teeth on the sun gear. Pillock. Replacement should be ready this week....I'll replace the downloaded drawings (eventually....)

[bobc]

Stuff arrived in the week; I've started assembly. So far so good, it all goes together, the triplex chain is tight, heres a photo. I got some seamless (DOM) tube to make the planet carrier bearing supports and some 6mm plate to make the tapered inserts; I forsee some more lathing sunday afternoon (kinks)

[Whiplash]

Looking great! Very anxious to see it in motion!

[Green Machine]

Omg this is sick...

Things are coming together real quick! Really amazing stuff.

[dbaker]

Can't wait to see this working

[bobc]

OK I've done a short demonstration video; only one side is on and none of the locating bearings and it's all new and stiff, so it's a bit clunky but you can see the unfeasably large reduction ratio (!). It will be a while before the target bike is ready unfortunately.

My impressions from doing the test on the lathe are that it will all be rather too "busy" if I'm looking to get output speeds up to 100rpm. I reckon I should probably halve the reduction ratio from 20:1 down to 10:1 - this will make all the gubbins 1/2 size and they will run at 1/2 the revs. And put a greater proportion fo the "gearing down" in the primary timing belt between the motor and the gearbox/chainring. I'll get this one going first though ;^)

[Green Machine]

Holy jeebies!!!

Well done bob!

Amazing to see you go from concept, to drawing to working prototype so fast...

Really a beautiful piece.

I cant wait to see how this develops given your not even one month in.

[Whiplash]

Awesome! What keeps it all together when you are standing on the pedals...

[bobc]

Ball bearings fixed on the crank axle Whiplash - they should control positions of planet carrier & therefore outer ring rather better too. I'm pleased with the way the outer ring is solidly located (it floats on the chain loop. Thinking about the size and gearing I've been idly sketching up mk2 - this is just 10:1 reduction & thus much smaller. Now I can hide all the workings inside the toothed belt primary drive pulleys - here's the sketch.

I think this is much more practical: I'll need 6:1 or more reduction via the toothed belts but I don't think that's a problem...

[Miles]

I think this is much more practical: I'll need 6:1 or more reduction via the toothed belts but I don't think that's a problem...

It would make it more attractive if only one other reduction stage was needed. If you can get a 12:1 ratio, it would make that more doable, perhaps?

[spinningmagnets]

On the first page I had a feeling this was more of a theoretical discussion about an obscure option. The video of the prototype not only shows that the concept does work, it also shows how viable this is as a real-world option. This is a great amount of reduction in a very compact package!

[bobc]

Thanks for the generous comments ;^) Miles - I'd say any ratio you want between 10 and 20:1 is pretty easy. Spinningmagnets - what I like is how accessible it is - I made that in a fortnight by walking down to the local laser cutting shop with a pen drive in my pocket with dxf's on it, bearings off the web & an hour or two on my lathe. A spur or planetry gear would need more than one stage to get that degree of reduction and is an expensive technology way outside the scope of the amateur tinkerer. I'm fairly happy with it; I don't think the mk1 is a practical solution;- too much metal flying around too fast... but I think the mk2 will be. And the way the whistling input and wobbly chain motion translates to such a smooth slow output drive is, well it's just cool!

[Solcar]

It's really cool to see this approach actually built. Thanks for the video.

[amberwolf]

I love watching that in operation. I'm tempted to put one on my list of projects to do by hand, so that I can use it as a reduction drive on a treadmill motor for one of the bikes.

[bobc]

New drawings taken to the laser cutters today.
This is what I'm aiming for

There is 10:1 between the timing pulleys and the chainring, with the harmonic stuff all protected inside the timing pulleys. Plan is to clamp an outrunner to the seat stay driving the timing pulleys directly, giving me an overall 70:1 reduction.
Yes I've ordered the big chainring is stainless - should look good ;^)

[Miles]

Can't wait to see this one in action...