At my school, Kalamazoo Valley Community College, they have a wind sail they made and have been debating the best way to transfer the power from the sail to the generator. It is a low RPM application at under 100 RPM. Currently they use a 10sp cassette and derailleur but there are problems. Mainly is it in a smaller space and shorter chain allowing only three gears to be used. They also want to use a belt over a chain for better efficiency, but common industrial pulley's don't work for this.
I suggested using an internal geared hub with three or more speeds. I also looked into the belt-drive system by Gates (are there other bike belt manufacturers?) but it's very costly.
Any other ideas are welcome for this. It's a long-term project so I can't say when or if it will be implemented.
There's a few real engineers on this list so hopefully they'll have some good input. But I can tell you this:
- internal geared hubs are a little less efficient than external gear systems (5%-10% ?) So, good idea, but it would set you back a little right away.
- Again, I'm not an engineer, but I'm not so sure belts are inherently more efficient than chains. I think it depends on the belt/pulley interface design.
What might help is if you could tell us what kind of gear ratio range you need. There are internal gear hubs with pretty wide ranges. If the range is critical, it might be worth the small efficiency loss. Also, what is it about the small space that limits how many gears you can use on the current set up? Are the two chainrings so close that you get an unacceptable chain angle? Note that a steep angle on the chain is also going to introduce some drag which might point you back to going IGH anyway.
Thanks Dave for the reply. Yes, according to industrial standards belts have higher efficiency (98% minimum) compared to chains (around 95%). The spacing issue is exactly as you describe with a poor chain angle because of distance between centers. I'm not sure about going hybrid with an IGH considering the spacing for more moving parts (i.e. multiple shifters).
The generator itself can be cranked by hand and at higher RPM's around 60 it produces a lot of drag hence where gearing comes in handy. I'll dig for some more specifics on the ratios from sail to generator.
The Gates belt system we found wouldn't work because it is a unique pulley/belt combination and belts are only produced in cycling lengths.
Nigel- The primary reason not going with direct drive is because of radial loads on the shaft from wind gusts. Also, using a variable gear drive allows for more efficient options of power transfer considering the sail is low RPM.