How to combine the power of two rods spinning at a different speed into one rod spinning at a combined speed?

Question

I have two rods (that will be powered via something like two water wheels), and I need to combine their power so they both rotate the same rod. Sadly, they may spin at different speeds, and the difference may be more than a little bit (and may change), so I don't think a differential as proposed by this question would work. But, I have a shaft that I'd like to rotate using these two rods. I don't really care if the power from the two rods is averaged or summed, as I can use gears to adjust the torque/speed ratio if needed.

So, given two rods spinning at two different rates, how can I combine their speed?

Answer 1

You could have each rod drive a generator and then combine the electrical output of the two generators.

Answer 2

Use adjustable gearing combined with a pawl and spring coupling similar to a bicycle casette.

When the target output spins faster than the input, the ratcheting mechanism lets the output freely (a bit of energy lost from the ratchet) turn. You'll have to figure out the adjustable gearing bit and how to increase/decrease the ratios based on what is going on.

Although with adjustable gearing, you could just as easily use a differential. Benefit with bike parts is that they usually also have a nifty gear ratio adjustment mechanism from the sprocket and chain setups that you might be able to use to your benefit.

As for alternate adjustable gearing systems, I tend to like one that uses a V belt pulley system - spring loaded tensioners and pulley composed of a geometry that can be described as two intersecting cones.

Answer 3

I'll be combining the two other existing answers (so far).

If you are interested more in ease of implementation and low cost then the way to go is connect each rod to a generator produce electricity and then use a motor to get the power at the rpm you want (Eric S solution).

If you are interested in a solely mechanical system or if (at a lesser extent) efficiency is important, then you could design a mechanical system with a planetary gear and a continuous variable transmission --CVT-- (if you need a wide range or rpm). (basic idea behind Abel's solution)