pic: 2451 PWNAGE Planetary Drive

[Mr V]

Originally Posted by JohnFogarty

Besides weight saving and real-estate gain. Are planetary gearboxes more or less efficient than regular non-planetary gearboxes?

Originally Posted by pwnageNick

Generally they are less efficient. Due to this being a custom setup, to know what the efficiency of this setup would be a prototype would need to be built to find that out. A prototype would also test any issues with loads, and how stable the wheel would be.

In general a planetary reduction is less efficient than a spur gear reduction. However this set up isn't really a planetary reduction and should have a frictional loss essentially a simple spur gear reduction.

The extra frictional losses occur in a planetary reduction because of the increased number of gear to gear interfaces.

In your basic spur gear reduction you have 1 gear to gear interface per stage. With a planetary set up you have 6 or 8 gear to gear interfaces as well as the friction of the pins that the planets rotate on.

This set up however does not have a sun nor really any planets, it is just the driven gear turned inside out.

[Cothron Theiss]

Originally Posted by Mr V

In general a planetary reduction is less efficient than a spur gear reduction. However this set up isn't really a planetary reduction and should have a frictional loss essentially a simple spur gear reduction.

The extra frictional losses occur in a planetary reduction because of the increased number of gear to gear interfaces.

While what you've said is accurate for these purposes in a practical setting, if I understand some of the finer points about involute gear profiles correctly, a ring gear is always slightly less efficient than a regular spur gear. Since the gear teeth of the ring gear are directed inwards, there is an increased amount of interference, which would lead to SLIGHTLY less efficiency if you use the exact same gear teeth profiles and spacing as a regular spur gear reduction.

Now, you can regain this loss in efficiency by increasing the amount of cutout in the ring gear (changing the pressure angle), or decreasing the center distance between the ring gear and pinion gear. But since both of those solutions lead to less load carrying capacity in the gear mesh, it's a trade-off. Since the OP is going with a really interesting combination of the wheel and gearbox, I would NOT risk decreasing the load carrying capacity to regain a small percentage of efficiency.

***Disclaimer***
I may be totally wrong about all of what I just said, but that's what I think is accurate.