Self-locking gears prevent backdriving (2012)
gearsolutions.com
gearsolutions.com
A normal involute spur gear pair is about 97% efficient. It's a strictly rolling contact if everything is set up correctly.
While these arrangements are far, far better than the 30% or less I'd expect from a worm gear, that's a LOT of energy to lose as heat and wear instead of having a separate brake mechanism.
You'll need about 5 times the cooling compared to a normal gearbox using this arrangement. I'd expect the gearing lifetime to also be considerably shorter.
I wonder how wear affects the reliability though?
Worm gears are generally not back-drivable (aka self-locking), which is a good feature for a guitar tuner (you don't want the string to unwind itself), and it's why most guitar tuners are approximations of worm gears.
One limitation of worm gears is if you want to make them compact for finger operation, I think you're basically stuck with the 90 degree input/output shaft arrangement. I'd like to experiment with different orientations so I went looking for other self-locking gear types, and found this page.
It's definitely outside my wheelhouse but I thought it was really cool to learn that the locking feature can be achieved basically by varying the shape of the teeth.