I got the Pololu 37D gearmotor (70:1) and it works well. I made a 3D model of the motor, and a bracket to hold it, if anyone is interested:
My problem: I find this motor is rather noisy. I took off the gear train and I found that the gears by themselves (at low speed, anyway) are nearly noiseless. It seems to be the small spur gear on the motor shaft itself that makes most of the noise, of course that is the part that runs at the highest speed. Is there any option for a similar gearmotor (eg. 150 RPM @ 12V) that is quieter? That would mean, I assume, either a helical gear, or a worm gear.
Maybe I can make something using pulleys; that would probably be quiet.
If you are using a PWM-based motor driver to control the speed and it is capable of higher frequencies, you might try using an ultrasonic PWM frequency (e.g. 20 kHz). However, if the noise is really a mechanical noise (i.e. it still makes noise when the motor is connected straight to a power supply) then we do not really have any suggestions for you.
By the way, cool bracket and motor model. I also liked the universal joint with minibearings that you made.
Just as a followup- I found even some nice Swiss-made gearmotors (escap, maxon) are sort of noisy, particularly at speed and under load. There is almost no noise from pulleys and a GT2 timing belt, but of course a large reduction ratio takes many axles and a lot of space. Helical gears are supposedly quiet, but mostly un-affordable.
Your update was unclear if the nicer motors are quieter. My experience has been that they are much quieter but not silent. I suppose it’s just higher quality gears with less clanking, and often- shit- the term I suddenly forget…But gears that are engaging not as spur gears but as a gear within a circle of gears…:O, kind of surprising I forgot at the moment but I dont have time to google. They have more point of gear engagement though so that can also dampen the sound.
The higher-price motors are mostly only for one-off projects though, and can be hard to source specific ratios. but you can get pulled motors in great condition on ebay. Things that retail for $100-300, for $10-30.
The only other thing about the swiss/german/high-end motors is that they sometimes have low inductance, so you will want to run them with high frequency PWM and add a choke inductor in many cases.