Turn at a constant speed, better to use gearing or H bridge?

[corrado33]

I have to turn something at a relatively slow, constant 7.5 ish RPM for long periods of time (1 hour at a time) and was wondering something.  Obviously I'm using a geared DC motor, but most of them only go down to about 30 RPM or slightly lower.  (Except for the one motor with a 1000:1 gearbox I can find)

Anyway, my question is this.  Is it better to use gearing or an H-Bridge to slow it down even more?  The project already has an MCU, so adding an H-Bridge wouldn't be hard.

If gearing is the answer (which is suspect it might be), what would be better for very quiet operation, spur gears or timing belts with pulleys?  I know straight cut gears aren't usually the quietest things in the world (have you ever heard a racecar transmission), but does the added complication of the belts provide noticeable sound dampening at such low speeds? 

Hm, it seems like it'd just be easier to buy the higher ratio gearbox instead of trying to mess with either....  (But the reason I'm asking is because that motor/gearbox is more expensive.)   But with the added costs of the gears.... dangit why do I always work out my questions when I'm typing up a topic....  Anyway, I would still like to know which is quieter. 

[mstacho]

FWIW, if you have a speed sensor on the motor, it's probably a thousand times better to use the H-bridge (AND the gearbox?) that  way you can be assured you're at your 7.5 RPM.  Unless it's not critical?   What happens if, instead of the, say, 6V you were expecting, you get 6.2?  Your motor will run faster unless you have a controller and a sensor...

[corrado33]

Meh, speed isn't critical.  Anywhere around 7.5 RPM is fine.  That's just the target.

For the record, I figured out my problem, I was calculating gear ratios wrong (silly me).  It's always something simple.  I can actually accomplish what I want with two simple pulleys and a belt. 

But the question still remains, which is quieter, gears or belts?

And which is better to use, an h-bridge or gearing?

[mstacho]

haha neat.  If I were to answer the last question, I'd have to say both :-P  A simple solution like you found just needed the gear ratio, but I use H-bridges a lot to control the speed.  Plus, with H-Bridges (and sensors...) you get a range of speeds, while with gearing you only get one (or as many gears are in your transmission...).  And don't forget that most applications with DC motors use both :-P

MIKE

[jkerns]

Does it need to run both directions or just slower?

An H bridge is used when you want to run both forward and reverse - otherwise all you need is a single power device that you can control via a duty cycle output from your micro.

Or just run it at a lower voltage.