whisper mode!

just reeling you in with Blue Thunder references...

looking for a really quiet, low-rpm DC motor (quieter than a Reliapro). Anybody used one?

just reeling you in with Blue Thunder references...

You're either geeky, old, or more likely - both.

;D

I suppose that makes me the same, though - given that I remember when Blue Thunder came out...

;)

Regarding your question: Prepare to spend some money.

There are two main factors in motor design that determine how loud it will be when running: Gear type, and cogging.

You didn't give a model # for the gear motor you were talking about; I dug up this:

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=176050

Whether it is similar or not, I don't know - but it looks like most other similar low-speed, high-torque gear motors, which use a transmission typically composed of spur gears (generally metal or high-strength plastic).

Spur gears are loud, but inexpensive. What you want is a transmission that uses helical gears, which tend to be much quieter in operation, but more expensive to purchase. Since you'll be spending the money anyway, you might as well add an all ball-bearing construction to the list, too, as oilite bushings can be (or can get) noisy as well.

There are other methods to the transmission design that can quiet it as well - harmonic drives tend to be quiet and high-torque, but they aren't cheap or easy to find; there's also the possibility of a transmission having sealed fluid or replaceable fluid that can quiet the part (but you don't typically see that in small gearmotors).

Cogging occurs because of the number of rotor arms in relation to the number of magnets, and how they are oriented to the shaft. Permanent vs electromagnetic stators will also play a role; most small DC gearmotors use permanent magnets on the stator, though. You might also be able to find a brushless DC gearmotor, but it will require a proper driver module to run it (note that I am not talking about an R/C brushless motor - there are industrial DC gearmotors which use brushless motors - expect to pay big $$$ for the motor and driver module).

The number of magnets in the stator, and the relationship to the number of magnets in the rotor, both play a part in how loud the motor will be; typically, though, you will see two, sometime four, magnets (PM) in the stator, and 3-5 arms/poles on the rotor in a fairly quiet motor (odd number of poles on the rotor to an even number of poles on the stator reduces cogging; increasing the number of poles on either does the same).

The orientation of the rotor and/or stator poles in relation to the shaft also determines how much cogging will occur; you can find staggered or helical position rotors/stators (they look like they have a slight "twist" to them on the shaft). I would be also willing to bet that there are certain winding styles and counts that also come into play.

Another thing to look for is "coreless" motors - these have a rotor generally composed wire on a plastic or other non-ferrous form, forming a lighter weight rotor with (generally) high-torque to weight ratio. Lower mass helps reduce the noise (vibration mainly).

All of these special items in a motor's construction will add up very quickly; top it with the special gearbox, and I wouldn't be surprised if a motor the size of that Reliapro didn't clock in at well over $200.00 (for that matter, I've got some nice spur-geared Pittman DC gearmotors that cost over $100.00 new - so my guesstimate might be off by a bit). It will be quiet though. I am not sure it is worth it, depending on the application, though.

Ha! well i suppose i asked for that one. Thing is, i'm as geeky as i'll ever be, while the old just keeps piling on...

thanks very much for the insight, cr0sh. i knew a few of those pieces, but it's nice to have someone line it all up. the jameco motor you found is pretty much the same as the ones i've been using. i have a couple of dayton DCs with the 3" square gearbox that are really quiet, but they're a bit big for this application.

i don't mind paying more for the quieter helical drives, but having a hard time finding them definitively labeled by sellers, or below 3-digit rpm's. what do you think of the top-left one here? http://www.surplustraders.net/a/0184.shtml

do you know any suppliers, or maybe ways to recognize these if not labeled directly when sold surplus?