source of largish gears?

Posted by Joe Strout on April 19, 2007, 1:36 pm

> This is where a good local surplus outfit comes in handy. Most areas
> have at least one, but they don't always stand out. You gotta go looking
> for them.
>
> Failing that, check out the surplus mail order like AmSci, H&R, C&H,
> etc. I like to buy assortments, if they're cheap enough, for the junk
> box. I've never had to buy a new gear from SDP, Boston, or their ilk.

Good tips, thanks.

> Lastly, venture into ye good olde hobby store, and check out the
> replacement gears for electric R/C airplanes and cars (like the Traxxis
> E-Maxx). These are going to be molded nylon, but well made. I use a
> Traxxis replacement gear that retails for about $3 in stores in my 360
> turret: product:
>
> http://www.budgetrobotics.com/shop/?shop=1&cat 7

That's a neat product (and a neat idea). I'm starting to cool on the
idea of constructing my own gear train, though. I'm just not all that
mechanically adept -- the theory is easy, but the practice is hard for
me. If I can find a decent gearmotor (something strong with about 60
RPM of speed) that will fit into my case, I'll probably just buy a
couple of those instead.

On the same page as your nifty turret, I see a box gearmotor that looks
promising -- but it's hard to justify even a $6 purchase without any
idea how much torque it'll have. Couldn't you rig up something where
the motor pulls a spring until it stalls, and then you just read off the
torque from the spring length (having cleverly calibrated this
beforehand)?

Best,
- Joe

Posted by Gordon McComb on April 19, 2007, 4:41 pm

Joe Strout wrote:

> On the same page as your nifty turret, I see a box gearmotor that looks
> promising -- but it's hard to justify even a $6 purchase without any
> idea how much torque it'll have. Couldn't you rig up something where
> the motor pulls a spring until it stalls, and then you just read off the
> torque from the spring length (having cleverly calibrated this
> beforehand)?

For surplus motors that I don't have many of it's not really worth it to
build the rig. Static torque isn't all that worthwhile of a measure
anyway. For gearmotors it really needs to be done with a dynamometer.
Buy me one and I'll be happy to take the measurement for you!

You can guestimate the torque, BTW. One implicitly knows it uses a small
PM motor (Mabuchi FA-130 or equivalent) that operates in the 7-9K RPM
range; you know the final output speed; and you know the current draw at
that speed. I can do the guestimate, but I can't prove it, so I don't
publish it. I leave that as an exercise to the reader, as they say.

The other sources I listed typically have a good number of DC gearmotors
to chose from. As they are in the business of selling these kinds of
things they often have specs.

-- Gordon

Posted by Joe Strout on April 19, 2007, 6:03 pm

> For surplus motors that I don't have many of it's not really worth it to
> build the rig. Static torque isn't all that worthwhile of a measure
> anyway. For gearmotors it really needs to be done with a dynamometer.
> Buy me one and I'll be happy to take the measurement for you!

Heh. I had to go look up "dynamometer." Thanks for introducing me to
something new today!

> You can guestimate the torque, BTW. One implicitly knows it uses a small
> PM motor (Mabuchi FA-130 or equivalent) that operates in the 7-9K RPM
> range; you know the final output speed; and you know the current draw at
> that speed. I can do the guestimate, but I can't prove it, so I don't
> publish it. I leave that as an exercise to the reader, as they say.

I think I'm still missing some critical formula. One that sounds very
handy indeed! Let me see if I can infer the general principle: the
current being drawn is a combination of speed, torque, and waste (noise
and heat). For an estimate we probably assume that waste is either
negligible, or factor it in as some reasonable constant. But I still
don't see how to relate current, speed, and torque.

Just taking a wild stab, I'd expect current to be proportional to both,
which suggests I = Torque*Speed. Unit check: (N m)*(rotations/sec)
gives, er, Watts, not Amps. So the units don't work out.

I'm making an effort here, but I'm still a clueless newbie... can you
give me a hint?

> The other sources I listed typically have a good number of DC gearmotors
> to chose from. As they are in the business of selling these kinds of
> things they often have specs.

Yes, I'm finding that about half the places do, and half of them don't.
All Electronics, in particular, has a nice selection, but no torque
ratings on any of them. This makes it very hard for me to know what I'm
getting for my money. Should I generally assume that for the same RPM,
a higher current draw means a stronger motor? Or is that just going to
get me a noisy, hot-running one?

I don't need an actual number, since I haven't any idea how much torque
is required for my bot anyway (it's modular, and modules will come and
go frequently, changing its weight from day to day). I just know that
stronger is better, and I have only so-much money to spend, so I just
need a way to compare two motors of similar cost and RPM to know which
is the better value. Is there any trick you guys use when torque isn't
listed?

Thanks,
- Joe

Posted by Joe Strout on April 19, 2007, 6:23 pm

> Should I generally assume that for the same RPM,
> a higher current draw means a stronger motor? Or is that just going to
> get me a noisy, hot-running one?

Here's a case in point. Check out this Faulhaber 141:1 gearhead:

http://www.goldmine-elec-products.com/prodinfo.asp?number=G16279

This is described as "features high torque." But it's only drawing 20
mA at an RPM of 80. That's really low current -- compare it to Gordon's
DCM-210 gem, with a no-load RPM of 100 and a current draw of 150 mA:

http://www.budgetrobotics.com/shop/?shop=1&cartB0202&cat 6&

...or this Jameco motor, with a similar gear ratio and RPM, but a
current draw of 630 mA:

<http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1

&storeId001&catalogId001&productId@1007>

Both Gordon's and Jameco's models are half the price of the "high
torque" one at Electronic Goldmine, and draw a lot (or in the Jameco
case, a LOT) more current at similar voltage and RPM. What should a
poor newbie make of this? Is the "high torque" description of the
Faulhaber mere marketing puffery, and most likely these other two motors
have substantially more? Or is it twice as expensive because it somehow
outperforms the other motors while drawing a lot less current?

Gordon, I'm not trying to put you on the spot -- I love your stuff (have
shopped there in the past and will do so again), and I wouldn't ask or
expect you to badmouth the competition. But maybe you (or anyone else)
can speak in general terms... how would YOU go about comparing these
motors, which look similar to me in all respects except current draw?

Thanks,
- Joe

Posted by John Nagle on April 19, 2007, 8:38 pm

Joe Strout wrote:

>
>

>>Should I generally assume that for the same RPM,
>>a higher current draw means a stronger motor? Or is that just going to
>>get me a noisy, hot-running one?

>
>
> Here's a case in point. Check out this Faulhaber 141:1 gearhead:
>
> http://www.goldmine-elec-products.com/prodinfo.asp?number=G16279
>
> This is described as "features high torque." But it's only drawing 20
> mA at an RPM of 80.

That's the no-load current. What you want are numbers like the stall
current or a graph of current vs. torque.

That Faulhaber unit is a modestly-priced motor
with a planetary gearhead, so it doesn't need much current to
run with no load. Here's its data sheet:

http://www.faulhaber-group.com/uploadpk/e_1524SR_DFF.pdf

Rated no-load current is 21mA. Full load current is 630mA.
Full load torque (at the motor shaft, not the gearhead)
is 2.5 mNm, at 10,000 RPM. With a 141:1 gearbox, at the output
shaft expect 352 mNm at 71 RPM, less frictional losses of maybe
35% in the gear train.

The gearhead is a separate component bolted onto the motor.
Plastic gears, incidentally; this is a light duty unit and
rated only to 5,000 RPM (input) and 150 mNm torque. So the gearbox
is weaker than the motor here.

So that's your answer. That drive is good for 35 RPM outputting
100 mNm torque continuous, about twice that intermittently.

100 mNm means 0.1 Newton-meters, or about 14 inch/oz. Which means
that if you put a 1" radius wheel on the thing and wrapped a cord
around it, it could lift 14 oz, a little over a pound.

John Nagle

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