source of largish gears?

Hey Joe. The best source I have found so far is Stock Drive Products, =
found at: https://sdp-si.com/eStore/  . They're the *only* place I have =
managed to find both matching timing pulleys and belts so far.

McMaster-Carr is also a good place to look & get educated, if nothing =
else: http://www.mcmaster.com/ . So is Stock Drive.

Hope that helps !
JCD

    SDP is more for small stuff.  As you move up in size, you
go to Boston Gear (www.bostongear.com).

    Take their "Gearology" on line course:

http://www.bostongear.com/pdf/gearology/all_gearology-chapters.pdf

    If you take the quizzes and pass, they send you a "Gearology"
certificate.

                    John Nagle

Thanks for the tips, guys.  Boston Gear looks like they may have some
nice stuff, but geez, I've never seen a company make it so difficult
even to find out exactly what products they have.  Fortunately there's a
local distributor nearby -- I'll go in today and look at a catalog.

Best,
- Joe

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.

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

They do have some with larger diameter, but the thickness is about what
you're looking for.

-- Gordon

Good tips, thanks.


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

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

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


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?


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

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

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

    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

Higher quality motors are more efficient, sometimes significantly so.
Faulhaber motors are higher quality. If you can afford those, get them.
(Of course, know they're still surplus. New Faulhabers cost $100+.)
Another advantage is that the manufacturer of these typically provide
torque ratings, sometimes right on the case of the motor.

The point is if you know a little about the basic motor used in a
gearbox, you can at least approximate the output torque. Torque/current
curves are predictable. Less predictable is friction loss in plastic
gearings, but these can be approximated. I was being a bit faceteous
about "knowing" the motors in the box gearmotors were Mabuchi FA-130s or
equivalents; if you know your toy motors, you know this motor simply by
appearance. In any case, you can take it as gospel that the motor is an
FA-130 or reasonable equivalent.

Because the FA-130 is probably the second most commonly used small PM
motor in the known universe  for toy products you can often go by
empirical comparison, and/or very basic calculations. If the no-load
speed of the FA-130 is about a conservative 9000 rpm, and the no-load
rpm of the gearbox is about 100 rpm (itself an approximation; I didn't
use a tachometer), that's a 90:1 ratio. The FA-130 has a stall torque of
36 g-cm at 3 volts. Complicating things is that I tested the motor at
4.5 volts, which is a more common voltage when using a simple H-bridge
like the L293; the toy these came from operated at 6 volts. You can
apply some simple math as you calculate values at the intended voltage
(and current) you will be using.

In the end the *hobby* of robotics means the adventure of
experimentation, especially as it relates to surplus.

-- Gordon

    Those are low end Faulhaber motors.  They have bronze sleeve
bearings and plastic gears.  They're intended for light load
applications, even though they have largeish bevel gears
stuck on the output end.

                John Nagle

You might consider toothed belts or timing belts. They are more tolerant
of inaccuracies on spacing (get a longer or shorter belt) they are
quieter than gears and you don't need to pick a gear that supplies the
mechanical spacing dictated by the rest of your design, just get a
different belt.

For bearings, you might try Enco: http://www.use-enco.com   They have
ball and roller bearings at pretty reasonable prices. They also have a
lot of tools.

I built a differential drive bot with one motor in front of the shafts
and one behind the shafts. In addition to the gearmotors, I had encoders
on the back of each motor so they were about 4cm longer. It worked out
really well. I used toothed belts from SDP for the drive.

Good Luck,
Bob

Thanks, that's some good insight.  I'll give it some thought.

Best,
- Joe

why not look on ebay?

id look on ebay ifin i were you