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Posted by Gordon McComb on April 20, 2007, 1:29 pm
Joe Strout wrote:
> 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?
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
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Posted by John Nagle on April 20, 2007, 11:20 pm
Gordon McComb wrote:
> Joe Strout wrote:
> 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.
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
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Posted by BobH on April 19, 2007, 7:06 pm
Joe Strout wrote:
> I have two largish (15 cm) surplus Pittman motors, which I'm trying to
> cram into a circular shell 24 cm across in order to power the two
> primary wheels (for a differential-drive robot). Obviously I can't just
> lay them out side by side; I'm going to have to have one motor in the
> front, and the other in the back, which means connecting to the wheels
> via gears, chains, or belts.
>
> Of these options, I like gears best and chains least. But now I'm at a
> loss -- where would I find reasonably-priced gears beefy enough to suit
> these motors? I'm expecting this to be a fairly heavy robot, which is
> why I'm glad to have such strong motors. The axle-to-axle span of these
> gears only needs to be about 4 cm, but it's no good if they're skinny
> little axles that introduce an obvious weakness compared to the rest of
> the system -- the gears built into the motor assemblies are all 5 mm
> thick or more.
>
> So, in brief: I need (for each wheel) a pair of gears with a total
> radius of 4 cm or so, and a thickness of 5 mm or more. (I'll also need
> fitting shafts and bearings, but presumably those are easier to find
> than the gears themselves.) Any suggestions?
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
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Posted by Joe Strout on April 19, 2007, 8:19 pm
> 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.
Thanks, that's some good insight. I'll give it some thought.
Best,
- Joe
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Posted by on April 21, 2007, 4:06 pm
> I have two largish (15 cm) surplus Pittman motors, which I'm trying to
> cram into a circular shell 24 cm across in order to power the two
> primary wheels (for a differential-drive robot). Obviously I can't just
> lay them out side by side; I'm going to have to have one motor in the
> front, and the other in the back, which means connecting to the wheels
> via gears, chains, or belts.
> Of these options, I like gears best and chains least. But now I'm at a
> loss -- where would I find reasonably-priced gears beefy enough to suit
> these motors? I'm expecting this to be a fairly heavy robot, which is
> why I'm glad to have such strong motors. The axle-to-axle span of these
> gears only needs to be about 4 cm, but it's no good if they're skinny
> little axles that introduce an obvious weakness compared to the rest of
> the system -- the gears built into the motor assemblies are all 5 mm
> thick or more.
> So, in brief: I need (for each wheel) a pair of gears with a total
> radius of 4 cm or so, and a thickness of 5 mm or more. (I'll also need
> fitting shafts and bearings, but presumably those are easier to find
> than the gears themselves.) Any suggestions?
> Thanks,
> - Joe
why not look on ebay?
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> 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?