Re: source of largish gears?

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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