# Need help selecting a motor

4 views

### Ed

Aug 19, 2005, 12:25:39 AM8/19/05
to
I am building what is essentially a motorozed clothesline. The length
of the clothesline is 5 ft long (~10 feet of rope).

I intend to connect a DC motor (reversible) to the axle of one of the
spindles to make the clothesline move forward and back. The
clothesline will not have more than one item on it and the items will
weight less than 1 pound. The item on the clothesline should be able
to move from one end to another in about 4 seconds. The diameter of
the spindle is 2.25 inches. As such, the RPM needs to be about 127 RPM
by my calculations. The motor need to be small and light (less than a
pound, hopefully just a few ounces) and hopefuly less than \$20US, and
can be up to 12VDC.

I tried this with a motor I removed from a tape deck, and it did not
have nearly enough torque to get the job done.

My questions are:

1) Does a motor exist that fits my requirements? What torque do I
need?
2) Where can I buy just one?
3) How can I best connect the spindle (which has a .25 hole in it) to
the motor shaft?
4) Is there any type of motor I should use or stay away from (stepper,
etc)?

Ed

### D Herring

Aug 19, 2005, 2:18:02 AM8/19/05
to
Specs:
<\$20, 130 rpm, 12V DC motor to move 1lb by 5 feet in 4 seconds.

Calculating torque requirements:

Force=mass*acc -> acc=Force/mass; assuming constant force, vel=(F/m)t
and pos=(F/2m)t^2. Solving for Force gives Force=2*mass*pos/t^2
Note that "1 lb" is really 1lb of force (due to gravity), and only
1/32lb of mass.

If it can slam into the wall, then we have
F=2*(1/32)*5/16=0.019 ft-lb/s^2(=lbf) linear force (=0.09 Newtons)
Converting to torque, torque=radius*force gives 0.04 lbf-in or 0.7 oz-in
(=0.0049 N-m).

If it must not slam into the wall, then allocate a 2s period of
acceleration followed by a 2s period of deceleration.
F=2*(1/32)*2.5/4=0.039 lbf (=0.17 N)
torque: 1.4 oz-in

Providing room for friction and slightly larger objects, spec something
with 5 oz-in of torque or more.

Thus your torque needs aren't all that high; standard servos have
roughly 40 oz-in of torque. Unfortunately, they only spin at 43 rpm;
high speed servos top out around 90 rpm. Maybe you could find a 7"
pulley and use a standard servo modified for continuous rotation? If
not, you should be able to find a small gearbox DC motor that fits your
needs. Lynxmotion.com sells a planetary gear motor for \$23 that should
fit your needs. Solarbotics.com has smaller gear motors under \$8 that
should work with a little modification (detailed on their site). There
are hundreds of other places as well.

As far as mounting the motor, small DC motors will have output shafts
much less than 0.25". You will need to build a shaft coupler that fits
over the motor shaft and connects to your pulley.

Later,
Daniel

### Ed

Aug 19, 2005, 8:35:36 AM8/19/05
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What do you guys think of these?

http://www.pololu.com/products/gearbox.html

### Si Ballenger

Aug 19, 2005, 10:23:29 AM8/19/05
to

You may want to look at DC powered electric drills and
screwdrivers. You can get a 6v black&decker electric screwdriver
at walmart for ~\$9. I got a 12v variable speed electric drill
from the harbor freright store on sale for ~\$10.

Message has been deleted

### how...@zaxcom.com

Aug 19, 2005, 2:58:20 PM8/19/05
to
Use this power window motor (I have some of these):

http://www.allelectronics.com/cgi-bin/category.cgi?category=400&item=DCM-243R&type=store

CATALOG# DCM-243R

its only \$17.00, its quiet and will run from 3V to 16V

at only 3V I cant stop the shaft with my fingers.

Its output shaft is about 120 RPM (2 revolutions per second)

Epoxy a pulley to the output shaft and your in like Flynn.

-howy

### Wayne Lundberg

Aug 20, 2005, 2:29:08 PM8/20/05
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I'd go to the nearest auto junk yard and get a windshield wiper motor along
with the microswitches that stop the motion at wiper blade end and also
incorporate the one shot circuit so when you turn it off, the device will
return to home and then stop. 12 volts and all the power you will need and
should not cost more than five bucks. Or the motor and gears that drive the
windows.

Wayne

"Ed" <edis...@yahoo.com> wrote in message