Need help selecting a motor

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)?

Thanks in advance for your consideration.


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Specs: 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. sells a planetary gear motor for $23 that should fit your needs. 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

Reply to
D Herring

What do you guys think of these?

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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.

Reply to
Si Ballenger

I bought a few of the 90 degree angle output shaft motors (GM3 or GM9, can't remember off hand). They are pretty good little motors. Unfortunately I haven't had time to use them too much. I like the easy with which they mount and they have two output shafts, perfect for adding an optical encoder.

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Mark Haase

Use this power window motor (I have some of these):

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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.


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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.


Reply to
Wayne Lundberg

Thanks to everyone for the excellent suggestions. I'll get this thing to work one way or another using your advice.


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