Need help selecting a motor

Posted by Ed on August 19, 2005, 12:25 am

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.

Ed

Posted by D Herring on August 19, 2005, 2:18 am

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

Posted by Ed on August 19, 2005, 8:35 am

What do you guys think of these?

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

Posted by Mark Haase on August 19, 2005, 11:30 am

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

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.

--
|\/| /| |2 |<
mehaase(at)sas(dot)upenn(dot)edu

Posted by Si Ballenger on August 19, 2005, 10:23 am

>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.
>Ed

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.

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