# I need help choosing a mini motor for kinetic movement

1. Jan 26, 2008

### Bob Schwartz

My sculptor wife has asked me to develop an electrical system for a kinetic sculpture that creates a 30 RPM movement at a whisper quiet level and yet fits into the palm of a hand. I'm looking at a 17mm diameter x 55 mm long mini-gearhead motor listed at "352:1 ratio that runs at 24 RPM @ 12 Vdc @ 22 mA, no-load specs". Am I on the right track in thinking this would be sufficient to move a weight less than 3 oz. a distance of 1 inch? I'm trying to mimic the back-and-forth/to-and-fro motion of a pendulum, similar to the speed of the pendulum of a grandfather clock. If you think this motor is too fast, what must I do to reduce the speed? Should I be looking at a DC motor (the slowest of which lists a staggering 7,000 RPMs)? Any input would be greatly appreciated!

2. Jan 28, 2008

### Phrak

You shouldn't find an ungeared DC motor or torque motor that runs anywhere near as slow as you wish. You definitely need a gear motor for your app, so you're on the right track. -or an AC clock motor.
I happen to have spent the past 2 weeks developing a process to select a DC motor myself, so I'm only half dangerous on the subject. All DC motors, geared or not, can be characterized by a few parameters like I_0, the no load torque, K_V the rpms per volt, K_T, the RMPs per milliamps, and such.
To really answer you question you need to know the torque exerted by the pendulum on the motor, and over what angular distance.
It leaves me curious as to how you plan to get a rotating motor shaft to drive a pendulum arangment back and forth.

3. Jan 28, 2008

### Danger

Actually, the mechanical linkage for that would be pretty simple.
I am wondering, however, if a stepper motor might be the proper approach to this.

4. Jan 28, 2008

### Phrak

I'm interested in the mechanical linkage because to would tell me something about required toque and motor power since I don't know what moving three oz. one inch means exactly --and with what leverage.

A stepper motor would require selecting a driver of some sort as well as the motor, and also a controller if it doesn't just run at a constant speed. Probably overkill and not cost effect.

5. Jan 29, 2008

### Danger

Yeah, I can see that being a problem, Phrak. Just tossing out ideas.

6. Jan 29, 2008

### Phrak

Danger-
No worries, mate.

7. Jan 31, 2008

### Bob Schwartz

I'm taking over for Bob because I'm making the art.

This is the "Wife", and I thought I'd better take over to explain this sculpture that needs this little motor. I know I can't get a direct pendulum movement from a rotary shaft...but I can attach a circular plate with protruding "pins" that rotate to contact a wire connected at one end to one side of a pivoting image, and at the other end to an immovable anchor. The other side of the pivoting image is secured by a spring. When the pins attached to the rotating plate come in contact with the wire, they either push or pull the wire aside, causing it to flex, causing the image to tilt against the pull of the spring. As the pins rotate away from the wire, the spring causes the image to retract and move the opposite direction. No, I haven't tested it. As soon as I identify a motor slow enough to rotate the plate no more than 30-40 RPMs, and that can flex the wire a distance of about 1" against a pull of about 3-5 ounces of weight (the image & spring), I can buy it, test it, and hopefully succeed. If you think this is am impossible scenario, please don't hesitate to cut me short before I spend a fortune on motors. Oh, and the motor has to fit inside a space approximately 5" square x 4" deep. And it would be nice if I could hook it up to a transformer with a slim cord instead of some monster electrical lamp cord. Looks are everything here!

8. Feb 1, 2008

### NoTime

If I understand what you said it looks like your torque requirement is under 500 g-cm.
This may not be true as I don't know exactly what your setup is like.
For example if you want very quiet a solenoid to keep the pendulum swinging at it's natural frequency might be a better choice as all motors make some noise.
Your space requirements may be large enough for battery operation as most of the motors are less than 1.5"D by 2"L.
Also since the motors are reversible you could use some simple switches to set the swing and reverse points.

Here is a selection page for motors.
http://www.jameco.com/webapp/wcs/st...1&catalogId=10001&langId=-1&categoryId=351540

9. Feb 1, 2008

### Bob Schwartz

Thanks, NoTime, for the input, but...

I'm embarrassed to say I'm an engineering idiot, and I'm a little lost by 500 g-cm...not to mention Solenoid (although I have seen that word in my handy dandy Mini Electronics catalog). I will immediately google that term and be better prepared when I next submit.

I would definitely love a battery-operated set up...but have no idea how to go about constructing it or what components I need. At this point I have already purchased a motor (see earlier threads) and am hoping I can connect it to a power source and regulate the speed to fit my needs. Oh okay...so I'm hoping my hubbycan connect it to a power source. I could also use some help with the mechanical part of this sculpture. Would I be wise to access the mechanical engineering section of the forum and get someone to help me with the motion? And speaking of help, can you recommend a basic design book in electrical engineering to give me a way of visualizing some of these components & arrangements?

10. Feb 1, 2008

### NoTime

You can post pictures and questions here, if the moderators feel you would be better off if in another section they will move it for you.

I'm not sure what to recommend since you say you have no experience at all.
Radio Shack has (or did have) an electronics experiment learning kit that assumed no prior knowledge.
Or perhaps look at the offerings of your local community college.
Which one might be best probably depends on your learning style.

11. Feb 2, 2008

### Danger

Maybe I'm not understanding what you're trying to accomplish, because it seems to me that the pin-wheel is overkill as was pointed out about the stepper. Wouldn't a simple crank and push-rod assembly do it?

12. Feb 2, 2008

### Phrak

Bob's Wife and you all-

Hmm. It looks like you have a motor--maybe a windshield wipper motor--that puts out about 7 oz-in of torque per second. And the motor revolves at 1/2 turn per second (unloaded). These small motors are about 20% efficent. So it looks to be sized just fine.
You'll need 8 alkaline batteries to power it, or 6 if it's too fast.

Based on what you have said, I'm assuming you have some sort of two dimensional thing with an image on it, and you want it to swing back and forth in a smooth motion suspended from the top. Assuming it's not going to have too much friction from rubbing on things, this hardly takes any force once it's moving and you're pushing on it sideways.

It I haven't gotten this wrong I'd do it like this:-

Use your disk on the motor with the pin sticking out of it. One one side of the sculpture, you can have a pin also.. Use some rod (stiff wire), like coat hanger wire to connect the two together. Put a small loop on both ends of the wire. The loops go over the pins. For the pins use #6 machine screws (3mm or so) with some nuts to hold them in place. The heads are good for keeping the wire loops from coming off.

This arrangment will rock the sculpure back and forth once every 2 seconds.

The hard part will be getting some sort of disk attached to the motor shaft. I'd just take my problem and the motor, to the local hardware store--not the Home Despot, and let them try to figure it out. Either that or ask for some drill collars at the local despot. Use epoxy glue to attach a pin, or a pin and disk to the shaft collar.

good luck, deCraig

Last edited: Feb 2, 2008