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Friday, May 23, 2008


Tiny motors are wonderful for driving little robots (like Appetizer or Chicago) or turning small parts (like sensors) on big robots. Illustrated above, from left to right:

* Blue m&m's candy for size comparison
* Cell-phone vibrating motor (salvaged)
* Solarbotics #TPM tiny pager motor. Replaced by TMP2 ($4.95) -- comes with clean shaft (no weight)
* Solarbotics #RPM pager motor. Replaced by RMP2 ($3.95) -- comes with clean shaft (no weight)

Electronic Goldmine has a number of pager motors with weights on their shafts: G12809 ($0.99), G13566 ($1.29), G15241 ($0.99), G15768 ($0.99), G15768 ($0.99 for 2), G16153 ($1.00).
BGMicro has a pager motor with a weight on its shaft: MOT1030 ($0.99).
All Electronics has a pager motor with a weight on its shaft: #DCM-204 ($1.25).

Solarbotics sells tiny geared pager motors!!

Geared motors are far superior to ordinary pager motors for robotics, since they have more torque (pushing power) and rotate more slowly. See the GM10 ($12) and GM15 ($19) in the Motors section of Solarbotics. Although slightly larger than most pager motors, the GM11, GM11a, GM12a, GM13a, GM18, GM19 ($19.75-$23 each) are still relatively small and are worth a look.


Before discarding a damaged or obsolete cell phone, pager, or force-feedback joystick (aka game controller), crack it open and recover the vibrating element! The vibrating part usually consists of a miniature motor with an offset-weighted shaft. Because the piece of metal on the shaft is not centered, when the motor spins it causes the device to shift back-and-forth as the weight of the metal piece shifts around and around the motor shaft. Vibration.

Dremel High-Speed Rotary Tool Failure

The offset weight must be removed to make an effective robot motor.

My first attempt at removal was to use a cutting wheel. Then, I tried to use a grinding stone. Although I successfully removed the weight, the process was difficult and took a long time. More significantly, the motor shaft got damaged. In the above photos, the shaft is nearly ground to half diameter. That's not good!

Someone with greater skill and tool-accessory knowledge could probably perform the operation better than I could. The Dremel isn't at fault.

Seriously, always wear goggles or protective eye wear when working with tools!

Locking Pliers

The metal attached to my cell-phone vibrating motors is extremely strong. It doesn't appear to be made of lead or some other malleable element. The metal ended up dulling or damaging the heads of standard pliers and cutters. Even when I could get a grip, I ended up pulling out the entire shaft of the motor. That's not good either!

While attempting to get a better grip on the motor, I accidentally discovered a reliable, repeatable, and easy way to remove the metal weight. Simply applying the locking force of locking pliers to the weight seems to crush or deform the metal very slightly, but enough to pop the crimp.

Unless you've got super powers, normal pliers won't work. The leveraged force of locking pliers is absolutely necessary.

Position, Squeeze, and Tug:

Grip the metal weight at the end of the locking pliers like popping off a beer cap with your molars. (The American Dental Association would like to point out that the prior sentence is very reckless and is likely to result in the enrichment of your local dental specialist.) Try to grip across only the crimped portion of the metal weight, so that it can squish out rather than crimp further. The motor body shouldn't be touching the pliers at all.

Use the greatest squeezing force the pliers can reasonably apply. The more the initial resistance to the squeeze, the easier it is to remove the motor shaft. When positioned and squeezed just right, I barely had to tug on the motor body to pull the shaft from the weight.

Use only your hand to pull on the motor body. No other pliers or tools should be necessary.

Alternate Method: Slitted Metal:

Depending on your access to precision machine tools (like a lathe and a milling machine), it may be possible for you to make a custom gear puller. If not, then a slightly more crude method can produce similar results.

Find the thickest piece of flat stock metal sheet (can be from a junk drawer) that will fit between the end of the motor and the gear or other object you want to remove. Saw or cut a slit part of the way through the metal sheet. See item (1) in above picture. The slit should be wide enough that the motor shaft can be slid on, leaving the gear on one side of the sheet and the motor on the other side of the sheet.

Then, find a rod (like from a consumed Dremel grinding bit) or make a punch tool that can fit through the object to make contact with the motor shaft. See item (2) in the above picture.

Place this setup on a vise or something similar to support the metal sheet. With the rod making contact with the motor shaft, gently tap the rod with a hammer. This should force the motor shaft out of the gear. At some point, the motor will fall to the floor. Be sure to put a blanket or something there to catch the motor.

Better still, if you have an arbor press, you can use the same tools to ease the motor free, rather than tapping on it with a hammer.

1 comment:

Anonymous said...

Nicely done man! :)
I was looking for that kind of info and you really hit the nail with this one. Great tips! ;)