reversible gear motor

Hi All, I need to buy a geared motor and noticed some companies advertising a REVERSIBLE geared motor. I would have assumed all geared motors should be
reversibe just by changing the polarity of the voltage in. Am I missing something?
Robert
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robert wrote:

Not all motors are reversible. The windings, brushes, or type of gearing can influence if the motor is CW, CCW, or dual. Single-direction DC motors are not uncommon for the larger gear motors you may find in surplus. Burden's has always listed their motors in this fashion, for example.
Freescale has some fairly nice motor tutorials that explain some of this.
http://www.freescale.com/files/microcontrollers/doc/train_ref_material/MOTORTUT.html
I like it when companies specify that a motor is reversible or dual.
-- Gordon
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Thanks Gordon.
Hadn't realised there were so many differenced within DC motors. The tutorials you linked to were great.
Robert.

http://www.freescale.com/files/microcontrollers/doc/train_ref_material/MOTORTUT.html
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