There are many manufacturers of 12V DC gear motors, including:
Faulhaber Group:
A German company that specializes in micro motors, including DC gear motors.
Maxon Motor:
A Swiss company that produces a wide range of DC gear motors for various applications.
Portescap:
A company that specializes in small motors and has a range of DC gear motors available.
Oriental Motor:
A Japanese company that produces a wide range of motors, including DC gear motors.
Johnson Electric:
A global company that produces motors for various applications, including DC gear motors.
Nidec Motor Corporation:
A global company that produces a wide range of motors, including DC gear motors.
AMETEK Precision Motion Control:
A company that produces motors and drives for various applications, including DC gear motors.
Kollmorgen:
A company that produces motors and drives for various applications, including DC gear motors.
Baldor Electric Company:
A company that produces motors and drives for various applications, including DC gear motors.
Bonfiglioli:
An Italian company that produces a wide range of motors and drives, including DC gear motors.
There are many other manufacturers of 12V DC gear motors as well, and it is always a good idea to shop around to find the best product for your specific needs.
