These DC motors include gear box and encoder. It uses 2 phase or 2 channels (quadrature) incremental encoders to identify the rate of the motor, path of rotation and the distance it has travelled.
Encoder feedback data enables you to control your motor rate and path Drive Chain efficiently. DC motors with encoder feed back signal usually called DC servo motors.
DC motors with encoder possess many applications in robots, robot arm, CNC machine and several other applications that require accurate control of rate, direction and placement. You can control your electric motor using microcontroller or arduino.
DC Motor Specifications
Voltage: 6:12 VDC
Output Power: 
2.2 W
Gear Ratio: 1:75
Rated Speed: 133 RPM
Ranked Torque: 25.4 N.cm
Stall Current: 3A
Encoder Type: Hall effect quadrature encoder 5v (monitor position and direction of rotation)
Size: 25 x 54 mm
Weight: 120 g
Shaft Diameter: 4mm