Pololu 120:1 Mini Plastic Gearmotor Offset 3mm D-Shaft Output
This compact, inexpensive, low-current gearmotor (brushed DC motor with a 120:1 reduction gearbox) is well suited for use in small robots. At 4.5 V, it has a free-run speed of 120 RPM and a stall torque of approximately 20 oz-in (however, the built-in safety clutch might start slipping before the stall torque is reached). The D-shaped output shaft has a diameter of 3 mm and works with the Pololu wheels and 3mm universal aluminum mounting hub. This gearbox output shaft is offset from but parallel to the motor shaft.