Internal motor spins a cylindrical plate with a weight (a screw) placed on one side creating an up down rapid vibrating movement. This vibration is then conveyed to 3 rods that smoothly propel the model forward in a sliding type movement.
This is a set of wheelchair motors and make great heavy duty robot motors. One left motor and one right motor. They are right angle gear drives with a release and electric brakes. They are 24VDC and the operating speed is 120 RPM.