All of the GT2 3mm Pitch Pulleys, with the exception of the 12 Tooth Pulley, have flanges to keep the belt on track. This is because the belts tend to thrust to the side when in motion. It is recommended that at least one pulley in the system have flanges to keep the belt from slipping. In situations where the center distance between shafts is more than 8 times the diameter of the smaller pulley or when the drive is operating on vertical shafts, both pulleys should have flanges on both sides.