These trollies are dependable, easy, and safe to use. Designed to easily adjust to the width of virtually any wide flange or S type I-beam. Width is adjusted with manual screw mechanism. Includes locking ring to prevent trolley width from changing accidentally. Manual screw mechanism may also be tightened to prevent trolley from moving (similar to a Beam Clamp). Each trolley includes four rollers with sealed bearings for long life. 6000 lbs Capacity.