U.S. Motors brand CORRO-DUTY motors are considered one of the toughest built to withstand a variety of severe duty applications. These motors are rated Class B rise at 1.0 service factor and Class F rise at 1.15 service factor with full Class F or better insulating materials. All CORRO-DUTY motors are constructed with cast iron, including frame, conduit box, fan cover, bearing caps, and end shields.