Material: Glass reinforced nylon, with steel shafts and steel-clad flange for greater wear resistance. Speed range infinitely variable within standard ratio limits. Pulleys are shipped with a blue, heavy srength spring unless another srength spring is requested. For the FZN-A1 series, we recommend the heaviest blue spring be used if the pulley is to be run with 3/8 - inch width V-belt. Set screw supplied. *Economical *Lighweight *Strong