The shells and sheaves are made of high dielectric, fiberfilled naturalnylon. The shaft is of silicon bronze and the sheave bearings are oilimpregnated bronze. Blocks are available with either polypropylene orPoly-Dacron rope for minimum stretch and long service life. DielectricStrength: Dielectric rating in dry weather is 30,000 volts between thebearing and the mounting bolt nut. Maximum 1/2" rope.