The safety factor on our Portable Winch Co. pulleys are as follows: At 2x WLL: Pulley is still operable – sheaves are turning freely. At 4x WLL: Pulley is still in one piece, but side plates are deformed and prevent shave from turning. Breaking point is much higher. For ISC self-locking pulleys: They are engraved with Minimum Break Strength (40 kN). Appropriate safety factors should be decided by the end user depending on their application.