The Three Gorges Project(TGP) ship lift employs 4 safety mechanisms, of which one consists of a rotary locking screw and a nut jaw column. The thread pair clearance(TPC) of safety mechanisms is set at 60 mm. Owing to influential factors,the TPC changes randomly in the upward/downward-stroke of the ship chamber. If it diminished to 0, the safety mechanism would be jammed, thus resulting in disastrous accidents. By the bearing test of the drive system, 7 influential factors have been studied; 15 other influential factors(including 8 factors of manufacture and installation deviation, 3 factors of chamber offset, 2 factors of external load and 2 factors of wear) have been analyzed based on the design data. Results by the limit superposition reveal that the TPC change varies from -43.8 mm to +48.4 mm when the water level of the chamber ranges from 3.4 m to 3.6 m. According to the Gaussian distribution, the probability of the TPC change varied from -53.7 mm to +58.8 mm in the most detrimental status is99.74%, therefore, the TPC remains in a safe condition. This paper puts forward that two-phase operation of the drive system should be adopted so as to reduce the maximum TPC change to -44.6 mm. The Three Gorges Project (TGP) ship lift employs 4 safety mechanisms, of which one consists of a rotary locking screw and a nut jaw column. The thread pair clearance (TPC) of safety mechanisms is set at 60 mm. Owing to influential factors, the TPC changes randomly in the upward / downward-stroke of the ship chamber. If it diminished to 0, the safety mechanism would be jammed, thereby resulting in disastrous accidents. By the bearing test of the drive system, 7 influential factors have been studied ; 15 other influential factors (including 8 factors of manufacture and installation deviation, 3 factors of chamber offset, 2 factors of external load and 2 factors of wear) have been analyzed based on the design data. Results by the limit superposition reveal that the TPC change varies from -43.8 mm to +48.4 mm when the water level of the chamber ranges from 3.4 m to 3.6 m. According to the Gaussian distribution, the probability of the TPC change varied from -53.7 mm to +58.8 mm in the most detrimenta The status of the TPC remains in a safe condition. This paper puts forward that two-phase operation of the drive system should be adopted so as to reduce the maximum TPC change to -44.6 mm.