(1) The main damage form of the large trapezoidal screw, the general screw (≤7 accuracy) is bending and wear; the ≥6 accuracy screw is wear and loss of precision or pitch size change.
　　 (2) Large trapezoidal screw material should have sufficient mechanical properties. Wuxi Xianghui screw rod has excellent processing performance, is not easy to produce abrasion, and can obtain low surface roughness and low processing residual internal stress. The screw rod has higher hardness after heat treatment, and has less quenching deformation and retained austenite. Materials that do not require overall heat treatment to high hardness, such as 45, 45# quenched and tempered, 40Mn, 40Cr, T10, T10A, T12A, T12, etc. Hardened screw materials include GCr15, 9Mn2V, CrWMn, GCr15, SiMn, 38CrMOAlA, etc.
　　(3) Heat treatment of large trapezoidal screw: general screw: normalizing (45 steel) or annealing (40Cr), stress relief treatment and low temperature aging, quenching and tempering, high frequency quenching and tempering precision of journal and square head. Unhardened screw:
　　 In addition to stress treatment, low temperature aging, spheroidizing annealing, quenching and tempering spheroidizing, if the original structure is bad, it needs to be normalized at 900°C (T10, T10A)-950°C (T12, T12A) before spheroidizing annealing. Direct quenching and spheroidizing precision hardened large screw: annealing or annealing after high temperature normalizing, stress removal treatment, quenching and low temperature aging.