1. Material: Ordinary carbon structural steel (Q yield strength), high-quality carbon structural steel (with an average carbon mass fraction of 20/10000), alloy structural steel (with an average manganese mass fraction of about 2% in 20Mn2), cast steel (ZG230-450 yield point not less than 230, tensile strength not less than 450), cast iron (HT200 gray cast iron tensile strength).
2. Common heat treatment methods: annealing (slow cooling in the furnace), normalizing (cooling in air), quenching (rapidly cooling in water or oil), tempering (reheating the quenched part to a certain temperature below the critical temperature, holding for a period of time and then cooling in air), quenching and tempering (the process of quenching+high-temperature tempering), chemical heat treatment (carburizing, nitriding, carbonitriding).
3. Failure manifestation of fasteners: fracture due to insufficient strength; Excessive elastic or plastic deformation; Excessive wear, slippage, or overheating of the friction surface; Loose connection;
4. Fatigue failure manifestation: The failure under the action of variable stress is called fatigue failure. Characteristics: Sudden fracture after multiple applications of a certain type of stress; The maximum stress under stress during fracture is much lower than the yield limit of the material; Even for plastic materials, there is no significant plastic deformation when they break. When determining the fatigue limit, the magnitude of stress, number of cycles, and cycle characteristics should be considered.
5. Types of threads: ordinary threads, pipe threads, rectangular threads, trapezoidal threads, serrated threads.
6. Basic types of threaded connections: bolted connections (ordinary bolted connections, bolted connections with hinged holes), double headed bolted connections, screw connections, and tight screw connections.
7. Anti loosening of threaded connections: friction anti loosening (spring washer, double nut, elliptical self-locking nut, transverse cut nut), mechanical anti loosening (open pin and groove nut, stop washer, round nut stop washer, serial steel wire), permanent anti loosening (punching method, end welding method, bonding method).
8. Methods to improve the strength of bolt connections: avoid generating additional bending stress; Reduce stress concentration.
9. Processing knowledge after heat treatment: The precision holes (through holes) after quenching require wire cutting processing; Blind holes require rough machining before quenching and precision machining after quenching. Non precision holes can be made in place before quenching (leaving a quenching allowance of 0.2mm on one side). The minimum allowance for rough machining of quenched parts is 0.4mm, and the allowance for rough machining of non quenched parts is 0.2mm. The thickness of the coating is generally 0.005-0.008mm, and it should be processed according to the pre plating dimensions.
10. The mechanical performance requirements for ordinary bolts of the same grade are slightly higher than those for high-strength bolts, but high-strength bolts have an additional acceptance requirement for impact energy compared to ordinary bolts. The strength of high-strength bolts does not lie in their designed load-bearing capacity, but in the high stiffness, high safety performance, and strong resistance to damage of their designed nodes. The essence of its high strength is that during normal operation, the node is not allowed to undergo any relative slip, that is, the elastic-plastic deformation is small and the node stiffness is high. The core difference between high-strength bolts and ordinary bolts is not the strength of the material used, but the form of force applied. The essence is whether to apply pre tension force and use static friction force to resist shear.
Post time: Jan-06-2025