According to the development of social level, the high strength bolt needs to be improved constantly. However, there are still some problems in the high strength bolt at present. According to the experience, the four common problems in high strength bolt are told.
1. Ensure the load, wedge load is not qualified. Guarantee load and wedge load are the key items to assess the mechanical properties of products under tensile load, class A., especially important for high strength hexagon nut fasteners. Guarantee load is to assess the product's ability to resist plastic deformation under the specified tensile load. If this index fails to meet the requirements, plastic deformation may occur due to the effect of preload and working load, so as to reduce the preload and loosen the connection. Wedge load test is to assess the tensile strength and head rod bonding strength of bolt products, this index is not qualified, in large working load, especially under the action of impact load or support surface under the action of partial load, fracture or U-turn, resulting in connection failure, and even safety accidents.
2. Hardness is not qualified. Hardness is an important index of assessment fasteners, mechanical properties, for each performance level product adopted by the material is different, fasteners, choose when mechanical performance level is an important basis, the installation and use of working load should be determined according to the level of performance and installation torque, for a specific material, hardness should be controlled in a reasonable level, high hardness may reduce the fatigue resistance of the products Performance, some enterprises in order to improve the strength of screw products, hardness control at a higher level, resulting in some products exceed the standard.
3. Decarbonization layer exceeds the standard. Decarbonization layer is also A type of project, due to decarbonization, the surface hardness and strength of the gasket parts are greatly reduced seriously affecting the surface contact strength and fatigue life of fasteners, especially the damage to the thread part is more prominent. The main reasons for decarbonization are the decarbonization of raw materials, the process of material restructuring and the process of product heat treatment.
4. The size is out of tolerance. Beijing Bolt nut Washer Fastener factory is a general parts, interchangeability requirements are high, size out of tolerance will directly affect its interchangeability, and even affect the connection strength, anti-loose performance or life. The main reasons for the unqualified size are first, improper control of the technological process. In order to save material, part of the size is controlled near the lower limit, and the equipment is not adjusted in time in the production process, and the out-of-standard mold is used. Second, the inspection is ineffective, and the process inspection and delivery inspection are not strict, so that some unqualified batches pass the test. Third, in the use of measuring tools can not be regular measurement verification, even the use of more than the wear limit of the gauge.
Different types of bolts are selected according to the actual needs of use, and high-strength bolts are made of high-strength materials. The screws, nuts and washers of high-strength bolts are made of high-strength steel. High-strength bolts apply pretension and transfer external force by friction. Ordinary bolt connection by shearing bolt rod and the pressure on the hole wall shear, tighten the nut when pulling force is small, its influence can be neglected, and high strength bolts, besides its material strength is very high to the bolt exerted a lot of tension, make connections between components to produce extrusion pressure, so that the perpendicular to the direction of the screw much friction, and strong, resistance to sliding coefficient and types of steel It directly affects the bearing capacity of high strength bolts.
