Four common problems of high-strength bolts


According to the continuous improvement of social devel […]

According to the continuous improvement of social development, high-strength bolts also need to be continuously improved. However, there are still some problems with high-light bolts in my country.

1. Guarantee load, wedge load is unqualified. Guaranteed load and wedge load are the key items for assessing the mechanical performance of the product under tensile load. Class A. It is especially important for high-strength hexagonal nut fasteners. Guaranteed load is to assess the ability of the product to resist plastic deformation under the specified tensile load. If this index does not meet the requirements, plastic deformation may occur due to the pre-tightening force and working load, thereby reducing the pre-tightening force and loosening the connection . The wedge load test is to assess the physical tensile strength of the bolt product and the bonding strength of the head rod. This index is unqualified. Under the action of large working load, especially impact load, or the supporting surface is subjected to eccentric load, fracture or Turn around, causing the connection to fail, or even a safety accident.

2. The hardness is unqualified. Hardness is also an important indicator for assessing the mechanical properties of fasteners. The materials used in products of various performance levels are different. The mechanical performance level is an important basis for the selection of fasteners. The working load should be determined according to the performance level during installation and use. And installation torque, for specific materials, the hardness should be controlled at a reasonable level. High hardness may reduce the fatigue resistance of the product. In order to improve the strength of the screw product, the hardness is controlled at a higher level, causing some products to exceed the standard. .

3. The decarburization layer exceeds the standard. The decarburization layer is also a category A project. Due to decarburization, the surface hardness and strength of the gasket parts are greatly reduced, which seriously affects the surface contact strength and fatigue life of the fastener, especially the harm to the threaded part. The main cause of decarburization is decarburization of raw materials, decarburization in the process of material reforming, and decarburization in the process of product heat treatment.

4. The size is out of tolerance. Bolt, nut, washer and fastener are general-purpose parts and require high interchangeability, and the size tolerance will directly affect their interchangeability, and even affect the connection strength, anti-loose performance or life. The main reason for the unqualified size is the process. Improper process control, in order to save materials, some dimensions are controlled near the lower limit, the equipment is not adjusted in time during the production process, and the out-of-tolerance tool and die are used: second, the inspection is not strong, the process inspection and the factory inspection are not strict, which makes some unqualified batches pass the customs; The third is that the measuring tools cannot be regularly checked, and even the gauges that exceed the wear limit are used.

Choose different types of bolts according to actual needs, and high-strength bolts are made of high-strength materials. The screws, nuts and washers of the high-strength bolts are all made of high-strength steel. The high-strength bolts exert pretension and transmit external force by friction. Ordinary bolt connections rely on the shear resistance of the bolt rod and the pressure of the hole wall to transmit the shear force. The pre-tension force generated when the nut is tightened is small, and its effect can be ignored. In addition to its high material strength, high-strength bolts also apply a lot to the bolt. The pre-tension force produces an squeezing force between the connecting components, so that there is a great friction force perpendicular to the screw direction. Moreover, the pre-tension force, the anti-slip coefficient and the type of steel directly affect the bearing capacity of the high-strength bolts.

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