Technical requirements for high strength bolts

Bolts made of high-strength steel, or bolts that require a large pre-tightening force, can be called high-strength bolts. High-strength bolts are mostly used for the connection of bridges, rails, high-pressure and ultra-high-pressure equipment. The fracture of such bolts is mostly brittle fracture. The high-strength bolts used in ultra-high pressure equipment require a large prestress in order to ensure the sealing of the container.

The mechanical properties and calculation methods of ordinary bolts and high-strength bolts are different. The high-strength bolts are first stressed by applying a pre-tension P inside them, and then between the connected parts Frictional resistance is generated on the contact surface to bear the external load, while ordinary bolts directly bear the external load. High-strength bolt connection has the advantages of simple construction, good mechanical performance, and detachable It is a very promising connection method due to its advantages of replacement, fatigue resistance, and no loosening under dynamic loads.

1. Subject content and scope of application This technical requirement specifies the technical requirements for the steel structure high-strength bolt pair connectors in the manufacturing, installation and inspection process of mobile machinery and equipment. Contents not specified in this technical requirement shall be implemented in accordance with relevant national standards. This technical requirement applies to mobile machinery steel structures that require high-strength bolt connections. This technical requirement applies to quality control and construction methods in the manufacturing plant and on-site installation.

2. Joint surface treatment

(1) For friction-type high-strength bolt connections, the joint surfaces at the joints are required to be in close contact and have sufficient friction coefficients. When the design drawings do not specify the treatment requirements for the joint surface, the treatment shall be carried out according to the following regulations: sandblasting or shot blasting shall be carried out on the joint surface of high-strength bolts to remove impurities such as rust and oil on the surface, and meet the Sa2.5 standard , The roughness is 50~75μm, and its coefficient of friction shall not be lower than 0.40. When there are regulations on the drawings, the regulations on the drawings shall be followed. (2) Protective measures shall be taken for the friction surface of the treated high-strength bolt joints to prevent contamination with dirt and oil. It is strictly forbidden to make any marks on the friction surface of the high-strength bolt connection. During storage in the factory, or during transportation, and storage at the installation site, special precautions should be taken to prevent contamination of the connection surface. The installation unit should pay special attention to protecting the cleanliness of the connecting plate of the high-strength bolt and the connecting surface of the mother body, and the characteristics of the friction surface. It is not allowed to use a grinder to grind the connecting surface of the connecting plate and the connecting surface of the mother body.

3. Inspection of anti-slip coefficient of high-strength bolt friction surface

The inspection of the anti-slip coefficient shall take the steel structure manufacturing batch as the unit, and every 2000t of a single project shall be regarded as a manufacturing batch, and those less than 2000t shall be regarded as a batch. Each surface treatment process needs to be inspected. Three sets of test pieces per batch. If the connection is diffused to external enterprises, each corresponding enterprise shall conduct anti-slip coefficient inspection. (1) The test piece used for the anti-slip coefficient test should be processed by the factory or the diffusion enterprise. The test piece and the steel structure member represented should be of the same material, produced in the same batch, adopt the same friction surface treatment process and have the same surface state, and apply the same batch of high-strength bolt connection pairs of the same performance level, and store them under the same environmental conditions. The anti-slip coefficient test is carried out according to the test method of GB50205 "Code for Acceptance of Construction Quality of Steel Structure Engineering". (2) The minimum value of the anti-slip coefficient inspection must be equal to or greater than the design specified value. When the above specified value is not met, the friction surface of the component should be re-treated. After the treatment, the friction surface of the component is re-inspected.

4. Connection and installation of friction-type high-strength bolts for steel structures

(1) Preparations before installation (2) Select qualified bolts, nuts and washers. The guarantee period of the coupling torque coefficient is six months from the date of delivery. (3) Bolts, nuts, and washers are unqualified if the following conditions apply, and their use is prohibited. a. The source (manufacturer) is unknown; b. Those whose mechanical properties are unknown; c. Those whose torque coefficient k is unknown; d. Cracks, scars, burrs, bends, rust, thread wear, oil stains, wet or defective; e. Those without a performance test report; f. Mixed with other batches of bolts; g. Bolts with insufficient length, that is, the bolt head does not expose the end face of the nut after tightening. Generally, the length protruding from the end face of the nut is preferably 2~3 threads. h. The torque coefficient of the connecting pair exceeds the guarantee period. Pay special attention to waterproofing during transportation and storage. (4) Before the construction of high-strength bolts with large hexagonal heads, the torque coefficient of the high-strength bolt connection pair should be re-inspected according to the factory batch. Each batch should be re-inspected for 8 sets, and the average torque coefficient of the 8 sets should be within the range of 0.110~0.150. The standard The deviation should be less than or equal to 0.010. The re-inspection method of the torque coefficient is carried out according to the provisions of GB50205 "Code for Acceptance of Construction Quality of Steel Structure Engineering". The installation of high-strength bolts should be carried out within a short period of time after the test..