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HSFG Bolt Standard:

  • IS 3757 (Indian Standard): This standard is specific to India and outlines the requirements for high-strength structural bolts, including HSFG bolts. It covers dimensions, materials, and mechanical properties.
  • ISO 7412 (International Organization for Standardization): ISO 7412 is an international standard that provides specifications for HSFG bolts. It ensures compatibility and interchangeability of these bolts across different countries and regions.
  • EN 14399 (European Standard): This standard is part of the European Norms (EN) and specifies high-strength structural bolting assemblies for preloading. It covers various aspects, including bolts, nuts, washers, and other related components.
  • BS 7371-8 (British Standard): This British Standard is part 8 of the BS 7371 series, focusing on high-strength structural bolting assemblies. It includes specifications for HSFG bolts and their components.

What is HSFG Bolt

High-Strength Friction Grip Bolts, or HSFG bolts, are bolts engineered for exceptional strength. Their load-bearing capacity surpasses that of ordinary bolts of the same specifications. The term “friction grip” highlights a key distinction – these bolts rely on intentional pre-tensioning during tightening, generating significant frictional forces perpendicular to the screw direction. This friction, in turn, facilitates the transmission of shear forces during the connection process.

Structural Variances of HSFG Bolt

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While the general shape and connection structure of HSFG bolts mirror those of ordinary bolts, the primary distinction lies in their operational principles. Ordinary bolts transmit shear forces through the rod’s compression and shear resistance, with minimal consideration for the pre-tension generated during tightening. In contrast, HSFG bolts intentionally apply substantial pre-tension, creating compression forces between the connected surfaces. This compression results in significant friction forces, essential for transferring shear forces in the connection.

Characteristics of HSFG Bolts

  • Material Composition
    HSFG bolts are crafted from high-strength materials, such as 45# steel, alloy steel (e.g., 20MnTiB, 35VB), or other premium materials. Heat treatment further enhances their strength, ensuring optimal performance in demanding applications. It’s essential to drill holes for HSFG bolts to precise dimensions to guarantee proper fit and function.
  • Size Specifications
    The diameter of the holes for friction-type HSFG bolts typically exceeds the nominal diameter (d) by 1.5 to 2.0 mm, while pressure-type HSFG bolts require holes 1.0 to 1.5 mm larger than the nominal diameter.

Distinguishing HSFG Bolts from Ordinary Bolts

  • Load-Bearing Capacity:
    HSFG bolts outperform ordinary bolts in terms of load-bearing capacity for a given specification, indicating a higher level of strength.
  • Material Analysis:
    Ordinary bolts are commonly made from Q235 steel (A3, also known as plain carbon steel), while HSFG bolts utilize high-strength materials like 45# steel, alloy steel, or other premium options.
  • Strength Grades:
    HSFG bolts are available in higher strength grades, with 8.8 and 10.9 being common, while ordinary bolts have lower strength grades like 4.4, 4.8, and 5.6. The use of 8.8S and 10.9S is specific to steel structures.
  • Nut Compatibility:
    HSFG bolts of different strength grades correspond to nuts of the same grade (e.g., 8.8 bolts with 8-grade nuts, 10.9 bolts with 10-grade nuts). This deliberate mismatch in hardness aims to protect the bolt during repeated assembly and disassembly, compensating for wear and tear.

High-Strength Bolt Torque Inspection System

Ensuring the integrity of high-strength bolt connections is paramount in various industries. To uphold quality standards, a systematic approach to post-tightening inspections is essential. Here’s a comprehensive guide for inspecting high-strength bolts after the final torque application:

  1. Notification and Preparation:
    After the final torque application of high-strength bolts, stainless steel bolt suppliers must promptly notify on-site technical personnel to issue a final torque inspection notice to the laboratory. The laboratory, in turn, is responsible for thorough preparations before conducting the final torque inspection.
  2. Timely On-Site Inspection:
    The on-site final torque inspection must be completed within 4 to 24 hours after the completion of high-strength bolt tightening. The inspection task falls under the responsibility of on-site testing personnel within the construction area, who should collaborate effectively with other on-site activities.
  3. Torque Wrench Calibration:
    Prior to the inspection, calibration of torque wrenches is mandatory. The torque wrenches should undergo calibration checks to ensure that torque errors do not exceed ±3% of the specified torque value, maintaining accuracy in the inspection process.
  4. Initial Inspection:
    The initial inspection is conducted by on-site technical personnel. Using a small hammer weighing approximately 0.3kg, the nut’s one side is gently tapped. Simultaneously, the inspector grips the other side of the nut tightly with fingers. Excessive vibration is considered a defect, and if detected, the initial tightening must be redone.
  5. Final Torque Inspection:
    Before conducting the final torque inspection, the stainless steel bolt factory and on-site technical personnel should observe all high-strength bolt connections post-torque application. The inspector checks the relative positions of bolts and nuts marked with paint circles after the initial tightening to verify if any bolts or nuts were missed during the final torque application.

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