What are Performance Grades of Bolts and Screw threads

  • Bolts are an omnipresent component in various industries, playing a crucial role in connecting and fastening structures. However, have you ever wondered about the significance of those grade markings? In this article, we delve into the performance grades of bolts and screw threads.

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    Performance Grades of Bolts

    Bolts used in steel structure connections come in various performance grades, numbering more than ten, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. Bolts rated 8.8 and higher are crafted from low-carbon alloy steel or medium-carbon steel, treated through heat processes, and are commonly known as high-strength bolts. The rest are typically referred to as ordinary bolts. The grade markings on bolts consist of two figures, representing the nominal tensile strength value and the yield strength ratio of the bolt material.

    For instance, a bolt with a performance grade of 4.6 signifies that the nominal tensile strength of the bolt material reaches 400 MPa, and the yield ratio is 0.6. In contrast, a high-strength bolt marked as 10.9 attains even greater strength after heat treatment, with a nominal tensile strength of 1000 MPa and a yield ratio of 0.9, achieving a nominal yield strength of 900 MPa.

    It's important to note that the bolt performance grade is an internationally recognized standard. Bolts with the same performance grade exhibit uniform performance characteristics, regardless of their material or origin. Consequently, when designing structures, engineers rely on the performance grade as the key parameter.

    In the realm of bolts, strength is typically expressed as "X.Y," where X * 100 equals the tensile strength, and X * 100 * (Y/10) equals the yield strength, given that yield strength/tensile strength equals Y/10, following standard identification regulations.

    Performance Grades of Screw Threads

    Screw threads are integral to countless applications, characterized by their uniform spiral profiles on solid surfaces, both internal and external. They can be classified into three primary categories based on their structural features and intended use.

    Common Screw Thread: Featuring a triangular tooth profile, common threads are primarily used for connecting or fastening components. These threads come in coarse and fine variations, with fine threads offering enhanced connection strength.

    Transmission Screw Thread: Transmission threads encompass trapezoidal, rectangular, sawtooth, and triangular tooth profiles, serving specific transmission purposes.

    Sealing Screw Thread: Reserved for sealing connections, this category includes pipe threads, taper threads, and taper pipe threads.

    The concept of thread fit refers to the degree of looseness or tightness between threaded components. Thread fit grades are defined by a combination of deviation and tolerance on both internal and external threads.

    For instance, in the unified inch system, external threads have grades 1A, 2A, and 3A, while internal threads have grades 1B, 2B, and 3B. The higher the grade, the tighter the fit. Grade 1A and 1B offer loose tolerances, suitable for less critical applications, while grade 3A and 3B provide a tight fit suitable for safety-critical designs.

    In metric threads, external threads are categorized as 4h, 6h, and 6g, while internal threads are categorized as 5H, 6H, and 7H, each with distinct applications based on their tolerance zones.

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    Thread Markings

    Understanding thread markings is essential when dealing with fastener components:

    Major Diameter/Tooth of Outer Diameter (d1): Represents the nominal diameter of the thread size.

    Minor Diameter/Tooth Root Diameter (d2): Denotes the diameter where the thread root coincides.

    Tooth Pitch (p): Refers to the axial distance between adjacent teeth on the meridian. In the imperial system, pitch is measured in teeth per inch, equivalent to 25.4mm.

    Metric and Inch Self-Tapping Screws: These screws come in various specifications and pitches, allowing for precise applications.

    In summary, understanding the grade markings of bolts, screws, and screw threads is vital, as different grades and markings indicate distinct applications and functions of bolts with different types in various industries and structures. Whether it's for high-stress steel connections or precise threading requirements, these markings provide essential information for engineers and manufacturers alike.