What is tower bolts, nuts, and washers?

This document provides a detailed technical overview of the specialized fasteners—bolts, nuts, and washers—used in the construction of structures like transmission towers, wind turbine towers, and other critical steel installations. These components are engineered for high strength, durability, and environmental resistance.

1. Components and Primary Functions

A tower fastener assembly typically consists of three key elements, each with a specific role:

· Bolts: These are threaded rods, often with a hexagonal head, designed to pass through pre-drilled holes in the connected structural members. Their primary function is to withstand tensile (stretching) and shear (sliding) forces. The most common types used are Hex Head Bolts and Anchor Bolts (for securing the tower base to the foundation) .
· Nuts: These internally threaded components mate with the bolts to clamp the connection together. Hex Nuts are standard, and for critical high-strength connections, a Thin Nut (or jam nut) is sometimes used in tandem with a standard nut to prevent loosening from vibrations .
· Washers: These are flat rings placed under the nut or bolt head. They serve several vital functions:
  · Distribute Pressure: Protect the connected steel surfaces from damage by distributing the clamping force over a larger area .
  · Prevent Loosening: Spring Washers (also called disc springs) provide a compensating force that helps prevent the nut from loosening under vibration .
  · Compensate for Gaps: Help account for minor misalignments in drilled holes .

2. 外形尺寸 (Dimensions)

The dimensions of these fasteners are standardized and correspond to their thread size and strength grade.

· Thread Sizes: For heavy-duty tower structures, common metric thread sizes range from M10 up to M64 and larger. Standards like NB/T 31082-2016 specify dimensions for washers used with bolts from M36 to M64, indicating the scale of fasteners used in applications like wind turbine towers .
· Bolt Length: The length is determined by the thickness of the materials being joined, plus the space required for the nut and washers. A general rule is that the bolt's threads should protrude past the nut by 1-2 thread pitches for a secure connection .
· Washer Dimensions: Washers are dimensioned by their inner diameter (d), which must fit the bolt's shank, and their outer diameter (dc), which determines the bearing surface. For example, a washer for an M42 bolt has an inner diameter of 43.4 mm and an outer diameter of 78 mm according to NB/T 31082-2016 .

3. 级别 (Grade/Class)

The "grade" or "class" indicates the mechanical strength of the fastener, which is critical for structural integrity.

· Bolts: The performance grade for bolts is marked on the head with two numbers separated by a dot (e.g.,6.8, 8.8, 10.9).
  · The first number represents the nominal tensile strength (in MPa) divided by 100. An 8.8 grade bolt has a nominal tensile strength of 800 MPa .
  · The second number is the yield ratio (the ratio of yield strength to tensile strength). An 8.8 grade bolt has a yield strength of 800 * 0.8 = 640 MPa .
  · Higher grades like 10.9 and 12.9 are used for the most critical, high-stress connections .
· Nuts: Nut strength classes are denoted by a single number (e.g., 8, 10, 12), which should generally match or exceed the bolt's grade to ensure the threads do not strip under load .
· Specialized Grades: Standards like T/CISA 193-2021 exist for Weathering Steel Bolts and Nuts, which are specifically designed for atmospheric corrosion resistance in structures like transmission towers .

4. 表面处理 (Surface Treatment)

Surface treatments are essential for preventing corrosion, which can severely compromise the strength of the connection over time.

· Hot-Dip Galvanizing bolt (HDG): This is the most common and robust protection for outdoor tower fasteners. The components are immersed in a bath of molten zinc, resulting in a thick, bonded coating that provides excellent corrosion resistance for decades. It is specifically mandated in standards like DL/T 284-2012 for transmission tower bolts and nuts . A downside is that the coating can be thick and affect thread fit, requiring care during assembly.
· Mechanical Plating: This process, which involves tumbling parts with zinc powder and glass beads, creates a uniform coating without using electricity. A key advantage is that it virtually eliminates the risk of Hydrogen Embrittlement, a dangerous phenomenon that can cause high-strength steel to fracture .
· Phosphating: This chemical treatment creates a crystalline phosphate layer on the surface that is not particularly corrosion-resistant on its own but is excellent at retaining oil. When coated with oil, it provides good protection and is especially valuable for high-strength bolts (10.9 grade and above) because, like mechanical plating, it avoids hydrogen embrittlement. It also has good anti-galling properties .
· Oxidation (Blackening): This is a low-cost process that creates a thin magnetite layer on the surface, offering minimal corrosion protection on its own. It is typically supplemented with an application of oil. "Blackening + oiling" is a popular, economical choice for industrial fasteners where corrosion resistance is not a primary concern .
· Dacromet: A zinc-aluminum flake coating that offers superior corrosion resistance compared to standard electroplating and also avoids hydrogen embrittlement. However, it is softer and can be damaged during installation, and environmental concerns exist around its chromium content .

5. 国际标准 (International Standards)

The manufacturing and use of these fasteners are governed by a wide array of international, national, and industry-specific standards to ensure safety, performance, and interchangeability.

· General Terminology: GB/T 3099.3-2017 (aligned with ISO 1891) defines the terminology for fastener surface treatments .
· Industry-specific Standards:
  · Power Transmission Towers: T/CISA 193-2021 covers weathering steel bolts and nuts for these structures . DL/T 1236-2013 specifies requirements for anchor bolts and nuts used in transmission pole structures .
  · Wind Turbine Towers: NB/T 31082-2016 details the high-strength bolt assemblies, including washers, used for wind turbine tower flanges .
· International & Regional Standards:
  · ASTM (U.S.): Standards like ASTM F436 specify requirements for hardened steel washers .
  · EN (Europe): The EN14399 series governs high-strength preloaded structural bolt assemblies, including various types of washers .
  · ISO (International): Standards like ISO 7416 cover hardened and tempered steel washers for high-strength bolting .

In summary, tower bolts, nuts, and washers are not ordinary fasteners. They are a carefully engineered system where the dimensions, high strength grade, and robust surface treatment (like hot-dip galvanizing) are all selected to work in concert, ensuring the long-term safety and stability of the vital structures they hold together.