A Hexagon Socket Button Head Bolt (also known as an Allen Button Head Bolt) is a type of mechanical fastener characterized by its low-profile, dome-shaped head and an internal hexagonal drive (socket) that requires an Allen key or hex wrench for installation. When manufactured from carbon steel and hot-dip galvanized (HDG), it provides a robust, high-strength solution with excellent corrosion protection, suitable for various heavy-duty and outdoor applications. The strength grades 6.8, 8.8, and 10.9 denote the bolt’s tensile and yield strength capabilities.

Key Features:

• Button Head Design: The low-profile, rounded (dome) head provides an aesthetic finish and minimizes obstruction, making it suitable for applications where a flush or smooth surface is desired.
• Hexagon Socket Drive: Requires an Allen key or hex wrench for installation and removal. This internal drive minimizes cam-out and allows for higher torque application compared to external drives for its size.
• Carbon Steel Material: Provides inherent strength and durability, essential for load-bearing applications.
• Hot-Dip Galvanized (HDG) Finish: The bolt is immersed in a bath of molten zinc, forming a metallurgical bond that creates a thick, resilient, and highly corrosion-resistant layer. HDG is particularly effective for outdoor and harsh environments.
• High Strength Grades (6.8 / 8.8 / 10.9): These grades indicate the mechanical properties of the bolt:
  • Grade 6.8: Medium strength, suitable for general structural applications. (e.g., Tensile Strength ~600 N/mm², Yield Strength ~480 N/mm²)
  • Grade 8.8: High strength, often referred to as “structural grade” or “high tensile” bolt. Suitable for critical structural connections where higher loads are present. (e.g., Tensile Strength ~800 N/mm², Yield Strength ~640 N/mm²)
  • Grade 10.9: Very high strength, used in demanding applications requiring maximum tensile and yield strength. (e.g., Tensile Strength ~1000 N/mm², Yield Strength ~900 N/mm²)

Specifications:

• Material: Carbon Steel
• Finish: Hot-Dip Galvanized (HDG)
• Strength Grades: 6.8, 8.8, 10.9 (as per ISO 898-1 or equivalent standards)
• Head Type: Hexagon Socket Button Head (also known as dome head or round head with internal hex)
• Thread Type: Typically Metric Coarse (e.g., M4, M6, M8, M10, M12, etc.). Imperial sizes (UNC/UNF) may also be available.
• Diameter: Available in a wide range of standard metric diameters.
• Length: Various standard lengths to suit different material thickness and application requirements.
• Standards: Often manufactured to standards like ISO 7380 (for button head socket screws).

Typical Applications:

• Construction: Fastening structural steel components, cladding, framing in outdoor or corrosive environments.
• Automotive: Chassis assembly, engine components (where low-profile heads are beneficial).
• Machinery and Equipment: Assembly of industrial machinery, conveyors, and heavy equipment, especially in outdoor or agricultural settings.
• Building & Fabrication: Attaching plates, brackets, and panels where a smooth, unobtrusive finish is desired and resistance to corrosion is critical.
• Outdoor Furniture and Fixtures: Assembling public benches, playground equipment, and outdoor installations that are exposed to weather.
• Marine (non-direct immersion): Used in boat structures above the waterline, docks, and other components requiring good corrosion resistance and strength.
• General Engineering: Any application requiring a strong, durable, visually appealing fastener with good corrosion protection.

Advantages:

• Corrosion Resistance: HDG provides an excellent, long-lasting barrier against rust and corrosion, extending the bolt’s lifespan in harsh environments.
• High Strength: Available in various strength grades to meet demanding load requirements for structural and heavy-duty applications.
• Aesthetic and Low Profile: The button head offers a clean, smooth, and less intrusive appearance compared to hex heads.
• Secure Drive: The internal hex drive reduces the risk of stripping and allows for higher tightening torques.
• Versatility: Suitable for a broad range of industries due to its strength, aesthetics, and protective finish.

Considerations:

• HDG Thickness: The galvanization process adds significant thickness, which may affect thread engagement with standard nuts. Oversized tapped holes or specifically manufactured “oversized” nuts (e.g., tapped to 6G/6H) are often used with HDG bolts to ensure proper fit.
• Hydrogen Embrittlement: High strength bolts (Grade 10.9 and above) that undergo hot-dip galvanizing must be carefully processed to mitigate the risk of hydrogen embrittlement, which can lead to brittle failure.
• Tooling: Requires an appropriate hex key for installation.