Views: 1000 Author: Site Editor Publish Time: 2025-05-29 Origin: Site
A carriage bolt, also known as a coach bolt or round head square neck bolt, is a specialized fastener instantly recognizable by its distinctive dome-shaped, smooth head and a square section (neck) directly beneath the head. This unique design serves a crucial functional purpose, making it a staple in applications where a smooth, finished appearance and resistance to rotation during tightening are essential.
Head: Smooth, rounded, low-profile dome (mushroom head). The underside is flat or slightly curved to bear against the surface. The smooth head provides a finished look and reduces snagging.
Square Neck: Immediately below the head is a short, square-sectioned shoulder. This is the defining feature.
Shank: The body of the bolt below the square neck is typically partially or fully threaded (though unthreaded shanks exist for specific uses). The shank is usually circular.
Threads: Machine threads designed to engage with a corresponding nut.
Carriage bolts are manufactured from various materials to suit application requirements:
Carbon Steel: The most common and economical choice. Offers good strength for general purposes.
Stainless Steel: (e.g., 304, 316) Provides excellent corrosion resistance for outdoor, marine, or chemical environments.
Alloy Steel: (e.g., ASTM A449) Used for higher strength requirements, often heat-treated.
Brass: Primarily chosen for its corrosion resistance and aesthetic appeal in decorative applications.
Silicon Bronze: Offers high strength and superior corrosion resistance, especially in marine settings.
Surface treatments enhance corrosion resistance, appearance, and sometimes lubricity:
Zinc Plating (Galvanizing): The most common treatment. Can be bright (electroplated) or dull grey (hot-dip galvanized). Hot-dip offers thicker, more durable protection.
Cadmium Plating: Good corrosion resistance and lubricity, but environmental concerns limit its use.
Chrome Plating: Primarily for decorative appeal and moderate corrosion resistance.
Phosphating/Oil: A thin phosphate coating often dipped in oil provides minimal corrosion protection and some lubricity for assembly.
Mechanical Galvanizing: Zinc applied via tumbling, offering good coverage on threads.
Plain/Unfinished: Typically carbon steel without coating, prone to rust unless painted or used indoors.
| Standard | Head Type | Scope | Material Grades |
|---|---|---|---|
| GB/T 12 | Small round head | M6–M20, C-class | Carbon steel, A2-50/70 |
| GB/T 14 | Large round head | M5–M20 | Carbon steel, stainless steel |
| DIN 603 | Large round head | M5–M20 | 4.8–10.9 Grade |
Common Sizes (Imperial/Metric):
Diameters: #10 to M20 (5–20 mm).
Lengths: 15–300 mm (common: 20–200 mm).
Drill Hole: A hole is drilled through the materials being joined. The hole diameter through the first material must be sized precisely to match the square neck of the bolt (a snug fit is crucial). The hole through subsequent materials can be slightly larger to accommodate the shank.
Insert Bolt: The carriage bolt is inserted through the holes from the "finished" side (where the smooth head will be visible). The square neck is driven into the first material (usually wood or a pre-punched square hole in metal). This is critical: The square neck bites into the material, locking the bolt head firmly in place and preventing it from rotating when the nut is tightened.
Add Washer (Recommended): A flat washer is often placed under the nut to distribute the load, prevent the nut from digging into the material, and provide a more secure connection.
Tighten Nut: A standard hex nut (or sometimes a lock nut) is threaded onto the bolt end protruding from the back side. A wrench is used to tighten the nut against the washer/material. Since the head is locked by the square neck, only the nut needs to be turned.
Carriage bolts are ideal for applications requiring a smooth, finished appearance on one side and resistance to rotation during assembly. Common uses include:
Wood Construction: Deck building, timber framing, furniture assembly, fence construction, play structures (securing beams, ledgers, posts, brackets).
Metal Fabrication: Connecting metal brackets to wood or other materials, securing components where a smooth head is desired on one side (e.g., truck beds, trailers, agricultural equipment).
Concrete Formwork: Securing forms together (head side against the plywood face).
Playground Equipment: Securing components where smooth heads prevent snagging.
Furniture: Especially outdoor furniture, bed frames, and assemblies where the bolt head is visible.
Agricultural Implements: Securing parts on machinery.
General Fastening: Anywhere the "square neck lock" feature and smooth head are advantageous.
In summary, the carriage bolt is distinguished by its smooth dome head and integral square neck. This neck is key, allowing the head to lock securely into the material it passes through, preventing rotation while the nut is tightened on the opposite side. Available in various materials, strengths, sizes, and finishes, carriage bolts are a fundamental fastener for creating strong, rotation-resistant connections with a clean, finished appearance on one side.
