I. Design Analysis: Why a "Round Head" and "Square Neck"?
The carriage bolt design is a perfect example of form following function, with each feature serving a specific installation challenge.
Round Head: The Union of Aesthetics and Safety
The head of a carriage bolt is a smooth hemisphere or dome shape, lacking sharp edges. Once installed, this design not only offers a clean and streamlined appearance but, more importantly, effectively prevents accidental snags, scratches, or injuries to personnel during daily maintenance or passage, thereby enhancing the overall safety of the system.
Square Neck: The Revolutionary Anti-Rotation Core
This is the soul of the carriage bolt. Directly below the round head is a section with a square cross-section, known as the neck. During installation, this square neck is pre-inserted into a specially punched square hole or round-square hole in the bracket or clamp.
Anti-Rotation Mechanism: When the nut is tightened, the significant torque applied tends to make the bolt itself rotate. However, the square neck is already firmly lodged and locked within the square hole, much like a key stuck in a lock cylinder, preventing any rotation. This allows the installer to simply use a single tool on the nut side for tightening, greatly simplifying the operation. This advantage is particularly evident in confined spaces or hard-to-reach areas.
Powerful Clamping Force
With the square neck effectively preventing rotation, the threads on the bolt shank can withstand very high preload forces. By tightening the nut, immense and uniform clamping force is generated between the rail, clamp, and bracket, creating a solid, rigid connection that effectively resists vibration and loosening caused by wind loads, snow loads, and thermal expansion/contraction.
II. Core Application in Solar Systems: Securing Rails and Clamps
The characteristics of carriage bolts make them an irreplaceable choice for specific connection points in PV mounting structures.
Application Scenario: Primarily used to fix aluminum or steel rails to support clamps, which in turn are connected to the roof structure or ground mount. This constitutes the "skeletal" connection point of the entire PV array.
Why is it Irreplaceable Here?
Efficient Installation Process: In large-scale solar farm installations, thousands of such connection points need to be handled. The "single-sided operation" feature of carriage bolts – requiring only a single wrench on the nut end – saves significant installation time compared to standard hex bolts requiring tools on both ends, dramatically improving construction efficiency.
Excellent Anti-Loosening Performance: PV systems are exposed long-term to outdoor conditions, enduring continuous wind-induced vibration and temperature fluctuations. Ordinary screws might loosen under repeated stress. The mechanical anti-rotation design of carriage bolts eliminates the possibility of the bolt itself rotating loose, ensuring long-term connection stability.
Reliable Connection Strength: Solar rails need to support the full weight of PV modules and resist external forces. The tight fit between the carriage bolt's square neck and the square hole ensures the connection point can withstand not only vertical pressure but also effectively resist lateral shear forces, guaranteeing the structural integrity of the array.
III. Material and Corrosion Resistance: The Cornerstone of 25-Year Lifespan
As critical components for outdoor use, the material of carriage bolts directly determines their service life.
Primary Material: Stainless Steel
A2-70 (304 Stainless Steel): Offers good general corrosion resistance, suitable for most inland environments, and represents a cost-effective option.
A4-80 (316 Stainless Steel): Contains molybdenum, significantly enhancing its corrosion resistance, particularly against chloride ions (salt spray). It is the preferred material for coastal areas, high-pollution industrial zones, and all projects demanding maximum lifespan. Using 316 stainless steel carriage bolts is a key detail in ensuring the PV system lasts as long as the modules (25+ years).
Surface Treatment:
In fewer cases, high-strength carbon steel carriage bolts with top-grade surface treatments, such as zinc flake coating (Dacromet), might be used to provide corrosion protection comparable to stainless steel. However, stainless steel remains the absolute mainstream due to its overall balanced performance.
IV. Synergy with Other Solar Screws
Within a complete PV mounting system, carriage bolts do not work in isolation. They collaborate with other specialized screws to form a comprehensive fastening solution:
Self-Drilling Screws: Used for attaching roof hooks to metal roofing.
Hex Socket Press Bolt Screws: Used for securing the clamps that hold down PV modules.
Carriage Bolts: Specifically tasked with connecting rails to clamps.
This division of labor ensures that every connection point in the entire mounting system, from the roof surface to the modules, achieves optimal strength and durability.