What Is Fastener Proof Load?

Fastener proof load is the minimum force a fastener can withstand for a specific period without permanently deforming. It’s a critical measure used in industries to ensure bolts, screws, and nuts can perform safely under standard operating conditions. By meeting proof load standards, fasteners are classified by grade, making matching them with the right applications easier.

In this guide, we’ll explore proof load, proof strength, and related terms like yield strength and tensile strength, giving you a clear understanding of these essential concepts.

What Is Meant by Proof Strength?

Proof strength refers to the maximum load a fastener can endure without exceeding its elastic limit, meaning it will return to its original shape once the load is removed. Qualifying bolts and other fasteners for specific grades or property classes is a benchmark. For example, SAE J429 specifies different proof strength values depending on fastener grade and size, such as 92,000 psi for Grade 5 bolts.

Proof strength testing ensures a fastener is strong and ductile, essential for safe and durable connections in structural and industrial applications.

How Is Proof Load Calculated for Fasteners?

Proof load is determined through testing, a standardized procedure to assess a fastener’s capacity. Here’s how the process works:

1. Preparation: Measure the initial length of the fastener using precise equipment to establish a baseline.
2. Testing Setup: Secure the fastener in a tensile testing machine with a nut on the threaded end and a wedge under the head. The wedge angle (4-10 degrees) depends on the fastener’s size and configuration.
3. Load Application: Gradually apply a load equivalent to 90–93% of the fastener’s minimum yield strength. Maintain the load for a specified time, typically 10 seconds, as standards like ASTM F606 require.
4. Evaluation and Verification: After the test, remeasure the fastener’s length. If the elongation is under 0.0005 inches, the fastener passes the test. Otherwise, it fails.

This careful process ensures fasteners work as they should and stay strong under pressure, helping to prevent them from breaking in everyday use.

FAQs

What Is the Difference Between Breaking Strength and Proof Load?

• Breaking strength is the maximum load a fastener can handle before it fractures and fails.
• Proof load is the highest load a fastener can take without causing any permanent deformation.

In essence, breaking strength determines when a fastener will fail, while proof load tests ensure it remains reliable under regular stress without damage.

Proof Strength vs. Yield Strength

• Proof strength measures the maximum force a fastener can endure elastically, meaning it springs back to its original shape without permanent deformation.
• Yield strength identifies the load where a fastener begins to stretch and deform permanently, losing its structural integrity.

Proof strength shows how much force a fastener can take without staying bent, making it important to keep things safe when used in real life. Yield strength is used to study small pieces of material and helps determine its strength before it stretches too much. Both are key to understanding how fasteners work.

Conclusion: Why Choose CNRL Fasteners?

At CNRL Fasteners, we specialize in manufacturing high-quality fasteners that meet strict proof load and strength standards. Our products, like the hexagon lock nut with flange and metric zinc-plated quick hex nuts, are rigorously tested to ensure durability and reliability.

With decades of expertise and cutting-edge manufacturing capabilities, CNRL Fasteners is your trusted partner for industrial and commercial applications. Explore our range today and experience precision-engineered fasteners that deliver unmatched performance.