Fastener Proof Load – Complete Guide by Pankaj International

By Pankaj International | Trusted Fastener Manufacturer in India

When it comes to industrial projects, construction work, heavy machinery, or automotive assemblies, one small component plays a very big role — fasteners. Bolts, nuts, screws, and studs hold everything together.

But how do we know a bolt is strong enough for your application?

The answer is: Proof Load.

In this guide by Pankaj International, we will explain fastener proof load in simple, easy-to-understand language. If you are an engineer, purchase manager, contractor, or business owner, this article will help you choose the right fasteners safely and confidently.

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What is Fastener Proof Load?

Fastener proof load is the maximum load (force) that a bolt can handle without permanently stretching or getting damaged.

In simple words:

Proof load tells you how much weight or force a bolt can take safely without losing its strength.

It is NOT the breaking point.

It is the safe limit before permanent damage happens.

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Why is Proof Load Important?

If a bolt stretches permanently:

• The joint becomes loose

• Clamping force reduces

• Vibration increases

• Equipment may fail

• Safety risks increase

That’s why understanding proof load is very important in industries like:

• Construction

• Infrastructure

• Oil & Gas

• Automotive

• Power Plants

• Heavy Engineering

At Pankaj International, we ensure every fastener meets the required proof load standards for maximum safety.

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Difference Between Proof Load and Tensile Strength

Many people get confused between these terms.

1. Proof Load

Maximum safe load without permanent damage.

2. Yield Strength

Point where the bolt starts permanently stretching.

3. Tensile Strength

Maximum load before the bolt breaks.

Proof load is always slightly lower than yield strength. This gives a safety margin.

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How Fastener Proof Load Testing is Done (Step-by-Step Procedure)

Proof load testing is performed using a calibrated Universal Testing Machine (UTM) as per ASTM F606 or ISO 898-1 standards.

Below is the complete procedure followed at Pankaj International.

Step 1 – Sample Selection and Inspection

A sample bolt is selected from the production batch.

Before testing, we check:

Surface finish

Thread accuracy

Dimensional tolerance

Visible cracks or defects

Heat treatment condition

Only defect-free fasteners proceed for testing.

Step 2 – Proper Thread Engagement

The bolt is screwed into a hardened test nut or fixture.

Important conditions:

Full thread engagement

Proper alignment

No bending stress

Nut hardness compatible with bolt grade

Improper setup can result in inaccurate testing.

Step 3 – Mounting in Universal Testing Machine (UTM)

The bolt assembly is placed inside a calibrated tensile testing machine.

The machine:

Applies axial tensile load

Measures force in lb (pounds) or kN

Increases load gradually

Records test data automatically

Machine calibration is essential for accurate results.

Step 4 – Applying the Specified Proof Load

The load is gradually increased until the required proof load value is reached.

For example:

If the bolt’s proof load requirement is 25,000 lb,

the machine applies exactly 25,000 pounds of tensile force.

Shock loading is not allowed. The load must increase smoothly and steadily.

Step 5 – Holding the Load

Once the specified proof load is reached:

The load is held for 10 to 15 seconds (as per standard)

The bolt remains under constant tensile stress

This ensures the bolt does not cross its yield strength.

Step 6 – Releasing the Load

After the holding period:

The load is gradually reduced

Sudden unloading is avoided

Step 7 – Checking for Permanent Deformation

After the load is removed, the bolt is inspected for:

Permanent elongation

Thread distortion

Cracks

Structural damage

PASS Condition:

The bolt returns to its original length with no permanent deformation.

FAIL Condition:

If permanent stretching is observed, the bolt fails the proof load test.

Proof Load Calculation Formula

Proof Load (lb) = Tensile Stress Area (in²) × Proof Stress (PSI)

Example:

Bolt Size: 1/2”-13 UNC

Tensile Stress Area: 0.142 in²

Proof Stress (Grade 8): 120,000 PSI

Proof Load = 0.142 × 120,000

Proof Load = 17,040 lb

This means the bolt must withstand 17,040 pounds without permanent stretching.

How is Proof Load Tested?

Proof load testing is done using special testing machines.

The process includes:

1. The bolt is fixed in a testing machine

2. A specific load is applied

3. The bolt is held under pressure

4. It is checked for permanent stretching

If the bolt returns to its original shape — it passes the test.

At Pankaj International, we follow strict quality control procedures and international standards to ensure reliable testing.

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Fastener Grades and Their Proof Load Capacity

Different bolt grades have different strength levels.

Grade 8.8 Fasteners

• Common in industrial use

• Medium strength

• Good for general engineering applications

Grade 10.9 Fasteners

• High strength

• Used in heavy machinery and automotive

Grade 12.9 Fasteners

• Very high strength

• Suitable for extreme load conditions

Stainless Steel Fasteners (A2, A4)

• Corrosion resistant

• Used in marine and chemical industries

Choosing the correct grade ensures better performance and longer service life.

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Factors That Affect Fastener Proof Load

Several things impact the proof load of a fastener:

• Material type (Carbon steel, Alloy steel, Stainless steel)

• Heat treatment process

• Bolt diameter and thread design

• Surface coating (Zinc, Hot Dip Galvanized, Black Oxide)

• Manufacturing quality

That’s why buying from a trusted fastener manufacturer in India like Pankaj International is important.

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Applications Where Proof Load Matters Most

Construction Projects

Structural bolts must handle heavy loads safely.

Automotive Industry

Engine and chassis bolts must maintain clamping force.

Oil & Gas Industry

High pressure environments demand strong fasteners.

Heavy Equipment

Machines that face vibration need high proof load bolts.

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Why Choose Pankaj International?

Pankaj International is a leading fastener manufacturer and supplier in India. We provide high-quality industrial fasteners designed for strength and durability.

Our Product Range:

• High Tensile Bolts

• Hex Bolts

• Stud Bolts

• Socket Head Cap Screws

• Structural Fasteners

• Custom Fasteners

Why Our Customers Trust Us:

• Premium raw materials

• Advanced manufacturing process

• Strict quality testing

• Competitive pricing

• Timely delivery

• Custom solutions available

We understand that fasteners are critical safety components. That’s why we never compromise on quality.

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How to Select the Right Fastener Based on Proof Load

Before selecting a bolt, consider:

• Total load on the joint

• Safety factor required

• Environmental conditions

• Temperature changes

• Vibration level

If you are unsure, our technical team at Pankaj International can guide you in choosing the right fastener grade and size.

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Common Questions About Fastener Proof Load

Is proof load same as working load?

No. Working load is usually lower for extra safety.

What happens if proof load is exceeded?

The bolt may stretch permanently and lose strength.

Can I use a lower grade bolt to save cost?

Not recommended. It may cause joint failure and safety risks.

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Conclusion

Fastener proof load is a key factor in ensuring safety, strength, and durability in any project. Choosing the right bolt grade and supplier makes a big difference in long-term performance.

If you are looking for a reliable fastener manufacturer in India offering high-quality bolts with proper proof load testing, Pankaj International is your trusted partner.

Contact us today to discuss your requirements and get the best fastening solutions for your project.