Bolt Strength Calculator

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Bolt Strength Calculator

SAE Grade 5 (Imperial) SAE Grade 8 (Imperial) SAE Grade 2 (Imperial) Metric Class 10.9 Metric Class 8.8 Metric Class 12.9 ASTM A307 (Low Carbon Steel)
1/4″ (0.250 in) 5/16″ (0.3125 in) 3/8″ (0.375 in) 7/16″ (0.4375 in) 1/2″ (0.500 in) 9/16″ (0.5625 in) 5/8″ (0.625 in) 3/4″ (0.750 in) 7/8″ (0.875 in) 1″ (1.000 in)

Calculation Results

function calculateBoltStrength() { // 1. Get input values var gradeValue = document.getElementById("boltGrade").value.split(','); var proofStrengthPsi = parseFloat(gradeValue[0]); var tensileStrengthPsi = parseFloat(gradeValue[1]); var tensileArea = parseFloat(document.getElementById("boltDiameter").value); var safetyFactor = parseFloat(document.getElementById("safetyFactor").value); var resultDiv = document.getElementById("result"); var resultContainer = document.getElementById("result-container"); // 2. Validate inputs if (isNaN(proofStrengthPsi) || isNaN(tensileStrengthPsi) || isNaN(tensileArea) || isNaN(safetyFactor)) { resultDiv.innerHTML = 'Error: Please ensure all inputs are valid numbers.'; resultContainer.style.display = 'block'; return; } if (tensileArea <= 0 || safetyFactor <= 0) { resultDiv.innerHTML = 'Error: Bolt diameter and safety factor must be greater than zero.'; resultContainer.style.display = 'block'; return; } // 3. Perform calculations // Ultimate Tensile Strength (Breaking Point) var ultimateTensileLoadLbs = tensileStrengthPsi * tensileArea; // Proof Load (Max load without permanent stretch) var proofLoadLbs = proofStrengthPsi * tensileArea; // Ultimate Shear Strength (Approx. 60% of Tensile Strength) var ultimateShearLoadLbs = ultimateTensileLoadLbs * 0.6; // Recommended Working Load (Tensile) with Safety Factor var workingTensileLoadLbs = ultimateTensileLoadLbs / safetyFactor; // Recommended Working Load (Shear) with Safety Factor var workingShearLoadLbs = ultimateShearLoadLbs / safetyFactor; // 4. Display results resultDiv.innerHTML = 'Ultimate Tensile Strength (Breaking Point): ' + ultimateTensileLoadLbs.toLocaleString(undefined, {maximumFractionDigits: 0}) + ' lbs' + 'Proof Load (Max Load w/o Damage): ' + proofLoadLbs.toLocaleString(undefined, {maximumFractionDigits: 0}) + ' lbs' + 'Ultimate Shear Strength (Breaking Point): ' + ultimateShearLoadLbs.toLocaleString(undefined, {maximumFractionDigits: 0}) + ' lbs
' + 'Recommended Max Working Load (Tensile): ' + workingTensileLoadLbs.toLocaleString(undefined, {maximumFractionDigits: 0}) + ' lbs' + 'Recommended Max Working Load (Shear): ' + workingShearLoadLbs.toLocaleString(undefined, {maximumFractionDigits: 0}) + ' lbs'; resultContainer.style.display = 'block'; }

Understanding Bolt Strength

Bolts are fundamental components in construction, automotive, and mechanical engineering, but not all bolts are created equal. Their ability to clamp parts together and resist forces is defined by their material, size, and grade. This calculator helps you estimate the key strength characteristics of a standard bolt to ensure your application is safe and secure.

How to Use the Bolt Strength Calculator

To determine the strength of a bolt, you need three key pieces of information:

  1. Bolt Grade/Material: Select the grade of your bolt. This determines its intrinsic material strength. Common imperial grades are SAE Grade 2, 5, and 8, marked by radial lines on the bolt head. Metric bolts use a class system like 8.8, 10.9, or 12.9. Higher numbers indicate greater strength.
  2. Bolt Diameter: Choose the nominal diameter of the bolt. A larger diameter provides a greater cross-sectional area to resist forces. The calculator uses the standard "tensile stress area" for each diameter, which accounts for the reduced cross-section at the threads.
  3. Safety Factor: This is a crucial multiplier that accounts for uncertainties like dynamic loads, vibration, temperature changes, and risk to human life. A safety factor of 1 means you are designing for the bolt to be at its breaking point. Common safety factors range from 2 to 5 or higher for critical applications.

Once you input these values, the calculator provides several key metrics in pounds (lbs) of force.

Key Bolt Strength Terms Explained

  • Ultimate Tensile Strength: This is the absolute maximum pulling (tensile) force a bolt can withstand before it fractures or breaks. It's also known as the "breaking point."
  • Proof Load: This is the maximum load a bolt can handle without undergoing permanent deformation. Once you exceed the proof load, the bolt will be permanently stretched and will not return to its original length. It is the highest load you can apply to a bolt that will still allow it to be safely reused.
  • Ultimate Shear Strength: This is the maximum force a bolt can withstand when forces are applied perpendicular to its axis, essentially trying to slice it in half. For steel bolts, this is commonly estimated as 60% of the ultimate tensile strength.
  • Recommended Working Load: This is the most important value for design purposes. It is the ultimate strength (either tensile or shear) divided by your chosen safety factor. You should never design a connection where the expected load exceeds this value.

Example Calculation

Let's say you are building a structure and need to select a bolt for a connection that will experience a maximum tensile load of 6,000 lbs. You want to use a SAE Grade 8 bolt with a safety factor of 3.

Which bolt diameter should you choose?

  1. Select "SAE Grade 8" and set the Safety Factor to "3".
  2. Try a 3/8″ bolt. The calculator shows a Recommended Max Working Load (Tensile) of 6,250 lbs. This is greater than your required 6,000 lbs, making it a suitable choice.
  3. If you had tried a 5/16″ bolt, the working load would be only 4,375 lbs, which is insufficient.

This process allows you to quickly size fasteners for your project's specific needs.

Disclaimer

This calculator provides estimates based on standard engineering formulas and material properties. It is intended for educational and preliminary design purposes only. For critical applications, especially those involving safety, dynamic loads, or extreme conditions, always consult a licensed professional engineer. The calculations do not account for factors like thread engagement, lubrication, fatigue, or combined shear and tensile loads.

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