🔩 Bolt Stretch Calculator
Estimate target preload, elastic elongation, installation torque, clamp loss allowance, and nut rotation for structural, alloy, and metric bolting jobs.
📌 Presets
⚙ Calculator Setup
🎯 Results
📊 Material / Spec Comparison
📖 Proof Strength and Modulus Table
| Grade / Spec | Proof Strength | Modulus E | Typical Use |
|---|---|---|---|
| SAE Grade 5 | 85 ksi | 30 Msi | Fixtures, brackets, shop tooling |
| SAE Grade 8 | 120 ksi | 30 Msi | Hubs, heavy clamps, machine frames |
| ASTM A325 | 85 ksi | 29 Msi | Structural steel connections |
| ASTM A193 B7 | 105 ksi | 30 Msi | Pressure boundary flanges |
| B8M Class 2 | 65 ksi | 28 Msi | Corrosion service flanges and covers |
| Metric 8.8 | 600 MPa | 210 GPa | Frames, guards, general machinery |
| Metric 10.9 | 830 MPa | 210 GPa | Motor mounts, structural machinery |
| Metric 12.9 | 970 MPa | 210 GPa | Dies, compact high-strength assemblies |
📏 Thread Pitch and Stress Area Table
| Fastener | Pitch | Stress Area | Common Application |
|---|---|---|---|
| 3/8-16 UNC | 16 TPI | 0.0775 in² | Pump covers and medium brackets |
| 1/2-13 UNC | 13 TPI | 0.1419 in² | Structural clips and plate splices |
| 5/8-11 UNC | 11 TPI | 0.2260 in² | Flanges and base connections |
| 3/4-10 UNC | 10 TPI | 0.3340 in² | Hub clamps and press frames |
| M10 × 1.5 | 1.5 mm | 58.0 mm² | Equipment guards and welded frames |
| M12 × 1.75 | 1.75 mm | 84.3 mm² | Motor feet and machine mounts |
| M16 × 2.0 | 2.0 mm | 157.0 mm² | Die shoes and high-force clamps |
| M20 × 2.5 | 2.5 mm | 245.0 mm² | Tower joints and heavy machine bases |
📈 Nut Factor and Torque Practice Table
| Condition | Typical K | Torque Effect | Field Note |
|---|---|---|---|
| Dry carbon steel | 0.20 | Highest torque for same preload | Large scatter if threads are rough |
| Zinc plated light oil | 0.18 | Moderate torque reduction | Common for machinery service bolts |
| Moly or anti-seize | 0.15 | Lower torque, same clamp load | Watch for over-tightening by habit |
| Prevailing torque locknut | 0.18 to 0.24 | Drag must be added separately | Subtract measured drag from wrench reading |
📋 Common Joint Targets Table
| Application | Suggested Proof % | Stretch Priority | Typical Check |
|---|---|---|---|
| Structural slip-critical splice | 70 to 75% | Clamp retention | DTI, turn-of-nut, or calibrated wrench |
| Pressure flange joint | 65 to 75% | Gasket seating balance | Cross-pattern torque plus hot retorque plan |
| Precision die or mold shoe | 75 to 85% | Repeatable stretch | Ultrasonic or micrometer elongation |
| Stainless cover bolt | 55 to 65% | Gall resistance | Lubricated torque with slower tightening |
💡 Tips
Use this calculator to compare bolt preload, stretch, and torque from one setup sheet. It helps translate thread data and proof strength into a measurable elongation target.
Torque and bolt stretch are two differently methods that can be use to measure the tension that is placed into a bolt. The use of torque, however, does not always indicate the correct amount of clamp forces that is present within the bolt. Clamp force is the force that hold two part of a structure together.
Clamp force is created when a person tightens a bolt. As a person tightens the bolt, the bolt stretch. The stretch of a bolt is a better measurement of the preload of that bolt then the use of torque measurements would be, as torque can change with the presence of dirt or lubrication on the bolt.
Measuring Bolt Tension: Torque and Bolt Stretch
Bolt stretch can be measured by calipers or ultrasonic tool. Using these tools to measure bolt stretch allow for a person to understand the exact amount of tension that is placed into a bolt. In understanding bolt stretch, a person can better ensure that the bolt that is being use in a structure is providing the correct amount of clamp force to the two component that that bolt is fastening.
In calculating bolt stretch, it is first necessary for a person to understand the effective length of the bolt. The effective length of a bolt is the distance from the underside of the bolt head to the point at which the threads engages with the nut or hole. This length is important in understanding that the unthreaded portion of the bolt does not stretch in the same way as the threaded portion of the bolt.
The thread pitch of the bolt is another important variable in calculating bolt stretch. Coarse threads will have a larger advance than fine threads. Because of this, bolts with coarse threads will require few degrees of rotation to achieve a specific stretch in the bolt as compared to bolts with fine threads.
The proof strength of the bolt is another variable that need to be accounted for in calculating bolt stretch. Proof strength is the maximum load that a bolt will endure before it begin to deform. Bolt designs typically target a stretch of between 65 and 80 percent of the proof strength of the bolt.
This percentage allow for the bolt to account for any fatigue or embedment loss of the clamp force. Embedment loss can occur with material like aluminum or gaskets, which may compress over time. To calculate bolt stretch, several variable must be accounted for.
These variables include the force that is applied to the bolt, the length of the bolt, the stress area of the bolt, and the modulus of the material of which the bolt is made. The stress area is the area of the bolt that is subjected to the tensile force, and not the same as the shank diameter of the bolt. The modulus of elasticity is a value that indicate the stiffness of the material, and for steel bolts the modulus is 30 million psi.
For bolts made of stainless steel, however, the modulus is less than that of carbon steel, meaning that the bolts made of stainless steel will stretch more than bolts made of carbon steel under the same amount of load. Another factor in calculating bolt stretch is the nut factor, which is represented by the K value. The nut factor relate the preload to the torque applied to the bolt.
The nut factor changes with the surface of the bolt, such as when the bolt is oiled. For instance, if the bolt is oiled steel, the nut factor is 0.18. If a person uses anti-seize lubricant on the bolt, the nut factor will decrease.
The use of a lower nut factor will require adjustment to the torque applied to the bolt. Finally, the prevailing torque of locknut or seals will also have an impact on the tension of the bolt. If a person tightens a bolt with locknuts or seals, the prevailing torque will reduce the total tension of the bolt if only a torque wrench is utilize to set the tension.
The temperature of the bolt and the cyclic loads that are placed upon the bolt are also two variable that will impact the tension of the bolt. Changes in temperature will impact the clamp force of the bolt; an increase in temperature will reduce the preload of the bolt. Cyclic load will also impact the tension of the bolt, as these loads will cause the threads of the bolt to fatigue.
Fatigue of the threads of a bolt will occur if the load placed upon the bolt is above 50 percent of the proof strength of the bolt. By measuring the stretch of a bolt, therefore, one can verify the tension of the bolt.
