Estimate the required tightening torque for a given bolt diameter and target preload based on friction characteristics.
Last updated: March 2026
Dry: 0.20, Lubricated: 0.15
Disclaimer: This calculator provides estimated torque values based on simplified assumptions and a standard torque-preload formula. Actual torque-to-preload relationships vary significantly depending on lubrication, thread condition, surface finish, bolt material, and environmental factors. Torque-preload scatter is typically ±25–30%. Always follow manufacturer specifications or engineering standards for critical applications. Improper torque can cause joint failure or bolt breakage.
Bolt torque is the rotational force applied to a bolt during tightening. When you turn a wrench, you create torque, which translates into tension within the bolt shank. This tension creates a clamping force called preload that holds the joint together and prevents loosening under vibration or load.
The relationship between torque and preload is not linear—it depends on the friction between the threads, the underside of the bolt head, and the materials being clamped. The friction factor (K-factor or nut factor) accounts for these variables. Typical K-factors range from 0.15 for lubricated bolts to 0.20 for dry, unlubricated fasteners. Proper torque ensures the bolt is neither under-tightened (risking joint failure) nor over-tightened (risking bolt breakage).
For imperial units, divide by 12 to convert from lb-in to lb-ft. For metric units, divide by 1000 to convert from N-mm to N-m. The K-factor typically ranges from 0.12 (well-lubricated, precision threads) to 0.25 (rusty or corroded threads).
Calculate torque for a 1/2" diameter bolt with 10,000 lbs preload, dry threads (K = 0.20):
The K-factor (or nut factor) accounts for friction in the threads and under the bolt head. A typical dry bolt has K = 0.20, while a lubricated bolt might be K = 0.15 or lower.
Preload ensures that the joint remains under compression even when external loads are applied. This prevents fatigue failure, loosening under vibration, and maintains joint integrity.
A common rule is to target 75% of the bolt's proof load for permanent joints, or 60-70% for joints that may need disassembly. Consult bolt grade specifications.
Yes, absolutely. Bolt grade determines its strength (proof load). Higher-grade bolts like Grade 8 or Class 10.9 can handle much higher preloads and torques than lower grades.
It depends. Lubrication reduces friction (lower K), allowing higher preload for the same torque. However, some applications (like safety-critical aerospace) specify dry torque values.
Overtightening can yield or break the bolt, strip threads, or crush the clamped material. Always use a calibrated torque wrench and follow manufacturer specifications.
Select K based on thread condition and lubrication; consult manufacturer data or engineering tables. When in doubt, use conservative (higher) K to avoid under-torquing.
Some assemblies benefit from retorquing after initial service or thermal cycles, but retorquing practices vary—follow equipment-specific maintenance guidelines.
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