Bolt Torque & Preload Calculator

This commercial-grade calculator determines the optimal tightening torque for flanged joints and structural bolts. It uses the Standard Torque Equation ($T=KFD$) with a comprehensive library of friction coefficients (K-Factors) and Bolt Material Yield Strengths (ASTM/SAE/ISO). It ensures the bolt is preloaded to the correct percentage of yield (typically 60-75%) to prevent joint separation or bolt fatigue.

1. Fastener Specifications

Geometry
Material Grade

2. Lubrication & Target Preload

Tribology
Target

Engineering Insights: The Science of Bolting

1. Torque is a Proxy, Not the Goal

The goal of bolting is Clamping Force (Preload), not Torque. We use Torque because it is easy to measure, but it is an indirect method. We apply rotational force to creating axial tension.

The relationship is $T = K \cdot F \cdot D$. Only about 10% to 15% of the applied torque actually stretches the bolt (creating preload). The remaining 85-90% is wasted overcoming friction under the nut head and in the threads. This makes friction control ($K$) the single most critical variable.

2. The K-Factor: Friction is Everything

The "Nut Factor" ($K$) summarizes all friction variables. A small change in lubrication changes the preload drastically for the same torque.

  • Dry / Rusty ($K=0.30$): High friction. Most of your torque fights rust. Result: Low Preload (Loose Joint).
  • Machine Oil ($K=0.20$): Standard condition for "lightly oiled" black bolts.
  • Moly Paste ($K=0.11$): Extremely slippery. Low friction. If you torque a Moly-coated bolt to "Dry" specs, you will over-tension and snap the bolt because all that torque goes into stretch.

Rule: Always re-calculate torque if you change the lubricant!

3. Why 70% of Yield?

We typically aim for 60-75% of the bolt's yield strength. Why not 100%? Or 10%?

  • Too Low (< 40%): The bolt acts like a loose spring. Vibration will cause the nut to back off. The joint may separate under process pressure, causing leaks.
  • Too High (> 90%): You risk yielding the bolt during installation (stripping threads or necking). Also, external loads (pressure, thermal expansion) add to the bolt tension, pushing it over the yield point into permanent deformation or failure.

4. Know Your Grades

ASTM A193 B7: The workhorse of the petrochemical industry. High strength Cr-Mo steel, good for high temps. Typical Yield: 105 ksi.
SAE Grade 8 / Metric 10.9: High strength structural bolts.
Stainless (B8 / 304): Lower strength (Yield ~30 ksi). Prone to galling. Requires anti-seize and lower torque.

5. Accuracy of Methods

  • Hand Wrench: $\pm 30\%$ Accuracy. (Guesswork).
  • Torque Wrench: $\pm 25\%$ Accuracy. (Depends on K-factor stability).
  • Hydraulic Tensioning: $\pm 10\%$ Accuracy. (Directly pulls bolt, bypasses friction).
  • Ultrasonic / Load Cell: $\pm 2\%$ Accuracy. (Measures actual stretch).