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Torque, Preload & EC3 Joint Sizer

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This module is a dual-purpose engineering tool. Its primary function is to calculate highly accurate assembly torques and field preloads using the VDI 2230 / ISO or API 6A methodologies. Its secondary function is a rigorous Eurocode 3 (EN 1993-1-8) connection sizer to verify the structural capacity of the bolts under external loads.

1. Practical Use Cases
Reverse Engineering (Swapping Lubricants)

If you have an existing manual or drawing that specifies a torque for a specific lubricant, but you need to assemble the joint using a different lubricant, you can use this tool to find the exact equivalent torque:

  1. Replicate the Original State: Set the Bolt Size, Class, and the Original Lubricant.
  2. Match the Torque: Adjust the Target Stress % until the value in the Total Torque (MA) column perfectly matches your known drawing torque.
  3. Swap the Lubricant: Without changing the Target Stress %, change the Lubricant dropdown to your new paste/grease.
  4. Get the New Torque: The tool instantly recalculates the new Total Torque (MA) required to achieve the exact same internal clamping force using the new friction coefficients.
Generating Drawing Notes & Manuals

The tool includes built-in export features designed specifically for CAD draftsmen and technical writers:

  • Automated Drawing Notes: Click directly on any Bolt Name (e.g., M24) in the table. This copies a perfectly formatted drawing note to your clipboard, containing the Make-Up Torque, Tool Accuracy, Material, Lubricant, and Design Basis.
  • Multiple Lubricant Notes: Click the Export Settings button. Under "Drawing Notes," check multiple lubricants. Now, when you click a bolt name, the copied note will list the specific torque required for every selected lubricant simultaneously.
  • Excel Export for Manuals: Use the Export Settings to check/uncheck specific columns (like Pitch, Area, or Thread Torque) and rename the headers to match your company's standard language. Check the boxes next to the bolts you want, and click Copy Selected to Excel to instantly paste a formatted table into your manual or drawing spreadsheet.
2. Core Parameters Explained
  • Target Stress Phase:
    • During Torquing (Combined): Limits the stress while the wrench is actively turning the nut. This includes both axial stretching (tension) and twisting (torsion). This is the safest and most common basis.
    • After Preload (Axial): Limits the stress after the wrench is removed and the torsion relaxes out of the bolt.
  • Tool Accuracy (±%): The mechanical tolerance of your wrench. A standard click-wrench is usually ±5%, while a hydraulic impact might be ±20%.
  • Friction Scatter (±%): Environmental unpredictability. Lab conditions might be ±5%, while rusty, uncontrolled field assembly might be ±30%.
  • Tightening Factor (αA): Derived directly from your Tool and Friction scatter. It represents the ratio between the Maximum possible field preload and the Minimum possible field preload (Fmax / Fmin).
3. Deep Dive: Eurocode 3 (EN 1993-1-8) Sizer

Clicking the EC3 Bolt Sizer button opens a structural validation module. It takes the tightening parameters from the main table and checks them against external Tension (Ft,Ed) and Shear (Fv,Ed) forces.

A. Connection Categories

The math adapts strictly based on EN 1993-1-8 Table 3.2:

  • Category A (Bearing): Shear loads are resisted by the bolt shank bearing against the hole. Structural preloading is not required by code, but the tool conservatively targets a 50% yield makeup torque to ensure standard field execution.
  • Category B (Slip-Resistant SLS): Friction between the plates must resist shear at the Serviceability Limit State.
  • Category C (Slip-Resistant ULS): Friction must resist shear at the Ultimate Limit State.
  • Category E (Preloaded Tension): Joints loaded in tension that must not separate under load.
B. Field Scatter vs. Structural Capacity

Because tools and friction are imperfect, you never hit your exact target preload. You get a "Mean" preload, but the actual bolt might land anywhere between the "Min" and "Max".

The Min Preload Toggle: The EC3 module features a critical switch: "Base Slip & Separation Capacity on MINIMUM Field Preload". When active, the tool assumes the worst-case scenario where field scatter results in the lowest possible clamping force.

Separation Warnings: If the external tension (Ft,Ed) exceeds your active clamping force (FM), the plates will physically separate. The tool will throw a warning flag indicating the joint fails the separation criteria.
C. Utilization Ratios (U)

The table outputs utilization percentages. Anything ≤ 100% is safe.

  • Tension (Ut): Applied Tension divided by Design Tension Resistance (Ft,Rd).
  • Shear (Uv): Applied Shear divided by Design Shear Resistance (Fv,Rd). A toggle is available to determine if the shear plane passes through the threaded area (As) or the thicker unthreaded shank (Agross).
  • Slip (Uslip - Cat B/C only): Evaluates the friction capacity between the plates. The active clamping force is dynamically reduced by external tension (Fclamp = FM - 0.8 × Ft,Ed) before multiplying by the slip factor (μ) and hole factor (ks).
  • Combined Interaction (Ucomb): Calculates the quadratic interaction limit state for bolts experiencing simultaneous tension and shear:
    Ucomb = Uv + ( Ut / 1.4 ) ≤ 1.0