Calculation Parameters

Based on ISO 898, ASME B1.1, and VDI 2230









Calculation Results

Recommended Torque:
Target Preload Force:
Friction Coefficient (k-factor):
Min-Max Torque Range:

Tightening Guidelines:

Step 1: Hand tighten until snug
Step 2: Apply 30% of final torque
Step 3: Apply 70% of final torque
Step 4: Apply 100% of final torque in smooth motion

How the Calculator Works

Reference Standards

International Standards:

  • VDI 2230:2015 – Systematic calculation of highly stressed bolted joints
  • ISO 898-1 – Mechanical properties of fasteners – Bolts, screws and studs
  • ASME B1.1 – Unified Inch Screw Threads
  • DIN 946 – Torque-preload relationship for threaded fasteners
  • ECSS-E-HB-32-23A – Threaded fasteners handbook (ESA)

Basic Torque Formula

The required tightening torque is calculated using:

T = k × F × d

where:

  • T — tightening torque (N·m)
  • k — torque coefficient (typically 0.15-0.25)
  • F — desired preload force (N)
  • d — nominal bolt diameter (m)

Preload Force Calculation

The preload force is determined by:

F = σ_y × A_s × utilization

where:

  • σ_y — yield strength of bolt material (MPa)
  • A_s — tensile stress area (mm²)
  • utilization — percentage of yield strength used

K-Factor (Torque Coefficient)

The k-factor depends on thread and bearing surface friction:

  • Dry/unlubricated: k = 0.20-0.25
  • Lightly oiled: k = 0.15-0.18
  • Molybdenum disulfide: k = 0.10-0.12
  • PTFE coating: k = 0.08-0.10

Bolt Strength Classes

Metric bolt classes indicate tensile and yield strength:

  • Class 8.8: 800 MPa tensile, 640 MPa yield
  • Class 10.9: 1000 MPa tensile, 900 MPa yield
  • Class 12.9: 1200 MPa tensile, 1080 MPa yield

Important Considerations

  • Always use calibrated torque tools
  • Clean threads before assembly
  • Apply torque in gradual steps for critical joints
  • Consider torque relaxation in soft joints
  • Account for prevailing torque in lock nuts
  • Re-torque after initial settling if required

Safety Factors

  • Static loads: 75-85% of yield typical
  • Dynamic loads: 50-65% of yield recommended
  • Critical safety: Additional analysis required
  • Reused bolts: Reduce torque by 10-20%

Usage Examples & Value Selection Guide

Example 1: Pump Flange Connection

Scenario: Connecting pump to motor with coupling guard

  • Bolt Size: M12
  • Grade: 8.8
  • Lubrication: Lightly oiled
  • Preload: 75% (standard)
  • Joint Type: Hard (steel to steel)
  • Method: Manual torque wrench
  • Result: 78 N·m (58 ft·lb)
  • Note: Tighten in cross pattern
Example 2: Pressure Vessel Flange

Scenario: High pressure steam line with spiral wound gasket

  • Bolt Size: M20
  • Grade: 10.9
  • Lubrication: Molybdenum disulfide
  • Preload: 85% (high performance)
  • Joint Type: Soft (with gasket)
  • Method: Hydraulic tensioner
  • Result: 340 N·m (251 ft·lb)
  • Critical: Follow ASME PCC-1 sequence
Example 3: Engine Head Bolts

Scenario: Automotive engine cylinder head assembly

  • Bolt Size: M10
  • Grade: 12.9
  • Lubrication: Engine oil
  • Preload: 90% (maximum)
  • Joint Type: Tapped hole
  • Method: Torque + angle
  • Result: 65 N·m + 90° turn
  • Note: Torque-to-yield application

How to Choose Values

Bolt Grade Selection
  • Class 4.6/Grade 2:
    • Non-critical applications
    • Light loads only
    • Low cost option
  • Class 8.8/Grade 5:
    • General engineering
    • Most common choice
    • Good strength/cost ratio
  • Class 10.9/Grade 8:
    • High strength applications
    • Dynamic loads
    • Reduced bolt count possible
  • Class 12.9:
    • Maximum strength
    • Critical applications
    • Special handling required
Lubrication Selection
  • Dry (k=0.20): Inconsistent results, avoid if possible
  • Light Oil (k=0.15): Standard choice, consistent
  • Moly Paste (k=0.10): High loads, stainless steel
  • PTFE (k=0.08): Lowest friction, precise preload
  • Anti-seize: Use specified k-factor from manufacturer
Preload Selection Guide
  • 50% Yield:
    • Vibrating equipment
    • Frequent disassembly
    • Aluminum components
  • 75% Yield:
    • Standard static joints
    • Steel assemblies
    • Most applications
  • 85-90% Yield:
    • Critical joints
    • No joint separation allowed
    • Engineered applications only
Joint Type Considerations
  • Hard Joint:
    • Metal to metal contact
    • Minimal relaxation
    • Standard torque values apply
  • Soft Joint:
    • Gaskets, O-rings present
    • Expect 10-20% relaxation
    • May need retorque
  • Tapped Holes:
    • Check thread engagement (2×D min)
    • Beware of bottom-out
    • Consider helicoil for aluminum
Tightening Sequence
  • 4-bolt pattern: Cross pattern (1-3-2-4)
  • 6-bolt pattern: Star pattern
  • Circular flange: 180° opposites, then 90°
  • Multiple passes: 30% → 70% → 100% → verify
  • Large flanges: Use ASME PCC-1 legacy method