Bolt Tightening Torque Calculator
Calculate required torque to achieve proper bolt preload
Calculation Parameters
Based on ISO 898, ASME B1.1, and VDI 2230
Calculation Results
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Tightening Guidelines:
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:
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:
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
📘 Complete Guide: Bolt Tightening Torque Calculator
🎯 What This Calculator Does
This calculator determines the required tightening torque for threaded fasteners to achieve proper preload force.
Critical for ensuring reliability and safety of bolted joints in all industries.
🌍 Understanding VDI 2230 and ISO 898 Standards
VDI 2230:2015 is the German directive "Systematic calculation of highly stressed bolted joints." The most comprehensive and recognized document worldwide for calculating bolted connections. Used in aviation, automotive, and power generation.
ISO 898-1: Bolt Strength Classes
Defines mechanical properties of fasteners with designation X.Y:
- 4.6: Yield strength 240 MPa (ordinary steel)
- 8.8: Yield strength 640 MPa (standard high-strength)
- 10.9: Yield strength 900 MPa (high-strength)
- 12.9: Yield strength 1080 MPa (ultra-high-strength)
Core Principle
Bolt must be pre-tensioned (stretched) to 75-90% of yield strength. This ensures:
- Joint remains sealed (no gapping)
- Handles variable loads (no loosening)
- Even load distribution between bolts
💼 Real-World Applications
1️⃣ Flange Assembly
Pipeline DN200, pressure 16 bar. 12 bolts M16, class 8.8. Calculator determines: 130 N·m per bolt. Tighten with torque wrench in cross pattern.
2️⃣ Critical Equipment Mounting
Gearbox mounting to frame. Bolts M20, class 10.9. Required torque: 420 N·m. Tighten in 3 passes: 30% → 70% → 100%.
3️⃣ Equipment Repair
Compressor cylinder head gasket replacement. Studs M12, class 8.8. Torque: 68 N·m. Tighten from center outward in spiral pattern.
4️⃣ Assembly Quality Control
Check tightening after installation. Measure breakaway torque. Should be 80-100% of nominal. If less - bolt loosened, re-tighten required.
📊 Practical Example: Pump Flange Connection
Application: Flange DN150, PN25
- 8 bolts M16, class 8.8
- Gasket: graphite composite
- Lubrication: graphite paste
Calculated: 95 N·m per bolt
Tightening: Cross pattern, 3 passes (30→70→100%)
Result: Joint sealed, no leaks ✓
📖 Technical Glossary
- Tightening Torque (T)
- Rotational moment applied to bolt during tightening. Measured in N·m (newton-meters) or lb·ft (pound-feet).
- Preload Force (F)
- Axial tension force in bolt after tightening. Ensures joint compression and connection reliability.
- K-factor (Torque Coefficient)
- Dimensionless coefficient linking torque and force: T = k × F × d. Depends on thread and bearing surface friction.
- Dry steel: 0.20
- Oil lubrication: 0.16
- Molybdenum grease: 0.11
- PTFE/Teflon: 0.09
- Yield Strength Percentage
- Recommended preload as percentage of yield:
- 75%: standard joints
- 85%: critical joints
- 90%: aerospace applications
- Hard Joint
- Metal-to-metal connection without gaskets. Minimal preload relaxation.
- Soft Joint
- Connection with gasket or seal. Typical relaxation 10-20%, requires re-tightening.
- Torque Wrench
- Tool for precise torque control. Types: click-type, dial-type, digital.
- Thread Galling
- Metal seizure during tightening. Common with stainless steel. Requires lubrication.
⚠️ Critical Safety Points
- Over-torquing can strip threads or break bolts
- Under-torquing leads to joint failure and leaks
- Always calibrate torque wrenches regularly
- Clean threads before assembly - dirt changes friction
- Use specified lubricant - wrong lube dramatically changes K-factor
🎓 Best Practices
- Tightening Pattern: Cross pattern for 4 bolts, star pattern for 6-8 bolts, spiral from center for flanges
- Multi-Pass Tightening: Use 2-3 passes for critical joints (first 30%, second 70%, final 100%)
- Mark Bolts: Mark bolt heads to detect loosening visually
- Re-torque: Check torque after 24 hours for soft joints
- Documentation: Record torque values for critical assemblies