Knowledge Base
Engineering reference for bolted joint design and assembly. Tightening torque tables M4–M42 for grades 8.8/10.9/12.9/A2-70/A4-70. Preload fundamentals, nut factor values, thread engagement requirements for steel/aluminium/plastic, thread locking methods, and drive type comparison.
Fundamentals
A bolted joint functions by clamping the joined parts together using the elastic stretching of the bolt. The clamping force (preload) must exceed the external service loads to maintain joint integrity. Understanding preload is essential for avoiding joint failure. VDI 2230-1
| Parameter | Symbol | Formula / Value | Notes |
|---|---|---|---|
| Bolt preload | F_M | F_M = k × A_s × R_p0.2 | k = tightening factor (typically 0.6–0.9) |
| Tightening torque | M_A | M_A = F_M × (0.159P + 0.578 × d₂ × μ_th + 0.5 × d_w × μ_h) | P = pitch; μ_th = thread friction; μ_h = head bearing friction |
| Torque-tension relationship | — | M_A ≈ K × d × F_M | K = nut factor (0.11–0.20 typical dry) |
| Nut factor — dry uncoated | K | 0.17–0.22 | Unlubricated carbon steel on steel |
| Nut factor — lightly oiled | K | 0.13–0.17 | Oil or wax lubricated |
| Nut factor — PTFE / Xylan | K | 0.10–0.12 | Consistent, low friction coating |
| Nut factor — hot-dip galvanised | K | 0.18–0.25 | Variable; dip lubricant recommended |
| Nut factor — cadmium plated | K | 0.11–0.15 | Low and consistent |
| Stress area | A_s | π/4 × ((d₂+d₃)/2)² | See ISO 261 / thread data table |
| Clamp force retention | — | ~80–90% of initial preload | After embedding and relaxation |
VDI 2230-1 is the primary reference for systematic bolted joint calculation. Nut factor K varies significantly with surface condition — always verify for critical joints.
| Method | Scatter in Preload (±%) | Notes |
|---|---|---|
| Torque wrench (dry) | ±25–35 | High variability due to friction |
| Torque wrench (lubricated) | ±15–25 | Improved consistency with defined lubricant |
| Torque + angle (torque-angle) | ±5–10 | More consistent; angle controls stretch directly |
| Torque-to-yield (TTY) | ±3–5 | One-time use; bolt approaches yield; common in automotive |
| Direct tension indicator (DTI) | ±5–10 | Washer-based visual/tactile confirmation |
| Ultrasonic bolt load measurement | ±1–3 | Direct elongation measurement; precision critical joints |
For structural applications, ISO 4993 (friction grip bolting) and EN 14399 series define preloading methods. VDI 2230-1 covers engineering joints.
ISO 16047
Indicative tightening torques for metric fasteners. Values shown assume: dry uncoated steel-on-steel (μ ≈ 0.14), utilisation of 70% of proof load. Always verify torque for your specific fastener, coating, lubricant and joint conditions. ISO 16047
| Size | Pitch (mm) | 8.8 Dry (Nm) | 10.9 Dry (Nm) | 12.9 Dry (Nm) | A2-70 Dry (Nm) | A4-70 Dry (Nm) |
|---|---|---|---|---|---|---|
| M4 | 0.7 | 3.0 | 4.3 | 5.1 | 1.9 | 1.9 |
| M5 | 0.8 | 6.0 | 8.6 | 10.0 | 3.7 | 3.7 |
| M6 | 1.0 | 10.3 | 14.8 | 17.3 | 6.4 | 6.4 |
| M8 | 1.25 | 25 | 36 | 43 | 15 | 15 |
| M10 | 1.5 | 49 | 71 | 83 | 30 | 30 |
| M12 | 1.75 | 85 | 122 | 143 | 52 | 52 |
| M14 | 2.0 | 135 | 193 | 226 | 83 | 83 |
| M16 | 2.0 | 210 | 300 | 351 | 130 | 130 |
| M18 | 2.5 | 290 | 415 | 485 | 179 | 179 |
| M20 | 2.5 | 410 | 585 | 685 | 253 | 253 |
| M22 | 2.5 | 560 | 800 | 935 | 346 | 346 |
| M24 | 3.0 | 710 | 1015 | 1185 | 438 | 438 |
| M27 | 3.0 | 1050 | 1500 | 1755 | 648 | 648 |
| M30 | 3.5 | 1430 | 2045 | 2390 | 883 | 883 |
| M33 | 3.5 | 1940 | 2775 | 3245 | — | — |
| M36 | 4.0 | 2500 | 3575 | 4180 | — | — |
| M39 | 4.0 | 3200 | 4575 | 5350 | — | — |
| M42 | 4.5 | 4000 | 5720 | 6685 | — | — |
Values ±30% depending on friction, lubrication and surface condition. A2/A4-70 values based on Rp0.2 = 450 MPa at 70% utilisation. For lubricated joints, reduce torque by approximately 20–30%. Always use a calibrated torque tool.
Engineering Reference
Thread engagement length is the axial length of thread contact between bolt and nut or tapped hole. Insufficient engagement leads to strip-out before the bolt reaches its tensile capacity. VDI 2230-1
| Parent Material | Minimum Engagement | Recommended Engagement | Notes |
|---|---|---|---|
| Steel (hardened / property class 8.8+) | 0.8 × d | 1.0 × d | Bolt and nut of matched property class |
| Steel (mild, S235/S275) | 1.0 × d | 1.25 × d | Lower-strength steel requires more engagement |
| Cast iron | 1.0 × d | 1.5 × d | Brittle; use full thread run-in depth |
| Aluminium alloy (6061, 7075) | 1.5 × d | 2.0 × d | Lower shear strength; insert preferred |
| Aluminium alloy with steel insert | 1.0 × d | 1.25 × d | Helicoil / threaded insert restores steel-equivalent strength |
| Magnesium alloy | 1.5–2.0 × d | 2.5 × d | Very low thread shear strength |
| Thermoplastic (ABS, PC, Nylon) | 2.0 × d | 3.0 × d | Moulded-in inserts or heat inserts strongly recommended |
| Thermoset / GRP | 1.5 × d | 2.0 × d | Brittle; cross-thread and strip risk; inserts preferred |
d = nominal bolt diameter. Engagement length assumes full-form thread in both components. Interrupted threads, blind holes and coated fasteners require additional review.
| Insert Type | Installation Method | Typical Use | Pull-out vs. plain tapped hole |
|---|---|---|---|
| Wire thread insert (Helicoil) | Special tap + installation tool | Aluminium, magnesium, plastic repair | Typically 200–300% improvement |
| Key-locking insert (Keensert) | Drilled, tapped, keys driven | High torque, heavy-duty aluminium | High pull-out and torque-out resistance |
| Moulded-in insert (brass/steel) | Insert moulded in during manufacture | Thermoplastics, composites | Full metal thread strength |
| Heat-set insert | Heated and pressed in | Thermoplastics (ABS, PC, Nylon) | Moderate; good for light assemblies |
| Self-tapping insert | Driven/screwed in, no pre-tap needed | Field repair, plastic | Variable; application-dependent |
Thread inserts should be specified on drawings where parent material thread strength is insufficient. DIN 8140 (wire thread inserts) and manufacturers' data sheets define installation requirements.
ISO 2320 / ISO 7042
Vibration, dynamic loading and thermal cycling can cause bolted joints to loosen progressively. Locking methods are classified as mechanical (positive locking) or friction-based (prevailing torque or clamping). ISO 2320
| Method | Type | ISO / Standard | Re-usable | Notes |
|---|---|---|---|---|
| All-metal prevailing torque nut | Friction / deformed thread | ISO 7042, ISO 7043 | Limited (3–5×) | Elliptical thread form or distorted top threads resist loosening |
| Nylon insert nut (Nyloc) | Friction / polymer insert | ISO 7042, ISO 10511 | No | Nylon patch grips thread; temperature limit ~120°C |
| Serrated flange nut | Mechanical / friction | ISO 4161 | No (damages surface) | Serrations bite into bearing surface; permanent grip |
| Spring washer (split ring) | Friction / clamping | DIN 127 (obsolete) | Limited | Minimal locking effect; DIN no longer recommends for vibration applications |
| Toothed lock washer | Mechanical / bite | DIN 6798 A/J/I, ISO 8738 | No (damages surface) | Internal/external/fern teeth bite into head and surface; good for soft materials |
| Tab washer / lock tab | Mechanical / positive | DIN 462, DIN 463 | No (deforms) | Bent tab locks nut mechanically; requires castellated slot or feature |
| Castle nut + split pin | Mechanical / positive | DIN 935 + DIN 1 / ISO 1234 | Limited (new pin) | Fully positive locking; aerospace, structural, safety-critical |
| Wire locking (safety wire) | Mechanical / positive | NASM 33540 | No (wire) | Common in aerospace; wire prevents rotation; torque must be correct direction |
| Thread locking adhesive (anaerobic) | Chemical | — | No (heat removal) | Fills thread clearance; cure prevents loosening; rated by breakaway torque |
| Conical spring washer (Belleville) | Load/friction | DIN 6796 | Yes | Maintains clamp force during relaxation; stack for higher deflection |
| Prevailing torque bolt (patched) | Friction / chemical patch | ISO 10513 | No | Polyamide or adhesive patch on bolt threads; factory applied |
DIN 127 split-ring spring washers are no longer considered effective for vibration locking by DIN and ISO (per DIN report). For dynamic applications, use all-metal prevailing torque nuts, anaerobic adhesive, or positive mechanical locking.
Reference
Drive type determines the torque transmission capability, access requirements and tool compatibility. Selection affects fastener head size, assembly speed and risk of cam-out or damage. ISO 1173
| Drive Type | ISO Standard | Bit / Socket Ref | Cam-Out Risk | Max Torque Capability | Notes | |
|---|---|---|---|---|---|---|
| Hexagon (external) | ISO 272 / ASME B18.2.1 | Spanner / socket WAF | N/A (external) | Very high | Standard for structural, hex head bolts | |
| Hexagon socket (Allen / hex key) | ISO 4762 | Hex key by WAF mm | Low | High | Cap screws, machine screws; 6 sizes M3–M20 | |
| Torx / 6-lobe (TX) | ISO 10664 | T6–T100 | Very low | High | Excellent torque transfer; automotive, precision | |
| Torx Plus (IP) | Proprietary (Acument) | IP6–IP60 | Very low | Very high | Deeper drive form; resists overwear | |
| Pozidriv (PZ) | ISO 8764-1 | PZ0–PZ4 | Low | Moderate | Distinct cross-recess with alignment ribs; not interchangeable with Phillips | |
| Phillips (PH) | ISO 8764-1 | PH0–PH4 | Moderate | Moderate | Designed to cam out at limit torque; common for wood/drywall screws | |
| Slotted | ISO 2380-1 | SL1.0–SL4.0 | High | Low | Legacy; not suitable for power driving | |
| Robertson (square) | — | R0–R4 | Low | High | Common in North America; good torque transmission | |
| Hex flange (combo) | — | WAF mm | N/A | High | Hex head + flange eliminates separate washer | |
| Tri-Wing / Pentalobe | Proprietary | — | N/A | Moderate | Consumer electronics security fasteners |
Pozidriv (PZ) and Phillips (PH) look similar but are NOT interchangeable — using the wrong driver causes rapid wear and cam-out. Torx (TX) is preferred over Phillips for automated assembly and high-torque applications.
References
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Welcome back. If you're sourcing fastenings, cleanroom consumables or ESD products, we'd be happy to help.