IDB-BLT-025
Fasteners · preload · torque · property classes · thread engagement
Bolted joint reference
Designing a bolted joint around preload — property classes, torque-to-tension, thread engagement, locking methods, and the failure modes that come from getting clamp force wrong.
Abstract
A bolted joint carries load through preload (clamp force), not by the bolt acting as a shear pin. You tighten the bolt to stretch it; the stored tension clamps the members. Because the clamped joint is far stiffer than the bolt, most of any external tensile load goes into relaxing the clamp rather than adding to bolt stress — which is why a properly preloaded joint barely feels fatigue and won't self-loosen.
Section 1 explains how the joint works. Section 2 covers property classes and materials. Section 3 is preload and tightening (torque-to-tension, the nut factor, method accuracy). Section 4 is thread engagement and stripping. Section 5 is joint design rules. Section 6 covers loosening and locking methods. Section 7 is failure modes and a tightening checklist.
1.How a bolted joint works
Tightening a bolt stretches it like a spring; the stored tension is the preload (Fᵢ), and the equal-and-opposite compression in the members is the clamp force. At rest they balance.
When an external tensile load Fₑ is applied, it is shared between the bolt and the joint in proportion to their stiffness. Because the clamped members are much stiffer than the slender bolt, the bolt picks up only a small fraction — the load factor Φ = k_bolt / (k_bolt + k_members), typically 0.1–0.3. The rest of Fₑ simply relaxes the clamp. So a highly preloaded bolt sees only a small stress swing under cyclic load — the key to fatigue life — and the joint stays clamped as long as Fₑ never exceeds the preload.
1.1Key terms
Design rule of thumb: preload high (≈75% of proof), keep external load below preload, and give the bolt enough grip to stretch. Those three decisions carry most joints.
2.Property classes and materials
Steel metric bolts are graded by property class, marked on the head. The first number ×100 ≈ tensile strength (MPa); the second ×10 ≈ yield as a % of tensile.
| Class | Proof Sₚ (MPa) | Yield 0.2% (MPa) | Tensile Sᵤ (MPa) | Notes |
|---|---|---|---|---|
| 4.8 | 310 | 340 | 420 | Low-carbon, general purpose |
| 8.8 | 580 (≤M16) / 600 | 640 | 800 | Med-carbon, quenched & tempered — the workhorse |
| 10.9 | 830 | 940 | 1040 | Alloy steel Q&T, compact high-load |
| 12.9 | 970 | 1100 | 1220 | Highest standard class; embrittlement-sensitive |
| A2-70 (304 SS) | — | 450 | 700 | Austenitic stainless, cold-worked |
| A4-80 (316 SS) | — | 600 | 800 | Marine / corrosion service |
Use 8.8 by default; step to 10.9/12.9 only when space forces a smaller, higher-strength bolt — and watch hydrogen embrittlement on plated high-strength parts. Stainless (A2/A4) trades strength for corrosion resistance and galls easily when run dry.
3.Preload and tightening
Target preload is normally ≈ 75% of proof load for reusable joints: Fᵢ = 0.75 · Sₚ · Aₛ. Permanent, yield-controlled joints go higher.
Torque is the usual but indirect way to reach that tension:
T = K · Fᵢ · d — where d is the nominal diameter and K is the nut factor (torque coefficient), which is dominated by friction under the head and in the threads.
| Thread / underhead condition | Nut factor K |
|---|---|
| Clean & dry steel | 0.20–0.30 (galls on stainless) |
| As-received, light oil | 0.20 (default assumption) |
| Zinc plated | 0.20–0.22 |
| Black oxide | 0.15 |
| Oil / grease lubricated | 0.12–0.15 |
| MoS₂ / anti-seize / wax | 0.10–0.12 |
| PTFE coated | 0.10–0.12 |
Because K varies so much, torque is a scattered way to set preload — the lubrication state matters as much as the wrench:
| Tightening method | Preload scatter |
|---|---|
| Torque by hand feel | ±35% |
| Calibrated torque wrench | ±25% |
| Torque + angle (turn-of-nut) | ±15% |
| Torque-to-yield | ±8% |
| Bolt stretch / ultrasonic / tensioner | ±3–10% |
Guideline torque, class 8.8, K = 0.20, ~75% proof (use the Screw torque & preload tool for the exact value with your K):
| Size | Torque (N·m) | Size | Torque (N·m) | |
|---|---|---|---|---|
| M3 | 1.3 | M8 | 25 | |
| M4 | 3.0 | M10 | 51 | |
| M5 | 6.0 | M12 | 89 | |
| M6 | 10.5 | M16 | 215 |
For class 10.9 multiply by ≈1.4; for 12.9 by ≈1.7. Always lubricate consistently or the same torque gives wildly different preload.
4.Thread engagement and stripping
Size the engagement so the bolt breaks before the threads strip — a bolt fracture is ductile and visible; a stripped thread fails silently. Minimum engagement length depends on the nut/tapped material, not the bolt:
| Nut / tapped material (steel bolt) | Min engagement Lₑ |
|---|---|
| Steel, ≥ bolt class | 0.8–1.0 × d |
| Cast iron | ~1.25 × d |
| Aluminium | 1.5–2.0 × d |
| Magnesium / plastics | 2.0–2.5 × d |
Add 1–2 thread pitches in blind tapped holes for incomplete lead threads. The Thread engagement tool sizes Lₑ exactly for a given bolt class and parent material.
5.Joint design rules
- Give the bolt grip. A long, slender bolt stores more stretch, so it holds preload through embedment and thermal cycling and sees a smaller fatigue swing. Short, stubby bolts lose preload from tiny settlements.
- Avoid soft joints in fatigue. Gaskets and stacked coatings creep and relax the clamp. Through-bolt to metal where possible, or specify a re-torque after seating.
- Washers spread the bearing load and protect the surface under the turned elementalways use them on slotted holes, soft materials, and oversized clearance.
- Spacing & edge distance: edge distance ≥ 1.5–2 × d, bolt spacing ≥ 2.5–3 × d.
- Shear joints: decide slip-critical (preload carries shear by friction) vs bearing (shank bears on the hole). Slip-critical needs full, verified preload.
- Don't over-clearance the holelarge clearance plus soft members invites slip and embedment.
6.Loosening and locking
Self-loosening is driven by transverse (sideways) vibration that momentarily overcomes thread friction and lets the nut back off (the Junker mechanism). The first defense is always high, maintained preload with adequate grip — most "loose bolts" are really under-preloaded bolts.
| Locking method | Mechanism | Vibration retention | Reusable |
|---|---|---|---|
| Correct preload + grip | clamp friction | high | yes |
| Nylon-insert nut (nyloc) | insert friction | medium | limited (≤ ~120 °C) |
| All-metal prevailing-torque nut | deformed thread | med–high | few reuses |
| Wedge-lock washer pair (e.g. Nord-Lock) | wedge cam under head | very high | yes |
| Threadlocker adhesive | cured resin fills threads | high (by grade) | single-use; degrease first |
| Split / spring lock washer | — | ~none vs transverse vibration | — |
| Jam (double) nut | thread-tension reversal | med–high | yes |
| Castle nut + cotter / lockwire | positive form lock | very high | single-use pin |
Note: the common split lock washer does not reliably prevent transverse self-loosening in modern testing — don't rely on it for vibration.
7.Failure modes and checklist
| Mode | Cause | Fix |
|---|---|---|
| Fatigue fracture (head fillet / first thread) | low preload, short grip, stress raiser | preload ~75% proof, longer grip, rolled threads, head fillet |
| Thread stripping | too little engagement, soft nut, over-torque | increase Lₑ, harder nut/insert, control torque |
| Embedment / relaxation | rough or soft faces, coatings, gaskets settle | re-torque after seating, fewer interfaces, harder faces |
| Over-torque yield | wrong K, inconsistent lube | control method, use correct K |
| Hydrogen embrittlement | plated high-strength (≥10.9) | bake after plating, mechanical zinc, avoid in H₂ service |
| Galling (stainless) | dry threads seize on run-down | anti-seize, slower speed, dissimilar alloy pairing |
| Galvanic corrosion | dissimilar metals + electrolyte | matched/coated fasteners, isolation |
7.1Tightening checklist
- Class8.8 by default; 10.9/12.9 only when space forces a smaller bolt; A2/A4 for corrosion.
- Size & preloadsize by tensile stress area Aₛ; set target preload ≈ 75% proof.
- Tightening spectorque via T = K·Fᵢ·d with the actual K, or angle / yield / stretch for accuracy.
- Engagementverify Lₑ for the parent material (Section 4 / tool).
- Grip & washersadequate grip length; washers under the turned element; avoid soft/gasketed joints in fatigue.
- Lockingmatch the method to vibration, temperature and reuse needs.
- Specify on the drawingclass, finish/coating, torque (and method), locking feature, washer.