This lesson explains the principles in our own words. For pressure equipment always follow your site bolting procedure, the gasket maker's values and ASME PCC-1 (or EN 1591-4 competence rules in Europe).
Torque is only a way to make preload
A bolt holds a joint together by being stretched like a stiff spring. That stretch creates a tension called preload (or clamp load), which squeezes the gasket and stops the joint from separating. Torque is simply the easy-to-measure way of producing that tension.
- Preload target: about 75 % of the proof load for ordinary reusable steel bolts; about 40–70 % of yield for flange studs (often 50 %), set by the gasket.
- Stress area (not the nominal area) carries the load. Inch threads use ASME B1.1, metric threads ISO 898-1.
The nut factor K
K lumps all the friction into one number. Typical values:
| Condition | K (typical) | Effect on torque for the same preload |
|---|---|---|
| Dry, plain steel | 0.20 | Baseline |
| Lightly oiled | 0.18 | −10 % |
| Machine oil | 0.15 | −25 % |
| Moly paste / anti-seize | 0.10–0.14 | −30 to −50 % |
Never use a dry-bolt torque on lubricated bolts: with K 0.12 the same torque gives about 1.7 times the preload, which can yield the bolt or crush the gasket. Even with good control, torque gives roughly ±25 % preload scatter; critical joints use hydraulic tensioners or bolt-stretch measurement.
Bolt grades: inch and metric
| System | Grade / class | Strength used | Typical use |
|---|---|---|---|
| Inch (SAE J429) | Grade 2 / 5 / 8 | Proof 55 / 85 / 120 ksi | Machinery, structures, supports |
| Inch (ASTM) | A193 B7, B16; A320 L7 | Yield 105 ksi (to 2½ in) | Flange studs; B16 hot, L7 cold service |
| Inch (ASTM) | A325 / A490 | Proof 85 / 120 ksi | Structural steel connections |
| Metric (ISO 898-1) | 4.6 · 5.6 · 8.8 · 10.9 · 12.9 | Proof 225 · 280 · 580–600 · 830 · 970 MPa | 8.8 is the everyday class; 10.9/12.9 high strength |
| Metric stainless (ISO 3506-1) | A2-70, A4-70, A4-80 | Yield 450 / 450 / 600 MPa | Corrosive areas; use anti-seize (galling) |
The class number tells the strength: for 8.8, the first 8 means 800 MPa tensile strength and ".8" means yield is 80 % of that (640 MPa).
Flange bolting tables
The flange standard fixes how many bolts, what size and on which circle:
- ASME B16.5 (NPS ½–24", Class 150–2500): inch studs, UNC up to 1", 8UN above. Example: 4" Class 300 = 8 × ¾"-10 UNC on a 7⅞" bolt circle.
- EN 1092-1 (DN15–DN600, PN10–PN40): metric bolts. Example: DN100 PN16 = 8 × M16 on a 180 mm pitch circle.
The calculator below holds both tables; pick the flange and press Use these studs to load the bolt into the torque calculation.
Star pattern and passes
Tightening one bolt fully and then the next pulls the flange faces out of parallel and crushes one side of the gasket. Instead, number the bolts 1 to N clockwise and tighten across the flange:
| Pass | Target | Pattern |
|---|---|---|
| 1 | Snug by hand; check gasket is centred and the gap is even | Star |
| 2 | 30 % of final torque | Star |
| 3 | 60 % | Star |
| 4 | 100 % | Star |
| 5 | 100 %, check pass | Clockwise round, repeat until no nut turns |
Finally measure the flange gap at four points 90° apart (it should be even) and re-check at 100 % after about four hours, because gaskets relax.
Live calculator
Choose inch or metric, the bolt, grade and lubrication, or pick an ASME B16.5 / EN 1092-1 flange and load its bolts. The pass table and animated star order update automatically. It also works offline in the Instrument Apps Hub.
Common mistakes
- Dry torque on oiled bolts, or anti-seize used without changing the torque.
- Going round in a circle instead of across: the gasket is crushed on one side and leaks on the other.
- One pass to full torque. Always build up in steps.
- Reusing gaskets or yielded studs; spiral-wound gaskets are single use.
- Rusty or damaged threads: friction goes up, so preload goes down for the same torque. Clean and lubricate.
- Uncalibrated torque wrench, or wrench used as a breaker bar.
- Mixed bolt grades on one flange. Check the head or stud marking (B7, 8.8, A4-70).
Q&A quiz
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