01How the process works
Tightening a bolt stretches it and clamps the joint. Torque is only an indirect measure of preload because most of it is consumed by friction, so lubrication, washers and tool calibration matter. Critical joints use torque-angle, tension or direct preload measurement.
Flange joints need a tightening sequence to compress gaskets evenly; structural preloaded joints need controlled methods and verification. Wrong bolt grades and uncalibrated tools are common audit findings.
- Bolt and component check. Grade, coating, lubrication.
- Assembly. Washers, gaskets, alignment.
- Tightening. Sequence and passes with calibrated tool.
- Marking. Torque marks applied.
- Verification. Audit torque or preload check.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Insufficient preload | Friction, wrong tool setting | Audit torque, ultrasonic bolt gauge |
| Overtightening and yield | Wrong torque | Visual, elongation |
| Wrong bolt grade | Mixed stock | Head marking, certificates |
| Uneven gasket compression | Sequence ignored | Gap measurement, leak test |
| Missing washers | Assembly error | Visual |
| Hydrogen embrittlement | Plating of high strength bolts | Certificates, process audit |
03Where an inspector should look
01
Before production
- Bolt certificates and grades
- Tightening procedure and torque values
- Tool calibration
02
During production
- Sequence and passes
- Lubrication as specified
03
After production
- Audit torque sampling
- Torque marks
- Leak or functional test
04Standards and references
ISO 898-1 (bolt properties), EN 14399 (preloadable structural bolts), ISO 16047 (torque and clamp force), ASME PCC-1 (flange assembly).
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.
05How we support buyers of bolting and torque parts
Steel Structure and Fabrication InspectionInspection of structural steel and fabricated assemblies: materials, welding, dimensions, bolting, coating and EN 1090 execution class requirements.
Mechanical Equipment InspectionInspection of rotating and static mechanical equipment: assembly, hardness, mechanical, load, pressure, leak and running tests.
SAT, Installation and Commissioning SupportInstallation inspection, site acceptance test witnessing, commissioning support, punch lists and handover documentation review.