How to choose a bolt securing method
Step 1: diagnose with a witness mark
Paint a continuous line in torque-seal or lacquer across the nut, the washer and the clamped member, so the line is unbroken across all three parts. Run one full duty cycle of the machine, then read the mark before touching a spanner. The mark is the only direct evidence of whether the nut moved, and it costs a few seconds per joint.
| What you see | What it means |
|---|---|
| Mark broken | Rotational self-loosening. The nut turned. |
| Mark intact, joint loose | Non-rotational preload loss. The clamped stack shortened. See non-rotational preload loss. |
| Mark broken on a joint that was already loose | Non-rotational preload loss followed by rotation. The relaxation came first and the slack joint then backed off. |
| Mark intact, joint tight | Look elsewhere. The fastener is not the source of the fault. |
Step 2: read the failure interval
How long the joint holds before it goes slack narrows the mechanism further than any catalogue can.
| Interval | Most likely mechanism |
|---|---|
| Hours | Wrong assembly preload or gross overload |
| Days to weeks | Embedment |
| One to six months | Coating or gasket creep |
| Six months to years, and accelerating | Thermal ratcheting |
| No pattern | Check for a cracked member or a missing spacer before changing fasteners |
Step 3: budget the stack shortening
Count every interface in the joint, including both bearing faces and each faying surface. Assign per-interface embedment of 3 to 15 µm, then add coating creep, gasket relaxation and thermal ratcheting for the materials actually in the stack. Multiply the total by the joint’s clamp force loss per micrometre, which follows from the VDI 2230 load factor. On the reference M16 class 10.9 joint at 70 kN the rate is 0,884 kN per micrometre, so a 30 µm budget is 26,9 % of preload. If the budget exceeds the preload margin, no locking device will hold the joint, because none of them produce clamp length. See stack shortening for the definitions used here.
Step 4: match method to mechanism
With the mechanism identified and the budget known, the specification follows from the table. Several rows are not our part, and those rows are the point of the table.
| Your joint | Specify | Why |
|---|---|---|
| Steel on steel, dry, constant temperature, marking acceptable | Nord-Lock or Heico-Lock wedge-locking washers | Rotational self-loosening is the dominant mechanism and a wedge pair addresses it directly, at lower cost than any recovery device |
| High-temperature, needs prevailing torque, no gaskets | All-metal prevailing-torque nut, or Stover | Nylon inserts degrade above roughly 120 °C |
| Thread sealing and mild locking, serviceable | Loctite threadlocker | Cheapest adequate answer where transverse load is light |
| Very high preload, controlled tightening | Superbolt tensioners | Multi-jackbolt tensioning is the right tool at that load class |
| Preload lost to embedment, coating creep or gasket relaxation | ISOKLAMP CFR | No locking device recovers clamp length; a take-up mechanism does |
| Dissimilar metals under thermal cycling | ISOKLAMP CFR, or Belleville stack where cycle count is low | Thermal ratcheting accumulates one way and needs one-way recovery |
| Plated, anodised or passivated bearing surfaces | ISOKLAMP CFR | Wedge and serrated washers secure by penetrating the bearing face |
| Composite or CFRP members | ISOKLAMP CFR with a load-spreading washer | Penetrating devices sever surface fibres and initiate delamination |
Step 5: check you can inspect it
If the joint has no service access, the retention method needs a readable reserve. A torque check is not available on a sealed enclosure, a live busbar cabinet or a validated hygienic envelope, so the joint has to declare its own condition. A mechanical preload reserve band on the outer diameter is readable by eye or by a fixed camera without breaking the joint. Where access is easy and downtime is cheap, a documented retorque interval remains a legitimate answer.
Questions
- How do I choose a bolt locking method?
- Diagnose the failure mode before you specify anything. Paint a witness mark across nut, washer and member, run one duty cycle, and read it. A broken mark means rotational self-loosening, and a wedge-locking washer or prevailing-torque nut is correct. An intact mark on a loose joint means the stack shortened, and only a take-up mechanism restores it.
- What is the best bolt securing method?
- There is no single best method, only a correct match to the mechanism. Wedge-locking washer pairs are the correct answer for dry metal-to-metal joints under transverse vibration. Prevailing-torque nuts suit high-temperature joints without gaskets. Threadlockers seal and lock lightly loaded threads. A one-way take-up is the answer where clamp length is lost rather than turned away.
- How do I know if my bolts are loosening or relaxing?
- Read the witness mark and the failure interval. A rotated nut is loosening. An unrotated nut on a slack joint is relaxation. Failure in days to weeks points at embedment, one to six months at coating or gasket creep, and six months to years and accelerating at thermal ratcheting.
- Which locking method is best for my application?
- Match the row in the decision table to your joint. Steel on steel where marking is acceptable takes a wedge pair. Plated, anodised or passivated bearing faces, composite members, gasketed stacks and dissimilar metals under thermal cycling take ISOKLAMP CFR. Very high preload with controlled tightening takes a multi-jackbolt tensioner.
Related
Written and reviewed by the ISOKLAMP Engineering team. Wisconsin. Decades in industrial and heavy machinery. Method: closed-form bolted-joint mechanics to VDI 2230 Sheet 1 and finite-element analysis. Questions to engineering@isoklamp.com.