Prevailing-Torque Lock Nuts: Performance Requirements and Selection per GB/T 3098.9
Executive Summary
Prevailing-torque lock nuts rely on a non-circular thread or a nylon insert to generate continuous friction resistance, making them one of the most mature anti-loosening solutions under vibration. This article interprets the performance requirements of GB/T 3098.9-2020: upper and lower prevailing torque, five screw-on/off cycles, proof load, hardness and material requirements, with selection criteria between all-metal and nylon-insert types.
Table of Contents
Switched to Lock Nuts, Yet the Joint Still Loosened
A bracket joint showed clear preload decay during vibration testing. Engineers replaced all plain nuts with nylon-insert lock nuts; the decay only dropped from 15% to 13%. The problem was not the nut itself, but choosing the wrong jointing method: the dominant loosening mechanism there was slow preload decline caused by transverse slip at the joint faces, and the nut showed no obvious rotation—under such conditions, no lock nut can do anything.
A lock nut addresses one path: "relative rotation of the thread pair." To use it well, you must first understand how its prevailing torque is generated, how large it is, and how long it lasts.
How Prevailing Torque Works
After a plain nut is tightened, the friction resistance of the thread pair changes with preload: the higher the preload, the higher the resistance. Once preload decays, the resistance falls in step.
The design idea of a prevailing-torque lock nut is to introduce an extra resistance independent of preload:
- All-metal type: one end of the nut is crimped into an ellipse or necked in; as it runs on, the thread is elastically spread, creating a continuous radial clamping force. This resistance depends only on the elastic deformation of the nut material, not on assembly preload.
- Nylon-insert type: a nylon ring is embedded in the nut with an inner diameter smaller than the pitch diameter; as the bolt runs on, the nylon is compressed to produce friction resistance. Likewise independent of preload.
Therefore the value of a lock nut is: when preload drops due to embedding, relaxation or slight slip, enough rotational resistance remains in the thread pair to prevent reverse rotation of the nut.
Core Requirements of GB/T 3098.9
GB/T 3098.9-2020 (Prevailing torque type steel lock nuts) specifies performance requirements, with five core indicator families:
| Indicator | Physical Meaning | Typical Requirement |
|---|---|---|
| Prevailing torque (first screw-on) | Torque needed to turn the nut on the thread under non-clamped conditions | Defined lower and upper limits |
| Prevailing torque (5th screw-off) | Remaining turning torque after five screw-on/off cycles | Still not below the specified lower limit |
| Proof load | No stripping or permanent deformation under the specified axial load | Value corresponding to the nut property class |
| Hardness | Reflects material strength and locking-structure elasticity | Staged by property class |
| Appearance and thread accuracy | No cracks or burrs; thread 6H | The locking structure must not damage the thread |
Of these, the two prevailing-torque measurements are the most engineering-significant part of the standard: they turn "anti-loosening ability" from a qualitative description into a measurable torque value, and use the 5th-screw-off torque to constrain durability.
All-Metal vs. Nylon-Insert Types
| Item | All-Metal Prevailing-Torque (GB/T 6185.1, GB/T 6187.1, etc.) | Nylon-Insert Type |
|---|---|---|
| Locking element | Nut body necked in (ellipse or three-point crimp) | Embedded nylon ring |
| Source of prevailing torque | Radial clamping from metal elastic deformation | Elastic compression of nylon |
| Max service temperature | Up to 300 °C+ (depending on material and plating) | Generally not over 120 °C (nylon softens) |
| Chemical resistance | Good | Affected by solvents and oils |
| Thread protection | Necking may damage bolt threads; mind plating scratches | Does not damage threads |
| Reusability | Poor; torque drops markedly with reuse | Some reusability, but torque also drops |
| Typical use | High temperature, heavy vibration, engines and exhaust systems | General machinery, electronics, appliances, light structures |
How to Read the Prevailing-Torque Acceptance Criteria
The standard defines an upper and lower limit for prevailing torque; both ends matter:
- Lower limit: guarantees anti-loosening ability. Too little torque cannot provide enough rotational resistance.
- Upper limit: guarantees assemblability. Too much torque causes excessive run-on torque, tool slip and thread wear; in serious cases it cannot reach target preload, or breaks the bolt.
A commonly overlooked engineering issue: locking torque is added on top of tightening torque. If locking torque is 3 N·m and target tightening torque is 20 N·m, the assembly tool actually needs to output about 23 N·m to obtain the design preload. Under torque control, this difference is miscalculated into preload.
| Target Tightening Torque | Locking Torque (Example) | Relative Effect | Handling |
|---|---|---|---|
| Effectively negligible (> 50 N·m and locking torque ≤ 2 N·m) | ≤ 2 N·m | < 4% | Negligible |
| Needs correction (20~50 N·m) | 2~5 N·m | 10%~25% | Add locking torque to the torque value |
| Must be corrected (< 20 N·m) | 2~5 N·m | > 25% | Switch to torque-angle method, or recalculate preload |
This is why small-size joints using lock nuts often show "tightened to position but insufficient preload"—locking torque consumes most of the tightening torque. For small-size, low-torque precision joints, use prevailing-torque lock nuts cautiously or switch to pre-applied adhesive.
Selection and Incoming-Inspection Checklist
Confirm before selection
- Service temperature: above 120 °C, nylon-insert types are unsuitable; all-metal types are required.
- Target tightening torque: confirm whether the ratio of locking torque to tightening torque is acceptable (recommended < 15%).
- Nut property class and bolt matching: the nut proof load should exceed the bolt minimum tensile load, ensuring failure occurs in the bolt rather than nut stripping.
- Coating and material matching: when stainless bolts are paired with stainless lock nuts, consider seizure risk (see anti-seize treatment).
- Whether sealing is needed: lock nuts do not seal; if sealing is required, add a sealing washer or choose a sealing-type product.
Incoming-inspection recommendations
- Check each batch's prevailing-torque inspection report, focusing on first-screw-on and 5th-screw-off values.
- Spot-check run-on torque: measure the maximum torque during run-on with a torque meter and compare with the standard range.
- Inspect the locking structure for cracks (at the neck) or a missing, offset or insufficiently tall nylon ring.
- Check the thread-entry chamfer for burrs or neck deformations that damage the thread.
Conclusion
The value of prevailing-torque lock nuts is providing a rotational resistance decoupled from preload: relative rotation of the thread pair is continuously prevented even when preload drops due to embedding or relaxation. It constrains one clear path—reverse rotation of the nut.
Therefore, before selecting, answer two questions: is this joint really "rotating"? If the dominant mechanism is joint-face slip or preload relaxation, lock nuts have limited effect. Will locking torque consume the tightening torque? If target torque itself is small, the locking structure may instead destroy preload.
Answer these two questions clearly, then inspect against the prevailing-torque indicators of GB/T 3098.9—and lock nuts can be used where they truly work.
