MIL-DTL-18240 — Self-Locking Fastener Element, 250°F Max
MIL-DTL-18240 defines the self-locking element applied directly to the threads of externally threaded fasteners — bolts and screws — as an alternative to lock-wiring, for service up to 250°F. Revision F (2 June 1997) is the current issue, superseding MIL-F-18240E.
At a Glance
Overview
MIL-DTL-18240 covers the self-locking element applied to the threads of an externally threaded fastener, rather than the fastener itself. It's intended for bolts and screws with UNC, UNJC, UNF, UNJF, UNRC, or UNRF threads used in tapped holes, where temperatures will not exceed 250°F — as a replacement for lock-wiring the fastener in place.
The specification defines two configurations of the locking element: a strip configuration (Type L), installed via a strip cut through the threads running parallel to the fastener's length, and a patch configuration (Type P), applied without removing any fastener material. A specific type is called out only when required by the design or application.
- Prevailing-torque values in the specification are conformance requirements tied specifically to the locking-torque test procedure's test conditions — they may not translate directly to a different service application.
- A qualification lot must consist of fasteners of the same diameter and length, fabricated by the same process, heat treated the same way, and produced as one continuous run.
- Vibration testing follows MIL-STD-1312-7, with sample fasteners assembled to a specified torque, disassembled and reassembled four times, then vibration tested.
Element Type Comparison
Compare the two self-locking element configurations covered by MIL-DTL-18240.
Verification Requirements
| Test | Requirement |
|---|---|
| Prevailing Torque | On/off torque within Table II limits at the specified test temperature |
| Vibration | Per MIL-STD-1312-7, after 4 assembly/disassembly cycles |
| Thread Gauging | Test nut gauged with go/no-go plug gage before test |
| Thread Damage | No distortion or scratches deep enough to reduce thread efficiency |
Manufacturing & Quality Notes
A qualification lot consists of fasteners of identical diameter and length, fabricated by the same process and heat treatment, from a single continuous production run.
Test nuts are 300-series stainless steel with a 63 microinch or better minor-diameter finish; threads not otherwise listed conform to MIL-S-8879 Class 3B.
Prevailing-torque values apply only under the exact test conditions in the locking-torque test procedure — actual service conditions that differ may require additional evaluation.
Specific element type (L or P) should only be called out when the design or application specifically requires it; otherwise the manufacturer's standard offering applies.
MIL-DTL-18240 — Frequently Asked Questions
Is MIL-DTL-18240 a bolt specification?
Not exactly — it specifies the self-locking element applied to a bolt or screw's threads, not the fastener geometry itself. The base fastener's own drawing (e.g. an MS or NAS bolt standard) calls out MIL-DTL-18240 compliance when a self-locking thread feature is required.
What's the difference between Type L and Type P elements?
Type L (strip) is installed via a strip cut through the threads parallel to the fastener's length. Type P (patch) is applied without removing any fastener material. Which type is used is typically driven by design or application requirements.
What's the maximum service temperature for MIL-DTL-18240 elements?
250°F. For higher-temperature self-locking requirements, other specifications such as NASM25027 cover nuts rated to 450°F or 800°F depending on material.
Need MIL-DTL-18240 Self-Locking Fasteners?
Ananka Fasteners supplies bolts and screws with MIL-DTL-18240 self-locking elements, fully tested and certified.
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