MS21044N9 – Aerospace Elastic Stop Nut (Self-Locking, Nylon Insert)
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Overview Section
The MS21044N9 is a high-strength, self-locking hex nut governed by the Military Standard (MS) system. It is part of the MS21044 family, which defines the standard for "Nut, Self-Locking, Hexagon, Prevailing Torque, Nylon Insert, 250 Degrees F, Steel."
The key feature of this nut is the non-metallic (nylon) insert at the top of the threads. As a bolt enters the nut, it creates a frictional "prevailing torque" against the nylon, preventing the nut from loosening under the intense vibration common in aerospace environments. The N9 designation identifies the specific size: 9/16-18 UNF-3B threads. This nut is designed for structural applications where temperatures do not exceed 250 degrees F (121 degrees C).
Key Features Section
Nylon Self-Locking Insert: Provides a reliable prevailing torque lock that does not damage bolt threads, allowing for limited reuse.
Vibration Resistance: Specifically engineered to prevent hardware backing off in high-vibration areas such as engine mounts, airframes, and landing gear.
Cadmium Plating: Finished with Type II, Class 2 Cadmium plating (plus yellow chromate) for superior corrosion resistance and consistent torque-tension characteristics.
Standard Hexagonal Drive: Designed for installation with standard aerospace wrenches and sockets.
Low-Profile Structural Strength: Provides a high strength-to-weight ratio, meeting strict military aerospace load requirements.
Temperature Limited: Suitable for environments up to 250 degrees F; for higher heat zones, all-metal locking nuts (e.g., MS21042) must be used.
Technical Specifications Section
Part Number: MS21044N9
Thread Size: 9/16-18 (9/16 inch diameter, 18 threads per inch)
Thread Series: UNF-3B (Fine Thread, Aerospace Class 3 fit)
Material: Carbon Steel or Alloy Steel (Heat Treated)
Locking Element: Nylon Insert (Usually Purple, Blue, or Green)
Finish: Cadmium Plated per QQ-P-416, Type II, Class 2
Nut Height: Approximately 0.641 inches (Standard height)
Width Across Flats: 7/8 inch (Wrench size)
Maximum Operating Temperature: 250 degrees F (121 degrees C)
Governing Specification: MIL-DTL-25027 / MS21044
Aircraft Compatibility Section
General Aviation and Commercial Transport: Used in nearly all Western-built aircraft for structural and non-structural assembly.
Heavy Structural Joints: The 9/16 size is typically used for larger landing gear pivots, engine mounting brackets, and major wing-to-fuselage attachment points.
Military Airframes: Fully compliant with military specifications for tactical and transport aircraft.
Note: The MS21044N9 is a structural fastener. The aircraft Structural Repair Manual (SRM) or Illustrated Parts Catalog (IPC) is the final authority for determining the correct nut for any specific application.
Applications Section
Structural Bolting: Securing major airframe components where high clamping force and vibration resistance are mandatory.
Control System Linkages: Used in primary flight control pivot points to ensure hardware remains secure throughout the aircraft life cycle.
Engine Accessories: Mounting external components to the engine (outside of high-heat zones).
Why Choose the MS21044N9?
The MS21044N9 is a safety-critical fastener. In aerospace, the loss of a single nut can lead to catastrophic structural failure. Using a commercial-grade "nyloc" nut instead of a certified MS21044N9 risks: 1) Material Failure: Commercial nuts lack the heat treatment and material purity required for flight loads; 2) Corrosion: Improper plating leads to hydrogen embrittlement or rapid oxidation; and 3) Locking Failure: Non-certified nylon inserts may degrade or lack the specific prevailing torque required by MIL-DTL-25027. Choosing the certified MS hardware ensures that the bolt remains tensioned and the nut remains locked, regardless of flight conditions.
The mechanical integrity of the MS21044N9 is defined by its prevailing torque. Unlike free-spinning nuts that require a lock washer, the nylon insert in the MS21044N9 is smaller than the major diameter of the mating bolt. This forces the bolt to compress the nylon, creating a constant radial load that "grips" the bolt threads. This design provides two benefits: it prevents rotation even if the bolt loses its initial tension, and it acts as a sealant to prevent moisture from entering the thread area, reducing the risk of internal thread corrosion. Because the nylon insert is elastic, the nut can be reused, provided it still meets the minimum prevailing torque requirements specified in the maintenance manual.
Note: The MS21044N9 is a structural component. It should always be installed with a cadmium-plated washer to protect the airframe surface. Never use an MS21044 nut in areas where temperatures exceed 250 degrees F, as the nylon insert will melt and lose its locking capability.
Documentation
MS21044 Military Standard Drawing: Contains exact dimensional tolerances and material requirements.
MIL-DTL-25027: The performance specification for self-locking, prevailing torque nuts.