NAS43HT3-13 – Spacer, High-Temperature, Hardened Alloy Steel
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Overview Section
The NAS43HT3-13 is a precision-machined, flight-critical Spacer governed by the National Aerospace Standard (NAS) system. In aerospace engineering, spacers like the NAS43HT3-13 are not simple washers; they are precision shims used to maintain a highly accurate, non-compressible dimensional gap between two components, particularly in bolted joint assemblies subject to high loads, vibration, or thermal expansion. This specific part is designated as a High-Temperature (HT) spacer, indicating its construction from a specialized, hardened alloy steel, making it suitable for use in engine sections, hydraulic manifold interfaces, or near high-heat exhaust components where maintaining joint preload and alignment is paramount.
The designation breaks down as: NAS43 (Spacer family), HT (High Temperature/Hardened), 3 (Nominal Inner Diameter code), and -13 (Nominal Length/Thickness code). Its exact dimensions and tolerances are defined by the NAS standard, ensuring absolute interchangeability and reliability when used in conjunction with high-strength tension bolts (e.g., NASM6 or NASM13 series).
Key Features Section
National Aerospace Standard (NAS) Compliance: Guarantees strict adherence to dimensional tolerances, material composition, and surface finish, ensuring airworthiness and interchangeability.
Hardened Alloy Steel Construction (HT): Manufactured from a high-strength, heat-treatable steel (often equivalent to 4130 or 4140 series steel, or a specialty tool steel alloy), providing superior compressive strength.
High-Temperature Service: Designed for applications where operating temperatures exceed the limits of standard aluminum or mild steel spacers, preventing thermal relaxation or creep within the joint.
Precision Thickness Control: Machined to a tight thickness tolerance to precisely establish the grip length of a bolted joint, which is critical for achieving the correct preload tension.
Corrosion Protection: Typically finished with a protective coating (e.g., Cadmium plating or specialized zinc-nickel alloy) to resist corrosion in harsh engine environments.
Non-Compressible Structure: The spacer's hardened nature ensures it does not deform under the high clamping force required for high-strength aerospace fasteners.
Technical Specifications Section
Part Number: NAS43HT3-13
Component Type: Precision, Plain Cylindrical Spacer
OEM Manufacturer: Various NAS-approved suppliers (Supply Chain Traceability is Mandatory)
Material: Hardened Alloy Steel (HT - High Temperature Grade)
Inner Diameter (ID Code 3): Nominal diameter (must be confirmed with NAS43 specification table)
Outer Diameter (OD): Defined by NAS43 specification relative to the ID.
Thickness/Length (Dash 13): Nominal thickness/length (Exact dimension in inches must be confirmed by the NAS43 table).
Finish: Cadmium Plated per QQ-P-416 or equivalent specialized finish.
Hardness: Heat treated to a specific Rockwell C scale hardness for high compressive strength (exact range specified in NAS43HT).
Governing Specification: National Aerospace Standard (NAS) 43HT.
Certification Status: Must be accompanied by a Certificate of Conformance (CoC) and traceable back to the manufacturer and raw material lot.
Aircraft Compatibility Section
Engine and Nacelle Structures: Used in mounting brackets, pylon attachments, and cowling interfaces where high heat is present.
Hydraulic and Pneumatic Manifolds: Used to accurately space manifold blocks and control valves.
Primary and Secondary Flight Control Actuators: Essential for shimming the base mounts of actuators to maintain zero-play alignment.
All Military and Commercial Aircraft: Used ubiquitously across all airframes (Boeing, Airbus, Lockheed, etc.) requiring NAS standardized hardware.
Note: The application is highly dependent on the specific structural drawing. The aircraft's Structural Repair Manual (SRM) or Illustrated Parts Catalog (IPC) is the final authority for verifying the required diameter and thickness of the spacer.
Applications Section
Grip Length Correction: Used to fill gaps and achieve the precise grip length required for a tension-loaded bolt, ensuring the nut and bolt thread combination engages correctly and preload is maintained.
Component Alignment: Used in stacked assemblies to ensure components are spaced correctly, preventing binding or improper mechanical contact.
High-Heat Joint Preload: Maintaining the bolt preload in areas exposed to engine exhaust or bleed air, where thermal expansion could otherwise relax the joint tension.
Why Choose the NAS43HT3-13?
The NAS43HT3-13 is a classic example of Critical Hardware. The integrity of any high-load bolted joint assembly—particularly those involving movement or heat—is compromised by spacers that fail to meet dimensional or material specifications. Using a non-NAS or incorrectly sized spacer will result in: 1) Incorrect bolt grip length, leading to inadequate fastener preload; 2) Spacer deformation (if the material is too soft), causing joint relaxation and potential fatigue failure; or 3) Thermal misalignment in high-heat areas. This NAS-certified, high-hardness alloy steel spacer is essential for maintaining joint stiffness and preventing fretting corrosion, thus extending the fatigue life of the entire assembly.
The performance of the NAS43HT3-13 is rooted in materials science and compressive yield strength. The HT designation signifies the use of a heat-treated alloy steel, processed through controlled quenching and tempering to achieve a martensitic structure. This results in an extremely high compressive yield strength, ensuring the spacer will not plastically deform when the high-strength tension bolt (e.g., $160 \text{ ksi}$ or $180 \text{ ksi}$ ultimate tensile strength) is torqued to its specified preload value. In the physics of the bolted joint, the spacer ensures that the clamping force is transferred uniformly and precisely across the entire face of the mating structure. The ability of the hardened material to resist brinelling (localized surface indentation) under these high compressive loads is critical for preventing the loss of preload, which is the primary mechanism for maintaining the fatigue life of the joint.
Note: The NAS43HT3-13 is a flight-critical component. The use of the correct NAS diameter and most importantly, the correct thickness/length (-13 dash number) is mandatory. Substitution is strictly prohibited unless authorized by OEM engineering data. Installation must verify that the spacer is free of burrs and is not installed on a non-perpendicular surface.
Documentation
National Aerospace Standard (NAS) 43HT Specification: Contains all dimensional tables, material requirements, and hardness levels.
MIL-HDBK-5: Reference for the mechanical properties of the alloy steel used.