EN 4551:2014

Overview

EN 4551:2014 is a European Standard developed by CEN that specifies the characteristics of swivel nuts used in aerospace pipe couplings. These swivel nuts are designed for inch series pipe couplings with a 37° fitting angle made from heat-resisting steel. The standard applies to aerospace applications requiring high durability and resistance to heat, ensuring reliable performance under demanding conditions. The nominal pressure rating is Class D, in accordance with ISO 6771, which covers pressure and temperature classifications for aerospace fluid systems.

This standard supersedes the previous EN 4551:2003 and supports consistent manufacturing, quality assurance, and interchangeability of aerospace-grade pipe coupling components across European countries.

Key Topics

• Swivel Nuts Specification
  Defines dimensions, configurations, and tolerances for swivel nuts compatible with 37° pipe couplings in inch series. Includes detailed measurements provided in millimeters - applicable before lubrication unless silver plating is used.

• Materials
  Specifies use of heat resisting steel conforming to aerospace grade steel standards EN 3468 or EN 3487, each with a minimum hardness (HB > 140). These materials ensure mechanical strength and resistance to harsh aerospace environments.

• Surface Treatments
  Highlights possible surface coatings for swivel nuts to enhance corrosion resistance and lubrication performance, including:

  • No treatment
  • Molybdenum disulphide dry lubricant coating per EN 2491
  • Electrolytic silver plating per EN 2786

• Locking Wire Hole Options
  The standard allows for swivel nuts with or without locking wire holes, offering design flexibility to meet different assembly or safety requirements.

• Designation and Marking
  Provides a clear system for part identification using codes that describe dimensional size, surface treatment, and locking wire hole options. Marking follows the EN 2424 aerospace marking standard for traceability.

• Compliance and Normative References
  Integrates references to related aerospace standards such as EN 4560 (pipe coupling specifications) and ISO standards (thread dimensions and pressure classifications), ensuring cohesive system compatibility.

Applications

• Aerospace Fluid Systems
  EN 4551 swivel nuts are critical components in aircraft hydraulic, fuel, and pneumatic pipe assemblies where reliable, leak-proof connections are essential under high pressure and temperature.

• Aerospace Manufacturing and Maintenance
  Component standardization facilitates quality control in manufacturing and enables maintenance engineers to source certified replacement parts that meet stringent aerospace requirements.

• Aerospace and Defence Industries
  Widely utilized in designing fluid transfer systems resistant to thermal and mechanical stresses typical of aerospace environments, promoting operational safety and performance consistency.

Related Standards

• EN 4560: Aerospace series - Pipe coupling, 37°, spherical - Inch series
  Technical specifications complement EN 4551 by detailing the overall pipe coupling system.

• EN 2424: Marking of aerospace products
  Governs marking to ensure product traceability.

• EN 2491: Molybdenum disulphide dry lubricants - Coating methods
  Defines the standard processes for applying dry lubricant coatings on aerospace components.

• EN 2786: Electrolytic silver plating of fasteners
  Details silver plating methods enhancing corrosion resistance.

• ISO 3161: Aerospace - UNJ threads - General requirements
  Specifies thread limits and dimensions relevant to swivel nuts.

• ISO 6771: Aerospace - Fluid systems and components - Pressure and temperature classifications
  Provides pressure class definitions, including Class D applicable to this standard's swivel nuts.

EN 4551:2014 ensures high-quality, heat-resistant swivel nuts for inch series 37° pipe couplings, supporting safe, reliable aerospace fluid system assemblies. Standardized dimensions, materials, and surface treatments make it an essential reference for aerospace manufacturers and maintenance professionals seeking durable, compliant coupling components.