MS28775-018 - Military Standard Packing O-Ring

MS28775-018 – Military Standard Nitrile (Buna-N) Hydraulic O-Ring Packing

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Overview

The MS28775-018 is a military standard O-ring packing seal precision-engineered for critical applications in aircraft hydraulic systems. Manufactured from a highly durable Nitrile Butadiene Rubber (NBR) compound, commonly known as Buna-N, this component is specifically formulated for long-term compatibility with petroleum-based hydraulic fluids such as MIL-H-5606. Its material properties and dimensional tolerances are governed by the MS28775 military specification, ensuring absolute reliability and performance within an operational temperature range of -65°F to +275°F.

Conforming to the AS568 dimensional standard for size -018, this O-ring is designed to create a positive, leak-proof seal in a variety of static and dynamic hydraulic applications. The inherent chemical resistance of the NBR elastomer prevents swelling, hardening, or degradation when exposed to hydrocarbon fluids, while its excellent physical properties resist abrasion and compression set. For any maintenance action requiring a seal of this specific dimension, the MS28775-018 provides the certified quality and traceability essential for maintaining aircraft airworthiness and system integrity.

Key Features

• Military Specification Compliance: Guaranteed to be manufactured in strict accordance with the MS28775 standard, ensuring material integrity and performance.

• Petroleum Fluid Resistant: Formulated from a specific Nitrile (Buna-N) compound for superior, long-term resistance to petroleum-based hydraulic fluids, oils, and fuels.

• AS568 Standard Sizing: Precisely dimensioned to the AS568-018 standard (0.739" ID, 0.070" CS) for a perfect fit in corresponding glands.

• Extreme Temperature Performance: Engineered to maintain elasticity and sealing force in harsh thermal environments from -65°F to +275°F (-54°C to +135°C).

• Robust Mechanical Properties: Exhibits high tensile strength and superior resistance to abrasion, ensuring durability in both static and reciprocating dynamic seals.

• Positive Sealing Mechanism: Designed to be energized by system pressure, creating a progressively tighter seal as hydraulic pressure increases.

Technical Specifications

• Part Number: MS28775-018

• Component Type: Hydraulic O-Ring Packing Seal

• OEM Manufacturer: Manufactured to Qualified Products List (QPL) standards.

• Material: Nitrile Butadiene Rubber (NBR), Buna-N, 75 Durometer Shore A (Typical).

• Finish: N/A

• Dimensions (AS568-018):

  • Inside Diameter (ID): 0.739 inches (18.77 mm)

  • Cross-Section Diameter (CS): 0.070 inches (1.78 mm)

• Operational Temperature Range: -65°F to +275°F (-54°C to +135°C)

• Compatible Fluids: MIL-H-5606, MIL-PRF-83282, MIL-PRF-87257, and other petroleum-based hydrocarbon fluids.

• Governing Specifications: MS28775, AS568

• Certification: Shipped with a Certificate of Conformance to MS28775.

Aircraft Compatibility

The MS28775-018 is a standard hardware component used on a multitude of aircraft platforms that utilize petroleum-based hydraulic systems. Its use is determined by the specific design requirements of a component, not a particular aircraft model. It is found in systems across:

• Military fighter, transport, and utility aircraft

• Commercial and regional aircraft (older models)

• General aviation piston and turboprop aircraft

• Rotorcraft hydraulic systems

Note: Verification of applicability is the responsibility of the installer. Always consult the current version of the aircraft or component's Illustrated Parts Catalog (IPC) and maintenance manual to confirm part number eligibility for a specific application.

Applications

• Static Sealing: Provides a reliable seal for larger hydraulic line fittings (AN/MS), component flanges, plugs, and housings.

• Dynamic Sealing (Reciprocating): Used as piston and rod seals in hydraulic actuators and shock struts where an AS568-018 dimension is specified.

• Component Sealing: Integrated into hydraulic pumps, motors, servo-valves, and accumulators.

Why Choose MS28775-018?

The use of a certified, traceable MS28775-018 O-ring is a non-negotiable requirement for airworthy hydraulic system repair. A generic, non-certified part of the same nominal size lacks the guaranteed material composition, quality control, and dimensional accuracy of a Mil-Spec component. The incorrect elastomer can lead to rapid degradation and failure when exposed to hydraulic fluid, while even minute dimensional deviations can prevent a proper seal. The result of such a failure is a hydraulic leak, which can lead to loss of flight controls, landing gear, or braking systems.

The material science of the MS28775-018 is centered on its Nitrile Butadiene Rubber (NBR) composition, a synthetic elastomer created by copolymerizing acrylonitrile and butadiene. The high proportion of acrylonitrile in the polymer chain introduces polar nitrile groups which give the material its characteristic oleophobicity, or strong resistance to non-polar hydrocarbon fluids like MIL-H-5606. The physics of its sealing function are based on the principle of converting applied pressure into sealing force. The O-ring, an elastomeric torus, is installed into a gland with a predetermined diametrical squeeze. This creates an initial line of contact and a low-pressure seal. When the hydraulic system is pressurized, the fluid, acting according to Pascal's Law, exerts uniform pressure on the energized side of the O-ring. This pressure deforms the elastomer, forcing it against the clearance gap on the low-pressure side of the gland, which increases the sealing stress to a value that exceeds the system pressure, thus preventing leakage.

Note: This component is a flight-critical seal and is subject to a finite shelf life as defined by aerospace standard ARP5316. Always verify the cure date of the O-ring prior to installation. It is considered a single-use part in all dynamic and critical static applications. Installation must be performed by a certified mechanic in a clean environment. Use of the system's specified hydraulic fluid as a lubricant is required, and care must be taken to avoid nicking, cutting, or twisting the O-ring during installation.

Documentation

The military standard governing this part is publicly available through the Department of Defense ASSIST database.

• ASSIST Document Summary Page for MS28775: http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=45248