MS28773-05 - Textron Aviation OEM PTFE Backup Retainer Ring

MS28773-05 – Textron Aviation PTFE Backup Ring (Anti-Extrusion Ring)

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The MS28773-05 is a high-performance, anti-extrusion backup ring manufactured in strict accordance with Military Standard MS28773. While specified in Textron Aviation parts catalogs, it is a universal component used across the aerospace industry. It is crucial for technicians to understand that this is not a primary sealing device; it is a mechanical support ring that works in tandem with an elastomeric O-ring (typically AS568-010 size for this dash number) in high-pressure hydraulic and pneumatic systems. Its sole function is to prevent the mechanical failure of the O-ring seal by blocking the path of extrusion, thereby allowing the system to operate reliably at pressures of 3000 PSI and above.

Fabricated from solid Polytetrafluoroethylene (PTFE), commonly known by the trade name Teflon®, this ring is engineered for its unique combination of high compressive strength, chemical inertness, and an extremely low coefficient of friction. The MS28773 is a split (scarf-cut) ring, allowing for easy installation without permanently deforming the material. The integrity of high-pressure hydraulic systems in landing gear, flight controls, and braking systems depends on the proper function of these essential components.

Key Features

• Military Standard (MS) Certified: Manufactured to the exact material and dimensional tolerances of the MS28773 specification, ensuring performance and interchangeability.

• Prevents O-Ring Extrusion: Provides a rigid barrier that blocks the clearance gap in a sealing gland, preventing the soft O-ring from being "nibbled" or forced out under high pressure.

• Solid Polytetrafluoroethylene (PTFE) Construction: Made from 100% PTFE, providing exceptional resistance to virtually all aviation fluids (hydraulic oils, fuels, solvents) and a wide operating temperature range.

• Low-Friction and Chemically Inert: The inherent lubricity of PTFE prevents abrasion or damage to the O-ring seal, while its chemical inertness prevents material degradation.

• Scarf-Cut for Easy Installation: The split design allows the ring to be carefully expanded or compressed for installation into internal grooves or over external shafts without specialized tooling.

Technical Specifications

• Part Number: MS28773-05

• Component Type: Backup Ring / Anti-Extrusion Ring

• Governing Specification: MS28773

• OEM Applicability: Textron Aviation (Cessna, Beechcraft), and all other manufacturers utilizing MS-standard hydraulic components.

• Material: Polytetrafluoroethylene (PTFE).

• Configuration: Single-Turn, Scarf-Cut (Split).

• Mating O-Ring Size: Designed for use with an AS568-010 O-ring.

• Typical System Pressure: Enables O-rings to reliably seal systems operating at pressures from 1500 PSI to over 3000 PSI.

Aircraft Compatibility

The MS28773-05 is a standard hardware component used in the assembly and overhaul of high-pressure fluid power components across all categories of certified aircraft. It is not specific to any one airframe but is called for by part number in the component maintenance manuals for countless hydraulic and pneumatic parts.

Crucial Note: The correct part number for a specific application must be verified against the Illustrated Parts Catalog (IPC) or Component Maintenance Manual (CMM) for the component being serviced.

Applications

The MS28773-05 is used to support O-ring seals in demanding, high-pressure dynamic and static applications, including:

• Landing Gear Actuators and Struts

• Flight Control Actuators (aileron, elevator, rudder)

• Hydraulic System Valves and Accumulators

• High-Pressure Brake System Components

• Thrust Reverser Actuators

Why Choose MS28773-05?

Choosing a genuine MS28773-05 backup ring is a critical requirement for a durable hydraulic system repair. An O-ring operating above 1500 PSI without a backup ring is guaranteed to fail prematurely through extrusion. The use of a non-certified part risks incorrect material or dimensions, which can lead to a poor fit, allowing the O-ring to extrude and fail. This can result in a loss of hydraulic pressure to a critical system. The MS specification guarantees the certified PTFE material and precision dimensions needed for the ring to function correctly as a robust anti-extrusion device.

The function of the MS28773-05 is rooted in the mechanics of high-pressure sealing and the material properties of Polytetrafluoroethylene (PTFE). In a hydraulic gland operating at high pressure, the elastomeric O-ring behaves as a highly viscous, incompressible fluid. The pressure differential across the seal attempts to force this "fluid" into the microscopic clearance between the moving and stationary metal components, known as the extrusion gap. The MS28773 backup ring, made of solid PTFE with a much higher shear modulus and compressive strength than the elastomer, is installed in the gland on the low-pressure side of the O-ring. It physically blocks the extrusion gap. When pressure is applied, the O-ring is energized and presses against the backup ring, which in turn is pressed against the wall of the gland, effectively reducing the extrusion gap to zero from the O-ring's perspective. The PTFE's extremely low coefficient of friction is also critical, as it allows the O-ring to slide and move against the backup ring's surface during pressure cycles without abrasion or "nibbling," which could otherwise create a failure point on the primary elastomeric seal.

Note: This is a critical hydraulic system component. Backup rings should be replaced any time the corresponding O-ring is replaced. Care must be taken during installation to ensure the ring is not nicked, scratched, or twisted. It must be installed on the LOW-PRESSURE side of the O-ring. For sealing pressure in two directions (a double-acting piston), two backup rings must be used, one on each side of the O-ring. Installation must be performed by a certified A&P mechanic in strict accordance with the applicable Component Maintenance Manual (CMM).

Documentation

The definitive technical requirements for this component are detailed in Military Standard MS28773. These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.