MS28782-19 - Contoured PTFE Backup Ring for Hydraulic Systems

MS28782-19 – Contoured PTFE Backup Ring for Hydraulic Systems

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The MS28782-19 is a high-performance, contoured backup ring manufactured in strict accordance with Military Standard MS28782. This is a flight-critical component specifically designed to work in conjunction with an elastomeric O-ring (typically MS28775-119 EPDM for this standard) in high-pressure hydraulic systems, particularly those using phosphate ester-based fluids like Skydrol®. Its sole, indispensable function is to prevent the extrusion of the softer O-ring seal into the clearance gap between mating metal parts, thereby enabling the seal to function reliably at system pressures of 3000 PSI and beyond.

Fabricated from virgin Polytetrafluoroethylene (PTFE), commonly known by the trade name Teflon®, this ring is engineered for its high compressive strength, exceptional chemical inertness, and extremely low coefficient of friction. The MS28782 features a unique contoured cross-section, designed to provide optimal support to the O-ring under pressure. It is a single-turn (scarf-cut/split) ring, allowing for easier installation into internal grooves or over shafts. The integrity of high-pressure hydraulic systems in flight controls, landing gear, and braking systems depends critically on the proper use and function of these anti-extrusion devices.

Key Features

• Military Standard (MS) Certified: Manufactured to the exact material, dimensional, and quality tolerances of the MS28782 specification.

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

• Solid Polytetrafluoroethylene (PTFE) Construction: Made from 100% virgin PTFE, providing exceptional resistance to virtually all aerospace fluids, including phosphate ester hydraulics (Skydrol®), fuels, and solvents, across a wide temperature range.

• Contoured Design: The concave cross-section is specifically shaped to mate with the curved surface of the O-ring, providing maximum support against pressure-induced deformation.

• 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.

• Single-Turn (Scarf-Cut) for Installation: The split design allows the ring to be carefully manipulated for installation without specialized tooling.

Technical Specifications

• Part Number: MS28782-19

• Component Type: Backup Ring / Anti-Extrusion Ring, Contoured

• Governing Specification: MS28782

• OEM Applicability: Textron Aviation (Cessna Citation/Hawker), Boeing, Airbus, and all other manufacturers of aircraft using phosphate ester hydraulic systems requiring MS-standard seals.

• Material: Polytetrafluoroethylene (PTFE).

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

• Mating O-Ring Size: Designed for use with an AS568-119 size O-ring (e.g., MS28775-119 for Skydrol systems).

• Nominal Mating O-Ring ID: 0.924 inches (Based on AS568-119 standard).

• Typical System Pressure: Enables O-rings to reliably seal systems operating at pressures up to and exceeding 3000 PSI.

Aircraft Compatibility

The MS28782-19 is a standard hardware component used in the assembly and overhaul of high-pressure hydraulic components across all categories of certified aircraft utilizing phosphate ester fluids. It is not specific to any one airframe but is called for by part number in the component maintenance manuals for countless hydraulic 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 MS28782-19 is used to support O-ring seals in demanding, high-pressure dynamic and static applications within phosphate ester (Skydrol®) hydraulic systems, including:

• Flight Control Actuators (spoilers, ailerons, elevators, rudder)

• Landing Gear Actuators and Struts

• Hydraulic System Valves, Pumps, and Accumulators

• High-Pressure Brake System Components

• Thrust Reverser Actuators

Why Choose MS28782-19?

Choosing a genuine MS28782-19 backup ring is a critical requirement for a durable, high-pressure 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, or an incorrect flat backup ring (like MS28774) where a contoured one is specified, risks incorrect fit, allowing the O-ring to extrude and fail, potentially resulting in a loss of hydraulic pressure to a critical system. The MS specification guarantees the certified PTFE material and precision contoured dimensions needed for the ring to function correctly as a robust anti-extrusion device in high-pressure Skydrol systems.

The MS28782-19's contoured geometry provides a significant advantage over flat backup rings in high-pressure applications. In a hydraulic gland under pressure, the elastomeric O-ring attempts to deform into the extrusion gap. A flat backup ring (like MS28774) provides a basic barrier, but the high pressure can still cause localized high stress and "nibbling" on the O-ring where it bridges the corner between the flat backup ring and the gland wall. The MS28782's concave radius, machined to closely match the O-ring's cross-sectional radius, provides full support around a larger portion of the O-ring's circumference. This cradling effect distributes the stress more evenly and minimizes the unsupported area of the O-ring exposed to the extrusion gap. This superior support significantly increases the O-ring's resistance to extrusion and nibbling, allowing the sealing system to operate reliably at higher pressures and extending the service life of the primary elastomeric seal, particularly in dynamic applications with relative motion.

Note: This is a critical hydraulic system component. Backup rings should generally 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. CRITICAL: 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, with their contoured faces oriented towards 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 MS28782. These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.