MS9581-09 - Keyed Lock Washer

MS9581-09 – Spindle Key Washer for 7/8" Axles

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Overview

The MS9581-09 is a flight-critical, positive locking device manufactured to a precise Military Standard for securing spindle and axle nuts. This is not a standard washer; it is an engineered component designed to mechanically lock a wheel bearing retainer nut in place, preventing any possibility of rotation due to operational forces. Its most common and critical application is on the main and nose landing gear axles of a wide variety of aircraft.

Manufactured from a high-strength yet ductile cadmium-plated carbon steel, the washer features an inner key that engages with a slot on the axle to prevent the washer itself from rotating. After the axle nut is torqued to the precise specification to set the bearing preload, one of the outer tabs is bent up against a flat on the nut, creating an infallible, physical lock. As a component built to a military standard, it guarantees the material properties and dimensional tolerances required for this critical safety-of-flight application.

Key Features

• Positive Mechanical Lock: Provides a physical barrier that prevents the axle nut from rotating, a more secure method than friction-based locking for critical rotating assemblies.

• Internal Key for Anti-Rotation: A tab on the inner diameter engages with a keyway on the spindle or axle, preventing the washer from turning with the nut.

• Bendable Locking Tabs: The outer diameter features multiple tabs, allowing one to be bent up against the nut for a secure lock regardless of the nut's final rotational position.

• Cadmium Plated Carbon Steel: Manufactured from a specific steel alloy that is strong enough to resist shear forces, yet ductile enough to be bent into its locking position without fracturing.

• MS9581 Standard Compliance: Meets all dimensional, material, and quality requirements of the military standard for spindle key washers, ensuring airworthiness.

• Single-Use Safety Device: Engineered for a single installation to guarantee the integrity of the locking tab.

Technical Specifications

• Part Number: MS9581-09

• Component Type: Key Washer / Spindle Lock Washer

• Governing Specification: MS9581

• Material: Carbon Steel

• Finish: Cadmium Plated per QQ-P-416

• For Spindle Thread Size: 7/8-14 UNF

• Inner Diameter: 0.885 inches

Aircraft Compatibility

The MS9581-09 is a standard hardware component used across the aerospace industry on landing gear assemblies. It is not specific to one aircraft but is a fundamental component found on:

• Military Aircraft

• Commercial and Transport Category Aircraft

• Business and Corporate Jets

• General Aviation (Piston and Turbine)

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

Applications

• Sole Application: To provide a positive, mechanical lock for spindle nuts, most commonly on main and nose landing gear axles to secure the wheel and bearing assembly and maintain correct bearing preload.

Why Choose MS9581-09?

The safe operation of an aircraft's landing gear is non-negotiable. The failure of a wheel bearing assembly due to an improperly secured axle nut can be catastrophic, potentially leading to a wheel departing the aircraft. The MS9581-09 key washer is the specified safety device to prevent this. Only a certified MS-spec part is guaranteed to have the correct material properties (strength and ductility) and precise dimensions for this critical, high-stress application. The use of a non-certified part would create an unacceptable risk of landing gear failure.

The material science of the MS9581-09 key washer is a critical balance between strength and ductility. The washer is typically fabricated from a low-to-medium carbon steel that is processed to a specific temper. The material must possess sufficient shear strength in its main body to withstand the rotational forces exerted by the nut during torquing, preventing the inner key from shearing off. However, the outer tabs must be ductile enough to allow for a 90-degree bend without fracturing or initiating a stress crack. This is achieved through careful selection of the steel alloy and a controlled annealing or tempering process after stamping. The final cadmium plating not only provides corrosion resistance but also serves to prevent hydrogen embrittlement, a phenomenon where absorbed hydrogen can cause a high-strength steel to fracture under stress, which would be catastrophic for the locking tabs.

Note: This is a flight-critical, single-use hardware component. A new key washer must be used every time the axle nut is removed and re-installed. The tabs are not designed to be bent more than once. Installation must be performed by a certified A&P mechanic in strict accordance with the aircraft maintenance manual, which provides the detailed, multi-step procedure for torquing the axle nut to set the correct bearing preload before the lock tab is bent.

Documentation

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

• ASSIST Document Summary Page for MS9581: http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=38558