9.1 Plain Bearings, Bushings & Spherical Bearings

9.1 Plain Bearings, Bushings & Spherical Bearings

Bearings are precision structural elements designed to support, guide, and position rotating, oscillating, or reciprocating shafts while minimizing frictional resistance and dissipating mechanical loads. In aircraft construction, bearings are broadly divided into two major families:

1. Plain Bearings (Sliding Contact) — where moving surfaces slide directly against one another, separated only by a microscopic lubricant film or low-friction dry liner.
2. Rolling-Element Bearings (Rolling Contact) — where balls or rollers separate the moving surfaces, substituting rolling friction for sliding friction.

Under EASA Part-66 Module 06 (Materials and Hardware — Sub-module 6.9 Bearings), maintenance certifying staff must understand the operating mechanics, material selection criteria, precision fitting and reaming techniques, retention mechanisms, and wear limits of plain bearings, bushings, and spherical joints used across airframes, landing gear, and powerplants.

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Plain Bearing Mechanics and Lubrication Regimes

A plain bearing (frequently referred to as a journal bearing, sleeve bearing, or bushing) consists of a cylindrical sleeve supporting a rotating or oscillating shaft called the journal. Because plain bearings distribute loads over a broad contact area rather than concentrated points or lines, they provide superior static load ratings, exceptional resistance to impact shock loads, high structural damping, silent operation, and very compact radial dimensions.

   HYDRODYNAMIC FLUID FILM FORMATION IN A JOURNAL BEARING

         At Rest                     Starting Up                  Full Speed
   (Boundary Contact)            (Mixed Lubrication)         (Hydrodynamic Wedge)
      .------------.                .------------.              .------------.   
     /   Housing    \              /   Housing    \            /   Housing    \  
    |    .--------.  |            |    .--------.  |          |    .--------.  | 
    |   / Journal  \ |            |   / Journal  \ |          |   / Journal  \ | 
    |  |  (Rest)    ||            |  |  (Rotates) ||          |  |  (Floats)  || 
    |   \    v     / |            |   \   ^      / |          |   \   ^      / | 
    |    '---+----'  |            |    '-+------'  |          |    '-+------'  |
     \   Metal-to-  /              \   Climbing   /            \ Convergent  /  
      '-- Contact -'                '-- Wedge ---'              '-- Wedge ---'   
    Eccentricity e = max          Friction Peaks (Mixed)      Journal lifted off bore

The Stribeck Curve and Lubrication Regimes

The frictional behavior and wear rate of a plain journal bearing depend on the operating lubrication regime, typically characterized by the dimensionless Hersey parameter:

$\text{Hersey Parameter} = \frac{\mu \cdot N}{P}$

Where $\mu$ is dynamic viscosity of the lubricant, $N$ is rotational speed (RPM), and $P$ is the projected bearing unit pressure (Load / Projected Area $L \times D$).

The Hydrodynamic Wedge Mechanism

When the journal is stationary at rest, it settles to the bottom of the bearing clearance, squeezing out the oil film and establishing direct metal-to-metal contact (boundary regime). As rotation begins:

1. Viscous drag forces the lubricating oil into the narrowing, wedge-shaped gap between the eccentric journal and the bearing wall.
2. The convergent geometry forces the incompressible oil into a high-pressure hydrodynamic wedge.
3. The generated hydrodynamic fluid pressure exerts an upward and lateral lifting force that physically elevates the journal off the bearing wall.
4. At design operational speed, the journal floats entirely on a continuous hydrodynamic oil film, establishing an equilibrium position defined by an eccentricity ($e$) and an attitude angle relative to the vertical load vector.

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Aerospace Plain Bearing and Bushing Materials

Because plain bearings rely on sliding contact, the bearing material must exhibit specific mechanical, thermal, and chemical characteristics:

• Compressive Strength: Ability to carry heavy operational and shock loads without extrusion, permanent brinelling, or plastic deformation.
• Conformability: Ability of the bearing surface to deform elastically or plastically to accommodate minor shaft misalignment, housing deflection, or geometric taper.
• Embeddability: Ability of the bearing alloy to absorb and embed circulating microscopic abrasive particles (dirt, grit, carbon debris) beneath its surface, preventing shaft scoring.
• Scuffing and Seizure Resistance (Anti-Weldability): Resistance to adhesive wear, galling, or micro-welding when the fluid film breaks down during startup or peak shock loads.
• Corrosion Resistance: Resistance to acidic breakdown products of oxidized synthetic lubricants and atmospheric moisture.

The Sintered Bronze (Oilite) Maintenance Trap

Sintered bronze bushings are produced by pressing copper and tin powders into precision dies and sintering them below the melting point. This creates a sponge-like structural matrix containing 15% to 20% interconnected void volume. The finished bushings are vacuum-impregnated with refined lubricating oil (typically SAE 20 or SAE 30 turbine-grade mineral oil).

During operation, frictional heat warms the bushing, causing the metal to expand and the oil inside the pores to decrease in viscosity. Thermal expansion and capillary forces pump the oil out onto the inner bearing surface, maintaining a hydrodynamic or mixed film. When motion stops and the assembly cools, capillary action draws the lubricating oil back into the porous bronze reservoir.

Exam Warning / Critical Maintenance Trap: Never wash or degrease a sintered porous bronze (Oilite) bushing using volatile solvent degreasers such as methyl ethyl ketone (MEK), acetone, trichloroethylene, or mineral spirits! Solvents instantly leach the impregnated lubricating oil out of the internal capillary pores, leaving a completely dry, unprotected porous metal matrix. If reinstalled dry, the bushing will overheat, squeal, and seize within minutes.

• If an Oilite bushing is accidentally washed in solvent or has dried out during long storage, it must be re-impregnated: submerge the bushing in a heated bath of clean engine/turbine oil maintained at 70°C to 80°C (158°F to 176°F) for at least 15 to 30 minutes, then allow it to cool submerged in the oil before installation.

Dry Self-Lubricating Liners (PTFE / Teflon)

Modern aircraft structures rely extensively on maintenance-free dry bearings to eliminate grease nipples, avoid hazardous grease migration near composite structures, and reduce routine line maintenance. These bushings feature a thin woven fabric liner composed of polytetrafluoroethylene (PTFE) and high-strength aramid (Nomex/Kevlar) fibres encapsulated in a thermoset phenolic or epoxy resin matrix, bonded to a stainless steel, aluminium alloy, or titanium backing shell.

• Operating Rule: Dry PTFE liners must NEVER be lubricated with grease or oil. Introducing oil or grease forms a sticky paste that attracts airborne runway dust, sand, and carbon brake dust. These abrasive particles embed into the soft liner, rapidly shredding the microscopic PTFE transfer film and destroying the bearing within dozens of flight cycles.

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Bushing Installation, Fitting and Anti-Rotation Methods

Bushings are fitted into airframe structural lugs and housings using precision interference fits (press fits or shrink fits) to ensure that the outer diameter of the bushing remains rigidly locked against the housing bore under all operational flight loads.

   BUSHING INSTALLATION AND RETENTION METHODS

       Thermal Shrink Fit                      Bore Contraction After Press Fit
       (Dry Ice / Liq. N2)                     
                                                       Bore Contraction (~75% of Interference)
     Liquid Nitrogen (-196°C)                                    | |   
           | |                                                   v v   
        .-------.  <-- Bushing shrinks             .-------------------------------. 
        |       |      radially by ~0.001"         | Housing Lug                   | 
     ===|=======|===                             ===|===========|                 |=== 
     ===|=======|===                             ===|===========|                 |=== 
        |       |                                   |  Bushing  |                 |   
        '-------'                                   |  Inner    |<-- Ream to size |   
            |                                       |  Diameter |    after press  |   
            v                                       |           |                 |   
     Drops smoothly into                         ===|===========|                 |=== 
     Housing without galling                     ===|===========|                 |=== 
                                                   '-------------------------------' 

1. Mechanical Press Fit vs. Thermal Shrink Fit

• Mechanical Press Fit (Arbor Press): The bushing is forced into the bore using a hydraulic or manual arbor press equipped with a stepped pilot mandrel. The pilot must fit closely inside the bushing bore, with a square shoulder supporting the full end face of the bushing. Hazard: Forcing an interference fit dry can cause severe galling, pickup, scoring, and metal transfer between the bushing and the housing, potentially scrapping an expensive landing gear forging.
• Thermal Shrink Fit (Preferred Aerospace Practice):
  • Dry Ice ($CO_2$) Cooling: The bushing is placed in an insulated container packed with solid carbon dioxide (dry ice) at -78.5°C (-109.3°F) for approximately 30 to 45 minutes.
  • Liquid Nitrogen ($LN_2$) Cooling: For heavier interference fits or titanium/steel housings, the bushing is submerged in liquid nitrogen at -196°C (-320°F).
  • Housing Preheating: If authorized by the Aircraft Maintenance Manual (AMM), the aluminium or steel housing lug may be warmed using a calibrated hot air blower or heating blanket (typically limited to a maximum of 100°C to 120°C / 212°F to 248°F to prevent altering the metallurgical temper or degradation of structural sealant).
  • Assembly: The thermally contracted bushing drops effortlessly into the housing by hand without mechanical force. Once room temperature is re-established, an unbreakable, stress-free interference joint is formed with zero bore scratching.

2. Bore Contraction and Final Sizing (Reaming vs. Burnishing)

When an interference-fit bushing is pressed or shrunk into a rigid housing, the compressive hoop stress exerted by the housing forces the thin-walled bushing inward. This causes bore closure (contraction):

$\text{Bore Closure} \approx 70\% \text{ to } 80\% \text{ of Total Diametral Interference}$

Because the internal diameter shrinks unpredictably, bushings cannot be manufactured to their final operating clearance prior to installation.

• Final Reaming: After installation and thermal stabilization, the bushing inner bore must be reamed to its final drawing tolerance using a precision adjustable or fixed spiral-flute hand reamer guided by a pilot sleeve. Spiral flutes prevent chattering and ensure a chatter-free, mirror finish.
• Ball Burnishing: Alternatively, an ultra-hard, precision-ground tungsten carbide ball (sized slightly larger than the desired bore) is lubricated and pressed hydraulically through the installed bushing bore. Burnishing displaces micro-asperities plastically, work-hardens the bronze surface, closes surface porosity, and produces an ultra-smooth, low-friction sliding finish.

3. Anti-Rotation Retention Methods

Under heavy oscillating or torsional loads, the frictional grip of an interference fit may relax due to differential thermal expansion (especially when a bronze bushing is installed in an aluminium housing). To prevent rotation and axial migration, positive mechanical retention is employed:

• Flanged Bushing Design: An integral radial flange bears against the housing face to resist axial thrust loads.
• Keys and Keyways: A longitudinal keyway machined into the bushing OD engages a matching key or slot in the housing.
• Dowel Pins and Grub Screws: A radial hole is drilled through the housing and bushing wall to accept an interference dowel pin or threaded locking pin.
• Flange Attachment Screws: High-load landing gear bushings often feature a bolted outer flange secured directly into the housing lug with safety-wired bolts.
• Retaining Compounds: Anaerobic retaining adhesives (e.g., Loctite 609 / 680) are applied during press fitting to fill microscopic surface gaps and prevent fretting corrosion.

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Spherical Bearings and Rod End Joints (Heim / Rose Joints)

In mechanical flight control systems, push-pull control rods transmit mechanical inputs from pilot cockpit controls (control column, rudder pedals) to primary aerodynamic surfaces (ailerons, elevators, rudder) and auxiliary systems (trim tabs, spoiler mixers, engine throttle runs). Because wing flexure, fuselage structural bending, and complex 3D bellcrank arcs generate continuous angular misalignment, rigid pin joints would bind catastrophically.

Spherical Bearings (commonly known as Heim joints, Rose joints, or spherical rod ends) provide low-friction rotational freedom while permitting omnidirectional angular self-alignment (typically up to $\pm 10°$ to $\pm 25°$ of angular tilt).

   SPHERICAL ROD END BEARING (HEIM / ROSE JOINT) ANATOMY

                 Hardened Steel Inner Ball (Convex Spherical Surface)
                       |      Through-Hole for Attachment Bolt
                       |      |
                       v      v
                   .-------[  ]-------.
                  /   .---.    .---.   \  
                 |   /     \  /     \   |  <-- Self-Lubricating Liner (PTFE)
                 |  |       ||       |  |  <-- Outer Race / Housing Eye
                 |   \     /  \     /   |
                  \   '---'    '---'   /
                   '-------.  .-------'
                           |  |
                           |  |  <-- Threaded Shank (Male or Female)
                           |  |      (Attached to Push-Pull Control Tube)
                           |__| 

Construction and Varieties

1. Inner Ring (Ball): Precision-ground, spherical outer profile manufactured from high-carbon chromium bearing steel (SAE 52100) or martensitic stainless steel (AISI 440C), hardened to 58–62 HRC and hard chrome plated for wear and corrosion resistance. A cylindrical bore accommodates the through-bolt.
2. Outer Ring (Housing / Eye): High-strength alloy steel (4130 / 4340) or stainless steel body containing a concave spherical mating race.
3. Bearing Interface:
   • Lined (Maintenance-Free): A self-lubricating PTFE/fabric composite liner is permanently bonded to the inner spherical race of the outer ring. This represents the civil transport aircraft standard.
   • Metal-to-Metal: Heat-treated steel ball sliding directly against a bronze or beryllium-copper race, equipped with a grease lubrication port.

Housing Retention Methods: Staking and Swaging

When a cartridge-style spherical bearing is installed into a machined structural housing (such as a flight control actuator bellcrank or flap drive arm), it must be positively retained against axial ejection:

• Roller Swaging (360° Groove Swage): The housing bore or bearing outer ring features a thin, ductile retention lip (swaging groove). A specialized three-roller rotary swaging tool mounted in a drill press or portable fixture rotates while axial pressure is applied. The spinning rollers smoothly deform the ductile lip outward into a $45°$ chamfer machined in the housing, creating a continuous, stress-free $360°$ mechanical interlock.
• Anvil Staking (Die Staking): Matched top and bottom circular punch dies containing radial teeth or serrations are compressed under a hydraulic press, swaging discrete points of the retention lip over the housing edge. Roller swaging is widely preferred in aerospace because it eliminates notch-sensitive impact stresses and fatigue cracks.

In-Service Inspection, Play Limits and Rejection Criteria

During scheduled A-checks and C-checks, certifying technicians inspect spherical bearings and rod ends for physical security, smooth movement, and mechanical wear:

1. Visual and Tactile Assessment:
   • Check for smooth, continuous articulation throughout its full angular range. The ball should rotate with a slight, smooth hand drag; any binding, notchiness, or gritty resistance requires immediate removal.
   • Inspect the liner: Look for evidence of liner extrusion (Teflon fabric bunching or peeling out of the ball gap), cracking, or discoloration.
   • Verify staking retention: Ensure the swaged lip remains tight against the housing chamfer without looseness or cracking.
2. Radial and Axial Play Measurement (Dial Test Indicator):
   • In flight control push-pull rods, excessive play leads to control surface flutter, reduced aerodynamic authority, and pilot control lag.
   • A Dial Test Indicator (DTI) is rigged to measure play while applying a calibrated push-pull reversing load (typically $20\text{ to }50\text{ N}$ / $5\text{ to }10\text{ lbf}$) along the radial and axial axes:
   • Maximum Permissible Radial Play: Typically $0.002\text{ to }0.005\text{ inches}$ ($0.05\text{ to }0.12\text{ mm}$) depending on the specific AMM or structural repair manual (SRM).
   • Maximum Permissible Axial Play: Typically $0.005\text{ to }0.010\text{ inches}$ ($0.12\text{ to }0.25\text{ mm}$).
   • If measured play exceeds structural limits, or if any free rattle is detectable by hand, the assembly must be replaced before flight.