10.1 Bearing Classifications, Load Types & Lubrication Practice

10.1 Bearing Classifications, Load Types & Lubrication Practice

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

Bearings are precision mechanical assemblies engineered to support, locate, and guide moving aircraft members while transmitting operational forces and minimizing rotational or sliding friction. Within aircraft systems—ranging from high-bypass turbofan mainshafts rotating at over 15,000 RPM to low-velocity, high-stress flight control hinge pivots—bearings operate under demanding operational environments. They must withstand extreme vibrational harmonics, severe gyroscopic moments, wide thermal variations (-55°C to over 250°C), and cyclic g-loading while maintaining micron-level dimensional stability. For the aircraft maintenance engineer, understanding bearing physics, classification boundaries, preloading mechanics, and lubrication discipline is essential to airworthiness and failure prevention.

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Fundamental Classifications: Plain vs. Rolling-Element Bearings

Aircraft bearings are divided into two fundamental mechanical categories based on their relative surface motion: plain bearings (sliding friction) and rolling-element bearings (rolling friction).

1. Plain Bearings & Journal Bushings

Plain bearings—often called bushings, sleeve bearings, or journal bearings—guide rotating, oscillating, or reciprocating shafts through direct surface-to-surface sliding contact across a sacrificial, low-friction boundary layer. They lack moving internal parts, providing high structural simplicity, compact radial dimensions, exceptional resistance to shock and impact loads, and quiet operation.

• Sintered Bronze Bushings (Oilite): Manufactured by compacting powdered tin-bronze alloys and sintering them in a controlled furnace atmosphere. The resulting metal structure remains porous (typically 18% to 25% oil-void porosity). The bushing is vacuum-impregnated with refined petroleum or synthetic lubricating oil. During shaft rotation, frictional heat expands the oil out of the porous matrix to form a hydrodynamic lubricating wedge. Upon cooling at standstill, capillary action draws the lubricant back into the sintered bronze pores. Commonly installed in throttle quadrant levers, landing gear door hinges, and low-speed actuator linkages.
• PTFE-Lined Composite Bushings (DU / Glacier Type): Comprise a structural steel backing strip, a porous sintered bronze interlayer, and an impregnated surface overlay of polytetrafluoroethylene (PTFE) and lead. These bushings operate completely dry without external oil or grease servicing. The PTFE transfer film bonds to the mating steel pin, establishing an ultra-low-friction sliding interface. They exhibit outstanding chemical resistance and perform reliably in flight control push-pull rod bellcranks and cabin environmental air valves.
• Woven Fabric Composite Bearings (Kamatics / Fiberglide): Utilize synthetic resin-bonded woven PTFE and Dacron fibers backed by stainless steel or aluminium shells. Capable of supporting dynamic bearing loads exceeding 200 MPa (30,000 psi) without external lubrication. They are standard equipment on modern commercial transport flap and slat tracks, spoiler hinges, and rotorcraft swashplate scissor links.

2. Spherical Bearings & Rod Ends (Heim Joints)

Spherical plain bearings consist of a precision-ground, spherical inner ring (ball) housed inside a spherical outer ring. In rod-end assemblies, the outer ring is forged directly into an internally or externally threaded shank.

• Self-Aligning Capability: Accommodates up to ±15° of angular misalignment and airframe torsional twist without binding. This makes them indispensable in flight control push-pull rods, engine mount strut links, and landing gear torque links.
• Liner Construction: Aviation rod ends are predominantly self-lubricating, utilizing an internal swaged PTFE liner between the ball and outer race. Maintenance engineers must never lubricate self-lubricating PTFE rod ends with grease or spray penetrants; foreign oils attract grit and wash away the microscopically thin PTFE transfer film, causing rapid wear.

+-------------------------------------------------------------------------+
|                   AIRCRAFT BEARING CLASSIFICATION MAP                   |
|                                                                         |
|   [ AIRCRAFT BEARINGS ]                                                 |
|     |                                                                   |
|     +---> [ Plain Bearings (Sliding Contact) ]                          |
|     |       |-- Sintered Porous Bronze (Oilite / Self-lubricating)      |
|     |       |-- PTFE-Lined Metal-Polymer (DU Bushings)                 |
|     |       |-- Woven Composite Bushings (Kamatics / Flap Tracks)       |
|     |       '-- Self-Aligning Spherical Rod Ends (Heim Joints)          |
|     |                                                                   |
|     '---> [ Rolling-Element Bearings (Rolling Contact) ]                |
|             |-- Ball Bearings: Deep-Groove Radial, Angular Contact       |
|             '-- Roller Bearings: Cylindrical, Spherical, Tapered, Needle|
+-------------------------------------------------------------------------+

3. Rolling-Element Bearings (Antifriction Bearings)

Rolling-element bearings replace sliding friction with rolling friction by interposing hardened metallic or ceramic balls or rollers between two concentric hardened steel raceways. Rolling friction coefficients (0.001 to 0.003) are an order of magnitude lower than unlubricated sliding friction, significantly reducing rotational drive torque, starting friction, and operating heat generation.

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Rolling-Element Geometries & Load Vectors

Bearing selection is dictated by the vector orientation of the applied forces relative to the rotational axis of the shaft:

• Radial Load: Forces acting purely perpendicular (at 90°) to the shaft centerline.
• Axial (Thrust) Load: Forces acting purely parallel to the shaft centerline along its longitudinal axis.
• Combined Load: Simultaneous acting radial and axial load vectors resulting in an oblique resultant vector.

+-------------------------------------------------------------------------+
|                    BEARING LOAD VECTOR RESOLUTION                       |
|                                                                         |
|           Pure Radial Load               Pure Axial (Thrust) Load       |
|                 |                                                       |
|                 v F_radial                                              |
|             +-------+                   =======> F_thrust               |
|       =====[| Shaft |]=====        -----[ Bearing ]-------------------  |
|             +-------+                   =======>                        |
|                 |                                                       |
|                                                                         |
|                         Combined Load Vector                            |
|                                 F_resultant                             |
|                                   ^   /                                 |
|                                   |  / Contact Angle (alpha)            |
|                                   | /                                   |
|                         F_radial  |/________> F_thrust                  |
+-------------------------------------------------------------------------+

Rolling-Element Types & Characteristics

1. Deep-Groove Radial Ball Bearings (Conrad Type):

   • Construction: Feature continuous, uninterrupted circular raceway grooves on both the inner and outer rings. The groove curvature radius closely conforms to the ball radius (typically 51% to 53% of ball diameter), maximizing contact area under load.
   • Capabilities: Primary capacity is radial load. Due to deep raceway shoulders, they can also support moderate bidirectional axial thrust loads (up to approximately 50% of their radial rating) simultaneously.
   • Applications: Aircraft electric motors, alternators, fuel boost pump drives, and gearbox idler shafts.

2. Angular Contact Ball Bearings:

   • Construction: Designed with one high raceway shoulder and one relieved (steeply cut) counter-shoulder on the outer ring. The line connecting the contact points between the ball and raceways forms an acute contact angle (typically 15°, 25°, 30°, or 40°) relative to the radial plane.
   • Capabilities: Specifically engineered to carry heavy combined radial and unidirectional axial thrust loads. Higher contact angles yield greater axial thrust capacity at the expense of radial rating. They must always be mounted against an opposing thrust load or paired with a matching bearing to prevent the balls from climbing out of the raceway.
   • Applications: Gas turbine compressor and turbine mainshaft thrust bearings, propeller governor drive shafts, and centrifugal fuel pumps.

3. Cylindrical Roller Bearings:

   • Construction: Hardened cylindrical rollers guided by ground flanges on one or both raceways. Contact between roller and raceway is a theoretical straight line (line contact), rather than the elliptical point contact of a ball bearing.
   • Capabilities: Massive radial load capacity and superior impact shock resistance. Standard designs carry zero axial thrust load. By allowing the inner ring to slide axially relative to the rollers, cylindrical roller bearings accommodate thermal longitudinal expansion of high-speed turbine shafts without introducing axial binding forces.
   • Applications: Gas turbine engine turbine bearings (expansion end), gearbox planetary output shafts, and starter-generator drive ends.

4. Spherical Roller Bearings:

   • Construction: Feature two rows of barrel-shaped symmetrical or asymmetrical rollers running inside a common outer ring with a continuous spherical raceway. The center of the outer raceway sphere coincides with the bearing center axis.
   • Capabilities: Extreme radial load capacity combined with self-aligning capability (up to ±2.5° angular deflection). Compensates dynamically for airframe structural bending, wing flexure, and shaft deflection under high g-loads.
   • Applications: Helicopter main rotor swashplates, landing gear trunnion pivots, and heavy transport flap drive torque tubes.

5. Tapered Roller Bearings:

   • Construction: The rolling elements are truncated cones (frustums). The extensions of the tapered conical surfaces of the rollers, inner raceway (cone), and outer raceway (cup) all converge at a common point on the bearing centerline axis (the apex). This geometry ensures pure rolling motion without sliding friction across the raceways.
   • Capabilities: Designed to handle severe simultaneous radial and unidirectional thrust loads. Axial load capacity increases with the cup angle.
   • Applications: Aircraft main and nose landing gear wheel hubs, where they support the entire aircraft weight on touchdown while absorbing severe lateral side-scrubbing thrust loads during crosswind landings.

6. Needle Roller Bearings:

   • Construction: Cylindrical rollers having a small diameter relative to their length (length-to-diameter ratio $L/D \ge 4$, with roller diameter typically $\le 5\text{ mm}$). Often installed without an inner raceway, running directly on a case-hardened shaft.
   • Capabilities: Delivers the highest radial load capacity per unit of radial cross-section of any rolling bearing. However, they cannot carry axial thrust and have lower maximum rotational speed ceilings.
   • Applications: Variable-pitch propeller blade pitch change linkages, piston engine rocker arms, flight control surface bellcranks, and epicyclic planet gear idler hubs.

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Bearing Anatomy & Retainer (Cage) Engineering

A precision rolling-element bearing comprises four primary structural components:

1. Inner Ring: Pressed onto and rotates with the shaft; features a precision-honed raceway.
2. Outer Ring: Fitted into the stationary housing; guides the rolling elements.
3. Rolling Elements: Precision-graded balls or rollers manufactured to tolerances within 0.00005 inches (0.001 mm).
4. Cage (Separator / Retainer): Crucial internal component that spaces the rolling elements equidistant around the circumference.

Functions of the Cage (Separator)

Without a cage, adjacent balls or rollers would contact each other. Because adjacent rolling elements rotate in the same rotational sense (e.g., both clockwise), their contact surfaces slide against each other in opposite directions at twice the surface velocity. This creates intense boundary friction, rapid galling, ball scuffing, and catastrophic lockup. The cage:

• Maintains uniform peripheral spacing between rolling elements to prevent ball-to-ball rubbing.
• Distributes the dynamic load uniformly across all active load-zone rolling elements.
• Prevents rolling element skewing and guiding them smoothly into and out of the loaded zone.
• Retains the rolling elements in separable bearings (e.g., tapered or cylindrical roller bearings) during bench assembly.

Cage Materials & Failure Modes

• Pressed / Stamped Steel: Lightweight and low cost. Standard for general aviation, accessory gearboxes, and medium-speed applications. Susceptible to fatigue cracking at pocket corners under heavy torsional vibration.
• Machined Brass / Bronze: High structural strength, superior thermal conductivity, and excellent natural anti-galling boundary lubricating properties. Guided on either the inner ring, outer ring, or rolling elements. Standard for high-speed gas turbine gearboxes and commercial transport APU bearings.
• Phenolic Resin / Non-Metallic Composites: Fabric-reinforced phenolic polymers. Extremely lightweight with low inertia and high oil absorption. Designed for ultra-high-speed applications exceeding 40,000 RPM, such as flight instruments, air-cycle machine (ACM) turbines, and gyros. Limited to operating temperatures below 120°C.

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Preloading Angular Contact Bearings & Duplex Mounting

In high-precision, high-speed aviation mechanisms, bearings must never operate with loose internal clearance. Uncontrolled clearance leads to shaft chatter, gear tooth misalignment, and dynamic ball skidding (where rolling elements slide rather than roll across raceways during rapid engine throttle transients, causing severe surface smearing). To achieve high assembly rigidity and zero clearance, angular contact bearings are installed with a permanent mechanical preload.

Preloading Principles

Preloading applies a deliberate, calibrated axial load to the bearing assembly prior to operational service. This force displaces the inner ring relative to the outer ring, squeezing the balls tightly against the raceway shoulders. This eliminates all internal radial and axial free play ($C_0$), pre-deforms microscopic surface asperities, and significantly increases dynamic shaft positioning accuracy.

Duplex Mounting Arrangements

Two matched angular contact bearings are paired into a single structural unit called a duplex set. The inner and outer rings are manufactured with precision-ground offset faces. When clamped together on a shaft with an arbor nut, the faces contact, establishing the exact engineering preload:

+-------------------------------------------------------------------------+
|                 DUPLEX ANGULAR CONTACT BEARING SETS                     |
|                                                                         |
|   1. BACK-TO-BACK (DB / 'O' Setup)     2. FACE-TO-FACE (DF / 'X' Setup) |
|      Contact lines DIVERGE outwardly      Contact lines CONVERGE        |
|                                                                         |
|          /|\         /|\                      \|/         \|/           |
|         / | \       / | \                      |           |            |
|        /  |  \     /  |  \                    / \         / \           |
|       /   |   \   /   |   \                  /   \       /   \          |
|      <----+----> <----+---->                <----+----> <----+---->     |
|       \   |   /   \   |   /                  \   /       \   /          |
|        \  |  /     \  |  /                    \ /         \ /           |
|         \ | /       \ | /                      |           |            |
|          \|/         \|/                      /|\         /|\           |
|       Rigid against tilting moments        Accommodates slight shaft    |
|       (Ideal for overhung loads)           misalignment; lower moment   |
|                                                                         |
|   3. TANDEM (DT Setup)                                                  |
|      Contact lines PARALLEL                                             |
|          /|\         /|\                                                |
|         / | \       / | \                                               |
|        /  |  \     /  |  \                                              |
|       Shares high unidirectional thrust load equally across both rows   |
+-------------------------------------------------------------------------+

1. Back-to-Back (DB - "O" Arrangement):
   • Geometry: The contact angle lines diverge as they pass outward toward the bearing centerline axis. The effective load centers fall outside the physical width of the bearing assembly.
   • Characteristics: Provides the highest moment rigidity and resistance to angular shaft tilting. It is the premier arrangement for cantilevered or overhung loads, machine tool spindles, and aircraft propeller gearbox input shafts.
2. Face-to-Face (DF - "X" Arrangement):
   • Geometry: The contact angle lines converge as they pass outward toward the bearing centerline axis. The effective load centers fall inside the bearing footprint.
   • Characteristics: Accommodates slight angular shaft misalignment and housing flexure. However, it has significantly lower moment stiffness compared to the DB arrangement.
3. Tandem (DT Arrangement):
   • Geometry: The contact lines run mutually parallel in the same direction.
   • Characteristics: Both bearings share a heavy unidirectional axial thrust load equally. It cannot resist thrust in the opposite direction, requiring a third bearing mounted in opposition.

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Lubrication Principles: Oil vs. Grease Discipline

Lubrication is the lifeblood of an aircraft bearing. It separates rolling elements from raceways via an elastohydrodynamic (EHL) oil film (thickness 0.1 to 1.0 microns), dissipates frictional heat, flushes wear debris, and prevents environmental oxidation.

1. Oil Lubrication Systems

• Recirculating Pressure Jet Oil: Mandatory in gas turbine engine mainshaft sumps and main helicopter transmissions. Cooled, de-aerated synthetic ester oil (conforming to MIL-PRF-23699 or MIL-PRF-7808) is sprayed under pressure directly into the bearing cage pocket. The high oil volume flushes away tremendous thermal energy transferred from turbine combustion stages.
• Oil-Mist / Air-Oil Systems: Compressed bleed air delivers microscopic droplets of oil to high-speed auxiliary gearbox bearings, eliminating hydraulic churning drag while maintaining positive internal pressure to keep dust and water out of bearing sumps.

2. Grease Lubrication Mechanics

Grease is a semi-solid lubricant consisting of three constituents: base oil (70%–90%), thickener / soap structure (10%–20%), and chemical additives (1%–5%).

• The Sponge Analogy: Think of the thickener (lithium complex, clay bentonite, or polyurea) as a microscopic sponge. The sponge retains the lubricating oil until mechanical shearing and heat from rotating bearing elements squeeze oil out into the contact track. When motion stops, the thickener reabsorbs the oil.
• Aviation Grease Specifications:
  • MIL-PRF-81322 (e.g., Mobilgrease 28 / Aeroshell Grease 22): Synthetic polyalphaolefin (PAO) base oil with an inorganic bentonite clay thickener. Operating range -54°C to +177°C. Standard for transport aircraft wheel bearings, flap actuators, and landing gear trunnions.
  • MIL-PRF-23827 (e.g., Aeroshell Grease 7 / 33): Synthetic ester base with lithium soap or complex. Formulated for high-load airframe mechanisms and flight control systems down to -73°C.
  • Incompatibility Warning: Never intermix greases with different thickeners (e.g., bentonite clay with lithium complex). Incompatible soaps react chemically, liquefying into a runny sludge that separates from the oil, completely flushing out of the bearing cavity.

Grease Quantity Is Component-Specific

A critical workshop discipline tested in EASA Part-66 examinations is grease fill packing volume. A 30% to 50% fill is a common training example for some bearing applications, but the component maintenance instructions control type and quantity:

$$\text{Permissible Grease Packing Volume} = 30\% \text{ to } 50\% \text{ of Total Internal Free Void Space}$$

+-------------------------------------------------------------------------+
|                    THE GREASE PACKING VOLUME RULE                       |
|                                                                         |
|     CORRECT: 30% to 50% Void Fill          FATAL TRAP: 100% Full        |
|           (One-Third Full)                   (Completely Packed)        |
|                                                                         |
|          .-----------------.                 .-----------------.        |
|         /   [Air Space]     \               /  [Solid Grease]   \       |
|        |     (50%-70%)       |             |   No Air Space!     |      |
|        |  =================  |             |  =================  |      |
|        |    Grease Film      |             |  Solid Grease Mass  |      |
|         \   (30%-50%)       /               \                   /       |
|          '-----------------'                 '-----------------'        |
|                                                                         |
|    Results: Stable operating temp,     Results: Severe hydraulic        |
|    free expansion, zero seal damage.   churning, rapid >150°C thermal   |
|                                        spike, seal blowout, seizure!   |
+-------------------------------------------------------------------------+

The Catastrophic Over-Packing Hazard

A common but dangerous workshop misconception is that "more grease provides better lubrication." In rolling bearings, over-packing is fatal:

1. Hydraulic Fluid Churning: When the cavity is 100% packed, the rolling elements and cage cannot push the grease aside into free void space. They are forced to continuously plow through the thick, viscous grease mass.
2. Extreme Frictional Heat Generation: Viscous shear friction within the churned grease generates intense heat. Operating temperatures spike rapidly past 150°C to 180°C within minutes of operation.
3. Seal Blowout: As trapped air and grease expand under severe heat, internal pneumatic pressure builds, blowing rubber lip seals and dust shields out of the housing.
4. Thermal Grease Breakdown: Elevated temperatures cause base oil evaporation and chemical oxidation. The thickener hardens into a carbonized, abrasive crust. Starved of liquid lubricating oil, the raceways score, balls skid, and the bearing suffers catastrophic thermal seizure.

Sealed-for-Life vs. Re-Greasable Bearing Assemblies

• Sealed-for-Life Bearings (2RS / 2Z): Fitted with Buna-N or Viton contact lip seals (RS) or non-contact metal shields (Z). Filled at the factory with a precise 30% charge of high-stability synthetic grease. They require zero maintenance and must never be washed in solvent or drilled for grease injection. When worn or reaching life limits, they are discarded.
• Re-Greasable Assemblies: Housed in pillow blocks or hub assemblies equipped with grease nipples (Zerk fittings) and purge relief valves. Maintenance manuals prescribe exact shot counts (using calibrated grease guns) to purge old grease without over-pressurizing internal seals.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

An aircraft maintenance technician is overhauling an inboard main landing gear wheel assembly on an Airbus A320. The assembly utilizes two large tapered roller bearings (inner and outer cone assemblies).

1. Cleaning: The technician removes old grease using clean mineral spirits, avoids spinning the dry rollers, and verifies complete absence of lint using clean lint-free cloths.
2. Inspection: Cone assemblies, rollers, and pressed cup raceways are inspected under 10x magnification. Cup raceways show smooth, mirror-like contact surfaces with zero step wear, no discoloration, and no spalling craters.
3. Packing Procedure: The technician mounts the clean cone on a mechanical pressure bearing packer. Mobilgrease 28 (MIL-PRF-81322) is injected until grease exudes uniformly around the entire circumference between the rollers and cage pockets. The technician uses a clean finger to wipe away excess grease from the outside faces, verifying the total grease volume occupies between 35% and 40% of the wheel hub void cavity. The remaining hub cavity is left empty to accommodate thermal grease expansion.
4. Preload Adjustment: The wheel is mounted onto the axle. The axle nut is tightened to the initial bedding torque (e.g., 150 ft-lbs) while rotating the wheel to seat the tapered rollers squarely against the cone thrust rib. The nut is then backed off and torqued to the final running torque (e.g., 35 ft-lbs) before locking with a cotter pin. Axial end play is verified with a dial indicator to be within the 0.001" to 0.005" AMM tolerance.

Common Exam Traps

• Trap 1: Assuming a bearing should be packed 100% full of grease. Remember: overpacking can cause churning and heat, but the specified quantity for the bearing is the acceptance rule. Packing 100% full causes violent churning, extreme overheating, seal failure, and rapid seizure.
• Trap 2: Confusing DB and DF angular contact duplex arrangements. Back-to-Back (DB) has diverging contact lines and provides high resistance to angular shaft tilting/moments; Face-to-Face (DF) has converging lines and accommodates slight misalignment.
• Trap 3: Using oil/solvents on self-lubricating PTFE rod ends. Self-lubricating rod ends must remain completely dry. Adding solvent or oil destroys the transfer film and attracts abrasive grit.
• Trap 4: Forgetting that standard cylindrical roller bearings carry ZERO axial thrust. Cylindrical roller bearings support pure radial loads and permit axial shaft expansion; they cannot react axial thrust without specialized rib configurations.