Hàng không Industry Bearings: Codes & Applications

Aerospace bearings are rolling and plain elements certified to aerospace standards — ABEC 7/9 (ISO P4/P2), MIL-B-17931, and AS9100 — including angular contact ball bearings in M50 steel and hybrid ceramic designs, installed in aircraft engines, flight control actuators, landing gear assemblies, and auxiliary systems. They operate under conditions that exceed any industrial application: temperatures from −54°C to +300°C, rotational speeds to 30,000 rpm, g-forces to 10g during maneuver, and zero tolerance for in-service failure.

Southeast Asia's MRO (Maintenance, Repair, and Overhaul) sector is expanding rapidly as regional air traffic recovers past pre-2020 levels. Vietnam Airlines, VietJet, and Bamboo Airways collectively operate over 150 narrowbody aircraft, primarily Airbus A320 family and Boeing 737 family, generating significant demand for FAA/EASA Part 145-certified MRO bearing replacement. This article covers the four main bearing positions in civil aerospace applications, the tolerance standards that govern them, material selection, and a case study from Southeast Asia's MRO sector.

What are aerospace bearings?

Aerospace bearings differ from industrial bearings in five dimensions: tolerance class (ABEC 7/9 versus standard ABEC 3), material (M50, M50 NiL, Si₃N₄ hybrid ceramic versus 52100 steel), surface finish (Ra ≤0.05 μm versus Ra ≤0.2 μm), documentation (FAA Form 8130-3 or EASA Form 1 release authorization), and traceability (lot number, heat number, hardness test records to the individual bearing serial number).

The four primary bearing groups in civil aerospace:

A bearing that does not carry FAA Form 8130-3 or EASA Form 1 documentation cannot be installed on a certificated aircraft — regardless of dimensional equivalence, material, or the manufacturer's industrial reputation. Documentation is not a bureaucratic formality; it is the traceability chain that connects the bearing in service to the material test records at manufacture.

Jet engine bearings

Jet engine main shaft bearings are the most technically demanding bearings in civil aviation. They operate at shaft speeds of 5,000–30,000 rpm, temperatures of 150–300°C at the bearing, thrust loads of 50–150 kN from compressor and turbine pressure differentials, and must sustain function through transient events including bird strike, blade-off, and aborted takeoff sequences.

Angular contact ball bearings (ACBB) for thrust load positions (compressor front, turbine rear):

• 7020 BECBP (d=100, D=150, B=24 mm, contact angle α=40°) — high-pressure compressor front bearing in 20,000–40,000 lbf thrust class engines; M50 NiL steel for thermal stability to 315°C; ABEC 9 (P2) tolerance
• 7028 BECBP (d=140, D=210, B=33 mm) — HP compressor rear bearing; larger bore for higher shaft diameter in high-bypass turbofan; hybrid ceramic option (Si₃N₄ balls) reduces centrifugal ball load at high speed

Cylindrical roller bearings (CRB) for radial load positions (compressor rear, turbine front):

• NU1024/HC5 (d=120, D=180, B=28 mm, C=260 kN) — turbine shaft radial bearing; HC5 = full complement of Si₃N₄ ceramic rollers; C5 (very wide) clearance compensates for extreme thermal differential between inner ring (hot shaft) and outer ring (cooled housing)
• 6028/HC5C3 (d=140, D=210, B=33 mm, C=186 kN) — accessory gearbox input shaft; HC5 hybrid ceramic, C3 clearance; jet oil (MIL-PRF-23699) lubrication through the engine oil system

Accessory gearbox bearings:

• NUP216 ECP (d=80, D=140, B=26 mm, C=186 kN, C₀=228 kN) — accessory gearbox intermediate shaft; NUP type provides both radial and one-direction axial support; ECP = polymer cage for fuel/oil splash environment

Main shaft bearing installation is performed exclusively at engine overhaul facilities under controlled conditions. Field replacement during line maintenance is not applicable — these are shop-level removals with engine stand required.

Flight control bearings: self-lubricating spherical plain

Flight control linkages — elevator, rudder, aileron rod ends, and primary actuator attachment points — use spherical plain bearings (self-lubricating) rather than rolling element bearings. The functional requirement is angular misalignment accommodation under high cyclic axial loads, with maintenance-free operation over the 25,000+ flight cycle design life of the airframe.

PTFE-lined self-lubricating spherical plain bearings:

• GE...TXE-2LS series (Schaeffler/INA designation) — PTFE fabric liner bonded to inner ring, stainless steel outer ring; no external lubrication required; contact angle ±12°; installed at primary flight control rod ends on A320 family aircraft
• GEZ 100 ES (d=100, D=150, B=67 mm, C=1,250 kN) — heavy-duty spherical plain for elevator actuator attachment; ES = stainless steel outer ring; designed for aircraft structure bearing hole interface
• GEZ 200 ES (d=200, D=290, B=130 mm, C=4,000 kN) — primary structure joint bearing; landing gear door hinge and wing attachment applications; extremely high static load requirement

These bearings must meet AS81820 (formerly MIL-B-81820) specification — the PTFE liner composition, bonding adhesive, and inner/outer ring material are defined and tested to this standard. Industrial-grade PTFE-lined spherical plain bearings that do not carry AS81820 conformance cannot substitute, even if dimensions match.

The key technical parameter is static load rating C₀ rather than dynamic load rating C — flight control joints see relatively low cycle rates but extremely high peak loads during maneuver limits (3.75g for A320) and ground load cases.

Actuator bearings: miniature DGBB and ACBB

Aircraft actuators — hydraulic actuators, electromechanical actuators (EMA), environmental control system (ECS) valves, and fuel control actuators — use miniature to small-size bearings in the ABEC 7 (P4) tolerance class. These positions demand dimensional accuracy that maintains actuator null position accuracy over thousands of cycles.

Miniature DGBB for valve and feedback mechanisms:

• 609 2Z/C3 (d=9, D=24, B=7 mm) — feedback potentiometer bearing in fuel control unit; 2Z metal shields (not 2RS rubber seals — rubber outgasses at altitude and is incompatible with hydraulic fluid); C3 clearance
• 618/8 (d=8, D=16, B=4 mm) — thin-section bearing for compact rotary actuator mechanisms; 618 series = extra-thin cross-section

Angular contact ball bearings for EMA (electromechanical actuator) motor shafts:

• 7201 BECBP (d=12, D=32, B=10 mm, C=6.20 kN, α=40°) — EMA motor bearing, DB mounting (back-to-back pair); ABEC 7 (P4) tolerance; M50 or 440C stainless for corrosion resistance; preloaded pair for bidirectional axial stiffness
• 7202 BECBP (d=15, D=35, B=11 mm, C=7.65 kN, α=40°) — secondary flight control EMA; higher load rating for larger actuator force output

Precision screw jack bearings for linear actuators:

• NSK 15TAC47BSUC10PN7B — 15 mm bore, 47 mm OD ball screw support bearing for electromechanical trim actuators; P7 (super-precision) tolerance; designed for simultaneous high axial and radial load in screw jack applications; this code is an NSK proprietary specification used across Airbus and Boeing electrical trim systems

Landing gear bearings

Landing gear structures carry the highest instantaneous loads in the airframe — a 70-tonne aircraft landing at 3 m/s sink rate generates vertical reactions exceeding 2,500 kN across both main gear assemblies. Bearings at gear strut pivots, torque link joints, and trunnion pivots must absorb these shock loads while maintaining dimensional stability through 20,000+ landing cycles.

Spherical roller bearings for trunnion pivots:

• 23240 CCK/W33 (d=200, D=360, B=128 mm, C=3,350 kN, C₀=5,000 kN) — main gear trunnion on widebody aircraft; K = tapered bore for adapter sleeve mounting; W33 = lubrication groove and holes; Cronidur 30 nitrogen-alloyed steel for corrosion resistance without cadmium plating
• 23234 CCK/W33 (d=170, D=310, B=110 mm, C=2,500 kN) — nose gear trunnion; lower load requirement but same corrosion resistance requirement for unpressurized wheel well environment

Spherical plain bearings for torque links and pivot pins:

• GEZ 200 ES (d=200, D=290, B=130 mm, C=4,000 kN) — main gear drag brace pivot; high-strength stainless steel inner and outer rings; self-lubricating PTFE liner; no maintenance lubrication required over design service life
• NA 4905 (d=25, D=42, B=17 mm, C=44.0 kN) — needle roller and cage assembly for torque link pin joints; compact cross-section required within the torque link geometry; grease-lubricated with access fitting

Landing gear bearing replacement is a Maintenance Task performed under aircraft maintenance manual (AMM) procedures at Part 145 certified organizations. Each bearing installation requires a Maintenance Release (FAA Form 8130-3 or EASA Form 1) from the supplier and a Maintenance Record entry in the aircraft technical log — no exceptions for any installation regardless of urgency.

Materials: M50, M50 NiL, Si₃N₄, Cronidur 30, AISI 440C

Material selection separates aerospace bearings from industrial bearings more clearly than any other specification:

52100 bearing steel (the universal industrial standard) is not used in aerospace main shaft or high-temperature positions — its maximum operating temperature of 120°C and susceptibility to hydrogen embrittlement in jet oil environments disqualify it for engine applications.

Si₃N₄ hybrid bearings (ceramic rolling elements in steel rings) provide three performance advantages: lower centrifugal force from 60% reduced roller/ball mass (allowing higher speed limits), higher hardness reducing contact stress under high load, and electrical insulation eliminating EDM pitting from engine alternator stray currents.

SKF Aerospace, FAG, Timken, NSK for MRO

The four primary suppliers for aerospace-certified bearings in Southeast Asia's MRO sector are SKF Aerospace (Sweden), FAG/Schaeffler Aerospace (Germany), Timken (USA), and NSK (Japan).

SKF Aerospace operates a dedicated aerospace division separate from SKF's industrial bearing operations — different manufacturing facilities, different quality systems (AS9100 Rev D), different documentation. SKF Aerospace supplies main shaft bearings for CFM56, CFM LEAP, PW1100G, and GE90 engines, with FAA/EASA dual-release documentation. SKF Aerospace is on the Approved Vendor List (AVL) for Airbus and Boeing, both of which supply approved repair stations in Southeast Asia.

FAG/Schaeffler Aerospace is the primary supplier for Airbus-platform flight control bearings (GE...TXE spherical plain series) and landing gear spherical rollers on A320/A330/A350 family aircraft. Schaeffler holds EASA Production Organization Approval (POA) and supplies into Europe-based MRO facilities that service aircraft operating in Vietnam.

Timken holds a strong position in landing gear and gearbox bearings — specifically the large-diameter spherical rollers for gear trunnion pivots on Boeing 737 and 777 aircraft. Timken's Aerospace division provides dual FAA/EASA Form 8130-3 release documentation.

NSK is the approved source for ball screw support bearings (TAC series) used in Boeing and Airbus electrical trim and flap actuator systems. NSK Precision (the aerospace division) holds AS9100 certification and provides JAXA-compliant documentation for Japanese carrier fleet aircraft.

ZVL is not on the Approved Vendor List for certificated aviation bearing positions and does not hold the aerospace quality system certifications required for aircraft installation. ZVL bearings are not applicable in any position covered by this article.

Case study: CFM56 engine overhaul in Southeast Asia

A Part 145-certified MRO facility in the region performs CFM56-5B engine overhauls for A320 operators, including aircraft from Vietnamese carriers. During a performance restoration shop visit, the facility identified a main shaft bearing (HP compressor front ACBB) showing raceway spalling on the inner ring, consistent with a material fatigue origin rather than contamination or EDM pitting.

Investigation: Bearing removed, photographed, and submitted to SKF Aerospace's laboratory under the Engine Bearing Analysis Program. Root cause identified: subsurface fatigue from a material inclusion — a known failure mode for M50 steel at the boundary of its fatigue life. The bearing had accumulated 18,000 flight cycles, approximately 85% of the expected design life for this position.

Replacement decision: The facility replaced all three HP compressor bearings as a matched set — replacing only the failed unit while leaving the others at 85% life would have required another engine removal within 2,000–3,000 flight cycles, a disproportionate cost versus the bearing material cost. SKF Aerospace supplied three units with dual FAA Form 8130-3 and EASA Form 1 release, traceable to individual lot numbers.

Challenge during procurement: The facility's standard supplier had one unit of the three in stock. The other two were sourced from a second authorized SKF Aerospace distributor in Singapore. A third-party offered the same bearing codes at significantly lower price from non-aviation stock — the facility declined. Non-aviation stock of the same part number lacks the AS9100 heat-number traceability required for installation on certificated aircraft, and the price difference was negligible against the labor cost of reinstalling a failed bearing in flight.

Outcome: Engine reinstalled and aircraft returned to service. The material analysis report was shared with the operator's continued airworthiness management organization (CAMO) to update the bearing life fraction tracking in the operator's reliability program.

Key lesson: aerospace bearing procurement is governed by documentation, not price. The decision to source from a second authorized distributor rather than non-aviation stock was not a preference — it was a regulatory requirement under EASA Part 145.A.42.