Tàu hỏa Industry Bearings: Codes & Applications

Railway bearings are rolling elements certified to railway-specific standards — EN 13260, UIC 615-4, and AAR M-101 — primarily tapered roller bearings for axleboxes and insulated ball bearings for traction motors, installed in wheelset axleboxes, traction motors, and gearboxes of locomotives, passenger coaches, freight wagons, and metro rolling stock.

Vietnam's North–South mainline spans 1,726 km, operating mixed freight and passenger traffic with axle loads of 15–18 tonnes. Metro systems in Hanoi (Line 2A) and Ho Chi Minh City (Line 1) are entering revenue service, adding a new urban rail segment to the existing mainline fleet. At these operating conditions, axlebox bearing failure means unscheduled stops or, in the worst case, a hot axle event that derails the train. This article covers the four main bearing positions in railway rolling stock, the certification standards that govern them, brand selection, and a case study from Vietnam's North–South mainline.

What are railway bearings?

Railway rolling stock bearings differ from industrial bearings in four critical respects: they are certified to railway-specific standards rather than general industrial standards; they carry wheelset axle loads up to 30 tonnes per bearing; they must demonstrate service life exceeding 1,000,000 km before scheduled replacement; and they operate in an environment of vibration, track irregularities, and thermal cycling that destroys uncertified bearings prematurely.

The four main bearing groups in railway rolling stock:

Certification is mandatory for axlebox and traction motor positions — uncertified bearings cannot be installed on revenue-service rolling stock regardless of dimensional equivalence.

Axlebox bearings: integrated bearing units

The axlebox bearing is the most safety-critical component in the wheelset assembly. It carries the full static axle load (typically 130–250 kN per bearing for passenger and freight applications) plus dynamic shock loads from track irregularities — impact factors of 1.5–2.5× static load are standard design assumptions.

Modern railway axlebox bearings use the Integrated Bearing Unit (IBU) or Tapered Bearing Unit (TBU) format: a complete bearing assembly with outer ring, inner ring assembly, tapered rollers, cage, and integrated seals — supplied as a single unit and pressed directly onto the axle journal. This eliminates the assembly errors possible with loose bearing components and ensures correct preload from the factory.

Major certified IBU specifications for 15–18 tonne axle loads (Vietnam's mainline standard):

• SKF TAROL series (d=130 mm, D=240 mm, C=930 kN, C₀=1,650 kN) — certified to UIC 615-4 and EN 13260; used on Vietnam Railways passenger coaches and Type D freight wagons; operating temperature range −40°C to +120°C, design life 1,000,000 km
• Timken 381A/382A (d=130 mm, D=230 mm, C=306 kN per row, combined unit) — AAR-certified for North American freight wagon axles; installed on some Vietnam Railway freight wagons imported via the AAR fleet; grease-lubricated, sealed for life
• NSK 130KBE22+L (d=130 mm, D=230 mm, B=168 mm assembly width, C=980 kN) — JIS D 4001-certified; used on Japanese-standard metro cars (Hanoi Line 2A Siemens Inspiro uses NSK-sourced axlebox assemblies); integrated ABS speed ring
• FAG 130TAROL (d=130 mm, D=240 mm, C=940 kN) — DIN/EN-certified, Schaeffler Railway supply; interchangeable with SKF TAROL series in wheelset geometry; used on European-derived rolling stock

Axlebox bearings are sealed-for-life grease-lubricated units — the integrated seal retains the factory-charged grease (typically lithium-calcium complex or polyurea grease rated for the full service interval) and excludes track contamination. Re-greasing intervals do not apply; replacement at the scheduled wheelset overhaul interval (typically 600,000–1,000,000 km) is the correct maintenance approach.

Hot axle detection: modern rolling stock uses trackside Hot Axle Box Detectors (HABD) that record axle temperature at 0.5–2 km intervals. Temperature thresholds: warning at ΔT+40°C above ambient, danger at ΔT+80°C. A bearing failing from contamination ingress, grease degradation, or mechanical damage will show rising temperature across multiple detector stations — the pattern distinguishes a failing bearing from a brake heating event.

Traction motor bearings: insulated DGBB

Traction motors in electric locomotives and electric multiple units (EMU) generate stray shaft currents from PWM inverter switching. Without insulation, these currents discharge through the bearing rolling contact, creating electrical pitting (EDM pitting) on raceways and rolling elements — a failure mode that progresses rapidly from first pitting to complete bearing destruction, typically within 200,000–500,000 km if unaddressed.

The solution is electrically insulated bearings: a ceramic (Al₂O₃) coating is applied to the outer ring outside diameter or inner ring bore, providing electrical resistance of >500 MΩ at DC. These bearings carry standard ISO 492 dimensional codes but are specified with insulation suffix designations:

• SKF 6324/C3VL0241 (d=120, D=260, B=55 mm, C=186 kN, C₀=212 kN) — traction motor drive-end (DE) bearing for 120 kW–200 kW AC motors; VL0241 = insulated outer ring, resistance >500 MΩ; C3 clearance for thermal expansion
• FAG 6320-M-C3-J20AA (d=100, D=215, B=47 mm, C=122 kN, C₀=132 kN) — insulated bearing for smaller traction motors (60–120 kW); J20AA = Schaeffler insulation designation; brass cage (M) for high-speed operation
• NSK 6320VV C3 EL (d=100, D=215, B=47 mm) — NSK's electrically insulated series; EL designation; used on Kawasaki and Nippon Sharyo rolling stock for HCMC Metro Line 1
• Timken 6330C3 Insocoat (d=150, D=320, B=65 mm, C=305 kN) — high-capacity insulated bearing for heavy traction motors (>300 kW) in freight locomotive applications

Installation requirement: insulated bearings must be installed with insulating end covers and insulating grease to prevent current bypass through the bearing housing. A standard bearing installed on the non-drive end (NDE) as a current return path negates the insulation on the drive end — both ends must be treated.

Inspection of traction motor bearings after disassembly: EDM pitting appears as a frosted or matte grey surface on the raceway, distinct from the polished bright grey of normal wear. A bearing showing EDM pitting must be replaced and the source of stray current (inverter grounding, shaft grounding brush condition) investigated before reinstallation.

Gearbox bearings: TRB and cylindrical roller

Railway traction gearboxes transmit motor torque to the axle, typically at gear ratios of 4:1 to 7:1. The pinion shaft and gear wheel shaft carry high combined radial and axial loads, requiring bearings with high dynamic load ratings and dimensional stability over the full maintenance interval.

Two primary bearing types:

Tapered roller bearings (TRB) for the pinion shaft input and gear wheel output positions, where combined loads and precise axial positioning are required:

• 32220 (d=100, D=180, B=49 mm, C=315 kN, C₀=450 kN) — pinion shaft on 120–200 kW traction gearboxes; mounted face-to-face (O-arrangement) to handle bidirectional axial thrust during acceleration and regenerative braking
• 32226 (d=130, D=230, B=67.75 mm, C=475 kN, C₀=700 kN) — gear wheel shaft on high-speed mainline applications; larger cone angle for higher axial load capacity under regenerative braking conditions

Cylindrical roller bearings (CRB) for positions requiring pure radial load support with axial freedom (floating arrangement):

• NU 2220 ECM/C3 (d=100, D=180, B=46 mm, C=350 kN, C₀=430 kN) — floating end of pinion shaft; ECM = machined brass cage for high-speed operation; C3 clearance for thermal expansion in gear oil
• NU 2226 ECM/C3 (d=130, D=230, B=64 mm, C=530 kN, C₀=670 kN) — floating end of gear wheel shaft on high-capacity gearboxes

Gearbox bearings operate submerged in synthetic gear oil (ISO VG 320 or PAO equivalent) at 60–100°C continuous, with thermal peaks during maximum load cycles. Relubrication interval: gearbox oil change at 300,000–500,000 km depending on OEM specification.

SKF, Timken, NSK, FAG for railway

The four primary suppliers for railway-certified bearings in Vietnam are SKF (Sweden), Timken (USA), NSK (Japan), and FAG/Schaeffler (Germany).

SKF holds the largest installed base on Vietnam Railways' mainline fleet — TAROL axlebox units on passenger coaches and freight wagons sourced through SKF Railway's certified supply chain. SKF provides full lifecycle documentation required by Vietnam Railways' maintenance management system (VMMS), including traceability from heat number to installation date and odometer at replacement.

Timken supplies the AAR-certified freight wagon fleet, with a Vietnam Railway-authorized distribution network in Hanoi and Ho Chi Minh City. Timken's Bearing Damage Analysis (BDA) program provides root cause analysis for in-service failures — used by Vietnam Railways to classify bearing failures (contamination, overload, installation error, EDM pitting) and improve procurement criteria.

NSK is the primary supplier to metro systems using Japanese rolling stock standards — Hanoi Metro Line 2A (Siemens) and HCMC Metro Line 1 (Hitachi) specified NSK for axlebox IBU units. NSK's railway product line includes integrated ABS speed sensors, required by modern metro CBTC signaling systems.

FAG/Schaeffler supplies European-derived rolling stock in Vietnam's expanding metro network and the Chinese-supplied Hanoi Metro Line 1 (Cat Linh–Ha Dong) which uses FAG-equivalent bearings through Schaeffler's Chinese manufacturing operations.

All four brands require purchase through certified railway supply channels — commercial aftermarket versions of the same part numbers (sometimes available through industrial distributors) do not carry the railway certification documentation and cannot be installed on revenue-service rolling stock.

ZVL does not hold UIC 615-4, EN 13260, or AAR railway certification for axlebox IBU units — ZVL is not an approved vendor for mainline railway axlebox or traction motor bearing positions. ZVL bearings are used for auxiliary positions (compressors, cooling fans) where standard ISO industrial certification is sufficient.

Real-world case study: North–South mainline axlebox

Vietnam Railways' maintenance facility in Da Nang operates a wheelset overhaul program for the North–South mainline fleet. During a scheduled intermediate wheelset overhaul at 600,000 km, maintenance engineers found three axlebox units on a Type SE3 passenger coach showing temperature anomalies flagged by HABD stations between Hue and Da Nang over the preceding 80,000 km.

Investigation: Disassembly of the three flagged units revealed progressive contamination ingress through seal lip degradation. Inspection confirmed: seal lip wear on the primary seal, dark (oxidized) grease in the bearing cavity versus the clean pale-yellow factory grease in unflagged units, and light surface pitting on the inner ring raceway consistent with abrasive particle contamination rather than EDM pitting.

Root cause: The coach had operated a high proportion of track kilometers in the Hue–Da Nang coastal corridor, where salt-laden sea air and high ambient humidity accelerate seal aging. The seals in the affected units were at the expected end-of-life for this operating environment; the remaining units on the same coach were at 70–75% of expected seal life and were replaced preventively.

Resolution: All six axlebox units on the affected bogie were replaced with new SKF TAROL units. Installation followed Vietnam Railways Technical Instruction VN-TI-04: hydraulic press mounting to the specified axle journal interference fit (0.04–0.08 mm), torque verification of the end cover bolts, and installation records entered into VMMS with bearing serial numbers and odometer reading.

Outcome: Post-installation HABD readings normalized at all monitoring stations. The coach returned to revenue service within the scheduled 72-hour maintenance window. Maintenance planners adjusted the scheduled overhaul interval for coastal-corridor coaches from 600,000 km to 500,000 km to catch seal degradation before contamination progression.

Key lesson: HABD temperature trend data provides actionable early warning significantly before catastrophic failure — the three-station rising-temperature pattern identified the failing units 80,000 km before the scheduled overhaul. Systematic analysis of HABD trend data, rather than only threshold alarms, reduces unscheduled stops.