Bearings and lubrication
Axle box bearings are the rolling element bearings that carry a European rail vehicle’s static and dynamic load onto the wheelset, permitting rotation under radial and axial loads that may exceed 100 kN per bearing.
Two bearing types dominate railway axle box applications, one of the safety-critical rolling stock components with the tightest failure tolerances: tapered roller and cylindrical roller bearings. Tapered roller bearings are supplied as sealed, pre-greased cartridge units as standard; cylindrical roller bearings are widely fitted as separate bearings, greased within the axle box housing itself.
Bearing internal clearance and pre-load are set at manufacture; the axle box design controls the fit dimensions that determine in-service load distribution.
Bearing types
Tapered roller bearings manage combined radial and axial loads in a single unit and are common in freight applications, where axial loading from buffer impact and curve negotiation is significant.
Cylindrical roller bearings carry high radial loads and are used in high-speed passenger applications, typically combined with a separate axial bearing.
Failure modes
The principal bearing failure modes in railway service are spalling — fatigue-initiated flaking of the raceway surface — fretting corrosion between the bearing outer ring and the axle box housing, and electrical erosion from stray traction return currents passing through the bearing.
Electrical erosion produces a characteristic fluted or frosted raceway surface and can progress rapidly to catastrophic failure. Standard practice insulates the non-drive-end bearing to break the current path on smaller motors, and insulates both ends on larger motors for the highest level of protection; shaft current diversion brushes are used as a further countermeasure on motorised axles.
Failure classification follows ISO 15243, the general damage terminology standard for rolling bearings. The railway-specific framework sits in EN 12080, EN 12081 and EN 12082 for axle box bearings, and in EN 15437 for the condition-monitoring interface between trackside detectors and rolling stock.
Detection and monitoring
Hot axle box detectors (HABD), installed at intervals along mainline routes, measure the surface temperature of the axle box housing by infrared sensing as trains pass — not the bearing itself, which is why detection lags actual bearing condition. Infrastructure managers work with graduated threshold levels; only the highest level triggers an immediate stop-and-inspect alarm, with lower levels generating a warning for follow-up.
HABD gives network-wide coverage, but it detects only late-stage thermal failure.
Acoustic bearing defect detection systems, deployed at trackside or onboard, analyse the frequency spectrum of bearing noise. Raceway spalling produces vibration at bearing defect frequencies above shaft rotational speed, while cage-related defects show up as a distinct sub-synchronous frequency; spectral analysis can flag these patterns before a HABD threshold would be reached.
Fleet management systems use the detection data to schedule bearing replacement ahead of thermal alarm level.
Lubrication
Sealed axle box bearings use lithium-complex or polyurea base greases formulated for the railway temperature range (−40°C to +130°C) and the long service intervals typical of rail applications. No field regreasing is required, or possible, on these sealed cartridge units during normal service intervals.
Gear units in traction drive systems run on synthetic extreme pressure (EP) gear oil, with drain-and-fill intervals running into hundreds of thousands of kilometres depending on manufacturer specification and duty cycle.
Wheel flange and rail lubrication reduces flange wear and lateral rail forces. It is applied either by wayside applicators at curves or by onboard flange lubricators triggered by curve entry signals.