Railway axle box bearings: arrangement, lubrication and failures
An axlebox bearing carries the weight of the vehicle, the forces of the curve and the shocks from rail joints, for millions of kilometres. A failure here is a safety issue, so the bearings arrive as sealed, pre-greased units that are fitted as a whole.
Typical duty
| Speed | Load | Temperature | Typical life |
|---|---|---|---|
| Up to about 1,700 rpm (300 km/h) | Heavy radial plus axial from curves; impacts from wheel flats | Watched by trackside detectors | Millions of km between overhauls |
How the bearings are arranged
Compact tapered bearing unit (CTBU) · outside axle journal
Used when: Freight wagons, locomotives and metro vehicles
Two rows of tapered rollers sit in one sealed cartridge on the end of the axle. The cartridge locates the axle in both directions, so there is no adjustment during fitting. Seals on both ends keep the factory grease in and water out.
Common failures and how to prevent them
| Failure | What you see | Why it happens | Prevention |
|---|---|---|---|
| Hot box Overheating & lubricant breakdown | High housing temperature detected by trackside sensors, burnt grease. | Seal damage letting water in, loss of grease, or a damaged bearing. | Replace seals at overhaul, use sealed pre-greased units, and monitor with trackside detectors. |
| Impact damage from wheel flats Overload deformation (true brinelling) | Dents on the raceways at roller spacing, noise. | Flat spots on wheels or rail joints produce high shock loads. | Re-profile wheels with flats and inspect bearings after severe impact events. |
| Fretting between rings and backing ring Fretting corrosion | Corrosion marks at the ring faces. | Micro-motion under changing loads. | Use the polymer spacer or coated parts specified by the bearing maker. |
| Water in the grease Moisture corrosion | Rusty or pitted raceways, emulsified grease. | Worn seals or cleaning with high-pressure jets. | Replace seals on schedule and control pressure washing near axleboxes. |
| Fatigue spalling Subsurface-initiated fatigue | Spall on the loaded zone of the raceway. | High axle loads over a long mileage or overloaded wagons. | Stay within axle load limits and renew at the overhaul interval. |
| Current from traction or welding Excessive current erosion | Pitted raceways with burnt-looking craters. | Traction return current or welding current passing through the bearing. | Make sure return paths are via earthing brushes and not via the bearings. |
Lubrication
Greased for life
Units are packed with grease at the factory and sealed. They are not relubricated in service and are replaced as units at overhaul.
Overhaul
At overhaul, the unit is dismounted, cleaned, inspected or replaced and refilled according to the rail operator’s procedure.
Sealing
- LL (lip-labyrinth) seal: A labyrinth with a light lip. Much lower friction than a garter seal, so lower temperature and longer grease life.
- Labyrinth seal: No contact at all. Zero friction, preferred for high-speed and very high-speed vehicles.
- Garter seal: The traditional freight car seal. Higher friction but low cost.
What to watch
- Trackside hot-box detection and acoustic detection.
- Housing temperature at inspections.
- Condition on arrival at the depot, such as grease leak or noise.
- Records of wheel flats and axle load events.
Questions
Which bearings are used in railway axleboxes?
Compact tapered bearing unit (CTBU) · outside axle journal: Freight wagons, locomotives and metro vehicles.
What are the common bearing failures in railway axleboxes?
Hot box: Seal damage letting water in, loss of grease, or a damaged bearing. Impact damage from wheel flats: Flat spots on wheels or rail joints produce high shock loads. Fretting between rings and backing ring: Micro-motion under changing loads.
How should railway axleboxes bearings be lubricated?
Greased for life: Units are packed with grease at the factory and sealed. They are not relubricated in service and are replaced as units at overhaul. Overhaul: At overhaul, the unit is dismounted, cleaned, inspected or replaced and refilled according to the rail operator’s procedure.