Thermal cracking
Friction heat on a sliding surface cracked the steel. Urgency
The steel could not carry the stress: broken in one event, or cracked progressively.
What thermal cracking looks like
- Fine cracks side by side, crossing the sliding direction, on ring faces, shoulders or raceways
- Straw, brown or blue temper colours and smeared or burnished patches nearby
- In bad cases, a ring split right through along the cracks
Rotec AI schematic illustration. It shows the typical pattern and is not a photograph of a real bearing.
How thermal cracking develops
Gross sliding between loaded surfaces (a ring spinning on its seat, a cage rubbing a shoulder, a stalled rolling element) creates intense local heat followed by rapid cooling. The surface turns hard and brittle and cracks as it expands and contracts.
How fast it progresses: Once cracks form they can run through the ring. Treat any sliding contact as urgent.
Causes of thermal cracking
Lubrication
- No lubricant at the sliding contact
Design & seats
- A loose fit that lets the ring spin on its seat
Operating conditions
- Parts rubbing; rapid speed changes causing skid
How to confirm it is thermal cracking
- Cracks perpendicular to the sliding direction, with heat tint
- Look for what was rubbing: the seat, the cage or an adjacent part
- Check for ring creep marks on the bore or outside surface
How to fix and prevent thermal cracking
- Lubricate sliding surfaces or apply coatings to cut peak temperatures
- Stop the ring turning: correct the fit or the clamping
- Remove the source of rubbing or skid
- Right lubricant, right quantity, right time
Easily confused with
- Adhesive wear (smearing & skidding): Smearing leaves torn, smudged metal; thermal cracks are fine and regular.
- Fatigue fracture: Fatigue cracks grow progressively from a defect; thermal cracks come in rows across a sliding surface.
Other fracture modes
Questions about thermal cracking
What causes thermal cracking in a bearing?
No lubricant at the sliding contact. A loose fit that lets the ring spin on its seat. Parts rubbing; rapid speed changes causing skid.
How do you fix or prevent thermal cracking?
Lubricate sliding surfaces or apply coatings to cut peak temperatures. Stop the ring turning: correct the fit or the clamping. Remove the source of rubbing or skid.
What does thermal cracking look like?
Fine cracks side by side, crossing the sliding direction, on ring faces, shoulders or raceways. Straw, brown or blue temper colours and smeared or burnished patches nearby.