Fatigue fracture
A crack grew a little with every revolution until the ring let go. Urgency
The steel could not carry the stress: broken in one event, or cracked progressively.
What fatigue fracture looks like
- A crack running across a ring, often starting at a spall, a stress raiser or a poorly supported area
- Progressive "beach" ridges on the fracture face when it is opened up
- Typically follows heavy spalling or fretting plus a poor seat
Rotec AI schematic illustration. It shows the typical pattern and is not a photograph of a real bearing.
How fatigue fracture develops
Repeated bending or hoop stress (from uneven support, a distorted housing, a tight fit or heavy spalling) opens a crack bit by bit. It can start at a surface defect or a hard particle trapped under the ring. When the remaining steel cannot carry the load, the ring breaks.
How fast it progresses: The final break is sudden, but the crack has been growing for a long time. Machines running on after severe spalling are the classic case.
Causes of fatigue fracture
Design & seats
- Seats that do not match the specification; split-housing halves that do not join true; uneven support
Mounting, handling & storage
- Swarf or chips trapped under the ring
Operating conditions
- Running on after severe spalling; heavy repeated overloading
How to confirm it is fatigue fracture
- Check the housing seat for chips, shims and flatness
- Look for fretting marks that point at a poor seat
- Examine the fracture face for beach marks, ideally under magnification
How to fix and prevent fatigue fracture
- Make seats conform to the drawing and keep them clean
- Check split housings fit concentrically
- Use correct tools and mounting methods
- Act on spalling warnings before the damage progresses
Easily confused with
- Forced fracture: Forced fractures look abrupt with no progression marks.
Worth knowing
Reading a cracked ring
An inner ring cracked along its length with slightly rounded fracture edges cracked in operation and cycled. Sharp edges mean it cracked when it was dismounted. Causes include slippage of the ring on the shaft, poor lubrication, a fit that is too tight, a grooved shaft, an out-of-round seat or grazing against nearby parts. A cracked outer ring, often a little past the middle of the raceway under axial load, points to poor support in the housing.
Other fracture modes
Questions about fatigue fracture
What causes fatigue fracture in a bearing?
Seats that do not match the specification; split-housing halves that do not join true; uneven support. Swarf or chips trapped under the ring. Running on after severe spalling; heavy repeated overloading.
How do you fix or prevent fatigue fracture?
Make seats conform to the drawing and keep them clean. Check split housings fit concentrically. Use correct tools and mounting methods.
What does fatigue fracture look like?
A crack running across a ring, often starting at a spall, a stress raiser or a poorly supported area. Progressive "beach" ridges on the fracture face when it is opened up.