inspection

1. Reading the Bearings
Aircraft Engine Bearing Analysis
by John Schwaner
www.mechanicsupport.com

2. Quick visual inspection will tell you if the crankcase and
crankshaft like each other:
• Crankcase alignment
• Crankshaft Alignment
• Crankcase Journal Clearances
• Connecting Rod Alignment
If the bearings show normal contact signatures then
the crankshaft, crankcase, and bearings are all working
as designed. If not... then there is a problem.

3. Step 1 - Turn the bearings over and examine the backside.
Polishing = bearing moving
in bore
Shiny back means that the
bearing was loose in the
bore. Red arrow shows
original surface - this is
what the backside should
look like.

4. Bearing back polishing tells you that:
• The bearing boss is too large, or
• Insufficient crankcase clamping force - inspect for fretting
on journal supports.
Fretting on journal support surface
shown by rough surface (no
machining marks, and dark fretting
corrosion residue. This bearing has
shifted to the left.

5. Bearing moving out of bore due to insufficient clamping
forces.

6. This is what can happen if the bearing starts moving around.
Possible symptoms include erratic oil pressure and excessive oil
thrown out engine breather. If you have an air-oil separator
then you might not notice impending bearing failure.

7. Step 2 - Front-side Analysis
Normal, symmetrical pattern side to side and end to end.
Crankshaft is running true in bearing. I call this an
"hour-glass" pattern.

8. Poor Alignment and beginning of bearing failure. Small wiggly lines on left
side (spider or hens tracks) are small cracks from too much loading. White
area on right shows no contact from rod journal. Poor rod alignment or
bearing boss alignment - something is bent!

9. Minutes to engine destruction
Contact pressure from the journal is more concentrated at the lower
edge leading to delamination. This releases oil pressure from the
journal.

10. The pattern on the right is a type of wear called 'wiping"
where the oil film is breached resulting in journal to bearing
contact. Caused by mis-alignment between crankcase and
crankshaft journal or dry engine starts. Notice pattern is
heavier on the lower side indicating poor alignment.

11. Bearings are victims. Light and dark areas on connecting rod show
high and low spots where the bearing back was making and not
making contact. The bearing forms itself into the bore. The high
spots in the bore become high spots in the bearing. Bearing
distortion causes overloading, fatigue failure and eventual
delamination from the high contact pressures.

12. Connecting rod bearing delamination (fatigue failure). This will go unnoticed
until it breaks-out at the bearing edge. Then oil pressure is lost and the rod
bolt breaks. You cannot detect impending fatigue failure - safety thru life
limits (TBO) and robust construction.

13. Delamination and fatigue failure of bearing babbit. Very
dangerous as the delamination has reached the edge where it
releases oil pressure. Pilot has 10 seconds of wobbly oil
pressure and then the rod cap breaks off.

16. Bearing bits found in sump.

17. Bearings act as filters
trapping and embedding any
particles. Bits of aluminum in
this picture.

18. Tiger Strips
High and low spots in bearing journal caused by line boring
crankcase with cutter driven by gearing rather than belt.
Gears transmit vibration - belts absorb vibration