Reading the bearings

Reading the Bearings

Aircraft Engine Bearing Analysis

by John Schwanerwww.mechanicsupport.com

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.

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.

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.

Bearing moving out of bore due to insufficient clamping forces. 

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.

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.

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!

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. 

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.

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. 

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.

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.

Bearing bits found in sump.

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

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