since 1940

Bearing Wear  

When overhauling a Lycoming or Continental aircraft engine the wear patterns on old parts can be informative. The samples below illustrate how main bearing wear patterns can tell whether the crankcase and crankshaft are in alignment. It is much easier to inspect the bearings for crankcase alignment while the bearings are still in the case so that each bearing is viewed in its proper position.

Normal	Abnormal
Normal Lycoming main bearing wear. Symmetrical pattern - side to side and end to end. Crankshaft is running true in bearing. I call such a pattern a "hour-glass" pattern.	Fatigue failed Lycoming 320 main bearing. Contact pressure from the crankshaft seems to be more concentrated and concentrated at the lower edge.

Abnormal bearings

	Main bearing wear. Contact occurs only on upper half of bearing. Crankshaft is not running true in bearing. Either crankshaft is bent or crankcase journals are mis-aligned
	Abnormal, notice piece missing above oil hole.  Notice also how the wear markings (witness marks) are displaced from the center of the bearing to the left in this photo. During normal engine operation a hydrodynamic oil film forms between the journal and the bearing which displaces the load-line and the point of minimum oil film thickness in the direction of rotation. Any damage, such as particles, is also displaced in the direction of rotation.  Damage or wiping at the center has to form before a hydrodynamic film is formed - an indicator of a dry start!
	Breakdown of oil film Severe abnormal main bearing wear located at right side of bearing and the bottom edge. The pattern on the right is called "wiping"
early sign of bearing failure may be drop off of idle oil pressure or momentary loss of propeller control	Bearing material flaking off due to fatigue failure of bearing surface from high loads and high temperature. When missing material reaches edge of the bearing,  oil pressure (formed by the pumping action of the journal in the bearing bore) no longer forms between the bearing and the journal. Since oil is the medium used to separate the rotating shaft from the bearing,  without oil pressure (which can be as high as 6,000-8000 psi), the journal rotating on the dry bearing melts and destroys the bearing. Increased clearance caused by the melted bearing material results in the rod bolts pounding and eventually failing. Fatigue starts at the load zone which is usually in the center. Tri-metal bearings (steel-copper-tin/lead alloy) or older "silver" bearings have several properties that enhance their fatigue strength over aluminum-tin bearings. Higher capacity of conformity. In other words the tin/lead surface is more capable of conforming to localized overloading. High load capacity Better lubrication properties
	Close up of main bearing in Lycoming engine Misalignment between crankcase and crankshaft. Notice the no contact area on the right and wear contact on the left. The small lines on the left are spider-web fatigue cracks that are caused by bearing overload. Eventually, small chunks of bearing material crack off the substrate.

	Remains of connecting rod bearing shell after oil system interruption
	Lycoming connecting rod bearing failure. Babbitt delimitation out to the edge of the bearing releases the oil pressure and causes journal to bearing contact. The rod journal acts as an oil pump creating a wedge of oil that lifts the journal so that it rides on a thin oil film. This oil film can't form if there is a passageway for the oil to flow out the side of the bearing. The question is why did the bearing fail?
	Sometimes the bearing is the victim. This is the connecting rod  bore that the above bearing was installed in. Notice the light and dark areas. These are high and low spots where the bearing back was making contact (light polished areas) and not making contact (dark areas). Notice that the high spot in the green square is in the same area that the Babbitt delaminated in the upper photo. The bearing forms  itself to the bore its installed in. The high spots in the bore became high spots in the bearing. Bearing distortion caused overloading, fatigue failure and eventual delimitation from the high contact pressures. Its always a good idea to inspect bearings, bearing backs, and bearing bores on any engine that you are taking apart. Did the parts "like" each other. If not, as in this example, then the connecting rod is going to require replacement or machining. Just installing a new "good" bearing will create another bearing failure and another victim

Pieces of main bearing material found in sump. These pieces are often too large to get past the suction screen and into the oil filter and too large to be detected using oil analysis.

Pilot perspective - how do you detect this type of damage?

Bits of bearing material may end up settling in the sump and may not be present in oil samples or the oil filter. In the engine above, small bits of bearing material were found in the sump. I'm trying to get a photograph of the material, so that if you ever see any in the oil you will know where it came from.

The crankshaft journal is held off  the bearing by the hydraulic pressure created by the rotation of the crankshaft journal acting as a pump (elastohydrodynamic lubricating regime) creating a wedge of oil. The bad thing about the bearing above is that the missing material extends to the edge of the bearing thereby providing a exit path for oil. Without an oil film the crankshaft journal contacts the bearing material; friction forces melt the material leaving just the bearing shell; the increased journal clearance allows the crankshaft to flex and eventually breaks in cyclic fatigue. Pilot indication is a fluctuating oil pressure gauge.

Failure of connecting rod bearings is slightly different as one rod bolt breaks and the rod cap peals off the journal. Little pilot indication before failure.

Mechanic perspective - how do you prevent this type of damage?

There are probably three possible causes or combination of cause:

1. defective bearing
2. overload
3. alignment problem
4. improper crankshaft surface finish. Thin oil films require a smooth surface.

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