Chapter 11: Failure Analysis
104
Insufficient lubrication
The bearing surfaces in an engine are not smooth. As
a result of the machining processes to make the engine
parts, there are little peaks and valleys that are only
visible on a microscopic scale. These peaks are called
asperities. As the engine breaks in, the asperities
break off leaving plateaus that become the bearing sur-
face. The valleys become reservoirs for the lubricant.
When an engine is properly lubricated, all of the mov-
ing parts glide on a thin film of oil. If that film breaks
down or carries enough grit to bridge the film, damage
will occur.
1. When the parts are at rest, they push the lubri-
cant or oil away resting on the bearing surfaces.
As the parts rotate, they climb over the oil, pull-
ing the oil between the bearing and the part,
riding on a film of oil.
The asperities are the first thing to make contact
between two moving engine parts with an insuffi-
cient oil film between them. This creates friction
and causes a transfer of metal between the
parts. The heat and friction further breaks down
the oil film, accelerating the process.
2. Insufficient lubrication failures include:
• Low oil level
• Wrong oil for the application
• Contaminated oil
• Degraded oil (heat, age, acids)
3. Metal transfer is the primary indicator that the
film of oil between two engine parts has been
violated.
If the damage is localized, a general failure of
the lubrication system is probably not the cause.
As an example: a piston skirt shows metal trans-
fer to the cylinder wall. The connecting rod and
wristpin show some signs of excessive heat. The
main bearings and camshaft are not damaged.
This would indicate that the problem was proba-
bly related to cylinder temperature.
The hall mark of a lubrication failure is the pres-
ence of discoloration and/or metal transfer on all
friction surfaces within the engine.
See Figure 11.8.
An important thing to note is that just because there are
signs of insufficient lubrication, that does not mean that
was the cause of the failure. It may only be a symptom
of the real cause of the failure.
Larger size abrasive particles can render the lubricants
ineffective, leading to an engine failure. An overheated
engine can cause the oil to break down leading to a
failure. In an engine overspeed, the oil is pushed away
from the bearing surface leading to a failure.
In all three of the above cases, the signs of insufficient
lubrication are symptoms not the cause. There will also
be signs of heat or discoloration around the parts
affected by the lack of lubrication. See Figure 11.9.
Figure 11.8
Metal to metal
transfer
Discoloration
Figure 11.9
Discoloration
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