Bearing lubrication is critical from the second the engine turns over until it is shut down. If not engine bearing failure damage will result costing you major repairs or possibly a new engine.
All engine bearings depend on a film of oil to keep shafts and bearing surfaces separated. At rest, the shaft and bearing are always in contact. On start up the shaft contacts the bearing briefly. Running, the shaft pulls oil from the clearance space into the wedge shaped area between the shaft and bearing.
This oil wedge lifts the shaft off the bearing surface and supports it during engine operations. Under normal operating conditions and a continuous supply of clean oil, the shaft and bearing surfaces will remain separated.
Proper selection of bearings will go a long way toward a successful engine rebuild. It is a good idea to inspect your old bearings before starting any new rebuild or repair. They do tell a story and will help find the problem before just replacing parts.
Identifying the root cause of a bearing failure is crucial in preventing the recurrence of the failure, as a simple replacement of the bearings typically will not address the factors that led up to the failure in the first place. It is important to note that in many cases the premature bearing failure is due to a combination of several of the causes. If you do not find the original cause engine bearing failure damage will repeat itself.
The following notes and illustrations will assist you in the diagnosis and causes of bearing failures:
Inadequate lubrication will cause Engine bearing failure damage
This includes all failures resulting from metal-to-metal contact. So as already discussed, causes include unsuitable oil, insufficient oil, inadequate clearance, etc. Unless seizure has occurred, the bearing surface will have an abnormally shiny appearance.
The presence of dirt in the bearing surface indicates one of two things, dirty assembly or inefficient filtration.
The softer the bearing alloy, the more tolerant it is,
- Babbitt is particularly good.
- Copper-lead is very bad.
- Aluminium-tin falls somewhere between these two.
Bearings which fail for this reason often display scraping or tracking marks.
Acidity of the lubricating oil can result in this type of failure. Copper-lead bearings are the most likely to suffer in this way, the acid attacking the lead content of the alloy. This weakens the structure, causing it to collapse under load. Bear in mind, however, that high temperatures can result in a similar decomposition of the lead. Other factors that can contribute to acid buildup include a restricted PCV system, engine operation during extremely cold or hot weather, excessive crankcase blowby (worn rings or cylinders) or using poor quality oil or fuel.
As specific power outputs rise, so do crankshaft speeds and inertia loads. This imposes severe stresses on main and big end.
bearings (the latter especially). As a result the bearing material is not equal to the task, it will flex and crack. As these cracks develop, large areas of the bearing surface tend to break away.
Engine bearing failure damage can occur as a result of faulty machining or careless assembly.
Sometimes, failures are the result of simple installation errors. For example, if a bearing half without an oil hole is improperly put into a position where the hole is needed, that journal will receive no lubrication. If a connecting rod or main bearing cap is installed in the wrong position, or a bearing isn’t set into place securely, lubrication will be insufficient and cause failure.
Dirt between bearing and housing
This is an assembly fault. The bearing will not seat properly if dirt is trapped between the bearing and housing bore. As a result this will impair heat dissipation and may adversely affect bearing life.
Furthermore, the bearing is likely to be deformed, leading to localized areas of abnormal wear.
Misaligned Bearing Cap
Reversing or interchanging bearing caps may result in this situation, causing excessive wear at one end of the cap and the opposite end of the housing. Sometimes, failures are the result of simple installation errors. If a bearing half without an oil hole is improperly put into a position where the hole is needed. Consequently that journal will receive no lubrication.
The term “crush” refers to the outward force created by the portion of the bearing that extends above the housing bore when the bearing halves are set into place. This “extra” material holds the outside diameter of the bearings firmly against the housing bore when the assembly is torqued to specification.
To ensure that the bearing shells are held firmly in place, it is usual for the ends to protrude slightly beyond the parting faces.
When the cap is assembled, the ends butt together to provide the desired degree of interference. This is the term known as bearing crush. If crush is excessive, the ends of the shells bulge inwards.
When clearance exists between the ends of the two shells, they are free to move within the housing. Lack of radial pressure impairs heat dissipation, thus shortening bearing life. A highly polished appearance on the back of the bearing can identify this condition.
You can use the following method to check bearing crush. One bolt (or nut) is torqued to a figure of 10 lb.ft. and the gap between the cap and housing checked at the opposite end. It should be possible to insert a 0.0015in feeler gauge at this point. Verify that the ends of the shells are in firm contact.
Misaligned connecting rod
Although a connecting rod is unlikely to bend in service, handling damage is by no means unknown. Localized wear will take place on opposite sides of upper and lower shells. There will also be evidence of piston misalignment. Even when a bearing fails, it’s usually not the bearing’s fault. With a bit of investigation, an engine specialist or technician can discover and eliminate one of literally dozens of reasons for premature wear or failure.
In addition main bearing bores out of alignment. Engine overheating. Improper tightening of engine components (bearing caps, heads, manifolds, etc.) Engine not properly or uniformly supported (large stationary engines). This will probably result in misalignment of the main bearings, with a consequent reduction in oil-film thickness. In extreme cases, this can lead to metal-to-metal contact. In addition a bent crankshaft will have the same effect.
Normal wear usually results in ovality, together with a little taper. If a connecting rod is bent, the crank pin may be worn into a barrel form. Incorrect grinding may cause an hourglass effect.
Other causes of Engine bearing failure damage,
– Excessive idling can result in an oil film that can not support the load needed.
– Engine lugging can distort the crankcase and/or crankshaft, affecting the connecting rod and/or main bearings.
– Hot rodding, or forcing excessive loads, can similarly affect the bearings.