Put simply, the importance of proper lubrication being available to the crankshaft in any engine under all operating conditions cannot be overstated. However, in practical terms, there is a big difference between "proper lubrication", and a "sufficient quantity" of oil being delivered to the crankshaft- even if the oil pressure is within OEM specified values. Let us explore this difference in some detail, beginning with explaining the damage to the crankshaft in the above image-
Although the crankshaft in this image is from a heavy truck engine, the same type of damage can, and does occur on crankshafts in all types of engines, so what exactly does this image show?
In this example, the damaged journal supported two connecting rods that were attached to pistons in opposing banks of cylinders, but note the holes in the journal. In all engine designs, these holes are present in all bearing journals and are the terminations of a system of oil galleries and passages that pass through the length of the crankshaft. Here is how this systems works-
When pressurised engine oil leaves the oil pump, it moves through a system of galleries and passages in the engine block that terminate in the part of the engine block that supports the crankshaft.
As the crankshaft rotates, the holes in the crankshaft journals briefly correspond with the holes in the engine block, thus allowing pulses of pressurised oil to flow through the crankshaft to maintain the lubricating film between all the bearing journals and bearing inserts. In engines that are in good mechanical condition, the pulses of pressurised engine oil are the only things that prevent direct metal-to-metal contact between the crankshaft and the bearing inserts.
Moreover, if the engine is running on clean oil that is of the correct type, grade, and formulation, the pulses of lubricating oil will provide the crankshaft with adequate lubrication under all engine speeds and loads, which begs the question of-
What caused the damage to this crankshaft?
In this example, the oil supply to the damaged bearing journal failed, which caused the protective lubricating film between the journal and bearing inserts to collapse. This type of failure is typically caused by excessive amounts of sludge in the engine that eventually build up in the oil galleries in the engine block until the free flow of oil to one or all, bearing journals becomes restricted, or is cut off completely.
In this particular example, the collapse of the protective lubricating film caused the bearing journals and the bearing inserts on the damaged journal to come into direct metal-to-metal contact. Thus, since there was no lubrication present, the resulting heat caused by excessive friction welded the two metal surfaces together almost immediately after the oil supply failed. However, since the crankshaft’s rotation was maintained by the combustion processes in the cylinders, the first weld was broken loose and reestablished repeatedly during the crankshaft’s rotation.
In practice, every time the excessive heat welded the journal and bearing journal together, progressively more material was transferred from the journal to the bearing insert and from the bearing insert to the bearing journal. Eventually, the continued transfer of material between the two surfaces and the resultant mixing of the two different metals generated sufficient friction to prevent the crankshaft from rotating freely. Note though that in this case, the bearing failure caused such loud knocking noises during each engine revolution that the engine was taken out of service before the crankshaft could fail catastrophically.
So while the crankshaft in the above image is likely repairable, it often happens that bearing journals and bearing inserts become welded together so firmly that the crankshaft’s rotation is halted abruptly, which causes the crankshaft to break, as shown in the image below-
This type of catastrophic failure can occur in both large and small engines, but note that this type of failure also often causes extensive damage to the engine when one or more connecting rods break as a result of the crankshaft failure. In these cases, the broken (and unguided) connecting rods can, and often do, break through the walls of the engine block, which effectively destroys the engine block. Moreover, pistons that were connected to broken connecting rods can slam into the cylinder head with enormous force, which not only destroys the valves in affected cylinders but often also the cylinder head itself.
It is also important to note that even if the oil in the engine is clean and of the correct type, grade, and formulation, too little oil in the engine, an insufficient supply of oil to the oil pump, and/or too much oil in the engine can cause all of the damage described above. Let us look at these scenarios in turn, starting with-
Low oil level
In the case of an insufficient supply of oil, the oil pump cannot deliver enough oil to the lubrication system to provide all the sliding and/or rotating parts with adequate lubrication. However, since the crankshaft is first in line (so to speak) to receive pressurised oil, the crankshaft is usually the first part to be affected, or to fail, with other rotating parts, such as the camshafts, failing almost directly after.
Note, though, that a low oil level will usually affect all the bearing journals, albeit not always equally. Whether (or not, as the case may be) all, or only some bearing journals are affected depends on factors like-
- the amount of mechanical wear on bearing journals and bearing inserts
- the volume of oil the engine needs for the oil level to be “up to the mark” on the dipstick
- the amount of time the engine had been running with a low oil level
- the quality of the engine oil, as opposed to its (claimed) grade, type, and/or formulation whether the engine had been rebuilt
- the quality of the replacement bearing inserts, and the physical dimensions of a repaired or reconditioned crankshaft
- the quality of the workmanship during the assembly of a rebuilt engine
It is important to note though, that the phrase “low oil level” means different things to both different people and different engines. Put differently, this means that while many engines, including expertly rebuilt engines, will operate reliably with marginally low oil levels, marginally low oil levels may cause the oil in other engines to overheat.
Although excessively hot oil may not always cause immediate crankshaft failures, excessively hot engine oil loses its lubricating properties fairly rapidly, meaning that even though the engine oil level is low, but still above the minimum allowable level, the reduced, but degraded engine oil can, and often does cause catastrophic crankshaft failures.
An insufficient supply of oil to the oil pump
While this can happen for several reasons, the most common cause of this condition is a restricted or clogged strainer/screen on the oil pump’s pick-up tube.
In practice, clogged oil pick-up strainer screens are most often caused by a build-up of sludge on the screen that forms when-
- the engine oil is not replaced at recommended or specified intervals
- engine oil of different brands, types, grades, and formulations are used instead of only the engine oil that is recommended or specified by the vehicle's manufacturer.
It should be noted that many engine oil brands, types, grades, and formulations are NOT compatible. Thus, using whatever engine oil is at hand, as opposed to what is required during oil top-offs or oil changes, often produces oil mixtures that can have-
- reduced, or often, almost no lubricating properties
- or oil mixtures that produce a type of jelly-like substance that the oil pump cannot pick up
Regardless of the actual cause of an insufficient oil supply to the oil pump though, this condition almost always results in crankshaft damage and failure, albeit not always catastrophically if the condition is detected early enough.
High oil levels
All engines are designed so that the crankshaft does not come into contact with the oil in the sump when the oil level is "up to the mark" on the dipstick, or even when the oil level is marginally above the maximum allowable level as indicated on the dipstick.
The reason for this is simple; if the crankshaft were allowed to come into contact with the oil in the sump, the rapidly rotating counterweights on the crankshaft will whip the oil into foam, much like an eggbeater whips air into eggs to produce foam. More to the point though, since all rotating oil pumps create a vacuum to “suck” oil out of the sump, and the engine oil is now a foam that consists of more air than oil, this vacuum collapses, which means that the oil pump effectively stops working.
As a result, the crankshaft is starved of lubricating oil, which inevitably causes severe damage to not only the crankshaft but to other rotating and/or sliding parts, as well. It should be noted though, that all engines are designed to have an oil level safety margin, which means that overfilling an engine with up to about 250ml of oil will usually not bring the crankshaft into contact with the oil in the sump.
Nonetheless, to prevent issues, it is always best to fill an engine with oil only up to the maximum allowable oil level indicated on the dipstick.