Status check
While crankshaft bearings are a must during engine overhauls, old bearings should be kept for a closer look at their condition, which can provide a lot of useful information about the overall condition of the engine. On the accompanying illustration examples of typical defects of bearing shells are given.
Typical examples of bearing wear
FATIGUE DEFORMATION
DIRTY INTRODUCTION
EXCESSIVE WEAR
POOR FIT
LACK OF LUBRICANT
NECK TONE
Bearing failure can occur due to lack of lubrication, dirt particles, motor overload and corrosion. Regardless of the nature of the defects, the cause of the damage to the liners must be eliminated during the engine overhaul to avoid recurrence.
For inspection, remove the bearing shells from their beds in the engine block/connecting rod heads and main/connecting rod caps and lay them in order of installation on a clean work surface. The organization of the placement of the liners will make it possible to link the nature of the identified defects to the state of the corresponding shaft journals.
Dirt and foreign particles enter the engine in various ways. They can be left inside the unit during assembly of the unit, or they can get through filters or the crankcase ventilation system. All particles that get into the engine oil eventually, sooner or later, end up in the bearings. Often, metal filings are embedded in the soft material of the liners, which are formed during the normal operation of the internal components of the engine. There is a high probability of the presence of traces of abrasive in the bearings, especially when due attention was not paid to cleaning the block after the completion of the engine reconditioning. Regardless of the way in which foreign particles enter the engine, they are highly likely to be embedded in the soft surface of the crankshaft bearing shells and are easily identified by visual inspection of the latter. Large particles usually do not linger in the liners, but leave noticeable marks in the form of scratches, cavities and scuffs on their surface and the surface of the shaft journals. The best guarantee against this kind of trouble is a responsible attitude to cleaning components after the completion of an engine overhaul and careful attention to cleanliness during assembly. Frequent, regular engine oil changes can also significantly extend bearing life.
Oil starvation can result from several different but often related phenomena. So, overheating of the engine leads to dilution of the engine oil and its displacement from the working clearances of the bearings. Lack of bearing lubrication can be attributed to excessive running clearances as well as normal leaks (internal or external). A common cause of oil being forced out of bearing clearances is constant over-revving of the engine. Violation of the patency of oil flows (usually associated with misalignment of holes when installing components) also leads to a reduction in the supply of lubricant to the bearings. A typical result of oil starvation is the complete or local wiping/pitting of the surface layer of liners from the metal substrate. In this case, the operating temperature can rise to such a level that the substrate acquires a bluish tint as a result of overheating.
The driving style of the car owner also has a significant impact on the service life of the bearings. Driving at low speed in high gear leads to significant overloads of the bearings, accompanied by the displacement of the oil film from their working gaps. This kind of overload leads to an increase in the plasticity of the liners and the appearance of cracks in the surface layer (fatigue strain). In this case, the surface material begins to crumble and separate from the steel substrate. Operation of the car in the urban cycle (frequent trips over short distances) leads to the development of bearing corrosion due to the fact that insufficient heating of the engine leads to condensation and the release of chemically aggressive gases. These products accumulate in engine oil, forming slags and acids. If such oil gets into the bearings, aggressive substances contribute to the development of corrosion of the liners.
Improper installation of liners during engine assembly can also cause their rapid destruction. Too tight fit does not provide the required working clearance of the bearings, which leads to their oil starvation. The result of getting under the liners (during their installation) foreign particles is the formation of elevations, the surface layer from which is quickly wiped off.
Insert selection
In case of wear or damage to the bearing shells, and also when the correct working clearance cannot be achieved, the situation can be corrected in the manner described below, by selecting and installing new shells. Regardless of the method used to determine the required bushing size, the running clearances of the bearings must then be checked using the Plastigage measuring set.
If it is not possible to properly correct the value of the working clearances by selecting the liners, the shaft should be replaced.
Usually, the selection of liners is carried out by specialists who made the groove of the shaft necks. The size of the new liners is determined by the depth of the neck groove. Grooving is usually carried out in increments of 0.25 mm. In some cases, the main and connecting rod journals are machined to different depths. The machined shaft is usually marked accordingly. So, for example, type marking «10-10» usually means both sets of necks are machined to a depth of 0.010 inches (0.25mm) and earbuds of the appropriate size should be selected with reduction. The identification marking on the back side of the bushing is used by specialists when determining the size group of bearings.
Visitor comments