When foreign particles find their way into the lubrication gap between the bearing and the shaft journal, there is a great risk of bearing damage. Due to the very low thickness of the lubricating film, even small particles may disrupt operation and cause mixed friction. They may become embedded in the sliding layer and therefore made "harmless". The edges created in the process are flattened during contact with the shaft. Particles which are larger than the thickness of the sliding layer cannot be completely embedded. The protruding part causes wear of the shaft journal in the form of grooves. Strongly pronounced grooves lower the expected service life and may promote bearing seizure. Even during production or reconditioning of an engine, particles can enter the engine block and become stuck. This may be the case, for example, during the sand or glass blasting of an engine block. Dirt particles may also "arise" during operation, however (for example, soot or carbon), or be introduced.
Inadequate maintenance of the lubricating system or extreme external influences also promote the intake of dirt into the lubricant circuit. Damaged adjacent bearings or other damaged engine components can also
introduce particles into the oil circuit. Generally, the danger of damage due to the influence of particles in the main bearing is greater than in the connecting rod bearing. Connecting rod bearings are supplied with
oil from the main bearings via holes in the crankshaft, meaning that the oil first runs through the main bearings (see Fig.). Larger particles are embedded in the main bearing and so generally do not reach the connecting rod bearing.
To gain an insight into where the particles have come from, it may be useful to analyse the bearing and a sample of the oil.
- Mounting was not clean: due to carelessness or inadequate cleaning of the engine components during mounting, dirt may enter the engine block
- Residues such as metal chips or residual blasting agent from production or reconditioning may form deposits in the engine block which then come away during operation – often these are also deposits from attachments, such as the oil cooler, which were not cleaned adequately during the engine reconditioning
- Damage to the seals in the area of the engine: if a seal is excessively stressed or damaged during installation, it no longer fulfils its function and particles can enter the system
- Lack of maintenance of the lubricant system: inspection intervals that have been exceeded or blocked oil filters may cause an increase in dirt in the oil
- Cavitation: particles are broken off of the bearing material and carried on by the oil – depending on the size, these may cause scoring or fine embedding in the bearing or in the adjacent bearing
- Seizure: seized engine components (pistons, bearing shells) cause numerous particles to enter the lubricant circuit, which in turn can cause damage to other components
- Fatigue damage: if material has broken off of engine components, this broken-off material can be further carried by the oil into the bearings and can cause damage here
Generally, despite scoring or embedded particles, bearings can still be used. This depends on the extent of the damage, however. If, for example, there are numerous large indents from particles with initial marks of mixed friction due to material accumulation, it is advisable to replace the bearing. Fine particle indents do not impair the function of the bearing. In both cases, the cause should be clarified in any case:
- Cleaning of all components before mounting: it is important to rinse out all of the oil bores in the shaft and housing before initial start-up. It is also important that the bearing seats are cleaned so that small chips and particles from production and/or reconditioning are removed – the oil channels of attachments, such as oil cooler and turbocharger, must also be thoroughly cleaned
- Check the seals for functionality
- Always replace the oil filter and oil according to the manufacturer's specifications: ensure that the inspection intervals are complied with and only oil and oil filters to the right standard are used
- Filtering of the intake air: service the filters regularly; replace if necessary
- Check other engine components for damage such as cavitation, fatigue or seizure – engine bearing damage due to the influence of particles is often consequential damage
- If the influence of particles cannot be ascertained, the analysis of damaged bearing shells and an oil sample may provide an insight: if particles are still embedded in the bearing or present in the oil, their chemical composition can be determined – if it is a case of material from the crankshaft, for example, a more precise check for damage can be carried out here
- Stroke-like recesses in the sliding direction with material accumulation at the edges
- The areas of material accumulation are partially levelled out again due to wear, bright and light
- Generally in line with scoring or embedded particles in the crankshaft or adjacent bearings
Particles which find their way into the lubricant gap and are not embedded into the bearing material are repeatedly forced through the gap, causing grooves in the process. Depending on the thickness of the edges created, these cannot be flattened during further operation and, due to increased mixed friction, there is an increase in temperature in the event of shaft contact. This often leads to initial rubbing marks and seizure.
The scoring may also be the result of the influence of mixed friction. Here, the grooves are fine and formed on the surface, however, and appear at both interacting sliding parts.
If grooves with pronounced material accumulation are present at the edges, the bearing must be replaced. If grooves are present with areas of material accumulation which have been flattened, and no further influence of particles can be expected, the bearings can still be used.
- Scarred surface
- Particle imprints (still containing particles in places), surrounded by material accumulation which is visible as a light shining point due to wear
- Often in conjunction with scoring in the journal and bearing
- In severe cases, initial rubbing marks from the embedding are visible
Particles which find their way into the lubricant gap may become embedded in the bearing material. Depending on the thickness of the sliding layer, we can differentiate between deep and shallow embedding. In the case of deep embedding, the particles are completely integrated into the sliding layer. This is only possible when the particle is smaller than the thickness of the layer. The material accumulation created during the embedding is flattened during the subsequent contact with the shaft due to wear. Shallow embedding takes place when the size of the particle is greater than the thickness of the layer. The particles are not completely embedded and protrude from the bearing surface. They cause wear and scoring on the journal surface. Due to the material accumulations at the edges or protrusions from particles which are not completely embedded, the formation of the lubricant film is disrupted, and mixed friction conditions may be created. So-called "wire-wool" wear is a possible consequence. Here the embedded particles cut into the surface of the shaft and remove material (chipping wool). The particles that break away, which are then embedded again, accelerate the bearing damage, and often complete failure of the journal and bearing cannot be avoided. Initial rubbing marks and seizure can therefore result from particle embedding.
If large particle embedding in conjunction with initial wear of the journal and bearing is present, the bearing must be replaced. If fine particle embedding is present, the material accumulation of which has been flattened and no further influence of particles can be expected, the function of the bearing is not impaired.
Dirt progression marks
- Individual indents, one after the other, are arranged in tracks. At the end of these, there may still be particles embedded
- They generally run diagonally to the edge of the bearing
- Starting from the oil groove or lubricating holes
- Often in conjunction with scoring in the journal and scoring/particle embedding in the bearing
Very large and hard particles which find their way into the lubricant gap cannot become embedded in the bearing material. They are forced through the lubricating gap, but then always become stuck again. Frequently, the pattern appears from oil grooves or oil bores as the particles were introduced here. Pronounced material accumulation along the progression track causes initial rubbing marks and seizure.
If there are pronounced areas of material accumulation along the progression track, or signs of an initial rubbing mark, the bearing must be replaced. The bearings can still be used, however, if the material accumulations have been flattened and there is no longer the risk of the influence of particles.
Foreign particles on bearing back
- Locally limited deviation of the wear pattern
- Light wear point in the sliding surface
- Often particle residues/imprints on the steel back of the bearing
- In severe cases pronounced marks of mixed friction in the form of initial rubbing marks and signs of material fatigue in the bearing sliding surface are visible
Due to dirt or oil residues (carbon) at the back of the bearing, local pressure points arise, which become noticeable on the bearing sliding surface. Due to the pressure, on the inside of the bearing there is increased wear compared to the remaining areas of the bearing. This can be recognised as a noticeable, mostly bright light deviation to the wear mark. Depending on the extent of the pressure points, initial rubbing marks and seizure, as well as fatigue damage, may be the result.
Whether the bearing can still be used depends on the progress of the wear of the sliding layer. As soon as initial rubbing marks or signs of material fatigue such as cracks or nicks in the area of the pressure
point occur, the bearing should be replaced, otherwise there is the threat of complete failure. Broken-off material can cause consequential damage in the same or an adjacent bearing.