Two-stroke bearings are mainly composed of aluminum or tin-based alloys, allowing the bearing to bed and transfer the load over a broader area. If the bearing surface experiences excessive stress, the bearing shell will normally continue to function as the load is distributed across a greater area. As a result, most white metal fragment detection will simply be an indication of bearing deterioration that requires immediate attention, as material fatigue would result in white metal separation.
Example of main bearing deterioration
When metal particles are detected in the crankcase, it is crucial to pinpoint their source. The most likely source is the white metal bearings, which may be confirmed by examining the magnetic characteristics of the particles using a magnet. White metal is nonmagnetic and appears silvery white. To determine the source of the particles, it is essential to record the shape and size of the particles that are discovered, as well as the place where these particles are discovered (sometimes due to natural trim of the vessel the particles may move during operation).
The LO filters and the mud discharge filter should be inspected to determine if the engine running has dislodged more (thin) particles. It is essential to eliminate any metal particles from the engine’s crankcase and LO filter. The overall area of the bearing surface area that has been destroyed must be determined, and images of the particles showing their size must be taken.
It is necessary to be aware of the different types of bearings installed on the engine, such as thick or thin shell type main bearings.
It is of the utmost importance to always have spares of each type of engine bearing and they must be included on the ship’s vital spares list. Spare bearing shells must be maintained/protected and kept in a manner that allows them to be used when necessary; ensuring that all tools required for bearing replacement are on board.
It is important to know that on MAN engines, if fatigue failure is found on a conventional bearing shell, it is advised to switch to a Blended Edge design.
Fatigue failure is the most typical form of failure that is seen in the form of dislodged white metal particles from the bearing shell and is observed in vessels where the bearings have been subjected to cyclic stress for an extended period of time. Frequently, fatigue cracks result from a high top clearance.
In general, the bearings are examined by measuring the top clearances which is an indicator to detect the state of the bearing (periodic checks are performed without opening the bearing housing), but also serves as a verification of the correct reassembly of the bearing. The clearances for new bearings must fall within the parameters indicated in the maintenance manual.
Failure due to contact damage on crosshead bearings on 2-stroke engines is caused because these bearings are more sensitive than other bearings due to the inability to build a hydrodynamic LO film for lubrication, which is exacerbated by operating at low loads, such as with the turbocharger cut out (TCCO) system for low-speed operation.
At low load operations, the crosshead pin does not display pin lift in the same manner as when the engine is working at a higher speed and this poses a challenge to the hydrodynamic oil film thickness. Reduced oil thickness is the result of an unbalanced relationship between upward and downward forces in the reciprocating system, which causes the crosshead bearing to always rest at the bottom of the lower shell. In a higher-speed engine with all turbochargers operational, the forces of inertia drive the pin to the upper shell once per revolution, resulting in a thicker oil layer. Consequently, contact damage is more prevalent on vessels operating with a light load.
If a bearing develops damage, it swiftly progresses to the edge and finally forms a hole at the edge, allowing white metal to fall into the crankcase below the bearing support.
It is very difficult to know with certainty from where the white metal comes, however crosshead bearing damage is considered more critical than crankpin and main bearing damage.
It is important to note that white metal particles that are less than 0.5 mm or are the same size as the bearing clearance and have smooth surfaces on both sides show that the white metal is being squeezed out of the bearing and this means that the bearing is being overloaded, which should be looked into before the ship departure.
Maximum permissible area of white metal discovered and assessment can be found if you subscribe into the Seafarer’s World Forum (powered by chiefengineerlog.com).
Close visual inspection must be performed on the bearings in the vicinity of the cylinder units where the metal particles are located, giving particular attention to the severely loaded bearing edges with a powerful flashlight in search of any dark spots (mirrors could be used for areas which are difficult to view and access). Listed below are the crucial areas for the various bearings:
- Upper half edge for the bottom end bearing of the connecting rod;
- Lower half edge for the cross head and main bearing;
- Crosshead guide shoes and slippers for marks across the length of the bearing surface.
A wire feeler check could be carried out on the main, cross head and connecting rod bearings to further confirm the finding during visual inspection.
In the form of service letters, engine makers have given inspection instructions and set the conditions that must be met to be able to check the condition of the bearings on an engine (service bulletins SL2012-552 and RT-188).
Any white metal particles found in the crankcase must be confirmed with an open-up inspection to find out where they came from. However, if the number of white metal particles found adds up to more than the area listed in the table found in Seafarer’s World Forum (powered by chiefengineerlog.com), or if the white metal particles have a thickness that looks like they were squeezed out of the bearing shell, the source must be found and confirmed before the engine can be run.
Most of the new engines come with a Main bearing temperature monitoring system and a bearing wear monitoring system. These systems are very reliable and work well to give an early warning before engine parts come into contact with each other and can be used to check the condition of the engine’s bearings.
If there is water in the LO, it could get into the bearings and damage them. This usually happens at the crosshead bearing, where water can cause corrosion at the top layer and then damage the bearing mechanically. The amount of water in the LO should always be less than 0.1%. If the change is more than 0.1 percent, further investigation should be carried out. Engine can’t be run at more than 0.2 % water content there is a specific order and recommendation. If the engine is run with more than 0.2 percent water in it, the crosshead bearing should be checked by opening it up.
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Source and Bibliography:
- MAN Diesel video training
- Wartsila Service Letter RT-188
- MAN Diesel & Turbo Service letter SL2012-552