What you need to know about inspection and monitoring of crankshaft bearings

Main, crank pin, and crosshead bearings usually wear out very slowly. Wear rates of up to 0.01 mm/10,000 hours are considered acceptable, but under normal conditions, wear rates will be lower. On the other hand, operating conditions that aren’t normal, like dirty lubricating oil, scratches on the journals, or spark erosion (for more information click HERE), can lead to much faster wear and, eventually, bearing failure (for more information about bearing failure click HERE).

Example of a damaged bearing

If you keep running a machine with a broken bearing that you don’t know about, it may get too hot and damage the shaft and bearing housing. In particular, if the main bearing fails, there may be a lot of work to do to fix it.
Checking the bearings on a regular basis is recommended to reduce the risk of damage to the bearings and any damage that could happen as a result.

The work that goes into taking apart a bearing takes time and costs money. Also, it runs the risk of messing up a working bearing by letting foreign matter in or making a mistake when putting it back together. So, it’s usually best to only open bearings to check them if it’s clear from the outside that it’s necessary.

With regard to external inspection of bearings, the following inspections are recommended to be carried out at regular intervals:

    • Inside engine:
      • Bearing clearance measurements
      • Bearing edge check
      • Inspection of crankcase for bearing metal
      • Crankshaft deflection measurement
    • Lubrication system:
      • Inspection of oil filters
      • Oil analysis as described in the “Operation” section of the instruction book

Close visual inspection must be performed on the bearings, 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.

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.

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. The edge can be checked with a wire feeler that is able to follow the bearing edge against the journal the whole way round on each side. If white metal is missing at the edge, the tip of the wire feeler will enter the hole and the damage is located. In most cases this hole can be seen with naked eye as a dark spot when using a strong flashlight.

Example of main bearing wire feeler check

The aft and foremost bearings are difficult to access on the whole circumference on each side, but it is often possible to bend the wire feeler to suit the situation and thus reach as far round as possible.

It is easy and possible to make a feeler onboard as it can be made from a piece of steel wire (welding rod), approximately 0.6 – 1 m long depending on the engine type, and 2 to 3 mm thick. Approximately 7 to 10 mm of the wire should be bent approximately to 65 ° to form a feeler tip. Grind the tip smooth to obtain the shape and dimension shown on below sketch.

Example of wire feeler fabrication

Note that the thickness of the white metal is approximately 1.5 mm, which is why the tip should be less than 1.2 mm thick and bigger than the maximum top clearance. At the other end of the tip, a handle should be made by bending an eye or similar in the same direction as the tip. These dimensions are as guidelines only and may depend on the engine type as well as individual, personal designs.

For engine crankshaft deflection click HERE to find more information and explanation.

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.

Example of metal particles found inside oil filter

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.

More bearing protection systems are in place nowadays, serving to detect particular conditions of the bearing environment that may cause bearing damage (e.g. Shaft Earthing Device Monitoring and Water in Oil Monitoring).

About Shaft Earthing Device Monitoring you can read in HERE….

Water in Oil Monitoring – In several cases, water in the lubricating oil has resulted in poor bearing performance. The lead-based over-layer used in crosshead bearings is sensitive to water (corrosion), but also main bearings of both the White-Metal and Tin-Aluminum types have been seen to suffer from water contamination.

Recommended inspection intervals for engines can be found in manufacturer’s instruction manuals. For engines without monitoring systems it is recommended to open the crosshead and crank pin bearings for inspection for every three piston overhauls of the cylinder in question because the risk of dirt particles entering the bearings during piston overhaul.

Besides the earlier-mentioned scheduled inspections, some observations of the lubricating oil system can conveniently be made on a more frequent basis, preferably on a daily basis:

      • A visual check of an oil sample for appearance and smell can give early information about deterioration or contamination by particles or water. A detailed description of these checks can be seen in the instruction book.
      • Most lube oil systems have a filter unit where back flushing will be trigged by high differential pressure over the filter. In case of abnormal wear in one of the engine bearings, the back flushing frequency is likely to increase. Therefore, frequent observations of the back flushing frequency are advisable.

It is not recommended to open up bearings for inspections unless this is found justified by the above-mentioned external inspections or by other observations. The only exception is the open-up inspection of crosshead bearings and crank pin bearings for every three piston overhauls on engines not fitted with the monitoring systems recommended.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.

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Source and Bibliography:

  • MAN Diesel video training
  • MAN Diesel & Turbo Service letter SL2012-552
  • You tube video source and credit: Gadget Rishi

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