Answer MEP Question 22

Q. A. Describe the procedure for opening a bottom end bearing for inspection making reference to the positioning of the crank and the safety precautions to be observed. 

B. State how the bearing clearance may be checked and adjusted when necessary 

C. State TWO defects, which may be encountered during inspection of the bottom end bearing and crankpin giving possible causes of EACH. 

D. State TWO checks, which should be made before returning the engine to service following an overhaul of the bottom end bearing.

The answer here is a precise form of detailed article on Bottom end bearing Inspection. 

Continuous survey of machinery (CSM)

In accordance with defined procedures by the classification society, ship operators can arrange for the examination of surveyable machinery items by the Chief Engineer on board, with a view that the results of examination may be used to credit the machinery item(s) towards the survey. Chief Engineers may carry out examinations of selected machinery items on a ship operating under a Continuous Survey Machinery (CSM) cycle, however, the applicable procedures will depend on whether or not the ship is operating an approved Machinery Planned Maintenance Scheme, (MPMS). The procedures are outlined by the class the vessel is registered to. For ships not operating an approved Machinery Planned Maintenance Scheme, following the examination by the Chief Engineer a statement; recording the results of the examination is to be prepared and submitted to the Surveyor attending for confirmatory surveys. This statement is to be reported using the template Chief Engineer's Statement of Examination of Surveyable Machinery Items.

General Precaution:

Survey to be planned when a sufficient time is available to carry out the same. 

Immobilization and permit to work are to be obtained from port authorities. A risk assessment is to be carried out. All involved persons must be familiar with the procedure. 

Read manufacturers instruction manual for the correct procedures and clearances. 

Keep all special tools, hydraulic jacks, lifting gear ready after an examination and are within the certification. 

Turn the engine for at least two revolutions. 

Safety precautions: 

Stopped engine 

Shut off starting air supply – At starting air receiver 

Block the main starting valve 

Shut off starting air distributor/distributing system supply 

Shut off the control air supply 

Engage turning gear 

Stop lubricating oil supply

A. Procedure for opening a bottom end bearing for inspection

Remove the Crankcase doors, and turn the crank of the unit to TDC.

Lockings to be removed, and the nuts slackened by hydraulic jacks.

Suspend the chain blocks, as shown on each side. With wire slings, shackles and eye-bolts, lower the bottom half into the Crankcase.

Lift out the bottom end bearing outside the crankcase, by appropriate shackles and chain blocks.

Fit the Crosshead holding pin device to the Guide as shown in second figure. Turn the engine till the crosshead rests on the holding pins.

Support the connecting rod adequately, and continue turning the engine, till the crankpin is clear of the upper half, in order to have sufficient space for the inspection..

B. State how the bearing clearance may be checked and adjusted when necessary

To check the bottom end bearing clearance - turn the engine to BDC, Isolate the turning gear, climb down into the crank pin and measure the clearance using feeler gauge.

The clearance is measured on the bottom half of the bearing because the mass of the running gear acting downwards means the top half of the bearing is sitting on the journal. On a large engine 960mm bore this clearance can be 0.8mm.

With modern shell bearings, the clearances are manufactured into the shells.

When the clearances have reached maximum as laid down in the instruction manual the bearing is changed.

Thick wall shell bearings fitted on some engines have the clearances adjusted by fitting shims between the bearing halves. The sims are of equal thickness on both sides of the bearing housing.

C. Possible defects

Scoring: It is caused by foreign bodies in the lube oil. Due to bad filtration or because of tin oxide from Corrosion in the bearing from (salt) water contamination that affects bearing and journal. 

Pitting: It is usually caused by acidic attack or corrosion. Although spark erosion can be a cause. Acidity in lube oil can be due to oxidization, bacterial attack or because the oil has lost its alkalinity reserve. Affects bearing and journal. 

Wiping: It Can be partial or complete. It is caused by overheating of the bearing material causing it to melt. This happens due to the breakdown of hydrodynamic film it can be because of overloading, Water contamination, excessive clearance, lack of oil, low oil viscosity. Affects bearing with possible scoring of the crankpin. 

Ovality: It can be caused due to varying downward load and angularity of control. If ovality exceeds 25% of bearing clearances, hydrodynamic lubrication can be affected. Which affects bearing and journal.

D. Tests are carried out on completion of the survey

On completion of work, ensure all tools are removed from the crankcase. Measure and record bearing clearances to ensure that it is within manufacturers limits. Check oil flow through bearing, without oil the bearing will wipe within seconds. Turn engine on turning gear for at least 2 revolutions, observing ammeter, comparing with readings taken before. This will ensure there is not a tight spot that may result in failure. Run the engine for 30 seconds on low load, open crankcase doors and check bearing temperature, and any evidence of white metal in the crankcase. Repeat after 5 minutes and a 30-minute run. This will highlight any abnormal overheating and may prevent damage to the crankpin should the bearing fail. After the 30 minute check, the engine load can be gradually increased over a 2 hour period to full load. This will allow the bearing to "bed-in".