Question

(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 the checks, which should be made before returning the engine to service following overhaul of the bottom end bearing.

Answer 1

(a) Procedure for Complete Inspection of a Bottom End Bearing:

Planning:

Ensure you have sufficient time, manpower, and all necessary tools, spares, and the manufacturer's manuals.
Organize the workspace, ensuring all safety measures are in place.

With the engine shut down, lubricating oil pump stopped, start air locked off, turning gear engaged, Risk assessment and a permit to work obtained, proceed as follows for checking the condition of the bottom end bearing surface of a large slow-speed engine.

Open the crankcase door at the relevant cylinder and ventilate, as the crankcase is an enclosed space (Follow the enclosed space entry procedure).
Turn the relevant cylinder to BDC and check the bearing clearance. This is because, after squaring up, the clearance should be the same.

To remove the Bottom end bearing (bottom-side)

Now turn the cylinder to TDC. Mount eyebolts on each side of the crankpin bearing cap and suspend two tackles from the lifting brackets in the athwartship direction.
Using shackles and wire ropes, hook on the tackles and haul tight.
Loosen the crankpin bearing studs using hydraulic jacks. Remove the palm nuts.
Lower the bearing cap while carefully ensuring that the studs do not damage the crankpin journal. Land the bearing cap on a couple of planks placed in the crankpit.
Using another tackle mounted on top of the crankcase door, carefully lift the bearing cap out of the crankcase and place it on wooden planks.

To remove the Bottom end bearing (top side)

Fit four guide shoe retaining blocks into the crosshead guides. This will prevent the crosshead and conrod from moving down when the engine is turned.
Wrap a strop around the bottom of the conrod and attach it to a chain block mounted on the side of the engine.
Turn the engine while pulling the lower part of the conrod with the lifting tackle so that the crankpin turns out of the top half of the bearing. Ensure that the crankpin does not foul on the top edge of the bearing.
The crankshaft can be turned to the bottom dead centre, and the top half of the bearing examined/ removed.

Inspect the bearing surface for any signs of scoring, pitting, wiping, ovality, cracks, or corrosive attack. Take photographs of the bearing and make relevant paperwork. The clearance of the bearing should be recorded before opening and after square up. Square up the bearing in the reverse order of dismantling.

Part (b)

Checking the bottom end bearing clearances:

Turn the engine to bring the particular unit to BDC
The top half of the bottom end bearing is always loaded, so the clearances are measured by the feeler gauge at the bottom half
For a engine with a bore dia 600mm, the standard clearance may be between 0.35 ~ 0.45mm with a max limit of 0.70mm

Adjustment

For modern engines after maximum wear down, the bearings are renewed
Some engines have thin shell bearings, for which clearances can be adjusted by fitting shims between the two bearing halfs on both sides.

(c) Possible Defects: (write any two)

Fretting: Insufficient tightness of the hydraulic nut can cause fretting.
Ensure proper tightening torque is applied.
Scoring: Foreign particles in the lube oil can cause scoring.
Clean the oil system and replace filters.
Pitting/Corrosion: Acidic attack in the lube oil can cause pitting or corrosion.
Replace the contaminated lube oil and investigate the source of acidity.
Wiped Out Bearing: Breakdown of the lube oil film due to overloading or overheating can lead to a wiped-out bearing.
Replace the bearing and investigate the cause of overloading or overheating.
Fatigue Cracks: Lack of lubrication can cause fatigue cracks.
Replace the bearing and investigate the cause of lubrication failure.
Hot Spots: Lack of lubrication can cause hot spots.
Replace the bearing and investigate the cause of lubrication failure.
Ovality: Varying loads can cause ovality.
Replace the bearing and ensure proper load distribution.

(d) Checks carried out before returning the engine to service: (write any two)

Measure the bearing clearance to ensure it is within limits as specified in the maker's manual
Check the crankshaft deflection to confirm proper alignment and that there are no undue stresses.
Start the lube oil pumps and check for proper oil flow to the bearings
Turn the engine on the turning gear and monitor amperage to detect any obstruction.
Test run the engine on air, followed by a load test. During operation, monitor the bearing temperature closely to detect any abnormalities.
After stopping the engine, inspect for any white metal fragments, which could indicate wear or damage.
Check for hot spots that could signify improper lubrication or bearing fitment issues.

✓ Verified Model Answer (Text Solution)

Checking the bottom end bearing clearances: