Bearing Defects, Inspection, Clearances on Ships

Bearing Defects

  1. Abrasive damage:  Fine scratches caused by particles in the lub oil. Very common on HFO burning engines.
  2. Erosion damage : Removal of the overlay in strips caused when the oil supply pressure is low orrapid journal movements occur. More usual on medium speed engines.
  3. Fatigue damage:  The overlay becomes detached from the lining when the bearing load becomes too high. The bearing surface loads cracked paving.
  4. Corrosion:  Discoloration and roughening of the bearing surface indicates that the oil has become acidic.
  5. Wiping:  This is overlay removal by melting wiping can be re-alignment of the bearing to journal, but if too much metal has been removed then clearances may be affected.

Journal Defects

  1. Cracks:  These will appear at the high stress points of the fillet radii and oil holes. These cracks may be removed by light grinding, but engine de-rating would be required if deep / numerous cracks are found.
  2. Scoring:  Similar problem to the abrasive damage.
  3. Overheating:  As the bearing is weaker than the shaft, the bearing should fail first. However if the engine is run on a failed bearing then shaft overheating will occur. This ‘bluing’ of the shaft increases the hardness of shaft and hence the shaft is less able to resist crack growth. Classification states a maximum hardness for crankshaft journal.

Bearing Inspection

  1. Edge wear
  2. Score & scratch (striation wear)
  3. Overheating surface (blur/violent colour show heating cracks)
  4. Cavitation & erosion (10% bearing surface)
  5. Corrosion
  6. Crack in galvanic layer
  7. Pitting & fretting

Factors Affecting Bearing Clearance

  • Desired operating temperature – extremely critical
  • Engine speed
  • Oil flow
  • Oil film thickness
  • Working viscosity of lubricant 
  • Load carrying capacity
  • Operating temperature     
  • Engine ambient temperature.

Measurement of Bearing Clearance on Ships

It is important that regular checking of bearing clearance is carried out, as the clearance determines the effectiveness of lubrication.

Lead wire

Traditional method, but requires that bearing are tightened just to obtain clearance. Accurate as long as load is not over squeezed. Lead is not to squeeze blow 1/3 rd of original diameter.

  • Turn the crank shaft and set the crank at TDC position.
  • Remove locking arrangements, mark the nut position.
  • Slacken the nut and lower the bottom half with bolts.
  • Then three lengths of lead wires would be laid circumferentially in the bottom half at three places.
  • Place the bottom half into position and tighten the nut to its tightening torque.
  • Lower down the bottom half again.
  • Remove the lead wires and take the measurement.
  • It must have within the limit, if out of limit, the bearing shell must be replaced with new ones or readjust the clearance by adjusting shims.

Feeler gauge

Quick method, but more difficult to be accurate when using the long feelers as measuring point may not be the minimum point.

  • Turn the crank shaft and set the crank at BDC.
  • Insert the feeler gauge between lower half and crank pin.
  • Take the measurement readings.

Plastigauge

Relies on the width of a plastic strip after compression. More accurate than leads.

Bridge gauge 

Depends on bedplate condition and crankshaft rigidity. Bridge gauge is an instrument for main bearing wears down measuring.

  • Remove the lube oil supply pipe.
  • Remove upper bearing half and fit the bridge gauge.
  • Then take the measurement by inserting feeler gauge.

Micrometer

  • More accurate

 What are the effects of excessive bearing clearance ?

  1. Low LO pressure
  2. Reduce load carrying capacity
  3. Pounding will case and bearing will damage.
  4. High impact load on crankshaft.

 

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