Bearing Failure, Corrosion and Wiping in Marine Engines

What are different types of bearing failures or defects on ships ?

  1. Crack
  2. Fatigue failure of white metal
  3. Squeezing of white metal, so oil grooves are partially blocked.
  4. Wiping
  5. Faulty casting and faulty machining.
  6. Tin oxide Corrosion
  7. Acid Corrosion
  8. Thermal Ratcheting
  9. Electrical Potential
  10. Fretting
  11. Cavitation Erosion

What is meant by wiping of bearings ?

  • Wiping is a slight transient phenomenon & is undetected until the machinery is opened up for survey.
  • In serious cases, complete bearing failure occurs due to over heating of bearing metal which occur owing to –

**********  Temporary lack of oil

**********   Very slow start up of engine

**********  Too small bearing clearance    

**********  Misalignment of pin and bearing                                                                                              

**********   Fabricated cross girder of bedplate

**********  Tin oxide corrosion

What is meant by fretting of bearings ?

  • Happens in dynamic loaded bearings / pivoted pad bearing i.e. thrust pads of thrust bearing.
  • Fretting occurs on the back of support surface where the interference fit / nip is insufficient for dynamic forces involved.
  • Caused by the housing, which is insufficiently rigid for the load cycle involved.

What is meant by fatigue failure of bearings ?

  • Bearings carrying high dynamic loads are liable to fatigue damage
  • Caused by a concentration of load due to mechanical imperfection i.e. poor geometric form, misalignment and distortion.
  • White metal bearings are particularly prone to fatigue since any high loading not only increases the stress in the lining, but the associated temperature rise reduces the strength.
  • Causes of fatigue cracking  is due to poor bonding of white metal to its steel shell.

What is meant by Tin Oxide Corrosion in bearings ?

  • Tin oxide is extremely hard & brittle and corrosion takes place at tin phase of white metal
  • This breaks off rapidly, causing wear of the surfaces & breakdown of oil film
  • Appearance  – Grey at initial stage, becomes darker as its thickness increases & particle become detached.
  • With high loads when the oxide layer becomes thick, the bearing temperature may rise sufficiently to melt the underlying metal & failure occurs by wiping.
  • Cause  – Water mixes with LO promoting electro-chemical reaction.
  • Prevention  – Regular & continuous removal of water from lubricating oil prevent tin oxide formation.

What is meant by acid corrosion in bearings ?

  • Takes place in high temperature condition
  • Bearing alloy is attacked by acid (condensation of SO2) from high sulphur content fuel.
  • Steel working parts corrode more than bearing alloy
  • Solution  – Add rust & corrosion inhibitor in lubricating oil and select proper material.

What is meant by thermal ratcheting in bearings ?

  • Caused  by alternate cooling & heating of bearing
  • Results in bearing deformation
  • Indication is high bearing temperature
  • Occurs mainly in thrust pad bearing surface

What is meant by electrical potential damage in bearings?

  • This type of damage occurs frequently in electrical machinery due to stray currents.
  • The damage consists of uniformly distributed pitting, the pits being generally hemispherical with the intensity increasing to a maximum in the zone of thinnest oil film.
  • Caused  by incorrect earthlings system which cause spark erosion damage.
  • Prevention  – Insulate the non-drying end bearing (pedestal bearing) of electrical machines and sometimes in both bearings.

What is meant by cavitation erosion in bearings?

  • Severe damage to complete bearing area.
  • Cavities are usually around at low pressure areas i.e. oil groove or oil holes.
  • Caused by an implosion of gas or air bubbles released from a lubricating oil film under particular conditions
  • The pressure set up locally during theses implosions are very high , possibly 220 bar & may cause a pitting / cavitation
  • Prevention  – May be reduced by viscous oil because of damping effect high viscous oil & viscosity must be in limit.

Cause of white metal squeezing

  • When bearing metal is pressed out into the oil groove due to load on bearing exceed its compressive strength.

Cause of faulty casting and machining 

  • It is due to premature failure even under normal running conditions. 

Causes of bearing overheat

  • Improper viscosity of oil (lower)
  • Insufficient lubrication
  • Improper oil clearance
  • Foreign matters in oil
  • Misalignment of shaft and bearing
  • Scored journal
  • Poorly fitted bearing

Big end/crank pin bearing bolts failure

  • Over stressed on bolts (due to piston seizure, over tightening, propeller strike some obstruction)
  • Too long in service
  • Wear of bolts & enlarge of holes can cause the easing of nuts.
  • Too much clearance of bearings, shock resulting fatigue in bolts

 Nitride surfaces

  • Surface to be machined at least by 0.025 mm to prevent bearing damage.
  • Stainless steel shafts & white metal bearing surface – wiping, pick up & seizure
  • Failures  – Due to lack of compatibility and the problem is worst at high specific load.
  • Ni or Cr Plating : on journals / pins must be voided which results in scuffing seizure.

 Reference:

Oral Guide by – MIN ZAR TAR

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