What are different types of bearing failures or defects on ships ?
- Fatigue failure of white metal
- Squeezing of white metal, so oil grooves are partially blocked.
- Faulty casting and faulty machining.
- Tin oxide Corrosion
- Acid Corrosion
- Thermal Ratcheting
- Electrical Potential
- 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
- 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.