Classification of bearings-
♦ Main bearing
♦ Top end bearing = Cross Head / Gudgeon Pin bearing
♦ Bottom End Bearing / Crank pin bearing
♦ Thrust Pad bearing
♦ Pedestal bearing (generator alternator side insulated bearing)
1. MAIN BEARINGS
Function : Support crankshaft and keep it aligned.
To remove heat produced by friction
2. TOP END BEARINGS
Cross head Engines: Transmit load from cross head pin to connecting rod
Allows relative movement of con rod & cross head pin
Trunk Piston Engines: Transmit load from gudgeon pin to connecting rod
Allows relative movement of con rod & gudgeon pin
3. BOTTOM END / CRANKPIN BEARINGS
Function: Transmit load from con rod to crankshaft
Allows relative movement of con rod & journal
Bearing Operation: Depends on
♦ Operating temperature of bearing
♦ Working temperature of bearing
♦ Minimum oil film thickness
♦ Rate of oil flow
♦ Rate of heat production
♦ Power loss of bearing.
Bearing Loads: Combustion forces, Inertia forces & Centrifugal force of rotating masses
Varying resultant load from gas forces & inertia forces
Two stroke engine No load reversal
Four stroke engine Load reversals at the end of exhaust stroke hence, wear uniform & lubrication better.
Fluctuating gas force results fatigue failure in bearing
Bearing Material Properties
Mechanical Strength
♦ Fatigue & compressive strength to carry load – depends upon thickness
♦ 0.3 mm white metal can withstand 141 bar pressure and 0.08 mm white metal can withstand 211 bar pressure
♦ Thin lining has poor conformability and too soft material tends to flatten under heavy loads
♦ Too hard material withstands high loads, posses high frictional characteristics & may be brittle with poor fatigue characteristics.
Soft & low melting point material
♦ Softness & modulus of elasticity of bearing alloy should be as low as possible but hard enough to withstand heaviest continuous loading or chock loading without plastic deformation
♦ Soft metal flows locally without damaging the harder steel called conformability
♦ Allows abrasive particles to embed to prevent damage to journal
Corrosion resistance – to withstand corrosive attack from lub oil
Compatibility between bearing & journal under boundary condition
♦ Anti-weld & anti-score property between shaft & journal during start up & stop and by using turning gear.
Antifriction & wear properties –depends upon type of oxide film that material forms on reaction without lube additives.
Bearing Material
White Metal = Tin (Sn) +_ Antimony (Sb) + Copper (Cu)
♦ Thin walled bearings, stiff cross head assembly 88% Sn + 8% Sb + 4% Cu
♦ Thick walled bearing, flexible crosshead & Bottom end bearing 87% Sn + 9% Sb + 4% Cu
♦ Tin forms soft matrix to accommodate misalignment
♦ Antimony forms hard cubes to withstand load of journal. Tends to float and segregate during casting
♦ Copper holds antimony in evenly dispersed pattern, solidifies first.
Copper Lead & Lead Bronze = Brass (Cu + Zn) & Bronze (Cu + Sn)
♦ Can withstand 3 times higher load than white metal
♦ Copper / Bronze matrix supplies the strength
♦ Lead remains in free state, provides bearing properties and Steel strips provides backing
♦ Overlay of 0.024 -0.04 mm thickness of lead –tin, lead –tin –copper.
♦ Running in prevents acid attack against lead but poor embeddability & conformability
Aluminum Tin = Al Matrix + Si (minor) + Overlay7 (Pb+Sn) + Steel Backing
♦ Soft Aluminum forms the matrix and provides embedability & conformability
♦ Tin held in suspension provides bearing properties
♦ Lead Tin Overlay of 0.02 mm for initial running in
♦ 3 times load carrying capacity than white metal but requires hardened journal
♦ Resistant to acid attack and fatigue strength same as Cu & Pb
Bearing Material – Shaft Material – Lubricant
High local pressure at the point of contact
Localized welding at these points
Alloy formed at welds
Shear strength at welds.
Shear strength (alloy) > Shear strength (metal)
THIN SHELL BEARING
Wall thickness to diameter ratio varies 0.05 mm for 40 mm shaft diameter and 0.02 mm for 400 mm shaft diameter.
Interference fit or bearing crush
Fretting – Interference fit resists relative movement, prevents fretting.
Locating Tags – For correct axial location of shell but not intended to resist motion
– Recessed below bearing joint face.
Free spread – Bearing shell in snapped into bearing housing
– Bearing can be held in place when inverted during assembling.
What is nip ?
The external circumference of a pair of bearing shell is slightly larger than the bore of housing. The difference is called nip.
Advantages of Thin Shell Bearings
High load carrying capacity; approximately 5 time > conventional bearing
Uniform wall thickness permits better metallurgical control of white metal casting process.
High Bond Strength and ultrasonic method of bond testing between layers is accurate.
Reduced thickness & absence of keying grooves results in higher fatigue strength
Blistering on bearing surface due to H2 emission form is less.
Oil Grooves on Bearing Shell
Oil Grooves to avoid at pressure areas as oil tend to escape high to low pressure zones
Circumferential grooves to compensate with increase length of the shell
Longitudinal groove is not extended to ends to avoid excessive side leakage.
Main Bearing Groove
Circumferential groove most effective and satisfactory
Oil supply at all angle and wide variation of load angle.
Different between conventional and thin shell bearing ?
♦ Conventional bearing
(a1) It is made of forged steel and running face is lined with white metal.
(a2) Vertical clearance is adjusted by shims.
(a3) Not easy to replace and must be done remodeling.
(a4) Not easy to handle, transport and store.
(a5) Suitable oil grooves design is required.
(a6) Lower load carrying capacity.
(a7) More cost in manufacturing.
♦ Thin shell bearing
(b1) It is made of tri-metal, they are steel shell, copper or lead alloy and thin layer of soft metal surface.
(b2) Easy replacement incase of bearing worn out. ( Re-metalling method no longer required)
(b3) No need to adjust by shim ( can not be adjusted by shims.)
(b4) Easy handling, transport and storage as spare.
(b5) Higher bearing load carrying capacity.
(b6) More economy in manufacturing.
(b7) No need to take lead reading.
Cause of thin shell bearing shifting ?
♦ Defective tag
♦ Insufficient nip clearance
♦ Suddenly applied extreme load.( pounding)
♦ Improper fitting
♦ Incorrect size of bearing use
♦ Due to over tightening bolts
♦ Frictional force from the back of the shell and keep.
Crosshead Bearing Construction Features.
Thin shell bearings are used and bearing on either end of crosshead pin.
No shim used with thin shell bearing
Oil grooves or gutter used on bottom half to distribute oil.
Grooves do not extend to end and grooves are small because of loaded half.
Grooves to be limited otherwise reduce bearing surface.
Lubricating oil is directly supplied to crosshead bearing
Bearing material usually Sn-Al with Pb-Sn overlay.
Crosshead Bearing Working Condition
High sudden load – Effect of combustion is directly on bearing
High bearing pressure – Bearing is placed high in engine. – Space limitations. – Assembly reciprocating.
Diameter & length – Diameter & length of bearing are low. – Bearing area limitations. –High specific loading
Possibility of bearing distortion – Bending moment & deflection are maximum at center. Pin bored at center (earlier model engines). Less stiffness & high stress concentration. – Bearing surface deflection. – Alignment difficulty.
Lubrication – Unsatisfactory or difficult oscillating moment. – Con rod swings over 25° -30°.
Oil supply disturbed – Difficult smooth & uninterrupted oil flow.
2 stroke engine – Unidirectional load.
Modification of Crosshead Bearings Over Last Few Decades:
Conjugate Deflection – Sulzer Engines (crosshead pin bored at center)
Crosshead mounted mechanical lub oil pump – MAN Engines (oil supplied when load is lowest & oil film is not broken at highest pressure)
Continuous full length bottoms half of crosshead bearing – MAN B & W, Sulzer Engines.
Eccentric bored bearing & machining shell – Fiat Engines
Large diameter stiff crosshead pins L/D ratio less (small con rod and crank throw ratio. Sliding velocity high & lub oil film improved)
Hardened cross head pin high degree of surface finish < 0.1μ
Thin shell bearing & improved material.
Bearing Housing Design Feature
♦ Bearing shells are in place by interference fit
♦ There is no relative movement of housing & shell
♦ Effective heat transfer between shell & housing is essential
♦ Cap holding bolts are to be closely pitched to prevent distortion
♦ Housing is robust to prevent excessive strains on shell.
♦ Housing is not too stiff to prevent localized load concentration on bearing.
No fretting marks at the back of bearing shell and crush at bearing shell ends within limits
Medium & High speed engine’s Con Rod bottom end bearing housing tendency to distort.
BEARING CAPS
Load is always on down wards & construction is light
Load rotates but bearing cap is rigid
Bolts centers are kept close together.
Two halves of bearing housing is kept concentric by fitted bolts, stepped cap & serrate cap
BEARING BOLTS
Adequate tensile strength
High resilience, capacity to absorb maximum strain before yielding
Reducing diameter to bottom of thread over the length of the bolt reduces localized stress except at fitted.
INSPECTION OF BEARING & JOURNAL FOR DEFECTS
Bearing should be inspected at the overhaul / survey for the following defects
BEARING
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 or rapid 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 indicate4s 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 clear4aqnces may be affected.
JOURNAL
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 derating would be required if deep / numerous cracks are found.
2) Scoring: Similar problem to the abrasive bearing
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 checking
1) Edge wear
2) Score & scratch (striation wear)
3) Overheating surface (blur/violent colour show heating cracks)
4) Cavitations & erosion (10% bearing surface)
5) Corrosion
6) Crack in galvanic layer
7) Pitting & fretting
Bearing Clearance: Depends on —
♦ Desired operating temperature – extremely critical ♦ Engine speed
♦ Oil flow ∝ (clearance) 3 ♦ Oil film thickness
♦ Working viscosity of lubricant ♦ Load carrying capacity
♦ Operating temperature ♦ Engine ambient temperature.
Bearing Clearance Methods:
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 1/3rd 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
Effect of excessive bearing clearance ?
Low LO pressure
Reduce load carrying capacity
Pounding will case and bearing will damage.
High impact load on crankshaft.
C /E’s Procedure for Complete Inspection of a Crosshead Emphasizing Areas of Significant Interest (extract form B & W manual)
The cross head bearings consists of steel shells with 1.0 to 1.5 μmm of white metal (WM) having a 25 μm lead based overlay for running in.
Complete inspection may be carried out on a time basis i.e. after 8000 operating hours, for a 4 years survey of following inspection carried out without opening up.
1. Check without opening up
Just after stopping feel over bearing, check that uniform oil jets appear form all the oil outlet grooves in the lower shell.
Check clearance (on top) with feeler gauge and compare with records.’
Visually inspect sides of bearing for signs of white metal squeezed / missing
Dismantle & inspect if oil jets are oblique / twisted / reduced / missing / if white metal gives cause for concern or if clearances have increased.
2. Inspection & Overhaul
Crosshead opened up, condition of white metal and journal surfaces noted and entered in engine room log. White metal should be checked for wear / wiping / cracking / discoloration due to corrosion / bonding defects.
It is quite normal for the overlay to be disturbed at the most highly loaded areas.
Overlay or WH squeezed into the oil wedges and oil grooves or small spots, which have loosened, can be removed with a scraper.
If wiping is less than hand size scrape to blue marking cracks formation which will eventually cause WM to become loosened and dislodged may be due to lack of bonding strength or geometric irregularities causing local overloading.
Areas of small local crack formation discovered at an early stage should be relieved by scraping.
The back side of the shell should be inspected for even contact fretting or cavitation.
Journals to be inspected for roughness and ovality; slight ovality is acceptable.
Change journal if _
♦ Loaded part is heavily worn
♦ More than 1/3 of the contact area is scratched.
♦ Roughness has caused a large area of the WM to be wiped
♦ Manual polishing with hemp rope will not then be satisfactory.
Coin test for roughness, No vibration heard or felt when lightly held coin is passed over the surface
Surface roughness New 0.05 μm, Run in 0.1 μm, Trouble possible 0.125 μm.
Roughness will most likely be due to abrasive or corrosive (acid or SW) contamination of the lube oil.
Note that 1% SW contamination of the lub oil can promote galvanic attack of the WM formation of very hard black tin oxide (Sno) which will roughen journal surfaces.
What point to be check after removing X head bearing ?
♦ Check bearing thoroughly.
♦ Check X head pin ovality.
♦ Check bearing clearance.
♦ Lubrication system and oil holes.
♦ Check Guide shoe wear down.
Types or Crosshead Bearing failure:
Various types of failure occur in cross head bearing lining. If it is found early, can be rectified and continue in service. Failure of the white metal in less- severe forms usually progress so that the bearing must be remetalled.
Cracking of white metal
Fatigue failure of white metal
Squeezing of white metal so that oil grooves are partially blocked; oil holes may be partially blocked or wholly blocked in extreme cases.
Failure of white metal when the bearing surface of the white metal becomes plastic or even melts.
Corrosion depends on the nature of the contamination of the lubricating oil.
Causes of Crosshead Bearing failure:
Bearing failures may result from any one or combination of the following causes.
Deterioration of surface finish or cross head pins.
Poor quality of white metal
Insufficient supply of lubricant
Impure lubricant or water contamination
Excessive firing pressure in cylinder
How to check the bearing ? (Bearing overhaul) ****** Before removal
a. Check locking device and nuts tightness.
b. Check for wiped out of loose white metal at bearing end.
c. Check bearing clearance ( roughly ) by tongue gauge.
After removing
a. Check pin or key or tag.
b. Check holding down bearing surface.
c. Check white metal bearing surface (crack and damage)
d. If over 30 % of wear or crack in the contact area it should be renewed.
e. Check oil grooves and passage holes.
f. Check pin diameter & pin ovality
What points to be check after removing main bearing ?
♦ Check bearing thoroughly ( tag, oil grooves and holes, bearing surface)
♦ Journal ovality-take measurement at least 3 spaces.
♦ Checking bearing clearance-0.4 to 0.6 mm for 550mm shaft diameter.
What points to be checked after removing big end bearing ?
♦ After cleaning, inspect the bearing thoroughly at crank pin ovility two halves of bearing together with oil holes and grooves.
♦ Thoroughly, examined the bolts ( no cracks, no extension and no twisting )
♦ Check sign of movement of the joint point such as two halve of bearing joint and between top halve and connecting rod foot.
♦ Bearing clearance.
What points do you check after removing upper half bearing ? ****
♦ Check upper half bearing & bearing keep
♦ Check the bearing wear down by using bridge gauge & feeler gauge ovality of journal pin
♦ Check crack pin condition, oil holes
♦ Check upper bearing clearance by lead wire method
♦ Visual check to edge of lower bearing half, bearing pocket
What points do you check after removing crank pin bearing ?
♦ Check pin ovality & oil holes
♦ Check two bearing halves with oil holes and grooves (tag, crack, wear,)
♦ Check the bolt (crack & stretching)
♦ Check the movement at the joining point such as two halves of bearing joint and between the top half and connecting rod foot
What points do you check on thin shell bearing during overhaul ?
1. Visual inspection of any wiping & squeezing
2. Check axial play
3. Check of local temperature after test run
For new thin shell
1. Fit into housing and check the contact area after thorough cleaning
2. After clean, check bearing running surface (any crack, grooves, tap)
3. After renew test run for 30 minute and again, after 5 hours operation, manual checking bearing temperature.
Check new bearing before fitted
Bearing thickness, length, crack, surface smoothness, edge, oil hole, groove, tap
Casing & keep surface
Pin ovality,
How to decide that shell to be replaced ?
1. Shell with galvanic layer worn down over 30% of developed working surface to be replaced (X head 5% )
2. Running hour excess of 40, 000 to be replaced in any case (O/H or not)
3. Running hour excess of 30, 000 to be replaced when engine overhaul
How to check bearing wear down (Main bearing) without bearing removal ?
(1) Remove LO pipe connection from keep a bore has in the keep
(2) Hole also be provided in upper bearing half
(3) Clean holes & insert the depth gauge & take reading
The different of present reading & previous reading give lower bearing wear down
Bearing Problems & Diagnostics
Types of bearing defects ?
♦ Crack
♦ Fatigue failure of white metal
♦ Squeezing of white metal, so oil grooves are partially blocked.
♦ wiping
♦ Faulty casting and faulty machining.
♦ Tin oxide Corrosion
♦ Acid Corrosion
♦ Thermal Ratcheting
♦ Electrical Potential
♦ Fretting
♦ Cavitation Erosion
Wiping of Bearings Surface
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
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.
Fretting
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.
Fatigue
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.
Tin Oxide Corrosion
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.
Acid Corrosion
Takes place in high temperature condition
Bearing alloy is attacked by acid (condensation of SO2) form high ‘S’ content fuel.
Steel working parts corrode more than bearing alloy
Solution – Add rust & corrosion inhibitor in lub oil and select proper material.
Thermal Ratcheting
Caused by alternate cooling & heating of bearing
Results in bearing deformation
Indication of high bearing temperature
Place mainly in thrust pad bearing surface
Electrical Potential
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.
Cavitation Erosion
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 is 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 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
1. Over stressed on bolts (due to piston seizure, over tightening, propeller strike some obstruction)
2. Too long in service (renew after 10 years)
3. Wear of bolts & enlarge of holes can cause the easing of nuts.
4. Too much clearance of bearings, shock resulting fatigue in bolts
What do you do if intermediate bearing or tunnel bearing or plumber block bearing temperature is increase ?
Overheating of plumber block bearing can be reduce by following ways
By applying maximum lubrication
By applying maximum cooling
By reducing to suitable engine speed.
By applying the air
By removing the cooling outlet pipe
By filling the L.O into the sump at the same time open the drain valve and drain out the hot oil
By reducing the engine speed to the suitable speed.
How to check plumber block at sea ?
Check L.O level and L.O temperature
Cooling water outlet temperature
Noise and vibration
Overheating of casing by hand touch feeling
Main bearing removing procedure.
Measure bearing clearance.
Turn crank shaft to efficient position.
Remove lube oil pipe, locking arrangement and nuts.
Took out bearing keep with thrust nut by means of wire sling and chain block or special tool. Took out upper bearing shell.
Took out lower bearing shell.
1. By fitting lower bearing taking out tool fitted at oil hole at crankshaft or adjacent crank web and turn the crank shaft (the direction is according to the maker instruction, usually opposite to the ahead running direction.)
2. By use of hydraulic jack to lift the crankshaft just clear, about 0.1 to 0.3mm and turn the bearing shell without rotating the crank shaft.
Eye bolt fitted to the back of the shell and lift it out of the engine.
Big end bearing removing and fitting procedure.
1) Measure bearing clearance.
2) Turn TDC position.
3) Remove locking arrangement and slackened the nuts. The bottom half lower a few and took out bearing clearance adjusting shim, each set being tied separately and note taken of the side to which each set belongs.
4) Chain blocks connected to eye bolts, screw into each bolt. After removing two nuts, bottom half lowered into the sump. If necessary it can be taken out from crankcase.
5) Putting hanging bar in position, connect chain blocks to crankcase door frame and eye bolts which is screwed into each side of the top half. Then turn the crankshaft to the position where the top half can be taken out.
6) Inspection on crank pin, bearings, oil holes, grooves, bolts cracks, sign of movement and elongation