Bearing

Classification 

1) Main bearing

2) Top end bearing = Cross Head / Gudgeon Pin bearing

3) Bottom End Bearing / Crank pin bearing 

4) Thrust Pad bearing 

5) 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 

1.Fatigue & compressive strength to carry load – depends upon thickness.

2. 0.3 mm white metal can withstand 141 bar pressure and 0.08 mm white metal can withstand 211 bar pressure.

3.Thin lining has poor conformability and too soft material tends to flatten under heavy loads.

4.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 

a 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 ? 

a) 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. 

b) 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 ? 

(01) Defective tag 

(02) Insufficient nip clearance 

(03) Suddenly applied extreme load.( pounding) 

(04) Improper fitting 

(05) Incorrect size of bearing use 

(06) Due to over tightening bolts

(07) 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 

1.Load is always on down wards & construction is light.

2.Load rotates but bearing cap is rigid.

3.Bolts centers are kept close together. 

4.Two halves of bearing housing is kept concentric by fitted bolts, stepped cap & serrate cap.

BEARING BOLTS 

1.Adequate tensile strength.

2.High resilience, capacity to absorb maximum strain before yielding.

3.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 --- 

J Desired operating temperature – extremely critical

J Engine speed

J Oil flow ∝ (clearance) 3 

J Oil film thickness

J Working viscosity of lubricant

J Load carrying capacity

J Operating temperature 

J 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.

 

1) Turn the crank shaft and set the crank at TDC position. 

2) Remove locking arrangements, mark the nut position.

3) Slacken the nut and lower the bottom half with bolts. 

4) Then three lengths of lead wires would be laid circumferentially in the bottom half at three places. 

5) Place the bottom half into position and tighten the nut to its tightening torque. 

6) Lower down the bottom half again. 

7) Remove the lead wires and take the measurement. 

8) 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.

1) Turn the crank shaft and set the crank at BDC. 

2) Insert the feeler gauge between lower half and crank pin. 

3) 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. 

1) Remove the lube oil supply pipe. 

2) Remove upper bearing half and fit the bridge gauge. 

3) 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)

1.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.

2.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 _ 

a.Loaded part is heavily worn 

a.More than 1/3 of the contact area is scratched. 

a.Roughness has caused a large area of the WM to be wiped.

a.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 J New 0.05 µm, J Run in 0.1 µm, J 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 ? 

(01) Check bearing thoroughly. 

(02) Check X head pin ovality. 

(03) Check bearing clearance. 

(04) Lubrication system and oil holes. 

(05) Check Guide shoe wear down.