What is the classification of bearings on ships based on functions ?
- 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)
What is the function of main bearing ?
- Support crankshaft and keep it aligned.
- To remove heat produced by friction
What is the function of 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
What is the function of bottom end / big end bearing ?
- Transmit load from con rod to crankshaft
- Allows relative movement of con rod & journal
What are the factors affecting bearing operation ?
- Operating temperature of bearing
- Working temperature of bearing
- Minimum oil film thickness
- Rate of oil flow
- Rate of heat production
- Power loss of bearing.
What are the properties of bearing materials ?
- Fatigue & compressive strength to carry load depends upon thickness of the bearing.
- 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.
- 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.
- Soft metal allows abrasive particles to embed to prevent damage to journal.
- Corrosion resistance is required to withstand corrosive attack from lube 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.
What are different bearing materials ?
- 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
- 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
What is meant by thin shell bearing ?
- For 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.
- Have interference fit or bearing crush
- Interference fit resists relative movement, prevents fretting.
- 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 of a bearing ?
The external circumference of a pair of bearing shell is slightly larger than the bore of housing. The difference is called nip.
What are the 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 Hydrogen emission form is less.
What are the properties of 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.
What are the peculiarities of main bearing groove ?
- Circumferential groove most effective and satisfactory
- Oil supply at all angle and wide variation of load angle.
What is the difference between conventional and thin shell bearing ?
- It is made of forged steel and running face is lined with white metal.
- Vertical clearance is adjusted by shims.
- Not easy to replace and must be done remodeling.
- Not easy to handle, transport and store.
- Suitable oil grooves design is required.
- Lower load carrying capacity.
- More cost in manufacturing.
Thin shell bearing
- It is made of tri-metal, they are steel shell, copper or lead alloy and thin layer of soft metal surface.
- Easy replacement in case of bearing worn out. ( Re-metalling method no longer required)
- No need to adjust by shim ( can not be adjusted by shims.)
- Easy handling, transport and storage as spare.
- Higher bearing load carrying capacity.
- More economy in manufacturing.
- No need to take lead reading.
What is the 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.
What are the Cross head 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.
What are Crosshead Bearing Working Conditions ?
- High sudden load – Effect of combustion is directly on bearing
- High bearing pressure – Bearing is placed high in engine, Space limitations, 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 to 30 degree.
- Oil supply disturbed – Difficult smooth & uninterrupted oil flow.
- 2 stroke engine – Unidirectional load.
What are the new modification of cross head bearings ?
- 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 < 0.1 microns
- 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.
- 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
- 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.
Oral Guide by – MIN ZAR TAR