Properties and Types of Bearings on Ships

What is the classification of bearings on ships based on functions ?

  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)

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 ?

  1. Operating temperature of bearing 
  2. Working temperature of bearing
  3. Minimum oil film thickness 
  4. Rate of oil flow
  5. Rate of heat production 
  6. Power loss of bearing.

What are the properties of bearing materials ?

  1. Fatigue & compressive strength to carry load depends upon thickness of the bearing.
  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.
  5. 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.
  6. Soft metal flows locally without damaging the harder steel called conformability.
  7. Soft metal allows abrasive particles to embed to prevent damage to journal.
  8. Corrosion resistance is required to withstand corrosive attack from lube oil.
  9. Compatibility between bearing & journal under boundary condition
  10. Anti-weld & anti-score property between shaft & journal during start up & stop and by using turning gear.
  11. Antifriction & wear properties  –depends upon type of oxide film that material forms on reaction without lube additives.

What are different bearing materials ?

White Metal

  • 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

  • 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 ?

Conventional bearing

  1. It is made of forged steel and running face is lined with white metal.
  2. Vertical clearance is adjusted by shims.
  3. Not easy to replace and must be done remodeling.
  4. Not easy to handle, transport and store.
  5. Suitable oil grooves design is required.
  6. Lower load carrying capacity.
  7. More cost in manufacturing.

Thin shell bearing

  1. It is made of tri-metal, they are steel shell, copper or lead alloy and thin layer of soft metal surface.
  2. Easy replacement in case of bearing worn out. ( Re-metalling method no longer required)
  3. No need to adjust by shim ( can not be adjusted by shims.)
  4. Easy handling, transport and storage as spare.
  5. Higher bearing load carrying capacity.
  6. More economy in manufacturing.
  7. No need to take lead reading.

What is the cause of thin shell bearing shifting ?

  1. Defective tag
  2. Insufficient nip clearance
  3. Suddenly applied extreme load.( pounding)
  4. Improper fitting
  5. Incorrect size of bearing use
  6. Due to over tightening bolts
  7. Frictional force from the back of the shell and keep.

What are the Cross head Bearing Construction Features ?

  1. Thin shell bearings are used and bearing on either end of crosshead pin.
  2. No shim used with thin shell bearing
  3. Oil grooves or gutter used on bottom half to distribute oil.
  4. Grooves do not extend to end and grooves are small because of loaded half.
  5. Grooves to be limited otherwise reduce bearing surface.
  6. Lubricating oil is directly supplied to crosshead bearing
  7. 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 ?

  1. Bearing shells are in place by interference fit
  2. There is no relative movement of housing & shell
  3. Effective heat transfer between shell & housing is essential
  4. Cap holding bolts are to be closely pitched to prevent distortion
  5. Housing is robust to prevent excessive strains on shell.
  6. Housing is not too stiff to prevent localized load concentration on bearing.
  7. No fretting marks at the back of bearing shell and crush at bearing shell ends within limits.
  8. 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.

 

Reference:

Oral Guide by – MIN ZAR TAR

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