crankshaft

Lets learn all about CRANKSHAFT- an important engine component.

INTRODUCTION:

• The working of other components of the engine, such as camshaft for fuel timing, firing order of each unit, opening of inlet valve and exhaust valve, operation of air start distributer etc depends on correct rotation of the crank shaft.
• Failure of crankshaft can stall the engine/ship.
• The parts forming a crankshaft are : journals( crankshaft), crank webs and the crankpins.
• They are too big to make a single unit and so they are constructed by joining together individual forgings.(will be discussed below how it is done).

MATERIAL:
  1. For slow speed marine diesel engines:
      steel castings or forgings
  2. For medium and high speed M.D.E
      Alloy steel

WEIGHT:
For 2-stroke crosshead engines- over 300 tonns.

CLASSIFICATON:
 They are divided into four depending upon the methods of construction-
(a) Fully built
(b) Semi-built
(c) Welded crankshaft
(d) Solid single piece used in very small engines.

Now we will know the above mentioned classifications one by one in a simple way.

(A) FULLY BUILT

• As already mentioned above the parts which forms a crankshaft: journal, webs,      crankpins.
• Journals and crankpins are made from forgings, and the webs may be forged or cast.They are made separately.
• These parts are shrink-fitted.
• Webs were heated up and the crank pin and journals fitted into the holes.The holes gets expanded due to heating.
• Were used on older engines. 

  Let's see the diagram:

BEARING

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 

• 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

 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 

1. Tin forms soft matrix to accommodate misalignment
2. Antimony forms hard cubes to withstand load of journal. Tends to float and segregate during  casting
3. Copper holds antimony in evenly dispersed pattern, solidifies first.