CRANKSHAFT, MAIN BEARINGS & SHAFT

ALIGNMENT

1. CRANKSHAFT,MAIN BEARINGS & SHAFT ALIGNMENT

1.1 DEFINITION OF A CRANKSHAFT

The crankshaft converts reciprocating motion in the cylinder into rotary motion of the propeller shaft.

1. CRANKSHAFT, MAIN BEARINGS & SHAFT ALIGNMENT

1.2 PARTS

The crankshaft is made up of throws and journals. A throw consists of a pin ( secured or attached to big / bottom end bearings ) and two webs or cranks. Journals rest or lie in the main bearings.

1. CRANKSHAFT,MAIN BEARINGS & SHAFT ALIGNMENT

1.3 STRESSES Bending ( when the piston is at TDC ); Shear stress ( in operation ); Torsion ( due to speed change, i.e. acceleration & deceleration )

1.4 MANUFACTURING

Solid forged built in a single piece ( small engines )

1.4 MANUFACTURING

Solid forged built in a single piece ( small-slow speed engines ) Semi-built design ( large engines ). Crankpins & webs are forged or cast in one piece and shrunk on to the journals.

1.5 MATERIALS Carbon steel

Alloy of nickel, chromium & molybdenum Specially alloyed grey cast steel

1.6 ARRANGEMENT OF CRANKS

Multi-throw shaft to provide for the engine firing order

2. MAIN BEARINGS / CRANKSHAFT BEARINGS

2.1 LOCATION

In the transverse saddles of the bedplate.

2. MAIN BEARINGS / CRANKSHAFT BEARINGS

2.1 LOCATION

In the transverse saddles of the bedplate.

2.2 PURPOSE

To provide support for the crankshaft

2.3 MATERIAL ( multilayer bearings )

Used for crankshaft bearings and connecting rod big end bearings

2. MAIN BEARINGS / CRANKSHAFT BEARINGS

2.1 LOCATION

In the transverse saddles of the bedplate.

2.2 PURPOSE

To provide support for the crankshaft

2.3 MATERIAL ( multilayer bearings ) Steel support shell ( basic element ); Bearing metal ( white metal, copper-lead or aluminium-tin alloy, leaded bronze ); Nickel barrier ( separating the two layers ); Galvanized layer ( good running-in and dry running properties ) Anti-corrosion layer ( lead or indium )

2.4 PARTS Upper & lower shells (fitted in bedplate seating / saddle )

2.4 PARTS Upper & lower shells (fitted in bedplate seating / saddle ) Thrust bolts Covers Shims ( for adjusting vertical bearing play )

2.5 LUBRICATION

Pressure lubricated ( low pressure )

2.6 CLEARANCE

2.6.1 Measurement lead wire wear gauge / bridge gauge feeler gauge “ kjaer “ feeler

2.7 BEARING DAMAGES

2.7.1 Indications increased temperature slight oil pressure drop ( sometimes followed by noise )

2.7.2 Kinds Squeezing of the overlay / white metal ( problems with oil film formation ); Fatigue cracking ( due to poor quality, shaft misaligment or local overload ); Dislodgement of overlay or white metal ( due to cracking ); Scoring ( striation due to presence of hard particles ); Wiping ( misshaped overlay or white metal due to high temperature )

3. CRANKSHAFT ALIGNMENTCrankshafts are aligned to determine load reactions on each bearing. This is done by laser techniques and computer calculations.

3.1 CHECKS

3.1.1 Frequency Once a year After replacing the main bearing If the ship has grounded

3.1.2 Techniques

The deflections are measured through one revolution of the crankshaft by inserting a dial gauge / dial indicator gauge between the webs.

Measurements are taken at TDC, BDC and two horizontal web positions.

3.1.3 Misalignment Wear of main bearings; Distortion of engine bedplate transverse members; Damage to supporting ship’s structure.

3.1.4 Consequences

Hogging is closing of the crank throw at TDC.

3.1.3 Misalignment Wear of main bearings; Distortion of engine bedplate transverse members; Damage to supporting ship’s structure.

3.1.4 Consequences

Hogging is closing of the crank throw at TDC.

Sagging is opening of the crank throw at TDC.

3.1.3 Misalignment Wear of main bearings; Distortion of engine bedplate transverse members; Damage to supporting ship’s structure.

3.1.4 Consequences

Hogging is closing of the crank throw at TDC.

Sagging is opening of the crank throw at TDC.

3.1.5 Reports Main / auxiliary engine crankshaft deflection record; Crankshaft and engine bearing data sheet.