5. Diesel crankshafts

Diesel crankshafts are made by machine-building plants solid and compound. Continuous and reliable operation of crankshafts depends on the correct grounding of the crankshafts at the stationary frame main bearings at the limits of boundary sizes for diesels of all diesel-making firms of the world. Boundary limits are defined as crank web clearance – it is algebraic difference of distance between the crankpin webs at two diametrically-opposed positions of the crankpin, for example: upper dead point and lower dead point is a vertical crank web clearance and also portside – starboard side is a horizontal crank web clearance.

Crank web (D-delta) is calculated on the formula:

Crank web Δ (Delta) = 0.0001xSmm

Where S – is a piston stroke in mms.

Severe requirements at limiting - available sizes of the crank web are resulted from the influence of the symbol changing loadings on the crankshaft beam, as to ground the crankshaft on zero meanings is very difficult, and deviations ± 0.01mm or ±0.02 – is also significant, as the crankshaft line and the crankshaft itself in mass is bending either up or down at each shaft revolution in different diesel calculated by crankshaft revolution from 80 rev/min to 1500 rev/min. The ball structure of the crankshaft metal in case of which the tiresome of the metal comes, cracks and breaks of crankshafts at the points of crankpin webs or at journals of the bottom-end bearings or main bearings.

The inner structure of the crankshafts is soft and not hardened, and journals bottom-end or main are saturated with worn out covering or make them larger in diameter and along for discharging the specific pressure on the surface of journals and bearings, which are rubbing. The lubricating clearance [=0.001 dos mm] where dos – is outer shaft diameter. At the absence of being worked elipticity, barrelity, conecity of crankshaft journals the lubricating clearance at the bottom-end bearings may be reduced at 0.002-0.003 mm at the absence of elipticity and coneuity of shaft journals. Bottom-end (tightening) bolts lengthened more than at 0.5 mm are necessary to be replaced for new, or reduce loading on cylinders or on engine at 10-15%.

Crankshafts, resonance, critical revolutions

Coincidence of the running internal combustion engine oscillations-diesel with the running internal combustion of the ship’s hull by which the large total innertional amplitude of oscillations of the power plant construction and ship’s hull is resulted:

• Ships engine, propeller shaft line, breaking of ship’s constructions, this resonance is named in instructions on all ship’s main diesels, gas turbines, steam turbine and nuclear power plants as critical revolutions, which are marked on tachometers by the red point in the engine-room on main diesels at Central control station, at ship’s wheel-houses Chief Engineer cabin.

The ranges of critical revolutions should be immediately (2-3 sec) transfered into larger or smaller revolutions, thus avoiding vibrational running of the propulsion plant.