Question

With reference to Marine Diesel engines:

(a) Describe how crankshaft alignment is checked.

(b) Identify with reasons the causes of crankshaft misalignment.

(c) State how the measurements are recorded.

Answer 1

Part (a)

Crankshaft alignment is checked by measuring the crankshaft deflection (the change in the distance between the crank webs as the crank rotates) using a deflection gauge (dial test indicator) mounted between the webs of each crank throw. The deflection is measured at the standard crank positions (BDC, 90 deg, TDC, 270 deg) and recorded.
The deflection readings indicate the bending/ distortion of the crankshaft and the alignment of the main bearings. A positive deflection (webs closing at TDC) indicates the shaft is sagging between bearings (bearings low); a negative deflection (webs opening at TDC) indicates the shaft is hogging (bearings high).
The alignment is also checked by measuring the main bearing clearances (bridge gauge readings) and by checking the bearing contact/ loading.
The deflection is compared with the maker's/ Class allowable limits.

Part (b)

Wear of the main bearings (uneven wear) causing the shaft to sag/ hog.
Settling/ deterioration of the engine foundation/ chocks (epoxy or metal chocks), causing the bedplate to distort.
Distortion of the engine frame/ bedplate (from thermal effects, or from the hull/ foundation).
A bent or distorted crankshaft (from a previous failure/ overload).
Incorrect main bearing clearances/ alignment at installation.
Changes in the hull/ foundation (e.g. from grounding, or from the vessel's loading/ hogging and sagging).
Wear of the crankpin/ journal bearings.
Thermal expansion/ distortion of the engine.

Part (c)

The deflection readings are recorded on a crankshaft deflection record sheet, which shows the crank throw number, the crank angle (position), and the deflection reading (in mm/ 0.01 mm) with the sign convention (whether the webs close or open).
The readings are recorded for each crank throw at the standard positions (BDC, 90 deg, TDC, 270 deg).
The record sheet is kept for trend comparison: the deflection is measured at each survey/ overhaul and compared with the previous readings and the maker's limits.
The main bearing clearances (bridge gauge readings) are also recorded on the same/ a related sheet.

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