Factors affecting the shaft alignment of ship's propulsion shafting

Factors:

• Thermal expansion/contraction of the hull and the machinery (engine, gearbox, thrust block, stern tube) as it runs and warms up, and of the shaft itself.
• Hull flexibility and deflection under load, particularly hull girder bending/sagging and hogging in different loaded conditions, and local stiffness of the engine room structure.
• Misalignment of the bedplate, engine seating and thrust block; settling/sinking of foundations.
• Deflections of the shaft due to its own weight between bearings and whirling/torsional effects.
• Bearing wear/clearances and bearing/seating heights and tilt (the alignment of the shaft axis through the stern tube and outboard bearing).
• The engine's own crankshaft/axle and coupling; the height of the thrust shaft relative to the tailshaft.
• External loads: propeller thrust, propeller weight, sea state causing loads on the tailshaft, and the effects of the stern tube seals and bearings.
• Misalignment during installation, propeller removal/refit, slack couplings, and distortion of the gearbox casing.
• Temperature gradients (hot oil/cool water) and the movement of the vessel (pitching).

Most effective methods for achieving the best possible alignment

• Optical alignment/Laser alignment of the line shaft using a sighting telescope or optical/laser target to place the shaft centreline coincident with the engine/gearbox axis.
• Parallellism/offset method: measure the shaft sag under its own weight and set each bearing to support the shaft using a defined "sag" so that the actual bow of the shaft is uniformly supported (a slight downward deflection between bearings is normal).
• Use of the "sag/deflection" (beam/via) method to calculate and set bearing heights to match the natural sag of the solid line shaft.
• Strain-gauge/whirling test: run a balancing/whirl test and use strain gauges on the coupling bolts to check connection/alignment and shaft alignment - tightening bolts in sequence and measuring bolt strain to achieve even loading.
• Micrometer/clearance and dial-gauge checks: with the shaft at rest, measure clearances at coupling flanges and bearing bores to detect misalignment; adjust by jacking/raising bearings.
• Use of thin chocks/packing and precision machining of the foundations and seatings; slackening and re-tightening of holding-down bolts correctly, and the use of shims to set exact bearing heights.
• Vibration and temperature-run analysis: check bearing temperatures and vibration signatures, and finally a sea trial with vibration measurement to confirm alignment.
• Adopting a flexible/mid-line bearing and correctly arranged flexible couplings and a resiliently mounted engine where appropriate; periodic recheck and re-alignment at overhaul.

The aim is that under the running (warm, loaded) condition the shaft centreline is as straight as possible in both planes, with correct bearing loads and no overloading of any bearing, and that the engine/gearbox/thrust/shaft are collinear.