Running checks should include:
Gear box – oil level; inspect for leaks around casing; ensure pins and bolts in mechanical linkages are secure.
Shaft bearings – check oil level in bearing sump; leakage through end seals; temperature of bearing.
Stern gland – a leaking stern gland is a normal routine and is adjusted as a matter of course. See stern tubes in section above.
Propeller shaft – check noise level to determine whether it is increasing.
Regular maintenance schedules will include:
Gearboxes – Adjust clutches and operating mechanism, clean filters and clean oil coolers.
Drive train – Engine and shaft bearing moutings, loose shaft coupling bolts (perhaps due to mis-alignment) may need to be tightened. Mis-alignment of intermediate bearings will show up by overheating of the bearing.
Survey servicing and repair schedules will include: Weardown survey- Allowance is determined by the manufacturer, but rules of thumb range from 3% of diameter as due for replacement to 6% being condemned (3% of a 50mm diameter shaft is 1.5mm permissible movement within the bearing).
Water lubricated tailshaft wear – The aft bearing is accessible via the small gap between the aft end of the stern tube and the front end of the propeller boss. If a rope guard is fitted over this gap, it must be removed. The shaft rests on the bottom half of the bearing. The gap between shaft and stern tube bearing can be measured by using long feeler gauges inserted at the top of the shaft. Many bearings have longitudinal grooves to allow water to circulate. Ensure measurement is taken at the bearing surface not the groove. If the gap above is too small to allow access of the feeler gauges another method of measuring the weardown is to clamp a dial indicator gauge to the hull with the pointer resting on the top of the shaft between the stern tube and propeller. Note the dial reading, then jack up the prop until resistance to jacking increases. Note the new reading. The difference is the wear down (the sum of the wear in the bearing and shaft wear).
Oil lubricated tail shafts wear – These shafts can have a mechanical seals at each end. The standard method of checking wear is by depth gauge. A collared plug in the gland housing or the stern tube just forward of the gland is removed. The depth from the face of the plug boss to the top of the shaft is measured. This should be compared to the original measurement when the shaft was installed. The difference is the wear down.
Checking intermediate shaft alignment
Misalignment will cause noise and vibration. Gearbox and shaft bearings can overheat and collapse more quickly. The flange couplings of the intermediate shaft of a vessel in service can be checked for parallel, concentricity and alignment using:
Dial indicators and/or
Feeler gauges and straight edges.
Utilising a dial indicator –
1. Inspect the intermediate shaft. If supported on two bearings go to step 2. If supported on a single bearing it is necessary to provide a temporary support to ensure the shaft does not tip. This could be a vee block set up to ensure the shaft maintains the same alignment.
2. Remove the coupling bolts at either end. If muff couplings are fitted, dismantle them and slide the muff/s away from the shaft ends.
3. Using an indicator gauge, clamp the gauge on the engine/gearbox side half coupling. Set the pointer on the top of the intermediate shaft coupling flange and record the reading on the indicator dial. Rotate the engine gearbox shaft and take readings of the dial at 90, 180 and 270 degrees. For perfect alignment, the indicator reading should remain the same at all angles. The procedure for a muff coupling is similar except that the indicator gauge is clamped on the shaft and the pointer set on the intermediate shaft.
4. As a double check transfer the dial indicator to the intermediate shaft. Set the pointer on the engine gearbox shaft and repeat the action in Step 3. The readings should be the same as before.
Using feeler gauges and straight edge –
1. Inspect the shaft. If supported on two bearings go to step 2. If supported on a single bearing, provide a temporary support to ensure the shaft does not cant or tip. (This could be a vee block carefully set up to ensure the shaft maintains the same alignment.)
2. Remove the coupling bolts at either end. If muff couplings are fitted, dismantle them and slide the muff/s away from the shaft ends. 3. Lay the straight edge across the outer diameters of the half couplings at 0, 90,180 and 270 degrees. Any mis-alignment will show as a gap between straight edge and one half coupling. This gap can be measured by feeler gauge. 4. Again as a double check, rotate one of the shafts 90 degrees and repeat step 3. Allowable mis-alignment will depend on the diameter of the shaft. The vessels operating manual or other documents will often provide the information.
