Wednesday, January 30, 2013

Surveyor Guide Notes on Tailshaft Survey

1. Propeller Shafts

1.1 Propeller Shaft Clearances - Measurement

Periodical docking surveys and periodical propeller shaft surveys are incomplete without propeller shaft clearances. These may be determined as bellows:
  •  When the shaft is removed for survey or maintenance, by calibration of journals and bearing bores.
  • When bearing is exposed with shaft in place for partial survey or seal maintenance, by leveler gauges, or soft wood wedge driven between shaft and bearing then measured with calipers.
  • Normal running condition, by poker gauge comparing readings with previous measurements.
  • In normal running condition, by lifting shaft and measuring the lift by dial gauge (Less accurate). Case needed to avoid undue force. Assessment of poker gauge readings can only be made if records of the previous readings are available and the relative shaft clearance is known. Poker gauge readings taken in isolation produce no reliable value of bearing clearance.

NOTE: When stem tube bearings are renewed or re-metallic, clearances will be back to original. On refit, it is essential that these clearances are recorded and also the corresponding new poker gauge readings for future comparison.

With most modem stem tube seal designs, the poker gauge measures from the seal box “OD” to the seal sleeve “OD”. Hence, whenever the seal is fitted with a new sleeve or the sleeve is machined, the poker gauge readings “as fitted” should be recorded in conjunction with the measured clearance, by feeler gauges, calibration or both, since the bearing will be accessible during seal overhaul.

1.2 Propeller Shaft Clearances - Initial and Maximum Allowed

1.2.1 The following clearances are intended for the guidance of Surveyors.
Recommendations of designers and manufacturers may differ and the Surveyor should take notice of these in deciding whether departures from the values given here are justified.
In the case of initial clearances drawings should be checked and for special materials such as “Tufnol” the manufacturer’s technical information should be heeded to allow for expansion of the bearing material on submersion in water.

1.2.2 Propeller shafts running in white metal bearings - oil lubricated.

1.2.3 Propeller shafts running in lignum-vitae or "Tufnol" type lined bearings water lubricated:

NOTE: The stern gland should be repacked or at least partially repacked at each periodical bottom survey.

1.2.4 Propeller shafts lubricated by grease.
1.3 Crack detection of propeller shafts

1.3.1 Area to be crack detected.

1.3.2 Crack detection may be by dye penetrant method or magnetic particle method. Magnetic particle inspection is preferred because it is more sensitive when properly carried out.
In both cases the area to be checked must be thoroughly cleaned. Where possible a qualified technician is preferred working to recognized standards.

The sensitivity of the magnetic particle inspection is easily checked using a field strength indicator (burmah castrol strip).

If cracks are detected and not easily removed by light polishing it is advisable to check the crack depth by ultrasonic before proceeding.

1.3.3 It is allowed to reduce the rule diameter by 3 % (corresponding to a decrease in torsion strength of 10 %) by machining or grinding. Therefore if crack depth deeper than 1.5 % or shaft diameter (shaft being rule size) the shaft shall be rejected. Repairs of corroded or cracked shafts within limits given above shall be smoothly ground out to reduce stress concentrations to a minimum. The hollows should be filled with propriety metal filler if in way of sealing rings etc.

Slight surface defects (corrosion or cracks) can be machined out provided that rule diameter is not reduced by more than 3 %.

1.3.4 In the case of a propeller-shaft in normal steel and the propeller hub in stainless steel, the holes of the propeller-flange and the propeller-flange itself have to be carefully inspected for electric-chemical corrosion.
Seawater has to be avoided and adequate protection is to be foreseen.

1.4 Repairs to Propeller shaft cone

Corrosion on the conical part of the propeller shaft may be repaired by machining the taper.
This will result in the propeller moving forward which must be contracted by fitting a spacer between the shaft couplings. The maximum thickness allowed for this spacer is 25 % of the intermediate shaft flange thickness. It is therefore the intermediate shaft flange thickness which determines the maximum amount which can be machined off the cone.
e.g. Intermediate shaft coupling flange thickness 100 mm, then maximum spacer which may be employed = 25 mm, if propeller shaft taper = 1 in 12 the radial amount which may be machined = 25/12= approx. 2 mm.

Surface contact of propeller bore to shaft cone should be checked using Prussian blue. There should be a minimum of 70 % contact equally distributed.

NOTE: As well as rectification of the damage, the cause must also be determined and repairs and preventative action taken to avoid a recurrence. The usual source of leakages are from a badly jointed and sealed lairing cone on the ack of the propeller or leakage past the sealing ring and/or gasket on the forward face - ensure the “O” ring is the correct size so that compression and sealing is achieved.

All free spaces between propeller shaft cone, propeller boss, nut and propeller cap are to be filled with a material insoluble in sea water and non-corrosive.

1.5 Protection of propeller shaft against corrosion

Arrangements are to be made to allow any air present in these spaces to escape at the moment of filling. It is recommended to test these spaces under a pressure at least equal to that corresponding to the immersion of the propeller in order to check, after filling, the tightness obtained.

1.6 Propeller shafts with bronze liners

1.6.1 General

Always check carefully the inboard part of the shaft where water from the stern gland may have caused corrosion cracking, characteristic X-shaped fissures particularity on the coupling Flange filled. This part of the shaft is best protected by a special coating.

Beware also of electro-chemical corrosion in the region of the end of the liner.
The bronze liners protect those parts of the shaft which would otherwise be in contact with sea water. 

Continuous liners -either in one piece or in several sections cover the shaft from forward propeller boss to forward part of the stern gland. Parts of liner most liable to wear are those in way of stern gland packing, or the liner can be eroded by vibration or hammering of the shaft. Su objected to torque reaction with the shaft; the liner can crack (on its surface, or in other places). Cracks may also result from occasional over-heating. Sea water can seep through the cracks and cause rapid corrosion of the shaft.

Other types of corrosion are the result of liner assembly. Liners are generally held to tail shaft by shrinkage. Shafts are submitted to helical stress and it is unlikely that the same distortions are equally applied over the whole length of liner.

Such fretting is often the cause of corrosion noted under aft extremity of liners where sea water filters in. What has been said about shaft and liner relative motions is also true for liner sections connected by (hammered or shrunk) red copper joints. Connections of two adjacent liner sections are submitted to torque reactions and if copper joints no longer ensure the necessary water tightness: corrosion is then frequent under joints and adjacent area.

1.6.2 Checking the fit and condition of liners

The Surveyor shall hammer test the liner and joints between sections - bearing in mind that liners may be chamfered beyond the bearing surfaces and will, therefore, give a different sound, not to be mistaken with that of a slackening liner.
Slackness is usually noted at extremities of liner. It may be caused by heating (of aft bearing, gland) or cracks in liner.

The Surveyor shall also examine carefully the surface of the liner for cracks or porosity, and if in doubt, he will carry out a non-destructive check.
Any cracking or slackening of the liner (especially all, next to propeller) or loosening of copper joints will result in corrosion of the shaft.

s for such defects, loose copper joints will need to be withdrawn and the liner surface in way machined off and other corroded areas should also be machined so that the shaft surface can be inspected.
The Surveyor will also check the bearing surface and wear in way of bush(es) and stern gland.

1.6.3 Renewal/Repairs of liners

Cracks, wear

Fully penetrating cracks always imply renewal of damaged part of liner.
Fissures can be machined down provided that liner's thickness remains within the limits given below for wear, otherwise the damaged part shall be renewed.
Major wear means that liner is unlit for use; the maximum wear allowed being as follows:
  • 25 % of rule thickness in way of the bearing area.
  • 50 % of rule thickness in way of out the stern-gland.
These measurements shall be taken alter machining of corroded or ribbed areas of the liner. When damage (fissures or wear) is important and requires the renewal of the liner, the Surveyor shall proceed as for the liner of a new shaft.

The new liner or liner sections ordered shall be submitted to the Society’s factory inspection test (quality of material, hydraulic test), and liner lit and good condition (no defects) shall be checked alter machining.

When renewal of the liner is not necessary and it is difficult or impossible to change only a section of it without removing the part of liner in good condition, it is often easier to have the damaged part re-metallic.

Some yards can replace the damaged liner portion by two hail shells fitted on to the shaft and welded together longitudinally. Any such repair shall be submitted to Technical Office for approval.

1.6.4 Ribbing in way of the stern-gland

Ribbing requires the complete - or part renewal of the liner only if thickness inside of grooves is inferior to hall the regulation thickness. When ribbing is slight, however, i.e. when depth of grooves is 3 mm, liner shall be machined over the whole length in way of aft stuffing box.
Slight ribbing needs only to be filed down or smoothed away with emery cloth.