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Propeller Tolerances and Inspection
Chapter 25
Propeller Tolerances and Inspection Chapter Outline 25.1 Propeller Tolerances 25.2 Propeller Inspection 25.2.1 Inspection During Manufacture and Initial Fitting
495 495
25.2.2 Inspection During Service References and Further Reading
499 500
496
Although part of the manufacturing process, the related subject of propeller tolerances and inspection deserves separate attention. This is because it is only by correctly specifying the tolerances and then checking these have been adhered to, that the intentions of the designer can be properly realized. Without proper attention to blade manufacturing tolerances many serious problems can be encountered in the service life of the propeller: for example, cavitation, power absorption, noise, fatigue failure, and so on.
25.1 PROPELLER TOLERANCES For general propeller design work the ISO specifications usually serve as the criteria for assessment. References 1 and 2 define the requirements for propellers greater than 2.5 m and between 0.80 and 2.5 m respectively. In certain cases, such as naval propellers, the purchasers of the propeller may impose their own particular tolerance specifications and methods of assessment: for example, the US Navy standard drawing method. In general tolerances are normally specified on the set of dimensions shown in Table 25.1. This is because they affect the performance of the propeller or adjacent components in some particular way. Table 25.1 deals only with geometric parameters, but a propeller should also be shown to meet both the required material chemical composition tolerances and the minimum mechanical properties. The latter are of course classification society requirements for those vessels built under survey. For all vessels, however, attention needs to be given to the actual characteristic of the material for strength and repair purposes and these are not generally represented by the cast test pieces. Of the geometric properties quoted in Table 25.1, each has some bearing on performance. It is, therefore, essential to understand the ways in which they influence the various propeller operational characteristics if the correct tolerance is
to be specified. Unfortunately, the importance of each characteristic requires particular consideration for a given design but, notwithstanding this, certain general conclusions can be drawn and these are shown in Table 25.2. In this table an attempt is made to distinguish between the primary and secondary effects of the various parameters specified in Table 25.1, but not necessarily in relation to the ISO requirements. As a consequence of the various effects detailed in Table 25.2, the designer and purchaser of the propeller need to determine what level of tolerance is required such that the propeller will be fit for the purpose for which it is intended. Indeed, one could specify the most stringent tolerance for every propeller; this, however, would be extremely wasteful in terms of additional costs of manufacture. The ISO specification defines four levels of tolerance: Classes S, 1, 2 and 3, these being in descending order of stringency. Again there is some latitude in deciding the correct tolerance level for a particular ship, but as a rough guide Table 25.3 has been prepared. Table 25.3 is generally self-explanatory and other ship types and the appropriate tolerance classes can be deduced from those given in the table. Of particular concern, however, are the small high-speed vessels such as patrol or chase boats. All too often, in the author’s experience, the subject of blade tolerances is completely neglected with these vessels leading to a host of cavitation related problems. Such vessels, by virtue of their speed, both in terms of ship and shaft rotational speeds, in association with a low static pressure head, should generally qualify the propeller for a Class S or 1 tolerance notation e sometimes more.
25.2 PROPELLER INSPECTION The inspection of propellers needs to be undertaken both during the manufacture of the propeller and also during its service life. In general the former is undertaken in the
Marine Propellers and Propulsion, Third Edition. http://dx.doi.org/10.1016/B978-0-08-097123-0.00025-3 Copyright Ó 2012 John Carlton. Published by Elsevier Ltd. All rights reserved.
495
496
Marine Propellers and Propulsion
TABLE 25.1 Normal Propeller Tolerances Specification Parameters Diameter Mean pitch Local section pitch Section thickness General section form (camber) Section chord length Blade form and relative location Leading edge form Rake and axial position Surface finish Static balance
relatively controlled conditions of the manufacturer’s works, whilst the latter frequently, but not always, takes place in a dock bottom. Both types of inspection are important: the former to ensure design compliance with the design and the latter to examine the propeller condition in service.
25.2.1 Inspection During Manufacture and Initial Fitting In the case of the fixed pitch propeller the inspection procedure is carried out with the aid of purpose-built
machines. Such machines, whether they be manually operated or part of a CAM system, are in general a variant of the drop height measurement system. With this system the measurements are either made on the cylindrical sections defining the propeller or, alternatively, at various points defining a matrix over the blade surface. In the former case direct comparison with the cylindrical design sections can be made, whereas with the latter, as this frequently requires an interpolation procedure to be invoked, may raise questions of the validity of the mathematical model defining the blade surface. In its most fundamental form the classical cylindrical measurement requires that the propeller be mounted in a gravitational or other known plane with a vertical pole, relative to the plane of mounting, erected on the shaft center line. To this pole is fixed a rotating arm which is free to rotate in a plane parallel to the plane on which the propeller is mounted; Figure 25.1. From this arm the various radii can be marked on the blade surface and drop heights to the blade surface measured at known intervals along the chord length. By undertaking this exercise on both surfaces of the propeller the blade section shape can be compared to the original design section. Whilst this is the basis of the measurement system, many refinements aimed at improving accuracy have been incorporated by manufacturers, each having their own version of the system. The area which causes most concern is the detail of the leading edge. In a great many cases this is checked with the aid of a template (References 1 and 2). However, there is
TABLE 25.2 Principal Effects of the Various Propeller Geometric Variations Parameter
Primary Effect
Secondary Effect
Diameter
Power absorption
e
Mean pitch
Power absorption
Cavitation extent
Local section pitch
Cavitation inception and extent
Power absorption
Section thickness
Cavitation inception, blade strength
Power absorption
General section form (camber)
Power absorption, cavitation inception
Blade strength
Section chord length
Cavitation inception
Blade strength power absorption
Blade form and relative location (excluding leading edge)
Generally small effects on cavitation inception and shaft vibratory forces at frequencies dependent on wake harmonics and blade irregularities
e
Leading edge form
Critical to cavitation inception
e
Rake and axial position
Minor mechanical vibratory forms
e
Surface finish
Blade section drag and hence power absorption
e
Static balance
Shaft vibratory loads
e
Chapter | 25
Propeller Tolerances and Inspection
497
TABLE 25.3 Typical Tolerances for Certain Ship Types ISO Tolerance
Typical Ships Where Tolerance Might Apply
S
Naval vessels* (e.g. frigates, destroyers, submarines, etc.). High-speed craft with a speed greater than 25 knots; research vessels; certain special purpose merchant vessels where nose or vibration is of paramount importance (e.g. cruise vessels, high-grade ferries).
1
General merchant vessels; deep sea trawlers; tugs, ferries, naval auxiliaries.
2
Low-power, low-speed craft, typically inshore fishing vessels, work boats, etc.
3
As for Class 2.
*Naval vessels are often specified on an ‘ISO S Class Plus’ basis.
also considerable use of optical methods for leading and trailing edge inspection: this is particularly true in the case of model propeller manufacture for model testing purposes where fine contol over dimensional tolerances is essential. The current trend in manufacturing tolerance checking is to progress towards the introduction of electronic and laser-based techniques. These developments embrace electronic pitchometers, numerically controlled geometric inspection through to fully integrated design, manufacturing and inspection capabilities, outlined in Chapter 20, and the use of laser measurement techniques which exhibit very fine accuracy in either their fixed or portable forms. In all cases, however, it is of fundamental importance to define a sufficient set of inspection points in order to fully define the actual blade surface adequately so as to act as a basis of comparison with the design specification. In addition to the blade profile tolerances, in the case of fixed pitch propellers, rigorous inspection needs to be given to the bore of the boss. The fitting of the propeller to the shaft requires considerable attention and the requirements
FIGURE 25.1 Mechanical pitch measurement principle.
for this are governed by the classification societies. For keyed propellers a satisfactory fit between the propeller and the shaft should show a light overall marking of the cone surface of the shaft taper with a tendency towards heavier marking in way of the larger diameter of the cone face. When conducting these inspections the final fit to the cone should be made with the key in place. In some cases the propeller is offered up to a shaft mandrel in the manufacturer’s works so that the proper degree of face contact can be developed as required by the classification society rules. In cases where hand fitting is required this must be done by scraping the bore of the propeller; it should never be done by filing of the shaft cone. With regard to the axial push-up required, Table 25.4 gives some typical guidance values for a shaft having a cone taper of 1 in 12. In this table Ds is the diameter of the shaft at the top of the cone and the axial push-up is measured from a reliable and stable zero mark obtained from the initial bedding of the propeller to the shaft. In cases where hydraulic nuts are used, great care needs to be exercised to ensure that the hub is not overstressed in way of the keyway and that the appropriate classification rule requirements are adhered to. In the case of the keyed propeller it is of the utmost importance to prevent the ingress of sea water into the cone and as a consequence inspection needs to be particularly rigorous in this area. When the sealing arrangement comprises a rubber ring completely enclosed in a recess in the propeller boss, ample provision must be made for the rubber to displace itself properly to form a good seal. Alternatively, if an oil gland is fitted the following points should be carefully considered: 1. To ensure that rubber rings for forming the seal between the flange of the oil gland sleeve and the propeller boss are of the correct size and properly supported in way of the propeller keyway. 2. The fair water cones protecting propeller nuts and the flanges of sleeves of oil glands should be machined
498
Marine Propellers and Propulsion
TABLE 25.4 Typical Axial Push-up Values for Copper Alloy Propellers
3.
4. 5. 6.
Propeller Material
Axial Push-Ups
Aluminium bronage
0.006Ds
High-tensile brass
0.005Ds
smooth and fitted with efficient joints at their connection to propellers. Drilling holes through the propeller boss should be discouraged but when these are essential to the design special attention needs to be paid to the efficient plugging of the holes. The arrangement for locking all screwed components should be verified. The propeller boss should be provided with adequate radius at the large end of the bore. When the design of the oil gland attachment to the propeller is similar to that shown in Figure 25.2, it is good practice to subject the propeller boss to a lowpressure air test, checking all possible sources of leakage with a soapy water solution in order to prove tightness.
When a keyless propeller is fitted to the shaft the inspector should pay particular attention to ensuring that the design and approved interference fit is attained. As a prerequisite for this procedure the inspector needs to know the start point load to be applied and the axial ‘push-up’ required. These should normally be supplied at two temperatures, typically 0 C and 35 C, to allow
FIGURE 25.2 Propeller shaft assembly.
interpolation between the two values to take place to cater for the actual fitting conditions. The inspector should carefully examine the final marking of the screw shaft cone fit; this should show a generally mottled pattern over the entire surface with harder marking at the large end of the cone. Two basic techniques are employed to fit keyless propellers: the dry press-fit or the oil injection method. With both methods the propeller is pushed up the shaft cone by means of a hydraulic nut, but the fitting procedure differs somewhat between the two methods and the manufacturer’s fitting procedure must be rigidly adhered to in all cases. As a final stage in the inspection it is essential that the propeller, both working and spare, be hard-stamped with information of the form detailed below on the outside of the boss away from any stress raisers or fillets: 1. Oil injection type of fitting i. Start point load (tonnes) ii. Axial push-up at 0 C (mm) iii. Axial push-up at 35 C (mm) iv. Identification mark on associated screw shaft 2. Press fit type of fitting (dry) i. Start point load (tonnes) ii. Push-up load 0 C (tonnes) iii. Push-up load 35 C (tonnes) iv. Axial push load 0 C (mm) v. Axial push load 35 C (mm) vi. Identification mark on associated screw shaft With regard to the process of fitting the propeller to the shaft a good review of methods is given by Eames and Sinclair.3 Casting defects always occur in propellers. On occasions those outcropping on the surface of the propeller are concealed by the use of unauthorized local welding. When subsequently polished and on a newly manufactured propeller which has been kept in a workshop, the existence of small amounts of surface welding can be very difficult to spot with the naked eye. This type of welding process, when not fully authorized, is a dangerous practice from the propeller integrity point of view. Therefore, if any doubt exists about the processes that have been undertaken the propeller surfaces should be lightly etched with an appropriate solution. This, depending upon the etching solution used, will reveal the presence of any such actions in a longer or shorter time. However, if the propeller has been left outside of the manufacturer’s compound and subjected to rain over a period of a week or so the acids in the rain will naturally etch the propeller surfaces and reveal any weld processes that have taken place. Similarly with propellers that have been in service, the action of the sea water has much the same effect and the welding history will be seen in dry dock.
Chapter | 25
Propeller Tolerances and Inspection
499
FIGURE 25.3 Typical blade inspection diagram.
25.2.2 Inspection During Service In-service inspections are normally carried out for maintenance or survey reasons: to examine the propeller after suspected damage has take place or to check for fouling, or, alternatively, to form part of a survey of the ship. In the former case this may be carried out in the water by a diver if a superficial check is required, or in a dry dock for a more
detailed survey. As a general comment on in-water and out-of-water inspections it should be remembered that a commercial diver is usually a highly trained person but is not normally a propeller technologist and, therefore, can give only generalized engineering reports. Furthermore, in many instances, typically in the North Sea area, while looking at one part of the propeller the diver may not be able to see the rest of the propeller due to the clarity of the
500
water. Irrespective of water clarity, the in-water survey is greatly enhanced if the diver can talk, preferably with the aid of video techniques, to a propeller specialist while undertaking the survey, so that important details will not be missed and other less important features given undue weight. When a propeller is operating any small cracks or defects tend to collect salt deposits. As a consequence, before conducting a blade surface inspection in a dry dock aimed at identifying cracks, the surface should be lightly cleaned to remove marine growth and then washed with a 10 per cent concentration of a sulphuric acid in water to dissolve the salts. If the propeller is removed from the shaft for this examination, then this is extremely helpful to the inspector and considerably increases the chances of finding small defects. It is, however, completely pointless to attempt an examination designed to look for small cracks in water. To undertake a general propeller inspection it is a prerequisite to have an outline of the propeller which, although not needing to be absolutely correct in every geometric detail, must represent the main propeller features adequately. The outline should show both the face and back of the blades and have suitable cylindrical lines marked on it at say 0.9R, 0.8R, 0.6R, 0.4R Without such a diagram serious misrepresentations of information can occur. In the author’s experience the best blade outline to use for damage recording is the developed blade outline, since this tends to represent the blade most closely to the way an inspector observes it. In addition to a blade outline there should also be a consistent way of recording information to signify, for example, cavitation damage, bending, missing portions, marine growth, and so on. Figure 25.3 shows a typical diagram for inspection purposes and in addition to showing the location of the damages on the blade, typical dimensions of damage length, width and depth need to be recorded. These records need to be taken individually for each blade, on both the back and face of the propeller, so as to answer questions of the similarities of blade damage since a propeller may exhibit damage from different sources simultaneously. In the case of a classification society inspection the propeller is normally required to be removed from the tail shaft at each screw shaft survey. On these occasions particular attention should be paid to the roots of the blades for signs of cracking.
Marine Propellers and Propulsion
If a new propeller is to be installed, the accuracy of fit on the shaft cone should be tested with and without the key in place. Identification marks stamped on the propeller should be reported for record purposes and if the new propeller is substantially different from the old one, it should be recalled that the existing approval of torsional vibration characteristics may be affected by significant changes in the propeller design. It is particularly important to ensure that rubber rings between propeller bosses and the aft ends of liners are the correct size and so fitted that the shaft is protected from sea water. Failure in this respect is often found at the ends of keyways due to the fact that the top part of the key itself is not extended to provide a local bedding for the ring in way of the recess in the boss. In such cases it may be found practicable to weld an extension to the forward end of the key. It should also be recollected that water may enter the propeller boss at the aft end and attention should therefore be paid to this part of the assembly. Filling the recess between the aft end of the liner and the forward part of the propeller boss with grease, red lead or a similar substance is not in itself a satisfactory method of obtaining watertightness. Sealing rings in connection with approved type oil glands should be similarly checked. If an oil gland is fitted the various parts should be examined at each inspection and particular attention paid to the arrangement for preventing the ingress of water to the shaft cone. All oil glands, on reassembling, should be examined under pressure and shown to be tight. If the ship has a controllable pitch propeller, the working parts and control gear should be opened up sufficiently to enable the inspector to be satisfied of their condition. In the case of directional propellers, at each docking the propeller and fastenings should be examined as far as practicable and the maneuvering of the propeller blades should be tested.
REFERENCES AND FURTHER READING 1. ISO 484/1. Shipbuilding e Ship Screw Propellers e Manufacturing Tolerances e Part 1: Propellers of Diameter Greater than 2.50m, 1981. 2. ISO 484/2. Shipbuilding e Ship Screw Propellers e Manufacturing Tolerances e Part 2: Propellers of Diameter Between 0.80 and 2.50m Inclusive, 1981. 3. Eames CFW, Sinclair L. Methods of attaching a marine propeller to the tailshaft. Trans IMarE 1980;92.
Propeller Tolerances and Inspection Chapter Outline 25.1 Propeller Tolerances 25.2 Propeller Inspection 25.2.1 Inspection During Manufacture and Initial Fitting
495 495
25.2.2 Inspection During Service References and Further Reading
499 500
496
Although part of the manufacturing process, the related subject of propeller tolerances and inspection deserves separate attention. This is because it is only by correctly specifying the tolerances and then checking these have been adhered to, that the intentions of the designer can be properly realized. Without proper attention to blade manufacturing tolerances many serious problems can be encountered in the service life of the propeller: for example, cavitation, power absorption, noise, fatigue failure, and so on.
25.1 PROPELLER TOLERANCES For general propeller design work the ISO specifications usually serve as the criteria for assessment. References 1 and 2 define the requirements for propellers greater than 2.5 m and between 0.80 and 2.5 m respectively. In certain cases, such as naval propellers, the purchasers of the propeller may impose their own particular tolerance specifications and methods of assessment: for example, the US Navy standard drawing method. In general tolerances are normally specified on the set of dimensions shown in Table 25.1. This is because they affect the performance of the propeller or adjacent components in some particular way. Table 25.1 deals only with geometric parameters, but a propeller should also be shown to meet both the required material chemical composition tolerances and the minimum mechanical properties. The latter are of course classification society requirements for those vessels built under survey. For all vessels, however, attention needs to be given to the actual characteristic of the material for strength and repair purposes and these are not generally represented by the cast test pieces. Of the geometric properties quoted in Table 25.1, each has some bearing on performance. It is, therefore, essential to understand the ways in which they influence the various propeller operational characteristics if the correct tolerance is
to be specified. Unfortunately, the importance of each characteristic requires particular consideration for a given design but, notwithstanding this, certain general conclusions can be drawn and these are shown in Table 25.2. In this table an attempt is made to distinguish between the primary and secondary effects of the various parameters specified in Table 25.1, but not necessarily in relation to the ISO requirements. As a consequence of the various effects detailed in Table 25.2, the designer and purchaser of the propeller need to determine what level of tolerance is required such that the propeller will be fit for the purpose for which it is intended. Indeed, one could specify the most stringent tolerance for every propeller; this, however, would be extremely wasteful in terms of additional costs of manufacture. The ISO specification defines four levels of tolerance: Classes S, 1, 2 and 3, these being in descending order of stringency. Again there is some latitude in deciding the correct tolerance level for a particular ship, but as a rough guide Table 25.3 has been prepared. Table 25.3 is generally self-explanatory and other ship types and the appropriate tolerance classes can be deduced from those given in the table. Of particular concern, however, are the small high-speed vessels such as patrol or chase boats. All too often, in the author’s experience, the subject of blade tolerances is completely neglected with these vessels leading to a host of cavitation related problems. Such vessels, by virtue of their speed, both in terms of ship and shaft rotational speeds, in association with a low static pressure head, should generally qualify the propeller for a Class S or 1 tolerance notation e sometimes more.
25.2 PROPELLER INSPECTION The inspection of propellers needs to be undertaken both during the manufacture of the propeller and also during its service life. In general the former is undertaken in the
Marine Propellers and Propulsion, Third Edition. http://dx.doi.org/10.1016/B978-0-08-097123-0.00025-3 Copyright Ó 2012 John Carlton. Published by Elsevier Ltd. All rights reserved.
495
496
Marine Propellers and Propulsion
TABLE 25.1 Normal Propeller Tolerances Specification Parameters Diameter Mean pitch Local section pitch Section thickness General section form (camber) Section chord length Blade form and relative location Leading edge form Rake and axial position Surface finish Static balance
relatively controlled conditions of the manufacturer’s works, whilst the latter frequently, but not always, takes place in a dock bottom. Both types of inspection are important: the former to ensure design compliance with the design and the latter to examine the propeller condition in service.
25.2.1 Inspection During Manufacture and Initial Fitting In the case of the fixed pitch propeller the inspection procedure is carried out with the aid of purpose-built
machines. Such machines, whether they be manually operated or part of a CAM system, are in general a variant of the drop height measurement system. With this system the measurements are either made on the cylindrical sections defining the propeller or, alternatively, at various points defining a matrix over the blade surface. In the former case direct comparison with the cylindrical design sections can be made, whereas with the latter, as this frequently requires an interpolation procedure to be invoked, may raise questions of the validity of the mathematical model defining the blade surface. In its most fundamental form the classical cylindrical measurement requires that the propeller be mounted in a gravitational or other known plane with a vertical pole, relative to the plane of mounting, erected on the shaft center line. To this pole is fixed a rotating arm which is free to rotate in a plane parallel to the plane on which the propeller is mounted; Figure 25.1. From this arm the various radii can be marked on the blade surface and drop heights to the blade surface measured at known intervals along the chord length. By undertaking this exercise on both surfaces of the propeller the blade section shape can be compared to the original design section. Whilst this is the basis of the measurement system, many refinements aimed at improving accuracy have been incorporated by manufacturers, each having their own version of the system. The area which causes most concern is the detail of the leading edge. In a great many cases this is checked with the aid of a template (References 1 and 2). However, there is
TABLE 25.2 Principal Effects of the Various Propeller Geometric Variations Parameter
Primary Effect
Secondary Effect
Diameter
Power absorption
e
Mean pitch
Power absorption
Cavitation extent
Local section pitch
Cavitation inception and extent
Power absorption
Section thickness
Cavitation inception, blade strength
Power absorption
General section form (camber)
Power absorption, cavitation inception
Blade strength
Section chord length
Cavitation inception
Blade strength power absorption
Blade form and relative location (excluding leading edge)
Generally small effects on cavitation inception and shaft vibratory forces at frequencies dependent on wake harmonics and blade irregularities
e
Leading edge form
Critical to cavitation inception
e
Rake and axial position
Minor mechanical vibratory forms
e
Surface finish
Blade section drag and hence power absorption
e
Static balance
Shaft vibratory loads
e
Chapter | 25
Propeller Tolerances and Inspection
497
TABLE 25.3 Typical Tolerances for Certain Ship Types ISO Tolerance
Typical Ships Where Tolerance Might Apply
S
Naval vessels* (e.g. frigates, destroyers, submarines, etc.). High-speed craft with a speed greater than 25 knots; research vessels; certain special purpose merchant vessels where nose or vibration is of paramount importance (e.g. cruise vessels, high-grade ferries).
1
General merchant vessels; deep sea trawlers; tugs, ferries, naval auxiliaries.
2
Low-power, low-speed craft, typically inshore fishing vessels, work boats, etc.
3
As for Class 2.
*Naval vessels are often specified on an ‘ISO S Class Plus’ basis.
also considerable use of optical methods for leading and trailing edge inspection: this is particularly true in the case of model propeller manufacture for model testing purposes where fine contol over dimensional tolerances is essential. The current trend in manufacturing tolerance checking is to progress towards the introduction of electronic and laser-based techniques. These developments embrace electronic pitchometers, numerically controlled geometric inspection through to fully integrated design, manufacturing and inspection capabilities, outlined in Chapter 20, and the use of laser measurement techniques which exhibit very fine accuracy in either their fixed or portable forms. In all cases, however, it is of fundamental importance to define a sufficient set of inspection points in order to fully define the actual blade surface adequately so as to act as a basis of comparison with the design specification. In addition to the blade profile tolerances, in the case of fixed pitch propellers, rigorous inspection needs to be given to the bore of the boss. The fitting of the propeller to the shaft requires considerable attention and the requirements
FIGURE 25.1 Mechanical pitch measurement principle.
for this are governed by the classification societies. For keyed propellers a satisfactory fit between the propeller and the shaft should show a light overall marking of the cone surface of the shaft taper with a tendency towards heavier marking in way of the larger diameter of the cone face. When conducting these inspections the final fit to the cone should be made with the key in place. In some cases the propeller is offered up to a shaft mandrel in the manufacturer’s works so that the proper degree of face contact can be developed as required by the classification society rules. In cases where hand fitting is required this must be done by scraping the bore of the propeller; it should never be done by filing of the shaft cone. With regard to the axial push-up required, Table 25.4 gives some typical guidance values for a shaft having a cone taper of 1 in 12. In this table Ds is the diameter of the shaft at the top of the cone and the axial push-up is measured from a reliable and stable zero mark obtained from the initial bedding of the propeller to the shaft. In cases where hydraulic nuts are used, great care needs to be exercised to ensure that the hub is not overstressed in way of the keyway and that the appropriate classification rule requirements are adhered to. In the case of the keyed propeller it is of the utmost importance to prevent the ingress of sea water into the cone and as a consequence inspection needs to be particularly rigorous in this area. When the sealing arrangement comprises a rubber ring completely enclosed in a recess in the propeller boss, ample provision must be made for the rubber to displace itself properly to form a good seal. Alternatively, if an oil gland is fitted the following points should be carefully considered: 1. To ensure that rubber rings for forming the seal between the flange of the oil gland sleeve and the propeller boss are of the correct size and properly supported in way of the propeller keyway. 2. The fair water cones protecting propeller nuts and the flanges of sleeves of oil glands should be machined
498
Marine Propellers and Propulsion
TABLE 25.4 Typical Axial Push-up Values for Copper Alloy Propellers
3.
4. 5. 6.
Propeller Material
Axial Push-Ups
Aluminium bronage
0.006Ds
High-tensile brass
0.005Ds
smooth and fitted with efficient joints at their connection to propellers. Drilling holes through the propeller boss should be discouraged but when these are essential to the design special attention needs to be paid to the efficient plugging of the holes. The arrangement for locking all screwed components should be verified. The propeller boss should be provided with adequate radius at the large end of the bore. When the design of the oil gland attachment to the propeller is similar to that shown in Figure 25.2, it is good practice to subject the propeller boss to a lowpressure air test, checking all possible sources of leakage with a soapy water solution in order to prove tightness.
When a keyless propeller is fitted to the shaft the inspector should pay particular attention to ensuring that the design and approved interference fit is attained. As a prerequisite for this procedure the inspector needs to know the start point load to be applied and the axial ‘push-up’ required. These should normally be supplied at two temperatures, typically 0 C and 35 C, to allow
FIGURE 25.2 Propeller shaft assembly.
interpolation between the two values to take place to cater for the actual fitting conditions. The inspector should carefully examine the final marking of the screw shaft cone fit; this should show a generally mottled pattern over the entire surface with harder marking at the large end of the cone. Two basic techniques are employed to fit keyless propellers: the dry press-fit or the oil injection method. With both methods the propeller is pushed up the shaft cone by means of a hydraulic nut, but the fitting procedure differs somewhat between the two methods and the manufacturer’s fitting procedure must be rigidly adhered to in all cases. As a final stage in the inspection it is essential that the propeller, both working and spare, be hard-stamped with information of the form detailed below on the outside of the boss away from any stress raisers or fillets: 1. Oil injection type of fitting i. Start point load (tonnes) ii. Axial push-up at 0 C (mm) iii. Axial push-up at 35 C (mm) iv. Identification mark on associated screw shaft 2. Press fit type of fitting (dry) i. Start point load (tonnes) ii. Push-up load 0 C (tonnes) iii. Push-up load 35 C (tonnes) iv. Axial push load 0 C (mm) v. Axial push load 35 C (mm) vi. Identification mark on associated screw shaft With regard to the process of fitting the propeller to the shaft a good review of methods is given by Eames and Sinclair.3 Casting defects always occur in propellers. On occasions those outcropping on the surface of the propeller are concealed by the use of unauthorized local welding. When subsequently polished and on a newly manufactured propeller which has been kept in a workshop, the existence of small amounts of surface welding can be very difficult to spot with the naked eye. This type of welding process, when not fully authorized, is a dangerous practice from the propeller integrity point of view. Therefore, if any doubt exists about the processes that have been undertaken the propeller surfaces should be lightly etched with an appropriate solution. This, depending upon the etching solution used, will reveal the presence of any such actions in a longer or shorter time. However, if the propeller has been left outside of the manufacturer’s compound and subjected to rain over a period of a week or so the acids in the rain will naturally etch the propeller surfaces and reveal any weld processes that have taken place. Similarly with propellers that have been in service, the action of the sea water has much the same effect and the welding history will be seen in dry dock.
Chapter | 25
Propeller Tolerances and Inspection
499
FIGURE 25.3 Typical blade inspection diagram.
25.2.2 Inspection During Service In-service inspections are normally carried out for maintenance or survey reasons: to examine the propeller after suspected damage has take place or to check for fouling, or, alternatively, to form part of a survey of the ship. In the former case this may be carried out in the water by a diver if a superficial check is required, or in a dry dock for a more
detailed survey. As a general comment on in-water and out-of-water inspections it should be remembered that a commercial diver is usually a highly trained person but is not normally a propeller technologist and, therefore, can give only generalized engineering reports. Furthermore, in many instances, typically in the North Sea area, while looking at one part of the propeller the diver may not be able to see the rest of the propeller due to the clarity of the
500
water. Irrespective of water clarity, the in-water survey is greatly enhanced if the diver can talk, preferably with the aid of video techniques, to a propeller specialist while undertaking the survey, so that important details will not be missed and other less important features given undue weight. When a propeller is operating any small cracks or defects tend to collect salt deposits. As a consequence, before conducting a blade surface inspection in a dry dock aimed at identifying cracks, the surface should be lightly cleaned to remove marine growth and then washed with a 10 per cent concentration of a sulphuric acid in water to dissolve the salts. If the propeller is removed from the shaft for this examination, then this is extremely helpful to the inspector and considerably increases the chances of finding small defects. It is, however, completely pointless to attempt an examination designed to look for small cracks in water. To undertake a general propeller inspection it is a prerequisite to have an outline of the propeller which, although not needing to be absolutely correct in every geometric detail, must represent the main propeller features adequately. The outline should show both the face and back of the blades and have suitable cylindrical lines marked on it at say 0.9R, 0.8R, 0.6R, 0.4R Without such a diagram serious misrepresentations of information can occur. In the author’s experience the best blade outline to use for damage recording is the developed blade outline, since this tends to represent the blade most closely to the way an inspector observes it. In addition to a blade outline there should also be a consistent way of recording information to signify, for example, cavitation damage, bending, missing portions, marine growth, and so on. Figure 25.3 shows a typical diagram for inspection purposes and in addition to showing the location of the damages on the blade, typical dimensions of damage length, width and depth need to be recorded. These records need to be taken individually for each blade, on both the back and face of the propeller, so as to answer questions of the similarities of blade damage since a propeller may exhibit damage from different sources simultaneously. In the case of a classification society inspection the propeller is normally required to be removed from the tail shaft at each screw shaft survey. On these occasions particular attention should be paid to the roots of the blades for signs of cracking.
Marine Propellers and Propulsion
If a new propeller is to be installed, the accuracy of fit on the shaft cone should be tested with and without the key in place. Identification marks stamped on the propeller should be reported for record purposes and if the new propeller is substantially different from the old one, it should be recalled that the existing approval of torsional vibration characteristics may be affected by significant changes in the propeller design. It is particularly important to ensure that rubber rings between propeller bosses and the aft ends of liners are the correct size and so fitted that the shaft is protected from sea water. Failure in this respect is often found at the ends of keyways due to the fact that the top part of the key itself is not extended to provide a local bedding for the ring in way of the recess in the boss. In such cases it may be found practicable to weld an extension to the forward end of the key. It should also be recollected that water may enter the propeller boss at the aft end and attention should therefore be paid to this part of the assembly. Filling the recess between the aft end of the liner and the forward part of the propeller boss with grease, red lead or a similar substance is not in itself a satisfactory method of obtaining watertightness. Sealing rings in connection with approved type oil glands should be similarly checked. If an oil gland is fitted the various parts should be examined at each inspection and particular attention paid to the arrangement for preventing the ingress of water to the shaft cone. All oil glands, on reassembling, should be examined under pressure and shown to be tight. If the ship has a controllable pitch propeller, the working parts and control gear should be opened up sufficiently to enable the inspector to be satisfied of their condition. In the case of directional propellers, at each docking the propeller and fastenings should be examined as far as practicable and the maneuvering of the propeller blades should be tested.
REFERENCES AND FURTHER READING 1. ISO 484/1. Shipbuilding e Ship Screw Propellers e Manufacturing Tolerances e Part 1: Propellers of Diameter Greater than 2.50m, 1981. 2. ISO 484/2. Shipbuilding e Ship Screw Propellers e Manufacturing Tolerances e Part 2: Propellers of Diameter Between 0.80 and 2.50m Inclusive, 1981. 3. Eames CFW, Sinclair L. Methods of attaching a marine propeller to the tailshaft. Trans IMarE 1980;92.