Technical field

[001] The present invention relates to a method of manufacturing a propeller blade assembly in accordance with the preamble of claim 1. Specifically the present invention relates to the manufacture of propellers for marine vessels.

Background art

[002] Marine vessels are most often provided with a propulsion arrangement having a propeller attached on a drive shaft by means of a hub. The propeller has a number of blades fastened or integrated to the hub. Such propellers are usually made of metal material whereby they are relatively heavy and the design freedom of the propeller blade geometry is limited by the respective material properties. For several decades attempts have been made to replace these metal propellers with ones manufactured of other materials, like for instance fibre reinforced composite materials. A problem area in the production of propeller blade assemblies of composite material may be found in the attaching of the composite blade section to the hub or drive shaft, either directly or indirectly. Indirect attachment requires the use of a metal root block assem- bly for fastening the blade assembly to the hub or the drive shaft. Thus, the fastening of a propeller blade on the hub or on the drive shaft takes place by first attaching each propeller blade of composite material to a separate metallic root block assembly to form a propeller blade assembly and then fastening each blade assembly by means of the separate metallic root block assembly on the propeller hub or directly on the drive shaft. This indirect method offers the advantage of being able to replace a separate propeller blade assembly for example in case of damage and to provide a suitable attachment method to different types of propellers like controllable pitch propellers.

[003] US 3,731 ,360 discusses a method of making a composite blade assembly of a composite blade and one or more root blocks, i.e. a root block assembly, in a single compacting and bonding operation to assure a secure attachment of the root blocks to the fiber layers and the fiber layers together. The blade construction is started by as- sembling a plurality of layers consisting of high strength fibers provided between a me- tallic backing sheet and sprayed on metallic coating in a matrix in stacked relation. The thus formed layers differ in dimension in order to produce the desired finished dimen- sion of the blade. In other words, the layers are cut to the desired dimension prior to the stacking operation. The blade layers may be assembled up as half-sections where there is a main center sheet of aluminum on the opposite sides of which certain of the plies are stacked and to which these plies are tack welded. There is a midsection sheet to which additional plies are attached and then an outer ply completes the assembly. The elements on each side of the center form blade half-sections. [004] After the plies are assembled, in contact within one another rather than spaced apart, to form a composite blade assembly, a wedge is positioned substantially midway of the assemblage of plies as against one side of the center sheet at the root end to spread the plies apart at this point for a more secure root attachment. Root blocks, i.e. the root block assembly, are then positioned on opposite sides of the assemblage of plies adjacent the same edge as the wedge, these blocks having inner surfaces shaped to conform to the finished contours of the compacted fibers positioned over the wedge. The entire assemblage is then positioned between suitable dies having opposed cavi- ties therein conforming to the finished shape of the opposed surfaces of the blade and also forming in conjunction with one another a cavity that is the complete shape of the blade with the root blocks applied thereto. Pressure and heat are then applied to the die to compact the fiber layers together and to cause the matrix surrounding the fibers to flow and fill all of the voids in the blade. The effect of this heat and pressure is further to bond the fibers and the matrix all together into a single compact entity and to securely attach the root blocks over their entire surfaces to the fiber material. [005] US-4,040,770 discusses a composite blade assembly in which the composite filament laminates of a composite blade are splayed at the foot part of the blade to receive solid metallic inserts therebetween. A transition material having a modulus of elasticity greater than that of the inserts and less than that of the laminates is bonded therebetween to more uniformly distribute the blade loadings into the root region. In other words, the foot part of the blade forming the dovetail is divided into several sec- tions by means of solid metallic wedges. The purpose is, naturally, to keep the dovetail foot of the blade in its outwardly tapering slot in the root block/s. [006] US-B2-8,272.841 discusses the manufacture of a propeller blade from carbon- fibre and glass-fibre reinforced epoxy resin with a central polyurethane foam core. The reinforcing fibres are laid up in appropriately shaped layers with the core to pre-form the propeller blade shape, and may be pre-impregnated with resin, or the fibre layers may be "dry" and the resin may be injected into the blade structure at a later stage (resin-transfer moulding, or RTM). The blade shape is formed by placing the fibre/core pre-form and the blade metal root into a mould with a cavity of the required blade shape, and applying heat and pressure to the mould while injecting resin into the cavity, in the case of RTM, or heat and pressure only in the case of pre-impregnated fibre layers. When preparing the dovetail foot part of the composite blade inside the blade metal root, before curing the composite with heat and pressure, the layers of reinforcing fibers inside the blade metal root and around the core are divided into packs of fibre layers such that wedge-shaped cavities are left between the packs and, thus, the foot part of the blade is spread into a dovetail- shaped element. Thereafter, in addition to filling the entire blade mould with resin, each wedge shaped cavity is filled with an appropriate wedge shaped fibre preform, and a final curing of the propeller blade is performed.

[007] Above discussed composite propeller blade assemblies require some sort of inserts/wedges to be applied already in the curing phase of the composite. The shape of the inserts/wedges need to be relatively accurate to avoid voids or areas with insuf- ficient strength after the curing phase. In other words, to reach a proper fiber volume fraction (approximately 60%), i.e. a desired degree of consolidation, the solid inserts have to be designed with high accuracy. The highly accurate inserts/wedges add to the costs of the final product, and still the fibre/resin ratio in all parts of the foot part may not be as good as it should.

[008] Thus, a problem worth consideration in the above discussed composite propel- ler blade assemblies may be found in their manufacture as the resin that has impreg- nated the layers of reinforcing fibres and thereby filled the cavity between the mould surfaces should cover about 40% of the volume of the article to be produced and be evenly distributed all over the layers of the reinforcing fibres. In case the amount of resin is higher, the strength properties of the product are reduced, and if the amount of resin is lower, there is a high risk that air is left in the reinforcing layers, i.e. a part of the fibres remains dry, whereby all the fibres are not bonded to one another, and again, the strength of the product is reduced. In other words, the layers of the composite ma- terials should be compressed to reach an optimum fiber volume fraction (approximately 60%) whereby an optimal or full strength of the composite is resulted.

[009] Another problem when manufacturing articles of composite materials is the ten- dency of the resin to shrink when curing. Such shrinking may result in the loosening of the bond between the root block assembly (consisting of one or more root blocks) and the composite material or between the wedge/s and the composite material. In a less severe case the bond between the metal surfaces and the composite material is sub- jected to such a tensile stress that easily makes the bond break when the blade as- sembly is subjected to a bending stress when running the propeller. The loosening of the bond results in minor movement of the composite blade inside the root block as- sembly whereby the composite material starts wearing and soon results in visible vi- bration and increased wear of the composite blade.

[0010] Yet another problem found in at least some prior art arrangements for fastening a composite propeller blade to its root block assembly is the use of solid metallic wedge-shaped inserts to spread the foot part of the blade to a dovetail shape. The metallic inserts have a surface to which the composite matrix resin does not easily attach, whereby any stress subjected to the wedge - matrix interface may break the bond and reduce the overall strength of the fastening of the blade to the root block/s. [0011] An object of the invention is to solve at least one of the problems of prior art composite propeller blades.

[0012] Another object of the present invention is to provide a method of manufacturing a propeller blade assembly such that the costs involved in the manufacture of a pro- peller blade are reduced without compromising the strength properties of the blade. [0013] Yet another object of the present invention is to provide a method of manufac- turing a propeller blade assembly such that the strength properties of the blade are improved.

[0014] A further object of the present invention is to provide a method of manufacturing a propeller blade assembly such that the composite is consolidated or compressed for the curing such that the fiber volume fraction is about 60%. [0015] A still further object of the present invention is to provide a method of manufac- turing a propeller blade assembly such that the reliability of the propeller blade assem- bly is improved.

Disclosure of the Invention

[0016] At least some objects of the present invention can be met substantially as is disclosed in the independent claim and in the other claims describing more details of different embodiments of the present invention.

[0017] According to an embodiment of the present invention the method of manufac- turing a propeller blade assembly comprises the steps of

a) preparing a set of fibrous reinforcement layers,

b) laying the set of reinforcement layers one on top of another for forming at least one stack of reinforcement layers to have a general shape of a propel- ler blade,

c) extending one end of the stack of reinforcement layers through a root block assembly for forming a foot end of the propeller blade,

d) dividing the reinforcement layers at the foot end to at least two packs of reinforcement layers for forming at least one wedge shaped cavity at the foot end of the propeller blade,

e) enclosing the at least one stack and the at least two packs of reinforcement layers inside a cavity formed by a first closing element, the root block as- sembly and a second closing element,

f) curing resin impregnated into the fibrous reinforcement layers ,

the method further comprising, after step (d), providing the at least one wedge shaped cavity at the foot end of the blade with at least one compressive mould insert, and subjecting the at least two packs of reinforcement layers to compression by means of the at least one compressive mould insert.

[0018] By means of the method of the present invention a composite propeller blade assembly for a marine vessel may be manufactured with considerably reduced costs when compared to the manufacture of prior art composite propeller blades. This is mainly due to the use of a flexible or inflatable mould insert, i.e. a compressive mould insert in place of a solid metal mould requiring accurate production. The method of the present invention also offers a chance to visually control the production quality of the propeller blade better between various production phases so that the strength proper- ties and the reliability of the propeller blade assembly is improved.

[0019] The exemplary embodiments of the invention presented in this patent applica- tion are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.

Brief Description of Drawings

[0020] In the following, the present invention will be described in more detail with ref- erence to the accompanying exemplary, schematic drawings, in which

Figure 1 illustrates a prior art propeller of a marine vessel,

Figure 2 illustrates a root block assembly in accordance with a first preferred embodi- ment of the present invention,

Figure 3 illustrates a root block assembly in accordance with a second preferred em- bodiment of the present invention,

Figure 4 illustrates, in its production stage, a partial section of a propeller blade in ac- cordance with the first and a third preferred embodiment of the present invention,

Figure 5 illustrates a partial section of a finalized propeller blade in accordance with the first and the third preferred embodiments of the present invention, and

Figure 6 illustrates, in its production stage, a partial section of a propeller blade in ac- cordance with the first and a fourth preferred embodiment of the present invention.

Detailed Description of Drawings

[0021 ] Figure 1 depicts schematically a prior art propeller of a marine vessel. The pro- peller 2 is formed of a hub 4 and four blades 6. The propeller 2 is fastened on the drive shaft 8 via the hub 4. The propeller may be a unitary cast piece of metal, which is then machined to its final dimensions and shrink fitted on the propeller shaft. The propeller may also be formed of a hub on which a desired number of separately manufac- tured/cast propeller blades is fastened. Each blade may, in fact, be a blade assembly formed of a blade having a foot part and a root block assembly consisting of one or more root blocks attached to the foot part of the blade. One or more blade assemblies are bolted on the hub or on the drive shaft, to form a propeller on the shaft 8, by using the root block assemblies. The use of the root block assembly is especially common in connection with propeller blade assemblies made of composite materials. [0022] Figure 2 illustrates a root block assembly 12 in accordance with a first preferred embodiment of the present invention. The root block assembly 12 may be a single metal part having a central opening 14 or formed of at least two separate parts, which when assembled together leave a corresponding central opening 14 for the foot part of the blade. The root block assembly 12 has a bottom surface 16, which lies against the hub or drive shaft when the blade assembly is fastened thereon. For the fastening the root block assembly 12 is provided with a number of holes 18 for fastening bolts or screws. The central opening 14 in the root block assembly 12 has a round, elliptical or oblong cross section and the opening 14 tapers away from the bottom surface 16 such that a dovetail shaped cavity 20 is left inside the root block assembly 12. Preferably, but not necessarily, the taper of the central opening 14 is curved in such a way that the walls 22 of the opening 14 turn into axial direction when reaching the outer rim 24 of the root block assembly 12. In practice such means that there is no need to make any sharp bend in the reinforcements when entering the root block assembly. However, as the inclination of the walls 22 of the opening is, preferably, but not necessarily, of the order of 6 to 15 degrees from the axial direction, the bend, even if the walls 22 are linear, is not too sharp.

[0023] Figure 3 illustrates a root block assembly 112 in accordance with a second pre- ferred embodiment of the present invention. Here, the only difference to the root block assembly of Figure 2 is that the walls 122 of the central opening 14 are linear. Here, the inclination of the walls 122 is given as angle b, which, as mentioned already above, is of the order of 6 to 15 degrees.

[0024] Figure 4 illustrates a partial longitudinal cross-section of a propeller blade as- sembly in accordance with a third preferred embodiment of the present invention. The propeller blade assembly of the present invention comprises a composite blade 30 and a root block assembly 12 formed of one or more root blocks by means of which the blade assembly is to be fastened to the propeller hub or to the drive shaft. The compo- site blade 30 is formed of numerous layers 32 of fibrous reinforcements, which may be either woven or non-woven fibrous reinforcements, and have either unidirectional or multiaxial structure. The material for the fibrous reinforcement may be at least one of glass, aramid and carbon, just to name a few options, without, however, limiting the options to the listed ones. The blade has a foot part 34, i.e. a part located inside the root block assembly 12, via which the blade 30 is fastened to the root block assembly 12. At the foot part 34 of the blade 30 the layers 32 of fibrous reinforcements are divided into at least two packs 36’ and 36” of reinforcements to form at least one wedge shaped cavity 38. The at least two packs of reinforcements 36’ and 36” have, preferably, but not necessarily, equal thickness.

[0025] In accordance with Figure 4, when manufacturing the blade assembly, the first step is to manufacture a mould plug 40 to have the shape of at least a part of the propeller blade 30 to be manufactured but in a smaller scale. Next, a suitable number of layers 32 of fibrous reinforcements are prepared, i.e. cut to appropriate shapes, and laid on both sides of the mould plug 40 such that stacks of reinforcement layers having a desired thickness is formed, and the blade has substantially correct dimensions. At least a part of the reinforcement layers 32 may be inserted inside the root block as- sembly 12 while forming the stacks of reinforcement layers on the mould plug 40, or a root block assembly 12 is either pushed on or assembled round the foot end of the blade after the stacks of reinforcement layers are formed such that the ends of the stacks of the reinforcement layers 32 are left in the central opening of the root block assembly 12. Next, a first closing element 42, preferably a bag 42, is arranged round the stacks of reinforcement layers 32 and the mould plug 40 such that the bag is sealed against the outer surface of the root block assembly 12. Then, the ends of the layers of the fibrous reinforcements inside the root block assembly 12 are divided into at least two packs 36’ and 36” of reinforcement layers having, preferably, but not necessarily, equal thickness to form at least one wedge-shaped cavity 38. Next, at least one inflat- able or flexible mould insert 44 is provided in the at least one wedge-shaped cavity 38, and a second closing element 46, to be specific an end mould bag 46 is provided to cover the end surface 16 of the root block assembly 12, the packs 36’ and 36” of the reinforcement layers therein and the inflatable or flexible mould insert 44. By providing suction inside the bags 42 and 46, and inside the root block assembly 12 the reinforce- ments layers 32 are consolidated to reach the desired fiber to resin ratio. The suction also forces the flexible mould insert 44 to compress the packs 36’ and 36” of the rein- forcements layers between the walls of the root block assembly 12 and the at least one wedge-shaped cavity 38 to consolidate properly. The mould insert 44 may be not only flexible but also inflatable, if so desired. Depending on the resin, also heat may be applied to speed up the curing of the resin.

[0026] After the resin within the various mould parts is cured, for instance after a period of 8 - 10 hours at a temperature of about 80 degrees Celsius, the moulds, i.e. the bags 42 and 46 as well as the flexible or inflatable, i.e. compressive, mould insert 44 are removed, and the at least one wedge-shaped cavity 38 is filled with appropriate mate- rial. A preferred choice for the filling material is a hardenable material that either main- tains or slightly increases its volume when hardening. Thereby it would be ensured that the packs 36’ and 36” of reinforcement layers 32 remain pressed against the wall of the root block assembly 12. Other possible choices for the filing material are the same resin, possibly epoxy, used in the curing of the reinforcement layers or the same resin plus an appropriate fibrous reinforcement filling. After the filling material in the cavity is cured, with or without heat, the foot part 34 of the blade assembly is machined flush with the bottom surface 16 of the root block assembly 12. The blade assembly is, then, attached to the hub or shaft that additionally prevents any possibility of the filling ma- terial from being pushed out of the cavity. Thus in principle requiring only sufficient compressive strength of the filling material.

[0027] As to the blade part of the blade assembly the edges of the blade have to be checked and, if needed, to be trimmed/machined to fulfil the requirements set for the shape of the blade. After the edge inspection and possible trimming the blade may be coated to provide the blade with desired surface finish.

[0028] Figure 5 illustrates a longitudinal cross-section of a finalized propeller blade assembly manufactured in accordance with the third preferred embodiment discussed in connection with Figure 4. In other words, the finalized blade assembly 50 comprises the blade 30 made of mould plug 40 and layers 32 of fibrous reinforcements and the filling 52 in the foot part 34 of the blade 30, and the root block assembly 12.

[0029] Figure 6 illustrates a partial longitudinal cross-section of a propeller blade as- sembly in accordance with a fourth preferred embodiment of the present invention. In all other respects the fourth embodiment is similar to the third one except for the second closing element, which is here a solid element 60, preferably but not necessarily a plate, pushed against the bottom surface 16 of the root block assembly 12 and tight- ened to the root block assembly 12. By tightening the second closing element 60 against the root block assembly, by means of bolts 62, for instance, a gas tight mould cavity is formed. Further, the second closing element comprises, in place of the at least one flexible mould insert of Figure 4, preferably but not necessarily, at least one inflat- able mould insert 64 that is inserted in the at least one wedge shaped cavity left be- tween the at least two packs 36’ and 36” of reinforcement layers. The inflatable mould insert 64 is connected to a source of pressure by means of a passage 66 through the second closing element 60.

[0030] A further option is that the solid element is only used for pushing the at least one flexible, or inflatable, mould insert in the at least one wedge-shaped cavity, and a bag is used for covering the solid element, too, in order to create the desired level of suction within the cavity provided for the blade.

[0031] The curing and further finalizing of the blade assembly are performed in the manner discussed in connection with Figure 4.

[0032] As to the use of the mould plug and especially its size it should be understood that the mould plug may also extend inside the root block assembly, contrary to what is shown in Figures 4, 5 and 6. The mould plug is preferably of a high density foam material, like polyurethane or styrene acrylonitrile foam, for instance. The phrase‘high- density foam’ refers to foams that do not compress under the pressure (normally less than 1 bar) used in the manufacture of the blade assembly and are able to resist the stresses as a result of the operational loads on the propeller blade. Also, any other material with sufficient compression strength could be considered, like for instance metal or plastic either machined or by additive manufacturing.

[0033] The fibrous reinforcement layers are either prepregs, i.e. fibrous reinforcements that have already been pre-impregnated with a sufficient amount of resin or dry rein- forcements, which require the use of a resin transfer method (RTM). [0034] As to the closing elements, it should be understood that the above is only a schematic representation of a possible mould structure, which is by no means intended to limit the coverage of the invention to the discussed options. Especially so, as the invention, i.e. the use of a compressive, flexible or inflatable mould insert in the pro- duction of the dovetail shape at the foot end of the blade, is entirely independent of the type of the mould used for shaping the blade. It should also be understood that the mould, i.e. the first closing element, used for shaping the blade may be, in place of the bag, at least two solid mould elements fastened to one another.

[0035] As to the inflatable, or broader, compressive, insert 64 discussed in more detail in Figure 6, it may be pressurized with any flowable media, liquid or gas, though gas is a preferred choice. Also, the inflatable media may be replaced with other compressive or flexible media. In the latter case the media has, in its free form, a volume larger than the cavity it is supposed to fill, whereby when the second closing element is put in place the mould insert is compressed and when doing so subjects the packs of reinforcement layers to compression, i.e. works like an inflatable and pressurized insert. In other words, the mould insert may be either a separate compressive element placed in the wedge shaped cavity before the second closing element is placed on the bottom of the root block assembly or a compressive element fastened to the second closing element. In accordance with one variation of the present invention the compressive element is such an inflatable element that has its inlet valve in connection with the second closing element.

[0036] While the invention has been described herein by way of examples in connec- tion with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the ap- pended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible. Thus, it is clear that the root block assembly of Figure 3 may be used in the production of the blade assemblies discussed in Figures 4 and 6.