Patent Grant
10722996
The subject matter disclosed herein relates to manufacturing of machine components comprising airfoil portions such as, but not limited to, rotor and stator blades or buckets for axial turbomachines, impellers for radial or axial-radial turbomachines and the like.
Axial turbomachines, such as axial compressors and turbines, comprise one or more stages, each stage being comprised of a circular arrangement of stationary blades or buckets and circular arrangement of rotor blades or buckets. The blades are provided with a root and a tip. An airfoil portion extends between the root and the tip of each blade.
In order to improve the turbomachine efficiency, the blades are usually subject to a polishing step. Additional treatments can be performed on the blades prior to polishing. For example a shot peening step is usually performed prior to polishing or finishing, for increasing the blade strength. Shot peening increases the surface roughness. The polishing step is currently performed by vibratory finishing, e.g. by vibro-tumbling. Vibro-tumbling provides for the blades to be placed in a rotating tumbler filled with pellets made of a natural abrasive or synthetic abrasive and a ceramic binder. The tumbler is caused to rotate and/or vibrate so that the pellets polish the surface of the airfoil profile. The final arithmetic average roughness (Ra) which can be achieved by vibro-tumbling ranges around 0.63 μm.
Lower roughness values could be achieved by continuing the vibro-tumbling treatment of the blades. However, the effect of the pellets on the airfoil profile not only modifies the surface roughness and texture, but also the airfoil geometry. Lowering the roughness below the abovementioned values would result in inadmissible alterations of the geometry. For this reason, lower roughness values cannot be obtained with the polishing methods of the current art
Shrouded impellers, e.g. for centrifugal compressors and pumps, are currently polished by means of so called abrasive flow machining. The abrasive flow machining process consists of generating a flow of a liquid suspension of abrasive material under pressure through the vanes of the impeller. Roughness values around 0.68 μm are achieved. Abrasive flow machining adversely affects the geometry of the blades, due to the abrasive action of the abrasive particles contained in the liquid suspension which is caused to flow under pressure through the vanes of the impeller. Moreover, the interaction between the blades and the abrasive flow is such that a non-homogeneous abrasive effect is obtained on the pressure side and suction side of each blade, due to the geometry of the latter. It is therefore not suitable to continue the abrasive flow machining process of an impeller beyond the above mentioned roughness values, since this would result in an unacceptable alteration of the blade geometry and therefore deterioration of the impeller efficiency.
The efficiency of a mechanical component comprised of an airfoil portion, such as an impeller or a blade, increases with reduced roughness, since energy losses due to friction are reduced. There is, therefore, a need for improving the finishing processes and methods in order to increase the efficiency of the airfoil profile by reducing the roughness thereof, without altering the geometry of the airfoil profile beyond an admissible threshold or tolerance.
An improved method is provided for polishing a machine component comprising at least one airfoil portion, comprised of a suction side, a pressure side, a leading edge and a trailing edge, which allows achieving particularly low roughness values on the airfoil surface.
In the present disclosure, including the annexed claims, unless differently specified, the surface texture and roughness are characterized by the arithmetic average roughness value (Ra). The arithmetic average roughness (Ra), also indicated as AA (arithmetic average) or CLA (Center Line Average) is the arithmetic averaged deviation of the actual surface from the mean line or center line within an assessment length (L) and is defined as
or:
Unless differently specified, the arithmetic average roughness (Ra) used herein is expressed in micrometers (μm). Unless differently specified, in the description and in the claims the term roughness shall be understood as being the arithmetic average roughness as defined above.
According to some embodiments, the method comprises:
arranging the machine component in a container and constraining the machine component to the container;
adding a polishing mixture in the container, the polishing mixture containing at least: abrasive powder, a liquid and metal particles;
vibrating the container and the machine component constrained thereto, thereby generating a polishing mixture flow along the airfoil portion until a final arithmetic average roughness is achieved.
In some embodiments, polishing is continued until a final arithmetic average roughness equal to or less than 0.3 μm is achieved on the machine component. It has been surprisingly discovered that the method disclosed herein can achieve such very low roughness values in a relatively short time and maintaining the geometry, i.e. the dimension and shape of the airfoil profile substantially unaltered, i.e. the roughness values mentioned above are achieved without adversely affecting the overall geometry of critical components such as turbine blades or buckets, turbomachine impellers and the like. Polishing methods according to the current art cannot be used to reach such low arithmetic average roughness values without causing unpredictable alterations of the airfoil profile, which would make the polished machine component actually unusable.
According to some embodiments, the treatment is applied until a final arithmetic average roughness equal to or less than 0.20 μm, may be equal to or less than 0.17 μm and more particularly equal to or less than 0.15 μm is obtained on the airfoil profile.
The container can be connected to a vibrating arrangement, for instance comprising a rotating cam and an electric motor. Arrangements can be provided for tuning the vibration frequency. According to some embodiments the method can thus further include a step of selecting a vibration frequency of the container and the machine component constrained thereto, which cause the metal particles advancing along the airfoil portion in adhesion thereto and generating a polishing action of the airfoil portion by means of abrasive powder between the airfoil portion and metal particles sliding there along. One or more vibration frequency values can be determined, depending e.g. upon the structural features and shapes of the machine components, which determine such a sliding advancement of the metal particles along the airfoil portion. Selection of the vibration frequency can be obtained experimentally, e.g. by gradually varying the rotation speed of an electric motor driving a cam which co-acts with the container. Suitable vibration frequencies can be selected by observing the movement of the metal particles or chips on the surface of the machine component.
In some embodiments, metal particles can be used having substantially planar surfaces. The metal particles can be caused to advance by vibration along the airfoil portion with the planar surfaces thereof in contact with the airfoil portion.
The machine components can be subjected to preliminary treatment processes, such as e.g. to a preliminary shot peening treatment.
According to some embodiments, the step of generating a flow of the polishing mixture along the airfoil portion comprises advancing the metal particles of the polishing mixture along the pressure side and the suction of the airfoil portion.
The machine component can be e.g. a blade or bucket of an axial turbomachine, having a root and a tip. The airfoil portion extends between the root and the tip, an airfoil chord being defined between the trailing edge and the leading edge in each position of the airfoil portion from the root to the tip.
In some embodiments of the method disclosed herein, the length of the chord is maintained substantially unaltered during the step of vibrating the machine component until a final arithmetic average roughness of 0.3 μm or less, may be 0.2 μm or less, more particularly of 0.17 μm or less is achieved. The chord length can be subjected to a variation which is less than an admissible tolerance value. For instance, the variation of the chord length can be equal to or less than 0.05% and more particularly equal to or less than 0.03%.
According to some embodiments, the variation of the chord length from the beginning to the end of the step of vibrating the container and the machine component constrained thereto can be equal to or less than 0.1 mm, may be equal to or less than 0.07 mm and even more particularly equal to or less than 0.02 mm.
A chord length variation during polishing, which remains equal to or below 0.1 mm and more particularly equal to or below 0.07 mm, results in the blade geometry and thus the blade functionality remaining substantially unaltered. Thus, according to some embodiments, when the machine component is a blade or a bucket of an axial turbomachine, the feature of maintaining the dimension and shape of the airfoil portion substantially unaltered means that the alteration of the chord length is equal to or less than 0.1 mm and more particularly equal to or less than 0.07 mm, e.g. equal to or less than 0.02 mm.
According to some embodiments, the machine component is a turbomachine impeller comprised of a hub with a central drive-shaft receiving bore and a plurality of blades arranged on the hub around the drive-shaft receiving bore. The blades form airfoil portions, each blade having a suction side and a pressure side. Vanes are defined between adjacent blades. Each vane has an inlet and an outlet and each blade has a leading edge at the inlet and a trailing edge at the outlet of the corresponding vane. By vibrating the machine component a polishing mixture flow is created, which circulates in and through the vanes of the impeller.
During the step of vibrating the machine component, the thickness of the blades of the impeller is reduced by less than 0.5% on average and may be by less than 0.4% on average, while a final arithmetic average roughness of the inner surface of the vanes is achieved, which can be equal to or less than 0.3 μm and more particularly equal to or less than 0.2 μm.
According to some embodiments, the variation of the blade thickness from the beginning to the end of the step of vibrating the container and the machine component constrained thereto can be equal to or less than 0.1 mm, may be equal to or less than 0.07 mm and even more particularly equal to or less than 0.02 mm.
A blade thickness variation during polishing, which remains equal to or less than 0.1 mm and more particularly equal to or less than 0.07 mm, results in the blade geometry and thus the blade functionality remaining substantially unaltered. Thus, according to some embodiments, when the machine component is an impeller for a turbomachine, e.g. an impeller for a radial pump or compressor, the feature of maintaining the dimension and shape of the airfoil portion substantially unaltered means that the alteration of the thickness of the impeller blades is equal to or less than 0.1 mm and may be equal to or less than 0.07 mm, e.g. equal to or less than 0.02 mm.
According to some embodiments, the impeller comprises a shroud comprised of an impeller eye. The shroud, the hub and adjacent impeller blades define flow vanes there between, each flow vane having an outlet aperture at the trailing edges of the blades. In some embodiments, the method provides for vibrating the impeller and generating a polishing mixture flow through the vanes, which causes the axial dimension of the outlet apertures to vary on average less than 0.05% and more particularly less than 0.04% with respect to the initial axial dimension.
In some embodiments the metal particles comprise metal chips. In particularly some embodiments, the metal particles comprise copper particles or copper chips.
In some embodiments the abrasive powder is aluminum oxide, ceramic or a combination thereof. The liquid can comprise or can be water. Additionally, a polishing medium can be added.
According to some embodiments the polishing mixture has the following composition by weight:
The step of vibrating the container and the machine component constrained thereto can last between 5 and 8 hours, more particularly between 6 and 7 hours.
According to other embodiments, the step of vibrating the container and the machine component constrained thereto can last between 1.5 and 10 hours.
In some embodiments, e.g. when axial turbomachine blades or buckets are polished, the vibrating step can last between 1 and 3 hours, e.g. between 1 and 2 hours.
According to a different aspect, the present disclosure also relates to a machine component comprising an airfoil portion, wherein the airfoil portion has an arithmetic average roughness equal to or less than 0.3 μm, may be equal to or less than 0.2 μm, more particularly equal to or less than 0.17 μm and even more particularly equal to or less than 0.15 μm. The machine component can be selected from the group comprising: an axial turbomachine blade or bucket; a turbomachine impeller.
Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.
A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.
Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.
Polishing of Blades of Axial Turbomachines
The axial compressor blade 1A shown in
The machine component 1A, 1B can be subjected to a surface-treatment step, for example a shot peening treatment. Once the machine component 1A, 1B has been pre-polished, it can be treated in a polishing machine. A schematic representation of an exemplary embodiment of a polishing machine 10 is shown in
In order to control the vibration of the vibrating table 13, in some embodiments one or more electric motors 21 are provided. The motor 21 controls rotation of an eccentric cam 23, which can rotate around a substantially horizontal axis 23A. The rotation of the eccentric cam 23 causes the vibrating table 13 and the container 11 constrained thereto to vibrate in a vertical direction, as schematically shown by a double-arrow f13.
In the container 11 one or more machine components 1A, 1B comprised of an airfoil portion can be arranged. In an embodiment, each machine component 1A, 1B is constrained to the container 11, so that the machine components 1A, 1B vibrate integrally with the container 11 and the vibrating table 13.
The container 11 is partly or entirely filled with an polishing mixture M. The polishing mixture can entirely cover the machine components 1A, 1B, so that the machine components are entirely submerged by the polishing mixture M. In other embodiments of the method disclosed herein a smaller amount of polishing mixture M can be used, only partially covering the machine components 1A, 1B, for example till 60%, 70% or 80% of the entire height H of the machine components 1A, 1B.
The polishing mixture M can be comprised of a liquid, for example water, metal particles and an abrasive powder. The metal particles can comprise metal chips, for example copper particles, such as copper chips. The abrasive powder can be selected from the group consisting of: aluminum oxide, ceramic particles, or combination thereof.
The metal particles can have a substantially planar shape, i.e. can be made of fragments of metal foils or laminae. In some embodiments the metal particles can have a thickness of between 1 and 2 mm. In some embodiments, the metal particles can have a cross-dimensions of between 3 and 5 mm.
The abrasive particles may have a grain side between 2 and 8 μm.
The polishing mixture M can further comprise a polishing medium. The polishing medium can be selected from the group consisting of: soap, passivizing liquid, or a mixture thereof.
The composition by weight of the polishing mixture M can comprise the following:
Once the polishing mixture has been introduced in the container 11, the latter is put into vibration by starting the motor 21. The vibration frequency can be suitably tuned, e.g. using a variable frequency driver 22. In an embodiment, treatment is performed at a vibration frequency which is set so that the metal particles of the polishing mixture advance slidingly along the surface of the airfoil portion 7 in contact therewith. The vibration frequency which causes this phenomenon can easily be selected for example by starting from a low frequency value and stepwise or continuously increasing the vibration frequency until the sliding movement of the metal particles is triggered, a condition which can be easily detected by the operator. Using a suitable variable frequency driver 22 for the electric motor 21 the vibration frequency can be tuned to the effective value which initiates the sliding advancement movement of the metal particles along the airfoil portion 7.
Since the advancing movement is determined by the vibration of the machine components 1A, 1B in the container 11, there is substantially no pressure applied against the surface of the airfoil portion 7 and the abrasive effect is extremely gentle.
As schematically shown in
Tests performed on several airfoil profiles of machine components show that the effect of this polishing method results in unexpectedly low roughness values, without adversely affecting the geometry of the airfoil profile.
The results of tests performed on a plurality of samples of stationary and rotary blades or buckets for axial turbines will be discussed here below, to show the effectiveness of the polishing method in terms of roughness achieved and conservation of the geometry of the profile.
The tests were performed on samples of buckets or blades of a heavy duty gas turbine available from General Electric, Evendale, Ohio, USA.
Tests were performed on rotor blade samples from the 2nd, 3rd, and 11th turbine stage and on stationary blades of the 5th, 6th, and 8th stage.
Among the several parameters describing the geometry of the blades and which can be used to check the effect of the polishing process over the overall geometry of the airfoil profile of the blades, the chord variation has been chosen. The chord has been measured at different distances from the blade root before and after the polishing process, to check how the polishing process affects this parameter.
As mentioned above, current art finishing processes negatively affect in particular the dimension of the blade chord due to the impact of the abrading pellets on the leading and the trailing edges of the blades, which lead to erosion of the edges, modification of their radius of curvature and alteration of the chord dimension. The chord dimension is therefore a critical parameter to be checked after polishing, to establish whether the polishing process has modified the geometry of the blade to such an extent that it can prejudice the blade efficiency.
The following Table n. 1 summarizes the main data of the blades tested. The table indicates the number of the rotor or stator of the gas turbine to which the tested blades or buckets belong, the number of the samples tested and the polishing cycle time. Aluminum oxide was used as abrasive and copper particles were used in the polishing mixture. The composition of the polishing mixture was as follows:
Referring first to the second rotor stage, the following Table n. 2 reports the arithmetic average roughness Ra measured on four different samples numbered 19, 12, 10, 26 in six different points of the suction side surface of each sample blade after shot-peening and before polishing. The samples are numbered with sample number (S/N) 19, 12, 10, 26. As mentioned above, the measurements are expressed in μm (micrometers). The position of the six points where the arithmetic average roughness Ra has been measured is shown in
Table 3 shows the arithmetic average roughness Ra measurements on the same rotor blade samples on the pressure side thereof in four different locations labeled P1 to P4, the position whereof is shown schematically in
The following Tables 4 and 5 report the roughness values Ra on the same samples and the same measurement points as well as the average value (last column, Avg) after a polishing process as described above:
The above reported data summarized in the diagrams of
The tests also show that the arithmetic average roughness improves very little after 120 minutes treatment time. The treatment time for each sample is shown in Table 1.
In order to check whether the final blade geometry obtained after polishing is consistent with the strict requirements applied to this kind of machine components, the extension of the chord profile has been measured before and after the polishing treatment on all four samples under test.
The data reported in
Tests performed on several turbomachine blades have shown that the total alteration of the chord dimension is less than 0.1 mm, usually equal to or less than 0.07 mm and that alterations as low as 0.02 mm can be achieved, while still obtaining the above mentioned desired arithmetic average roughness values on the pressure and suction sides of the blade.
The following Tables 6 to 9 report the roughness measurements on six rotor blade samples of the third turbine stage.
The following Table 7 shows the arithmetic average roughness values measured on four points P1-P4 on the pressure side (
The following Tables 8 and 9 show the arithmetic average roughness values measured on the same samples and in the same points as in Tables 6 and 7 after polishing:
The sample number (S/N) is reported in the first column.
The following Tables 10, 11, 12 and 13 report the measured arithmetic average roughness values on the suction side and the pressure side before polishing (Tables 10 and 11) and after the polishing (Tables 12 and 13) for six rotor blade samples (S/N 1, 35, 7, 19, 29, 26) belonging to the 11th turbine stage:
The arithmetic average roughness data reported in the above tables are summarized in the diagrams of
Tests performed on sample blades or buckets on 5th, 8th and 16th stator stage of the same turbine show similar results in terms of roughness values achieved and insignificant alteration of the blade geometry. The following Tables 14, 15, 16 and 17 report the measured roughness data on the suction side (Table 14) and pressure side (Table 15) before polishing and the roughness values on the suction side (Table 16) and on the pressure side (Table 17) after polishing, respectively.
Arithmetic average roughness values around or below 0.15 μm are obtained on both pressure side and suction side of the buckets.
The following Tables 18, 19, 20 and 21 show the roughness measurements before polishing (Table 18—suction side, Table 19—pressure side) and after polishing (Table 20—suction side, Table 21—pressure side) for six different samples of stator buckets of the 8th stage of the turbine. Arithmetic average roughness values under 0.2 μm, mainly around or below 0.15 μm are obtained. The arithmetic average roughness values (before and after polishing) on the suction side and the pressure side are depicted and summarized in
Finally, Tables 22, 23, 24 and 25 report the arithmetic average roughness values measured on the suction side and pressure side before polishing (Table 22—suction side; Table 23—pressure side) and after polishing (Table 24—suction side; Table 25—pressure side) for six stator bucket samples of the 16th stage of the turbine.
The diagram of
Polishing of Impellers
The above described polishing method may be used for polishing impellers for centrifugal compressors, pumps and radial or axial-radial turbomachines in general.
An exemplary embodiment of such an impeller is shown in
In some embodiments the shroud 33 forms a stepped outer profile for co-action with a sealing arrangement arranged in the stationary casing, where the impeller 30 is supported for rotation.
In
By tuning the frequency of the vibration, a frequency can be set at which the metal particles contained in the polishing mixture M slide along the inner and outer surfaces of the impeller 30 and in particular circulate inside the vanes 37. Abrasive powder between the treated surface of the impeller 30 and the metal particles is thus caused to act upon the treated surface due to the sliding movement of the metal particles along the surfaces under treatment, quite in the same way as described above in connection with
Contrary to what happens in abrasive flow machining procedures of the current art polishing processes, the polishing mixture M flows through the vanes of the impeller 30 at substantially no pressure, so that the geometry of the impeller remains unaffected by the polishing particles acting thereon, while the gentle treatment obtained by the displacement of the metal particles with the abrasive powder thereon along the impeller surfaces causes a substantial reduction of the arithmetic average roughness of the inner and outer surfaces of the impeller.
The following data have been obtained on a sample of a 2D centrifugal compressor impeller treated with the above described polishing process. These data show that the process is capable of reaching very low arithmetic average roughness values (Ra) without adversely affecting the geometry of the critical parts of the impeller, in particular the blades, defining the airfoil profiles of the impeller.
The polishing process was performed with a polishing mixture having the following composition:
The impeller was maintained under vibration for 7 hours and 30 minutes.
The following Table 26 reports the arithmetic average roughness measured before and after polishing in three different points along a vane between adjacent blades of the impeller, starting from the impeller outlet. The measurements were carried out on three different points at 10, 44 and 75 mm from the impeller outlet in radial direction.
Since measurement requires partial removal of the shroud, the measurements before and after polishing were carried out on different vanes. The shroud portion was first removed from one vane to get access to the interior thereof. After polishing a further shroud portion was removed from a different vane, so that the polishing treatment of the vane under measurement was performed with the vane being closed by the shroud.
The axial dimension of the impeller outlet and the blade thickness were used as significant parameters for checking the effect of the polishing process on the overall geometry of the blade.
The difference on the measurements before and after polishing is negligible and below the sensitivity (0.005 mm) of the instrument used, in both vanes considered and for all measurement locations.
The following Table 27 shows the thickness of three blades of the same impeller measured at the trailing edge thereof. The table reports the blade thickness before and after polishing. The difference between the measurements before and after treatment is negligible.
These data show that the polishing process has substantially no effect on the geometry of the impeller and of the profile of the blades.
A 3D impeller made of carbon steel schematically shown in
The process was performed for 6 hours in a polishing machine 10 as shown in
Table 28 show the arithmetic average roughness measured in the areas A-D prior to polishing and in the areas A-E after polishing:
As best shown in
Measurements carried out on the dimensions of these parts of the impeller before and after polishing show that these critical impeller dimensions are not altered by the polishing process, in spite of the extremely low arithmetic average roughness values reached at the end of the polishing process (Table 28).
The following Table 29 summarize the measurements made before and after polishing on the inner diameter of the hub, on the diameter of the five sealing rings R1-R5, and on the axial dimensions dx and sx of the vane outlet, respectively:
As evidenced by the data reported in the above Table 29, the critical parts of the impeller remain unaffected by the polishing process, which reaches extremely low arithmetic average roughness values, around 0.1 μm.
Tolerances on the mean blade thickness are usually around +/−5% and the tolerances on the mean output width are around +/−3%. The measurements carried on the samples treated with the method disclosed herein show that the modification of these critical measures is negligible, and well below the acceptable tolerances.
While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.
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