WO2016026927A1

WO2016026927A1 - Method for producing a rotor for a screw compressor and a kit of parts for a rotor for a screw compressor

Info

Publication number: WO2016026927A1
Application number: PCT/EP2015/069138
Authority: WO
Inventors: Jonas BERGENDAL; Marcus Persson
Original assignee: Höganäs Ab
Priority date: 2014-08-21
Filing date: 2015-08-20
Publication date: 2016-02-25

Abstract

According to one aspect of the inventive concept there is provided a method for producing a rotor for a screw compressor comprising: providing a shaft, and forming at least two rotor segments of compacted metal powder, said segments comprising an outer threaded surface and an inner surface defining a through-hole for receiving the shaft and wherein each segment is movable along the shaft,and arranging the segments on the shaft, wherein a boundary surface of at least a longitudinal section of the shaft and said inner surface of each segment are arranged to cooperate such that a continuous threaded surface is formed by the outer threaded surfaces of the segments when each of said segments is moved to a respective position along the shaft. There is also provided a kit of parts for producing a rotor.

Description

METHOD FOR PRODUCING A ROTOR FOR A SCREW COMPRESSOR

AND A KIT OF PARTS FOR A ROTOR FOR A SCREW COMPRESSOR

Technical field

The present inventive concept relates to a method for producing a rotor for a screw compressor and a kit of parts for producing the same.

Background

Screw compressors are used in many applications requiring pressurized gas, for example for driving compressed-air apparatus such as tools and machinery for industrial applications including among others construction and mining work. Screw compressors operate according to a rotating positive displacement mechanism. Commonly one screw rotor, or two meshing screw rotors, are used to move gas through a cavity from an inlet side to an outlet side. Screw compressors come in both dry and wet configurations. In either configuration, providing a screw compressor with a high compression capacity puts great requirements on the manufacturing tolerances for the rotor(s), to ensure an accurate fit between the rotor(s) and the cavity. In the prior art rotors are manufactured by turning and hobbing of a raw material such as a cast metal. The required surface roughness of the rotor is thereafter commonly obtained in an additional grinding operation which may increase both the time and the costs of the production.

Summary of the inventive concept

The inventors have realized that there is room for improving upon existing methods for producing rotors for screw compressors. A particular object of the present inventive concept is to enable rational production of a rotor for a screw compressor within a tight tolerance range. Further objects will be understood from the following.

According to a first aspect of the inventive concept there is provided a method for producing a rotor for a screw compressor comprising:

providing a shaft;

forming at least two rotor segments of compacted metal powder, said segments comprising an outer threaded surface and an inner surface defining a through-hole for receiving the shaft, wherein each segment is movable along the shaft; and arranging the segments on the shaft, wherein a boundary surface of at least a longitudinal section of the shaft and said inner surface of each segment are arranged to cooperate such that a continuous threaded surface is formed by the outer threaded surfaces of the segments when each of said segments is moved to a respective position along the shaft.

By the inventive method a rotor having a desired geometry may be formed from two or more segments of compacted metal powder. Compacted powder components lend themselves for inexpensive volume production. Meanwhile, compacted powder components may be produced with high precision in a repeatable manner and with a surface roughness meeting the often high requirements in screw compressor applications. The need for an additional grinding step is therefore reduced.

The inventive cooperation between the segments and the shaft enables guiding of the segments such that the outer threaded surfaces of adjacent segments join to form a rotor body with a continuous outer thread. The segments may be guided to an orientation about the shaft such that each thread of a first segment of said at least two segments aligns with a respective thread of an adjacent second segment at an interface between the first and second segments, when the first and second segments are arranged at their respective positions along the shaft. A smooth transition between adjacent segments and the threads thereof may thus be achieved.

Discontinuities between adjacent segments may otherwise prevent a close cooperation between the rotor(s) and the surrounding cavity.

In this context, a segment or a rotor having a threaded surface is to be interpreted broadly as covering a segment or a rotor comprising one thread (i.e. a single-start screw), or two or more threads (i.e. a multi-start screw). Each thread may be a ridge or groove formed on the surface of the segment or rotor. Each thread may extend about the segment or the rotor in a spiral- shaped path, i.e. in a circumferential and longitudinal direction with respect to the shaft. Each thread may extend about the segment or rotor in a helical path (i.e. a spiral-shaped path with a constant helix angle). The spiral-shaped or helical path may be right- or left-handed depending on the particular application.

According to one embodiment the method further comprises forming the longitudinal section of the shaft with first guiding means and forming each segment with second guiding means, wherein the first guiding means and the second guiding means of each segment are arranged to cooperate with each other to control an orientation of the outer threaded surface of each segment about the shaft. Thereby, a continuous thread may be formed by the outer threaded surfaces of the segments when each of said segments is moved to a respective position along the shaft.

According to one embodiment the method further comprises forming the longitudinal section of the shaft with a cross section having an outer peripheral shape deviating from circular shape, and forming each of the segments with a through-hole having a shape deviating from circular shape. A non-circular interface between the shaft and the segments enables guiding of the orientation of each segment about the shaft, and thus guiding of the orientation of the outer threaded surface of each segment. The outer peripheral shape of the shaft and the shape of the through-hole may match along at least a portion about the shaft to form an interface portion between the shaft and said segment. The interface portion may be arranged to prevent rotation of said segment about the shaft, when said segment is arranged at its position along the shaft.

According to one embodiment the method further comprises forming the longitudinal section of the shaft with a cross section having an outer peripheral shape which is polygonal and forming each of the segments with a through-hole having a polygonal shape. Polygonal shapes may provide a strong cooperation between the shaft and the segments and lends itself for efficient and precise production of the segments and the shaft. The outer peripheral shape of the shaft and the shape of the through-hole may match along at least a portion about the shaft to form an interface portion between the shaft and said segment. The interface portion may be arranged to prevent rotation of said segment about the shaft, when said segment is arranged at its position along the shaft. The interface portion may thus control the orientation of the segments about the shaft.

According to one embodiment the method further comprises forming the longitudinal section of the shaft with a cross section having an outer peripheral shape including at least one edge portion, and forming each of said segments with a gripping portion extending along a portion of the respective through-hole and about the through-hole, and being arranged to engage with said edge portion when said segment is moved to its respective position along the shaft. By the cooperation between the edge portion and the gripping portion the orientation of the segments about the shaft may be controlled. Additionally, a rotation of each segment with respect to the shaft may be prevented when the segment has been arranged at its position along the shaft. In particular the above-mentioned interface portion may comprise said edge and gripping portions.

According to one embodiment the method further comprises forming the inner surface of each segment with a groove and forming a protruding portion on the boundary surface of said longitudinal section, wherein the protruding portion is arranged to extend into the groove of each segment when each of said segments is moved to its respective position along the shaft. Thereby, means for guiding the orientation of the segments about the shaft may be provided in a simple manner. Advantageously, the longitudinal extension of the protruding portion in the longitudinal direction is less than the length of the shaft. Thereby the end portions of the shaft may be designed in a manner facilitating connection to other parts of the screw compressor, without the protruding portion interfering. In particular, the extension of the protruding portion may correspond to the combined length of the segments.

According to an alternative embodiment the method further comprises forming the inner surface of each segment with a protruding portion and forming a groove in the boundary surface of said longitudinal section, wherein the protruding portion is arranged to extend into the groove when each of said segments is moved to its respective position along the shaft. Thereby, means for guiding the orientation of the segments about the shaft may be provided in a simple manner.

In any of the above-mentioned embodiments including a cooperating groove and protruding portion, arranging each segment on the shaft may comprise arranging the protruding portion to extend into said groove. In particular, each segment may be arranged on the shaft by orienting the segment such that the groove (of the segment or the shaft) and the protruding portion (of the shaft or the segment) align and thereafter arranging the shaft to extend through the through-hole of said segment with the protruding portion extending into the groove.

According to one embodiment the number of grooves or protruding portions of each segment is equal to, or greater than, the number of threads of said segment. The grooves or protruding portions are preferably

symmetrically distributed about the through-hole, with respect to each other, on the inner surface of said segment. Provided also the threads are symmetrically distributed, this may simplify arranging the segments on the shaft such that the continuous outer threaded surface is formed by the segments. In particular, increasing the number of grooves or protruding portions beyond the number of threads makes it possible to use segments having a length dimension which is less than the pitch of the threaded surface and still have the threaded surfaces of the segments forming the continuous outer threaded surface, even for segments having identical geometries.

According to one embodiment, arranging the segments on the shaft comprises arranging an end portion of each segment to abut against an end portion of an adjacent segment when each of said segments is moved to its respective position along the shaft. Thereby, gaps in the final rotor body may be avoided and the rotor body may be formed by bringing adjacent segments together.

According to one embodiment mutually abutting end portions of the segments are formed with cross sections having matching outer peripheries. An outer periphery of a cross section of an end portion of a first segment may match an outer periphery of a cross section of an end portion of an adjacent second segment when each of said segments is moved to its respective position along the shaft. Optionally, an inner periphery of the cross section of the end portion of the first segment may match an inner periphery of the cross section of the end portion of the adjacent second segment.

According to one embodiment the segments are formed to present identical geometries. The rotor may thereby be produced in a rational manner. Each segment may be formed in a same mould or in moulds having identical geometries. Geometric deviations between segments may thereby easily be minimized, or at least reduced.

According to one embodiment the method comprises forming each segment with a length dimension corresponding to an integer multiple of the pitch of the threaded surface of said segment. Thereby, the threaded surfaces of adjacent segments, once arranged on the shaft, may connect to each other in a continuous manner and thereby together form the continuous threaded surface of the rotor. This embodiment may advantageously be combined with the above-mentioned embodiment wherein the segments are formed to present identical geometries. For example, rotors of different lengths may be produced by varying the number of identical segments on the shaft.

According to one embodiment the segments are formed to present identical geometries and each segment is formed to comprise, on the inner surface thereof and extending along the through-hole, at least a first groove and a second groove, or at least a first protruding portion or a second protruding portion, and further comprise a thread extending about a longitudinal axis of each segment, and wherein an angular separation between said first and second groove, or an angular separation between said first protruding portion and said second protruding portion, corresponds to an angular separation between a thread top of said thread at a first end of said segment and a thread top of said thread at a second end of said segment. Thereby the thread tops of two such adjacent segments may be conveniently aligned with each other on the shaft by aligning one of the grooves or protruding portions of one of the segments with one of the grooves or protruding portions of the other segment.

According to one embodiment each segment comprises a thread extending about a longitudinal axis of the segment, wherein there is, for each segment, a sub-section along said length section of the shaft, which subsection is arranged to support said segment and presents an outer peripheral cross-sectional shape such that, when the longitudinal axis of said segment and a longitudinal axis of said shaft are aligned, reception of said sub-section in the through-hole of said segment is allowed at only an integer number of relative orientations between said segment and said shaft, about the longitudinal axis of the shaft. This may simplify arranging the segments to align on the shaft since threading the segments on the shaft is allowed only for an integer number of relative orientations, and is prevented for all other relative orientations about the longitudinal axis of the shaft. Advantageously, this embodiment may be combined with the embodiment in which the segments are formed to present identical geometries.

According to one embodiment the method comprises forming the segments as green bodies, arranging the green body segments on the shaft and sintering the green body segments arranged on the shaft. This enables a rational production of the rotor.

According to one embodiment the method comprises sinter brazing the green body segments arranged on the shaft. By sinter brazing, the segments may be sintered and bonded together in a same operation. The bonding may prevent a relative rotation of the segments and also contribute to the continuity of the outer threaded surface of the rotor body. Moreover a brazing joint may be formed between the shaft and the segments, thereby reliably fixing the positions of the segments along the shaft.

According to a second aspect there is provided a kit of parts for a rotor for a screw compressor, the kit comprising: a shaft,

at least two rotor segments of compacted metal powder, said segments comprising an outer threaded surface and an inner surface defining a through-hole for receiving the shaft, wherein each segment is movable along the shaft; and

wherein a boundary surface of at least a longitudinal section of the shaft and said inner surface of each segment are arranged to cooperate such that a continuous threaded surface is formed by the outer threaded surfaces of the segments when each of said segments is moved to a respective position along the shaft.

The various embodiments of the shaft and rotor segments discussed above in connection with the first aspect apply correspondingly to the second aspect and will therefore not be repeated here. Moreover, the advantages discussed in connection with the first aspect and the embodiments thereof apply correspondingly to the second aspect.

According to a third aspect there is provided a rotor for a screw compressor, the rotor comprising:

a threaded body made of compacted metal powder and arranged on a shaft extending through the threaded body,

wherein at least a longitudinal section of the shaft, which section is enclosed by the threaded body, presents a cross section having an outer peripheral shape deviating from circular shape,

and wherein threaded body presents a cross section having an inner peripheral shape deviating from circular shape.

The rotor may be produced by a method in accordance with the first aspect, or from the kit of parts in accordance with the second aspect. The advantages discussed in connection with the first and second aspect thus apply correspondingly to the third aspect.

The cross section of the longitudinal section of the shaft may have an outer peripheral shape which is polygonal and the cross section of the threaded body may have an inner peripheral shape which is polygonal.

Polygonal shapes may provide a strong cooperation between the shaft and the segments and lends itself for efficient and precise production of the rotor. Especially, the outer peripheral shape of the shaft and the inner peripheral shape of the threaded body may match along at least a portion about the shaft to form an interface portion between the shaft and threaded body. The interface portion may be arranged to prevent rotation of the threaded body about the shaft.

According to one embodiment one of the longitudinal section of the shaft or the threaded body comprises a groove extending along the longitudinal direction and the other one of the longitudinal section of the shaft or the threaded body comprises a protruding portion arranged to extend into the groove. The cooperation between the groove and the protruding portion may among others reliably prevent rotation of the threaded body about the shaft.

In accordance with a separate, fourth inventive aspect, it has been contemplated that the above described method in accordance with the first aspect may present a more general applicability in the sense that a similar method may be used to form a threaded body, in particular an elongated threaded body such as a screw, suitable also for other uses than in screw compressors. In view of this insight, there is provided a method for producing a threaded body comprising:

providing a shaft;

forming at least two rotor segments of compacted metal powder, said segments comprising an outer threaded surface and an inner surface defining a through-hole for receiving the shaft, wherein each segment is movable along the shaft; and

arranging the segments on the shaft, wherein a boundary surface of at least a longitudinal section of the shaft and said inner surface of each segment are arranged to cooperate such that a continuous threaded surface is formed by the outer threaded surfaces of the segments when each of said segments is moved to a respective position along the shaft.

The thusly produced threaded body may for example form an elongated gear, such as a spiral gear, a screw for a screw conveyor, or even a screw for joining construction elements. By forming the threaded body from two or more rotor segments threaded bodies of arbitrary length and/or having a low thread pitch or a complex threading pattern may be formed from a number of more simple compacted powder segments. The advantages of metal powder compaction technology may thus be conferred to threaded bodies having designs which would be impracticable or even impossible to obtain from a single pressed segment.

The general advantages pertaining to inexpensive volume production, repeatability, surface roughness and smooth transition between adjacent segments discussed in connection with the first aspect apply mutatis mutandis to this fourth inventive aspect. Moreover the description of embodiments (as well as the general advantages thereof, disregarding advantages specific to screw compressors) of the first aspect are applicable also to this fourth inventive aspect.

However, as may be understood from the discussion in connection with the first aspect, the method is particularly advantageous for producing rotors for a screw compressor, owing inter alia to the fact that compacted powder components may be produced with high precision in a repeatable manner and with a surface roughness meeting the often high requirements in screw compressor applications, wherein the need for an additional grinding step is reduced. Moreover the inventive cooperation between the segments and the shaft makes it possible to avoid discontinuities between adjacent segments which otherwise could prevent a close cooperation between the rotor(s) and the surrounding cavity. As may be understood, this may be particularly advantageous for rotors for screw compressors in view of the often great requirements on the manufacturing tolerances.

Brief description of the drawings

The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present inventive concept, with reference to the appended drawings, where like reference numerals will be used for like elements, wherein:

Fig. 1 is a perspective view of a shaft and a number of segments for a rotor according to one embodiment.

Fig. 2 is a cross-sectional view of a segment and the shaft.

Fig. 3 illustrates the rotor during assembly.

Fig. 4 illustrates the rotor in an assembled condition.

Fig. 5 illustrates a rotor according to another embodiment.

Figs 6-10 are cross-sectional views of a segment and the shaft in accordance with further embodiments.

Detailed description of preferred embodiments

Detailed embodiments of a kit of parts and a method for producing a rotor 100 for a screw compressor, as well as a rotor 100 for a screw compressor in accordance with the present inventive concept will now be described with reference to the drawings. It should be noted that the geometry of the thread of the illustrated rotor 100 merely is provided as an illustrative example for facilitating understanding of the inventive concept and that other designs are equally possible without departing from the scope of the inventive concept as defined by the appended claims.

Moreover, the present inventive concept is not limited to a specific type of screw compressor but presents a more general applicability. The method in accordance with the present inventive concept may for example be used to produce a rotor for a single-rotor screw compressor as well as meshing male and female rotors for twin-rotor screw compressors. Screw rotors may be manufactured for use in dry screw compressors (i.e. using timing gears for synchronizing the rotors) as well as wet screw compressors (i.e. not using timing gears but including a rotor separating liquid such as oil). The detailed working and design of screw compressors as such is well-known and will therefore not be further described herein.

As illustrated Figs 1 -2 the method comprises providing a shaft 1 10. The shaft 1 10 is elongate and thereby defines a longitudinal direction. The longitudinal direction may also be referred to as a longitudinal axis of the shaft 1 10. The shaft is formed with a protruding portion 1 14 on the boundary surface 1 12 of the shaft 1 10. The boundary surface 1 12 forms an outer surface or envelope surface of the shaft 1 10. The protruding portion 1 14 extends between opposite ends of the shaft 1 10 and parallel to the

longitudinal direction. To facilitate mounting of the finished rotor 100 in a screw compressor, the shaft 1 10 may optionally be provided with an axial through-hole 1 16. The shaft 1 10 may be made of metal such as cast iron, steel, a steel alloy (e.g 16MnCr5) or aluminum to name a few. The shaft 1 10 with the protrusion 1 14 may be formed in for example an extrusion process. Alternatively, the shaft 1 10 with the protrusion 1 14 may be formed by casting, machining or a combination thereof.

The method further comprises forming a plurality of rotor segments 120 of compacted metal powder. The shaft 1 10 and the segments 120 form a kit of parts for producing the rotor 100. The illustrated segments 120 have an identical geometry. Thus, the following description applies correspondingly to each of the segments shown in Fig. 1 . The segments 120 may be formed by compaction of a metal powder composition such as stainless steel powders, iron based powder compositions, for example Fe-Cu-C powder compositions, etc. The powder composition may be arranged in a mould having a shape corresponding to the desired geometry of the segment 120 and thereafter be subjected to mechanical pressure along a direction coinciding with the longitudinal direction (i.e. axial direction) of the compacted segment. A compacted segment 120 may be referred to as a green body or green component. By virtue of the identity of the geometry of the segments 120, the segments 120 may be manufactured in the same mould or in moulds having identical geometries. Each segment 120 comprises an inner surface 122 defining a through-hole for receiving the shaft 1 10 in the longitudinal direction of the shaft 1 10. The through-hole extends along a longitudinal direction or longitudinal axis of the segment 120. The inner surface 122 may also be referred to as an envelope surface for the through-hole.

The inner surface 122 of the segment 120 is formed with a plurality of grooves 124. The grooves 124 are symmetrically distributed on the inner surface 122, about the through-hole and the longitudinal axis of the segment 120. Each groove 124 extends in the longitudinal direction. Each groove 124 extends from a first end of the segment 120 to a second end of the segment 120, which second end is opposite to the first end. Thus, each groove 124 is coextensive with the axial length of the through-hole. In the illustrated embodiment the segment 120 comprises a plurality of grooves 124, i.e. more grooves than protruding portions on the shaft 1 10. Moreover, when the segments 120 are arranged on the shaft 1 10 as described below, the grooves 124 of adjacent segments 120 will align to form a plurality of common grooves symmetrically distributed about the shaft 1 10. This is due to an angular separation, measured about the longitudinal axis of the segment 120, between a pair of adjacent grooves 124 of the segment 120, being at least approximately equal to an angular separation, measured about the

longitudinal axis of the segment 120, between a thread top of a thread at a first end of the segment 120 and a thread top of the same thread at the opposite end of said segment 120. These aspects may provide production- related advantages as will be described below.

As an example a punch, preferably a lower punch, may be provided with a shaft-like structure comprising axially extending protrusions for defining the through-hole of the segment 120 and the axially extending grooves 124. Following compaction, the compacted segment 120 may be removed from the cavity of the mould. The upper and/or lower punches may be journalled in bearings, thereby allowing the compacted segment 120 and the punch(es) to be synchronously rotated, or screwed, out from the mould in an axial direction. Alternatively, the mould may comprise two opposite and axially extending halves wherein, following compaction, the mould may be opened or divided to allow the compacted segment 120 to be removed.

Each segment 120 comprises an outer threaded surface comprising a plurality of parallel threads 126 formed as ridges. The threads 126 are symmetrically distributed about the segment 120. The segments 120 may thus be referred to as screw segments. According to the illustrated

embodiment, the axial dimension of each segment 120 equals the pitch of the threaded surface of said segment 120. "Pitch" is the axial distance between adjacent thread tops. If the threaded surface comprises more than one thread adjacent thread tops belong to adjacent threads. If the treaded surface comprises a single thread adjacent thread tops belong to the same thread. As may be seen in Fig. 1 , an angular position of a top of a thread 126 at a first end of the segment 120 coincides with an angular position of a top of a thread 126 at a second end, opposite the first end of the segment 120. Once arranged on the shaft 1 10, the threaded surfaces (and the thread tops) of adjacent segments may connect to each other in a continuous manner and thereby together form a continuous threaded surface.

The method proceeds with an assembly stage by arranging the segments 120 on the shaft 1 10 in . The shaft 1 10 and the through-hole of a segment 120 to be arranged on the shaft 1 10 are aligned with each other. The shaft 1 10 and/or the segment 120 are rotated about the longitudinal direction to obtain alignment between the protruding portion 1 14 of the shaft 1 10 and one of the grooves 124 of the segment 120. The shaft 1 10 may thereafter be received in the through-hole of the segment 120 wherein the segment 120 may be moved in relation to the shaft 1 10 to a desired position along the shaft 1 10 with the shaft 1 10 extending through the through-hole of the segment 120 and the protruding portion 1 14 travelling in the groove 124.

Due to the outer peripheral cross-sectional shape of the shaft 1 10 and the shape of the through-hole of each segment 120, when the longitudinal axis of a segment 120 and the longitudinal axis of shaft 1 10 are aligned, reception of the shaft 1 10 in the through-hole of the segment 120 is allowed at only an integer number of relative orientations between the segment 120 and the shaftl 10, about the longitudinal axis of the shaft (in the illustrated embodiment six orientations, i.e. the same as the number of grooves 124), and is prevented for all other relative orientations. Fig. 3 illustrates a first segment 120a which has been arranged at a respective position along the shaft 1 10 and a second segment 120b being moved along the shaft 1 10 towards the segment 120a. It should be noted that movement of the segment 120b along the shaft 1 10 may be achieved either by keeping the shaft 1 10 at a fixed position and moving the segment 120b to be mounted on the shaft 1 10 along the shaft or by keeping the segment 120b to be mounted on the shaft 1 10 at a fixed position and moving the shaft 1 10 (together with any segments 120a already mounted thereon, if any) through the through-hole of the segment 120.

The protruding portion 1 14 forms a first guiding means of the shaft 1 10.

Each groove 124 of each segment 120 forms a second guiding means. The protruding portion 1 14 is arranged to extend into a groove 124 of each segment 120 while said segment is moved along the shaft 1 10. The

protruding portion 1 14 thus provides edge portions and the groove 124 provides a gripping portion engaging with the edge portion. The cooperation at the interface portion formed between the protruding portion 1 14 and the groove 124 guides or controls the orientation of the segment 120 about the shaft 1 10. When the segment 120 is arranged at its respective position the cooperation between the protruding portion 1 14 and the groove 124 also prevents rotation of the segment 120 about the shaft 1 10, and thus in relation to other segments arranged on the shaft 1 10.

The cross-sectional dimensions of the protruding portion 1 14 may match the cross-sectional dimensions of the corresponding groove 124.

Thereby play between the protruding portion 1 14 and the grooves 124 may be avoided and a reliable rotational fixation of the segment 120 about the shaft 1 10 may be achieved. The protruding portion 1 14 and the grooves 124 need however not be formed with rectangular shapes but triangular as well as rounded shapes are also possible. The cross-sections of the protruding portion 1 14 and the grooves 124 may even not be the same. For example, the grooves may present a rectangular shape and the protruding portion 1 14 may be shaped as a semicircle with a diameter matching the width of the groove 1 14.

Fig. 4 illustrates the kit of parts in an assembled state with four segments 120 arranged at their respective positions along the shaft 1 10, the shaft 1 10 and the segments 120 thereby forming the rotor 100. An end portion of the segment 120a is arranged to abut against an end portion of the adjacent segment 120b. By the configuration of the protruding portion 1 14 and the groove 124 of the segments 120, the threads 126 of the segments 120 obtain an orientation about the shaft 1 10 such that a continuous outer thread is formed by the threads 126 of the segments 120. In particular, at the interface between the segment 120a and 120b, the outer periphery of the cross section of the end portion of the segment 120a matches the outer periphery of the cross section of the end portion of the segment 120b. Thus the outer surface of the end portion of the segment 120a joins the outer surface of the end portion of the segment 120b to form a continuous outer surface. In other words, the threads 126 of the segments 120a and 120b align at the interface between the segments 120a, 120b. This discussion applies correspondingly to the further segments. As may be seen in Fig. 4 the threads 126 of the segments 120 thus form a common outer threaded surface comprising a number of continuous threads, which number is equal to the number of threads 126 of each segment 120.

After the segments 120 have been arranged on the shaft 1 10 the rotor

100 may be subjected to sintering. Thereby the green body segments 120 may be transformed to sintered segments 120. The sintering step may comprise sinter brazing. A braze material may be applied to the segments 120. The braze material may for example be a braze compound such as a braze alloy. As a non-limiting example Sinterbraze90 obtainable from

Hoganas AB , S-263 83 Hoganas, Sweden may be used. The braze material may be applied at least at the interfaces between the segments 120. During sintering, a brazed joint may form between adjacent segments 120, for example between the mutually abutting end surfaces of the segments 120a and 120b. The individual green body segments 120 may thereby be bonded to each other and form a common rotor body. A brazing material may be applied also at interface portions between the shaft 1 10 and one or more of the segments 120. Thereby bonding may be achieved also between the common rotor body and the shaft 1 10. A brazing material may be applied to a segment 120 prior to arranging the segment 120 on the shaft 1 10. If less control over the quality of the bonding between the segments 120 is acceptable it may also be possible to apply the brazing material after the segments 120 have been arranged on the shaft. Brazing material may for example be applied at a free end of an outermost one of the segments 120. By orienting the shaft 1 10 with the segments 120 in the vertical direction the brazing material may become distributed along the shaft 1 10 and flow to the other segments 120 during the sintering step. In any case, the grooves 124 of the segments 120 which not are occupied by the protruding portion 1 14 may aid distribution of brazing material between the segments 120 and along the shaft 1 10.

As illustrated in Fig. 3, the relative cross-sectional dimensions of the through-hole of each segment 120 and the shaft 1 10 are such that the shaft 1 10 is movable through each segment 120. The relative dimensions may be such that an interference fit results between the shaft 1 10 and the segments 120. However, the interference fit should preferably not be tighter than to allow movement of the shaft 1 10 through each segment 120 without causing damage to the surface of the segments 120 or the shaft 1 10. Alternatively the dimensions may be such that a clearance fit results between the shaft 1 10 and the segments 120. However, the clearance should preferably not be so great that a substantial rotation of a segment 120 about the shaft 1 10 is allowed, when the segment 120 is arranged at its intended position along the shaft 1 10. A substantial rotation of a segment means a rotation allowing threads 126 of adjacent segments to deviate from alignment to such an extent that a required smoothness of the outer surface of the rotor 100 not is obtained.

Each one of the heat applied during the sintering step, the

transformation of the segments 120 during the sintering, as well as the subsequent cooling may result in changed dimensions of the shaft 1 10 and the segments 120. By appropriate choices of powder material, shaft material and process parameters during the sintering step dimensional changes may however be controlled and limited to an acceptable amount. In fact, a slight relative decrease of the cross-sectional dimensions of the through-hole of each segment 120 ,in relation to the corresponding dimensions of the shaft 1 10, may be allowed wherein a shrink fit between the shaft 1 10 and the segments 120 may be obtained.

Optionally, a stopper may be arranged on the shaft 1 10 to restrict movement of the segments 120 along the shaft 1 10. For example a clamp may be arranged on the shaft 1 10 to abut an outer one of the segments 120. A stopper may prevent movement of a segment 120 along the shaft 1 10 beyond the position of the stopper along the shaft 1 10. A second stopper may be arranged at an opposite outer segment 120. Thereby all segments 120 may be fixed in position along the shaft 1 10. This may be used in combination with the above-discussed bonding and shrink fitting between the segments 120 and the shaft 1 10 or as an alternative means for achieving fixation. It has further been contemplated that the segments 120 may be sintered prior to being arranged on the shaft 1 10. In that case, positions of the segments 120 along the shaft 1 10 may be fixed using the above-discussed stopper(s) wherein the segments 120 may be kept together and fixed in position by mechanical means. Alternatively, the segments 120 may be fixed to the shaft 1 10 using an adhesive provided at the interface between the shaft 1 10 and the segments 120. In any case, the segments 120 may also be joined by providing an adhesive at the interfaces between adjacent segments 120.

Although the illustrated embodiment comprises four segments 120, this is only one example. In particular, by virtue of the above-discussed relation between the length of the segments 120 and the pitch thereof, the length of the rotor 100 may be easily varied by removing segments 120, or by adding additional segments identical to the segments 120, to/from the shaft 1 10. Generally however, the number of segments may depend on the desired length of the finished rotor, the characteristics of the threading of the finished rotor, etc. A longer rotor, or a rotor having a smaller pitch, may for example require a greater number of segments to make forming of the segments practical.

It should further be noted that the particular cross-sectional shape, the pitch as well as the number of the threads 126 illustrated in Fig. 1 merely form one example and that other designs of the outer threaded surface of the segments 120 also are possible. For example, the segments may for example comprise a higher or a lower number of threads such as 1 (e.g. for forming a single-start screw rotor), 2-5 or 7 and more (e.g. for forming a multi-start screw rotor).

The illustrated segment 120 comprises a same number of grooves 124 as a number of threads 126 of the segment 120. Furthermore, the grooves 124 are symmetrically distributed about the through-hole. Thus, for the purpose of obtaining alignment between thread tops of abutting segments 120, when the segments 120 are arranged on the shaft 1 10, the particular choice of groove does not matter.

The illustrated segments 120 comprise six grooves 124. Thereby, each segment 124 comprises grooves which not will be occupied by the protruding portion 1 14. As discussed above, this may provide facilitate distribution of brazing material. However, this advantage may be obtained also with segments comprising only two or more grooves. Preferably, the unoccupied one or more grooves of each such segment align with each other when the segments are arranged on the shaft 1 10. Moreover, the symmetric distribution of the grooves 124 (as seen in e.g. Fig. 2) is also optional and non-symmetric distributions are also contemplated although a symmetric distribution may facilitate alignment between unoccupied grooves and thereby facilitate assembly of the rotor 100.

Additionally, the shaft 100 may be formed with one or more additional protruding portions, corresponding to the protruding portion 1 14. More than one protruding portion 1 14 may enable a stronger cooperation between the segments 120 and the shaft 1 10, thereby preventing rotation of the segments 120 about the shaft 1 10 even more strongly. Generally, the number of protruding portions should preferably be less than, or equal to, the number of grooves 124 of each segment 120. The distribution of the protruding portions about the shaft 1 10 should preferably correspond to the distribution of the grooves about the through-hole of each segment 120.

Although the identical geometry of the segments 120 provides advantages also segments having non-identical geometries are contemplated to fall within the scope of the present inventive concept. This may be used, for example, if production-related aspects make it beneficial to form segments not having a length which equals the pitch, or if the common threaded surface of the final rotor 100 should present a pitch which varies along the longitudinal direction.

Fig. 5 illustrates a further variation of the embodiment described in connection with Fig. 1 . The rotor 500 is identical to the rotor 100 however differs in that the protruding portion 514 of the shaft 510 extends only along a longitudinal section 518 of the shaft 510. In particular the length of the longitudinal section 518 corresponds to the combined length of the segments 520 arranged on the shaft 510. This may facilitate incorporation of the rotor 500 in a screw compressor by the shaft 510 providing smooth and circular end portions, and thus rotationally invariant end portions. For example, this may facilitate journaling of the shaft 510 in bearings.

According to a further embodiment, a shaft may be formed with a plurality of disconnected protruding portions, a respective protruding portion for each segment. Each respective protruding portion may be arranged at a respective sub-section along the shaft. Each respective protruding portion may for example have a length which is less than the length of a segment which is intended to be arranged at the sub-section. The respective protruding portions may be aligned with each other, or be provided at different angular positions about the shaft. In the latter case, each segment preferably comprises grooves at least at the same angular positions about the

longitudinal axis as the protruding portions of the shaft to allow the segments to be easily movable along the shaft.

According to a further embodiment, a shaft may be formed with a protruding portion similar to the protruding portions 1 10 or 510 however differing in that the protruding portion extends helically about the shaft 1 10. The groove(s) of the segments 120, 520 may be arranged to form an angle with the axial direction, which angle corresponds to the helix angle of the protruding portion, wherein movement of the segments along the shaft is allowed. Regardless of the number of grooves of each segment, each segment may comprise at least one groove at an angular position in relation to the outer threaded surface such that a continuous threaded surface is formed by the outer threaded surfaces of the segments when each of said segments is moved to its intended position, i.e. corresponding to a respective sub-section, along the shaft.

Fig. 6 illustrates the cross section of a segment and of a shaft, formed in accordance with a further embodiment. The cross sections illustrated in Fig. 6 differs from the cross sections illustrated in Fig. 2 in that the shaft 610 is formed with a number of longitudinally extending grooves, exemplified by groove 614, and the segment 620 is formed with a corresponding number of longitudinally extending projecting portions, exemplified by projecting portion 624. Thus, in Fig. 6, the groove 614 forms first guiding means and the projecting portion 624 form second guiding means, Especially, the groove 614 provides edge portions and the protruding portion 624 provides a gripping portion engaging with the edge portion. The cooperation at the interface portion formed between the protruding portion 624 and the groove 614 guides or controls the orientation of the segment 620 about the shaft 610. The variations of the protruding portion 1 14, 514 and the groove 124 discussed above (e.g. the number of grooves and protruding portions and/or the geometry and distribution thereof) may be correspondingly applied to the groove 614 and the protruding portion 624 in Fig. 6.

A common aspect for the shafts and the segments in the above- illustrated embodiments, as well as the above-described variations thereof, is that the shaft (or at least a longitudinal section thereof) is formed with a cross section having an outer peripheral shape deviating from a circular shape. Correspondingly, each segment is formed with a through-hole having a shape deviating from circular shape. Moreover, the outer peripheral shape of the shaft and the shape of the through-of each segment matches along at least a portion about the shaft wherein an interface portion between the shaft and each segment is formed. The interface portion prevents rotation of each segment about the shaft and thus determines the orientation of each segment about the shaft. In other words the shaft is provided with first guiding means in the form of a non-circular outer peripheral shape and the segments are provided with second guiding means in the form of non-circular through-holes.

According to some embodiments, the non-circular cross section and through-hole may present a polygonal shape. A polygonal shape provides a number of edge portions of the shaft and a number of gripping portions of each segment (i.e. edge portions of the polygon), which may cooperate to guide the orientation of the segments about the shaft. For example, with reference to Fig. 7, the shaft 710 may be formed with a rectangular outer peripheral shape, in particular a square shape. The segments 720 may be formed with a through-hole having a corresponding rectangular shape, in particular a square shape. With reference to Fig. 8, the shaft 810 may be formed with a hexagonal outer peripheral shape. The segments 820 may be formed with a through-hole having a corresponding hexagonal shape. In fact, the shape of the through-hole and the shaft need not even be identical. For example, with reference to Fig. 9, the shaft 910 may be formed with a hexagonal outer peripheral shape while the segments 920 may be formed with a through-hole having a rectangular shape, in particular a square shape.

Fig. 10 illustrates a further example of non-circular shapes, wherein the shaft 1010 and the through-hole of the segments 1020 are formed with corresponding elliptical shapes. The interface between the shaft 1010 and each segment 1020 prevents rotation of each segment about the shaft and thus determines the orientation of each segment 1020 about the shaft 1010.

In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily

appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims

1 . Method for producing a rotor for a screw compressor comprising:

providing a shaft;

2. A method according to claim 1 , further comprising forming said longitudinal section of the shaft with first guiding means and forming each segment with second guiding means, wherein the first and the second guiding means are arranged to cooperate with each other to control an orientation of the outer threaded surface of each segment about the shaft.

3. A method according to any one of claims 1 -2, further comprising:

forming said longitudinal section of the shaft with a cross section having an outer peripheral shape deviating from circular shape, and

forming each of said segments with a through-hole having a shape deviating from circular shape.

4. A method according to any one of claims 1 -3, further comprising:

forming said longitudinal section of the shaft with a cross section having a polygonal outer peripheral shape which is polygonal,

forming each of said segments with a through-hole having a polygonal shape.

5. A method according to any one of claims 1 -4, further comprising:

forming said longitudinal section of the shaft with a cross section having an outer peripheral shape including at least one edge portion, and forming each of said segments with a gripping portion extending along a portion of the respective through-hole and being arranged to engage with said edge portion when said segment is moved to its respective position along the shaft.

6. A method according to any one of claims 1 -5, further comprising forming the inner surface of each segment with a groove and forming a protruding portion on the boundary surface of said longitudinal section, wherein the protruding portion is arranged to extend into the groove of each segment when each of said segments is moved to its respective position along the shaft.

7. A method according to any one of claims 1 -5, further comprising forming the inner surface of each segment with a protruding portion and forming a groove in the boundary surface of said longitudinal section, wherein the protruding portion is arranged to extend into the groove when each of said segments is moved to its respective position along the shaft.

8. A method according to any one of claims 6-7, wherein arranging each segment on the shaft comprises arranging the protruding portion to extend into said groove.

9. A method according to any one of claims 1 -8, wherein an end portion of each segment is arranged to abut against an end portion of an adjacent segment when each of said segments is moved to its respective position along the shaft.

10. A method according to any one of claims 1 -9, wherein mutually abutting end portions of said segments are formed with cross sections having matching outer peripheries when the segments are arranged at their respective positions along the shaft.

1 1 . A method according to any of claims 1 -10, wherein said segments are formed to present identical geometries.

12. A method according to any of claims 1 -1 1 , wherein the length dimension of each segment corresponds to an integer multiple of the pitch of the threaded surface of said segment.

13. A method according to any of the claims 1 -12, wherein said segments are formed as green body segments, and the method further comprises sintering the segments arranged on the shaft.

14. A method according to claim 13, the method further comprising sinter brazing the green body segments arranged on the shaft.

15. A kit of parts for a screw rotor for a screw compressor, comprising:

a shaft, and

at least two rotor segments of compacted metal powder, said segments comprising an outer threaded surface and an inner surface defining a through-hole for receiving the shaft, wherein each segment is movable along the shaft;