ROTOR FOR A ROTARY SCREW MACHINE, A ROTARY SCREW MACHINE AND A PROCESS FOR MANUFACTURING A ROTOR

Abstract: The invention relates to a rotor for a rotary screw machine having helical lobes (12) and grooves (14). According to the invention a shell structure (18) forming the external surface of the rotor is made of pressed metal powder. The rotor is manufactured by cold isostatic pressing of a powder in a vessel having a surface shaping the rotor. The pressure is applied radially by means of a cast bag in which high pressure liquid acts. After the pressing process the rotor blank is sintered and attached to a shaft (16).

Description: ROTORFORAROTARYSCREWMACHINE,AROTARYSCREWMACHINE ANDAPROCESSFORMANUFACTURINGAROTOR. The present invention relates in one aspect to a rotor for a rotary screw machine with helical lobes and intermediate grooves on the external peripherical surface of the rotor. The invention also relates to a rotary screw machine being provided with at least one such rotor. In still another aspect the invention relates to a process for manufacturing such a rotor. Common types of such machines operating as compressors contain a pair of rotors of the kind mentioned above running in a working space formed by a surrounding casing, having a shape of two intersecting cylinders, each housing one of the rotors. Through the helical lobes and grooves the rotors intermesh, forming chevron-shaped working chambers between themselves and the surrounding casing, in which chambers the gaseous fluid is transported and compressed. One of the rotors is a male rotor, having the major part of each lobe located out¬ side its pitch circle and the other one is a female rotor, having the major part of each lobe located inside its pitch circle. A screw rotor of this kind is a complicated element, in particular concerning its geometry. In a transverse section through a rotor each flank of each lobe follows a curve which usually is of complex structure and has to cooperate with a corresponding complex curve on a lobe of the other rotor. In many cases the lobes are unsymmetrical so that the two flanks of a lobe are different. Since the lobes are helically extending along the rotor, the angular position of a transverse section through the rotor varies if the location of the section is moved axially along the rotor. The rotors intermesh along their entire axial extension, whereby the sealing lines forming the working chambers each receives a complicated three-dimensional configuration. For attaining an optimal shape of the rotor profiles a lot of considerations have to be made concerning clearances, leakage, efficiency, capacity, noise, vibrations and manufacturing aspects. Due to the complex geometrical structure of the rotors, the different requirements they must meet and the high accuracy that is necessary the costs for their manufacture are considerable. According to the conventional technique a rotor is machined from cylindrical steel blanks in a plurality of passes. This process is time-consuming and results in a high degree of waste material. In the field of screw rotor technology it therefore since long have been continuous efforts to attain rotors, which are more economical to manufacture. This has resulted in a lot of solu¬ tions using different manufacturing techniques and different materials including composite rotors. Examples of rotors being made by less conventional manufacturing techniques and/or other materials and material combinations can be found in the following patent documents: US 3 918 838 discloses a rotor in which a plastic is molded on a metal skeleton, DE 2409554 discloses a rotor made by glassfibre-reinforced plastic on a steel shaft, DE 3 424 148 discloses an injection-molded polymer rotor, DE 2364727 discloses a rotor built by a plurality of plates, WO 86/05555 discloses a plastic rotor, US 2714314 discloses a rotor made as a welded construction, GB 1 433 132 discloses manufacturing of a rotor by extruding and DE 3 124247 discloses a ceramic rotor made of silicon nitride. The object of the present invention is to achieve a rotor which can be more economically manufactured than by the conventional technique and to develop a process for manufacturing such a rotor. This has according to the invention been attained in that a rotor of the kind in question includes an external shell made of pressed metal powder of substantially uniform thickness forming said peripherical surface and in that the method of manufacturing the rotor includes that said shell is manufactured by cold isostatic form pressing in a die having an internal shape substantially corresponding to the external shape of the finished rotor, and is pressed by a pressing tool located within said die which pressing tool has a shape corresponding to the shape of the die but with smaller radial dimensions so that a gap of substantially uniform thickness is formed between said die and said pressing tool, in which gap said shell is formed, said shell is sintered and said shell is attached to a shaft. Although manufacturing articles by compacting metal powder under pressure has been used for several decades, there have been no earlier attempts to manufacture screw rotors or important parts thereof by this technique. Examples of manufacturing articles of complicated shape by cold isostatic form pressing are disclosed in SE 303 108, US 3 506383, DE 2 133724, DE 2 349776 and GB 1 470584 which hereby are included by reference. Using the powder pressing technique for manufacturing the outer layer of a screw rotor results in cost reduction in comparence with conventional technique, since only one or even none machining pass will be required for achieving the finished rotor profile, whereas at least two or three machining passes are required when conventionally manufactured, and since the amount of material used will be much lower. A rotor having an external shell made of pressed metal powder also has the advantage that the material for the shell can be composed, by mixing suitable powder qualities, for different application areas of the machine. As an example, the rotor can be given self-lubricating pro¬ perties, and the addition of the lubricating compounds will be integrated in the basic produc- tion process without any additional cost. In an advantageous embodiment the shell is attached to a shaft along the helical lines formed by the internal side of the shell at the bottom of the grooves. This leaves a large part of the rotor to consist of hollows, which rotor therefor will have low weight and low moment of inertia. Alternatively the interspace between the shell and the shaft can be filled with a polymer, through which they are connected to each other. The polymer should have a coefficient of elasticity lower than that of the shell. This contributes to avoid that the shell will losen from the polymer due to the centrifugal forces. For the manufacture of a rotor having a shell structure made of pressed metal powder an advantageous embodiment of the process includes that the shell is attached to the shaft along the helical lines which are formed at the inside of the groove bottoms. In the claims, further preferred embodiments of the rotor and the process for manufacturing the rotor are specified. The claims also include a rotary screw machine in which the invented rotor is used. The invention will be explained through the following detailed description of preferred embodiments of a rotor according to the invention and of preferred embodiments of a process for the manufacture of a rotor according to the invention and with reference to the accom¬ panying drawings of which, Figure 1 is a longitudinal section through a rotor according to the invention, Figure 2 is a section along line II-II of figure 1, Figure 3 is a section similar to that of figure 2, but showing a second embodiment of the invention and Figure 4 schematically illustrates the compacting step of the manufacture of a rotor according to the invented process. The rotor shown in figures 1 and 2 is a male rotor of a rotary screw compressor. The rotor has five lobes 12 extending helically along the rotor and intermediate grooves 14. The rotor is constituted by a shell structure mounted on a hollow steel shaft 16. One part 18 of the shell t UKIαV o α-ii»_valil-i structure forms the high pressue end surface of the rotor. The other end of the rotor is open, allowing the incoming air to reach the hollows in the rotor. The shaft 16 is attached to the shell structure 18, 20 at the inner side of each groove bottom 24. The shell structure 18, 20 is made of pressed metal powder. The major part of the powder is Fe-based powder. The powder also contains ingredients giving the surface of the shell structure 18, 20 self- lubricating properties. The shell structure is secured to the shaft during a sintering process. The hollows in the shell structure can alternatively be filled. Such is the case in the embodi¬ ment shown in figure 3. The interspace between the shell 18' and the shaft 16', which inter¬ space includes the hollows in the lobes is filled with a polymer forming a rotor body 26. This filling material forms the connection for the attachment of the shell 18' to the shaft 16', so that it is not necessary that the inside of the groove bottoms contact the shaft. The polymer has a coefficient of elasticity which is lower than that of the shell. The shell structure of the rotors of figures 1 to 3 is made by pressed metal powder being compacted in a so-called cold isostatic pressing process. Figure 4 schematically illustrates this process when used for manufacturing a rotor according to the invention. The metal powder 30 to be compacted is filled into a vessel 28 including a frame 34, 36 and a die or forming tool 38 being internally shaped so as to correspond to the shape of the finished rotor. A pressing tool in the form of a flexible cast bag 32 made of silicon/urethan and containing a liquid 44, is located centrally in the vessel. The external surface 48 of the cast bag 32 has a shape corresponding to the shape of the finished rotor, but has smaller radial dimensions. The die 38 and the cast bag 32 are positioned in such a way that their profiles are aligned. Thereby a gap is formed between them, which is of substantially uniform thickness. This gap will be filled up by the metal powder 30. The liquid 44 in the cast bag 32 is connectable to a pressue source (not shown). After filling the powder 30, the cast bag 32 is closed by a cover 40, and a closure part 42 of the frame is secured to the cylindrical wall 34 thereof. Thereafter the cavity containing the liquid 44 is brought into communication with the pressure source so that the liquid will be under pressure. This pressure is very high, normally in the range of 200 to 500 MPa. Due to the pressure in the cast bag 32 excelled by the liquid 44, the cast bag 32 will expand radially and compact the powder 30 to a shell structure in the gap between the cast bag 32 and die 38. The compaction reduces the gap to about 80 % of its initial thickness, whereas there is no reduction in the axial dimensions. _*-*_ιtεr linisiiiπ tuc compaction, the closure 4__. anu the cover 40 are removed anu tuc pressεu shell structure can be taken out from the die. The pressed rotor blank is then sintered. Before sintering, the shell structure of the rotor blank is placed on a shaft contacting the shell structure at the internal side of the groove bottoms. During the sintering the shell structure then will adhere to the shaft along the contact lines. This connection will be sufficient to transmit the torque when the rotor is in operation. In an alternative embodiment the shell structure is sintered before attachment to the shaft. In the sintered shell the shaft is inserted centrally without contacting the shell. A polymer is injected into the interspace between the shell and the shaft so that these parts are molded together as the polymer is solidified. The polymer is selected to have a coefficient of elasti¬ city lower than that of the shell.

Claims: CLAIMS 1. A rotor for a rotary screw machine with helical lobes (12) and intermediate grooves (14) on the external peripherical surface of the rotor, c h ar a c t e ri z e d i n t h a t the rotor includes an external shell (18) made of pressed metal powder of substantially uniform thickness forming said peripherical surface. 2. A rotor according to claim 1 including a shaft (16), said shaft (16) and said shell (18) contacting each other along helical lines (24) formed by the internal side of said shell (18) at the bottoms of said grooves (14) whereby said shell (18) is attached to said shaft ZΛ _r\ _ι . , ,: J Λ />-> Λ \ (lO) iυng s iu lines .( ,*+j. 3. A rotor according to claim 1 including a shaft (160 and a rotor body located between said shaft (16) and said shell (18'), said rotor body (26) being made of a material having a lower coefficient of elasticity than said shell (18'). 4. A rotor according to claim 3 in which said material is a polymer 5. A rotary screw machine provided with at least one rotor according to any of claims 1 to 4. 6. A process for manufacturing a rotor for a rotary screw machine having helical lobes and intermediate grooves on the external peripherical surface of the rotor, which rotor includes an external shell made of pressed metal powder of substantially uniform thick¬ ness forming said peripherical surface, c h a r a c t e r i z e d i n t h a t - said shell is manufactured by cold isostatic form pressing of metal powder (30) in a die (38) having an internal surface (46) with a shape substantially corresponding to the external shape of the finished rotor, and is pressed by a pressing tool (32) located within said die (38) which pressing tool (32) has a shape corresponding to the shape of the die (38) but with smaller radial dimensions so that a gap of substantially uniform thickness is formed between said die (38) and said pressing tool (32), in which gap said shell is formed, said shell is sintered, and said shell is attached to a shaft. 7. A process according to claim 6 in which said shell is attached to said shaft along the helical lines which are formed at the inside of said shell by the bottoms of said grooves. ' 8. A process according to claim 7 in which said shell is attached to said shaft by sintering. 9. A process according to claim 6 in which said shell is attached to said shaft in that said shaft is centrally positioned within said shell forming an interspace between said shaft and said shell, said interspace is filled up with a material in a plastic state, and said material is solidified. 10. A process according to claim 9 in which said material is a polymer which polymer in its solidified state has a lower coefficient of elasticity than said shell.

Inventors: TIMUSKA KARLIS, TIMUSKA, KARLIS
Applicants: SVENSKA ROTOR MASKINER AB
Application year: 1991
Main classification: F04C18
IPC codes: B22F3, B22F5, B22F7, B29C70, B30B11, F01C1, F04C18