ROTOR FOR A ROTARY SCREW MACHINE

Abstract: A rotor for a rotary screw machine, in particular a male rotor, having helical lobes and intermediate grooves. The rotor has a core consisting of a shaft (1) of a first material and an intermediate layer (2) of a second material mounted on the shaft. The surface of the rotor is coated with a plastic layer. The invention also concerns a rotary screw machine provided with such a rotor. The invention further relates to a method for manufacturing the rotor, whereby a rod is extruded to a shape substantially corresponding to the shape of the rotor and then being cut up into rotor bodies. The plastic layer is injection moulded around each of said rotor bodies. After making a hole in the rotor body it is mounted on the shaft. The rotor can be manufactured without any cutting machining and meets the required demands on shape accuracy and strength.

Description: ROTOR FOR A ROTARY SCREW MACHINE The present invention relates to a rotor for a rotary screw machine, in particular a male rotor, having helical lobes and intermediate grooves forming the working surface of the rotor, and being axially limited by two radial end surfaces, the rotor having a core and a surface layer of substantially uniform thickness made of a plastic, prefer¬ ably a composite plastic, coated on at least said working surface formed by the lobes and the grooves. The invention also relates to a method for manufacturing such a rotor. Screw rotors are normally manufactured by machine cutt¬ ing of solid metal blanks. Having a complicated geometry the screw rotors require high manufacturing precision, and; the amount of material to be cut away is very large. In combination these drawbacks result in long manufacturing times and high costs. Consequently, it has long been desired to produce screw rotors in a simpler fashion and with less stringent re¬ quirements on precision and accuracy, in order to enable the rotors to be manufactured in very large quantities at reasonable costs. Endeavours have been made as early as in 1953 to there¬ fore produce the rotors from plastic, as evident from US Patent 2,868,442. The document thus discloses a male rotor for a rotary screw compressor of the Lysholm type, in which the rotor is made of a plastic surrounding a metal core. The core consists only of the rotor shaft and the rotor holds a large quantity of plastic. To make a rotor with a _ large plastic body having varying thickness entails draw¬ backs of different kinds. Large and varying shrinking re¬ sults in bad accuracy to size, and it is furthermore hard to avoid that shrinking stresses develop in the plastic part of the rotor. Also the tensile properties of the rotor will be limited. Such a rotor therefore is appropriate only for small rotor dimensions. In order to achieve improved strength in a rotor con¬ sisting of a plastic surrounding a metal core it is further known to reinforce the plastic with metal. US patent 3,918,838 discloses a female rotor moulded from plastic around a metal shaft whereby the plastic is reinforced by a skeleton having radial metal discs substantially correspon¬ ding to the external shape of the rotor. Since the plastic also in this case reaches the rotor shaft the difficulties with built-in shrinking stresses remain, in particular if such a construction would be used for the male rotor having a la-rger amount of material". - • - Attempts also have been made to make a 'rotor of a metal. core substantially corresponding to the external shape of the rotor and coat the rotor with a thin plastic layer. An example of this type is disclosed in GB 1,306,352. The metal core in this case is made integral with the shaft. With regard to the material required for the rotor shaft the manufacture of the rotor body implies a moulding pro¬ cedure, whereby considerable unevennesses will occur on the external surface thereof. This complicates the moulding of the plastic layer around the metal core since the uneven¬ nesses creates constrictions in the narrow space between the rotor body and the surrounding mould which obstruct a uniform distribution of the plastic in said space. The un¬ evennesses also create unbalanced centrifugal forces in the rotor, in particular at high rp . These problems could be avoided by milling or grinding the working surface of the rotor body before applying the plastic layer although the manufacturing costs thereby would be increased. In spite of the achievements in this field - to elimi¬ nate the need for machine cutting of the rotors by making them partly of plastic - a satisfactory solution has up to now not been attained, neither through the above mentioned examples nor through other similar constructions. One object of the present invention thus is to attain a rotor for a rotary screw machine which does not require any machine cutting for its manufacture and which does not has the drawbacks entailing earlier attempts to attain this. This has according to one aspect of the invention been achieved in that the core of a rotor of the introductional- ly specified kind consists of a shaft of a first material on which an intermediate layer of a second material is mounted. Advantageous embodiments of the invention are specified in the dependent claims . The invention also relates tq an appropriate application of the rotor in a rotary screw machine. Another object of the invention is to develop a method . for manufacturing a rotor of the construction specified above. This has been achieved in that a rod of said second material continuously is extruded to a shape substantially corresponding to the shape of the working surface of the rotor having helical lobes and grooves, said extruded pro¬ file is cut up into rotor bodies of a length, corresponding to the length of the rotor, said plastic layer is injection moulded around said rotor body to the shape of the finished rotor profile, a centric hole is made in said rotor body and said rotor body is mounted on the shaft of said first metal . The invention is further explained in the following de¬ tailed description of a preferred embodiment of the inven¬ tion and with reference to the accompanying drawings . Figure 1 is a longitudinal sectional view through the upper half of a rotor according to the invention. Figure 2 is a vertical section through the rotor taken along line II-II of figure 1. The rotor illustrated in the figures is the male rotor of a rotary screw compressor. It has three helically exten¬ ding lobes and intermediate grooves to gearingly mesh with grooves and lobes of a female rotor in the compressor. The rotor consists of a steel shaft 1,on which a rotor body 2 made of aluminium or an alloy thereof is mounted. The rotor body is shrinked on to shaft, but also other kind of joints, e.g. of the key-type can be used. The lobes and grooves of the rotor are coated with a layer 3 of a composite plastic, preferably a thermoplastic containing up to 30 %, in certain cases up to 50 % reinfor¬ cing fibres. In a preferred embodiment the plastic used was Polyamid 66. The thickness of the layer is some millimetres and the length of .the fibres lies in the range from some tenths of millimetres to some ten millimetres. With the selected combination of materials the rotor bo¬ dy 2 has a coefficient of thermal expansion exceeding that of the shaft 1 , and the coefficient of thermal expansion of the plastic is at least as great as that of the aluminium rotor body. As a consequence thereof an advantageous dist- ribution of thermal stresses occuring during operation of the compressor will be attained. Also one end face of the rotor is coated with a plastic layer 4 securing an even end surface at the end of the ro¬ tor facing the high pressure end wall of the compressor. Thanks to its construction in three components the rotor can be manufactured without any machine cutting of the pro¬ file. By selecting aluminium for the rotor body it will be possible to attain a profile of satisfactory accuracy by extruding and without any subsequent tooling of the sur- face. The low density of the material also will reduce un- — — balanced centrifugal forces. By extruding, long profiled rods can be made, which thereafter are cut up into rotor bodies . When the rotor body has been mounted on the steel shaft the plastic layer is applied. This is made by injec- tion moulding, whereby the rotor body is placed in a mould with an inner surface having a shape corresponding to the shape of the finished rotor. Alternatively the plastic layer can be applied before the rotor body is mounted on the shaft. By the invention a rotor has been attained which is of a construction that eliminates the need for machine cutting of the rotor profile and which also meets required demands on shape accuracy and strength.

Claims: _ fi . CLAIMS 1. A rotor for a rotary screw machine, in particular a male rotor, having helical lobes and intermediate grooves form¬ ing the working surface of the rotor, and being axially li¬ mited by two radial end surfaces, the rotor having a core and a surface layer (3) of substantially uniform thickness made of a plastic, preferably a composite plastic, coated on at least said working surface formed by the lobes and the grooves, characterized in that the core consists of a shaft (1 ) of a first material on which an intermediate layer (2) of a second material is mounted. 2. The rotor according to claim 1 in which the materials of the rotor are so selected that the coefficient of thermal expansion of the plastic is at least as great as that of the second material and that the coefficient of thermal expansion of the second material exceeds that of the first material. 3. The rotor according to claim 2 in which said second material is aluminium or an alloy thereof. 4. The rotor according to any of claims 1 to 3 in which said plastic also is coated on at least one of said radial end surfaces (4) . 5. Rotary screw machine, characterized in that it is pro¬ vided with a rotor according to any of claims 1 to 4. 6. A method for manufacturing a rotor according to any of claims 1 to 4, the rotor having helical lobes and inter¬ mediate grooves forming the working surface of the rotor, and being axially limited by two radial end surfaces, the rotor having a core and a surface layer of substantially uniform thickness made of plastic, the core being composed of a shaft of a first material, on which an intermediate layer of a second material is mounted, characterized in that - a rod of said second material continuously is extruded to a shape substantially corresponding to the shape of the working surface of the rotor having helical lobes and grooves, - said extruded profile is cut up into rotor bodies of a length corresponding to the length of the rotor, - said plastic layer is injection moulded around said rotor body to'the shape of the finished rotor profile, - a centric hole is made in " said rotor body and - said, rotor body is mounted on the shaft of said first material.

Inventors: TIMUSKA KARLIS, TIMUSKA, KARLIS
Applicants: SVENSKA ROTOR MASKINER AB
Application year: 1987
Main classification: F04C18
IPC codes: F01C1, F04C18