Abstract:
A combination double screw rotor assembly includes a first and a second screw rotor arranged in parallel in a casing, two sets of bearings respectively mounted in the casing near the outlet to support the shafts, and a plurality of locking means respectively fastened to the shafts near the inlet. The first and the second screw rotor each has a low pressure screw rotor element, a high pressure screw rotor element, and a spiral thread formed of a first spiral thread segment at the high pressure screw rotor element and a second spiral thread segment at the low pressure screw rotor element, the first spiral thread segment having an uniform short pitch, the second spiral thread segment having an uniform long pitch, the first spiral thread segment and second spiral thread segment of the first and the second screw rotor being respectively meshed together.

Description:
This application is a continuation-in-part of my patent application, Ser. No. 09/639,944, filed Aug. 17, 2000 U.S. Pat. No. 6,341,951. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to double screw rotor assembly, and more particularly to a multi-segment or combination double screw rotor assembly for controlling a flow pressure, for example, for use in vacuum pumps, air compressors, etc. 
     FIG. 1 shows a double screw rotor assembly constructed according to U.S. Pat. No. 5,443,644. This structure of double screw rotor comprises two screw rotors  81  and  82  meshed together. Because the screw rotors  81  and  82  have an uniform pitch P′ and same height of tooth H′, the volume and pressure of the air chambers  810  and  820  are not variable. When operated through a certain length of time, a high pressure occurs in the area around the outlet  80 , and a significant pressure difference occurs when air is transferred to the outlet  80 , resulting in a reverse flow of air, high noises, and high energy consuming. 
     U.S. Pat. No. #5,667,370 (FIG. 2) discloses a horizontal type double screw rotor assembly. According to this design, the first pair of screw rotors  32 ′ and  33  and the second pair of screw rotors  34  and  35  have different outer diameters and pitches. Further, the installation of the partition plate  93  between two shells  91  and  92  greatly increases the dimension of the screw rotor assembly and complicates its structure. 
     FIG. 3 shows still another structure of horizontal type double screw rotor assembly according to the prior art. According to this design, the screw rotors  4 ′ and  5 ′ have a variable pitch. 
     However, because the processing of the screw rotors requires a specially designed processing equipment and cutting tool, the manufacturing cost of this structure of double screw rotor is high. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished to provide a combination double screw rotor assembly, which eliminates the aforesaid drawbacks. It is one object of the present invention to provide a combination double screw rotor assembly, which effectively prevents a reverse flow, and reduces power loss and operation noise. It is another object of the present invention to provide a combination double screw rotor, which is compact and requires less installation space. It is still another object of the present invention to provide a combination double screw rotor assembly, which is easy and inexpensive to manufacture. According to one aspect of the present invention, the combination double screw rotor assembly comprises a casing, a first screw rotor, and a second screw rotor. The casing comprises an inside wall defining a receiving chamber, an inlet, and an outlet. The first rotor comprises a shaft pivoted in the casing, a low pressure screw rotor element and a high pressure screw rotor element respectively mounted on the shaft in direction from the inlet toward the outlet, and a spiral thread raised around the periphery thereof and extended over the low pressure screw rotor element and high pressure screw rotor element. The spiral thread of the first rotor is comprised of a first spiral thread segment raised around the periphery of the low pressure screw rotor element of the first rotor and defining an uniform long pitch, and a second spiral thread segment raised around the periphery of the high pressure screw rotor element of the first rotor and defining an uniform short pitch. The second screw rotor comprises a shaft pivoted in the casing and disposed in parallel to the shaft of the first screw rotor, a low pressure screw rotor element and a high pressure screw rotor element respectively mounted on the shaft of the second rotor in direction from the inlet toward the outlet, and a spiral thread raised around the periphery thereof and extended over the low pressure screw rotor element and high pressure screw rotor element of the second rotor. The spiral thread of the second rotor is comprised of a first spiral thread segment raised around the periphery of the low pressure screw rotor element of the second rotor and defining an uniform long pitch, and a second spiral thread segment raised around the periphery of the high pressure screw rotor element of the second rotor and defining an uniform short pitch. The first spiral thread segment and second spiral thread segment of the spiral thread of the second screw rotor are respectively meshed with the first spiral thread segment and second spiral thread segment of the first screw rotor. According to another aspect of the present invention, two parallel sets of axle bearings are mounted in the casing near the outlet to support the shafts of the first screw rotor and the second screw rotor, and keyless axle bushes or like device are installed in the shafts of the first screw rotor and the second screw rotor to secure the axle gearings in place. According to still another aspect of the present invention, timing gears are respectively mounted on the shafts of the first screw rotor and the second screw rotor and meshed together for enabling the first screw rotor and the second screw rotor to be rotated without contact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a double screw rotor assembly according to the prior art. 
     FIG. 2 is a sectional view of another structure of double screw rotor assembly according to the prior art. 
     FIG. 3 is a sectional view of still another structure of double screw rotor assembly according to the prior art. 
     FIG. 4 is a sectional view of a combination double screw rotor assembly according to the present invention. 
     FIGS. 5 to  11  are sectional views of alternative embodiments of combination double screw rotor assemblies according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 4, a combination double screw rotor assembly is shown adapted for use in a vacuum pump, comprised of a casing  1 , a first screw rotor  2 , and a second screw rotor  3 . 
     The casing  1  comprises a top cover  11 , a peripheral shell  12 , and a bottom cover  13 . The top cover  11  has an inlet  111  connected to an enclosure to be drawn into a vacuum condition. 
     The peripheral shell  12  comprises an inside wall  121  defining a receiving chamber  10 . The bottom cover  13  comprises an outlet  131  disposed in communication with the atmosphere, and two parallel sets of axle bearings  46  and  46 ′ adapted to support respective shafts  4  and  5  of the screw rotors  2  and  3  on the bottom cover  13 . 
     The first screw rotor  2  comprises a low pressure screw rotor element  21  and a high pressure screw rotor element  22  axially connected in a line and extended in direction from the inlet  111  toward the outlet  131 , and a spiral thread  20  raised around the periphery thereof and extended over the low pressure screw rotor element  21  and the high pressure screw rotor element  22 . The spiral thread  20  is comprised of a first spiral thread segment  201  raised around the periphery of the low pressure screw rotor element  21  and defining an uniform long pitch P 1 , and a second spiral thread segment  202  raised around the periphery of the high pressure screw rotor element  22  and defining an uniform short pitch P 2 . The second screw rotor  3  comprises a low pressure screw rotor element  31  and a high pressure screw rotor element  32  axially connected in a line and extended in direction from the inlet  111  toward the outlet  131 , and a spiral thread  30  raised around the periphery thereof and extended over the low pressure screw rotor element  31  and the high pressure screw rotor element  32 . The spiral thread  30  is comprised of a first spiral thread segment  301  raised around the periphery of the low pressure screw rotor element  31  and defining an uniform long pitch P 1 , and a second spiral thread segment  302  raised around the periphery of the high pressure screw rotor element  32  and defining an uniform short pitch P 2  (the uniform long pitch P 1  and uniform short pitch P 2  of the first screw rotor  2  are identical to that of the second screw rotor  3  so that same respective reference signs P 1  and P 2  are used). 
     The assembly process of the present invention is outlined hereinafter with reference to FIG. 4 again. The shafts  4  and  5  are respectively mounted in the respective axle bearings  46  and  46 ′ at the bottom cover  13 , and then the high pressure screw rotor elements  22  and  32  of the first screw rotor  2  and the second screw rotor  3  are meshed together and respectively mounted on the shafts  4  and  5  and secured thereto by respective keys  41  and  51 , and then check if the top sides A and B of the high pressure screw rotor elements  22  and  32  are disposed at same elevation or not. If the top sides A and B of the high pressure screw rotor elements  22  and  32  are not horizontally aligned, insert a packing  40  in between the high pressure screw rotor segment  22  and the respective axle bearing  46 , enabling the top sides A and B of the high pressure screw rotor elements  22  and  32  to be adjusted to same elevation. After the top sides A and B of the high pressure screw rotor elements  22  and  32  have been adjusted to same elevation, mount two meshed timing gears  42  and  52  on the shafts  4  and  5  at one end, and then adjust the phase angle of the timing gears  42  and  52  and the clearance between the high pressure screw rotor elements  22  and  32 , and then fasten two keyless axle bushes  43  and  53  to the shafts  4  and  5  and the timing gears  42  and  52  to hold down the timing gears  42  and  52  in place. After installation of the timing gears  42  and  52  and the keyless axle bushes  43  and  53 , the timing gears  42  and  52  can then be driven to rotate the high pressure screw rotor elements  22  and  32 , keeping the predetermined clearance between the high pressure screw rotor elements  22  and  32 , and preventing friction between the high pressure screw rotor segments  22  and  32 . Therefore, less noise is produced during the rotation of the high pressure screw rotor elements  22  and  32 . 
     Thereafter, the low pressure screw rotor elements  21  and  31  are meshed together and respectively mounted on the shafts  4  and  5  at the other end. Because the first spiral thread segment  201  (or  301 ) and the second spiral thread segment  202  (or  302 ) are designed to form a continuously extended spiral thread  20  (or  30 ), the thread segments  201  and  202  (or  301  and  302 ) can easily be aligned. After installation, the low pressure screw rotor elements  21  and  31  are well adjusted to have the designed clearance left therebetween, and then respective keyless axle bushes  44  and  54  are installed to secure the low pressure screw rotor elements  21  and  31  to the shafts  4  and  5 . As stated above, axle bearings  46  and  46 ′ are installed in the high pressure side near the outlet  131  to support the shafts  4  and  5  positively in place. It is unnecessary to install additional axle bearings in the low pressure side near the inlet  111 . Because no axle bearings are required in the low pressure side near the inlet  111 , the invention prevents the possibility of reverse flow of evaporated lubricating grease from the double screw rotor assembly to the enclosure to be drawn into a vacuum condition. Therefore, the invention is practical for use in semi-conductor manufacturing equipment where the cleanness of the chamber is critical. 
     As shown in FIG. 4, the first spiral thread segment  201  of the low pressure screw rotor element  21  of the first screw rotor  2  and the first spiral thread segment  301  of the low pressure screw rotor element  31  of the second screw rotor  3  are meshed together and have an uniform long pitch P 1 ; the second spiral thread segment  202  of the high pressure screw rotor element  22  of the first screw rotor  2  and the second spiral thread segment  302  of the high pressure screw rotor element  32  of the second screw rotor  3  are meshed together and have an uniform short pitch P 2  (P 2 &lt;P 1 ). Therefore, the volume of the air chambers  204  and  304  in the high pressure screw rotor elements  22  and  32  is smaller than the volume of the air chambers  203  and  303  in the low pressure screw rotor elements  21  and  31 . During rotary operation of the double screw rotor assembly, the flow of air in the air chambers  203  and  303  is compressed in advance, preventing a significant pressure difference between the low pressure side near the inlet  111  and the high pressure side near the outlet  131 , and therefore the possibility of a reverse flow is greatly reduced, and less power loss and operation noise will occur. This design enables the double screw rotor assembly to be made compact. Because the processing of the component parts is easy, the manufacturing cost of the double screw rotor is low. 
     Hereunder demonstrates a variety of different combination of installing the high pressure screw rotor elements and the timing gears to their respective shafts. 
     FIG. 5 shows a sectional view of other embodiment. The structure of this embodiment is basically similar to FIG. 4 except, the timing gear  42   a  is mounted with a key  41   a  and is fastened by a screw nut  43   a  to the shaft  4   a.    
     FIG. 6 shows a sectional view of another embodiment. The structure of this embodiment is basically similar to FIG. 4 too, except the high pressure screw rotor element  32   b  of the second screw rotor  3   b  is fastened by a keyless axle bush  6   b  to the shaft  5   b.    
     FIG. 7 shows a sectional view of still another embodiment. The structure of this embodiment is basically similar to FIG. 4 too, except the high pressure screw rotor element  32   c  of the second screw rotor  3   c  is fastened to the shaft  5   c  with a keyless axle bush  6   c , and the timing gears  42   a ,  52   a  are mounted with keys  41   c ,  51   c  to the respective shafts  4   c .  5   c , and then fastened by screw nuts  43   c ,  53   c  respectively. 
     FIG. 8 shows a sectional view of further another embodiment. The structure of this embodiment is basically similar to FIG. 4 too, except the high pressure screw rotor elements  22   d ,  32   d  of screw rotors  2   d ,  3   d  are constructed with the respective shafts  4   d ,  5   d  to be a union (i.e., integrally connected. 
     FIG. 9 shows a sectional view of still further another embodiment. The structure of this embodiment is basically similar to FIG.  8 . The high pressure screw rotor elements  22   e ,  32   d  of screw rotors  2   e ,  3   d  are constructed with the respective shafts  4   e ,  5   d  to be a union. However, the timing gear  42   e  is mounted with a key  41   e  and is fastened to the shaft  4   e  by a screw nut  43   e.    
     FIG. 10 shows a sectional view of one another embodiment. The structure of this embodiment is basically similar to FIG. 4 too, except the high pressure screw rotor elements  22   f ,  32   f  of the screw rotors  2   f ,  3   f  are fastened to the respective shafts  4   f ,  5   f  with keyless axle bushes  6   f ,  7   f  respectively. After alignment of the thread, the low pressure screw rotor elements  21   f ,  31   f  are fastened to the respective shafts  4   f ,  5   f  with keyless axle bushes  44 ,  54  respectively. 
     FIG. 11 shows a sectional view of one more embodiment. The structure of this embodiment is basically similar to FIG.  10 . However, the timing gears  42   g ,  52   g  are mounted with keys  41   g ,  51   g  to the respective shafts  4   g .  5   g , and then fastened by screw nuts  43   g ,  53   g  respectively. 
     While only some embodiments of the present invention have been shown and described, it will be understood that various modifications and changes could be made thereunto without departing from the spirit and scope of the invention disclosed.