Abstract:
A vacuum pump includes an inlet port ( 14 ) and an exhaust port ( 86, 88 ). Gas from an enclosure connected to the inlet port is pumped to the exhaust port by first and second rotors ( 18, 52, 254 ) which are mounted on first and second shafts ( 30, 60 ) extending through a pump chamber ( 112 ). The rotors are connected with shaft sections ( 140, 150, 240, 250 ) which include a lobe ( 142, 172, 242, 242′ ) extending from the shaft sections and a mating channel ( 152, 182, 252, 252′ ) defined in the other. The lobes matingly engage the channels during rotation of the rotors to form a suction section ( 154 ). The suction section ( 154 ) compresses a volume of gas entering the pump from the inlet port ( 14 ) reducing the power consumed to move the volume of gas through the pump chamber more easily and increase pump efficiency.

Description:
[0001]    This application is a continuation-in-part of U.S. application Ser. No. 09/691,009, filed Oct. 18, 2001, now abandoned. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to the vacuum pump arts. It finds particular application in a helical screw rotor vacuum pump.  
           [0003]    Screw vacuum pumps include two pairs of helical rotors attached to shafts which are driven at high speed by an electric motor positioned below the shafts. The rotors have a plurality of teeth on their edge or arrayed on one or both of their faces and, in use, the teeth rotate within a pumping chamber and urge molecules of gas being pumped through the pumping chamber.  
           [0004]    A gearbox is usually positioned at the driven end of each shaft. The gearbox contains the shaft ends, bearings within which the shaft rotates, any timing gears and the motor positioned about the driven shaft.  
           [0005]    Oils and/or greases associated with lubrication of the gearbox need to be contained and isolated within the gearbox. This is to ensure cleanliness and prevent non-contamination of the gases being pumped in the pumping chamber and to avoid the possibility of transfer of such contamination back into the enclosure being evacuated.  
           [0006]    The conventional screw vacuum pump has working rooms for compressing fluid (gas) by decreasing its volume and working rooms which have no compression action on the fluid, but has merely a fluid feeding action. Therefore, in the conventional screw vacuum pump, the pressure rises up locally (at the portion which has the compression action), and this local rise-up of the pressure causes an abnormal temperature increase at parts of the rotors and the casing of the vacuum pump. That is, the temperature at the discharge side at which the working room reduces its volume and thus compresses the gas tends to abnormally rise up. As a result, the members constituting the screw vacuum pump are un-uniformly thermally expanded due to the local temperature increase, and thus the dimensional precision of the gap between the casing and the rotors and the engaging portion&#39;s gap between the male rotor and the female rotor cannot be set to a high value.  
           [0007]    In some prior art screw vacuum pumps, pressure adjustment devices are provided on the lower surface of the casing and in the axial direction of the rotors in order to prevent excessive rise-up of the pressure of the working rooms and thus prevent the abnormal temperature rise-up of the vacuum pump when the vacuum pump works in a state where the suck-in pressure is substantially equal to the atmospheric pressure.  
           [0008]    Minimizing power consumption in the pump is an on-going challenge. Existing pump systems include suction sections at the ends of the rotors adjacent the closed end plates. The roots portions are provided at each of the both ends of the screw gear portions; that is, they are provided at both the suck-in side and the discharge port. A roots stage is needed adjacent the end plates. Including the suction sections at the ends of the rotor results in a less efficient compression and a smaller reduction in temperature. The roots portions of the existing pumps are difficult to machine and do not result in an appreciably larger volume of gas being trapped and accordingly result in less efficient compression.  
           [0009]    Accordingly, it is considered desirable to develop an improvement to the power consumption of the pump condition which would reduce power needs at high pressures and reduce rotor sizes, which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with a first aspect of the present invention, a vacuum pump includes a pump chamber in which an inlet and exhaust port are defined. First and second rotors are mounted parallel to each other in the pump chamber adjacent the inlet and outlet ports. A lobe is mounted to the first rotor adjacent the inlet port and a channel is defined in the second rotor adjacent the inlet port. The lobe and channel cooperate to form a suction section adjacent the inlet port.  
           [0011]    In accordance with another aspect of the present invention, a method is provided for reducing the power consumed to move a volume of gas through a vacuum pump. A first shaft section is defined extending from a first rotor in a pump chamber adjacent an inlet port. A second shaft section is defined extending from a second rotor adjacent the inlet port. A lobe is provided on the first shaft section and a channel is defined in the second shaft section. The channel matingly engages the lobe to form a suction section between the rotors and the inlet port.  
           [0012]    One advantage of the present invention is that it reduces power needs at high pressures, thus improving pump efficiency.  
           [0013]    Another advantage of the present invention is that it reduces the temperature within the pump chamber due to lower power consumption.  
           [0014]    Another advantage of the present invention is that it allows reduction in size of the rotors, thus reducing production costs.  
           [0015]    Still another advantage of the present invention is that it reduces pump operating costs.  
           [0016]    Yet still another advantage of the present invention is that providing the insert at the center of the screw rotors instead of at the ends of the rotors reduces machining costs.  
           [0017]    Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.  
         [0019]    [0019]FIG. 1 shows a side elevational crosssectional view of the existing screw vacuum pump assembly.  
         [0020]    [0020]FIG. 2 shows a top elevational view of the existing screw vacuum pump.  
         [0021]    [0021]FIG. 3 shows a perspective view of a pair of rotors with the suction sections in accordance with the preferred embodiment of the present invention.  
         [0022]    [0022]FIG. 4 shows a perspective view of a pair of rotors with the suction sections in accordance with a second preferred embodiment of the present invention.  
         [0023]    [0023]FIG. 5A shows an elevational view of a screw rotor with a widened center gap.  
         [0024]    [0024]FIG. 5B shows a cross-sectional view of a rotor with a widened center gap.  
         [0025]    [0025]FIG. 6A shows an elevational view of a screw rotor with a V-shaped male lobe in the center gap.  
         [0026]    [0026]FIG. 6B shows a cross-sectional view of a screw rotor with a V-shaped male lobe in the center gap.  
         [0027]    [0027]FIG. 6C shows an elevational view of a screw rotor with a V-shaped female portion in the center gap.  
         [0028]    [0028]FIG. 7A shows an elevational view of a screw rotor with a radius-shaped male lobe in the center gap.  
         [0029]    [0029]FIG. 7B shows a cross-sectional view of a screw rotor with a radius-shaped male lobe in the center gap.  
         [0030]    [0030]FIG. 7C shows an elevational view of a screw rotor with a radius-shaped female portion in the center gap.  
         [0031]    [0031]FIG. 8 is a graph of thread pressure vs. thread volume without internal compression.  
         [0032]    [0032]FIG. 9 is a graph of thread pressure vs. thread volume with internal compression at the ends of the rotors.  
         [0033]    [0033]FIG. 10 is a graph of thread pressure vs. thread volume with internal compression at the center gap of the rotors.  
         [0034]    [0034]FIG. 11 is a graph of theoretic power vs. inlet pressure.  
         [0035]    [0035]FIG. 12 is a perspective view of a pair of rotors with suction sections in accordance with another embodiment of the present invention.  
         [0036]    [0036]FIG. 13 is a top view of the rotors of FIG. 12. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    With reference to FIG. 1, an existing screw vacuum pump comprises a vacuum pump  10  comprising a pump chamber  12  having a first end  13 , a second end  15 , a third end  17  and a fourth end  19 . The pump chamber  12  further comprises a central inlet port  14  located at the third end  17  of the chamber  12 , through which gas from an enclosure (not shown) connectable to the inlet can be pumped to a pump high pressure exhaust port  16  located at the fourth end  19 .  
         [0038]    The chamber further includes a first pair of rotors  18 ,  20  located within the chamber adapted for high velocity rotation horizontally within the chamber. The first pair of rotors  18 ,  20  are mounted on a first shaft  30  extending through the chamber  12  and into bearing mounts  32 ,  34  located at opposite ends of the shaft  30 . The bearing mounts  32 ,  34  are substantially isolated from the chamber by means of seals  42 ,  40 , respectively, which are mounted on the shaft  30  and located on opposite ends of the shaft  30 .  
         [0039]    The rotors  18 ,  20  have teeth  44 ,  46 , respectively, which when mated with a second set of rotors  52 ,  54  (shown in FIG. 2) create a plurality of closed chambers or cells  47  in the pump chamber  12  and urge molecules of gas to be pumped through the cells. The rotors each have low pressure inlet faces  48 ,  50  through which the inlet gas enters the rotor from the inlet port  14 . The teeth  44  on the rotor  18  advance in an opposite direction from the teeth  46  on rotor  20  by virtue of opposite helix direction, thus moving the gas in an opposite direction.  
         [0040]    Referring now to FIG. 2, the second pair of rotors  52 ,  54  are mounted on a second shaft  60 , which is parallel to the first shaft  30 . The second shaft  60  includes a bearing mount  62  and a seal  66  at one end of the shaft and a bearing mount  64  and a seal  68  at the opposite end of the shaft. The rotors  52 ,  54  have teeth  70 ,  72  which also advance in opposite directions from each other. The second set of rotors  52 ,  54  also have inlet faces  80 ,  82  through which gas enters the rotors from the inlet port  14 .  
         [0041]    The seals can be of a close tolerance but noncontact design. The seals  40 ,  68  are located adjacent an end plate  90  which is flush with ends  91 ,  93  of the rotor assemblies  18  and  52 . The seals  42 ,  66  are located adjacent end plate  92  which is flush with the ends  95 ,  97  of the rotor assemblies  20  and  54 .  
         [0042]    Referring again to FIG. 1, gas enters the pump through the low pressure inlet port  14 . The gas then moves in opposite directions along the helical rotors  18 ,  20 ,  52 ,  54  toward exhaust ports  86 ,  88  which are located at the first and second ends  13 ,  15  of the pump chamber  12  at end plates  90 ,  92 , respectively. End plate  90  is located at end plane  100  and end plate  92  is located at end plane  102 . The gas is essentially captured between the teeth of rotors  18 ,  20 ,  52 ,  54  and the fixed volume of gas is moved along the rotors  18 ,  20 ,  52 ,  54  toward the opposite end planes  100 ,  102 . Rotors  18  and  52  move the gas toward end plane  100 . Rotors  20  and  54  move the gas toward end plane  102 . As the rotors are rotated on shafts  30 ,  60 , the threads of the rotor threads move toward the end planes  100 ,  102 . The seals each include a stationary side  98 ,  104 ,  106 ,  108 , respectively, which are pressed into the end plates  90 ,  92 .  
         [0043]    Referring again to FIG. 2, the teeth  44  of the rotor  18  mesh with the teeth  70  of rotor  52  and push the fixed volume of gas toward the end plane  100 . The teeth  46  of rotor  20  mesh with the teeth  72  of rotor  54  and push another fixed volume of gas in an opposite direction toward the end plane  102 .  
         [0044]    A motor  110  drives the shafts  30 ,  60 . Referring to FIG. 2, the motor  110  is located beneath gearboxes  120 ,  122  at the motor drive end  112 . The bearing mounts  32 ,  34 ,  62 ,  64  surround the shafts  30 ,  60  and house bearings within which the shafts  30 ,  60  rotate. Referring to FIG. 1, On the motor drive end  112  of the shafts, there is a pair of angular contact bearings  114 ,  116  which position the shafts radially and hold them in place axially in the pumping chamber. On the opposite side of the shaft is a single ball bearing  130  which also provides radial and axial support for the shafts.  
         [0045]    As the gas enters the two exhaust ports  86 ,  88 , it is transported to a first exhaust cavity  126  located at exhaust port  86  and to a second exhaust cavity  128  located at exhaust port  88 . The first and second exhaust cavities lead to a third exhaust cavity  132  through which the gas flows into the high pressure exhaust port  16 .  
         [0046]    Referring to FIG. 3, rotors  18 ,  20 ,  52 ,  54  have screw thread sections  19 ,  21 ,  53 ,  55 , respectively, which extend in opposite directions from the center of the rotors. At the center of the rotors  18 ,  20 ,  52 ,  54  are center shafts  140 ,  150  which are positioned below the inlet port  14  within the pump chamber. The shafts  140 ,  150  are positioned in the center gaps of the rotors. The center gaps have been increased in width to form the shafts  140 ,  150 .  
         [0047]    A preferred embodiment of the present invention comprises the shaft  140  having a raised relief male lobe  142  and a female channel  143  which is 180° opposite to the lobe  142  and is the negative profile of the lobe. Lobe  142  engages a correspondingly hollow female or channel portion  152  in the second shaft  150 . Shaft  150  also has a lobe  153  which is 180° opposite channel  152  and is the negative profile of the channel. The male lobe  142  and the corresponding female portion or channel  152  are shown to be V-shaped in FIG. 3. The lobe  142  and channel  152  form a suction section  154 . Channel  143  and lobe  153  also form a suction section opposite section  154 .  
         [0048]    However, in a second preferred embodiment, shafts  170  and  180  include a male lobe  172  and a female channel  182  which are round or radius-shaped as shown in FIG. 4. This radius (R) may be increased up to and including R is equal to infinity; in which case, the leading edge of the insert would be a straight line. This straight line may b parallel to the shaft centerline. The lobe  172  and channel  182  form a suction section  184 . Similarly, shaft  170  also includes a channel  173  which is 180° opposite lobe  172  and shaft  180  includes a lobe  183  which is 180° opposite channel  182 . There are other embodiments of the suction sections including multi-lobed suction sections which are not shown.  
         [0049]    As seen in FIG. 1, the existing pump screws have small center gaps  160 . As seen in FIGS. 5A and 5B, the modification to the screw rotors includes increasing the width of the center gap shaft  190 . As shown in FIGS. 6A, 6B, and  6 C, a V-shaped insert is added to the center gap to forming male lobe  142  and correspondingly female channel  143  in shaft  140 . FIG. 6C illustrates female channel  152  in shaft  150  and correspondingly male lobe  153 . FIGS. 7A and 7B show a radius-shaped lobe  172  and female channel  173  in shaft  170 . FIG. 7C shows a corresponding radius-shaped female channel  182  and lobe  183  in shaft  180 .  
         [0050]    [0050]FIG. 3 illustrates the interaction of the male lobe  142  and the female channel  152 . Gas is sucked in through the inlet port  14  into the shaft sections  140 ,  150  and is compressed by the male lobe  142  and the female channel  152 . At the initial stage, the suction section  154  increases in volume as the rotors rotate, drawing gas into the pumping chamber. At the point where shaft  150  reaches maximum volume, a position equivalent to that shown for shaft  140  in FIG. 3, the male lobe closes the suction section  154  to the inlet opening. With further rotation, the male lobe compresses the trapped suction gas into the adjacent screw section(s). The gas tightness of the suction section  154  is kept by the male lobe  142  and the female channel  152 . The increase in compression of the gas resulting from the suction sections reduces the amount of power consumed to move a volume of gas through the pump.  
         [0051]    Under normal vacuum operation, the power consumption is predominately determined by the rotor diameter and the screw pitch at the exhaust ends of the rotor. With the increased intake volume created by the suction section, the screws are supercharged, moving a considerably higher quantity of gas, determined by the selected volume ratio (V r ), with the same power consumption. The amount of power saved is illustrated in FIG. 10.  
         [0052]    [0052]FIG. 8 is a graph illustrating power needed to move a volume of  100  cubic meters of gas per hour through the screw rotor without any internal compression. That is, the area within the curve is theoretical power consumed (3kW of power) at an inlet pressure (Pi) of 10 mbar and an exhaust pressure of 1100 mbar. The built-in volume ratio V r  is equal to 1 (one) since there is no internal compression. That is, the volume ratio is equal to the volume of gas trapped in the first screw thread at the inlet versus the volume of gas trapped in the last screw thread at the exhaust. Since there is no internal compression, the ratio is equal to 1. The cycle proceeds as follows. From state  0  to state  1 , the volume of the thread is increasing with rotation of the rotor. At state  1 , the first thread is closed to the inlet port. From state  1  to state  2 , the closed thread advances from the inlet end to the exhaust end with the corresponding increase in pressure and without any reduction in volume. At state  2 , the thread is opened to the exhaust plane. From state  2  to state  3 , the transported gas is expelled from the pump. This amount of power is roughly equivalent to that which would be consumed by a roots blower or by a screw pump to move a volume of gas without internal compression (i.e., without any end plates).  
         [0053]    Referring now to FIG. 9, the graph illustrates that a power savings is obtained when internal compression is added to the pump at the exhaust ends of the pump cavity. The gas begins entering the pump chamber at state  0 . This continues until maximum volume is achieved at state  1 . From state  1  to state  2 , the gas is transported from the inlet end to the exhaust end without any reduction in volume. At state  2 , the thread is not immediately exposed to the exhaust by virtue of a close clearance end plate with a timed exhaust opening. From state  2 , the thread arriving at the end plane is compressed against the end plate until the time when it is exposed to the exhaust opening at state  3 . Depending on the thread pressure realized at state  2 , and the selected Vr, there may be an over compression or under compression at state  3  (a slight over compression is shown). Upon exposure to the exhaust port, the thread pressure instantaneously achieves exhaust pressure (state  4 ). From state  4  to state  5 , the gas is expelled from the pump.  
         [0054]    The compression power needed to move a 100 cubic meter volume of gas per hour is 2.7 kW which is an approximately  10  percent savings in power from when there is no internal compression (3 kW of power). The built-in volume ratio (V r ) is 1.7. That is, the ratio of volume trapped in the first screw thread is 1.7 times the volume of gas trapped at the last screw thread at the exhaust.  
         [0055]    In FIG. 10, the graph illustrates the power savings due to internal compression which occurs in the preferred embodiment of the present invention. In the present invention, the internal compression occurs at the center gaps below the inlet port as the gas is pumped into the opposite screw sections. This results in an over 50 percent reduction in power consumed as compared to the power and when there is no internal compression. That is, the power consumed to move 100 cubic meters of gas per hour through the pump chamber to the exhaust is 1.3 kW as compared to 3 kW without internal compression. The built-in volume ratio V r  is 2.3. That is, the ratio of volume trapped in the suction section  154  is 2.3 times the volume trapped at the last screw thread at the exhaust.  
         [0056]    [0056]FIG. 11 illustrates various types of theoretical power versus inlet pressure. Isochoric pressure is shown which is pressure with constant volume pumping. Adiabatic pressure is shown which is pressure without heat exchange with the surroundings. The isothermal curve reflects power consumed when there is no change in temperature.  
         [0057]    A fixed V r  of 3 allows more power to be saved at low inlet pressure. That is, the higher the volume ratio, the more power is saved. Thus, at a V r  of 2.3 (corresponding to FIG. 10) where internal pressure occurs at the center gap, additional power is saved than where internal compression occurs at the end of the rotors (V r =1.7, FIG. 9). By varying the width of the center gap, the volume ratio can be altered thus changing the power consumption.  
         [0058]    As the volume is compressed, the temperature within the pump chamber increases. When the volume is compressed at the end of the rotors, the temperature rises at the ends of the rotors. Since the volume is gradually compressed, the heat within the screw is distributed over the length of the screw. With the preferred embodiment of the present invention, since less power is needed to move the volume of gas, there is less temperature increase in the pump chamber.  
         [0059]    With reference to FIGS. 12 and 13, a first rotor  218  includes a series of helical threads or teeth  244 . A first shaft section  240  extends from an end of the helical threads adjacent an inlet port. A second rotor  254  defines a second set of helical threads or teeth  270  which mesh with the helical threads  244  of the first rotor. As the first and second rotors rotate, the helical threads pump gases from an inlet port, along their length, to an exhaust port adjacent an opposite end thereof. The second rotor  254  has a second shaft portion  250  extending from an inlet port end thereof. The first shaft portion  240  carries a lobe  242  which is received in a complementary channel  252 . The second shaft section  250 , 180° displaced from the first lobe and channel arrangement, defines a lobe  242 ′ and the first shaft portion  240  defines a channel  252 ′.  
         [0060]    There are various ways the power consumption can be altered by the suction sections. The width of the center gap can be altered. Secondly, the shape of the male and female lobe connections can be varied by different geometric configurations. Third, a multi-lobed configuration could be used in lieu of a single-lobed configuration.  
         [0061]    The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon a reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.