Abstract:
A fluid pump ( 10 ) or motor ( 100 ) includes a pair of enmeshed tapered rotors ( 22,24,122,124 ) having intersecting axes of rotation. The first rotor ( 22,122 ) includes a small low pressure end ( 34,54,134,154 ) and a larger high pressure end ( 32,52,132,152 ) and a spiral thread ( 36,56,136,156 ) that increases in width and depth as it progresses from the high pressure end ( 28,128 ) to the low pressure end ( 26,126 ). The second rotor ( 24,124 ) enmeshes with the first rotor ( 22,122 ), and has an identical structure, except that its threads ( 36,56,136,156 ) progress in the opposite direction. Both rotors ( 22,24,122,124 ) are mounted on sliding splines ( 42,62,142,162 ) which permit them to move, to a limited extent, into and out of their respective receiving cavities. The pressure on the high side ( 28,128 ) of the pump ( 10 ) or motor ( 100 ) tends to urge the rotors ( 22,122,24,124 ) against the walls ( 16,20,116,120 ) of the receiving cavities thereby improving their sealing capabilities and the overall efficiency of the pump ( 10 ) or motor ( 100 ) as a whole.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of PCT/US2006/008524 filed on Mar. 9, 2006, and provisional U.S. application Ser. No. 60/660,224 filed Mar. 10, 2005 and entitled “The Tapered Screw Pump” by Alan Notis, the entire contents and substance of which are incorporated in total herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to dual screw-type motors and pumps in general, and, in particular, to both dry and lubricated vacuum pumps, pneumatic and air-conditioning compressors, hydraulic pumps, pneumatic motors, and hydraulic motors. 
     2. Description of Related Art 
     The prior art includes a number of efforts to produce effective screw-type rotor pumps and motors. In some rare cases, the rotor is tapered and the flights of the screw portion are graduated so as to be wider at one end. 
     Perhaps most typical of these special cases is the pump described in U.S. Pat. No. 6,672,855 issued to Michael Henry North on Jan. 6, 2004 and illustrated in  FIG. 1  as Prior Art. In this design the inner-cone diameter, referred to as the “root diameter”, increases as the screw moves from the inlet side to the outlet side. This design appears to allow for improved volume compression characteristics. The varying pitched threads also allow for greater pumping speed at relatively “low” (under 50 mbar) inlet pressures as well. 
     A similar earlier effort is described in U.S. Pat. No. 6,019,586 which discloses a screw compressor/pump including a graduated, contracted screw portion. The patent refers to “an inner cone tapered towards a suction port side”, and a rotor chamber “defining an outer cone tapered toward the discharge port side, thus forming a gradationally contracted cavity between the spiral groove and the conical wall surface of the rotor chambers.” The result is a pump/compressor which provides for volume compression as well as a shortening of the rotor and shaft. This design appears to improve pump sealing characteristics somewhat, but the patent reference only states that “the helix tooth has its top land surface approximating very closely to the inside wall of the rotor chamber to minimize the clearance and gas leakage.” 
     Of possible lesser relevance is the device described in U.S. Publication US 2001/0041145 A1, which discloses a vacuum pump including a body, a pumping chamber, and tapered rotors. 
     The general state of the art can also be found in the following: U.S. Pat. Nos. 4,952,125; 6,129,534; 6,217,305; and, 6,379,135. Note also the following U.S. Publications: 2001/0024620 A1; 2001/0041145 A1; and, 2001/0051101 A1. In addition note the following patent applications or references: Japanese Patent/Application 2001/304,156; 2000/073,976; and, 01267384. Also European application number 1130263 A1 and French patent number 2684417A1. 
     There tends to be the following shortcoming with regard to the foregoing prior art for screw-type pumps and motors: 
     First, they often have poor sealing characteristics, resulting in excessive energy consumption and lower pumping efficiency. This especially becomes a problem over time, because the gaps between enmeshed screws become larger as friction wears the screws against each other. 
     Second, they tend to have large rotor size and shaft sizes compared to their pumping capacities. Pump/motor size to output ratio is thus an issue. 
     Third, while some of the cited tapered screw pumps have limited volume compression, standard screw pumps have no volume compression along the rotor length. This makes it difficult to use such pumps/motors for gas applications, and also requires more power consumption than a pump which achieves compression along the rotor. 
     Fourth, such pumps/motors tend to have limited pressure differential for low viscosity flows. 
     The device described in U.S. Pat. No. 6,672,855 and some others attempt to address some of these issues, and while such inventions are useful, their versions still suffer from other significant drawbacks: 
     First, the pump designs do not relieve pressures within the pump that exceeds the high-side pressure. At the start of pump operations, when both sides are at the same pressure, the pump internally may develop pressures higher than the “high pressure” side. This causes a large loss of efficiency, and diminishes the seal which slows down pumping action. U.S. Pat. No. 6,672,855 suggests the use of an electronic regulator to reduce shaft speed at the initial stages of pump operation to minimize the problem, but at the cost of slower pumping speed. 
     Second, the outer cone, i.e. “thread diameter” taper described in U.S. Pat. Nos. 6,672,855 and 6,019,586 both work against optimal sealing efficiency. As the pumps operate, the pressure differential from the high side to the low side will naturally tend to push the screws out of the block, i.e. the rotor chamber. With respect to vacuum pumps, at the start of the pumping cycle, the intra-cavity pressure is higher than the output pressure (atmospheric pressure); thus the problem is exacerbated. The inventions described in U.S. Pat. Nos. 6,672,855 and 6,019,586 both include a truncated cone with the wider diameter at the inlet, i.e. low, pressure side. In a two dimensional view, the block is a trapezoid with the longer length at the inlet side. The outlet, i.e. high, pressure side tends to force the rotors out of the block, which reduces sealing characteristics between the block edge and the threads. 
     While both such prior art devices can adequately handle the pressure differential required for vacuum pumps (about 14.7 psi or 760 mmHg), both are unfit for pumping across much larger pressure differentials. In gas compressors, for instance, the pressure differential could be as large as 5,000 psi. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a tapered screw pump/motor in which both the inner cone, i.e. root, and the outer cone, i.e. thread, diameters increase when progressing from the low pressure to the high pressure sides of the device. The result is that the screw axes are not parallel, as is common in the prior art. This configuration of the block and screws now use the pump/motor&#39;s pressure differential to enhance sealing properties. This results in a pump/motor which can achieve pressure differentials much greater than those found in the prior art. 
     The pitch of the screw threads of the present invention varies across the length of the rotors. The pitch change is quite steep compared to that found in existing tapered screw machines. By achieving volume compressions comparable to or exceeding that of existing prior art pumps/motors by using fewer threads, a shorter, more compact pump size is achieved. This present invention can achieve volume compression ratios ranging from 1:1 to 15:1. 
     The present invention also introduces the concept of pressure relief valves/devices within the rotor cavities. Prior art devices acknowledge the problem of having “too many compressive forces across the screw mechanism” but seek to mitigate the problem by adding electronic devices to reduce pumping speeds (and thus yielding a slower output) for the starting interval. Pressure relief valves maximize pump efficiency by reducing internal pressure. When the compressive forces exceed the high pressure side, the relief valves open and lower the pump internal pressure to essentially the same pressure of the high side. In a vacuum pump, this allows for a greater volume differential for the same sized motor. 
     The invention may be more fully understood by referring to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art tapered screw vacuum pump such as described in U.S. Pat. No. 6,672,855. 
         FIG. 2  illustrates the basic vacuum pump configuration of the preferred embodiment of the invention which, in some cases, could also be a configuration for a motor or load at the high pressure side. 
         FIGS. 2A-2B  illustrate the block and tapered cavities in a vacuum pump configuration. 
         FIG. 2C  illustrates a three rotor embodiment. 
         FIG. 2D  illustrates a six rotor embodiment including at least one driven rotor. 
         FIG. 3  is a side elevational view of the pump configuration illustrated in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the rotor (i.e. screw) and sliding spline including a load/driving motor on the high pressure side. 
         FIG. 5  illustrates the preferred embodiment of the invention when used as a motor/pump where the low pressure side is at atmospheric pressure. 
         FIG. 5A  is a top view of the block and tapered cavities in a motor/pump configuration. 
         FIG. 6  illustrates a side elevational view of the basic motor configuration illustrated in  FIG. 5  and including a control circuit. 
         FIG. 7  is a cross-sectional view of the rotor (screw) and sliding spline of the load/motor on the low pressure side as shown in  FIG. 5 . 
         FIG. 8  illustrates a rotor (screw) component part in detail. 
         FIGS. 9A-9G  illustrate the sliding, rotating seal characteristic maintained by the rotors in both the pump and the motor configurations. 
         FIG. 10  illustrates the construction of a strand of the sliding, rotating seal and the manner in which the curves are generated. 
         FIGS. 11A and 11B  illustrate how the small ends of the rotors interact with each other. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     During the course of this description like numbers will be used to identify like elements according to the different views that illustrate the invention. 
     As previously discussed,  FIG. 1  illustrates a prior art Tapered Dual Screw Pump of the sort described in U.S. Pat. No. 6,672,885. The screws are complimentary, i.e. one is a right-handed thread and the other one is a left-handed thread. The core of the rotor taper decreases from the high-side to the low-side as the thickness of the thread increases. Note also that the axes of rotation of both rotors are parallel. Since the high-side is on the tapered end of the pump, the natural tendency is for the high pressure to push the dual screws out of the cavity. This tends to decrease their sealing capability and, accordingly, decrease its efficiency. The dual screw pump illustrated in  FIG. 1  is typical of the prior art. 
     A pump  10  according to the preferred embodiment of the invention is illustrated in  FIGS. 2-4 . The pump invention  10  includes a pump block  12  having a first cavity  14  including a cavity wall  16  and a second cavity  18  having a second cavity wall  20 . A first rotor  22  fits snugly in the first cavity  14  and comes into sealing contact with first cavity wall  16 . Similarly, a second rotor  24 , having threads of the opposite hand with respect to the first rotor  22 , and meshing therewith, sits in the second cavity  18  and comes into sealing contact with the cavity walls  20  of the second cavity  18 . The pump  10 , according to the preferred embodiment of the invention, like almost all pumps, includes a low pressure side  26  and a high pressure side  28 . 
     The construction of the first and second rotors  22  and  24 , respectively, are very similar. First rotor  22  includes a tapered core  30 , a large end  32  and a small end  34 . A first spiral flight  36  progresses from the low pressure, side  26  to the high pressure side  28 . The spiral flight  36  is thickest closer to the low pressure end  26  and becomes more narrow as it progresses towards the large end  32  near the high pressure side  28 . First rotor  22  includes a spiral outer edge  38  that contacts the walls  16  of the first cavity. A spline receiving cavity  40  is located inside of the long axis of the first rotor  22 . Spline receiving cavity  40  is intended to accept a spline attached to shaft  82  as shown in  FIG. 2 . The small end  34  of rotor  22  includes a face having a larger outer segment  44 , a smaller inner segment  46 , and a pair of “S” shaped transition zones  48  as shown in  FIG. 11A . Spline  42  includes a bevel gear  84   a  which engages with another bevel gear  84   b  connected to the second rotor  24  via spline  62 . 
     The second rotor  24  has a structure almost identical to that of the first rotor  22  except that it has a thread twist of the opposite hand from the first rotor  22  and it is not connected to a drive shaft  82 . Similar to rotor  22 , the second rotor  24  includes a tapered core  50  which is widest at its large end  52  and smallest at its small end  54 . Second rotor spiral flight  56  surrounds the core  50  and travels in a hand opposite from the spiral flight  36  on the first rotor  22  but meshes therewith in a relatively tight sealing arrangement. The spiral flight  56  includes an outer surface  58  that contacts wall  20 . A spline receiving cavity  60  is located along the long axis of the second rotor  24 . The small end face  54  includes a larger outer segment  64 , a small inner segment  66 , and a pair of “S” shaped transition zones  68 . 
     A plurality of relief valves  70  connected to relief passages  72  are shown in  FIG. 3 . Relief passages  72  extend to and through port  74  which is attached to a removable high pressure head  76 . Rotors  22  and  24  are biased by compression springs  78 . 
     As shown in  FIG. 2  a removable section (or piece)  80  is located at the high pressure end of pump  10 . Removal of section  80  permits access to bevel gears  84   a  and  84   b  as well as to bearings  96 . Conversely, if the device  10  is operated as a motor, then shaft  82  effectively becomes an output shaft. It is evident from the foregoing that the splines  42  and  62  ride inside of the cavities  40 ,  60  under the influence of pressure on the high side of the large end surfaces  32 ,  52 . The motor version of  10  or  100  can be controlled by a control box  88  as shown in  FIG. 6 . The controls of control box  88  are similar to those of a conventional motor control system, opening and closing valves in response to torque/speed requirements. Bevel gears  84   a ,  84   b  sit on ball bearings  96  so that they are free to rotate. Splines  42 ,  62  ride in and out of the spline receiving cavities  40 ,  60  and in the pump version bias screws  22 ,  24  via tension spring  90 , which sits on a spline nut  92  held in place by a screw  94 . 
     With the foregoing environment in mind, the pump version  10  and the motor version  100  of the present invention can be fully understood. 
       FIG. 2  shows the basic vacuum pump configuration  10 , which in some cases could also be a configuration for a motor or load at the high-pressure side. Note that the axes are at an angle of between 0 and 60 degrees with respect to each other. In the vacuum pump configuration, the spiral flights comprise roughly three in number, whereas in the prior art there tend be more flights. 
       FIG. 3  is a side view of the basic pump configuration  10  where pressure relief valves  70  connect to passageways  72  and ultimately to port  74  on the high pressure side. Removable high pressure head section  76  allows for the installation and/or replacement of the rotors  22 ,  24 . 
       FIG. 4  is a cross sectional view of rotor  22  and sliding spline assembly. Compression spring  78  urges the screw into the block on pump startup. One of the major features and advantages of the present invention is that the pressure on the high side of the invention tends to exert a force on the large end surfaces  32 ,  52  thereby forcing rotors  22 ,  24  into sealing engagements against the walls of the cavities  14 ,  18  in which they are located. This improves the seal and efficiency of the operation. The large ends  32 , 52  of the rotors  22 , 24  have a face which is relatively flat and takes up more than 70% of the area of the cavity in which it sits when in sealing engagement with the cavity. In an embodiment of the invention, the small ends  34 , 54  of the rotors  22 , 24  have a face that takes up more than 30% of the area of the low pressure side, but not more than 75% of the area. 
     The arrangement of the elements in  FIGS. 2-4  as a pump  10  can be modified slightly to yield excellent results as a motor/compressor  100  as illustrated in  FIGS. 5-7 . 
     The preferred embodiment of the motor version  100  includes a motor block  112  and a first cavity  114  having first cavity walls  116 . The motor block  112  also includes a second cavity  118  having second cavity walls  120 . A first rotor  122  is fit snugly in the first cavity  114  making sealing contact with the walls  116 . Similarly, a second rotor  124  is fit snugly in the second cavity  118  making sealing contact with cavity walls  120 . As is true also of the pumping embodiment  10 , the motor  100  includes a low pressure side port  126  and a high pressure side port  128 . 
     The first rotor  122  includes a first tapered core  130 , a large end  132 , and a small end  134 . A first spiral flight  136  surrounds the tapered core  130 . The outer edge  138  of the spiral flight  136  contacts walls  116 . A spline  142  is received in cavity  140  in the first rotor  122 . at Small end low pressure face  134  includes a larger outer segment  144 , a smaller inner segment  146 , and a pair of “S” shaped transition zones  148 . 
     Similarly, the second rotor  124  is virtually identical to the first rotor  122 , except that the direction of spiral flights  156  are opposite from those of spiral flights  136 . The second rotor  124  includes a tapered core  150 , a large end  152  and a small end  154 . The second rotor spiral flight  156  includes an edge  158  that contacts the walls  120 . A second rotor spline  162  is received in the spline cavity  160  in the second rotor  124 . Like the first rotor  122 , the second rotor  124  includes a small end having a large outer segment  164 , a small inner segment  166 , and a pair of “S” shaped transition zones  168 . The large ends  132 , 152  of the rotors  122 , 124  have a face which is relatively flat and takes up more than 70% of the area of the cavity in which it sits when in sealing engagement with the cavity. In an embodiment of the invention, the small ends  134 , 154  of the rotors  122 , 124  have a face that takes up more than 30% of the area of the low pressure side  122 , but not more than 75% of the area. 
     As shown in  FIG. 6 , the motor embodiment  100  includes tubing which goes to control box  88 . This allows high pressure to enter various points in the cavities  114 , 118  or allows fluid out from those points to the low pressure side, controlling speed and torque. A removable high pressure head  176  permits access to the high pressure side. Bevel gears  184   a ,  184   b  are fitted into the splines  142 ,  162  and mesh with each other. A control box  88  controls the operation of motor  100 . When embodiment  100  is used as a pump, tension spring  90  is attached to spline  142  at one end and spline nut  92  on the other end. Spline nut  92  is held in place by screw  94  to the rotor  122 . This provides limited pressure to the screw against the block during compressor start-up. 
       FIG. 5  illustrates the basic configuration for the motor  100  according to the preferred embodiment where the low pressure side is at atmospheric pressure. Bearings  196  hold the splines  142 ,  162  in place. Low pressure side plate  180  is removable so that the bevel gears  184   a ,  184   b  and bearings  196  can be removed or maintained. In the motor embodiment  100 , the output shaft  182  is connected to a load, while in the pump embodiment  10  the shaft  182  is connected to a drive motor. Bevel gears  184   a ,  184   b  distribute the load more evenly across both rotors  122 ,  124  to reduce screw-to-screw wear. The enmeshed rotors  122  and  124  rotate, forming a progressively changing volume allowing for internal expansion within the motor  100 . The high pressure port is shown as item  128 . The sliding splines  142 , 162  rotate, or are rotated by the rotors and allow the pressure differential to push the screws back against the block forming a positive seal. 
       FIG. 6  illustrates a side-view of a basic motor configuration  100 , in cross-section, wherein the control box  88  controls the pressure from the high side to various points of the screw cavity volume through the fluid hoses. This allows for the control of torque and speed. In an application where the motors are in series, the output pressure can be controlled when less than full pressure is required. 
       FIG. 7  is another cross-sectional cut-away view of a screw and a sliding spline arrangement for the pump  100  on the low pressure side. The motor version of  100  would be identical to  FIG. 7  except for the removal of tension spring  90 , spline nut  92 , and screw  94 . The pressure differential is allowed to push the rotors  122 ,  124  against the block  112  forming a better seal while the rotors  122 ,  124  turn or are turned by the spline, in the pump embodiment. 
     The spline cavity  114  and  116  allows the rotors  122 ,  124  to settle into the block  112  as a result of the pressure differential. Tension spring  90  is used to pull the rotors  122 ,  124  into the block  124  during pump start up. On the other side of the tension spring is a spline nut  92 . The spline nut is held in place by screw  94 . The tension spring  90  is mechanically attached or soldered to the spline  142  or  162  and spline nut  92 . 
     There are some fundamental aspects of the invention which do not vary by specific application.  FIGS. 8-10  highlight the operating properties which hold for the inventor&#39;s application regardless of whether it operates as a pump or motor embodiments  10  or  100 . 
       FIG. 8  illustrates the screw component parts of embodiment  10 , but are equally true for embodiment  100 . Item  30  is the tapered core of the rotor, while  38  is the outer edge of a spiraled flight. Both are tapered towards the low pressure side in contrast to the prior art, where the tapers of the inner and outer surfaces are in opposite directions. The outer surface of the thread or spiral flight forms a unique rotating seal. The compression/expansion ratios can vary from 1:1 to 15:1. 
       FIGS. 9A-9G  show the progressive movement of the sliding rotating seal  48 ,  68 ,  148 ,  168  of the S-shaped transition zone of each of the rotors. The invention, in all embodiments, maintains the sliding seal in all applications. The development of the seal is very useful. The construction of the S-shaped transition zone seal is illustrated in  FIG. 10 . With radius A, 
     a compass with pivot on Θ1 swings an arc from t which lies on the middle circle of the figure to Θ1 Min which is at the intersection of the inner circle and the radial line passing through Θ1. The outer arc is made using Θ2 as the pivot, swinging an arc from t to Θ2Max which is at the intersection of the outer circle and the radial line segment passing through Θ2. Line segments A, B, C, and D are all of equal length. Additional complexities arise with the use of non-parallel axes. The solution found was to put each strand of the described seal on the surface of a sphere whose radius is from the intersection of the two axes. Each strand and its mate on the other screw have all the same measurements as well as distance from the axis intersection. 
     While the invention has been described with reference to the preferred embodiments, it will be appreciated by those of ordinary skill in the art that modifications can be made to the structure and operation of the invention without departing from the spirit and scope thereof as a whole.