with reference to fig1 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 . 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 . the rotors 18 , 20 have teeth 44 , 46 , respectively , which when mated with a second set of rotors 52 , 54 ( shown in fig2 ) 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 . referring now to fig2 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 . 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 . referring again to fig1 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 . referring again to fig2 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 . a motor 110 drives the shafts 30 , 60 . referring to fig2 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 fig1 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 . 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 . referring to fig3 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 . 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 fig3 . the lobe 142 and channel 152 form a suction section 154 . channel 143 and lobe 153 also form a suction section opposite section 154 . 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 fig4 . 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 . as seen in fig1 the existing pump screws have small center gaps 160 . as seen in fig5 a and 5b , the modification to the screw rotors includes increasing the width of the center gap shaft 190 . as shown in fig6 a , 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 . fig6 c illustrates female channel 152 in shaft 150 and correspondingly male lobe 153 . fig7 a and 7b show a radius - shaped lobe 172 and female channel 173 in shaft 170 . fig7 c shows a corresponding radius - shaped female channel 182 and lobe 183 in shaft 180 . [ 0050 ] fig3 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 fig3 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 . 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 fig1 . [ 0052 ] fig8 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 ). referring now to fig9 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 . 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 . in fig1 , 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 ] fig1 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 . 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 fig1 ) 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 , fig9 ). by varying the width of the center gap , the volume ratio can be altered thus changing the power consumption . 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 . with reference to fig1 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 ′. 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 . 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 .