FIELD OF THE INVENTION

The present invention relates to screw-type vacuum pumps having pairs of enmeshed screw-type rotors.

BACKGROUND

Vacuum pumps are used in various technical processes to create a vacuum for the respective process. There are various types of vacuum pumps.

Screw-type vacuum pumps have pairs of screw-type rotors rotatively engaged with each other in a pump casing which cooperate to convey a gas along a longitudinal direction of the pump.

SUMMARY OF THE INVENTION

In an aspect, there is provided a screw-type vacuum pump comprising a housing comprising a pumping chamber, a first rotor having exactly two first helical lobes, and a second rotor having exactly three second helical lobes. The first helical lobes and the second helical lobes are enmeshed with one another in the pumping chamber and the first and second rotors are configured to convey a gas along a longitudinal direction of the screw-type vacuum pump.

The screw-type vacuum pump may be an oil-injected vacuum pump and further comprises oil injection means configured to inject an oil into the housing. The oil may provide sealing of the first and second rotors against the housing and/or each other. The oil may cool the rotors and/or a pumped gas. The first rotor and the second rotor may be in contact with one another, and one of the first rotor or the second rotor is configured to drive the other of the first rotor or the second rotor. The screw-type vacuum pump may be a dry vacuum pump. The screw- type vacuum may further comprise one or more gears configured to drive and synchronize rotation of the first and second rotors.

One or both of the first rotor and the second rotor may have variable pitch along a length of that rotor. The pitch of one or both of the first rotor and the second rotor may decrease linearly between 10% and 50% between a suction end face and a discharge end face of that rotor. The pitch of one or both of the first rotor and the second rotor may decrease linearly approximately 20% between the suction end face and the discharge end face of that rotor.

One or both of the first rotor and the second rotor may have a wrap angle of between approximately 165° and approximately 350°. One or both of the first rotor and the second rotor may have a wrap angle of selected from the group of angles consisting of approximately 203°, approximately 204°, approximately 305°, and approximately 306°.

One or both of the first rotor and the second rotor may have a length-to- diameter ratio of greater than or equal to 1.7.

One or both of the first rotor and the second rotor may have a tooth- depth ratio of greater than or equal to 2. the screw-type vacuum pump may further comprise one or more blow-off valves configured to release pressure in the pumping chamber responsive to the pressure in the pumping chamber exceeding a threshold value.

The first rotor may be a male rotor.

The second rotor may be a female rotor.

The screw-type vacuum pump may be a gas vacuum pump.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration (not to scale) of a side view cross section of a vacuum pump; Figure 2 is a schematic illustration (not to scale) of a top view cross section of a part of the vacuum pump;

Figure 3 is a schematic illustration (not to scale) of a perspective view of a pair of screw rotors of the vacuum pump; and Figure 4 is a schematic illustration (not to scale) showing profiles of the pair of screw rotors.

DETAILED DESCRIPTION

It will be appreciated that relative terms such as above and below, horizontal and vertical, top and bottom, front and back, and so on, are used herein merely for ease of reference to the Figures, and these terms are not limiting as such, and any two differing directions or positions and so on may be implemented rather than truly above and below, horizontal and vertical, top and bottom, and so on. Figure 1 is a schematic illustration (not to scale) of a side view cross section of an embodiment of a vacuum pump 100.

Figure 2 is a schematic illustration (not to scale) of a top view cross section of a part of the vacuum pump 100.

In this embodiment the vacuum pump 100 is a gas vacuum pump for pumping gas or gases.

In this embodiment the vacuum pump 100 comprises a housing 102, a first rotor 104, a first shaft 106, a second rotor 108, a second shaft 110, and a motor 112.

The housing 102 comprises an inlet port 114 located at or proximate to a first end of the housing 102. In this embodiment, the inlet port 114 is a radial port, i.e. the inlet port 114 is oriented in the radial direction which is perpendicular to a longitudinal axis of the vacuum pump 100. However, in other embodiments, the inlet port 114 may be an axial port or a combination of an axial port and a radial port. The housing 102 further comprises an exhaust outlet 116 located at or proximate to a second end of the housing 102 opposite to the first end of the housing 102. In this embodiment, the exhaust outlet 116 is a radial port. However, in other embodiments, the exhaust outlet 116 may be an axial port or a combination of an axial port and a radial port.

The first shaft 106 extends through the housing 102 from the first end of the housing 102 the second end of the housing 102. The first shaft 106 is rotatably mounted to the housing 102 via one or more bearing assemblies (not shown). The first shaft 106 is configured to rotate with respect to the housing 102 about a first axis 118.

In this embodiment, the first rotor 104 is a screw-type rotor comprising a first end 120, a second end 122 opposite to the first end 120, and a screw-type body or working portion 124 extending between the first and second ends 120, 122. The first end 120 is a suction end face of the first rotor 104. The second end 122 is a discharge end face of the first rotor 104.

The first rotor 104 is a male screw-type rotor, which may also be referred to as a Main rotor. Thus, the screw-type body portion 124 of the first rotor 104 is a male-lobed body or working portion. The first rotor 104 comprises exactly two helical lobes. Further details of the first rotor 104 are described in more detail later below with reference to Figures 3 and 4.

The first rotor 104 may be fixedly mounted to the first shaft 106. The first rotor 104 may be unitarily formed with, or be integral with, the first shaft 106. For example, the rotor profile may be machined out of the first shaft 106. The first rotor 104 is housed within an internal chamber (i.e. a pumping chamber) 126 of the housing 102 between the inlet port 114 and the exhaust outlet 116. In particular, in this embodiment, the first end 120 of the first rotor 104 is located at or proximate to the inlet port 114. Also, the second end 122 of the first rotor 104 is located downstream of the first end 120, between the inlet port 114 and the exhaust outlet 116 and preferably proximate to the exhaust outlet 116.

The second shaft 110 extends through the housing 102 from the first end of the housing to the second end of the housing 102. The second shaft 110 is rotatably mounted to the housing 102 via one or more bearing assemblies (not shown). The second shaft 110 is configured to rotate with respect to the housing 102 about a second axis 128.

In this embodiment, the second rotor 108 is a screw-type rotor comprising a first end 130, a second end 132 opposite to the first end 130, and a screw-type body or working portion 134 extending between the first and second ends 130, 132. The first end 130 is a suction end face of the second rotor 108. The second end 132 is a discharge end face of the second rotor 108.

The second rotor 108 is a female rotor, which may also be referred to as a Gate rotor. Thus, the screw-type body portion 134 of the second rotor 108 is a female-lobed body or working portion. In this embodiment, the second rotor 108 comprises exactly three helical lobes. Further details of the second rotor 108 are described in more detail later below with reference to Figures 3 and 4.

The second rotor 108 may be fixedly mounted to the second shaft 110. The second rotor 108 may be unitarily formed with, or be integral with, the second shaft 110. For example, the rotor profile may be machined out of the first shaft 106. The second rotor 108 is housed within the chamber 124 of the housing 102 between the inlet port 114 and the exhaust outlet 116. In particular, in this embodiment, the first end 130 of the second rotor 108 is located at or proximate to the inlet port 116. Also, the second end 132 of the second rotor 108 is located downstream of the first end 130, between the inlet port 114 and the exhaust outlet 116 and preferably proximate to the exhaust outlet 116.

In this embodiment, the male-lobed body portion 124 of the first rotor 104 is enmeshed with the female-lobed body portion 134 of the second rotor 108. In other words, the lobes of the first rotor 104 are enmeshed with the lobes of the second rotor 108.

In this embodiment, the vacuum pump 100 further comprises an exhaust port located at or proximate to the second ends 122, 132 of the rotors 104, 108. The exhaust port is upstream of the exhaust outlet 116, between the exhaust outlet 116 and the rotors 104, 108. The exhaust port may be an approximately butterfly-shaped port. The geometry of the exhaust port may be dependent on the shape of the rotors 104, 108 and/or the volume index of the vacuum pump 100.

The vacuum pump 100 further comprises one or more blow-off valves 136. The one or more blow-off valves 136 are disposed in one or more respective apertures through a wall of the housing 102. The one or more blow- off valves 136 are disposed between the internal chamber 126 of the vacuum pump 100 and the external atmosphere of the pump 100. The one or more blow-off valves 136 may be positioned at any appropriate locations on the vacuum pump 100. The one or more blow-off valves 136 are a pressure relief system configured to relieve pressure in the internal chamber 126 of the vacuum pump 100. The one or more blow-off valves 136 are configured to adopt closed positions when the pressure in the internal chamber 126 is less than a threshold pressure, thereby preventing or opposing gas flow therethrough. Also, the one or more blow-off valves 136 are configured to open when the pressure in the internal chamber 126 is greater than or equal to the threshold pressure, thereby allowing gas flow out of the internal chamber 126 to the external atmosphere. In this way, a pressure with the internal chamber 126 may be relieved. This advantageously tends to reduce load on the rotors 104, 108 and prevent over- compression of the gas being pumped.

In this embodiment, the vacuum pump 100 is an oil-injected vacuum pump. The housing 102 comprises an oil injection port 138 via which oil may be injected by an oil injection system (not shown). In this embodiment, in operation, oil is injected into the vacuum pump 100, as indicated in Figures 1 and 2 by an arrow and the reference numeral 140. The injected oil advantageously tends to provide improved sealing between the first and second rotors 104, 108 at points where the first and second rotors 104, 108 touch, thereby to reduce or eliminate backflow of fluid. The injected oil also tends to provide improved sealing between the rotors 104, 108 and the housing 102, thereby to reduce or eliminate backflow of fluid. The injected oil also tends to provide cooling, lubrication, and noise dissipation.

The motor 114 may be an electric motor. In this embodiment, the first rotor 104 is coaxial with the motor 114 and is supported by bearings on inlet and outlet sides of its screw-type body portion 124. In this embodiment, the motor 114 is configured to drive, i.e. rotate, the first shaft 106 about the first axis 118. When so driven in an operative first direction about the first axis 118, the first rotor 104 drives the second rotor 108 about the second axis 128 in a second direction which is opposite to the first direction. This enmeshed rotation of the screw-type body portions 124, 134 of the rotors 104, 108 draws fluid into the inlet port 114 (as indicated in Figures 1 and 2 by arrows marked by the reference numeral 142). Preferably, the geometry of the inlet port 114 is such that the flow of fluid is cut off at the time in the cycle when the volume in which fluid is received reaches its maximum value. After drawing fluid into the inlet port 114, the continued rotation of the rotors 104, 108 drives the fluid through the internal chamber 126 (as indicated in Figures 1 and 2 by arrows marked by the reference numeral 144), through the exhaust port (located at the second ends 122, 132 of the rotors 104, 108), and then out of the exhaust outlet 116 (as indicated in Figures 1 and 2 by arrows marked by the reference numeral 146).

What will now be described with reference to Figures 3 and 4 are further details of the first rotor 104 and the second rotor 108.

Figure 3 is a schematic illustration (not to scale) of a perspective view of the first rotor 104 and the second rotor 108.

Figure 4 is a schematic (not to scale) cross sectional view showing profiles of the first rotor 104 and the second rotor 108.

The first rotor 104 and the second rotor 108 have the screws directed oppositely to each other.

The first rotor 104 is the driving side rotor. The first rotor 104 is a clockwise helical screw rotor. The first rotor 104 comprises exactly two helical lobes, hereafter referred to as the “first lobes” 300. The first lobes 300 define exactly two helical grooves 302.

The second rotor 108 is the driven side rotor. The second rotor 108 is an anticlockwise helical screw rotor. The second rotor 108 comprises exactly three helical lobes, hereafter referred to as the “second lobes” 304. The second lobes 304 define exactly three helical grooves 306.

Referring to Figure 4, cross section curves are provided, in which the pair of screw rotors 104, 108 are engaged with each other.

The first axis 118 is a rotation centre of the first rotor 104. The second axis 128 is a rotation centre of the second rotor 108.

For the first rotor 104, each first lobe 300 comprises a circular arc portion 400, a first pseudo-Archimedean spiral curve portion 402, and a second pseudo-Archimedean spiral curve portion 404.

For each first lobe 300, a first end of the first pseudo-Archimedean spiral curve portion 402 is contiguous with a first end of the circular arc portion 400. Also, a second end of the first pseudo-Archimedean spiral curve portion 402 (opposite to the first end of the first pseudo-Archimedean spiral curve portion 402) is contiguous with a first end of the second pseudo-Archimedean spiral curve portion 404. Also, a second end of the circular arc portion 400 (opposite to the first end of the circular arc portion 400) is contiguous with a second end of a second pseudo-Archimedean spiral curve portion 404 of the other first lobe 300. Also, a second end of the second pseudo-Archimedean spiral curve portion 404 (opposite to the first end of the second pseudo-Archimedean spiral curve portion 404) is contiguous with a second end of a circular arc portion 400 of the other first lobe 300.

For each first lobe 300, the circular arc 400 extends approximately an eighth of a circle. The circular arc portion 400 corresponds to a root or base portion 406 of the first rotor 104 from which the first lobes 300 extend. A radial distance of the circular arc portion 400 from the first axis 118 is indicated in Figure 4 by a double-headed arrow and the reference numeral 408. The radial distance 408 is a radius 408 of the root or base portion 406. This radial distance 408 may be any appropriate value dependent upon application, for example between about 0.02 m and about 0.2 m, or more preferably between about 0.032 m and about 0.18 m, or more preferably between about 0.02 m and about 0.09 m, or more preferably between about 0.03 m and about 0.05 m, or more preferably between about 0.03 m and about 0.04 m, or more preferably about 0.035 m.

For each first lobe 300, the point at which the first pseudo-Archimedean spiral curve portion 402 meets the second pseudo-Archimedean spiral curve portion 404 defines the tip 410 of that first lobe 300. The maximum radius of the first rotor 104, i.e. the radial distance between the tip 410 and the first axis 118, which is indicated in Figure 4 by a double-headed arrow and the reference numeral 412, may be any appropriate value dependent upon application, for example greater than about 0.06 m, or more preferably between about 0.06 m and about 0.35 m, or more preferably between about 0.06 m and about 0.10 m, or more preferably between about 0.06 m and about 0.08 m, or more preferably about 0.07 m.

In this embodiment, the tooth depth of the first lobes 300 is the radial distance between the base portion 406 of the first rotor 104 and the tip 410 of a first lobe 300. Equivalently, the tooth depth of a first lobe 300 is the radial distance between the circular arc portion 400 and the maximum radius of the first rotor 104. The tooth depth is indicated in Figure 4 by a double-headed arrow and the reference numeral 414. The tooth depth 414 may be any appropriate value dependent upon application, for example, greater than 0.02 m, or more preferably between about 0.02 m and about 0.06 m, or more preferably between about 0.03 m and about 0.05 m, or more preferably between about 0.03 m and about 0.04 m, or more preferably about 0.035 m.

The tooth depth ratio of the first rotor 104 is the ratio between the tooth depth 414 and the radius 408 of the base portion 406. In this embodiment, the tooth depth ratio of the first rotor 104 is approximately 1. Preferably, the first rotor 104 has a tooth-depth ratio of greater than or equal to 1.5. More preferably, the first rotor 104 has a tooth-depth ratio of greater than or equal to 2, e.g. between 2 and 3, or between 2 and 2.5, or between 2 and 2.1. An example tooth-depth ratio for the first rotor 104 is 2.36. One benefit of having a larger tooth-depth ratio may be that the individual chamber volume and thus the throughput of the machine tend to be increased.

For the second rotor 108, each second lobe 304 comprises a third pseudo-Archimedean spiral curve portion 420, an end portion 422, and a fourth pseudo-Archimedean spiral curve portion 424.

For each second lobe 304, the third pseudo-Archimedean spiral curve portion 420 defines a convex surface of the second lobe 304. The end portion 422 defines circular arc, for example extending approximately a twelfth of a circle. The end portion 422 is disposed between the third pseudo-Archimedean spiral curve portion 420 and the fourth pseudo-Archimedean spiral curve portion 424. The fourth pseudo-Archimedean spiral curve portion 424 defines a concave surface of the second lobe 304.

For each second lobe 304, a first end of the end portion 422 is contiguous with a first end of the third pseudo-Archimedean spiral curve portion 420. Also, a second end of the end portion 422 (opposite to the first end of the end portion 422) is contiguous with a first end of the fourth pseudo- Archimedean spiral curve portion 424. Also, a second end of the third pseudo- Archimedean spiral curve portion 420 (opposite to the first end of the third pseudo-Archimedean spiral curve portion 420) is smoothly contiguous with a second end of the fourth pseudo-Archimedean spiral curve portion 424 of an adjacent second lobe 304. Also, a second end of the fourth pseudo- Archimedean spiral curve portion 424 (opposite to the first end of the fourth pseudo-Archimedean spiral curve portion 424) is smoothly contiguous with a second end of the third pseudo-Archimedean spiral curve portion 420 of an adjacent second lobe 304.

The points at which the second ends of the third and fourth pseudo- Archimedean spiral curve portions 420, 424 of adjacent second lobes meet define a root or base portion 426 of the second rotor 108 from which the second lobes 304 extend. A radius of the root or base portion 426 of the second rotor 108 from the second axis 128 is indicated in Figure 4 by a double-headed arrow and the reference numeral 428. The radius 428 is of the root or base portion 426. This radius 428 may be any appropriate value dependent upon application, for example between about 0.01 m and about 0.2 m, or more preferably between about 0.01 m and about 0.18 m, or more preferably between about 0.01 m and about 0.05 m, or more preferably between about 0.01 m and about 0.04 m, or more preferably about 0.01 m, 0.15 m, 0.02 m, or 0.03 m, or 0.04 m.

For each second lobe 304, the circular arc end portion 422 defines the tip 430 of that second lobe 304. The maximum radius of the second rotor 108, i.e. the radial distance between the tip 430 and the second axis 128, which is indicated in Figure 4 by a double-headed arrow and the reference numeral 432, may be any appropriate value dependent upon application, for example greater than about 0.03 m, or more preferably greater than about 0.05 m, or more preferably between about 0.03 m and about 0.15 m, or more preferably between about 0.05 m and about 0.10 m, or more preferably between about 0.05 m and about 0.06 m, or more preferably about 0.055 m

In this embodiment, the tooth depth of the second lobes 304 is the radial distance between the base portion 426 of the second rotor 108 and the tip 430 of a second lobe 304. This tooth depth is indicated in Figure 4 by a double headed arrow and the reference numeral 434. The tooth depth 434 may be any appropriate value dependent upon application, for example between about 0.02 m and about 0.06 m, or more preferably between about 0.03 m and about 0.05 m, or more preferably between about 0.03 m and about 0.04 m, or more preferably about 0.035 m.

The tooth depth ratio of the second rotor 108 is the ratio between the tooth depth 434 and the radius 428 of the base portion 426. In this embodiment, the tooth depth ratio of the second rotor 108 is approximately 1.75. Preferably, the second rotor 108 has a tooth-depth ratio of greater than or equal to 1.7 or greater than or equal to 2. More preferably, the second rotor 108 has a tooth- depth ratio of greater than or equal to 2.5, e.g. between 2.5 and 3.5, or between 2.6 and 3.3, or between 2.7 and 3.2. An example tooth-depth ratio for the first rotor 104 is about 2.75 to 3.15, e.g. 2.75 or 3.14. One benefit of having a larger tooth-depth ratio may be that the individual chamber volume and thus the throughput of the machine tend to be increased.

The pitch of a rotor is defined as the longitudinal distance along the central axis of that rotor between adjacent rotor/lobe tips.

In this embodiment, the pitch of the first rotor 104 varies along the length of the first rotor 104. In other words, the first rotor 104 has variable pitch along its length, i.e. in the direction of the first axis 118. Preferably, the pitch of the first rotor 104 decreases (e.g. monotonically or linearly) in a direction from the first end 120 of the first rotor 104 to the second end 122 of the first rotor 104. Preferably, the pitch of the first rotor 104 decreases (e.g. monotonically or linearly) between 10% and 50% between the first end 120 and the second end 122. More preferably, the pitch of the first rotor 104 decreases (e.g. monotonically or linearly) between about 10% and 30% between the first end 120 and the second end 122. More preferably, the pitch of the first rotor 104 decreases (e.g. monotonically or linearly) about 20% between the first end 120 and the second end 122. Reduction in pitch along the rotor length may be achieved by increasing the wrap angle along the rotor axis. The spacing or gap between the lobe tips and along the length of the root diameter of the shaft may decrease when viewed along the rotor axis.

In this embodiment, the pitch of the second rotor 108 varies along the length of the second rotor 108. In other words, the second rotor 108 has variable pitch along its length, i.e. in the direction of the second axis 128. Preferably, the pitch of the second rotor 108 decreases (e.g. monotonically or linearly) in a direction from the first end 130 of the second rotor 108 to the second end 132 of the second rotor 108. Preferably, the pitch of the second rotor 108 decreases (e.g. monotonically or linearly) between 10% and 50% between the first end 130 and the second end 132. More preferably, the pitch of the second rotor 108 decreases (e.g. monotonically or linearly) between about 10% and 30% between the first end 1230 and the second end 132. More preferably, the pitch of the second rotor 108 decreases (e.g. monotonically or linearly) about 20% between the first end 130 and the second end 132. The wrap angle of a rotor is defined herein to be the angle between the two end face sides of the rotor profile measured around the rotor axis. The wrap angle represents the twist of rotor profile between the suction and discharge end faces.

In this embodiment, the wrap angle of the first rotor 104 may be between approximately 250° and approximately 350°, or more preferably between approximately 280° and approximately 330°, or more preferably between approximately 300° and approximately 315°, or more preferably between approximately 305° and approximately 310°, or more preferably approximately 305° or 306°.

In this embodiment, the wrap angle of the second rotor 108 may be between approximately 165° and approximately 235°, or more preferably between approximately 185° and approximately 220°, or more preferably between approximately 200° and approximately 210°, or more preferably between approximately 203° and approximately 207°, or more preferably approximately 204°.

The wrap angle of the second rotor 108 may be a function of the wrap angle of the first rotor 108. For example, the wrap angle of the second rotor 108 may be two-thirds of the wrap angle of the first rotor 108. For example, if the first rotor 104 has a wrap angle of approximately 306°, then the second rotor 108 may have a wrap angle of approximately 204°.

In this embodiment, as noted above, the radius 412 of the first rotor 104 may be about 0.07 m. In some embodiments, the length of the first rotor 104 between its first and second ends 120, 122 may be any appropriate length dependent upon application. Preferably the length of the first rotor 104 is greater than or equal to 0.14 m. Preferably, the first rotor 104 has a length-to- diameter ratio of greater than or equal to 1.5. More preferably, the first rotor 104 has a length-to-diameter ratio of greater than or equal to 1.7. More preferably, the first rotor 104 has a length-to-diameter ratio of greater than or equal to 2, for example about 2.1. In this embodiment, as noted above, the radius 432 of the second rotor 108 may be about 0.055 m. In some embodiments, the length of the second rotor 108 between its first and second ends 130, 132 may be any appropriate length dependent upon application. Preferably the length of the second rotor 108 is greater than or equal to 0.14 m. Preferably, the second rotor 108 has a length-to-diameter ratio of greater than or equal to 1.7, or greater than or equal to 2, or greater than or equal to 2.5, e.g. about 2.7. The length-to-diameter ratio of the second rotor 108 may be dependent on, i.e. a function of, that of the first rotor 104.

The use of rotors having a relatively low number of lobes, i.e. the use of a two-lobed rotor in combination with a three-lobed rotor, tends to be beneficial for vacuum applications.

A lower number of rotor lobes tends to mean that the rotors have less mass, resulting in greater pump efficiency.

Furthermore, a lower number of rotor lobes tends to provide for increased inter-lobe volume. This advantageously tends to allow the rotor to drive a greater volume of gas for a given number of rotations. Thus, pump efficiency tends to be improved.

Furthermore, a lower number of rotor lobes tends to provide for easier rotor manufacture. In particular, there are fewer lobes that need to be machined. Also, there tends to be less rotor surface area to machine or finish. Moreover, having a lower number of lobes tends to provide for greater inter-lobe volume, providing easier access to machining tools and the like.

Furthermore, a lower number of rotor lobes tends to provide for reduced leakage within the pumping chamber of the pump, and thus greater pump efficiency. The lower number of rotor lobes tends to provide for larger chamber volume. The ratio of chamber throughput to leakage losses due to inter-lobe and radial leakage tends to be larger than it would be on a rotor lobe combination of higher order. Also, a lower number of rotor lobes tends to provide for a smaller inter-lobe leakage length and a smaller tip leakage length. This advantageously tends to reduce back-leakage within the pump. Conventionally, in screw-type vacuum pumps, the volume index of the pump is controlled by controlling the geometry of the exhaust port (which may be a so-called “butterfly valve”) of the pump. The present inventors have realised that lowering the number of lobes in a screw-type vacuum pumps tends to lead to the geometry of the exhaust port becoming more difficult to specify, more restrictive, and/or less machinable. The present inventors have further realised that, advantageously, the volume index of the vacuum pump can be controlled by controlling the parameters or geometry of the helical lobes of the rotors, including for example the rotor pitch, the lobe shape, and the wrap angle. The present inventors have realised that, by having a non-uniform rotor pitch along the length of a rotor as in the above embodiments, it tends to be possible to achieve a high volume ratio without being reliant on the exhaust port geometry. Furthermore, by having relatively fewer lobes, it also tends to be possible to have a satisfactorily large pitch that would accommodate a larger reduction in rotor pitch throughout the length of the rotor.

Advantageously, the above-described blow-off valves tend to allow for adjustment of the volume index of the pump. For example, the positions and threshold pressures of the one or more blow-off valves may be controlled to control the volume index of the pump. Thus, it tends to be possible to achieve a high volume ratio without being reliant on the exhaust port geometry.

Advantageously, the above-described rotor profiles and rotor designs tend to provide for a beneficial or desirable volume index of the pump without or with reduced reliance on the timing or shape of the exhaust port or valve.

In the above embodiments, the vacuum pump is an oil-injected vacuum pump. The pump may comprise oil injection means configured to inject an oil into the housing, whereby the oil provides sealing of the first and second rotors against the housing and/or each other. The first rotor and the second rotor may be in contact with one another. The first rotor may drive the second rotor or vice versa. Flowever, in other embodiments, the vacuum pump is not an oil-injected vacuum pump. Also, in some embodiments, one rotor does not drive the other rotor. For example, the vacuum pump may be a dry vacuum pump. In some embodiments, the vacuum pump comprises one or more gears configured to drive and synchronize rotation of the first and second rotors.

In the above embodiments, the vacuum pump comprises two rotors, namely the first and second rotors. However, in other embodiments, the apparatus comprises a different number of rotors, such as more than two rotors. For example, in some embodiments, there may be multiple female rotors engaged to a given male rotor or vice versa.

In the above embodiments, the vacuum pump comprises a single inlet port. However, in other embodiments the apparatus comprises a different number of inlet ports, i.e. more than one inlet port.

In the above embodiments, the vacuum pump comprises a single exhaust outlet. However, in other embodiments the apparatus comprises a different number of exhaust outlets, i.e. more than one exhaust outlet.

In the above embodiments, the first rotor is a male rotor and the second rotor is a female rotor. However, in other embodiments the first rotor is a female rotor and the second rotor is a male rotor.

In the above embodiments, the first rotor drives the second rotor. However, in other embodiments, the first rotor does not drive the second rotor. For example, in some embodiments the second rotor drives the first rotor. In such embodiments, the second rotor may be driven by a motor. In some embodiments, the first and second motors are separately driven by respective motors or a common motor. In such embodiments, the first and second rotors may be spaced apart such that they do not contact each other in operation.

In the above embodiments, the vacuum pump comprises one or more blow-off valves. However, in other embodiments, the blow-off valves are omitted.

In the above embodiments, the profile of the first rotor is as described in more detail earlier above with reference to Figure 4. However, in other embodiments, the first rotor has a different rotor profile. In the above embodiments, the profile of the second rotor is as described in more detail earlier above with reference to Figure 4. However, in other embodiments, the second rotor has a different rotor profile.

In the above embodiment, the rotors have variable pitch along the length of the rotors. However, in other embodiments, one or more of the rotors does not have variable pitch along its length.

REFERENCE NUMERAL LIST 100 - vacuum pump 102 - housing 104 - first rotor 106 - first shaft

108 - second rotor 110 - second shaft 112 - motor 114 - inlet port 116 - exhaust outlet

118 - first axis 120 - first end 122 - second end

124 - screw-type body or working portion 126 - internal or pumping chamber

128 - second axis 130 - first end 132 - second end

134 - screw-type body or working portion 136 - blow-off valves

138 - oil injection port 140 - oil flow direction 142, 144, 146 -fluid flow direction

300 - first lobes 302 - helical grooves 304 - second lobes 306 - helical grooves 400 - circular arc portion

402 - first pseudo-Archimedean spiral curve portion 404 - second pseudo-Archimedean spiral curve portion

406 - root or base portion of first rotor 408 - radial distance of circular arc portion from first axis 410 - tip of first lobe 412 - maximum radius of first rotor 414 - tooth depth of first lobe

420 - third pseudo-Archimedean spiral curve portion 422 - end portion

424 - fourth pseudo-Archimedean spiral curve portion 426 - root or base portion of second rotor 428 - radius of root or base portion from second axis

430 - tip of second lobe 432 - maximum radius of second rotor 434 - tooth depth of second lobe