Vacuum pump

Abstract

A vacuum pump 100 comprising a housing 104 with a pumping chamber 131; a shaft 107 rotating within the housing about its lengthwise axis, the lengthwise axis extending in an axial direction 110; a rot

Description

FIELD OF THE INVENTION The present invention relates to vacuum pumps, including but not limited to multi-stage vacuum pumps such as multi-stage vacuum Roots-type vacuum pumps. BACKGROUND Conventional multi-stage vacuum pumps, such as multi-stage vacuum Roots-type vacuum pumps, comprise multiple pumping chambers arranged in series. Each pumping chamber contains a pair of intermeshing rotors, for example Roots-type profile rotors, that are securely attached to respective shafts. In operation, these rotor pairs rotate within each pumping chamber to evacuate a volume connected to the vacuum pump's inlet port by compressing gas drawn from the evacuated volume. During pumping, the compression of gas generates heat, known as compression heat. To prevent overheating of the pump housing, which could affect pumping performance, this compression heat is typically dissipated to the atmosphere. This dissipation is achieved by transferring the heat from the outer surface of the housing to the surrounding air. Cooling water may be circulated through a cooler attached to the housing to help regulate the temperature and maintain optimal operating conditions. As a result, the housing of the multistage vacuum pump may remain at a stable temperature, ensuring efficient and reliable performance. However, during the operation of the multi-stage vacuum pump, the pumping chambers are effectively evacuated, resulting in minimal heat radiation to the gas within each chamber from the rotors and shafts. Consequently, the temperature of each rotor pair and its shafts may gradually increase compared to the housing. The housing and shafts tend to expand proportionally with their respective temperatures. Significant temperature differences between the housing and shafts may lead to different degrees of expansion therebetween. This can lead to variations in the positioning of the rotors relative to the housing. Despite the multi-stage vacuum pump being designed to maintain a consistent axial clearance between the rotors and the inner surfaces of the pumping chambers, this clearance may progressively increase or decrease due to the significant thermal expansion disparity between the shaft and the housing. This variability in between the rotors and the housing can lead to the rotors coming into contact with the inner surface of the pumping chamber and / or increased gas leakage, for example, between pumping stages. SUMMARY OF THE INVENTION The vacuum pump disclosed herein tends to be free from the above-described drawbacks. In a first aspect, there is provided a vacuum pump comprising: a housing in which a pumping chamber is formed; a shaft arranged for rotation within the housing about its lengthwise axis, the lengthwise axis extending in an axial direction; a rotor located in the pumping chamber, the rotor being fixedly attached to the shaft; and a disc located in the pumping chamber, the disc being fixedly attached to the shaft at a first side of the rotor. A first end of the shaft is substantially immovable relative to the housing in the axial direction. A second end of the shaft is movable relative to the housing in the axial direction, the second end being opposite to the first end. A wall of the pumping chamber comprises a recess formed therein. At least a part of the disc is located in the recess. The vacuum pump may further comprise a further disc located in the pumping chamber, the further disc being fixedly attached to the shaft at a second side of the rotor opposite to the first side of the rotor. A further wall of the pumping chamber may comprise a further recess formed therein. At least a part of the further disc may be located in the further recess. The vacuum pump may be a multi-stage vacuum pump comprising: a plurality of pumping chambers formed in the housing; a plurality of rotors, each rotor of the plurality of rotors being located in a respective pumping chamber of the plurality of pumping chambers, the plurality of rotors being fixedly attached to the shaft; and a plurality of discs, each disc of the plurality of discs being located in a respective pumping chamber of the plurality of pumping chambers, each disc of the plurality of discs being fixedly attached to the shaft at a first side of a respective rotor of the plurality of rotors. A respective wall of each pumping chamber of the plurality of pumping chambers may comprise a respective recess formed therein. At least a part of each disc of the plurality of discs may be located in a respective recess. The vacuum pump may further comprise a plurality of further discs, each further disc of the plurality of further discs being located in a respective pumping chamber of the plurality of pumping chambers, each further disc of the plurality of further discs being fixedly attached to the shaft at a second side of a respective rotor of the plurality of rotors opposite to the first side of that rotor. A respective further wall of each pumping chamber of the plurality of pumping chambers may comprise a respective further recess formed therein. At least a part of each further disc of the plurality of further discs may be located in a respective further recess. The vacuum pump may be selected from the group of vacuum pumps consisting of a Roots-type pump, a screw pump, and a claw pump. The vacuum pump may be a multi-stage Roots-type vacuum pump. The housing may comprise a plurality of pumping chambers formed therein. The plurality of pumping chambers may be arranged in series and in fluid communication with one another. A first pumping chamber of the plurality of pumping chambers which is at a first end of the series may act as an initial stage pumping chamber. A second pumping chamber of the plurality of pumping chambers which is at a second end of the series may act as a final stage pumping chamber. The vacuum pump may comprise a plurality of Roots-type pump sections occupying the respective pumping chambers, each of the Roots-type pump sections comprising a pair of intermeshed Roots-type profile rotors and a pair of first discs. The vacuum pump may comprise a pair of shafts arranged for rotation within the housing about their lengthwise axes in contra-rotational direction, the lengthwise axes each extending in the axial direction, the pair of shafts being fixedly attached to the pair of intermeshed Roots-type profile rotors and the pair of first discs in each of the Roots-type pump sections, a first end of each of the shafts being substantially immovable relative to the housing in the axial direction, a second end of each of the shafts being movable relative to the housing in the axial direction. At least a part of each first disc may be located in a respective first recess formed in a wall of a pumping chamber of the plurality of pumping chambers. Each of the Roots-type pump sections may further comprise a pair of second discs. The first discs and the second discs may be fixedly attached to the shafts such that each Roots-type profile rotor is sandwiched between a respective pair of first and second discs. At least a part of each second disc may be located in a respective second recess formed in a wall of a pumping chamber of the plurality of pumping chambers. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration (not to scale) of a side-view crosssection of a vacuum pump; and Figure 2 is a schematic illustration (not to scale) of a cross-sectional view of the vacuum pump taken along line A-A in Figure 1. DETAILED DESCRIPTION Figure 1 is a schematic illustration (not to scale) of a side-view crosssection of an embodiment vacuum pump 100. Figure 2 is a schematic illustration (not to scale) of a cross-sectional view of the vacuum pump 100 taken along line A-A in Figure 1. In this embodiment, the vacuum pump 100 is a “Roots”-type multi-stage vacuum pump. The vacuum pump 100 comprises complementary housing members, hereinafter referred to as the first housing member 101 and the second housing member 102, which attach together to form a housing 104. The vacuum pump 100 further comprises a pair of opposing side members, hereinafter referred to as the first side member 105 and the second side member 106, which are coupled to opposite ends of the housing 104. The housing 104 comprises an integral inlet port 112 and an integral outlet port 114. Specifically, the first housing member 101 has formed at its upper side the integral inlet port 112. In this embodiment, the inlet port 112 is in fluid communication with a volume (not shown) that is to be evacuated by the vacuum pump 100. Also, the second housing member 102 has formed at its lower side the integral outlet port 114. The outlet port 114 is a port through which gas is exhausted to outside the vacuum pump 100 after passing through the housing 104. In this embodiment, the housing 104 defines five axially spaced wall partitions 121, 122, 123, 124, 125 which define six pumping chambers arranged in series and in fluid communication with one another. The six pumping chambers are referred to as the first stage pumping chamber 131, the second stage pumping chamber 132, the third stage pumping chamber 133, the fourth stage pumping chamber 134, the fifth stage pumping chamber 135, and the sixth stage pumping chamber 136. The first stage pumping chamber 131 is an initial stage pumping chamber of the vacuum pump 100 and is in fluid communication with the inlet port 112. The sixth stage pumping chamber 136 is a final stage pumping chamber of the vacuum pump 100 and is in fluid communication with the outlet port 114. In operation, the six pumping chambers 131-136 compress the gas drawn from the volume being evacuated in stepwise fashion. At a central portion inside the housing 104, a pair of parallel shafts is provided, hereinafter referred to as the first shaft 107 and the second shaft 108. The shafts 107, 108 are arranged to rotate within the housing 104 about their lengthwise axes in contra-rotational direction. The lengthwise axes of the shafts 107, 108 extend in an axial direction 110 of the housing 104. In this embodiment, the first shaft 107 is arranged to be rotated within the housing 104 about its longitudinal axis by an electric motor (not shown). The second shaft 108 is arranged to be rotated within the housing 104 in an opposite direction to the first shaft 107 by virtue of the second shaft 108 being coupled to the first shaft 107 by means a gears arrangement (not shown). The vacuum pump 100 further comprises two pairs of bearings 116, 118. A first pair of bearings 116 (only one is shown in Figure 1) is provided in the first side member 105. A second pair of bearings 118 (only one is shown in Figure 1) is provided in the second side member 106. Opposite ends of the first shaft 107 are supported for rotation by one of the first pair of bearings 116 and one of the second pair of bearings 118. Opposite ends of the second shaft 108 are supported for rotation by the other of the first pair of bearings 116 and the other of the second pair of bearings 118. The first and second pairs of bearings 116, 118 support the shafts 107, 108 in a parallel arrangement. In this embodiment, the second pair of bearings 118 have fixed axial positions relative to the second side member 106, and thus also relative to the housing 104. Thus, the second pair of bearings 118 are substantially immovable relative to the housing in the axial direction 110. In this embodiment, the first pair of bearings 116 are arranged in the first side member 105 such that movement of the first pair of bearings 116 in the axial direction 110 is permitted. Thus, the first pair of bearings 116 are coupled to the housing 104 such that movement of the first pair of bearings 116 in the axial direction 110 relative to the housing 104 is permitted. Accordingly, lengthwise expansion of the shafts 107, 108 (i.e., expansion of the shafts in the axial direction 110) is allowed by movement of the first pair of bearings 116 relative to the housing 104. In other words, the first pair of bearings 116 are moveable in the axial direction 110 relative to the housing 104. In each of the pumping chambers 131-136, the shafts 107, 108 support (and are fixedly attached to) a respective pair of Roots-type profile rotors, hereinafter referred to as the first pair of rotors 141a-b, the second pair of rotors 142a-b, the third pair of rotors 143a-b, the fourth pair of rotors 144a-b, the fifth pair of rotors 145a-b, and the sixth pair of rotors 146a-b. The first, second, third, fourth, fifth, and sixth pairs of rotors 141a-b, 142a-b, 143a-b, 144a-b, 145a-b, 146a-b are located in the first, second, third, fourth, fifth, and sixth stage pumping chambers 131-136, respectively, and are arranged to rotate relative to an inner surface of the housing 104 within the pumping chambers 131-136 whereby to pump gas through the pumping chambers 131-136. In this embodiment, in each of the pumping chambers 131-136, the shafts 107, 108 support (and are fixedly attached to) a respective two pairs of discs, i.e. the shafts 107, 108 support twelve pairs of discs. The discs are hereinafter referred to as the first pair of discs 161a-b, the second pair of discs 162a-b, the third pair of discs 163a-b, the fourth pair of discs 164a-b, the fifth pair of discs 165a-b, and the sixth pair of discs 166a-b, the seventh pair of discs 167a-b, the eighth pair of discs 168a-b, the ninth pair of discs 169a-b, the tenth pair of discs 170a-b, the eleventh pair of discs 171a-b, and the twelfth pair of discs 172a-b. The first and second pairs of discs 161a-b, 162a-b are located in the first pumping chamber 131 and are disposed on the shafts 107, 108 at opposite sides of the first pair of rotors 141a-b. In other words, one of the first pair of rotors 141a is disposed on the first shaft 107 within the first pumping chamber 131 and is sandwiched between one of the first pair of discs 161a and one of the second pair of discs 162a, while the other of the first pair of rotors 141b is disposed on the second shaft 108 within the first pumping chamber 131 and is sandwiched between the other of the first pair of discs 161b and the other of the second pair of discs 162b. Similarly, the third and fourth pairs of discs 163a-b, 164a-b are located in the second pumping chamber 132 and are disposed on the shafts 107, 108 at opposite sides of the second pair of rotors 142a-b. Similarly, the fifth and sixth pairs of discs 165a-b, 166a-b are located in the third pumping chamber 133 and are disposed on the shafts 107, 108 at opposite sides of the third pair of rotors 143a-b. Similarly, the seventh and eighth pairs of discs 167a-b, 168a-b are located in the fourth pumping chamber 134 and are disposed on the shafts 107, 108 at opposite sides of the fourth pair of rotors 144a-b. Similarly, the ninth and tenth pairs of discs 169a-b, 170a-b are located in the fifth pumping chamber 135 and are disposed on the shafts 107, 108 at opposite sides of the fifth pair of rotors 145a-b. Similarly, the eleventh and twelfth pairs of discs 171a-b, 172a-b are located in the sixth pumping chamber 136 and are disposed on the shafts 107, 108 at opposite sides of the sixth pair of rotors 146a-b. In this embodiment, walls of each pumping chamber 131-136 comprise recesses formed therein. More specifically, the opposing, axially spaced walls of each pumping chamber 131-136 each comprise one or more recess formed therein. A first wall 181 of the first pumping chamber 131 comprises one or a pair of first recess(es) 201 formed therein. The first wall 181 of the first pumping chamber 131 may be considered to be a upstream wall when considering the direction of gas flow through the first pumping chamber 131. At least a part of each of the first pair of discs 161a-b is located in the first recess(es) 201. A second wall 182 of the first pumping chamber 131 comprises one or a pair of second recess(es) 202 formed therein. The second wall 182 of the first pumping chamber 131 may be considered to be a downstream wall when considering the direction of gas flow through the first pumping chamber 131. At least a part of each of the second pair of discs 162a-b is located in the second recess(es) 202. A third wall 183 of the second pumping chamber 132 comprises one or a pair of third recess(es) 203 formed therein. The third wall 183 of the second pumping chamber 132 may be considered to be a upstream wall when considering the direction of gas flow through the second pumping chamber 132. At least a part of each of the third pair of discs 163a-b is located in the third recess(es) 203. A fourth wall 184 of the second pumping chamber 132 comprises one or a pair of fourth recess(es) 204 formed therein. The second wall 182 of the second pumping chamber 132 may be considered to be a downstream wall when considering the direction of gas flow through the second pumping chamber 132. At least a part of each of the second pair of discs 162a-b is located in the fourth recess(es) 204. A fifth wall 185 of the third pumping chamber 133 comprises one or a pair of fifth recess(es) 205 formed therein. The fifth wall 185 of the third pumping chamber 133 may be considered to be a upstream wall when considering the direction of gas flow through the third pumping chamber 133. At least a part of each of the fifth pair of discs 165a-b is located in the fifth recess(es) 205. A sixth wall 186 of the third pumping chamber 133 comprises one or a pair of sixth recess(es) 206 formed therein. The sixth wall 186 of the third pumping chamber 133 may be considered to be a downstream wall when considering the direction of gas flow through the third pumping chamber 133. At least a part of each of the sixth pair of discs 166a-b is located in the sixth recess(es) 206. A seventh wall 187 of the fourth pumping chamber 134 comprises one or a pair of seventh recess(es) 207 formed therein. The seventh wall 187 of the fourth pumping chamber 134 may be considered to be a upstream wall when considering the direction of gas flow through the fourth pumping chamber 134. At least a part of each of the seventh pair of discs 167a-b is located in the seventh recess(es) 207. An eighth wall 188 of the fourth pumping chamber 134 comprises one or a pair of eighth recess(es) 208 formed therein. The eighth wall 188 of the fourth pumping chamber 134 may be considered to be a downstream wall when considering the direction of gas flow through the fourth pumping chamber 134. At least a part of each of the eighth pair of discs 168a-b is located in the eighth recess(es) 208. A ninth wall 189 of the fifth pumping chamber 135 comprises one or a pair of ninth recess(es) 209 formed therein. The ninth wall 189 of the fifth pumping chamber 135 may be considered to be a upstream wall when considering the direction of gas flow through the fifth pumping chamber 135. At least a part of each of the ninth pair of discs 169a-b is located in the ninth recess(es) 209. A tenth wall 190 of the fifth pumping chamber 135 comprises one or a pair of tenth recess(es) 210 formed therein. The tenth wall 190 of the fifth pumping chamber 135 may be considered to be a downstream wall when considering the direction of gas flow through the fifth pumping chamber 135. At least a part of each of the tenth pair of discs 170a-b is located in the tenth recess(es) 210. An eleventh wall 191 of the sixth pumping chamber 136 comprises one or a pair of eleventh recess(es) 211 formed therein. The eleventh wall 191 of the sixth pumping chamber 136 may be considered to be a upstream wall when considering the direction of gas flow through the sixth pumping chamber 136. At least a part of each of the eleventh pair of discs 171a-b is located in the eleventh recess(es) 211. A twelfth wall 192 of the sixth pumping chamber 136 comprises one or a pair of twelfth recess(es) 212 formed therein. The twelfth wall 192 of the sixth pumping chamber 136 may be considered to be a downstream wall when considering the direction of gas flow through the sixth pumping chamber 136. At least a part of each of the twelfth pair of discs 172a-b is located in the twelfth recess(es) 212. The recesses 201-212 formed in the walls of the pumping chambers 131-136 and the discs 161a-b - 172a-b being at least partially located within those recesses 201-212 advantageously tend to prevent or oppose the back leakage of gas from a downstream pumping chamber to an adjacent upstream pumping chamber, i.e. from the exhaust area to the inlet area. This tends to be because the recesses 201-212 and discs 161a-b - 172a-b create a more convoluted or tortuous gas path for back leakage. A large proportion of the axial area for back leakage requires the gas to travel in a radial direction. Thus, a more restrictive gas path for back leakage is provided, which tends to limit, prevent, or oppose the leakage. The pumping chambers 131-136 are in fluid communication with each other by way of passages (not shown) formed in the housing 104. Each passage connects two adjacent pumping chambers 131-136, thereby to connect together the pumping chambers 131-136 in series. In operation, gas is drawn into the inlet port 112 (as indicated by arrow 151 in Figure 1) and undergoes a six-stage compression process by the six pumping chambers 131-136 in turn. The gas is compressed in stepwise manner by the six pumping chambers 131-136 between the inlet port 112 and the outlet port 114. The pumped gas is then exhausted outside the vacuum pump 100 via the outlet port 114 (as indicated by arrow 152 in Figure 1). The gas is heated by the compression process. More specifically, during operation of the vacuum pump 100, gas compression occurs in each of the six pumping chambers 131-136 which results in generation of compression heat at progressively higher temperatures in the pumping chambers 131-136. The generated compression heat is transferred to the rotors 141a-b, 142a-b, 143a-b, 144a-b, 145a-b, 146a-b, the shafts 107, 108, and the housing 104. Compression heat transferred to the housing 104 may be dissipated to the atmosphere and / or a cooler may provide cooling to the housing 104. On the other hand, compression heat transferred to the rotors 141a-b, 142a-b, 143a-b, 144a-b, 145a-b, 146a-b and the shafts 107, 108 may be dissipated to a lesser extent and / or less effectively. This may result in the temperatures of the rotors 141a-b, 142a-b, 143a-b, 144a-b, 145a-b, 146a-b and the shafts 107, 108 increasing to a greater extent and / or at a faster rate than the housing 104. Accordingly, the shafts 107, 108 may expand in the axial direction 110 relative to the housing 104. The axial expansion of the shafts 107, 108 tends to be permitted by the first pair of bearings 116 moving or sliding within the first side member 105 relative to the housing 104. Advantageously, each of the pumping chamber walls 181-192 having recesses 201-212 formed in opposite sides thereof, and the discs 161a-b -172a-b being at least partially located with those recesses 201-212, advantageously tend to provide improved leakage prevention, even in cases where the shafts 107, 108 expand axially relative to the housing 104. This tends to be because, even if the axial expansion of the shafts 107, 108 causes a disc 161a-b - 172a-b to move relatively further out of its corresponding recess 201-212 on one side of a pumping chamber wall 181-192 thus creating a larger gap / gas path between pumping chamber wall and disc, that same axial expansion of the shafts 107, 108 causes a different disc 161a-b - 172a-b to move relatively further into its corresponding recess 201-212 on the other side of that pumping chamber wall 181-192 thus creating a more restrictive gas path for back leakage. The above-described apparatus tends to prevent or oppose back leakage of pumped gas, for example due to tolerance stack-up gap requirement, and thermal expansion movement. Also, the above-described apparatus tends to prevent or oppose back leakage of pumped gas when the pump is running at a condition with low differential thermal expansion. The above-described apparatus tends to provide improved leakage prevention, for example in cases where machining tolerances lead to large clearances between the rotors and the housing. In the above embodiments, the vacuum pump is a “Roots”-type multistage vacuum pump. However, in other embodiments, the vacuum pump is a different type of vacuum pump. The vacuum pump may be, for example, a type selected from the group of vacuum pump types consisting of: a Roots-type pump, a screw pump, and a claw pump. The vacuum pump may be a multisage pump or may be a single-stage pump. In the above embodiments, the vacuum pump comprises six pumping chambers. However, in other embodiments, the vacuum pump comprises a different number of pumping chambers. In the above embodiments, the vacuum pump comprises two shafts, to which the rotors and discs are fixedly attached. However, in other embodiments, the vacuum pump comprises a different number of such shafts, such as only a single shaft, or more than two shafts. In the above embodiments, the vacuum pump comprises six pairs of rotors. However, in other embodiments, the vacuum pump comprises a different number of rotors. In the above embodiments, the vacuum pump comprises twelve pairs of discs. However, in other embodiments, the vacuum pump comprises a different number of discs. In the above embodiments, each rotor is sandwiched between a respective pair of discs. The rotors may be in contact with the discs between which it is sandwiched, or may be spaced aprt from one or both of those discs. However, in other embodiments, one or more of the rotors is not sandwiched between discs. In some embodiments, in one or more of the pumping chambers, a rotor may have only a single disc disposed at either its upstream or downstream side, and not at its opposite side. In the above embodiments, each pumping chamber comprises one or more recesses in each of its upstream and downstream walls, in which respective discs are at least partially located. However, in other embodiments, one or more of the pumping chambers does not include recesses in both its upstream and downstream walls. For example, one or more of the pumping chambers may comprise one or more recesses in its upstream wall only, and not in its downstream wall. This may be in embodiments in which a disc is located only at an upstream side of a rotor and not at the downstream side of the rotor. As another example, one or more of the pumping chambers may 5 comprise one or more recesses in its downstream wall only, and not in its upstream wall. This may be in embodiments in which a disc is located only at a downstream side of a rotor and not at the upstream side of the rotor. Reference numerals 100 - vacuum pump 101 - first housing member 102 - second housing member 104- housing 105 - first side member 106 - second side member 107 - first shaft 108 - second shaft 110- axial direction relative to housing 112- inlet port 114-outlet port 116- first pair of bearings 118- second pair of bearings 121, 122, 123, 124, 125-wall partitions 131, 132, 133, 134, 135, 136-pumping chambers 141 a-b - first pair of rotos 142a-b - second pair of rotors 143a-b - third pair of rotors 144a-b - fourth pair of rotors 145a-b - fifth pair of rotors 146a-b - sixth pair of rotors 151, 152 - fluid flow directions 161 a-b - first pair of discs 162a-b - second pair of discs 163a-b - third pair of discs 164a-b - fourth pair of discs 165a-b - fifth pair of discs 166a-b - sixth pair of discs 167a-b - seventh pair of discs 168a-b - eighth pair of discs 169a-b - ninth pair of discs 170a-b - tenth pair of discs 171 a-b - eleventh pair of discs 172a-b - twelfth pair of discs 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192-walls 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211,212 - recesses

Claims

1. A vacuum pump comprising:a housing in which a pumping chamber is formed;a shaft arranged for rotation within the housing about its lengthwise axis, the lengthwise axis extending in an axial direction;a rotor located in the pumping chamber, the rotor being fixedly attached to the shaft; anda disc located in the pumping chamber, the disc being fixedly attached to the shaft at a first side of the rotor; whereina first end of the shaft is substantially immovable relative to the housing in the axial direction;a second end of the shaft is movable relative to the housing in the axial direction, the second end being opposite to the first end;a wall of the pumping chamber comprises a recess formed therein; andat least a part of the disc is located in the recess.

2. The vacuum pump of claim 1, further comprising:a further disc located in the pumping chamber, the further disc being fixedly attached to the shaft at a second side of the rotor opposite to the first side of the rotor; whereina further wall of the pumping chamber comprises a further recess formed therein; andat least a part of the further disc is located in the further recess.

3. The vacuum pump of any preceding claim, whereinthe vacuum pump is a multi-stage vacuum pump comprising:a plurality of pumping chambers formed in the housing;a plurality of rotors, each rotor of the plurality of rotors being located in a respective pumping chamber of the plurality of pumping chambers, the plurality of rotors being fixedly attached to the shaft; anda plurality of discs, each disc of the plurality of discs being located in a respective pumping chamber of the plurality of pumping chambers, each disc of the plurality of discs being fixedly attached to the shaft at a first side of a respective rotor of the plurality of rotors; whereina respective wall of each pumping chamber of the plurality of pumping chambers comprises a respective recess formed therein; andat least a part of each disc of the plurality of discs is located in a respective recess.

4. The vacuum pump of claim 3, further comprising:a plurality of further discs, each further disc of the plurality of further discs being located in a respective pumping chamber of the plurality of pumping chambers, each further disc of the plurality of further discs being fixedly attached to the shaft at a second side of a respective rotor of the plurality of rotors opposite to the first side of that rotor; whereina respective further wall of each pumping chamber of the plurality of pumping chambers comprises a respective further recess formed therein; andat least a part of each further disc of the plurality of further discs is located in a respective further recess.

5. The vacuum pump of any preceding claim, wherein the vacuum pump is selected from the group of vacuum pumps consisting of: a Roots-type pump, a screw pump, and a claw pump.

6. The vacuum pump of any preceding claim, wherein:the vacuum pump is a multi-stage Roots-type vacuum pump;the housing comprises a plurality of pumping chambers formed therein;the plurality of pumping chambers are arranged in series and in fluid communication with one another;a first pumping chamber of the plurality of pumping chambers which is at a first end of the series acts as an initial stage pumping chamber;a second pumping chamber of the plurality of pumping chambers which is at a second end of the series acts as a final stage pumping chamber;the vacuum pump comprises:a plurality of Roots-type pump sections occupying the respective pumping chambers, each of the Roots-type pump sections comprising a pair of intermeshed Roots-type profile rotors and a pair of first discs; anda pair of shafts arranged for rotation within the housing about their lengthwise axes in contra-rotational direction, the lengthwise axes each extending in the axial direction, the pair of shafts being fixedly attached to the pair of intermeshed Roots-type profile rotors and the pair of first discs in each of the Roots-type pump sections, a first end of each of the shafts being substantially immovable relative to the housing in the axial direction, a second end of each of the shafts being movable relative to the housing in the axial direction; andat least a part of each first disc is located in a respective first recess formed in a wall of a pumping chamber of the plurality of pumping chambers.

7. The vacuum pump of any claim 6, wherein:each of the Roots-type pump sections further comprises a pair of second discs;the first discs and the second discs are fixedly attached to the shafts such that each Roots-type profile rotor is sandwich between a respective pair of first and second discs; andat least a part of each second disc is located in a respective second recess formed in a wall of a pumping chamber of the plurality of pumping chambers.

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