GB2609430A

GB2609430A - Shaft assembly

Info

Publication number: GB2609430A
Application number: GB2110982.2A
Authority: GB
Inventors: Van Hoof Nils; Antoine J Cools Ronny; Meeusen Wim; Vinck Glenn
Original assignee: Atlas Copco Airpower NV
Current assignee: Atlas Copco Airpower NV
Priority date: 2021-07-30
Filing date: 2021-07-30
Publication date: 2023-02-08
Also published as: WO2023006904A1; TW202332835A; BE1029641A1; BE1029641B1; GB202110982D0

Abstract

A shaft assembly comprising a first shaft 112 and a second shaft 130 fixedly attached to each other via a coupling element 200. An electrically insulative element 204 is coupled between the first shaft 112 and the coupling element 200 to electrically isolate the first shaft 112 from the coupling element 200. The first shaft 112 may be the rotor shaft of a vacuum pump, a compressor, or an expander, driven by the second shaft 130 that may be a motor shaft of a motor. The coupling element 200 can be an annular or tubular collar configured to fit around an end 202 of the second shaft 130 and is fixedly attached to the first shaft 112 by one or more fasteners 216. The fasteners 216 are electrically isolated from one or both of the first shaft 112 and the coupling element 200.

Description

SHAFT ASSEMBLY

FIELD OF THE INVENTION

The present invention relates to shaft assemblies for use in vacuum pumping apparatuses, compressors, expanders, and the like, and, in particular, shaft assemblies for coupling together two shafts such that the two shafts are electrically isolated from each other.

BACKGROUND

to Vacuum pumps are used in various technical processes to pump gases out of process chambers, thereby to create low-pressure conditions for the respective processes.

Positive-displacement vacuum pumps include screw vacuum pumps, Roots vacuum pumps, and the like, and are widely used for evacuating a process gas from vacuum chambers in semiconductor fabrication facilities.

Positive-displacement pumps typically comprise a pair of pump rotors mounted on rotor shafts, and a motor comprising a motor shaft. The motor shaft of the motor is fixedly attached to one of the rotor shafts. In use, the motor drives the rotors. Specifically, the motor shaft is rotated about its axis, thereby to cause the rotor shafts and the rotors mounted thereon to rotate within a pumping chamber of the vacuum pump and draw in and discharge a gas.

SUMMARY OF THE INVENTION

The present inventors have realised that, for many vacuum pumps including, but not limited to, oil-sealed rotary screw vacuum pumps with Variable Speed Drive (VSD), electrical current may be caused to flow through the bearings of that pump. The bearings of the pump rotatably support the rotor shafts to allow the rotors to rotate within the pumping chamber.

The present inventors have further realised that electrical current flowing through the bearings of the pump tends to damage the bearings. It is thus desirable to prevent or reduce electrical currents within the bearings of the pump.

The present inventors have further realised that prevention or reduction of electrical currents within the bearings of the pump tends to be achieved by electrically insulating, or improving electrical insulation between, the rotor shafts and the motor shaft.

In an aspect, there is provided a shaft assembly comprising: a first shaft having a first shaft end; a coupling element fixedly attached to the first shaft end, to the coupling element being configured to fixedly attach to a second shaft end of a second shaft; and an electrically insulative (or electrically resistive) element coupled between the first shaft and the coupling element thereby to electrically isolate the first shaft from the coupling element.

The shaft assembly may further comprise the second shaft, the second 15 shaft having the second shaft end. The coupling element may be attached to the second shaft end of the second shaft.

The first shaft may be a shaft selected from the group of shafts consisting of a rotor shaft of a vacuum pump, a compressor, or an expander, and a motor shaft of a motor for driving a rotor shaft of a vacuum pump, a compressor, or an expander.

The coupling element may be an annular or tubular collar configured to fit around the second shaft end of the second shaft.

The coupling element may be fixedly attached to the first shaft end of the first shaft by one or more fasteners. The one or more fasteners may be electrically isolated from one or both of the first shaft and the coupling element.

The first shaft may comprise a flange located at the first shaft end. The coupling element may be attached to the flange. The shaft assembly may further comprise one or more fasteners positioned through the flange from a first side of the flange to a second side of the flange opposite to the first side of the flange.

The one or more fasteners may extend into the coupling element. The coupling -3 -element may be located at the second side of the flange. The shaft assembly may further comprise one or more further electrically insulative elements disposed between the one or more fasteners and the flange thereby to electrically isolate the one or more fasteners from the first shaft.

The one or more further electrically insulative elements may comprise an annular electrically insulative element positioned around the first shaft. The annular electrically insulative element may be located at the first side of the flange. The shaft assembly may further comprise an annular metal disc disposed against the annular electrically insulative element.

The one or more further electrically insulative elements may comprise one or more electrically insulative washers, each electrically insulative washer being disposed between the first side of the flange and a part of a respective one of the one or more fasteners. The shaft assembly may further comprise one or more metal washers, each metal washer being in contact with a respective one of the one or more electrically insulative washers and disposed between the first side of the flange and a head of a respective one of the one or more fasteners.

The electrically insulative element may be formed of a material selected from the group of materials comprising a ceramic material, aluminium oxide, and/or zirconium oxide.

In a further aspect, there is provided a vacuum pump, compressor, or expander assembly comprising: a vacuum pump, a compressor, or an expander having a rotor disposed on a rotor shaft rotatably arranged in a housing; a motor having a motor shaft; wherein the motor shaft is coupled between the rotor shaft and the motor; the motor is configured to rotate the motor shaft thereby to drive the rotor shaft; and the motor shaft and the rotor shaft are electrically isolated from one another.

The assembly may further comprise a coupling element fixedly attached to an end of the motor shaft and an end of the rotor shaft, thereby to fixedly attach together the motor shaft and the rotor shaft. The assembly may further comprise an electrically insulative element disposed between either the motor shaft and the coupling element or the rotor shaft and the coupling element.

The motor shaft may comprise a flange located at the end of the motor shaft. The assembly may further comprise: one or more fasteners positioned through the flange from a first side of the flange to a second side of the flange opposite to the first side of the flange, the one or more fasteners extending into the coupling element, the coupling element being located at the second side of the flange; and one or more further electrically insulative elements disposed between the one or more fasteners and the flange; the electrically insulative element is disposed between the motor shaft and the coupling element; and the coupling element is an annular or tubular collar in which is received the end of the rotor shaft.

The vacuum pump may be an oil-sealed rotary screw vacuum pump.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration (not to scale) showing a side view cross 15 section of a vacuum pumping apparatus; Figure 2 is a schematic illustration (not to scale) showing a side view cross section of a coupling assembly of the vacuum pumping apparatus; and Figure 3 is a schematic illustration (not to scale) showing a side view cross section of a further coupling assembly.

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) showing a side view cross section of a vacuum pumping apparatus 100 comprising a vacuum pump 102 and a motor 104.

In this embodiment, the vacuum pump 102 is an oil-sealed rotary screw vacuum pump.

In this embodiment, the motor 104 is an alternating current (AC) electric motor. The motor 104 is configured to drive the vacuum pump 102. A controller comprising a variable frequency drive (VFD) (not shown) controls the frequency of the electrical power supplied to the motor 104. Accordingly, the rotational speed of the motor output, and thus of a rotor of the vacuum pump 102 may be adjusted. As such, the vacuum pump 102 is a pump having Variable Speed Drive (VSD).

to The vacuum pump 102 comprises a housing 106 defining a pumping chamber 108, and a pair of intermeshing male and female screw rotors 110 rotatably mounted within the pumping chamber 108. The screw rotors 110 are mounted to or integral with respective rotor shafts 112.

At one end of the housing 106 is formed a gas inlet 114. At another end of the housing 106, opposite to the end at which the gas inlet 114 is located, is formed a gas outlet 116.

At a first end portion of the pumping chamber 108 is formed a suction port 118, which is in fluid communication with the gas inlet 114. At a second end portion of the pumping chamber 108, opposite to the first end portion, is formed a discharge port 120, which is in fluid communication with the gas outlet 116.

The rotor shafts 112 are rotatably supported at their first end portions, i.e. at their suction sides, within a suction-side bearing casing 122 by one or more first bearing 124. The first bearings 124 may include, for example, cylindrical roller bearings for radial load whose outer rings may be held at predetermined certain positions through an appropriate spacing, and/or angular ball bearings for forward thrust load and/or reverse thrust load.

The rotor shafts 112 are rotatably supported at their second end portions, i.e. at their discharge sides, within a discharge-side bearing casing 126 by one or more second bearings 128. The second bearings 128 may include, for example, cylindrical roller bearings for radial load whose outer ring is held at a predetermined certain position, and/or angular ball bearings for forward thrust load and/or reverse thrust load.

The first end portion of one of the rotor shafts 112 (shown in Figure 1) is coupled for integral rotation to a motor shaft 130 of the motor 104. The motor shaft 130 may be considered to be an output shaft or drive shaft of the motor 104.

The rotor shaft 112 and the motor shaft 130 are coupled together by a coupling assembly 132. The coupling assembly 132 is described in more detail later below with reference to Figure 2.

The motor shaft 130 may be rotatably supported, e.g. at the end proximate to to the vacuum pump 102 by one or more third bearings. The third bearings may include, for example, cylindrical roller bearings for radial load whose outer ring is held at a predetermined certain position, and/or angular ball bearings for forward thrust load and/or reverse thrust load.

In operation, the screw rotors 110 are rotated by the motor 104. More specifically, operation of the motor 104 causes the motor shaft 130 to be rotated about its axis, thereby rotating the rotor shaft 112 and corresponding screw rotor 110 attached thereto by the coupling assembly 132. Rotational drive force is transferred between the male and female rotors 110, for example, by a synchronous gear or by direct contact between the screw rotors 110. Rotation of the screw rotors 110 by the motor 104 causes gas to be drawn into the gas inlet 114 and through the suction port 118 into the pumping chamber 108 in which the screw rotors 110 are rotating. Continued rotation of the screw rotors 110 moves the gas through the pumping chamber 108, from the suction port 118 to the discharge port 120. The gas is subsequently forced out of the discharge port 120, and out of the vacuum pump 102 via the gas outlet 116.

In this embodiment, the vacuum pump 102 is an oil-sealed pump. The vacuum pump 102 further comprises oil injection means (not shown) configured to inject an oil into the housing 106. The oil may provide sealing of the screw rotors 110 against the housing 106 and/or each other. The oil may cool the screw rotors 110 and/or the pumped gas. The oil may further provide cooling and/or lubrication to the bearings 124, 128. Oil may be fed through an oil flow path (not shown) to each of the bearings 124, 128 within the suction-side and discharge side bearing casings 122, 126.

Figure 2 is a schematic illustration (not to scale) showing a side view cross section of coupling assembly 132.

The coupling assembly 132 is an assembly in which an end of the motor shaft 130 is fixedly attached to an end of a rotor shaft 112.

The coupling assembly 132 comprises a shaft assembly comprising the motor shaft 130, a coupling element 200 fixedly attached to the end 202 of the motor shaft 130, and an electrically insulative element 204 coupled between the motor shaft 130 and the coupling element 200.

In this embodiment, the motor shaft 130 comprises a flange 206 at the end 202 of the motor shaft 130. The flange 206 has a first side 208 and a second side 210 opposite to the first side 208.

The electrically insulative element 204 is disposed against, i.e. abuts, the second side 210 of the flange 206. The electrically insulative element 204 may be a disc or annular-shaped element. The electrically insulative element 204 may have substantially the same diameter as the flange 206 and may be axially aligned with the motor shaft 130. The electrically insulative element 204 may be formed from any appropriate electrically insulative material, including, but not limited to, a ceramic material, aluminium oxide, and/or zirconium oxide. The electrically insulative element 204 may be embodied as a coating disposed on one or both of the second side 210 of the flange 206 and/or the side of the coupling element 200 facing the flange 206.

The coupling element 200 is disposed against, i.e. abuts, the electrically insulative element 204 at the opposite side of the electrically insulative element 204 to the flange 206. Thus, in this embodiment, the electrically insulative element 204 is sandwiched between the coupling element 200 and the flange 206 of the motor shaft 130. The electrically insulative element 204 spaces apart the coupling element 200 and the motor shaft 130. The electrically insulative element 204 substantially electrically isolates the motor shaft 130 from the coupling element 200. The thickness of the electrically insulative element 204 -s -between the surface that abuts the flange 206 and the surface that abuts the coupling element 200 may be application dependent. Preferably, the thickness of the insulative element 204 is between about 1 mm and 5 mm. More preferably, the thickness of the insulative element 204 is between about 1.2 mm and 1.8 mm.

More preferably, this thickness is about 1.5 mm. Having such a thickness tends to mean that the insulative element 204 is robust enough to reduce the likelihood of it being damaged during handling, installation, and use, while at the same time being relatively easy to manufacture.

In this embodiment, the coupling element 200 is an annular or tubular collar. The coupling element 200 is fixedly attached to the flange 206 by a plurality of fasteners 212. The fasteners 212 may be threaded fasteners.

In this embodiment, the fasteners 212 are disposed in respective through bores 214 through the flange 206, from the first side 208 of the flange 206 to the second side 210 of the flange 206. The fasteners 212 are further disposed through the electrically insulative element 204, and extend into the coupling element 200, thereby to fixedly attach together the motor shaft 130, the electrically insulative element 204, and the coupling element 200. The fasteners 212 may be threadedly engaged with the coupling element 200.

Fastener heads 216 of the fasteners 212 are located at the first side 208 20 of the flange 206. The fastener heads 216 are located within respective recesses or countersunk portions 218 at the first side 208 of the flange 206.

In this embodiment, the fasteners 212 are substantially electrically isolated from the motor shaft 130. In particular, in this embodiment. a respective electrically insulative washer 220 is disposed between each of the fastener heads 216 and the flange 206 (specifically, between each of the fastener heads 216 and the base of the respective countersunk portion 218 at the first side 208 of the flange 206). Also in this embodiment, the walls of the through bores 214 are spaced apart from the portions of the fasteners 212 positioned therethrough. Thus, air gaps exist between the walls of the through bores 214 and the portions of the fasteners 212 positioned therethrough. -9 -

The electrically insulative washers 220 may be formed from any appropriate electrically insulative material, including, but not limited to, a ceramic material, aluminium oxide, and/or zirconium oxide.

In this embodiment, a respective further washer 222 is disposed between each of the fastener heads 216 and the flange 206. The further wasters 222 are sandwiched between the fastener heads 216 and electrically insulative washer 220. Alternatively, the further fasteners 222 may be between the flange 206 and the electrically insulative washers 220 as long as electrical isolation between the fasteners 212 and the flange 206 is maintained. The further washers 222 may be formed from any appropriate material, such as metal, e.g. steel. Advantageously, use of the further washers 222 tends to more evenly distribute the loads applied to the electrically insulative washers 220. Thus, the likelihood of damage to or breakage of the electrically insulative washers 220, which may be relatively brittle in some embodiments, tends to be reduced or eliminated.

In this embodiment, the coupling element 200 is an annular or tubular collar configured to fit around the end of the rotor shaft 112. The coupling element 200 circumscribes a central axis and defines an axial bore 224. The end 226 of the rotor shaft 112 is received in the axial bore 224. The surface of the axial bore 224 has a mating surface formation defined thereon (e.g., a keyed surface, or a surface comprising one or more lugs or grooves) for engaging a compatible mating surface formation defined on the surface of the end 226 of the rotor shaft 112. The engaged compatible mating surfaces of the axial bore 224 of the coupling element 200 and the end 226 of the rotor shaft 112 prevent or oppose relative circumferential movement of the coupling element 200 and the rotor shaft 112, but may permit relative axial movement.

Thus, the coupling element 200 attaches together the motor shaft 130 and the rotor shaft 112. Advantageously, this attachment is in such a way that the motor shaft and the rotor shaft are electrically isolated from one another. This electrical isolation of the rotor shafts and the motor shaft tends to reduce or eliminate the flow of electrical current between and through the shafts.

Accordingly, electrical currents through the bearings that rotatably support the -10 -rotor shaft tend to be reduced or eliminated. This advantageously tends to reduce damage to or wear on the bearings.

Advantageously, the above-described electrically insulative coupling between the motor shaft and the rotor shaft tends to be robust to fatigue and 5 breakage, for example caused by corrosive and/or high temperature gases being pumped by the vacuum pump.

Advantageously, the above-described apparatus tends to be more compact compared to apparatuses which use a flexible coupling between motor and rotor shafts. Use of supplementary bearings tends to be avoided.

In the above embodiments, the vacuum pump is an oil-sealed rotary screw vacuum pump having VSD. The vacuum pump is a single stage pump. However, in other embodiments, the vacuum pump is a different type of apparatus, such as a compressor or an expander, or a different type of vacuum pump such as a dry vacuum pump, a positive-displacement pump, a Roots-type vacuum pump, or a multistage pump.

In the above embodiments, the motor is an AC electric motor. However, in other embodiments, the motor is a different type of motor.

In the above embodiments, the motor is controlled by a controller comprising a VFD. However, in other embodiments, the motor is controlled in a different way.

In the above embodiments, the vacuum pump comprises a pair of intermeshed rotors. However, in other embodiments, the vacuum pump comprises a pumping mechanism having a different number of rotors.

In the above embodiments, the fastener heads are countersunk in the first flange. However, in other embodiments, one or more of the fastener heads is not countersunk in the first flange.

In the above embodiments, the fasteners are substantially electrically isolated from the motor shaft by respective electrically insulative washers and by spacing apart the walls of the through bores from the portions of the fasteners positioned therethrough. However, in other embodiments, the fasteners are substantially electrically isolated from the motor shaft in a different way.

For example, in some embodiments, an annular electrically insulative element is positioned around the first shaft and abuts with the flange at the first side of the flange. The fasteners are arranged to pass through the annular electrically insulative element, through the flange, through the electrically insulative element, and into the coupling element. The annular electrically insulative element is disposed between the heads of the fasteners and the first side of the flange, thereby to space apart the heads of the fasteners and the first side of the flange. The annular electrically insulative element may be considered to provide the same electrically isolating functionality as the electrically insulative washers in the above embodiments, and the individual electrically insulative washers may be omitted.

Figure 3 is a schematic illustration showing such an embodiment of the coupling assembly 132. Elements common to this embodiment and the embodiment of Figure 2 are indicated by like reference numerals.

In this embodiment, the fastener heads 216 are not countersunk in the first flange 206.

In this embodiment, coupling assembly 132 comprises a further electrically insulative element 300. The further electrically insulative element 300 is disposed against, i.e. abuts, the first side 208 of the flange 206. The further electrically insulative element 300 may be a disc or annular-shaped element. The further electrically insulative element 300 may have substantially the same diameter as the flange 206 and may be axially aligned with the motor shaft 130. The further electrically insulative element 300 may be formed from any appropriate electrically insulative material, including, but not limited to, a ceramic material, aluminium oxide, and/or zirconium oxide. The further electrically insulative element 300 may be embodied as a coating disposed on the first side 208 of the flange 206. In some embodiments, the further electrically insulative element 300 is substantially the same as the electrically insulative element 204.

-12 -In this embodiment, coupling assembly 132 comprises a disc 302. The disc is disposed against, i.e. abuts, the further electrically insulative element 300 such that the further electrically insulative element 300 is sandwiched between the flange 206 and the further electrically insulative element 300. The disc 302 is spaced apart from and electrically isolated from the motor shaft 130. Alternatively, the disc 302 may be between the flange 206 and the further electrically insulative element 300 as long as electrical isolation between the fasteners 212 and the flange 206 is maintained. The disc 302 may be formed from any appropriate material, such as metal, e.g. steel. Advantageously, use of the disc 302 tends to more evenly distribute the loads applied to the further electrically insulative element 300, e.g. by the fasteners 212. Thus, the likelihood of damage to or breakage of the further electrically insulative element 300, which may be relatively brittle in some embodiments, tends to be reduced or eliminated. The disc 302 may be considered to serve a similar purpose to the further washers 222 of the previous embodiment.

The fasteners 212 pass through, and attach together the disc 302, the further electrically insulative element 300, the flange 206, the electrically insulative element 204, and the coupling element 200.

The electrically insulative element 204 and the further electrically insulative element 300 substantially electrically isolates the motor shaft 130 from the coupling element 200.

The thickness of the further electrically insulative element 300 between the surface that abuts the flange 206 and the surface that abuts the disc 302 may be application dependent. Preferably, the thickness of the further insulative element 300 is between about 1 mm and 5 mm. More preferably, the thickness of the further insulative element 300 is between about 1.2 mm and 1.8 mm. More preferably, this thickness is about 1.5 mm. Having such a thickness tends to mean that the further insulative element 300 is robust enough to reduce the likelihood of it being damaged during handling, installation, and use, while at the same time being relatively easy to manufacture.

-13 -Optionally, further washers (e.g. metal washers) may be implemented to distribute loads on the further electrically insulative element 300 and/or the disc 302, thereby reducing the likelihood of damage thereto.

In some embodiments, the through bores through the flange, in which the fasteners are located, may be coated with an electrically resistive or insulative material thereby to prevent or oppose the flow of electrical current between the fasteners and the flange. The electrically resistive or insulative material may coat the recessed or countersunk portions of the flange in embodiments in which they are present.

In the above embodiment, the rotor shaft and the motor shaft are coupled together by an annular or tubular coupling element that is fastened to a flange on the motor shaft, and is fitted around and securely receives the rotor shaft. However, in other embodiments, the motor shaft and the rotor shaft are coupled together in a different way that electrically isolates them from one another.

By way of example, in some embodiments, the rotor shaft comprises a flange at its end. The annular or tubular coupling element is fastened to the flange of the rotor shaft in the same way as that described above for the motor shaft. The coupling element fits around and securely receives the motor shaft in the same way as that described above for the rotor shaft.

As another example, in some embodiments, both the rotor shaft and the motor shaft comprise respective flanges at their respective ends. The flanges of the rotor shaft and the motor shaft may be coupled together in such a way that they are electrically isolated from each other. For example, an electrically insulative element may be disposed between the two flanges thereby to space apart the flanges. A plurality of fasteners may be disposed through both flanges and the electrically insulative element therebetween. The fasteners may be electrically isolated from one or both of the flanges, e.g. as described above for the motor flange. That is to say, the fasteners may be electrically isolated from one or both of the flanges by electrically insulative washers (disposed between fastener heads and a flange, and/or between fastener nuts and a flange) and air gaps between the fasteners and the walls of the through bores. As another -14 -example, the flanges of the rotor shaft and the motor shaft may be coupled together by one or more clamps that contact the flanges via electrically insulative material.

In the above embodiments, the fastener heads are located at the opposite side of the rotor shaft flange to the coupling element. However, in other embodiments, the fastener heads are located at the same side of the rotor shaft flange as the coupling element, and the fastener bodies pass into and connect to the flange.

REFERENCE NUMERAL LIST 100 -vacuum pumping apparatus 102 -vacuum pump 104-motor 106 -housing 108-pumping chamber -screw rotors 112-rotor shafts 114 -gas inlet 116 -gas outlet 118 -suction port 120-discharge port 122-suction-side bearing casing 124 -first bearings 126-discharge-side bearing casing 128-second bearings 130-motor shaft 132-coupling assembly -coupling element 202 -end of the rotor shaft 204 -electrically insulative element 206 -flange 208 -first side 210-second side 212 -fasteners -15 - -16 - 214 -through bores 216 -fastener heads 218-recesses or countersunk portions 220 -electrically insulative washer 222 -further washer 224 -axial bore 226 -end of the rotor shaft 300 -further electrically insulative element 302 -disc

Claims

-17 -CLAIMS1. A shaft assembly comprising: a first shaft having a first shaft end; a coupling element fixedly attached to the first shaft end, the coupling element being configured to fixedly attach to a second shaft end of a second shaft; and an electrically insulative element coupled between the first shaft and the coupling element thereby to electrically isolate the first shaft from the coupling element.
2. The shaft assembly of claim 1, further comprising the second shaft having the second shaft end, wherein the coupling element is attached to the second shaft end of the second shaft.
3. The shaft assembly of any preceding claim, wherein the first shaft is a shaft selected from the group of shafts consisting of a rotor shaft of a vacuum pump, a compressor, or an expander, and a motor shaft of a motor for driving a rotor shaft of a vacuum pump, a compressor, or an expander.
4. The shaft assembly of any preceding claim, wherein the coupling element is an annular or tubular collar configured to fit around the second shaft end of the second shaft.
5. The shaft assembly of any preceding claim, wherein the coupling element is fixedly attached to the first shaft end of the first shaft by one or more fasteners, the one or more fasteners being electrically isolated from one or both of the first shaft and the coupling element.
-18 - 6. The shaft assembly of any preceding claim, wherein the first shaft comprises a flange located at the first shaft end, and the coupling element is attached to the flange.
7. The shaft assembly of claim 6, further comprising one or more fasteners positioned through the flange from a first side of the flange to a second side of the flange opposite to the first side of the flange, the one or more fasteners extending into the coupling element, the coupling element being located at the second side of the flange.
8. The shaft assembly of claim 7, further comprising one or more further electrically insulative elements disposed between the one or more fasteners and the flange thereby to electrically isolate the one or more fasteners from the first shaft.
9. The shaft assembly of claim 8, wherein the one or more further electrically insulative elements comprises an annular electrically insulative element positioned around the first shaft, the annular electrically insulative element being located at the first side of the flange.
10. The shaft assembly of claim 9, further comprising an annular metal disc disposed against the annular electrically insulative element.
11. The shaft assembly of claim 8, wherein: the one or more further electrically insulative elements comprises one or more electrically insulative washers, each electrically insulative washer being disposed between the first side of the flange and a part of a respective one of the one or more fasteners; and -19 -optionally, the shaft assembly further comprises one or more metal washers, each metal washer being in contact with a respective one of the one or more electrically insulative washers and disposed between the first side of the flange and a head of a respective one of the one or more fasteners.
12. A vacuum pump, compressor, or expander assembly comprising: a vacuum pump, a compressor, or an expander having a rotor disposed on a rotor shaft rotatably arranged in a housing, a motor having a motor shaft; wherein the motor shaft is coupled between the rotor shaft and the motor; the motor is configured to rotate the motor shaft thereby to drive the rotor shaft; and the motor shaft and the rotor shaft are electrically isolated from one another.
13. The assembly of claim 12, further comprising: a coupling element fixedly attached to an end of the motor shaft and an end of the rotor shaft, thereby to fixedly attach together the motor shaft and the rotor shaft; and an electrically insulative element disposed between either the motor shaft and the coupling element or the rotor shaft and the coupling element.
14. The assembly of claim 13, wherein: the motor shaft comprises a flange located at the end of the motor shaft; the assembly further comprises: one or more fasteners positioned through the flange from a first side of the flange to a second side of the flange opposite to the first side of the -20 -flange, the one or more fasteners extending into the coupling element, the coupling element being located at the second side of the flange; and one or more further electrically insulative elements disposed between the one or more fasteners and the flange; the electrically insulative element is disposed between the motor shaft and the coupling element; and the coupling element is an annular or tubular collar in which is received the end of the rotor shaft.to
15. The assembly of any of claims 12 to 14, wherein the vacuum pump is an oil-sealed rotary screw vacuum pump.