Patent Description:
Rotating machines such as pumps are often designed with tight clearance gaps between the stationary and moving parts to improve performance. In particular, the pumping performance of a pump is related to the leakage of fluids between the rotor and stator during pumping and thus, clearance gaps are set to be small to improve performance. A problem associated with small clearances arises due to differential thermal expansion of the different components of the pump. This may be due to different components heating up at different speeds and by different amounts during operation of the pump and/or to different materials with different coefficients of thermal expansion being used for the different components.

This differential expansion can lead to clashing of the moving and stationary parts where the clearance gaps are smaller than the differences in thermal expansion. If clearances are eroded or reduced the rotating and static parts can touch or clash. If soft materials are used for one of the components, for example aluminium, it can be prone to gouging and generate burrs which may travel through the pump and damage other parts. Aluminium parts in a pump are also prone to seizing where clashing between the rotor and stator occur.

The publications <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> constitute prior art dealing with pumps (some of them multi stage root pumps), the material they are made from and the clearances between rotor and stator.

It would be desirable to be able to provide a pump where the clearance gaps are small, but the consequences of the rotor and stator clashing are mitigated.

A first aspect provides a pump according to claim <NUM>.

The inventors of the present invention recognised that although it may be advantageous to form the stator and rotor from different materials, where the stator is formed of a material that is softer than the material of the rotor this renders the stator prone both to being damaged and on occasion to seizing via galling of the material when clashes occur between the rotor and the stator.

The rotor and stator of a pump are often formed of different materials to mitigate the effects of differential expansion due to differential heating during operation. In this regard the rotor will generally heat up more than the stator during operation and thus, forming the rotor with a material of a lower coefficient of thermal expansion may help reduce differential expansion. However, forming the rotor of a different material may also affect the damage that may occur were clashing to happen as one of the materials may be harder than the other.

The inventors of the present invention have addressed these issues by treating at least a portion of the surface of the stator to harden it and configuring the pump so that the treated portion is a portion where clashing is most likely to occur. In this regard, the rotor and stator are configured with clearances and the clearances are selected such that clashing occurs preferentially on differential thermal expansion of the parts at certain places. Ensuring that the stator is treated at these places will help inhibit potential seizing of the rotor to the stator where there are clashes and will also inhibit burrs being gouged from the surface of the stator when it is contacted by the rotor.

In some embodiments, said stator and rotor are configured such that a lowest clearance gap is provided between said rotor and said at least a portion of said surface of said stator that has been treated.

As noted previously, rotors and stators are formed with clearance gaps between them such that the rotor can rotate during normal operation without contacting the stator but with relatively tight clearances to provide effective pumping. In order to ensure that any clashing that does occur occurs between the rotor and an anodised portion of the stator, the rotor and stator are configured so that the clearance gaps differ across the pump and are lowest at the part where the stator is anodised.

In some embodiments, said stator and rotor are configured such that a lowest axial clearance gap in both axial directions is provided between said rotor and portions of said stator where said surface of said stator has been treated.

Generally, axial expansion along the rotor between the bearing supports is significantly greater than radial expansion and thus, axial clashing is more likely to occur. A pump may be designed so that axial clearances are equally likely in either direction, and where this is the case the stator and rotor may be configured such that a lowest axial clearance gap in both of the axial directions is provided between the rotor and portions of the stator that are anodised.

Although, radial clashing is less likely to occur, it may be advantageous in some embodiments for the stator and rotor to be configured such that a lowest radial clearance gap is provided between the rotor and an anodised portion of the stator. In this case, even in the unlikely event that there is a radial clash the pump is has some protection from seizing and some protection from being damaged by being gouged by the rotor.

In some embodiments, said softer material comprises aluminium.

Aluminium has many properties that make it a suitable material for forming a stator. However, it is a relatively soft material and thus, there are likely to be particles of aluminium or burrs scraped from the surface of the stator where it to clash with a rotor of a harder material, and this may cause problems with the operation of the pump. Clashing may also lead to the pump seizing due to galling of the material. Treating a surface of the aluminium by for example, anodising it at portions where it is deemed clashes are likely to occur can mitigate these problems.

In some embodiments, the harder material of the rotor may be iron.

Iron has a lower thermal coefficient of expansion than aluminium making it a suitable material for a rotor which generally heats up more than the stator. Furthermore, although it is harder than aluminium and may gouge an aluminium surface, where it clashes with an anodised aluminium surface it is the iron that will be preferentially damaged and the iron dust created by such a clash is not as damaging to the pump as aluminium burrs are, the iron dust generally being able to exit the pump with the fluid being pumped.

Although the surface of the softer material may be treated in a number of ways in some embodiments, the surface of said softer component is treated by one of anodising said surface or coating said surface with a harder material.

In some embodiments, a head plate portion of said stator is formed of aluminium that has not been treated to harden said surface.

Although there are advantages of treating much of the stator to provide a surface that is both tougher and in some cases has a lower coefficient of friction, the treated surface may have other properties that are less advantageous. Anodised aluminium for example, has a rougher surface than non-anodised aluminium making it more difficult to seal against. Thus, it may be advantageous if the head plates of the pump and in particular the low vacuum head plate is formed of a material that has not been treated.

In some embodiments, the pump comprises a multiple stage roots-type pump.

Embodiments are particularly applicable to a multiple stage roots-type pump with an aluminium stator and in some embodiments with a rotor formed of iron.

In some embodiments, said stator and said rotor are configured such that axial clearance gaps are selected to be lowest between said rotor and a higher vacuum side of one of said vacuum stages of said stator and between said rotor and a lower vacuum side of another one of said vacuum stages of said stator, such that clashes preferentially occur between the rotor and these selected sides of these stages.

In order to cover axial expansion in either direction and to ensure it occurs on an anodised portion of the stator, in some embodiments, the higher vacuum side wall and the lower vacuum side wall of one or more stages have been selected as the clash sites of preference.

In some embodiments, said stator and said rotor are configured such that axial clearance gaps are set to be lowest between said rotor and a higher vacuum side of said ultimate low vacuum stage of said stator and between said rotor and a lower vacuum side of a penultimate stator low vacuum stage of said stator.

Generally in a multiple stage roots-type pump the tightest tolerances occur in the lowest vacuum stage adjacent to the exhaust and the low vacuum head plate. A potential problem with this conventional design occurs where a head plate is formed of non-anodised aluminium while other portions of the stator have been anodised to protect them were there to be any clash between the rotor and the stator. In order to address this potential problem, embodiments of the pump have been configured such that the tightest axial clearances are remote from the head plate. Thus, rather than being at either side of the lowest vacuum stage of the pump as is conventional, they are rather on the high vacuum side of the lowest vacuum stage and on the low vacuum side of the penultimate lowest vacuum stage of the pump. In this way, if a clash does occur due to differential axial thermal expansion it will not occur between the rotor and the low vacuum head plate but rather between the rotor and the treated wall of the stator.

In some embodiments, said stator comprises a portion having a clam shell configuration and being formed in two parts.

In some embodiments, said stator bore surface of said clam shell portion is treated to harden said surface.

A stator bore may be formed in two parts to allow ease of assembly, two parts having a clam shell type configuration. In such a case, it may be convenient to treat the clam shell portion of the stator particularly the surfaces within the stator bore that the rotor may contact.

In some cases, a whole outer surface of the clam shell portion of the stator is treated.

The treating process may require a component to be placed in a bath, as is the case for anodising for example, and thus, it is often simpler to treat a whole outer surface of a particular component rather than treating selected portions of it. Thus, in some cases the whole outer surface may be treated.

In other embodiments, said clam shell portion comprises at least one groove configured to accommodate at least one seal, said at least one groove not being treated.

There are however, advantages in not treating the whole of the stator surface in that the non-treated surface may provide a better surface for sealing to. Thus, in some embodiments there may be grooves within the clam shell portion of the stator that are configured to receive seals and it is advantageous if these grooves are not treated. This may be achieved by masking them during the treatment process or by removing the treated surface within the groove after the treatment process.

In some embodiments, the claim shell portion comprises assembly holes for receiving pins during assembly of said pump, said assembly holes not being treated.

A further portion of a stator that it may be preferable not to treat is where it has assembly holes for aligning the different parts of the stator and head plates. It is very important where clearances are tight that these are accurately aligned and thus, it is preferable if these surfaces are not treated but that the original surface is retained. Masking holes during treatment is not a complicated process.

A second aspect provides a method of manufacture of a pump, said method comprising: manufacturing a stator in a softer material than a material of a rotor; manufacturing said rotor configured to rotate within said stator in a harder material than said stator; treating at least a portion of a surface of said stator to harden said surface; and assembling said pump by mounting said rotor within said stator; wherein said steps of manufacturing said stator and said rotor are performed such that said rotor and stator are configured so that clashing will preferentially occur between said rotor and said at least a portion of said stator where the surface has been treated.

In some embodiments, said steps of manufacturing said stator and rotor are performed such that said rotor and said stator are configured to provide a lowest clearance gap between said rotor and a treated portion of said stator; and a head plate portion of said stator is not treated.

Features of the dependent claims may be combined with features of the independent claims as appropriate as long as consistent with the appended claims.

Before discussing the embodiments in any more detail, first an overview will be provided.

Embodiments provide a pump where at least a portion of the surface of the stator has been treated to make the surface harder and less prone to damage in the event of a clash with the rotor. In some embodiments the treatment of the surface also reduces its frictional coefficient making it less likely to seize if a clash occurs with the rotor. The stator is formed of a softer material than the rotor. In some embodiments the stator is formed of aluminium and the rotor is formed of iron. In embodiments, the material selected for the rotor is one which is both harder than the material for the stator and one which has a lower coefficient of thermal expansion. In other embodiments, the rotor may have a larger coefficient of thermal expansion.

In order to protect the stator from being damaged by the harder rotating rotor and to inhibit the possibility of the rotor seizing within the stator were a clash to occur, portions of the stator surface where it is determined that clashes are most likely to occur are treated to harden them. In some embodiments, this treatment involves anodising the material, while in other embodiments, the material may be plated or coated with a harder metal such as nickel. In some embodiments the whole of the stator bore portion of the stator may be treated and the head plates may not be treated and in other embodiments particular portions of the stator bore portion may be treated and others not.

It may be advantageous for certain portions of the stator which are configured to accommodate seals not to be treated. In some embodiments, portions of the stator which are used for aligning the stator with other portions of the pump such as other parts of the stator and head plate are also not treated.

In embodiments the configuration of the rotor and stator is such that the lowest clearances are provided in predetermined positions such that clashing does not occur at the head plates which are formed of non-treated material.

<FIG> shows a clam shell half of a stator <NUM> according to an embodiment. Clam shell half comprises a plurality of interstage walls <NUM> between the different stages in the multiple stage stator. The stator <NUM> also comprises grooves <NUM> for supporting seals and alignment holes <NUM> for receiving alignment pins. The interstage wall between the lowest vacuum stage and the next lowest vacuum stage is configured such that the tightest axial clearances are between the side of the wall facing the head plate and the other side of the wall facing the other stage. This means that were the rotor to move towards the high vacuum end of the pump it will preferentially clash with one side of wall 12a whereas if it were to move towards the lower vacuum end of the pump it will preferentially clash with the other side of this wall 12a and not with the aluminium head plate.

In this embodiment the stator is formed of aluminium and the claim shell portion is anodised while the aluminium head plate is preferably not anodised to improve sealing. Thus, a rotor contacting the head plate may gouge aluminium from the surface of the head plate and create burrs that will travel through the pump. In order to reduce the probability of this occurring the stator is designed so that clashes preferentially occur at other portions that have been anodised.

In this embodiment, substantially the whole of the clam shell stator has been anodised. The portions that have not been anodised are the grooves <NUM> and the alignment holes <NUM>. There may also be alignment holes in the end of this stator portion that are not shown for alignment with the head plate and these too may not be anodised.

<FIG> shows a section through a multiple stage roots-type pump when it is assembled. This pump shows the two halves of the stator <NUM> fixed together and with a rotor <NUM> rotatably mounted within these two halves. The non-anodised vacuum head plate <NUM> is also shown and arrows <NUM> and <NUM> indicate where tolerances have been changed compared to a conventional pump such that a clash that might have occurred on the aluminium head plate <NUM> at arrow <NUM> will now occur on the low vacuum side of wall 12a at arrow <NUM>.

<FIG> shows a flow diagram illustrating steps in a method according to an embodiment.

<FIG> shows a flow diagram illustrating steps in a method of manufacturing a pump according to an embodiment. In step S10 components of a multiple stage stator are manufactured from aluminium and a corresponding multiple stage rotor is manufactured from iron. The stator and rotor are configured such that the clearance gaps between them are smallest at the inter-stage wall between the lowest vacuum stage and the next lowest vacuum stage. In step S20, the components of the stator comprising the stator bore are anodised. In step S30 the pump is assembled by mounting the rotor within the stator.

Claim 1:
A multiple stage roots-type vacuum pump comprising:
a rotor (<NUM>) rotatably mounted within a stator;
said stator (<NUM>) being formed of a softer material than a material that said rotor (<NUM>) is formed from; characterised in that
said stator (<NUM>) is configured such that at least a portion of a surface of said stator has been treated to harden said surface, said rotor and stator being configured such that clashing will preferentially occur between said rotor and said at least a portion of said surface of said stator that has been treated; and
said stator and said rotor are configured such that axial clearance gaps (<NUM>,<NUM>) are selected to be lowest between said rotor and a higher vacuum side of one of said vacuum stages of said stator and between said rotor and a lower vacuum side of another one of said vacuum stages of said stator, such that clashes preferentially occur between the rotor and these selected sides of these stages.