Patent Description:
Vacuum pumps are known. These pumps are typically employed as a component of a vacuum system to evacuate devices. Also, these pumps are used to evacuate fabrication equipment used in, for example, the production of semi-conductors. Rather than performing compression from a vacuum to atmosphere in a single stage using a single pump, it is known to provide multi-stage vacuum pumps where each stage performs a portion of the complete compression range required to transition from a vacuum to atmospheric pressure. Such pumps are described in <CIT>, <CIT> and <CIT>.

Although such multi-stage vacuum pumps provide advantages, they also have their own shortcomings. Accordingly, it is desired to provide an improved arrangement for multi-stage vacuum pumps.

According to a first aspect, the present invention provides a rotor for a multi-stage roots-type vacuum pump, comprising: a plurality of rotary vanes, the plurality of rotary vanes being axially displaced and coaxially aligned; a pair of end shafts, each end shaft extending from opposing axial ends of the plurality of rotary vanes; and an inter-vane shaft extending between adjacent rotary vanes of the plurality of rotary vanes, the inter-vane shaft having a diameter which is greater than that of the end shafts. The inter-vane shaft comprises a collar fitted onto an internal shaft extending between the adjacent rotary vanes, and the collar comprises separable portions. The internal shaft and the adjacent rotary vanes are also unitary.

Accordingly, the increase in diameter of the inter-vane shaft is achieved using the collar which is fitted onto an internal shaft which extends between the adjacent rotary vanes, and the internal shaft and the rotary vanes are made from a single, unitary member, rather than being made from different, attachable, component parts. Providing the separable or split collar made of portions that may be disconnected or decoupled makes fitting the collar on to the internal shaft easier.

The first aspect recognises that when providing a plurality of rotary vanes arranged on a common shaft, the diameter of the shaft extending between adjacent rotary vanes may cause the modal frequency of the rotor to be close enough to the operating frequency of the rotor to cause difficulties. As mentioned above, each of the rotary vanes share a common axis and share a common shaft. The vanes are axially displaced or separated and coaxially or concentrically-aligned. The rotor is provided with a pair of end shafts. The end shafts extend or protrude from opposing or distal axial ends of the plurality of rotary vanes. An inter-vane shaft is provided which extends between or couples adjacent rotary vanes. The inter-vane shaft is configured with a diameter which is greater than that of the end shafts. In this way, the inter-vane shaft provided between each rotary vane has an increased diameter, which improves the stiffness of the shaft and changes the modal frequency of the rotor. Such a change in the modal frequency is typically sufficient to improve its operation.

In one embodiment, the rotary vanes have epicycloid portions and a central hypocycloid portion defined by surrounding hypocycloidic faces and the inter-vane shaft has a diameter which exceeds a distance of closest approach of the surrounding hypocycloidic faces. Accordingly, in a roots-type rotor, there are provided epicycloid portions (which define the radial lobes of the rotor) together with a central hypocycloid portion (which defines the radially-inner part of the rotor). The inter-vane shaft may be dimensioned to have a diameter which is greater than that of the central hypocycloid portion, which helps to stiffen the rotor and change the modal frequency of the rotor.

In one embodiment, the rotary vanes have a pair of epicycloid portions and a central hypocycloid portion defined by opposing hypocycloidic faces and the inter-vane shaft has a diameter which exceeds a distance of closest approach of the opposing hypocycloidic faces.

In one embodiment, the collar comprises a releaseably fixable pair of hemi-cylinders. The hemi-cylinders together make a cylinder of the required diameter. According to a second aspect, the present invention provides a multi-stage vacuum pump, comprising: a first stage pump; a second stage pump; and a rotor according to the first aspect extending within both the first stage pump and the second stage pump.

According to a third aspect, the present invention provides a method, comprising: providing a plurality of rotary vanes of a rotor for a multi-stage roots-type vacuum pump, the plurality of rotary vanes being axially displaced and coaxially aligned; providing a pair of end shafts, each end shaft extending from opposing axial ends of the plurality of rotary vanes; providing an inter-vane shaft extending between adjacent rotary vanes of the plurality of rotary vanes, the inter-vane shaft having a diameter which is greater than that of the end shafts; and fitting a collar fitted onto an internal shaft extending between the adjacent rotary vanes to form the inter-vane shaft. The collar comprises separable portions and the internal shaft and the adjacent rotary vanes are unitary.

Before discussing the embodiments in any more detail, first an overview will be provided. Embodiments provide an arrangement for a multi-stage roots-type vacuum pump. In such a vacuum pump, a rotor is provided with multiple rotary vanes, each sharing a common rotor shaft. Those rotary vanes are typically axially separated along the common shaft by an inter-vane shaft. The inter-vane shaft extending between the different rotary vanes typically undergoes high levels of stress during rotation of the rotor. The bending mode frequency of the rotor can be close to the operating frequency of the rotor, which leads to unacceptable mechanical deflection of the rotor during operation. Accordingly, embodiments provide arrangements which enlarge the diameter of the inter-vane shaft in order to modify the natural frequency of the rotor away from its operating frequency.

In accordance with the present invention, a collar is fixed on to the inter-vane shaft extending between the rotary vanes, whilst in arrangetments outside of the scope of the clams, shims or inserts are added to the inter-vane shaft, which has been machined to be indented or faceted during manufacture of the rotor, in order to restore that indented or faceted shaft back to its previous cylindrical form.

<FIG> illustrate a two-stage booster pump, generally <NUM>, according to one embodiment. A first pumping stage <NUM> is coupled with a second pumping stage <NUM> via an inter-stage coupling unit <NUM>. The first pumping stage <NUM> has a first stage inlet 20A and a first stage exhaust 20B. The second pumping stage <NUM> has a second stage inlet 30A and a second stage exhaust 30B.

The inter-stage coupling <NUM> is formed from a first portion 40A and a second portion 40B. The first portion 40A is releasably fixable to the second portion 40B. When brought together, the first and second portions 40A, 40B define a gallery <NUM> within the interstage coupling unit through which gas may pass during operation of the pump. The inter-stage coupling unit <NUM> defines a cylindrical void <NUM> which extends through the width of the inter-stage coupling unit <NUM>. The first portion 40A forms a first portion of the void <NUM> and the second portion 40B forms a second portion of the void <NUM>. The void <NUM> separates to receive a one piece rotor <NUM>, as will now be described in more detail.

<FIG> is a perspective view of the rotor <NUM>. The rotor <NUM> is a rotor of the type used in a positive displacement lobe pump which utilises meshing pairs of lobes. Each rotor has a pair of lobes formed symmetrically about a rotatable shaft. Each lobe <NUM> is defined by alternating tangential sections of curves. The curves can be of any suitable form such as circular arcs, or hypocycloidal and epicycloidal curves, or a combination of these, as is known. In this example, the rotor <NUM> is unitary, machined from a single metal element and cylindrical voids <NUM> extend axially through the lobes <NUM> to reduce mass.

A first axial end <NUM> of the shaft is received within a bearing provided by a head plate (not shown) of the first pumping stage <NUM> and extends from a first rotary vane portion 90A which is received within a stator of the first stage <NUM>. An intermediate axial portion <NUM> extends from the first rotary vane portion 90A and is received within the void <NUM>. The void <NUM> provides a close fit on the surface of the intermediate axial portion <NUM>, but does not act as a bearing. A second rotary vane portion 90B extends axially from the intermediate axial portion <NUM> and is received within a stator of the second stage <NUM>. A second axial end <NUM> extends axially from the second rotary vane portion 90B. The second axial end <NUM> is received by a bearing in a head plate (not shown) of the second pumping stage <NUM>. The rotor <NUM> is machined as a single part, with cutters forming the surface of the pair of lobes <NUM>. The axial portions <NUM>, <NUM>, <NUM> are turned to form the first rotary vane portion 90A and the second rotary vane portion 90B.

As will be understood, a second rotor <NUM> (not shown) is received within a second void <NUM> which also extends through the width of the inter-stage coupling <NUM> but is laterally spaced from the first void <NUM>. The second rotor <NUM> is identical to the aforementioned rotor <NUM> and is rotationally offset by <NUM>° thereto so that the two rotors <NUM>, mesh in synchronism.

Returning to <FIG>, the first pumping stage <NUM> comprises a unitary stator, forming a chamber therewithin. The chamber being sealed at one end by the head plate (not shown) and at the other end by the inter-stage coupling unit <NUM>. The unitary stator has a first inner surface 20C. In this embodiment, the first inner surface 20C is defined by equal semi-circular portions coupled to straight sections which extend tangentially between the semi-circular portions to define a void/ chamber which receives the rotors <NUM>. However, embodiments may also define a generally-figure-of-eight cross-section void. The second pumping stage <NUM> comprises a unitary stator forming chamber therewithin. The chamber being sealed at one end by the head plate (not shown) and at the other end by the inter-stage coupling unit <NUM>. The unitary stator has a second inner surface 30C defining a slightly figure-of-eight cross-sectional chamber which receives the rotors <NUM>. The presence of the unitary stators greatly increases the mechanical integrity and reduces the complexity of the first pumping stage <NUM> and the second pumping stage <NUM>. In an alternative embodiment, the head plate could also be integrated into each stator unit to form a bucket type arrangement, such an approach would further reduce the number of components present.

The first rotary vane portions 90A of the rotors <NUM>, mesh in operation and follow the first inner surface 20C to compress gas provided from an upstream device or apparatus at a first stage inlet 20A and provide the compressed gas at a first stage exhaust 20B. The compressed gas provided at the first stage exhaust 20B passes through an inlet aperture 120A formed in a first face 110A of the inter-stage coupling unit <NUM>. The first face 110A represents a boundary between the first pumping stage <NUM> and the gallery <NUM>. The compressed gas travels through a gallery <NUM> formed within the inter-stage coupling unit <NUM> and exits through an outlet aperture 120B in a second face 110B of the inter-stage coupling unit <NUM>. The second face 110B represents a boundary between the gallery <NUM> and the second pumping stage <NUM>. The compressed gas exiting the outlet aperture 120B is received at a second stage inlet 30A. The compressed gas received at the second stage inlet 30A is further compressed by the second rotary vane portions 90B of the rotors <NUM> as they mesh and follow the second inner surface 30C and the gas exits via a second stage exhaust 30B.

The assembly of the two-stage booster pump <NUM> is typically performed on a turnover fixture. The unitary stator of the first pumping stage <NUM> is secured to the build fixture. The head plate is attached to the stator and then the assembly rotated through <NUM> degrees.

The two rotors <NUM> are lowered into the first stage stator. The first portion 40A and the second portion 40B of the inter-stage coupling <NUM> are slid together over the intermediate axial portion <NUM> to retain first rotary vane portion 90A within the first pumping stage <NUM>. The first portion 40A and the second portion 40B of the inter-stage coupling unit <NUM> are then typically dowelled and bolted together. The assembled halves of the inter-stage coupling <NUM> are then attached to the unitary stator of the first pumping stage <NUM>.

The unitary stator of the second pumping stage <NUM> is now carefully lowered over the second rotary vane portion 90B and attached to the inter-stage coupling unit <NUM>.

A head plate is now attached to the unitary stator of the second stage pump <NUM>. The two rotors <NUM> are retained by bearings in the two head plates.

The rotor <NUM> was analysed to understand its natural frequencies. It can be shown that the transitional displacement of the rotor <NUM> under a <NUM>,000N uniformly-distributed load applied to one side of both the first rotary vane portion 90A and the second rotary vane portion 90B is up to <NUM>. As can be appreciated, dependent upon the tolerances and operational frequency of the two-stage booster pump <NUM>, this amount of displacement may lead to damage within the inter-stage coupling <NUM>.

<FIG> illustrates the bending modes of the rotor <NUM>. As can be seen, the first bending modes occur at <NUM>, which are close to the operating frequency of the rotor <NUM>.

<FIG> illustrates the provision of a collar, generally <NUM>, according to one embodiment. The collar <NUM>, shown more clearly in <FIG>, comprises a pair of hemi-cylindrical elements 210A, 210B dimensioned to be received on an outer surface of the intermediate axial portion <NUM>. The pair of hemi-cylindrical elements 210A, 210B together, once fixed onto the intermediate axial portion <NUM>, extend the diameter of the intermediate axial portion <NUM>. In this embodiment, the pair of hemi-cylindrical elements 210A, 210B extend the diameter of the intermediate axial portion <NUM> to <NUM>. In this embodiment, M8 screws are received by screw apertures <NUM> in order to mechanically secure the hemi-cylindrical elements 210A, 210B together. However, it will be appreciated that a variety of different techniques may be used to fix the hemi-cylindrical elements 210A, 210B together. Also, it will be appreciated that the collar <NUM> may be fabricated from parts of differing configuration.

It can be shown that the transitional displacement of the rotor <NUM> with the collar <NUM> under a <NUM>,000N uniformly-distributed load applied to one side of both the first rotary vane portion 90A and the second rotary vane portion 90B reduces to <NUM>.

As can be seen in <FIG>, the modal frequency of the rotor <NUM> with the collar <NUM> has increased significantly. The first modes are now at <NUM>. These are significantly further away from the operating frequency of the rotor <NUM>.

<FIG> illustrates a portion of a rotor 50A, according to an arrangement outside the scope of the claims. In this arrangement, the intermediate axial portion 80A is of an enlarged diameter of <NUM>. An indented face <NUM> is machined into the intermediate axial portion 80A during machining of the lobes 55A. In this arrangement, the diameter of the intermediate axial portion 80A is <NUM>. Inserts (not shown) are then fitted into these indented faces in order to restore the intermediate axial portion 80A to a cylindrical shape of constant diameter of <NUM>. Accordingly, the inserts are axially-elongated with intersecting opposing faces. The cross-section of the inserts is therefore defined by a segment intersecting a hypocycloid. It will be appreciated that the inserts may extend along the length of the intermediate axial portion 80A or at least a pair of inserts may be provided, disposed at either end of the intermediate axial portion 80A in the vicinity of the first face 110A and the second face 110B. The inserts may be initially machined with the hypocycloid inner face which engages with the indented face <NUM> and is fixed in place. The inserts may then be turned to form the cylindrical outer face.

<FIG> shows a portion of a rotor 50B, according to an arrangement outside the scope of the claims. In this arrangement, the rotor 50B has an intermediate axial portion 80B which has an enlarged initial diameter of <NUM>. An indented face is initially machined, as mentioned above, but then that face is milled to provide a flat surface <NUM> onto which cylindrical segments <NUM> (shims) are fitted in order to restore the intermediate axial portion 80B back to its original cylindrical shape with a constant external diameter. Accordingly, the cylindrical segments <NUM> are axially-elongate with intersecting opposing faces. The cross-section of the cylindrical segments <NUM> is therefore defined by a segment intersecting a straight line. It will be appreciated that the cylindrical segments <NUM> may extend along the length of the intermediate axial portion 80B or at least a pair of cylindrical segments <NUM> may be provided, disposed at either end of the intermediate axial portion 80B in the vicinity of the first face 110A and the second face 110B. It will be appreciated that manufacturing cylindrical segments is significantly easier than manufacturing the inserts mentioned above. The cylindrical segments <NUM> may be initially machined with the flat inner face which engages with the flat surface <NUM> and is fixed in place. The cylindrical segments <NUM> may then be turned to form the cylindrical outer face.

As can be seen in <FIG>, the modal frequency of the rotor 50B of <FIG> having a larger diameter formed with flats is increased significantly over shaft <NUM> case illustrated in <FIG>. The first mode is now <NUM>. This are significantly further away from the operating frequency of the rotor <NUM>.

Embodiments and arrangements provide two-stage booster rotor stiffening collar, inserts and/or shims. The mechanical strength of a one-piece rotor is increased by the addition of a rotor stiffening collar and/or faces onto which the inserts or shims fit. In one embodiment, the one piece rotor design is for a <NUM> / <NUM><NUM> booster.

As mentioned above, manufacturing a rotor by a slab-milling process uses large-diameter milling cutters. To cut the full profile, the cutter has to transverse the profile until the centre-line of the cutter has passed the end of the rotor profile. The cutter would therefore gouge into the inter-stage shaft diameter if the shaft diameter is larger than the root width. If the inter-stage shaft diameter was increased to a diameter larger than the root width of the rotor profile, then a mill turning process would be required to machine the rotor profile. This is time-consuming and requires an expensive mill turn machine. The rotor stiffening collar, inserts and/or shims enable slab-milling of the rotor profile and may be attached to the rotor shaft after grinding the shaft diameters. Rotor balancing may be done after the attachment of the stiffening collar.

Embodiments and arrangements maintain the easy manufacture and strength of a one-piece rotor but add a stiffening collar, inserts and/or shims to raise the natural frequency of the rotor. This can be used in multistage pumps particularly roots designs. This arrangement avoids the need to increase the root diameter of the rotor. Assuming the shaft centre distance and rotational speed is maintained, then the tip diameter must be reduced and this reduces the swept volume. To overcome this the shaft centre distance would need to be increased to enable a larger root and tip diameter to give the same displacement.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims.

Claim 1:
A rotor (<NUM>) for a multi-stage roots-type vacuum pump (<NUM>), comprising:
a plurality of rotary vanes (90A,90B), said plurality of rotary vanes (90A,90B) being axially displaced and coaxially aligned;
a pair of end shafts (<NUM>,<NUM>), each end shaft (<NUM>,<NUM>) extending from opposing axial ends of said plurality of rotary vanes (90A,90B); and
an inter-vane shaft extending between adjacent rotary vanes of said plurality of rotary vanes (90A,90B), said inter-vane shaft having a diameter which is greater than that of said end shafts (<NUM>,<NUM>), said inter-vane shaft comprises a collar (<NUM>) fitted onto an internal shaft extending between said adjacent rotary vanes (90A,90B)
characterised in that said collar (<NUM>) comprises separable portions (210A,210B) and in that said internal shaft and said adjacent rotary vanes (90A,90B) are unitary.