The present invention relates generally to rotors for rotary machines, and particularly to improvements in rotors suitable for use in rotary piston positive displacement pumps and other rotary machines. As used in this specification the term"pump"will be understood to include machines designed to work with any form of working fluid, whether liquid or gas, and to include compressors and vacuum pumps.

In the production of rotors for rotary machines there are a number of considerations which must be taken into account, of these the mass of the rotor and therefore the rotary inertia of the body, the accuracy with which the surface can be formed, and the nature of the material used, especially its wear resistant properties, are all important.

The rotor of the present invention finds particular application in the field of rotary piston positive displacement pumps, sometimes called Roots pumps (or blowers if used for air). As is known, rotary piston positive displacement pumps generally comprise two rotors working within an epitrochoidal chamber, the surface shape of each rotor being such that the surfaces of the two rotors, which are driven to turn in opposite directions about parallel shafts, maintain a closely spaced relationship so they act positively to displace fluid trapped in the space between them as they rotate.

Such rotors must be accurately balanced as they involve significant mass, so that out- of-balance forces can result in considerable vibration and noise. Conventionally, such

rotors are formed by casting or forging solid material. The present invention seeks to provide a more economic way of producing such rotors, which also allows other advantages to be gained.

According to one aspect of the present invention, therefore, there is provided a rotor for a rotary piston pump or other rotary machine, rotatable, in use, about a rotor axis, the rotor comprising an assembly of a plurality of modular elements, in which at least a part of the outer surface of the rotor is covered in a coating material.

Substantially the entirety of the outer surface of the rotor assembly may be covered in a coating material. By coating the modular elements once assembled the coating process can be simplified. In addition, by coating the assembly this makes it easier to restrict the coating to the outer surface if required.

By forming a partite rotor from a plurality of modular elements problems relating to the formation of a single rotor component, such as the mass of the rotor and the economics of forming a unitary rotor, can be avoided.

By using a coating layer the coating can be used to provide a smooth outer surface of the rotor formed from an assembly of modular elements joined together. A coating can be applied over the joins between the modular elements, which can not only be used to seal the joins but also to make sure that they are smooth.

The coating material can be used to give the outer surface of the rotor useful properties

such as wear resistance or resistance to chemical attack.

The coating layer can be used to contribute to the outer shape of at least part of at least part of the rotor. Accordingly the thickness of the coating layer can be varied around the periphery of the rotor to produce a desired shape.

The coating material may be a resin, such as an epoxy resin. Alternatively the coating material may be a ceramic, a sintered boride or other suitable material.

The coating material may be applied in a fluid form and allowed to set.

The rotor may include one or more chambers. The chambers can be present purely as a method of weight saving or could have some active function, such as for conducting coolant or cleaning fluid through the rotor assembly.

The (or if there is more than one, a) chamber may communicate with passages leading to the exterior and forming a path for a coolant fluid to flow through the rotor. The passages in the rotor may be radial passages linking the interior volume of the chamber with one or more axial passages extending through the shaft of the rotor, these being linked by a radial passage which may open into an annular groove in the outer surface of the shaft.

The rotor may comprise a plurality of modular-elements in face-to-face relationship with facing surfaces of adjacent modular elements lying transverse the axis of rotation

of the rotor.

In forming a rotor of this type a plurality of modular elements can be stacked in an array along the axis of rotation and secured together by any suitable means.

In one embodiment of the invention the modular elements are substantially laminar, which makes their production easy and economical, and preferably the laminar modular elements are secured together by adhesive.

Another advantage of utilising laminar members as the modular elements lies in the fact that each may then be laser-cut to shape. This offers considerable advantages of accuracy and economy, whilst the opportunity to cut relatively large openings in the elements resulting in significant chambers or voids within the interior of the body formed by a stack of such elements, offers considerable weight saving, and by making the shaft of a different material appropriate properties for the rotor body elements themselves are not compromised as may be the case in prior art devices where the rotor and body are of the same material.

It has been found, contrary to expectation, that the junction lines between adjacent laminar elements secured in face-to-face relationship do not detrimentally affect the performance of the pump. In order further to ensure that no fluid leakage between adjacent elements can take place, the outer surface of the rotor, or at least a part thereof, is covered in the coating material.

Conveniently the rotor may have a central opening through which can pass a rotor shaft the axis of which is substantially coincident with the axis of rotation of the rotor.

In embodiments in which the modular elements are stacked in face-to-face relationship each of the elements will require a central opening. This configuration has advantages in that it allows the choice of materials for the rotor shaft to be different from that for the rotor body itself, thereby enabling material having suitable properties for each particular function to be chosen.

In order to secure the shaft non-rotatably within the body of the rotor, at least a portion of the shaft may have a non-circular cross-section capable of form-engagement with a correspondingly shaped opening in the rotor to transmit rotary motion from the shaft to the body of the rotor as it rotates.

In its simplest form the non-circular cross-sectional shape of the shaft may comprise a flat formed along the cylindrical surface of the shaft. Other form-engaging shapes may, of course, be utilised.

For use in a positive displacement rotary piston pump it is preferred that the rotor has a plurality of lobes. There may be at least one chamber in each lobe. For modular elements stacked together face-to-face, therefore, an opening in the lobe of each element can be combined to form respective chambers or voids in each lobe of the rotor.

Although conventionally rotary piston positive displacement pumps have pairs of

rotors with two lobes, it would be possible to form rotors having three or more lobes of suitable shape comprising an assembly of suitably shaped modular laminar elements.

The shaft of a rotor formed in accordance with the present invention may have a coupling and spacer for fixing a drive gear to one end of the shaft.

The modular elements may be of different form, for example they may be elementary sections of a screw rotor which is formed when a plurality of the modular elements are stacked in a face-to-face array to form a screw rotor pump of a type having a substantially continuous helical channel and a generally cylindrical outer surface. The same concept may be applied to the structure of augers.

The modular elements of the rotor body may be secured to the central shaft by adhesive.

Where adhesive is used to secure the modular elements together or to secure the modular elements to the shaft, this may be an epoxy resin adhesive.

The present invention also comprehends a rotary piston positive displacement pump having a rotor or rotors as defmed herein. In such a pump the casing may have a plurality of external fins to promote cooling.

The present invention also extends to a method of making a rotor for a rotary machine,

comprising the steps a method of making a rotor for rotary machine, comprising the steps of forming a plurality of the modular elements into an array in face-to-face relationship, securing the modular elements together and fitting the array to a shaft in such a way as to be secured non-rotatably thereto. The method may include the further step of coating the array with a coating material.

Whilst a rotor formed from a plurality of laminar elements has many advantages, it also has some disadvantages. For example, the operation of applying adhesive to the individual laminates to bond them together can present problems. In addition due to tolerances there must be careful selection of plates to insure that a stack of laminates is the required length.

The rotor may be formed from a plurality of modular elements having a hollow section. For example, the elements may be tubes which are arranged to lie parallel to each other to form the rotor. By using tubes as the modular rotor elements they can be formed from relatively thin material; for example tubes may be formed by rolling a sheet of material. In addition the length of the tubes determines the length of the rotor so that there is no requirement for component matching as for laminar elements.

Tubular structures automatically provide a rotor with a chamber, which could be used to conduct cooling fluid, for example.

The tubes may be simple cylinders with a circular cross-section or they may be formed as cylinders and further processed to a desired shape. For example, the tube may be

processed to include a flat surface to allow it to be easily stacked on a further tube which has been similarly processed.

According to a further aspect of the present invention there is provided a rotor for a rotary piston pump or other rotary machine, rotatable, in use, about a rotor axis, the rotor comprising one or more rotor elements having a hollow section.

The hollow sectioned rotor may comprise one or more tubes. A tubuliform rotor has many advantages over a solid rotor including a reduction in weight and the automatic presence of at least one chamber, which could be used, for example, to conduct cooling fluid.

The rotor may comprise a plurality of tubular members, which may be assembled together in parallel relationship to each other.

By forming a partite rotor from a plurality of tubular members a desired shape of a rotor can be achieved more easily than a single members which must be formed into a potentially complex shape.

The rotor may have two or more lobes and could thereby be used in a rotary piston positive displacement pump.

At least part of the outer surface of the hollow sectioned rotor may be covered in a coating material to give advantages such as abrasion resistance or resistance to

chemical attack.

The or each tubular member may be formed from a sheet of material which is rolled into the tube structure. Rolled hollow section members with a required peripheral size can therefore be formed from relatively little material and require no casting or forging of solid materials. In very small rotors the tubes may simply be cylindrical rods.

The present invention also provides a method of making a rotor for a rotary piston pump or other rotary machine, comprising the steps of assembling a plurality of modular elements and applying a coating around at least part of the outer surface of the assembly.

The coating may be applied by introducing the assembly into a mould, the mould cavity of which is larger than the external dimensions of the assembly, and injecting a coating material into that part of the mould cavity not occupied by the rotor, and allowing it to set or curve around the modular element assembly to form a coating.

Various embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a face view of a modular element suitable for forming part of a rotor of a rotary piston positive displacement pump; Figure 2 is a schematic cross-sectional view through a part of a rotary piston pump incorporating the rotor formed in support of the present invention; Figure 3 is a schematic end view of a pump having a rotor formed in

accordance with the present invention; Figure 4 is a schematic perspective view of the rotor of Figure 2; Figure 5 is an enlarged schematic view illustrating the fitting of a gear to the end of the shaft of a rotor formed in accordance with the principles of the present invention; Figure 6 is a schematic view illustrating an alternative embodiment of the present invention formed as a helical screw auger; Figure 7 is a diagrammatic section of a rotor body assembly formed according to an alternative embodiment; Figure 8 is a section along line VIA-VIZ of Figure 7; and Figure 9 is a perspective view of a tri-lobe rotor formed according to an alternative embodiment; Figure 10 is a diagrammatic view of the rotor of Figure 7 following a coating step.

Referring first to Figures 1 and 2, a rotor for a positive displacement rotary piston pump is generally indicated 11, and comprises a stacked array of individual modular elements 12 one of which is shown in Figure 1. Each modular element 12 is a flat unitary body 13 of metal having a so-called"hourglass"outline comprising two enlarged lobes 14,15 and a central waist. The outer periphery 16 of the modular element 12 may be cut, for example by a laser cutter, to shape, which has advantages in producing the outline shape without the need for further processing such as grinding or de-burring.

Each lobe 14,15 has a central void, 18,19 respectively, the shape of which largely follows the outline 16 of the laminar element itself. A central opening or hole 17 in the body portion 13 is provided to receive a shaft 20. As can be seen in Figure 1 the hole 17, has two straight sides 21,22 engaging co-operating flats 29,30 of the shaft 20 (see Figure 4) to allow form engagement between the individual laminar element 12 of the rotor and the shaft 20.

As can be seen in Figures 2 and 3, the voids 18,19 combine in the finished assembled rotor to form two chambers 23,24 which have the effect of significantly reducing the mass of the rotor for a given peripheral dimension.

As can be seen more clearly in Figure 2, the assembled rotor, generally indicated 25 comprises a plurality of laminar elements such as the element 12 stacked in face-to- face relationship with a modified, that is un-pierced, but otherwise identical, laminar member 26,27 at each end of the stack to close the chambers 23,24.

The stack of elements 12 is fitted onto a rotor shaft 28 having flats 29,30 (see Figure 4) which engage the straight sides 21,22 of the hole 10 in the centre of each element 12. This form-engagement ensures a secure driving connection between the shaft 28 and the rotor stack.

Adjacent each end plate 26,27 the outer pierced laminar elements have radial passages 31,32 and 33,34 extending between the central opening 17 and the respective voids 18,19 which form the chambers 23,24 in the assembled stack. As

can be seen in Figure 1, each radial passage 30,31, 32 has an enlarged recess at the radially inner end to ensure registration of the passages 31,32 with respective radial passages 37,38 ; 39,40 in the shaft 20, the former communicating with an axial passage 41 in one end 42 of the shaft 28, and the other (the passages 39,40), communicating with an axial passage 43 in the other end 44 of the shaft 28. These passages allow a coolant fluid to be circulated through the interior of the rotor 25 from the axial passage 41 at the input end, through the communicating radial passages 37, 38 in the shaft 28 and the radial passages 31,32 in the central body portion 13 of the end plate 12 allowing communication with the chambers 23,24 from where the coolant fluid can flow through radial passages 33,34 in the laminar member 12 at the far end of the stack, into the radial passages 39,40 in the shaft 28 and out through the communicating axial passage 43. Providing the fluid is maintained constantly pressurised with no variation in material, especially no gas bubbles if the fluid is a liquid, no detrimental influence on the balance of the rotor is caused by the coolant fluid and this allows the rotors to be operated at higher speed without causing an unwanted rise in temperature of the pumped fluid.

Figure 3 illustrates the normal arrangement of a twin rotor two lobe positive displacement rotary piston pump showing the outer casing 45 defining an epitrochoidal chamber 48 within which rotate two rotors 46,47 of hourglass shape.

The pivot axes 49,50 about which the two rotors 46,47 rotate are located at the centre of curvature of respective semi-circular end faces 51,52 of the chamber 48, which are joined by two parallel flat faces 53,54 in which are formed respective inlet and outlet openings 55,56 respectively. By continuously counter-rotating the rotarypistons 46,

47 about their axes 49, 50 the pump functions continuously, without the need for separate valves, to draw fluid from the inlet side 55 and deliver it to the outlet port 56.

It will be appreciated that for this purpose the two rotors 46,47 must be synchronously driven. They are usually mechanically connected by gears mounted on the shafts which define the axes about which the rotors turn. In Figure 5 is illustrated the improved mounting arrangement for a gear 57 which meshes with the corresponding gear (not illustrated) of a second rotor in a pump assembly into which the rotor 25 is fitted. The gear 57 has a central opening 58 with a counterbore 59 into which are engaged two annular wedge rings 60,61 engaged axially by a spacer 62 held in place in such a way as to apply an axial force to the wedge rings 60,61 and thus a radial clamping force to the gear 57 by engaging in the counterbore portion 59 of the central opening 58.

As mentioned above the individual modular elements 12 are secured together by adhesive, by which they may also be secured to the shaft 28. A further coating around the outside of the rotor defined by the stack 25 of modular members is provided as represented by the reference numeral 63. This coating may be of a comparable or similar material to the adhesive, for example an epoxy resin, typically in the region of up to 1. 2mm thick.

Although in a rotary piston positive displacement pump the rotor surfaces do not run in constant frictional contact with each other or with the surface of the chamber 48 within which they are located, there being a small clearance between each surface in movement, there is nevertheless the possibility of some contact between relatively

moving surfaces, and the epoxy resin layer provides a wear surface resistant to abrasion. Other coating material such as ceramic, a sintered boride or the like may be used instead of a resin.

Although described herein in relation to twin lobed rotors it is, of course, possible to devise pumps having three or more lobes per rotor and corresponding provision to those described hereinabove apply as far as a three or more lobed rotor is concerned.

Figure 6 illustrates an alternative embodiment in which the same principle of stacked modular members is utilised to form a screw auger. Each modular member 64 comprises an element of a helical screw rotor 65, the elements 64 being mounted face- to-face with a sufficient angular offset to maintain the helical channel continuously around the rotor. Once again a coating 63 is provided as the outer wear surface of the rotor 65.

In order to improve the performance of rotary piston positive displacement pumps especially when used as air blowers, it is essential to extract as much heat as possible.

The heat is generated largely by the compression of the gas (usually air) being pumped, and for many applications low temperature delivered air is important, especially in the food industry. Rotary piston positive displacement pumps are used to generate relatively large volume low pressure air movements, and previous attempts to reduce the temperature of delivered air have included total immersion of the entire pump in a coolant bath. By proving a finned casing the cooling effect of the external fluid can be maximised, but even this is insufficient to allow the rating of the pump to

be increased to a duty level desired in order to obtain greater efficiency and a larger delivery of fluid from a pump of given size. The pump of the present invention may be adapted for pressurisation and exhaustion. The process for producing a rotor from a stack of laminations includes the steps of securing the laminations by one means or another. For example they may be held together with adhesive, or alternatively may be welded together. If adhesive is sued to process may include fitting the shaft thereto and coating the stack of laminations in a mould. The shaft may, alternatively, be fitted after the laminations have been coated in the mould. In addition to the gear shown in the drawings, a rotor 25 is borne on bearings (not shown) and suitable seals such as an oil thrower are provided at the bearings between rotary shafts and the casing.

As an alternative to the mould coating of the stack of laminations, thin coating, perhaps only a few microns thick, may be achieved by spray coating a rotor assembled from modular elements of accurately formed dimensions.

Other ways of holding the stack of laminar members together includes longitudinal (that is axially oriented) bolts, for which purpose the central shaft itself may be used, either above or in company with others, for example in the region of a periphery of the rotor. One way which has been found to be satisfactory is to hold the laminar member together in a stack, cut a groove along the tip of each lobe, and run a weld bead along the groove.

Referring now to Figures 7 and 8 there is shown a rotor according to an alternative embodiment. The rotor assembly 100 is formed from three tubular elements 110,120,

130. The tubular elements, 110,120, 130 are stacked one on top of the other and are coaxial, each running parallel to each other and to the axis of a rotor shaft 140 located in the central tubular element 120.

Because the elements 110,120, 130 are cylindrical with circular cross-sections they have no flat surfaces with which to engage each other. Accordingly flat plates 150, 160 are interspersed between the elements. The plates 150,160 are then welded to the respective adjacent elements at welding points 160.

Each of the tubular elements 110,120, 130 is formed by rolling a sheet of material into a cylindrical form. Accordingly each of the elements is hollow with a central chamber 111,121, 131. The rotor shaft 140 is secured within the chamber 121 by filling the space around the shaft 140 with a resin initially in fluid form which cures to a solid form and prevents rotation of the shaft 140 within the element 120.

Referring to Figure 9 there is shown a perspective view of a tri-lobular rotor formed from hollow section elements. The rotor comprises three main elements 210,230, 270 arranged coaxially at 120° intervals to form a tri-lobe assembly. A further element 220 is positioned centrally and is intended to receive a rotor shaft (not shown). The elements 210,230, 270,220 have an elliptical cross-section, and are secured together in the assembly shown by welding contacting surfaces.

Referring now to Figure 10 there is shown an assembly 100a of tubular elements 11 Oa, 120a, 130a connected to form a rotor assembly in the same way as that shown in

Figures 7 and 8. The assembly 10a is formed into a finished bi-lobular rotor by coating with a resin 163. The coating 163 is applied by first placing the element assembly 100a into a mould (not shown) which has the shape of the outline of the resin layer 163. The assembly 100a forms part of the mould such that when resin 163 is flowed into the mould it flows around the assembly and causes the rotor to have the hourglass shape shown in Figure 9. It will be noted that the thickness of the coating layer 163 varies significantly around the periphery of the assembly 100a and that in fact it is the resin 163 which determines the overall shape of the finished rotor.