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Title:
ROTATING MACHINE AND METHOD OF MANUFACTURE
Document Type and Number:
WIPO Patent Application WO/2018/033713
Kind Code:
A1
Abstract:
A method of manufacturing a rotating machine and a rotating machine are disclosed. The rotating machine comprises: at least one rotatable shaft supported on at least one bearing within a clearance bore; the bearing comprising an inner ring and an outer ring enclosing rolling elements The clearance bore comprises an inner diameter that is larger than a diameter of the outer ring of the bearing; and the rotating machine comprises a bearing biasing means for biasing the bearing in a radial direction away from a central position of the bore so as to constrain radial movement of the shaft.

Inventors:
HOLBROOK, Alan Ernest Kinnaird (Edwards Limited, Innovation Drive, Burgess Hill Sussex RH15 9TW, RH15 9TW, GB)
TATTERSALL, Jack Raymond (7/180 Nepean Hwy, Seaford, Victoria VIC 3198, VIC 3198, AU)
Application Number:
GB2017/052388
Publication Date:
February 22, 2018
Filing Date:
August 14, 2017
Export Citation:
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Assignee:
EDWARDS LIMITED (Innovation Drive, Burgess Hill Sussex RH15 9TW, RH15 9TW, GB)
International Classes:
F16C39/02; F16C25/08; F16C39/06; F04C18/12
Foreign References:
US6491131B12002-12-10
US20070196228A12007-08-23
EP1679447A12006-07-12
US20160031473A12016-02-04
US5036235A1991-07-30
Attorney, Agent or Firm:
RAWLINS, Kate (Edwards Limited, Innovation Drive, Burgess Hill Sussex RH15 9TW, RH15 9TW, GB)
Download PDF:
Claims:
CLAIMS

1 . A rotating machine comprising: at least one rotatable horizontally mounted shaft supported on at least one bearing within a clearance bore said rotatable horizontally mounted shaft comprising rotor elements;

said bearing comprising an inner ring and an outer ring enclosing rolling elements; and

said clearance bore comprising an inner diameter that is larger than a diameter of said outer ring of said bearing; and

said rotatable horizontally mounted shaft and rotor elements being housed within a stator bore, said stator bore and clearance bore being offset with respect to each other to compensate for said shaft being supported in a radially downward position away from said central position of said clearance bore.

2. A method of manufacture of a rotating machine comprising a rotatable shaft, said shaft being horizontally mounted in use within a clearance bore supported on at least one bearing, said shaft comprising at least one rotor element, said at least one rotor element being housed within a stator bore, said method comprising: mounting said rotatable shaft supported on at least one bearing within said clearance bore, said bearing comprising an inner ring and an outer ring enclosing rolling elements and said clearance bore comprising an inner diameter that is larger than a diameter of said outer ring of said bearing;

providing a stator having a stator bore to surround said at least one rotor element, said stator bore and said clearance bore being offset with respect to each other by an amount corresponding to an amount said shaft is offset away from a central position of said clearance bore due to said shaft resting on a lower surface of said clearance bore.

3. A method according to Claim 2, further comprising a bearing biasing means for biasing said bearing in a radial direction away from said central position towards said downwards direction of said bore so as to constrain radial movement of said shaft.

Description:
ROTATING MACHINE AND METHOD OF MANUFACTURE

FIELD OF THE INVENTION

The invention relates to rotating machines and to their method of manufacture.

BACKGROUND

Rotating machines need to be carefully designed and manufactured in order for the moving parts to cooperate with each other accurately. Radial clearances, for example, can result in the moving parts of a rotating machine seizing when they are too small, while when they are too large they can result in poor performance.

A complex rotating machine can have many different parts all of which are manufactured with different tolerances and many of which interact. These tolerances may each contribute to overall performance. As the sum of these tolerances may be large, the resulting performance can be badly affected. This problem can lead to failures in rotating machines, particularly in high precision rotating machines such as vacuum pumps. One solution to this might be to reduce the tolerances in each component by improved manufacturing processes. Although effective, this is both difficult and expensive.

It would be desirable to reduce the number of failures in a rotating machine without unduly increasing the costs.

SUMMARY

A first aspect of the present invention provides a rotating machine comprising: at least one rotatable shaft supported on at least one bearing within a clearance bore; said bearing comprising an inner ring and an outer ring enclosing rolling elements; and said clearance bore comprising an inner diameter that is larger than a diameter of said outer ring of said bearing; and said rotating machine comprising a bearing biasing means for biasing said bearing in a radial direction away from a central position of said bore so as to constrain radial movement of said shaft. When analysing the effect of the tolerances of different components on the machine, the inventors of the present invention recognised that some

components had a far greater effect on performance than others. In particular, tolerances in the position of the rotatable shaft in a rotating machine had a large effect on the overall function of that machine, in many cases variations in the tolerances contributing to over 50% of the overall variations. Rotatable shafts need to be mounted in a non-fixed manner to allow them both to rotate and to move axially. Axial movement is required during axial setting of the shaft and rotors during assembly and also during use where the shaft may expand and contract with temperature changes. Thus, these shafts are mounted on bearings which are in turn mounted in a clearance bore. Clearance between the bore and bearing is provided to allow for the axial movement. A clearance bore is any substantially cylindrical hole into which at least a portion of the bearing can be inserted.

The inventors of the present invention recognised, that although there may be a technical prejudice against biasing the bearings away from the central position such that the shaft is mounted eccentrically within the clearance bore as this seems to lead to an asymmetry and a resistance to axial movement, such biasing does provide the improvement of allowing the position of the shaft to be both more accurately predicted and also more stable by reducing variations in radial position that may arise during operation. In effect variations and uncertainties in the positioning of the shaft which are so important to overall function are reduced without having improved the manufacturing tolerances of the bearing, bearing elements and clearance bore.

It should be noted that although the shaft may be mounted with some clearance within the bearing, generally it is mounted in the bearing as an interference fit and the axial movement is provided between the clearance bore and bearing. In some embodiments, said bearing biasing means is configured to bias said bearing towards a known contact position with said clearance bore.

The use of the biasing means to bias a bearing in a radial direction away from the central position of the bore brings it into contact with the clearance bore and constrains or impedes the movement of the shaft in any radial plane that is in any plane perpendicular to the shaft's axis. Arranging the biasing means such that this contact position is a known position, allows the shaft's position to be accurately predicted and can be used when designing the other elements of the rotating machine.

In some embodiments, said rotating machine comprises at least one rotor mounted on said at least one shaft and a stator, said stator comprising a stator bore, said stator bore and said clearance bore being offset with respect to each other to compensate for said shaft being supported in a radially biased position away from said central position of said clearance bore.

As noted previously, by using a biasing means, the shaft is not mounted concentrically within the clearance bore, however, its position in that bore is more predictable. Thus, although variations in its position may be reduced its overall effect on other tolerances may not always be advantageous. One way of addressing this is to offset elements of the rotating machine in the same way that the mounting of the shaft within the clearance bore has been offset. In particular, where the clearance bore is offset to match the offset of the shaft position within the clearance bore, then radial clearance tolerances can be significantly reduced without having improved the tolerances of the individual elements. In this way, an improved rotating machine is provided.

In some embodiments, said bearing biasing means comprises one of a

mechanical spring, a gas spring and a magnetic means. The biasing means may be formed of a number of things provided it can provide a biasing force for biasing the bearing and therefore the shaft away from a central position. Examples of such biasing means are mechanical springs, gas springs and a magnetic means. The magnetic means may be permanent magnets or it may be an electromagnet.

In some embodiments, said mechanical spring comprises one of a leaf spring and a coiled spring, said mechanical spring extending into said clearance bore and biasing said bearing against an opposite surface of said clearance bore.

One effect of using a mechanical spring which extends into the bore is that it may provide increased wear and distortion of the bearing and in some embodiments, in order to reduce such effects, the contact surface for contacting said bearing of the mechanical spring is shaped to correspond with an outer surface of said bearing.

Providing the contact surface of the spring with a shape that corresponds with the outer surface of the bearing reduces frictional wear of the spring on the bearing and reduces such distortion.

In other embodiments, said bearing comprises a housing for housing said rollable elements, said housing comprising said biasing means, said outer ring being formed of flexible material and comprising a diameter that varies around said ring, said housing being such that a largest diameter of said bearing is when not distorted larger than said inner diameter of said clearance bore.

Rather than providing a separate biasing means, in some embodiments the biasing means may be made to be part of the bearing itself, the bearing having a housing for housing the rollable elements which is formed of a flexible material and has a variable diameter such that the larger diameter contacts the opposing surfaces of the clearance bore when mounted within it and the housing is under a distortion force such that it is held in position by this flexible force. In some embodiments, said clearance bore has alignment means for aligning the bearing biasing means with a predetermined position of the clearance bore. The use of biasing means enables the bearing and therefore the shaft to be held in a fixed position within a clearance bore. It is advantageous if this position is a known position and thus, providing alignment means which align the bearing biasing means with a predetermined position of the clearance bore provides such predictability.

Although the alignment means can be provided in a number of ways, in some embodiments said alignment means comprises an alignment recess for receiving a cooperating portion of said bearing biasing means. In some embodiments, said bearing biasing means is configured to bias said bearing towards a downward position within said clearance bore.

Although the biasing means may be arranged in a number of ways to bias the bearing in a number of directions, in some cases it is arranged to bias the bearings towards a downward position within the clearance bore. This is particularly effective where the rotating machine is a large machine with a heavy shaft and is arranged such that the shaft is horizontal. In such a case, using the biasing means to bias the shaft downwards enables it to work in cooperation with gravity and provides a combined downwards force, thereby requiring a lower force from the biasing means to overcome rotational imbalances. In other embodiments it may be advantageous to bias the bearing in a direction of gas load, which may be upward or outward depending on the configuration of the rotating machine. Where gas load is significant biasing in this way enables the biasing means to act in conjunction with the loading on the pump to achieve a stable position of the shaft. In some embodiments, said rotating machine comprises two parallel shafts each supported on bearings in a respective clearance bore, said bearing biasing means being mounted between said two clearance bores. Some rotating machines have two or indeed more shafts, each mounted on bearings. In some cases, a bearing biasing means arranged between the two bores and which acts on the bearings of both shafts can be used. In this way, a single bearing biasing means can be used to bias the bearings in each of the two shafts.

In some embodiments, said bearing biasing means extend into said two clearance bores and biases each bearing away from each other towards an outer wall of said respective clearance bores. In other embodiments, said bearing biasing means may be configured to bias each bearing towards each other. In this regard, the bearing biasing means may be a magnet which attracts magnetic material either of or mounted on the bearing elements or the bearing outer ring. In either case, the attractive force of the magnets may be used to bias the bearings. In alternative embodiments, the magnets may be mounted to repel the bearings. In either case, the bearing biasing means does not need to extend into the clearance bore which can reduce bearing distortion and wear.

In some embodiments, said bearing biasing means is arranged to bias each bearing in a same direction. Biasing the bearings in the same direction can be advantageous in that it reduces variations in the distance between the two shafts.

In some embodiments, said rotating machine comprises multiple bearing biasing means for biasing said bearing in a radial direction away from a central position of said bore so as to constrain radial movement of said shaft, each of said bearing biasing means being arranged such that at least one component of a biasing force exerted on said bearing by said bearing biasing means is towards a contact position of the bearing and clearance bore.

Although each bearing may be biased by a single biasing means, in some cases it may be advantageous to use two or indeed multiple biasing means. Where these biasing means are arranged such that they each have a component of the biasing force towards a contact position of the bearing on the clearance bore, then by having more than one biasing means, a more stable arrangement is provided which holds the shaft in a more predictable and non-variable position, more effectively counteracting rotation imbalances and counteracting loads from the gas or liquid being pumped.

In other embodiments, there may be a single biasing means acting on an intermediate member which contacts the bearing in two or more places. The use of an intermediate member such as a bridge piece, allows a single biasing means to act on more than one point providing the desired increase in stability and providing a more predictable and non-variable position of the shaft.

In some embodiments, said rotating machine further comprising an axial biasing means for biasing said at least one rotatable shaft against a plate such that one end of said shaft is axially fixed and an other end of said shaft is able to move axially in response to thermal changes; said bearing biasing means being configured with a biasing strength high enough to counteract rotational imbalances and said bearing and axial biasing means being configured with relative strengths such as to allow axial movement of said shaft in response to said axial biasing means.

Providing biasing means which biases the bearings in a radial direction away from the central position provides a means of counteracting rotational imbalances and a more predictable position of the shaft within the rotating machine, both of which can lead to improved performance and counteract the problems that arise due to tolerances within the elements forming the rotating machine. The strength of the biasing means can be selected according to desired characteristics. In particular, reduced rotational imbalances not only improve performance but also reduce noise and vibrations generated by the rotating machine. In some embodiments a radially mounted screw is provided towards a fixed end of the shaft, as a radial biasing means. The radially mounted screw firmly secures the bearing in an intended location. In some embodiments a pad may be mounted between the bearing and screw to reduce fretting and wearing of the bearing. In some embodiments, said at least one rotatable shaft is mounted on two bearings; said bearing biasing means of said bearing closest to said axially movable end being configured to be strong enough to counteract or inhibit any rotational imbalance and to be weak enough to allow axial shaft movement in response to said axial biasing means; and said bearing biasing means of said bearing closest to said axially fixed end being configured to be strong enough to counteract or at least inhibit any rotational imbalance. Where the rotatable shaft is mounted using an axial biasing means which biases the rotating shaft against a plate and the shaft is supported by two bearings, then the bearing closest to the axial biasing means should allow axial movement of the shaft such that it can expand and contract, the other bearing means does not need to provide this axial movement. Thus, when selecting the strength of the biasing means, the location of the bearing that it is biasing should be considered. A biasing means which is strong enough to effectively counteract rotational imbalances, provides advantages, however the bearing biasing means of the bearing that is closest to the axially moveable end of the shaft should not have a biasing strength that is too high as it needs to allow the axial movement of the shaft in response to the axial biasing means, such that as it contracts, following expansion it will move back towards its former position. One way to facilitate axial movement may be to use low friction coatings on the bearing and/or clearance bore. Although embodiments of the invention provide advantages to many different types of rotating machine, they are particularly advantageous when used for a vacuum pump. A vacuum pump is a high precision instrument where high precision tolerances are required and variations in these can lead to seizing of the pump. In particular, where the vacuum pump is a dry pump which does not use lubrication and therefore requires very high precision, such an arrangement can be particularly advantageous. Other vacuum pumps such as turbomolecular pumps which have a high rotational speed, rotary vane pumps or screw pumps can also benefit from embodiments of this invention.

A second aspect of the present invention provides a rotating machine comprising: at least one rotatable horizontally mounted shaft supported on at least one bearing within a clearance bore said rotatable horizontally mounted shaft comprising rotor elements; said bearing comprising an inner ring and an outer ring enclosing rolling elements; and said clearance bore comprising an inner diameter that is larger than a diameter of said outer ring of said bearing; and said rotatable horizontally mounted shaft and rotor elements being housed within a stator bore, said stator bore and said clearance bore being offset with respect to each other to compensate for said shaft being supported in a radially downward position away from said central position of said clearance bore.

Although the provision of biasing means in the first aspect has the benefit of providing a resistance to rotational imbalances and a predictability to a location of the shaft, this can also be provided to some extent by gravity where a shaft is horizontally mounted during operation and this is particularly so for larger machines with heavier shafts. In order to benefit from this predictability of location the stator and clearance bores can be designed with a relative offset that corresponds to the offset of the shaft within the clearance bore. Thus, where the clearance bore diameter is say designed to be 100 microns larger than an outer diameter of the bearing casing, the clearance bore can be designed to have an offset of 50 microns with respect to a centre of the stator bore. A third aspect of the present invention provides a method of manufacture of a rotating machine comprising a rotatable shaft, said shaft being horizontally mounted in use within a clearance bore supported on at least one bearing, said shaft comprising at least one rotor element, said at least one rotor element being housed within a stator bore, said method comprising: mounting said rotatable shaft supported on at least one bearing and comprising said at least one rotor within said clearance bore, said bearing comprising an inner ring and an outer ring enclosing rolling elements and said clearance bore comprising an inner diameter that is larger than a diameter of said outer ring of said bearing; providing a stator having a stator bore to surround said at least one rotor element, said stator bore and said clearance bore being offset with respect to each other by an amount corresponding to an amount said shaft is offset away from a central position of said clearance bore due to said shaft resting on a lower surface of said clearance bore.

In some embodiments, the method further comprises a bearing biasing means for biasing said bearing in a radial direction away from said central position towards said downwards direction of said bore so as to constrain radial movement of said shaft.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 schematically shows a cross section through a bearing mounted within a clearance bore according to the prior art; Figure 2 schematically shows a cross section through a bearing mounted within a clearance bore according to an embodiment;

Figure 3 schematically shows a cross section through bearings mounted within clearance bores of a twin shaft machine according to an embodiment;

Figure 4 shows a longitudinal section through a rotating machine;

Figure 5 shows a cross section through the same machine; and

Figures 6 to 1 2 provide examples of different biasing means for biasing bearings within a clearance bore according to embodiments. DESCRIPTION OF THE EMBODIMENTS

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

In rotating machines with the shafts supported on rolling element bearings, there is often a need for the bearing or bearings at at least one end of the shaft to slide axially so as to accommodate growth in the shaft length due to thermal or other effects. The axial slide of the bearing is usually made possible by housing the bearing in a clearance bore. However, this clearance allows substantial variation in the bearing's radial position, which is usually detrimental to a machine's performance. Variation in bearing position results in a corresponding variation in radial clearances, which can lead to part seizure or, in the case of pumps and engines, loss of compression and machine efficiency.

Specifically, in vacuum pumps, the ultimate pressure can be impaired resulting in a more expensive design being required to achieve satisfactory performance. Embodiments seek to improve the radial control of rotating parts in a machine by biasing the bearings in a fixed direction. By holding the bearings in a known contact position or region, the variability of radial clearances in a machine can be significantly reduced. The magnitude of the improvement can be as much a ten fold, depending on the nominal clearance and the part tolerances. If the variation in shaft position is made more certain (by biasing the bearings) then the radial clearances in a machine can be reduced and significant improvements achieved in performance. Alternatively, better control of bearing locations could enable some tolerances to be relaxed while still meeting certain performance

requirements, thus reducing manufacturing costs. Examples where embodiments could provide improved performance include: turbines, gas and fluid compressors, vacuum pumps and other precision rotating machines. Specifically in vacuum pumps, embodiments could potentially be used on rotary vane pumps, turbo-molecular pumps, screw pumps, multistage dry pumps and mechanical boosters.

Figure 1 schematically shows a cross section through a bearing 10 mounted within a clearance bore 1 2 according to the prior art. As can be seen its position within the bore can vary significantly. Figure 2 schematically shows the same cross section through a bearing mounted within a clearance bore but in this case a biasing load according to an

embodiment is applied to the bearing. By applying a lateral load to the bearing, as demonstrated the variability in location of the bearing 10 and rotational imbalances during operation are significantly reduced. The biasing means providing the lateral load is configured to provide a magnitude of lateral load so that it allows the bearing to thermally expand in the bore whilst overcoming or at least inhibiting typical bearing reaction forces during rotation. A bearing can be forcefully biased to a specified position in its bore by any practical and cost effective means. These means include: mechanical and gas springs, electro- magnetic energisation, permanent magnets and gravity.

For a twin shafted machine such as is shown in Figure 3, the two bearings 1 0 can be forced apart with a single spring 20. This could be a conventional coil or leaf spring, or any other energising device. One potential embodiment would be to use a ring shaped spring, as shown in Figure 3. As the biasing of a bearing places it off centre, it is possible and indeed desirable to re-position the bore to ensure the bearing and the shaft it carries are restored to the required central position. Alternatively other elements within the rotating machine may be offset to compensate for the predicted offset of the biased bearing and shaft.

Figure 4 shows a longitudinal section through a rotating machine while Figure 5 shows a cross section through the same type of twin shafted machine. The rotating machine has a motor 30 for driving the shafts 32. The shafts 32 have rotor elements 34, mounted within a stator 36, having a stator bore 37. The rotating machine is a mechanical booster pump and has two rotating shafts 32. Operation of the pump is affected by clearances, increased clearances reducing the pumping efficiency and smaller clearances providing increased risks of the pump seizing.

Clearances which affect the operation of this pump include the rotor clearance A between the rotor element 34 and the stator bore 37, the rotor to rotor clearance B shown in Figure 5 between the different rotor elements 34, and the bearing 10 to clearance bore 12 which are more easily seen in Figures 1 to 3. There are also clearances between the rotor elements and the through bores (not shown). All of these affect the operation of the rotating machine. However, on statistical analysis of the rotating machine performance it has been found that as the clearance between the bearing and clearance bore has an effect on many of the other clearances this contributes a surprising amount to the overall performance of the rotating machine. In this regard, it is found by statistical analysis that the shaft clearance tolerances often contributed more than 50% to the overall tolerances of the rotating machine. It was therefore determined that where these variations could be reduced then an improved machine could be provided. An improved machine could therefore be provided by biasing the bearing towards an edge of the clearance bore and offsetting the clearance bore to compensate for the shaft position not being in the centre of the clearance bore. The biasing means could be some form of spring or magnet as shown in Figures 6 - 10, or with a horizontally mounted shaft it could be simply due to gravity, the offset of the clearance bore with respect to the stator bore and the through bores providing an improved performance. In the roots pump of Figure 4, the shafts 32 are mounted on four bearings 1 0, 1 6 within respective clearance bores 1 2, 1 8. At one end of the shafts 32 they are held against plate 40, towards the motor, such that there is very limited axial movement at this end of the pump. Bearings 1 0 at this end of the pump can therefore be supported without the need to allow axial movement and as such may be mounted as an interference fit within clearance bores 12 or with strong biasing means to hold them in position. The bearings 16 towards the opposite end of the shafts 32 are mounted within respective clearance bores 18 in a way that allows axial movement and are further biased towards plate 40 by axial biasing means 50. Bearing biasing means (not shown) that bias bearings 1 0, 1 6 towards fixed position within respective clearance bores 1 2, 18 are therefore selected so as to inhibit radial movement due to rotation imbalances while allowing axial movement.

In addition to axial movement being required to allow for thermal changes, it is also required during pump assembly where the shafts 32 are slid into position against plate 40 with shims 21 being used to alter their axial position such that it they set appropriately to provide suitable axial clearances.

Figures 6 to 9 show examples of different type of biasing means for bearings 1 0, 1 6 that can be used to bias bearings in embodiments of the invention. The examples illustrated in figures 6 to 9, for bearings 1 0 and clearance 1 2, are equally applicable for bearings 1 6 and clearance 1 8.

Figure 6 shows a leaf spring 20 that serves as a biasing means for bearing 10. Leaf spring 20 is mounted in an alignment recess 14 within clearance bore 1 2 thereby providing a known predictable position for both the biasing means and the biased bearing.

Figure 7 shows a twin shaft system where a magnetic biasing means 20 is mounted between the two shafts and interacts with magnets mounted on the housing of the bearings 10 to bias the bearings in each clearance bore 12 in a same direction. Thus, the magnets mounted on the bearings on the left hand side of the Figure are repelled by magnet 20 and those mounted on the bearings shown on the right hand side are attracted to central magnet 20. There are advantages to biasing the bearings in the same direction as it reduces variations in distance between the shafts.

Figure 8 shows a twin shaft system similar to that shown in Figure 7 but in this case magnetic biasing means 20 mounted between the two shafts interacts with magnets mounted on the housing of the bearings 10 to bias the bearings in each clearance bore 12 in different directions, in this case away from the central position towards the outer edges of their respective clearance bores. Were a magnet of different polarity to be used as biasing means 20 the bearings might be attracted towards a central position, In either case the biasing means does not extend into the clearance bore and biases the bearings away from a central position.

Figure 9 shows schematically the use of plural biasing means 20 and how they improve the stability of the bearing and therefore shaft and act to further reduce rotational imbalances and the associated vibrations and noise. As can be seen multiple biasing means which each have an element of the biasing force in a common direction are used. In this example, the common direction is downwards and they therefore act in conjunction with gravity. In some cases the biasing means may be an integral part of the bearing. Figure 10 shows such an embodiment where bearing 10 has an outer casing made of flexible material which has a distorted circular shape. The inner casing 6 which holds the shaft is circular. One side of the outer casing has a larger diameter 8 than the other side. The overall diameter is adapted to be slightly smaller than the clearance bore in which it is mounted such that it is held in place by the flexible casing being distorted. The central bore of the inner casing 6 will be biased towards one side of the shaft due to the distorted shape of the outer casing.

Figure 1 1 shows an embodiment where an intermediate bridge piece 12 is used to provide two points of contact to the bearing from a single biasing means. This provides additional stability and a more predictable location of the shaft with a single biasing means. The arrow in the figure represents application of the biasing load. Figure 12 shows a further biasing means 24, 26, which can be used on the axially fixed end of the shaft as it provides a secure means of holding the bearing 1 0 in the intended location. Providing a curved plate 24, between the screw 26 and bearing 10 reduces fretting and wear on the bearing. In summary although it may seem to be counterintuitive to bias a shaft away from a central position as it would seem to increase any lack of concentricity in the machine and thereby decrease performance, it has been found that such a biasing provides a more predictable position of the shaft and reduces rotational imbalances. Furthermore, were the rotating machine to be designed to compensate for this offset by offsetting the clearance stator bores with respect to each other, the shaft is in effect moved back to a central position while still being held in a predictable and relatively stable position.

In this way embodiments provide:

· The potential to relax manufacturing tolerances, thus reducing machining costs.

• The potential to reduce nominal radial clearances, thus improving product performance.

• Improved product reliability through better dimensional control, leading to reduced seizures on test, resulting in less assembly re-work and scrap.

• Reduced seizures on test, resulting in less assembly re-work and scrap. • Reduced performance variation due to narrow distribution of rolling clearance.

• Repeatable shaft position may improve clarity of radial clearance

evaluation (paint dot tests etc), enabling rapid radial clearance

optimisation.

• Reduced noise and vibrations

Furthermore, implementation of this idea into rotational machines requires only a small number of component changes, such as the introduction of a biasing means for biasing the bearing.

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 and their equivalents.

REFERENCE SIGNS inner casing 6 outer casing 8 bearing 10, 16 clearance bore 12, 18 alignment recess 14 biasing means 20 shim 21

bridge piece 22 curved plate 24 screw 26

motor 30

shaft 32

rotor elements 34 stator 36

stator bore 37 plate 40

axial biasing means 50




 
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