Patent Publication Number: US-2023160470-A1

Title: Shaft mounting assembly

Description:
FIELD OF INVENTION 
     This invention relates to a shaft mounting assembly comprising a spring, and methods of assembling the same. 
     BACKGROUND TO THE INVENTION 
     Rotating machinery, for example gas turbines, can produce vibrations in use which may not be desirable, and which can lead to wear and subsequent damage to internal machine components. 
     Known solutions to the problem of unwanted vibrations include oil squeeze films, which can provide damping and reduce vibrations. A squeeze film may, for example, be a layer of oil between a bearing and a housing which increases the damping effect. This solution requires the presence of a radial clearance to accommodate the oil film. However, such a radial clearance provides a space which can lead to component misalignment problems. 
     Sealing rings have also been used to provide centralising, damping and sealing effects. However, the sealing rings can be subjected to high loads causing the sealing rings to wear out relatively quickly. Furthermore, when the gap between the shaft and the housing is filled with hot fluid, such as oil, the chemical structure of the material forming the sealing rings (e.g. rubber) can break down and cause the ring to deform. Over time, the sealing rings become less effective at sealing and providing the necessary damping/centralisation effects. 
     It is therefore an object of embodiments of the present invention to address at least one of the above disadvantages. 
     SUMMARY OF THE INVENTION 
     Accordingly, in a first aspect, the present invention provides a shaft mounting assembly comprising: an elongate shaft comprising an outer surface having a substantially circular cross-section; 
     a cylinder having an inner surface defining a bore, the bore housing the shaft, wherein the inner diameter of the cylinder is larger than the outer diameter of the shaft such that a gap is defined between the outer surface of the shaft and the inner surface of the cylinder; and, a spring comprising a substantially circular discontinuous band having correspondingly shaped axially arcuate inner and outer surfaces; 
     wherein one of the outer surface of the shaft and the inner surface of the cylinder comprises a groove; and wherein the groove extends around the circumference of the outer surface of the shaft or the circumference of the inner surface of the cylinder respectively; 
     wherein the spring is positioned in the groove such that both axial edges of the band are located within the groove; and, wherein the spring has a de-energised state in which the height of the band is greater than the depth of the groove, so that a portion of the band between the axial edges protrudes out of the groove, and the axial width of the band is less than the width of the groove; and an energised state in which the spring is compressed within the bore such that the height of the band is reduced, and the axial width of the band is increased compared to the de-energised state. 
     Accordingly, in a second aspect, the present invention provides a shaft mounting assembly comprising: 
     an elongate shaft having a substantially circular cross-section and a groove extending around the circumference of the shaft; 
     a cylinder having a bore housing the shaft and a side wall port communicating with the bore, wherein the inner diameter of the bore is larger than the outer diameter of the shaft such that a gap is defined between the outer surface of the shaft and the inner surface of the bore; and, 
     a spring comprising a substantially circular discontinuous band having correspondingly shaped axially arcuate inner and outer surfaces; 
     wherein the spring is positioned in the groove such that both axial edges of the band are located within the groove; and, wherein the spring has a de-energised state in which the outer diameter of the band is greater than the inner diameter of the bore and the axial width of the band is less than the width of the groove; and an energised state in which the spring is compressed within the bore such that the outer diameter of the band is reduced and the axial width of the band is increased compared to the de-energised state. 
     Accordingly, in a third aspect, the present invention provides a shaft mounting assembly comprising: 
     an elongate shaft having a substantially circular cross-section; a cylinder having a bore housing the shaft and a side wall port communicating with the bore, wherein the inner diameter of the bore is larger than the outer diameter of the shaft such that a gap is defined between the outer surface of the shaft and the inner surface of the bore; and, wherein the bore comprises a groove extending around the circumference of the bore; 
     a spring comprising a substantially circular discontinuous band having correspondingly shaped axially arcuate inner and outer surfaces; 
     wherein the spring is positioned in the groove such that both axial edges of the band are located within the groove; and, wherein the spring has a de-energised state in which the inner diameter of the band is less than the outer diameter of the shaft and the axial width of the band is less than the width of the groove; and an energised state in which the spring is compressed within the bore such that the inner diameter of the band is increased and the axial width of the band is increased compared to the de-energised state. 
     The spring can be a stiff, high-rate spring to provide radial anti-vibration action, and also to provide a centralising effect between the shaft and the cylinder. 
     The spring of the present invention can provide radial centralising of the shaft within the cylinder, thus allowing any sealing rings to act only as seals. This can help to increase the longevity of the sealing rings. 
     Furthermore, the axially arcuate, or crescent, shape of the inner and outer surfaces of the band, combined with the material the band is made of, means that the spring is both axially and radially resilient and compliant (flexible). In turn, this can allow the spring to withstand higher axial and radial loads in use compared to sealing rings alone. 
     The outer surface of the shaft may comprise the groove, such that the groove extends around the circumference of the outer surface of the shaft. Alternatively, the inner surface of the cylinder may comprise the groove, such that the groove extends around the circumference of the inner surface of the cylinder. 
     The groove can comprise a recessed base, and two substantially parallel side faces arranged perpendicular to the base. The groove can comprise an opening, such as opposite the recessed base, such that the spring can protrude from the groove through the opening. The opening can be flush with the inner surface of the cylinder, or the outer surface of the shaft. 
     In the energised state, the spring can be compressed within the bore between the outer surface of the shaft and the inner surface of the cylinder. The portion of the band between the axial edges can protrude out of the groove in the de-energised and the energised states. In the energised state, the portion of the band can extend across the gap to make a single point of contact with the inner surface of the cylinder or the outer surface of the shaft, depending on whether the groove is located around the outer surface of the shaft or the inner surface of the cylinder respectively. The single point of contact can be located at the mid-point between the two axial edges of the band. 
     The maximum, or outer, diameter of the outer surface of the band may be greater than the inner diameter of the cylinder, when the groove extends around the outer surface of the shaft. Alternatively, the minimum, or inner, diameter of the inner surface of the band may be less than the outer diameter of the shaft, when the groove extends around the inner surface of the cylinder. 
     In the de-energised state, the axially arcuate surfaces of the band can have an arc measure (the arc angle) of from 60 degrees to 80 degrees relative to the radius of the arc. The arc angle may be around 70 degrees relative to the radius of the arc. 
     The band can have correspondingly shaped arcuate inner and outer surfaces along the longitudinal axis of the band. The outer surface of the band may be axially convex in shape and the inner surface of the band may be correspondingly axially concave in shape. Alternatively, the outer surface of the band may be axially concave in shape and the inner surface of the band may be correspondingly axially convex in shape. 
     The portion of the band which protrudes out of the groove may be the peak, or apex, of the outer surface of the band, when the groove is located around the outer surface of the shaft. The apex can be located at the mid-point between the two axial edges of the band. Alternatively, the portion of the band which protrudes out of the groove may be the trough of the inner surface of the band, when the groove is located around the inner surface of the cylinder. The trough can be located at the mid-point between the two axial edges of the band. 
     The inner and outer surfaces may have a circular arc shape. As the spring is energised by radial compression (due to compression between the outer surface of the shaft and the inner surface of the cylinder), the width of the band increases as the axial edges of the band move away from each other in an axial direction. 
     In the de-energised state, the narrower width of the band compared to the width of the groove means that there is sufficient space to allow the band to axially spread (i.e. the axial width of the band increases) as the spring is energised. As explained further below, in addition to a centralising effect, this can help to provide a radial restorative effect. 
     The band can have a substantially regular or uniform profile. In both the de-energised and energised states, the band can have a substantially constant thickness. 
     The thickness of the band may be at least 0.12 mm. The ratio of the width of the band (chord of the axially arcuate surfaces) to the thickness of the band may be approximately 30:1 in the de-energised state. 
     The aspect ratio of the axial width of the band (the chord of the axially arcuate surfaces) to the height of the arcuate surfaces (the sagitta) can be 6:1 in the de-energised state. The sagitta in the de-energised state is greater than the sagitta in the energised state (when the spring is compressed within the bore). 
     The aspect ratio of the width of the groove to the depth of the groove can be 10:1. In both the energised and de-energised states, the height of the band can be greater than the depth of the groove, such that at least a portion of the band can protrude out of the opening of the groove. 
     The width of the band can be less than the width of the groove in the energised state. 
     The discontinuous band can comprise a gap in its circumference, such that the ends of the band are spaced apart. As the spring is compressed within the bore, the ends of the band can be brought closer together to allow the band to contract around the shaft, or the ends of the band are spaced further apart to allow the band to expand within the bore. The gap can become smaller when compared to the de-energised state as the band contracts around the shaft. The gap can become bigger when compared to the de-energised state as the band expands within the bore. In use, when the spring is in the energised state, the gap can allow the spring to expand or contract due to thermal fluctuations within the assembly. 
     In some embodiments, the discontinuous band may be comprised of spring steel. However, it will be appreciated that any other suitable material can be used. The material preferably has a stiff, high spring rate. For example, the spring rate can be in the range of 1800 N/mm to 2100 N/mm. 
     The assembly can additionally comprise a further spring comprising a substantially circular and discontinuous band having correspondingly shaped axially arcuate inner and outer surfaces. The further spring can be positioned in the groove such that both axial edges of the band of the further spring are located within the groove. The spring and the further spring can be double banked, such that they are stacked on top of the other within the groove. The depth of the groove may be increased by one spring thickness to accommodate the stacked spring arrangement. The combined spring rate of the stacked springs can be in the range of 5500 N/mm to 6500 N/mm, for example 6000 N/mm. 
     The further spring can also comprise a gap in its circumference. The gap of the spring and the gap of the further spring can be aligned. Alternatively, the gap of the spring and the gap of the further spring can be circumferentially offset. 
     Using a single spring arrangement, a greater spring rate can typically be achieved by increasing in the spring thickness. However, a thicker spring will have a reduced range of radial movement because the stress in the spring can become too high. Using a stacked spring arrangement instead, the spring rate can be increased without having to make the individual springs thicker. This means that the range of radial travel is very similar to a single spring arrangement. Therefore, a stacked spring arrangement can provide a greater spring rate compared to a single spring arrangement, whilst maintaining the same range of radial travel as the single spring arrangement. 
     The shaft can have a further groove extending around the circumference of the outer surface of the shaft. Alternatively, the inner surface of the cylinder can have a further groove extending around the circumference of the inner surface of the cylinder. A sealing ring can be positioned within the further groove. 
     The cylinder can have a side wall port communicating with the bore. The groove can be located between the further groove and the side wall port. The outer surface of the shaft can comprise a groove and a further groove on both sides of the side wall port. Alternatively, the inner surface of the cylinder can comprise a groove and a further groove on both sides of the side wall port. Each groove can be located between the respective further groove and the side wall port. Each groove can contain a spring, as described herein. Each groove can also contain a further spring, as described herein. Each further groove can contain a sealing ring. 
     The side wall port can provide a supply of a bearing fluid to the gap. The assembly can be a static piston assembly. A static piston assembly can be defined as non-rotational and non-reciprocating. 
     Accordingly, in a fourth aspect, the present invention provides a method of assembling the assembly according to the first aspect of the invention, the method comprising the steps of: 
     installing the spring within the groove, by either:
         spacing apart the ends of the band, positioning the band around the shaft, and releasing the ends so that the band seats in the groove located around the outer surface of the shaft; or,   bringing together the ends of the band, positioning the band within the bore, and releasing the ends so that the band seats in the groove located around the inner surface of the cylinder; and,       

     inserting the shaft into the bore and compressing and energising the spring such that the ends of the band are moved relative to each other. 
     In embodiments where the band is positioned around the shaft, the step of inserting the shaft into the bore and compressing and energising the spring causes the ends of the band to be brought closer together. In embodiments where the band is positioned within the bore, the step of inserting the shaft into the bore and compressing and energising the spring causes the ends of the band to be moved further apart. 
     Due to the shape of the spring, the net radial force acting on the shaft will be zero, which helps to keep the shaft concentric with the bore, and also provide a radial restorative effect. If the shaft is moved off-centre in a given direction, the spring will act to re-centre the shaft within the bore, by increasing the reaction force in the given direction whilst reducing the force in the opposite direction. As such, the load on the spring may not be even around the circumference of the spring, but the net force will remain zero. When the groove is located around the outer surface of the shaft, the action of the spring may be referred to as outspringing. When the groove is located around the inner surface of the cylinder, the action of the spring may be referred to as inspringing. 
     The method may further comprise installing the further spring within the groove, by either: 
     spacing apart the ends of the band, positioning the band around the shaft, and releasing the ends so that the band seats in the groove, and the further spring is stacked on top of the spring; or bringing together the ends of the band, positioning the band within the bore, and releasing the ends so that the band seats in the groove, and the further spring is stacked on top of the spring. 
     In embodiments comprising a spring and a further spring stacked on top of the other within the groove, this may minimise the force that is required to re-centre the shaft within the cylinder, and provide a stronger restorative effect. 
     Whilst the invention has been described above, it extends to any inventive combination set out above, or in the following description or drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be performed in various ways, and an embodiment thereof will now be described by way of example only, reference being made to the accompanying drawings, in which: 
         FIGS.  1   a  and  1   b    show a side view and a sectional view of the spring, and the terminology used to describe various aspects of its geometry; 
         FIG.  2    shows a front view (A), side view (B), and cross-sectional view (C) of the spring; 
         FIG.  3    shows a longitudinal cross-section through one half of a cylinder having a bore housing a shaft, and comprising the spring; 
         FIG.  4    plots the radial deflection against the radial load of the spring of  FIG.  2    and different sealing rings; 
         FIG.  5    shows a cross-sectional view of a stacked spring arrangement; 
         FIGS.  6   a  and  6   b    show a side view and a sectional view of a further embodiment of the spring, and the terminology used to describe various aspects of its geometry; 
         FIGS.  7   a  and  7   b    show a perspective and an enlarged view of the  FIG.  6   a  and  b    spring; 
         FIGS.  8   a  and  8   b    show a perspective and an enlarged view of the  FIG.  1   a  and  b    spring; 
         FIG.  9    shows a longitudinal cross-section through one half of a cylinder having a bore housing a shaft, and comprising the  FIG.  7   a    spring and the  FIG.  8   a    spring. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The term “axial” as used herein is defined as in a direction parallel to the longitudinal axis of the spring, which extends through the centre of the band. For example, “axial width of the spring” is defined as the width of the spring extending along the longitudinal axis of the spring. 
     The term “radial” as used herein is defined as in a direction perpendicular to the longitudinal axis of the spring, and extends from the longitudinal axis to a point on the circumference of the band. 
       FIGS.  1   a  and  1   b    show a side view of a spring and a cross section through line A-A. The figures are labelled with various terminology used to describe the geometry of the spring herein. 
       FIG.  2    shows a de-energised spring  10  comprising a discontinuous band  11  having a substantially circular cross-section with diameter d 1  (seen in  FIGS.  1   a    and  2 (B)). In this example, the diameter d 1  may be around 28 mm. As also shown in  FIGS.  8   a  and  8   b   , the discontinuity is formed by creating a gap  12  in the band, such as by axially cutting the band. The gap  12  causes the ends of the band to be spaced apart. This allows the spring to be easily fitted around a shaft, such as a piston, by separating the two ends of the band, and releasing them around the shaft (see  FIG.  3   ). In this example, the band  11  is made of spring steel which is stiff, with a high spring-rate, and ensures that the band is sufficiently resilient to not be permanently deformed by such manipulation. It will be appreciated that other suitable materials can also be used. 
     The band  11  comprises an outer surface  14  and an inner surface  16 , as seen in  FIG.  2 (C) . The outer and inner surfaces  14 ,  16  of the band are correspondingly arcuately shaped. The outer surface  14  has an axially convex shape, and the inner surface has a corresponding axially concave shape (see  FIGS.  1   b    and  2 (C)). In this example, the arc length is around 4 mm. The width (or chord) of the axially arcuate surfaces, w, can be seen in  FIG.  2 (C) . The width in this example is around 3.8 mm. The height, or sagitta, of the arc is demonstrated in  FIG.  1   b   , and is around 0.66 mm when in a de-energised state. When in use, in an energised state, the height of the arcuate shape is around 0.5 mm. Thus, the axially arcuate shape of the band has an aspect ratio (the ratio of the chord to the sagitta) of around 6:1. 
     As can be seen in  FIG.  2 (B) , the band  11  also comprises a regular or uniform profile and a constant thickness. In this example, the thickness of the band  11  is around 0.127 mm. 
     As shown in  FIG.  2 (A)  and (C), the outer surface  14  at boundaries  18   a ,  18   b  has a first radial distance r 1 , that is the radial distance from the axis of the band to the boundaries of the band. The outer surface  14  at apex  20  has a second radial distance r 2 , that is the radial distance from the axis of the band to the apex of the band. 
     The second radial distance r 2  is greater than the first radial distance r 1 . This defines the arcuate shape, with the apex  20  positioned at the mid-point between the boundaries  18   a ,  18   b  of the band. The difference between r 2  and r 1  defines the sagitta. The arcuate shape may also be referred to as a crescent shape. The outer surface  14  and the inner surface  16  have corresponding arcuate shapes. The (outer) diameter d 1  is defined by the apex  20  of the outer surface  14 . 
     In embodiments, a stacked spring arrangement  100  may be provided (see  FIG.  5   ). The stacked spring arrangement  100  comprises spring  10  and a further spring  50 . Spring  50  is identical to spring  10 , both structurally and functionally. As shown in  FIG.  5   , spring  10  and further spring  50  are stacked on top of each other. Spring  10  may be referred to as the inner spring, and further spring  50  may be referred to as the outer spring. Alternatively, the springs may be switched, such that spring  10  is the outer spring, and further spring  50  is the inner spring. 
     One use for spring  10 , or stacked arrangement  100 , is within a shaft mounting assembly  30 , part of which can be seen in  FIG.  3   . Specifically,  FIG.  3    shows a cross-section through one half of an elongate shaft  32  located within the bore of a cylinder or housing  35 . The shaft mounting assembly  30  can be non-reciprocating and non-rotational. That is, the shaft  32  and cylinder  35  are not intended to move relative to each other. The shaft mounting assembly  30  may be a static piston assembly. 
     The cylinder  35  comprises a side wall port  36 . The inner diameter of the cylinder  35  is larger than the outer diameter of the shaft  32 , thus creating a gap  40  between the outer surface of the shaft  32  and the inner surface of the cylinder  35 . The gap  40  is filled with a fluid  42 , such as oil. In use, the layer of fluid  42  between the cylinder  35  and the shaft  32  is supplied via the port  36 , and provides an anti-vibration damping effect, which in turn can help to minimise the vibrations and noise transmitted through the assembly when in use. 
     The shaft  32  has a substantially circular cross-section, and comprises two longitudinally spaced apart grooves  34 , and two further longitudinally spaced apart grooves  37 , around the circumference of the outer surface of the shaft  32 . The grooves  34  each contain a spring  10 , as described above. The spring  10  is entirely located within groove  34 , such that both axial edges of the spring  10  (boundaries  18   a ,  18   b ) are located within the groove  34 . Alternatively, the grooves  34  can comprise a stacked spring arrangement  100 , comprising the spring  10  and a further spring  50 . Both the spring  10  and the further spring  50  are entirely located within groove  34 , such that both axial edges of the spring  10  and the further spring  50  are located within the groove  34 . 
     The grooves  34  comprise a recessed base, two substantially parallel side faces arranged perpendicular to the base, and an opening located opposite the recessed base through which the spring  10  can protrude from the groove  34 . The opening is flush with the outer surface of the shaft  32 . 
     The grooves  37  each contain a seal in the form of an O-ring  38 . As can be seen in  FIG.  3   , the springs  10  (and grooves  34 ) are located between the O-rings  38  (and grooves  37 ) and the port  36 . 
     Conventionally, the O-rings alone would provide the damping and centralising effects, as well as providing the required sealing effect. The springs of the present invention carry the radial load so that the O-rings can act predominantly as seals, whilst the springs act as centralisers. This can help to prolong the lifespan of the O-rings. 
     The width (or chord), w, of the band  11  is less than the width g 1  of groove  34 . Whilst in the de-energised state, the height of the arc is greater than the depth of the groove, such that the apex  20  of the band protrudes out of the opening of groove  34 . There is also a gap between one or both side edges of the groove  34 , and one or both axial edges (boundaries  18   a ,  18   b ) of the band. The gap allows the band to axially expand when compressed and energised in use. In embodiments, the width of the groove may be 3 mm, and the width of the band (namely, the chord of the axially arcuate surfaces) may be less than 3 mm (in both the energised and de-energised states). When energised, the width of the band  11  remains less than the width of the groove  34 . 
     In the de-energised state, diameter d 1  defined by the apex  20  of the outer surface  14  is greater than the inner diameter of the cylinder. When energised, the sagitta/height of the arc remains greater than the height of the groove, such that the apex  20  of the band protrudes out of the opening of groove  34  and into and across the gap  40 . The outer surface  14  of the band then makes a single point of contact with the inner surface of the cylinder  35  (defining the bore), which helps the springs  10  to provide the required centralising effects. Furthermore, the springs  10  can provide an element of sealing, although in this embodiment, this is not their primary purpose. 
     The above description also applies to stacked arrangement  100 . The width of the band of further spring  50  may also be less than 3 mm (in both the energised and de-energised states). In embodiments utilising the stacked spring arrangement  100 , the apex of the band of the outer spring (further spring  50  in  FIG.  5   ) protrudes out of the opening of groove  34  and into the gap  40 . The outer surface of the band then contacts the inner surface of the cylinder  35 , which helps the stacked spring arrangement  100  to provide the required centralising effect. Furthermore, the depth of the groove  34  is increased by around one spring thickness compared to the groove depth for spring  10  alone in order to accommodate the further spring  50 . 
       FIG.  4    shows some test results obtained by the Applicant which show the relationship between the radial deflection of the spring  10  and O-rings  37  and the radial load applied. Both O-rings tested were manufactured by Rhondama, and comprise different spring rates (300 Nmm −1  and 600 Nmm −1 ). The spring  10  was manufactured by the Applicant. The graph clearly shows that springs  10  are able to withstand higher radial loads compared to O-rings alone. Therefore, the springs  10  are much better suited to providing the required centralising effects for the shaft compared to O-rings alone. By allowing the springs  10  to take most of the radial load, the O-rings are much more effective at providing the required sealing. 
     In use, the cylinder  35  and shaft  32  are assembled by firstly installing a spring  10  within each groove  34 , by spacing apart the ends of the band and releasing them around the respective groove  34 . At this point, a further spring  50  may also be installed within one or both of the grooves  34 , by spacing apart the ends of the band and releasing them around the respective groove  34 , such that spring  50  sits on top of spring  10  within the groove  34 . 
     The following process will be described with regard to the single spring arrangement comprising spring  10  within each groove  34 , but the process will also apply to the stacked spring arrangement  100  comprising spring  10  and further spring  50  within each groove  34 . 
     At this point springs  10  are de-energised, and there is a gap between the axial edges of the band (boundaries  18   a ,  18   b ) and the edges of the respective groove  34 . 
     The shaft  32  is then inserted into the cylinder  35  (from left to right in  FIG.  3   ). As the first spring  10  (the rightmost spring in  FIG.  3   ) enters the bore, it is deformed and compressed, and becomes energised. In the energised state, the ends of the band  11  are brought together around the shaft  32 . As the band is compressed between the outer surface of the shaft and the inner surface of the cylinder, the height of the band is reduced, and the edges of the band (boundaries  18   a ,  18   b ) move away from each other in the axial direction and towards the edges of the groove, and the spaced apart ends of the band are brought closer together around the shaft. However, at no point does the chord distance between  18   a  and  18   b  become equal to the groove width g 1 , that is the band arcuate shape is not controlled axially by the groove  34 , and the chord distance between  18   a  and  18   b  is always less than g 1 . The rightmost spring  10  is held in the energised position between the shaft  32  and the cylinder  35 . The apex  20  of the spring  10  makes a single point of contact with the inner surface of the cylinder  34 . 
     As the shaft  32  moves further into the bore, the rightmost spring  10  has to move past radial port  36 , which provides a side opening to the cylinder  35 . There may be up to four radial ports, set 90 degrees apart around the circumference of the cylinder  35 . As the rightmost spring  10  moves past port  36 , it can relax and at least partially de-energise. However, when the rightmost spring  10  reaches the other side of the port, it is again deformed and compressed, and becomes fully re-energised. 
     Conventionally shaped rings are more likely to catch on the corner where the port  36  meets the remainder of the cylinder  35 , thus making assembly more difficult. The axially arcuate shape of spring  10 , along with its resilience and compliance, makes assembly easier and more efficient. 
     As the shaft  32  moves even further into the cylinder  35 , the second or leftmost spring  10  of  FIG.  3    will eventually enter the bore, and will deform and compress, and become energised. The leftmost spring  10  does not move past radial port  36 . 
     When the shaft  32  is fully inserted into the bore of the cylinder  35 , a spring  10  is located on either side of radial port  36 . Both springs  10  are held in the energised position between the shaft  32  and the cylinder  35 . 
     Once the shaft  32  is fully inserted into the bore of the cylinder  35 , the gap  40  is then filled with oil  42  via port  36 . In use the fluid acts as a bearing. 
     In the energised position, the spring  10  exerts a net zero radial force against the inner surface of the cylinder  35 , which in use centres the shaft  32  within the cylinder  35 . The energised spring  10  also provides a radial anti-vibration effect, along with the oil  42  (which also acts as a bearing). 
     The net force acting on the shaft is zero, which helps to keep the shaft  32  centralised within the cylinder  35  (that is, the shaft and cylinder are concentric), and also provide a radial restorative effect. If the shaft is moved off-centre in a given direction, the spring  10  provides the required force to re-centre the shaft  32  within the cylinder  35 , by increasing the reaction force in the given direction whilst reducing the force in the opposite direction. As such, the load on the spring  10  may not be even around the circumference of the spring  10 , but the net force will remain zero. The spring  10  can react to approximately 1000 N/mm of force in the radial direction, with a maximum displacement of around 0.1 mm (as determined by the radial clearance provided by the gap  40 ). 
     It is therefore no longer the responsibility of the O-rings to provide a damping and centralising effect, and as such they can focus on sealing the shaft  32  in the cylinder  35 . In this embodiment, the spring  10  does provide an element of sealing, which can help to further increase the longevity of the O-rings by reducing the amount of hot oil  42  reaching the O-rings. 
       FIGS.  6   a ,  6   b ,  8   a ,  8   b    and  9  show a further embodiment of the spring  200 . The structural and functional features described above with respect to spring  10  also apply to spring  200 , with any differences being highlighted below. 
     The spring  200  is essentially an inverted version of spring  10 , that is the outer surface  214  has an axially concave shape, and the inner surface  216  has a corresponding axially convex shape. The inner surface  214  at boundaries  218   a  and  218   b  has a larger radial distance than the inner surface  214  at trough  220 , which defines the arcuate shape. The trough  220  is positioned at the mid-point between boundaries  218   a  and  218   b  of the band  211 . 
       FIGS.  6   a  and  6   b    show a side view of the spring  200  and a cross-section through line A-A. The figures are labelled with various terminology used to describe the geometry of the spring  200 .  FIGS.  7   a  and  7   b    show a de-energised spring  200  comprising a discontinuous band  211  having a substantially circular cross-section with (inner) diameter d 2  defined by the trough  220  of the inner surface  216 . The discontinuity is formed by creating a gap  212  in the band, which causes the ends of the band  211  to be spaced apart. This allows the spring to be easily fitted within a bore, by compressing the two ends of the band together, inserting the band  211  into the bore of a cylinder and releasing the band  211  so it seats in a groove  234  extending around the inner surface of the cylinder (see  FIG.  9   ). 
     Spring  200  can be used in a shaft mounting assembly as shown in  FIG.  9   . Specifically,  FIG.  9    shows a cross-section through one half of an elongate shaft  32  located within the bore of a cylinder or housing  35 . The bore defined by the inner surface of the cylinder  35  has a substantially circular cross-section. The inner surface of the cylinder  35  comprises a groove  234  extending around the circumference of the inner surface of the cylinder  35 . The groove  234  is the same as groove  34 , except it is located around the inner surface of the cylinder  35  instead of around the outer surface of the shaft  32 . The groove  234  contains a spring  200 , which is entirely located within groove  234 , such that both axial edges of the spring  200  (boundaries  218   a ,  218   b ) are located within the groove  234 . 
     Similar to  FIG.  3   , the inner diameter of cylinder  35  is larger than the outer diameter of the shaft  32 , thus creating a gap  40  between the outer surface of the shaft  32  and the inner surface of the cylinder  35 . Diameter d 2  defined by the trough  220  of the inner surface  216  of the band  211  is less than the outer diameter of the shaft  32 . In use, the shaft  32  is inserted into the cylinder  35 , and as the shaft  32  passes across spring  200 , the spring  200  is deformed and compressed, and becomes energised. As the band is compressed between the outer surface of the shaft  32  and the inner surface of the cylinder  35 , the height of the band  211  is reduced, and the edges of the band (boundaries  218   a ,  218   b ) move away from each other in the axial direction and towards the edges of the groove, and the spaced apart ends of the band are moved further apart within the bore. The trough  220  of the spring  200  protrudes out of the opening of the groove  234  and extends across the gap  40  to make a single point of contact with the outer surface of the shaft  32 . 
     For comparison,  FIG.  9    additionally shows a groove  34  located around the shaft  32  and comprising a spring  10 , as described above. Although spring  200  is an inverted version of spring  10 , it functions in exactly the same way as spring  10  in both the de-energised and energised states. In embodiments, a stacked spring arrangement may be provided, as described above, which comprises spring  200  and a further spring stacked on top of each other. 
     Although the invention has been described above with reference to different embodiments of the invention, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. For example, in some embodiments, the corner where the port meets the remainder of the cylinder may have an angled/tapered surface, such as a chamfered surface. Such an angled surface can assist with assembly, and help to return the spring to the energised position between the shaft and the cylinder, after passing the port.