Scroll pump and anti-rotation device for a scroll pump

Abstract

A scroll pump anti-rotation device for resisting relative rotational movement between two scrolls 20, 22, the anti-rotation device comprising a body portion 53 from which two pairs of arms 56, 58 exte

Description

FIELD OF THE INVENTION The field of the invention relates to scroll pumps and to scroll pump anti-rotation 5 devices. BACKGROUND A scroll pump comprises two interleaving scrolls one of which has an orbital motion with respect to the other thereby trapping and pumping or compressing 10 pockets of fluid between the scrolls. In some cases, one of the scrolls is fixed, while the other is mounted on a drive shaft with an eccentric cam such that it orbits eccentrically without rotating. Another method for producing the relative orbiting motion is by co-rotating the scrolls, in synchronous motion, but with offset axes of rotation. Thus, the two scrolls are mounted on parallel shafts and the 15 relative motion is the same as if one were orbiting and the other stationary. In the case of fixed and orbiting scrolls an anti-rotation device may be used connected to the scrolls to resist relative rotation between them and thereby allowing the radial clearances to be accurately maintained as the scrolls pump. 20 The anti-rotation device should resist rotational movement but also allow the relative orbiting motion required for the pumping. In the case of co-rotating scrolls, such a device can be used to transmit a drive force from one scroll to the other, in a similar way to a pair of gears or a drive belt and pulleys. 25 A bellows arrangement formed from a crimped pipe has been used as an anti-rotation device in a fixed and orbiting scroll pump. This arrangement is located on the orbiting scroll side of the pump and resists rotation of the orbiting scroll but is sufficiently flexible to allow the orbiting motion. 30 A drawback of such a device is that the bellows need to be quite long to limit stress in the bellows below fatigue limits, and thus, pumps with bellows are physically quite big for their displacement. An alternative more compact anti rotation arrangement is disclosed in WO2011 / 135324. The anti-rotation device here comprises a body portion from the outer surface of which two perpendicular pairs of arms extend. 5 It would be desirable to provide a scroll pump with a scroll pump anti-rotation device which is both compact and effective. SUMMARY One aspect provides a scroll pump anti-rotation device for resisting relative io rotational movement between two scrolls, said anti-rotation device comprising: a body portion from which two pairs of arms extend, one of said pairs of arms being configured for connection in a fixed relation to one of the two scrolls and the other of said pairs of arms being configured for connection in a fixed relation to the other of the two scrolls, one of said pairs of arms being configured to flex to 15 allow movement of the one scroll relative to the other scroll in a first direction and the other of said pairs of arms being configured to flex to allow movement of the one scroll with respect to the other in a second direction generally orthogonal to said first direction; wherein at least one of said pairs of arms is mounted within said body portion. 20 It was recognised that a more compact scroll pump might be desirable. The scroll pump of WO2011 / 135324 comprises a lightweight anti-rotation device that comprises two sets of orthogonal arms that extend outside of a main chassis or frame portion. This arrangement was substantially more compact than previous 25 anti-rotation devices. However, the inventors of the present invention recognised that the anti-rotation device might be made smaller and yet still function well and in some cases better, if at least one pair of the arms was taken within the body portion. 30 In some embodiments, said two scrolls are a fixed scroll and an orbiting scroll and said at least one of said pairs of arms mounted within said body portion is connected in a fixed relation to said orbiting scroll. It was also recognised that when reducing the size of a scroll pump, it might be particularly advantageous if the size and weight of any components that orbit could be reduced as this would decrease the unbalanced weight in the pump that 5 such components contribute to. Taking the arms that are connected in a fixed relation to the orbiting scroll within the body portion, not only allows the size of the anti-rotation device to be reduced, but also allow the arms connected to the orbiting scroll to be longer without a corresponding increase in the size of the orbiting scroll. In this regard owing to the circular cross section of the orbiting 10 scroll a longitudinal dimension passing through or close to its centre has a length similar to that of the scroll’s diameter, this longitudinal dimension decreases away from the centre. Thus, putting the arms within the body portion means that they attach to the orbiting scroll closer to its centre where it is longer, allowing the arms themselves to be longer. Longer arms experience reduced stress as they 15 bend and this allows them to be made with reduced thickness. In some embodiments, said body portion comprises a hollow frame-like structure comprising four sides each side being arranged generally perpendicular to an adjacent side. 20 In some embodiments, said arms in a respective pair of arms are mounted adjacent and parallel to opposing sides of said hollow frame structure. In some embodiments an end of each of said arms in said pair of arms that are 25 attached to said body portion are attached to said body portion at or close to opposing ends of a same side. The arms of the anti-rotation device may comprise elongate members, that are configured to flex in a direction perpendicular to their length and resist movement 30 along their length. In some embodiments the pairs of arms are arranged on opposite sides of a body portion, arms within a pair being substantially parallel to each other, and arms of the different pairs being substantially perpendicular to each other. One pair allows movement of the orbiting scroll relative to the housing in a first direction and the other pair allows movement of the orbiting scroll relative to the housing in a second direction generally orthogonal to the first direction. 5 In some embodiments, said scrolls comprise a fixed scroll and an orbiting scroll, said pair of arms attached in a fixed relation to said fixed scroll being configured to flex and allow movement in a substantially linear direction, said body portion following said movement in said substantially linear direction. 10 As the orbiting scroll orbits the arms attached to orbiting scroll flex in one direction but resist movement in a direction perpendicular to this, so the body portion does not follow the movement of the orbiting scroll in the linear direction that the arms attached to the orbiting scroll flex in, but does follow it in the 15 perpendicular direction to this, which is the direction that the other arms flex in. In some embodiments, said frame-like structure comprises curved inner and outer corners between adjacent sides. This provides increased stiffness in the plane of the anti-rotation device. 20 In some embodiments, said arms are distanced from said body portion by a distance that is between 25 and 60% of a width of one of said sides of said body portion. 25 The arms are arranged close to the sides of the body portion allowing them to be distanced from each other while not unduly increasing the size of the anti-rotation device. In some embodiments, said sides of said body portion comprise substantially a 30 same width. There are different forces that are felt by the different portions of the anti-rotation device, however, forming the anti-ration device with sides of substantially equal width may have advantages in the overall balance of the pump and in manufacturability. Widths are considered to be substantially the same if they are 5 within 10% of each other. In some embodiments, said outer arms extend from two of said outer curved corners, said two outer curved corners being adjacent curved corners. io In some embodiments said inner arms extend from close to adjacent inner corners. In some embodiments, said body portion comprises a plurality of recesses forming pockets wherein a thickness of said body portion at said pockets is 15 thinner than at other regions of said body portion. The body of the anti-rotation device undergoes reciprocal motion during operation and this can lead to imbalances during operation. It is therefore advantageous if the body portion has a low weight. Forming recesses in the body portion can 20 reduce the weight while with careful design, not unduly reducing the strength of the device. Furthermore, less material is required and where the anti-rotation device is injection moulded the paths that the plastic flow along are paths that are reduced in width which reduces shrinkage on cooling. 25 The recesses may extend from opposing surfaces and form substantially symmetrical pockets about a mid-point. The recesses may define paths of a full thickness of said anti-rotation device, said paths having a width that varies by less 30%. 30 The recesses may be configured such that some of said full thickness paths runs around an inner and an outer perimeter of said body portion and others of said full thickness paths run between said inner and outer perimeters between said recesses. The recesses may be configured such that alongside walls of said body portion 5 said recesses comprise alternately orientated triangles. In some embodiments, said anti-rotation device is made of plastic. Plastic is a robust and yet lightweight material, it is also easy to manufacture in a 10 desired shape and with any required surface recesses using for example, injection moulding techniques. In some embodiments, one end of each of said arms is connected to said body portion and the other end comprises at least one mounting means for receiving a 15 respective fixing means for fixing said arm in a fixed relation to a respective one of said two scrolls, each of said mounting means comprising a metal bushing acting as an intermediate element between said mounting means and said fixing means. 20 The fixing means may experience substantial forces during mounting on the pump and during operation and where they are for example made of plastic some distortion may result. Embodiments may provide metal bushings as an intermediary between the pump and the anti-rotation device and this can inhibit distortion and render the anti-rotation device more robust. 25 A further aspect provides a scroll pump comprising: a motor; two interleaving scrolls mounted such that rotation of said motor imparts an orbiting motion to one scroll with respect to the other scroll; and anti-rotation device according to a first aspect, one pair of arms being connected in a fixed relation to one scroll and the 30 other pair of arms being connected in a fixed relation to the other scroll. ln some embodiments, the one scroll is a fixed scroll and the other scroll is an orbiting scroll. In some embodiments, the scroll pump comprises a drive shaft configured to be 5 driven by said motor and comprising a concentric shaft portion and an eccentric shaft portion, said eccentric portion being connected to said orbiting scroll; wherein said fixed scroll comprises an opening through which said shaft extends, said shaft being connected to said orbiting scroll on an opposing side of the fixed scroll to the motor, a high vacuum region being located on an orbiting scroll side 10 of the scroll arrangement and a low vacuum region being located generally on a fixed scroll side of the scroll arrangement, said anti-rotation device being located in the high vacuum region for resisting rotation of the orbiting scroll and allowing said orbiting motion, and a bearing arrangement for supporting rotation of the concentric shaft portion and eccentric shaft portion being located in the low 15 vacuum region. 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 20 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. 25 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 shows a scroll pump having an anti-rotation device according to an 30 embodiment; and Figure 2 shows an anti-rotation device according to an embodiment; Figure 3 shows the anti-rotation device in place on a scroll pump; Figure 4 shows a further example of an anti-rotation device according to an embodiment; and Figure 5 shows the bushings that may be used between the anti-rotation device and fixing means for fixing the device to the pump. 5 DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided. io An anti-rotation device is provided that is lubricant free and thus, can be arranged in the high vacuum portion of the pump. The arrangement is a compact arrangement allowing it to be used with a reduced sized pump. Figure 1 shows a scroll pump 10 according to an embodiment Scroll pump 10 15 comprises a pump housing 12 from which a fixed scroll 22 extends and which has a cap portion 18. Scroll pump 10 has a drive shaft 14 driven by a motor 18 with an eccentric shaft portion 16 which may be in the form of a cam sleeve. The eccentric shaft portion 16 of the drive shaft is connected to an orbiting scroll 20 so that rotation of the shaft imparts an orbiting motion to the orbiting scroll 20 20 relative to the fixed scroll 22 for pumping fluid along a fluid flow path between a pump inlet 24 and a pump outlet 26. The fixed scroll is shown generally on the left and the orbiting scroll on the right. In this arrangement the fixed scroll comprises an opening through which the shaft 25 14, 16 extends. The shaft is connected to the orbiting scroll 20 on an opposing side of the fixed scroll to the motor 18. A high vacuum region 30 is located at the inlet 24 and a low vacuum or atmospheric region is located adjacent to the outlet 26. A first bearing 34 supports a concentric portion of the drive shaft for rotation. A second bearing 36 supports the eccentric portion 16 of the drive shaft relative 30 to the orbiting scroll 20 allowing angular movement of the orbiting scroll relative to the eccentric portion. The two bearings are located in the lower vacuum / higher pressure region of the pump. A counterbalance 44 balances the weight of the orbiting components of the pump including the orbiting scroll, the second bearing 36 and the eccentric portion of the drive shaft. An anti-rotation device 50 in the form of a “frog” is located in the 5 high vacuum region 30 of the pump. The anti-rotation device 50 comprises arms connected to the orbiting scroll and other arms connected in a fixed relation to the fixed scroll. The anti-rotation device 50 of an embodiment is shown in more detail in Figure 2 but essentially has a body portion and a plurality of arms extending from the body. The arms are arranged in two opposing pairs, one of io the pairs being connected in a fixed relation to the fixed scroll and one of the pairs being connected in a fixed relation to the orbiting scroll. Rotation of the motor causes the orbiting scroll to orbit and the arms connected in a fixed relation to the fixed scroll resist movement in a direction along the length of the arms and allow it in a direction perpendicular to the arms, the arms flexing to move 15 substantially in this direction, such that the body portion of the anti-rotation device moves in a linear reciprocating manner in phase with the rotation of the motor. Figure 2 shows an anti-rotation device according to an embodiment. The anti-rotation device is formed of plastic and has a body portion 53 with a frame like 20 structure having two pairs of opposing side walls or struts 53A and 53B that are perpendicular to each other and form a generally rectangular frame structure. There are two pairs of arms 56 and 58. One pair 58 is configured to be fixed in a fixed relation to the orbiting scroll, generally to the orbiting scroll itself and the other pair 56 is configured to be fixed in a fixed relation to the fixed scroll, that is 25 to the scroll itself or to the pump housing. Arms 56 are arranged adjacent to and generally parallel with the side walls 53A, while arms 58 are arranged adjacent to and generally parallel with the side walls 53B. In this embodiment the pair of arms 58 are arranged inside the body portion 53 of 30 the anti-rotation device, making the device more compact than were the arms outside of the body portion. This allows the device to be used on a reduced sized pump, while still providing effective resistance to rotational movement. In this embodiment the inner arms are configured for connection to the orbiting scroll, and this allows them to be longer as they are closer to the inner portion of the scroll which has a larger diameter than at the edge (see figure 3). 5 Each arm has two apertures 57 that act as mounting means for receiving fixing means such as a bolt or screw to fix the arm in a fixed relation to the respective scroll. The apertures each comprise a metal bushing 59 which acts to protect the aperture from distortion due to the pressure and relative movement of the fixing means. io Figure 3 shows an anti-rotation device or frog 50 mounted to a scroll pump. Anti-rotation device 50 has inner arms 58 that are attached to orbiting scroll 20. As can be seen as they are within the frame or body portion of the anti-rotation device 50 so that they attach to the orbiting scroll closer to a centre line of the 15 orbiting scroll than they would were they outside the frame. Here the diameter is larger than it is further from the centre line towards the edge of the scroll. This allows the arms to be longer. Longer arms experience reduced stress as they bend and this allows them to be made with reduced thickness. 20 Outer arms 56 are attached in a fixed relation to the fixed scroll. They may be attached to the fixed scroll itself or to the housing. In this embodiment, they are attached to the housing 12. Figure 4 shows an anti-rotation device or frog 50 according to an embodiment. 25 As can be seen, the inner and the outer comers of the frog are curved and the outer arms 56 extend from the outer curved surfaces. In this embodiment, the width T of the body portion sidewalls is substantially the same for each sidewall. In other embodiments the horizontal and vertical 30 sidewalls may have different thicknesses. The gap G between the arms and the side walls is also substantially the same for each of the arms and is between 25 and 60% of the width of the side walls. In this embodiment, the height H of the frog is slightly longer than the width W of the frog, being between 8% and 20% longer. Example dimensions are a gap G of between 3 and 5 mm and a width T of the 5 sidewalls of between 7 and 9mm. The width W of the entire frog may be between 125 and 130mm while the height H may be between 117 and 123mm. In this embodiment, there are recesses or pockets 54 in the plastic material that forms the frog and these are there to reduce the weight and are shaped both to io retain rigidity and to allow the plastic to flow freely during the injection moulding process. In this example they are substantially triangular in shape and are arranged in different orientations such that adjacent pockets may be seen as being 180° rotated with respect to each other. In this example the recesses or pockets at the corners have a different shape and this is for ease of 15 manufacturability and to encourage flow of the plastic material during injection moulding. Figure 5 shows the metal bushings which may be applied between the mounting apertures or mounting means 57 within the arms and the fixing means such as a 20 bolt or a screw for fixing the frog to the scroll pump. These metal bushings 59 extend beyond the thickness of the mounting apertures and protect them from distortion due to the forces during the actual mounting process and during operation. They may be press fitted in to the holes at the end of the arms, but are advantageously over-moulded that is placed within the mould during the 25 injection moulding process. 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 30 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 10 scroll pump 12 housing 14 shaft 5 16 crank sleeve 18 cap 19 motor 20 orbiting scroll 22 fixed scroll io 24 inlet 26 outlet 30 high vacuum region 34, 36 bearings 44 rotational counterbalance 15 50 frog 53 body portion 53A, 53B sidewalls 54 recesses 56 arms for connection to fixed scroll 20 57 mounting means 58 arms for connection to orbiting scroll 59 metal bushing 25

Claims

1. A scroll pump anti-rotation device for resisting relative rotational movement between two scrolls, said anti-rotation device comprising:5 a body portion from which two pairs of arms extend, one of said pairs ofarms being configured for connection in a fixed relation to one of the two scrolls and the other of said pairs of arms being configured for connection in a fixed relation to the other of the two scrolls, said one of said pairs of arms being configured to flex to allow movement of said one of the two scrolls relative to the io other of the two scrolls in a first direction and the other of said pairs of arms being configured to flex to allow movement of said one of the two scrolls with respect to the other of the two scrolls in a second direction generally orthogonal to said first direction;wherein said one of said pairs of arms is mounted within said body portion 15 and the other of said pairs of arms is mounted outside said body portion.

2. A scroll pump anti-rotation device according to claim 1, wherein the arms comprise elongate members that are configured to flex in a direction perpendicular to their length and resist movement along their length.

203. A scroll pump anti-rotation device according to claim 1 or claim 2, wherein, in each pair, the arms are arranged on opposite sides of the body portion.

4. A scroll pump anti-rotation device according to any preceding claim, 25 wherein, in each pair, the arms are parallel to each other.

5. A scroll pump anti-rotation device according to any preceding claim, wherein the arms of said one of said pairs of arms are perpendicular to each other.

306. A scroll pump anti-rotation device according to any preceding claim, wherein said body portion comprises a plurality of recesses forming pocketswhere a thickness of said body portion is thinner than other regions of said body portion.

7. A scroll pump anti-rotation device according to claim 6, wherein said5 recesses and outer edges of said body portion define paths of a full thickness of said anti-rotation device, said paths having a width that varies by less 30%.

8. A scroll pump anti-rotation device according to claim 6 or claim 7, wherein said recesses extend from opposing surfaces and form substantially symmetrical io pockets about a mid-point.

10. A scroll pump anti-rotation device according to any preceding claim, wherein said anti-rotation device is made of plastic.15 11. A scroll pump anti-rotation device according to any preceding claim,wherein one end of each of said arms is connected to said body portion and the other end comprises at least one mounting means for receiving a respective fixing means for fixing said arm in a fixed relation to a respective one of said two scrolls, each of said mounting means comprising a metal bushing acting as an20 intermediate element between said mounting means and said fixing means.

12. A scroll pump comprising: a motor;two interleaving scrolls mounted such that rotation of said motor imparts25 an orbiting motion to one scroll with respect to the other scroll; andan anti-rotation device according to any preceding claim, said one of said pair of arms being connected in a fixed relation to said one scroll and the other of said pair of arms being connected in a fixed relation to the other scroll.30 13. A scroll pump according to claim 12, wherein the one scroll is an orbitingscroll, and the other scroll is a fixed scroll.

14. A scroll pump according to claim 13, comprising a drive shaft configured to be driven by said motor and comprising a concentric shaft portion and an eccentric shaft portion, said eccentric portion being connected to said orbiting scroll; wherein5 said fixed scroll comprises an opening through which said shaft extends,said shaft being connected to said orbiting scroll on an opposing side of the fixed scroll to the motor, a high vacuum region being located on an orbiting scroll side of the scrolls and a low vacuum region being located generally on a fixed scroll side of the scrolls, said anti-rotation device being located in the high vacuumio region for resisting rotation of the orbiting scroll and allowing said orbiting motion, and a bearing arrangement for supporting rotation of the concentric shaft portion and eccentric shaft portion being located in the low vacuum region.16

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