Seal

Abstract

A scroll pump comprises of two scrolls that engage with one another when in operation. Each of the scrolls comprise of a base and a spiral wall defining a spiral channel. A channel seal 10 is arranged

Description

FIELD OF THE INVENTION The present invention relates to channel seals for scroll pumps. BACKGROUND Scroll pumps are a known type of pump used in various industries to pump fluid using the relative motion of two intermeshed scrolls. Each scroll includes a base from which a spiral wall extends to define a spiral channel. Scroll pumps include seals to prevent leakage of fluid from various parts of the scroll pump. Seals may include tip seals, positioned at the tip of the spiral wall, and channel seals, positioned at the base of the spiral channel. Channel seals have advantages in compactness and sealing performance over tip seals; however, challenges arise when matching the channel seals to the scrolls. Typically, a scroll pump includes a channel seal positioned at the base of each spiral channel. The height of the channel seals must match the scrolls to achieve sufficient sealing. For example, if one channel seal is too thick it will push the scrolls apart slightly, resulting in a small leakage path across the other channel seal. Since it is difficult to machine the channel seals and scrolls accurately to the tolerances required, a period of operation is required to wear the channel seals down until a near perfect match, and thus optimal sealing, is achieved. This period of operation is herein referred to as bedding-in. The present invention provides a channel seal optimised to speed up bedding-in, such that channel seals are matched after a shorter running period. SUMMARY OF THE INVENTION According to a first aspect, the present invention provides a channel seal for a scroll pump. The channel seal comprises a surface which is subject to wear during operation of the scroll pump, wherein at least a portion of the surface is textured. A scroll pump typically comprises two intermeshing scrolls, each having a base from which a spiral wall extends to define a spiral channel. In use, the channel seal is positioned on the base of a spiral channel of one of the scrolls to provide a seal between the base of that scroll and the tip of the spiral wall of the other scroll. Bedding-in is required to wear down the channel seal such that the height of the channel seal matches the scrolls. In other words, the channel seal is worn down until the height of the channel seal is equal to the spacing / axial offset between the base of the scroll and the tip of the wall of the other scroll. The textured portion of the surface acts to reduce the amount of material required to be removed or worn-away from the channel seal during bedding-in. In other words, the textured portion of the surface has a lower density than a comparatively smoother surface. This advantageously reduces the bedding-in time, since the textured portion of the surface will wear more quickly than a comparatively smoother surface. The textured portion of the surface may be provided by a pattern of ridges. The pattern of ridges may be regular or irregular. Alternatively, the textured portion of the surface may be provided by an array of regular or irregular protrusions, such as but not limited to pillars and / or depositions. Alternatively, the textured portion of the surface may be provided by an array of cavities, for example in the case of a foam or fibrous mesh. The pattern of ridges may comprise at least a first set of parallel ridges extending in a first direction and a second set of parallel ridges extending in a second direction at an angle to the first direction. Preferably, the first direction is substantially perpendicular to the second direction, such that the first and second sets of parallel ridges form a substantially grid-like pattern of ridges. The grid-like pattern of ridges may form a plurality of substantially square or rectangular pockets within the textured portion. Such a grid-like pattern of ridges advantageously simplifies the manufacture process of the channel seal, which will be explained in more detail in later aspects of the present invention. Furthermore, a pattern of intersecting ridges, whether grid-like or otherwise, advantageously acts to minimise leakage in multiple directions. In other words, leakage is prevented across the pockets formed between adjacent intersecting ridges. It should be appreciated that the pattern of ridges may include more than two sets of parallel ridges (for example, three, four, five, etc.). The ridges of each set of parallel ridges may follow any path along their length, for example straight, curved or winding. Each set of parallel ridges may extend in varied directions and at varied angles relative to one another, such that a wide range of patterns may be formed, including but not limited to triangular and hexagonal. Alternatively, the pattern of ridges may comprise a plurality of curved or circular ridges. The channel seal may comprise a base portion to which a texture is applied or on which a texture is formed. For example, the channel seal may comprise a base portion from which the ridges protrude. The textured portion has a first density and the base portion has a second density greater than the first. As the channel seal continues to wear during operation, the base portion engages with the spiral wall of the other scroll. Since the base portion has a higher density when compared to the textured portion, the base portion has a lower wear rate. This advantageously increases the wear life of the channel seal after the textured portion has worn away. The base portion is preferably a solid portion. Alternatively, the base portion may be a foam or a mesh. The base portion may be formed from the same material as the textured portion. Alternatively, the base portion may be formed from a harder material than the textured portion. Each ridge may protrude in a direction which is angled to the base portion (to define an “angle of protrusion”). For ridges with a cross-section comprising two parallel sides, for example a trapezoid, parallelogram or rectangular cross-section, the angle of protrusion is the smallest angle between a side and the surface of the base from which the ridge protrudes. For ridges with a triangular cross-section, the angle of protrusion is the angle between the bisector and the surface of the base portion from which the ridges protrude. For ridges with a rounded or semi-circular cross-section, the angle of protrusion is the angle between a line taken from the tip of the ridge to midway along the width of the ridge at the base portion and the surface of the base portion from which the ridge protrudes. The ridge cross-section is the cross-section perpendicular to the lengthwise direction of the ridge. Preferably, the angle of protrusion is a right angle. Preferably, the angle of protrusion is constant along the length of each ridge. Preferably, the angle of protrusion is equal for all ridges. The height of each ridge may range from 20 to 500 microns and preferably from 20 to 100 microns. The height of each ridge is the distance from the surface of the base portion from which the ridge protrudes to the tip of the ridge. The height of each ridge does not include the height of the base portion of the channel seal and hence is not equal to the height of the channel seal. Preferably, the height of each ridge is constant along its length. Preferably, the height is equal for all ridges. The width of each ridge may be constant along the height of the ridge. Alternatively, each ridge may narrow in a direction extending away from the base portion towards its tip. This narrowing may occur linearly or non-linearly in the direction extending away from the base portion towards the tip, to form different differing ridge crosssections, including but not limited to trapezoid, parallelogram, triangular or semicircular. This may result in a pointed or curved tip for example. Preferably, each ridge has a constant cross-section along its length (i.e. the narrowing is constant along the length of each ridge). Preferably, the ridge cross-sectional is the same for all ridges. The ridge cross-section is again the cross-section perpendicular to the lengthwise direction of the ridge. The surface may comprise an outer portion extending radially about the textured portion of the surface, and the outer portion may be smoother than the textured portion of the surface. In use, the outer portion of the surface is positioned between an outer portion of the scrolls to provide a higher degree of wear resistance in this region. The outer portion of the scrolls between which the outer portion of the surface of the channel seal is positioned is a portion of the scrolls falling outside each respective spiral wall. The surface of the outer portion that is subject to wear during operation of the scroll pump may be axially offset below the tip of each ridge. Preferably, the axial offset is equal to the height of each ridge of the pattern of ridges. In other words, the surface of the base portion from which the ridges protrude is in the same plane as the surface of the outer portion that is subject to wear during operation of the scroll pump. The outer portion typically functions as a bearing area, as opposed to a sealing area, hence providing an axial offset below the height of each ridge may eliminate the need to wear the outer portion during bedding in. This advantageously ensures that a minimum bedding-in period is maintained, even where an outer portion that is relatively smoother than the textured portion is present. According to a second aspect, the present invention provides a scroll pump comprising a first scroll and a second scroll intermeshing with the first scroll, each of the scrolls having a base and a spiral wall defining a spiral channel, and a drive for effecting relative orbiting motion between the scrolls. The scroll pump also comprises the channel seal of the first aspect arranged within the spiral channel of one of the scrolls for engaging the spiral wall of the other scroll. During operation of the scroll pump the textured portion of the channel seal engages with the tip of the spiral wall of the other scroll. Due to the high wear rate provided by the textured portion, the textured portion of the surface wears down quickly during bedding-in, until the height of the channel seal matches the scrolls, as previously described. This advantageously reduces the time required for bedding-in, when compared to a conventional channel seal formed with a uniform density across its height. Since bedding-in typically occurs during the manufacturing process, this advantageously reduces the manufacturing time of the scroll pump. As the channel seal continues to wear during operation, the base portion of the channel seal becomes exposed and engages with the tip of the other scroll. The base portion has a lower wear rate than the textured portion, to increase the wear life of the channel seal. The maximum distance between adjacent ridges is preferably less than the width of the spiral wall engaging the channel seal. In other words, in use, there is preferably at least one ridge engaging with the tip of said spiral wall at any given time to form a seal across the entire length of said spiral wall. This acts to minimise leakage across the pockets between adjacent ridges during bedding-in. Where the textured portion is provided by a pattern of pillars and / or alternative protrusions, the maximum distance between adjacent pillars and / or alternative protrusions is preferably less than the thickness of the spiral wall engaging the channel seal, such that there is at least one pillar and / or protrusion engaging with the tip of said spiral wall at any given time to form a seal across the entire length of said spiral wall at any given time. The scroll pump may comprise more than one channel seal. For example, the scroll pump may comprise a further said channel seal arranged within the spiral channel of the other scroll for engaging the spiral wall of said one of the scrolls. Where more than one channel seal is included within the scroll pump, bedding-in is considered complete once all channel seals have made contact with the tip of the spiral wall with which it is configured to engage. The scroll pump may be a scroll vacuum pump. In a further aspect, the present invention provides a method of manufacturing a channel seal for a scroll pump, the method comprising providing a base portion and texturing a surface of the base portion that is subject to wear during operation of the scroll pump. The method may also comprise cutting the textured base portion to define the channel seal. This may be applicable where, prior to texturing, the base portion does not fit the spiral channel of the scroll to which the channel seal is to be arranged during use, for example where the base portion forms a sheet from which multiple channel seals are cut. Alternatively, prior to texturing, the base portion may already fit the spiral channel of the scroll to which the channel seal is to be arranged during use. Texturing may be achieved by pressing, laser-etching, electro chemical machining (ECM) or any other suitable technique. Preferably, pressing is used as it is simple and cost effective. In an alternative aspect, the present invention provides a method for manufacturing a channel seal for a scroll pump comprising providing a mould and injecting material into the mould to form the channel seal, wherein the mould is shaped to define a channel seal having a textured portion which is subject to wear during operation of the scroll pump . It should be appreciated that the channel seal of the present invention may also be manufactured by additive manufacture. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 illustrates a first embodiment of a channel seal. Figure 2a illustrates a top plan view of a portion of the channel seal of Figure 1. Figure 2b illustrates a first perspective view of a portion of the channel seal of Figure 1. Figure 2c illustrates a second perspective view of a portion of the channel seal of Figure 1. Figure 3a illustrates a cross-sectional view of a second embodiment of a channel seal. Figure 3b illustrates a cross-sectional view of a third embodiment of a channel seal. Figure 3c illustrates a cross-sectional view of a fourth embodiment of a channel seal. Figure 4 illustrates a scroll pump comprising the channel seal of Figure 1. Figure 5 illustrates a detailed view of the channel seal of Figure 3a engaging with a spiral wall of a scroll during operation. DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a first embodiment of a channel seal 10. The channel seal 10 exhibits a spiral geometry to match the spiral channel of the scroll in which the channel seal is designed to be arranged during use. In this embodiment, the entirety of the surface which is subject to wear during operation of the scroll pump (hereafter referred to as wear surface 12) is textured. The texture is provided by a first set of parallel ridges 14 (hereafter referred to as first ridges), which extend in a first direction, and a second set of parallel ridges 16 (hereafter referred to as second ridges), which extend in a second direction. The first direction is at a right angle to the second direction, resulting in a grid-like pattern of intersecting ridges over the entirety of the wear surface 12. In use, the channel seal undergoes bedding-in, during which the first and second ridges 14,16 are worn down. The textured portion provided by the first and second ridges 14,16 has a lower wear resistance when compared to the comparatively smoother surface of conventional channel seals. In other words, the textured portion wears faster when compared to the comparatively smoother surface of conventional channel seals. This advantageously reduces the bedding-in time. Figure 2a illustrates a top plan view of a portion of the channel seal 10 of Figure 1. The first ridges 14 are evenly spaced apart, such that the distance between adjacent first ridges 14 is constant. Similarly, the second ridges 16 are evenly spaced apart, such that the distance between adjacent second ridges 16 is constant. The distance between adjacent first ridges 14 is equal to the distance between adjacent second ridges 16, resulting in a grid-like pattern of square pockets 18 across the textured portion. The maximum spacing between adjacent first and / or second ridges 14,16 is preferably less than the width of the spiral wall for engaging the channel seal 10 during use, as will be explained in more detail with reference to Figure 5. In the present embodiment, the maximum spacing between adjacent first and / or second ridges 14,16 is the diagonal distance D from one corner of each square pocket 18 to the other. Ensuring that this distance D is less than the width of the spiral wall for engaging the channel seal 10 during use guarantees that there is always at least one ridge 14,16 in contact with the spiral wall to form a seal during bedding-in. Figures 2b-c illustrate perspective views of a portion of the channel seal 10 of Figure 2a. The first and second ridges 14,16 protrude from a base portion 20 of the channel seal 10. In use, the channel seal 10 undergoes bedding-in, during which the first and second ridges 14,16 are worn down, exposing the smoother base portion 20 to the spiral wall and maximizing wear resistance of the channel seal 10. Figure 3a illustrates a cross-sectional view of a second embodiment of a channel seal 10’. The cross-sectional view is taken through a plane perpendicular to the lengthwise direction of the first ridges 14’ (second ridges not shown). The first ridges 14’ have a constant width from base to tip 15’. In other words, the first ridges 14’ have a constant width from the point at which they protrude from the surface of the base portion 22 to their tip 15’. Figure 3b illustrates a cross-sectional view of a third embodiment of a channel seal 10”. The cross-sectional view is taken through a plane perpendicular to the lengthwise direction of the first ridges 14” (second ridges not shown). The first ridges 14” have a width that reduces in the direction from base to tip 15”, such that the first ridges 14” narrow in the direction from base to tip 15”. In other words, the width of the first ridges 14” narrows from the point at which they protrude from the surface of the base portion 22’ to their tip 15”. The width reduces linearly, resulting in a triangular ridge cross-section. Figure 3c illustrates a cross-sectional view of a fourth embodiment of a channel seal 10”’. The cross-sectional view is taken through a plane perpendicular to the lengthwise direction of the first ridges 14’” (second ridges not shown). The first ridges 14’” have a width that reduces in the direction from base to tip 15’”, such that the ridges narrow in the direction from base to tip 15’”. In other words, the width of the first ridges 14’” narrows from the point at which they protrude from the surface of the base portion 22” to their tip 15’”. The width reduces non-linearly, resulting in a substantially semi-circular ridge cross-section. It should be appreciated that the cross-sections of the ridges is not limited to the above-described embodiments and one or more of a multitude of ridge crosssections may be used. It should also be appreciated that the cross-sections of any additional sets of ridges (not shown) may also vary in a similar way, though preferably the cross-sections of each set of ridges is the same. Each ridge 14’,14”,14’” protrudes from the base portion 20’,20”,20’” at an angle to the surface of the base portion 22,22’,22” which is subject to wear during use. In these embodiments, the angle of protrusion is a right angle, but the angle may be an acute angle. The height of each ridge hR is the distance from the surface of the base portion 22 from which it protrudes to the tip 15’ of the ridge and is illustrated in Figure 3a for reference. The height of each ridge hR may range from 20 to 500 microns, preferably from 20 to 100 microns. The height of the base portion hB is typically larger than the height of the ridges hR. The overall height of the channel seal hS is equal to the sum of the heights of the base portion hB and the highest of the ridges hR. Figure 4 illustrates a scroll pump 100 comprising the channel seal 10 of Figure 1. The scroll pump 100 comprises a first scroll 120 and a second scroll 130 intermeshing with the first scroll 120. Each of the scrolls 120,130 comprise a base 122,132 and a spiral wall 124,134 defining a spiral channel (not labelled). The first spiral wall 124 extends perpendicularly from the first base 122 towards the second base 132, such that the tip of the first spiral wall 124 is proximate but not in contact with an opposing surface of the second base 132. Similarly, the second spiral wall 134 extends perpendicularly from the second base 132 towards the first base 122, such that the tip of the second spiral wall 134 is proximate to but not in contact with an opposing surface of the first base 122. The first and / or second spiral walls 124,134 may be integrally formed with the respective first and / or second base 122,132. A channel seal 10 is arranged within each of the spiral channels (not labelled), such that one of the channel seals 10 is sandwiched between the end surface of the second spiral wall 134 and the opposing surface of the first base 122 and the other of the channel seals 10 is sandwiched between the end surface of the first spiral wall 124 and the opposing surface of the second base 132. The first scroll 120 and second scroll 130 may be biased together by a biasing apparatus (not shown) of the scroll pump (e.g. one or more springs). In this embodiment, the housing 110 and the first scroll 120 together define an overall housing of the scroll pump 100. However, it should be appreciated that the first scroll 120 may not define any of the overall housing of the scroll pump 100. A drive 150 is coupled to the second scroll 130 for effecting relative orbiting motion between the first and second scrolls 120,130 during operation of the scroll pump. During operation of the scroll pump 100, fluid is pumped from an inlet to an outlet of the scroll pump 100 (not shown) via the spiral channel. The precise physical mechanism by which fluid is pumped is well understood and will not be described herein for the sake of brevity. The channel seals 10 provide axial seals to minimise leakage of fluid axially between different sections of the spiral channels during operation of the scroll pump 100. To ensure maximum sealing, the height of the channel seals 10 must match the first and second scrolls 120,130. During operation of the scroll pump 100, due to the relative orbiting motion of the first and second scrolls 120,130, each of the relative end surfaces of the first and second spiral walls 122,124 rub against the relative channel seal 10. Thus, each of the channel seals 10 are subject to wear during use over time. The surface of the channel seals 10 that is subject to wear during operation of the scroll pump 100 is textured. As a result, it has a lower wear resistance when compared to a comparatively smooth surface of conventional channel seals. As a result, there is a period during initial operation of the scroll pump 100 in which the textured surface of the channel seals 10 wears relatively quickly until the base portion of the channel seal is exposed. This proves advantageous in reducing the time required for beddingin, as previously described herein. Figure 5 illustrates a detailed view of the channel seal of Figure 3a engaging with a spiral wall during operation. The maximum spacing between adjacent ridges of the set of parallel ridges 14’ is denoted S. The maximum spacing S is less than the width of the spiral wall, denoted W. In other words, S <W. It should be appreciated that the diagonal distance D of the embodiment of Figure 2a is equivalent to the maximum spacing S of the embodiment of Figure 5. This advantageously ensures that, during operation, at least one ridge engages with the tip of the spiral wall 124 at any given time to form a seal across the entire length of said spiral wall. This acts to minimise leakage across the pockets between adjacent ridges during bedding-in. Although several embodiments of the invention have been described in detail with reference to Figures 1 to 5, it should be appreciated that the invention is not limited to these precise embodiments and that various modifications may be made by the skilled person without departing from the scope of the invention as claimed herein. For example, the embodiment of Figure 4 provides a scroll pump 100 with two channel seals 10 according to the present invention. It should be appreciated however, that one or both of the channel seals may be a channel seal according to an embodiment of the present invention. In other words, one of the channel seals may be a conventional channel seal. Alternatively, it should also be appreciated that the scroll pump 100 may comprise only one channel seal 10 arranged within only one of the first or second spiral channels. REFERENCE NUMERALS 10, 10’, 10”, 10’” channel seal 12 wear surface 14, 14’, 14”, 14’” first set of parallel ridges (first ridges) 15’, 15”, 15’” tip 16 second set of parallel ridges (second ridges) 18 pocket 20, 20’, 20”, 20’” base portion 22, 22’, 22” surface of base portion 100 scroll pump 110 housing 120 first scroll 122 first base 124 first spiral wall 130 second scroll 132 second base 134 second spiral wall 150 drive D diagonal distance hR first ridge height hB base height hS channel seal height W spiral wall width S ridge spacing

Claims

1. A channel seal for a scroll pump, the channel seal comprising:a surface which is subject to wear during operation of the scroll pump, wherein at least a portion of the surface is textured.

2. A channel seal according to claim 1, wherein the textured portion of the surface is provided by a pattern of ridges.

3. A channel seal according to claim 2, wherein the pattern of ridges comprises at least a first set of parallel ridges extending in a first direction and a second set of parallel ridges extending in a second direction at an angle to the first direction.

4. A channel seal according to any of claims 2 or 3, wherein the height of each ridge of the pattern of ridges ranges from 20 to 500 microns, preferably from 20 to 100 microns.

5. A channel seal according to any of claims 2 to 4, comprising a base portion from which the ridges protrude, wherein each ridge narrows in a direction extending away from the base portion towards a tip.

6. A channel seal according to any preceding claim, wherein the surface comprises an outer portion extending radially about the textured portion of the surface, and wherein the outer portion is smoother than the textured portion of the surface.

7. A scroll pump comprising:a first scroll and a second scroll intermeshing with the first scroll, each of the scrolls comprising a base and a spiral wall defining a spiral channel;a drive for effecting relative orbiting motion between the scrolls; andthe channel seal of any preceding claim arranged within the spiral channel of one of the scrolls for engaging the spiral wall of the other scroll.

8. A scroll pump according to claim 7 when dependent from claim 2, wherein the maximum distance between adjacent ridges is less than the width of the spiral wall engaging the channel seal.

9. A scroll pump according to claims 7 or 8, comprising a further said channel5 seal arranged within the spiral channel of the other scroll for engaging the spiral wall of said one of the scrolls.

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