Plug and drain aperture for fluid reservoir

Abstract

A fluid reservoir has a drain aperture 200 and a corresponding drain plug 100 that cooperates with the drain aperture and the aperture and plug can be used in a vehicle. The plug has an elongate fastener portion 102 and a head 104 arranged at one end of the elongate fastener portion, the head including resilient arms 116 at its perimeter and these arms extend circumferentially about the plug’s longitudinal axis 108. The main body of the head has a fixed end region 130 adjacent a slot 122, and the end region provides anchorage for the arms and the slot provides a space for the resilient arms to deflect. The arms further include engagement features 136, that in use cooperate with a similar engagement feature 224 on the drain aperture. The arm engagement feature can be a protrusion, and the drain aperture engagement feature can be a recess. The cooperating elongate fastener portion can be a thread cooperating with a corresponding thread 206 of the drain aperture, and when the corresponding drain plug is screwed into the drain aperture an upper annular region 228 can act to urge the resilient arms radially inwardly for location within the drain aperture engagement portion. The arrangement provides a secure connection between the plug and the drainage aperture that is resistant to vibration and leakage.

Description

TECHNICAL FIELD The present disclosure relates to a fluid reservoir comprising a drain aperture and a plug for cooperation with the drain aperture. Aspects of the invention relate to a plug, a fluid reservoir, a kit comprising the fluid reservoir and corresponding plug, an assembly, and a vehicle. BACKGROUND Fluid reservoirs, such as oil sumps for machines and vehicles, usually have a drainage aperture which can be selectively opened by a user to drain the reservoir during maintenance activities. Typically, such fluid reservoirs are provided with a removable plug which screws into and seals the drainage aperture. During normal use, the machine or vehicle causes the fluid reservoir to vibrate which, in turn, causes the plug to gradually loosen from the drainage aperture, thereby allowing fluid to leak from the fluid reservoir. Unintentional loosening is accelerated by the fluid held within the reservoir lubricating the cooperating features of the plug and the drainage aperture. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a plug, a fluid reservoir, a kit of parts, an assembly, and a vehicle as claimed in the appended claims. According to an aspect of the invention, a plug for cooperation with a drain aperture of a fluid reservoir is provided. The plug comprises an elongate fastener portion and a head arranged at one end of the elongate fastener portion. The elongate fastener portion defines a longitudinal axis of the plug and is configured to hold the plug within the drain aperture upon rotation about the longitudinal axis in a first direction and to release the plug from the drain aperture upon rotation about the longitudinal axis in a second direction, the second direction being opposite to the first direction. The head comprises a resilient arm or member which defines an outer perimeter portion of the head and extends in a circumferential direction around the longitudinal axis from a fixed end region to a free end region to define a slot in the plug head. The resilient member may be configured to flex into the slot towards the longitudinal axis upon application of a radial load. The resilient member further comprises a plug engagement feature configured to engage with one or more corresponding drain aperture engagement features as the elongate fastener portion is rotated within the drain aperture. The plug engagement feature may be disposed at the free end region of the resilient arm. The plug engagement feature may take the form of a protrusion extending from the resilient arm and, in such embodiments, the protrusion may extends from the resilient arm radially away from the longitudinal axis of the plug. Additionally or alternatively, the protrusion may comprise an arced surface for engaging with one or more corresponding drain aperture engagement features. In embodiments, the plug may comprise a plurality of resilient arms spaced equidistantly apart from each other. Embodiments with three such resilient arms advantageously provide an even distribution of radial loads on the plug head, while minimising friction between the plug head and the corresponding drain aperture. According to another aspect of the invention, a fluid reservoir comprising a drain aperture is provided. The fluid reservoir comprises a housing having an interior surface and an exterior surface, and the drain aperture is configured for cooperation with a plug comprising a resilient arm having an engagement feature. The drain aperture comprises a through-hole which is configured to receive an elongate fastener portion of the plug, and extends through the fluid reservoir housing from the exterior surface to the interior surface to define a longitudinal axis of the drain aperture. The drain aperture further comprises an array of drain aperture engagement features which are arranged concentrically around the longitudinal axis of the drain aperture. The drain aperture engagement features may be equidistantly spaced apart from each other. Each drain aperture engagement feature is configured to engage with the plug engagement feature of the plug. The exterior surface of the fluid reservoir housing may define an annular surface of the drain aperture which is arranged concentrically with the through-hole. In other words, the drain aperture may comprise an annular surface defined by the exterior surface of the fluid reservoir housing. In such embodiments, the array of drain aperture engagement features are provided on the annular surface. In some embodiments, each drain aperture engagement feature may take the form of a recess extending into the annular surface and radially away from the longitudinal axis of the drain aperture. In embodiments, the annular surface may be defined by an annular projection comprised as part of the drain aperture. In such embodiments, the annular projection extends from the exterior surface of the fluid reservoir housing and is arranged concentrically with the through-hole to define the annular surface on a radially inner side of the annular projection. Another aspect of the invention provides a kit of parts comprising the plug described above and a corresponding drain aperture, wherein the drain aperture comprises the annular surface described above. In this kit, the diameter of the plug head is larger than the inner diameter of the annular surface such that the or each resilient arm of the plug is flexed towards the longitudinal axis of the plug when the plug is secured within the drain aperture, i.e. even when the plug engagement features are seated within and engaged with the drain aperture engagement features. According to another aspect of the invention, an assembly comprising the fluid reservoir and a corresponding plug is provided. A vehicle comprising the plug and fluid reservoir assembly is also provided. It will be appreciated that preferred and / or optional features of one aspect of the invention may be incorporated alone or in appropriate combination in other aspects of the invention also. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1 and 2 show perspective views of a plug according to embodiments of the invention; Figure 3 shows a plan view of the plug shown in Figures 1 and 2; Figure 4 shows a perspective view of a drainage aperture for cooperation with the plug shown in Figures 1 to 3; Figure 5 shows a plan view of the drainage aperture shown in Figure 4; Figure 6 shows a close-up perspective view of part of the drainage aperture shown in Figure 5; Figure 7 shows a plan view of the plug shown in Figures 1 to 3 assembled with the drainage aperture shown in Figures 4 to 6; Figure 8 shows a close-up plan view of part of the assembly shown in Figure 7; Figure 9a shows a plan view of the plug shown in Figures 1 to 3 assembled in a first position within the drainage aperture shown in Figures 4 to 6; Figure 9b shows a plan view of the plug shown in Figures 1 to 3 assembled in a second position within the drainage aperture shown in Figures 4 to 6. DETAILED DESCRIPTION In general terms, embodiments of the invention provide a plug (or stopper) and a fluid reservoir comprising a corresponding drainage hole (or drain aperture). The plug is configured to removably fit into and block the drain aperture to prevent fluid from flowing out of the fluid reservoir. The fluid reservoir may be an oil sump for a vehicle, for example. The plug and the drain aperture are configured so that the plug can be tightened into a secure ‘plugging’ position by inserting and rotating the plug into the drain aperture. The plug and the drain aperture are provided with corresponding engagement features which engage as the plug is rotated within the drain aperture. Engagement of the engagement features increases the torque required to rotate the plug, meaning that a minimum torque must be applied to the plug to both tighten and loosen its position within the drain aperture. This minimum torque is greater than typical torque loads which might be transferred to the plug during normal “plugged” use of the fluid reservoir. Accordingly, the engagement features of the plug and drain aperture prevent the plug from unintentionally loosening while the fluid reservoir is in use, e.g. due to vibrations and I or lubrication from fluid held within the reservoir, yet allow a user to use a drive tool to easily remove the plug to drain the reservoir when required. Figures 1 and 2 show a plug 100 according to an embodiment of the invention. The plug 100 is configured for cooperation with a drain aperture 200 of a fluid reservoir which is described in more detail later with reference to Figures 4 to 6. The plug 100 comprises an elongate fastener portion 102 and a head fixed 104 at one end of the elongate fastener portion 102. In the example shown, the head 204 and the elongate fastener portion 102 are integrally formed together. The fastener portion 102 takes the form of a shaft extending from the head to a free end 106 to define a longitudinal axis 108 of the plug 100. The elongate fastener portion 102 is substantially cylindrical and is threaded for screwing into a corresponding tapped portion of the drain aperture 200. That is to say, the elongate fastener portion 102 is configured to hold the plug 100 within the drain aperture 200 upon rotation about the longitudinal axis 108 in a first direction and to release the plug 100 from the drain aperture 200 upon rotation about the longitudinal axis 108 in a second, opposite direction. In this example, the first direction is clockwise (when viewed from the head end of the plug) and depicted by arrow C. The head 104 of the plug 100 has a generally disc like form having a constant cross section in the longitudinal direction. The head 104 is arranged concentrically with the elongate fastener portion 102 so as to be centrally arranged at the end of the elongate fastener portion 102. In other words, the elongate fastener portion 102 extends perpendicularly from the centre of one face of the head 104. Said face of the plug head 104 may be referred to as the inward face 110 because it faces inwardly into the fluid reservoir when the plug 100 is inserted into the drain aperture 200. The other, opposing face of the plug head 104 may referred to as the outward face 112 of the plug. As shown in Figure 2, the plug 100 comprises a drive feature 114 for receiving a drive tool. The drive feature 114 is provided centrally on the outward face 112 of the head 100. In this example, the drive feature 114 is a hexagonal socket formed in the plug head 104 so that the outward face 112 is substantially flat and without any projections. It will be appreciated, however, that the socket 144 may be any suitable shape for applying torque to the plug 100, and need not be hexagonal. In other embodiments, the drive feature 114 may instead take the form of a fixed nut projecting from the outward face 112 of the plug head 104. The head 104 comprises a plurality of resilient arms 116, or members, arranged around its periphery. There are three identical resilient arms 116 and they are arranged equidistantly around the centre of the head so that the plug head 104 has a rotational symmetry of order three. However, in other embodiments, the head 104 may comprise fewer or more resilient arms 116 similarly arranged. One of the resilient arms 116 will now be described in more detail with reference to Figure 3 which shows the plug head 104 as viewed from the outward face 112. As mentioned above, the head 104 has a generally disc like form which defines an outer circumferential periphery 118 (indicated by the dotted line in Figure 3). The outer circumferential periphery 118 denotes a circular boundary which aligns with portions of the outer perimeter 120 of the plug head 104 when the plug head 104 is in a neutral position with no loads applied. The resilient arm 116 is defined by a slot 122 which extends into the head 104 from the outer circumferential periphery 118. The slot 122 extends radially from the outer circumferential periphery 118 towards the longitudinal axis 108 before extending in a circumferential direction around the longitudinal axis 108 to define a cantilever finger like form which is the resilient arm 116. The term ‘slot opening region’ may be used to refer to the opening 124 of the slot 122 at the outer circumferential periphery and the term ‘slot end region’ may be used to refer to the closed end 126 of the slot 122 where it terminates. The boundary of the slot defines corner regions 128 which are radiused to minimise the occurrence of stress concentrations in these regions. In this example, the slot 122 extends from the slot opening region 124 to the slot end region 126 in a clockwise direction when the plug 104 is viewed from the outward face 112. That is to say, the resilient arm 116 extends in an anticlockwise direction when the plug is viewed from the outward face 112. In other examples, the slot may instead extend in the anticlockwise direction and the resilient arm in the clockwise direction. The resilient arm 116 extends along the outer circumferential periphery 118 (i.e. in a circumferential direction) from a fixed end region 130 to a free end region 132, where the fixed end region 130 is adjacent to the slot end region 126 and the free end region 132 is adjacent to the slot opening region 124. A radially outer side 134 of the resilient arm 116 defines a portion of the outer perimeter 120 of the plug head 104 which aligns with the circular boundary denoted by the outer circumferential periphery 118. The resilient arm 116 is formed to act as a cantilever which is configured to bend or flex radially into the slot 122 towards the longitudinal axis 108 of the plug 104 when a radial load is applied to the free end region 132 or the radially outer side 134 of the resilient arm 116. The resilient arm 116 comprises an engagement feature 136 disposed at its free end region 132. In this embodiment, the engagement feature 136 takes the form of a protrusion on the radially outer side 134 of the resilient arm 116. That is to say, the protrusion 136 extends outwardly from the resilient arm 116, away from the longitudinal axis 108, and past the outer circumferential periphery 118 of the plug head 104. Accordingly, the protrusion 136 defines a portion of the outer perimeter 120 of the plug head. The protrusion 136 comprises an arced surface which gives it a substantially semi-circular convex profile when viewed looking towards the outward face 112 of the plug head 104. As will become clear from further discussion below, the protrusion 136 is specifically shaped to engage with corresponding recesses formed in the corresponding drainage aperture 200. The features of a fluid reservoir comprising such a corresponding drainage aperture 200 will now be described with reference to Figures 4 and 5. Figures 4 and 5 show a drainage aperture 200 formed in a housing 210 of a fluid reservoir (not shown). In this example, the fluid reservoir is an oil sump for a vehicle and is arranged for holding oil therein. The housing 210 comprises an internal surface (not shown) which contacts the oil when in use, and an external surface 208 which is exposed to the atmosphere when in use. The drain aperture 200 comprises a central through hole 202 which extends through the housing 210 from the exterior surface 208 to the interior surface to define a longitudinal axis 204 of the drain aperture. The through hole 202 is configured to receive the elongate fastener portion 102 of the corresponding plug 100 and so, in this example, comprises a tapped portion 206 which corresponds to the thread on the plug 100. Accordingly, the plug 100 can be inserted and held within the drain aperture 200 by aligning the longitudinal axis 108 of the plug 100 with the longitudinal axis 204 of the through hole 202 and screwing the elongate fastener portion 102 into the tapped portion 206 of the through hole in the clockwise direction. In the example shown, the drain aperture 200 comprises an annular projection 212 which extends from the exterior surface 208 of the housing 210 and is arranged concentrically with the through hole 202. The annular projection 212 extends substantially perpendicularly from the exterior surface 208 and comprises a radially outer surface 216, which faces outwardly away from the longitudinal axis 204 of the drain aperture 200, a radially inner surface 214, which faces inwardly towards the longitudinal axis 204 of the drain aperture 200, and an upper surface 218, which is between the radially inner and outer surfaces 214,216 and is substantially parallel with the exterior surface 208 of the housing 210. For the purposes of the following description, the radially inner surface 216 of the annular projection 212 may simply be referred to as the annular surface. In another example, the annular surface 216 may be provided by a circular recess formed in the exterior surface 208 of the housing. A shoulder region 220 is defined between the radially inner surface 214 of the annular projection 212 (i.e. the annular surface) and the through hole 202 of the drain aperture 200. The inner diameter (d) of the annular projection 212 measures from one side of the annular surface 214 to the other, through the longitudinal axis 204 of the drain aperture 200. The inner diameter (d) is such that the plug head 104 of a corresponding plug 100 may be seated against the shoulder region 220 when the fastener portion 102 is inserted within the drain aperture through hole 202. In other words, the annular surface 214 and the shoulder together 220 define a cavity 222 for receiving the head 104 of a corresponding plug 100. As seen, the drain aperture 200 comprises a series or array of recesses 224, or (drain aperture) engagement features, formed in the annular surface 214. Each of these recesses 224 extends into the annular projection 212 away from the longitudinal axis 204 of the drain aperture 200. There are twelve recesses in total and they are equidistantly spaced around the circumference of the annular surface 214 so that the drain aperture 200 has a rotational order of symmetry about the longitudinal axis 204 of twelve. In other examples there may be fewer or more recesses 214, but generally, there should be a number of recesses 214 which is a multiple of the number of corresponding projections 136 on the corresponding plug 100. This is so that all of the projecting engagement features 136 on the plug 100 can be simultaneously engaged with corresponding recessed engagement features 224 in the drain aperture 200. Figure 6 shows a close up perspective view of two adjacent recesses 224. As shown, the recesses 224 are shaped to engage with the protruding engagement features 136 of a corresponding plug 100. As such, in this example, each recess 224 comprises an arced surface 226 which gives it a substantially semi-circular concave profile when viewed looking towards the exterior surface 208 of the housing 210. The profile of each recess 224 is substantially equal and opposite to the profile of the resilient arm protrusions 136 on the corresponding plug 100 so that the protrusions 136 can fit within and engage with the recesses 224. In this example, the engagement features of the drain aperture 200 (drain aperture engagement features 224) are recesses and the engagement features of the corresponding plug 100 (plug engagement features 136) are protrusions. However, in other examples, the profiles may be reversed so that the drain aperture comprises protruding engagement features and the corresponding plug comprises recessed engagement features. The region of the annular surface 214 between two adjacent recesses 224 may be generally referred to as an annular region 227. As can be seen, there is an upper annular region 228 and a lower annular region 230. The lower annular region 230 is substantially perpendicular to the shoulder region 220 of the drain aperture 200, while the upper annular region 228 is angled slightly away from the longitudinal axis 204 so as to be frustoconical. Accordingly, the inner diameter (d) of the annular projection 212 is larger at the opening of the cavity 222 cavity than towards the base of the cavity 222 nearto the shoulder region 220. Thus, when the corresponding plug 100 is screwed into the drain aperture 200, the upper annular region 228 acts to urge the resilient arms 116 radially inwardly. Each intersection between a recess 224 and the adjacent annular region 227 is radiused to provide a transition region 232. A transition region on the clockwise side of the recess may be referred to as a clockwise transition region 232’ and a transition region on the anticlockwise side of the recess may be referred to as an anti-clockwise transition region 232”. Figure 7 shows an assembly 300 comprising the plug 100 described above with reference to Figures 1 to 3 inserted and secured within the drain aperture 200 described above with reference to Figures 4 to 6. The plug 100 is in a secured position, meaning that the plug 100 has been screwed into the tapped portion of the through hole 202 such that the inward face 110 of the plug head 104 is tightly abutted against the shoulder portion 220 of the drain aperture 200 and that each of the engagement features 136 protruding from the resilient arms 116 are seated within and engaged with the corresponding recessed engagement features 224 of the drain aperture 200. In this secured position, the plug 100 seals the drain aperture 200 so that fluid is unable to pass through and out of the fluid reservoir housing 210. The diameter (d) of the cavity 222 defined by the drain aperture annular surface 214 and shoulder region 220 is substantially the same as that of the outer circumferential periphery 118 of the plug head 104. This is so that, when the plug 100 is in the secured position, the resilient arms 116 are in an unbent or undeflected (neutral) position and the outer perimeter 120 of the of the plug head 104 contacts and conforms to the annular surface 214. In other words, the plug head 100 fits snuggly within the corresponding cavity 222. However, In some embodiments, the cavity diameter (d) may be slightly smaller than the outer circumferential periphery 118 of the plug 100 so that, when the plug 100 is in the secured position, the resilient arms 116 are slightly bent or deflected radially towards the longitudinal axis 108. In this way, friction between the outer perimeter of the plug and the annular surface is increased and the torque required to rotate the plug head 104 within the cavity 222 is also increased. The plug head 104 has a substantially uniform longitudinal depth and the drain aperture cavity 222 has a longitudinal depth that is the same or greater. This is so that, when the plug 100 is fully inserted and secured within the drain aperture 200, the outward face 112 of the plug head 104 is flush or inset from the upper surface 218 of the annular projection 212. To undo the plug 100 from this secured position requires a minimum anticlockwise torque (T) to be applied to the plug head drive feature 114. When an anticlockwise torque is applied to the plug 100, a contact force between each protruding engagement feature 136 on the plug head 104 and the anticlockwise transition region 232’ of the engaged recess 224 increases. With reference to Figure 8 (which shows a close up of a protruding engagement feature 136 engaged within a corresponding recess 224), the minimum anticlockwise torque is the torque at which the radial component of that contact force (Fr) is sufficient to bend the resilient arm 116 radially into the slot 122, thus allowing the protrusion 136 to slide out of engagement with the recess 224, past the transition region 232’ and into contact with the adjacent annular region 227 as the plug 100 is rotated. Continued anticlockwise rotation of the plug 100 will move the plug head into the position shown in Figure 9a, in which the contact of the annular region 227 against each protrusion 136 causes each resilient arm 116 to remain bent into the slot 122 as the plug 100 is rotated. As will be appreciated and as shown in Figure 9B, yet further anticlockwise rotation of the plug 100 will see each resilient arm 116 return to its unbent position as each protrusion 136 partially engages with the next recess along 224’. As above, the minimum anticlockwise torque will need to be applied to the plug 100 to bend the resilient arm 116 again so that the protrusion 136 can be disengaged from and moved past this second recess 224’. The minimum anticlockwise torque will be required to move the protrusion 136 past each recess 224 encountered until the elongate fastener portion 102 is sufficiently unscrewed from the tapped through-hole 202 such that the plug head 104 is clear of the annular projection 216. In other words, the minimum anticlockwise torque (T) must be applied to loosen the plug 100 from its tightened and secure position within the drain aperture 200. The minimum torque is sufficiently high that it can only be applied to the plug intentionally through use of a drive tool. Accordingly, the plug is prevented from being loosened unintentionally, for example by vibrations caused when the fluid reservoir is in use on a vehicle. Installing the plug 100 within the drain aperture 200 and tightening it to a secure position is similar to undoing the plug 100 but in reverse. First, the longitudinal axis 108 of the plug 100 is aligned with the longitudinal axis 204 of the drain aperture 200. The plug 100 is then rotated (either by hand or using a drive tool with the drive feature 114 on the plug head 104) about its longitudinal axis 108 in the clockwise direction (when viewed looking at the outward face 112 of the plug head 104) so that the thread of the fastener portion 102 screws into the corresponding tapped portion of the drain aperture through-hole 202. The screwing motion draws the plug 100 into the drain aperture 200 until the projecting engagement features 136 on the plug’s resilient arms 116 abut with the upper surface 218 of the annular projection 212. The plug 100 is rotated further so as to continue drawing the plug head 104 longitudinally towards the shoulder region 220. Meanwhile each resilient arm 116 bends slightly in the longitudinal direction to allow the projecting engagement features 136 thereon to be drawn along the upper surface of the annular projection 212 until each of the projecting engagement features 136 align with a corresponding recess 224 in the annular surface 214. At this point, each resilient member 116 returns to its neutral position as the projecting engagement feature 136 thereon drops into partial engagement with the aligned corresponding recess 224. Continued clockwise rotation of the plug 100 causes a contact force between each protruding engagement feature 136 and the clockwise transition region 232 of the corresponding partially engaged recess 224 to increase. As above, the minimum torque (T) must be applied (this time in a clockwise direction) such that the radial component of the contact force pushes the resilient arm 116 into the slot 122 so that the protrusion 136 can slide past the recess 224 to allow the plug head 104 to be further rotated. As will be understood, continued clockwise torquing of the plug 100 will push the resilient arms 116 into and out of engagement with successive recesses 224 until the plug 100 is fully drawn into the drain aperture 200 with the plug head 104 abutted tightly against the shoulder region 220, as shown in Figure 7. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A plug for cooperation with a drain aperture of a fluid reservoir, the plug comprising:an elongate fastener portion defining a longitudinal axis of the plug, wherein the elongate fastener portion is configured to hold the plug within the drain aperture upon rotation about the longitudinal axis in a first direction and to release the plug from the drain aperture upon rotation about the longitudinal axis in a second direction, the second direction being opposite to the first direction;a head arranged at one end of the elongate fastener portion, wherein the head comprises a resilient member defining an outer perimeter portion of the head, wherein the resilient member:extends in a circumferential direction around the longitudinal axis from a fixed end region to a free end region to define a slot in the plug head; andcomprises a plug engagement feature configured to engage with one or more corresponding drain aperture engagement features as the elongate fastener portion is rotated within the drain aperture.

2. A plug according to claim 1, wherein the resilient member is configured to flex into the slot towards the longitudinal axis upon application of a radial load.

3. A plug according to claims 1 or 2, wherein the plug engagement feature is disposed at the free end region of the resilient arm.

4. A plug according to any preceding claim, wherein the plug engagement feature takes the form of a protrusion extending from the resilient arm.

5. A plug according to claim 4, wherein the protrusion extends from the resilient arm radially away from the longitudinal axis of the plug.

6. A plug according to claim 4 or 5, wherein the protrusion comprises an arced surface for engaging with the one or more corresponding drain aperture engagement features.

7. A plug according to any preceding claim comprising three identical resilient arms spaced equidistantly apart from each other.

8. A fluid reservoir comprising a drain aperture for cooperation with a plug, wherein:the plug comprises a resilient arm having a plug engagement feature; andthe fluid reservoir comprises a housing having an interior surface and an exterior surface, and the drain aperture comprises:a through-hole configured to receive an elongate fastener portion of the plug, wherein the through-hole extends through the fluid reservoir housing from the exterior surface to the interior surface to define a longitudinal axis of the drain aperture; andan array of drain aperture engagement features arranged concentrically around the longitudinal axis of the drain aperture, wherein each drain aperture engagement feature is configured to engage with the plug engagement feature of the plug.

9. A fluid reservoir according to claim 8, wherein the drain aperture engagement features of the array are spaced equidistantly apart from each other.

10. A fluid reservoir according to claim 8 or 9, wherein the exterior surface defines an annular surface of the drain aperture arranged concentrically with the through-hole, wherein the array of drain aperture engagement features is provided on the annular surface.

11. A fluid reservoir according to claim 10, wherein the drain aperture comprises an annular projection extending from the exterior surface of the fluid reservoir housing and arranged concentrically with the through-hole to define the annular surface on a radially inner side of the annular projection.

12. A fluid reservoir according to claim 10 or 11, wherein each drain aperture engagement feature of the array takes the form of a recess which extends radially away from the longitudinal axis of the drain aperture into the annular surface.

13. A kit comprising the fluid reservoir of claim 10 or 12 and a corresponding plug according to any of claims 1 to 7, wherein the diameter of the plug head is larger than an inner diameter of the annular surface such that the or each resilient arm of the plug is flexed towards the longitudinal axis of the plug when the plug is secured within the drain aperture.

14. An assembly comprising a fluid reservoir according any of claims 8 to 12 and a corresponding plug according to any of claims 1 to 7.

15. A vehicle comprising the assembly of claim 12.12

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