PREMIUM TRIPOD HOUSING WITH HYBRID ELLIPTICAL TRACKS

Abstract

Tripod arrangement comprising: a housing body (30) extending between a first housing end (36) and a second housing end (38) along a longitudinal axis (22), wherein the housing body (30) defines a plurality of guide channels spaced from one another by a separating element (50), the guide channels (52) extending from the first housing end (36) to the second housing end (38), each of the guide channels (52) having a first wall (70), a second wall (72), and a third wall (74) extending between the first wall (70) and the second wall (72), a first outer transition region (112) extending between the first wall (70) and the third wall (74), and a second outer transition region (114) extending between the second wall (72) and the third wall (74), a first inner transition region (116) extending between the first wall (70) and a first separating element (50), and a second inner transition region (118),extending between the second wall (72) and a second separating element (50), wherein at least one of the first outer transition region (112), the second outer transition region (114), the first inner transition region (116) and the second inner transition region (118) has a concave elliptical, non-circular contour, and , a separate roller assembly disposed in each of the guide channels (52), each roller assembly (83) comprising an inner roller element (85) and an outer roller element (81), the inner roller element (85) being disposed on a rotor hub element (14) and the outer roller element (81) being disposed in engagement with the first wall (70), the second wall (72), and the third wall (74), the outer roller element (81) having a convex outer surface (124) configured for engagement with the first outer transition region (112) and the second outer transition region (114), and a convex inner surface (126) configured for engagement with the first inner transition region (116) and the second inner transition region (118).

Description

BACKGROUND OF THE INVENTION

[0001] Tripod joints connect the rotating shaft elements of a vehicle. The tripod joint comprises a tripod housing having an inner surface that provides a plurality of roller tracks for guiding a plurality of roller assemblies.

[0002] Current tripod housings may allow excessive tilting of a roller assembly, and in particular an outer roller thereof, within a roller track, wherein the tilting is observed along a plane extending transversely to a longitudinal axis of the tripod housing.

[0003] Excessive tilting of the roller assembly and its outer roller relative to the roller track, combined with a lack of restoring torque from the roller assembly, can lead to a binding condition of the roller assembly. A binding condition is highly undesirable because binding prevents the intended low-friction rolling motion of the roller assembly, which can lead to premature wear and failure. In addition to promoting wear, binding can also result in highly undesirable noise and vibration (NVH), which is highly undesirable and readily audible to a passenger in the vehicle.

[0004] DE 101 95 950 T5 discloses a tripod assembly comprising: a housing body extending between a first housing end and a second housing end along a longitudinal axis, the housing body defining a plurality of guide channels spaced apart from one another by a separating element, the guide channels extending from the first housing end to the second housing end, each of the guide channels having a first wall, a second wall, and a third wall extending between the first wall and the second wall, a first outer transition region extending between the first wall and the third wall, and a second outer transition region extending between the second wall and the third wall, a first inner transition region extending between the first wall and a first separating element, and a second inner transition region extending between the second wall and a second separating element,wherein each of the first outer transition region, the second outer transition region, the first inner transition region, and the second inner transition region has a circular contour, i.e., a contour with a constant radius. The tripod assembly further comprises a separate roller assembly disposed in each of the guide channels, each roller assembly having an inner roller element and an outer roller element, the inner roller element being disposed on a rotor hub element and the outer roller element being disposed in engagement with the first wall, the second wall, and the third wall, the outer roller element having a convex outer surface configured for engagement with the first outer transition region and the second outer transition region, and a convex inner surface configured for engagement with the first inner transition region and the second inner transition region.

[0005] From DE 11 2004 002 573 T5, a tripod arrangement is also known, comprising: a housing body extending between a first housing end and a second housing end along a longitudinal axis, wherein the housing body defines a plurality of guide channels spaced from one another by a separating element, the guide channels extending from the first housing end to the second housing end, each of the guide channels having a first wall, a second wall, and a third wall extending between the first wall and the second wall, a first outer transition region extending between the first wall and the third wall, and a second outer transition region extending between the second wall and the third wall, a first inner transition region extending between the first wall and a first separating element, and a second inner transition region,extending between the second wall and a second separating element, wherein each of the first outer transition region, the second outer transition region, the first inner transition region, and the second inner transition region has a round contour in the shape of a circular arc, and a separate roller assembly arranged in each of the guide channels, each roller assembly comprising an inner roller element and an outer roller element, wherein the inner roller element is arranged on a rotor hub element and the outer roller element is arranged in engagement with the first wall, the second wall, and the third wall, wherein the outer roller element has a convex outer surface and a convex inner surface.

[0006] US 39 715 E teaches a tripod assembly comprising: a housing body extending between a first housing end and a second housing end along a longitudinal axis, the housing body defining a plurality of guide channels spaced apart from one another by a separating element, the guide channels extending from the first housing end to the second housing end, each of the guide channels having a first wall, a second wall, and a third wall extending between the first wall and the second wall, a first outer transition region extending between the first wall and the third wall, and a second outer transition region extending between the second wall and the third wall, a first inner transition region extending between the first wall and a first separating element, and a second inner transition region extending between the second wall and a second separating element,wherein each of the first outer transition region, the second outer transition region, the first inner transition region, and the second inner transition region has a concave round contour of constant radius, and a separate roller assembly disposed in each of the guide channels, each roller assembly comprising an inner roller element and an outer roller element, the inner roller element being disposed on a rotor hub element and the outer roller element being disposed in engagement with the first wall, the second wall, and the third wall, the outer roller element having a convex outer surface and a convex inner surface.

[0007] It is the object of the present invention to provide a tripod assembly, a tripod housing and a method for producing such a tripod housing, wherein the tripod assembly and the tripod housing have roller tracks which make it possible to prevent a stuck state of roller assemblies within the roller tracks, which minimize the tilting of roller assemblies within the roller tracks, wherein the tilting is seen along a plane which extends transversely to a longitudinal axis of the tripod housing, and which optimize a restoring moment of the roller assemblies within the roller tracks.

[0008] This object is achieved by a tripod arrangement having the features of claim 1, a tripod housing having the features of claim 9 and a method for producing a tripod housing of claim 16.

[0009] Advantageous embodiments are specified in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the claims at the end of the specification. The foregoing and other features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a perspective view of a tripod joint in accordance with one aspect of the disclosure; Fig. 2 a side view of a tripod housing of the tripod arrangement of Fig. 1 is; Fig. 2A is an enlarged partial view of a circled area 2A of the tripod housing of Fig. 2, which illustrates a contour of a ball track of the tripod housing; and Fig. 2B is a partial cross-sectional view of a roller assembly disposed on a rotor hub, viewed generally parallel to a central axis of the roller assembly. DETAILED DESCRIPTION

[0011] Referring now to the figures, in which the invention is described with reference to specific embodiments without limitation thereto, it should be understood that the disclosed embodiments are merely illustrative of non-limiting embodiments of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or reduced in size to show details of particular components. Therefore, the specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for enabling one skilled in the art to practice the present invention.

[0012] With reference to Fig. 1, a tripod joint 10 enables the transmission of torque between two rotatable shaft members, potentially changing their axial or angular position relative to each other. The tripod joint 10 is configured to transmit torque from the first shaft member 12 to a second shaft member (not shown) through a rotor hub member 14 operatively connected to the second shaft member, through various speeds, joint angles, or telescopic positions, as will be well known and appreciated by those of ordinary skill in the art of tripod joint technology.

[0013] The tripod joint 10 includes a tripod housing 20 extending from the first shaft member 12 along a longitudinal axis, illustrated as a longitudinal central axis 22. The combination of the tripod housing 20 and the first shaft member 12 is rotatable in a fixed, interconnected relationship about the longitudinal central axis 22. A direction of travel of a roller assembly, also referred to as a ball set, ball element, or ball assembly 16, rotatably mounted on the rotor hub member 14, is substantially parallel to the longitudinal central axis 22.

[0014] The tripod housing 20 includes a housing body 30. The housing body 30 has an outer surface 32 and an inner bore 18 defined by an inner surface 34, each extending along the longitudinal central axis 22 between a first housing end 36 and the second housing end 38. The first shaft member 12 is fixedly connected to the housing body 30 in close proximity to the second housing end 38 and may be formed as a single piece of material integral with the housing body 30, or as a separate piece of material that is subsequently fixed to the housing body 30, purely by way of example and without limitation, for example via a welded connection.

[0015] With reference to Fig. 1-2B, the inner surface 34 of the housing body 30 defines at least one and preferably a plurality of separators, shown as three separators 50, and a plurality of roller tracks, also referred to as ball tracks, lubricated ball bores, or guide channels 52. Each separator 50 extends radially from an outermost region of the inner surface 34 toward the longitudinal central axis 22 to a radially inwardly facing free end, also referred to as a tip 24. The separators 50 extend axially from the first housing end 36 to the second housing end 38. The separators 50 may be formed with a chamfer 60 located proximal to the first housing end 36.The chamfer 60 extends from the inner surface 34 of the housing body 30 to the tip 24 of the separator 50, with the chamfer 60 extending in an oblique relationship to the inner surface 34 and the tip 24 of the separator 50.

[0016] Each guide channel 52 extends axially along and substantially parallel to (substantially shall mean that there may be a deviation from true parallel of up to 5 degrees, rather than being truly parallel) or truly parallel to the axis 22. Each guide channel 52 extends radially inwardly from a respective pair of apexes 24 toward the outer surface 32 of the housing body 30, between circumferentially spaced-apart separating elements 50. The tripod joint 10 includes three guide channels 52 that are circumferentially spaced from each other about the axis 22 by intermediate separating elements 50, such that each guide channel 52 is partially defined by and disposed between a pair of separating elements 50.Each guide channel 52 is configured to receive a portion of a pivot pin 26 of the rotor hub member 14, on which a respective ball assembly 16 is disposed, the rotor hub member 14 being received for translation along the longitudinal center axis 22 within the inner bore 18 of the housing body 30 of the tripod housing 20.

[0017] Each guide channel 52 of the housing includes a first wall 70, a second wall 72, and a third wall 74. The first wall 70 and the second wall 72 extend radially away from the axis 22 toward the third wall 74.

[0018] The first wall 70 is disposed opposite the second wall 72 such that the first wall 70 and the second wall 72 are in a facing, substantially parallel relationship to each other (substantially means that the first and second walls 70, 72 may not actually be parallel to each other due to any curvature). The third wall 74 extends between the first wall 70 and the second wall 72. The third wall 74 is disposed farther from the axis 22 than the first wall 70 and the second wall 72 to define a radially outermost surface of the inner surface 34. The first wall 70 and the second wall 72, at least in a transition region with the third wall 74, have a generally arcuate or a generally concave profile when viewed in a cross-section taken along a plane transverse to the longitudinal central axis 22, as shown in Fig. 2A.

[0019] The first wall 70 defines a first functional region 80. The first functional region 80 extends between the separating element 50 and the third wall 74. The first functional region 80 is arranged on the first wall 70 and at least a portion of the separating element 50 that is arranged adjacent to the guide channel 52.

[0020] The first functional area 80 is designed to be connected to a portion of the rotor hub element 14 ( Fig. 2B), such as an outer roller element, also referred to as outer ball element 81 of a roller assembly, also referred to as ball assembly 83, disposed on the rotor hub member 14. The ball assembly 83 includes an inner roller element, also referred to as inner ball element 85, disposed directly on the pivot pin 26 of the rotor hub member 14, with rollers, such as needle rollers 87, disposed between the outer ball element 81 and the inner ball element 85 to enable low-friction relative rotation of the outer ball element 81 and the inner ball element 85 relative to each other.

[0021] The second wall 72 defines a second functional region 90. The second functional region 90 faces the first functional region 80. The second functional region 90 extends between another separating element 50 and the third wall 74. The second functional region 90 is arranged on the second wall 72 and at least a portion of the separating element 50 that is arranged adjacent to the guide channel 52.

[0022] The second functional area 90 is designed to be connected to a portion of the rotor hub element 14 ( Fig. 2B), such as the outer ball element 81 of the ball assembly 83, which is arranged on the rotor hub element 14 in a similar manner as the ball assembly 83 in contact with the first functional region 80.

[0023] The third wall 74 may be defined purely by way of example and without limitation by a first wall portion 100, a second wall portion 102, and a third wall portion 104. The first wall portion 100 extends between one end of the first wall 70 and the second wall portion 102. The second wall portion 102 extends between the first wall portion 100 and the third wall portion 104. The second wall portion 102 is configured as a convex projection or extension 106 that extends radially inward toward the longitudinal central axis 22. The third wall portion 104 extends between the second wall portion 102 and one end of the second wall 72.

[0024] At least a portion of the third wall 74 defines a third functional area 110. The third functional area 110 is defined at least in part by the second wall section 102. In at least one embodiment, the third functional area 110 may be defined by at least one or more of the first wall section 100, the second wall section 102, and the third wall section 104.

[0025] The third functional region 110 is configured to contact a portion of the rotor hub member 14, such as the outer ball member 81 of the ball assembly 83 disposed on the rotor hub member 14.

[0026] How best in Fig. 2A, a first outer transition region 112 extends between the first wall 70 and the third wall 74, blending the first and third walls 70, 74, and a second outer transition region 114 extends between the second wall 72 and the third wall 74, blending the second and third walls 72, 74. A first inner transition region 116 extends between the first wall 70 and a first separator 50a, blending the first wall 70 and the first separator 50a, and a second inner transition region 118 extends between the second wall 72 and a second separator 50b, blending the second wall 72 and the second separator 50b.At least one of the first outer transition region 112, the second outer transition region 114, the first inner transition region 116, and / or the second inner transition region 118 is provided by a surface having a contour profile defined by a second-order quadratic function, in contrast to known transition regions that have a straight, linear first-order contour profile. The second-order contour profile is defined herein to provide an elliptical contour. The elliptical contour minimizes the tilting of ball assemblies 83 within the guide channels 52, inhibiting the potential for a binding condition of the ball assembly 83 and optimizing a restoring moment of the ball assemblies 83 within the guide channels 52 to promote the retention and return of the ball assemblies 83 to their intended low-friction running orientation within the guide channels 52.

[0027] As in Fig. 2A, the first outer transition region 112 and the second outer transition region 114 each have an outer elliptical contour, and the first inner transition region 116 and the second inner transition region 118 each have an inner elliptical contour. As illustrated, the outer elliptical contours of the first outer transition region 112 and the second outer transition region 114 may be the same, and the inner elliptical contours of the first inner transition region 116 and the second inner transition region 118 may be the same. Additionally, the outer elliptical contours and the inner elliptical contours may be the same; however, for optimal low rolling friction and aligning torque performance, they are different from each other, as further explained below.

[0028] The outer elliptical contours of the first outer transition region 112 and the second outer transition region 114 each have an outer major diameter OMD, and the inner elliptical contours of the first inner transition region 116 and the second inner transition region 118 each have an inner major diameter IMD, wherein the outer major diameters OMD are equal to one another and the inner major diameters IMD are equal to one another. However, the outer major diameters OMD differ from the inner major diameters IMD. In particular, the outer major diameters OMD are larger than the inner major diameters IMD in order to maximize the low rolling friction and the restoring moment for the ball assemblies 16. Accordingly, the outer elliptical contours are larger than the inner elliptical contours ( Fig. 2A).

[0029] The outer major diameter OMD of each of the first outer transition region 112 and the second outer transition region 114 and the inner major diameter IMD of each of the first inner transition region 116 and the second inner transition region 118 extend generally transverse to the first wall 70 and the second wall 72. Accordingly, the outer major diameters OMD and the inner major diameters IMD are generally parallel to each other.

[0030] As in Fig.2B, each roller assembly 83 includes the inner roller member 85 and the outer roller member 81. The inner roller member 85 is disposed on the generally spherical pivot bearing member 26 of the rotor hub member 14, and the outer roller member 81 is disposed for engagement with the first wall 70, the second wall 72, and the third wall 74. The outer roller member 81 has a convex outer surface 124 configured for engagement with the first outer transition region 112 and the second outer transition region 114, and a convex inner surface 126 configured for engagement with the first inner transition region 116 and the second inner transition region 118.The convex outer surface 124 and the convex inner surface 126 may each have a constant radius R, where the radius R may be the same for both or different from each other as best suited for optimal interaction with the respective elliptical contact surfaces of the transition regions 112, 114, 116, 118.

[0031] According to another aspect, a method for manufacturing a tripod housing 20 is provided. The method includes providing a housing body 30 extending between a first housing end 36 and a second housing end 38 along a longitudinal central axis 22. Further, it includes forming a plurality of guide channels 52 extending axially relative to the longitudinal central axis 22 from the first housing end 36 to the second housing end 38, wherein the plurality of guide channels 52 are spaced from one another circumferentially about the longitudinal axis 22 by separating elements 50.Furthermore, it also includes forming each of the guide channels 52, which have a first wall 70, a second wall 72, and a third wall 74 extending between the first wall 70 and the second wall 72, with a first outer transition region 112 extending between the first wall 70 and the third wall 74; a second outer transition region 114 extending between the second wall 72 and the third wall 74; and a first inner transition region 116 extending between the first wall 70 and a first separating element 50a; a second inner transition region 118 extending between the second wall 72 and a second separating element 50b, and forming at least one of the first outer transition region 112, the second outer transition region 114, the first inner transition region 116 and / or the second inner transition region 118 to have an elliptical contour.

[0032] According to another aspect, the method may further comprise forming both the first outer transition region 112 and the second outer transition region 114 to have an elliptical contour.

[0033] In another aspect, the method may include forming both the first inner transition region 116 and the second inner transition region 118 to have an elliptical contour.

[0034] In another aspect, the method may further comprise forming each of the first outer transition region 112, the second outer transition region 114, the first inner transition region 116, and the second inner transition region 118 to have an elliptical contour.

[0035] According to another aspect, the method may further comprise forming the first outer transition region 112 and the second outer transition region 114 to have an outer elliptical contour with an outer major diameter OMD, and forming the first inner transition region 116 and the second inner transition region 118 to have an inner major diameter IMD, wherein the outer major diameter OMD is different from the inner major diameter IMD.

[0036] According to another aspect, the method may further comprise forming the outer major diameter OMD such that it is larger than the inner major diameter IMD.

[0037] In this specification, the terms "attach", "fastening", "connected", "coupled", "coupling", "mount" or "assembling" shall be construed to mean that a structural component or element is connected or in contact with another element in some manner, either directly or indirectly through at least one intervening structural element, or that it is formed integrally with the other structural element.

Claims

[1] Tripod arrangement comprising: a housing body (30) extending between a first housing end (36) and a second housing end (38) along a longitudinal axis (22), wherein the housing body (30) defines a plurality of guide channels spaced from one another by a separating element (50), the guide channels (52) extending from the first housing end (36) to the second housing end (38), each of the guide channels (52) having a first wall (70), a second wall (72), and a third wall (74) extending between the first wall (70) and the second wall (72), a first outer transition region (112) extending between the first wall (70) and the third wall (74), and a second outer transition region (114) extending between the second wall (72) and the third wall (74), a first inner transition region (116) extending between the first wall (70) and a first separating element (50), and a second inner transition region (118),extending between the second wall (72) and a second separating element (50), wherein at least one of the first outer transition region (112), the second outer transition region (114), the first inner transition region (116) and the second inner transition region (118) has a concave elliptical, non-circular contour, and, a separate roller assembly disposed in each of the guide channels (52), each roller assembly (83) comprising an inner roller element (85) and an outer roller element (81), the inner roller element (85) being disposed on a rotor hub element (14) and the outer roller element (81) being disposed in engagement with the first wall (70), the second wall (72), and the third wall (74), the outer roller element (81) having a convex outer surface (124) configured for engagement with the first outer transition region (112) and the second outer transition region (114), and a convex inner surface (126) configured for engagement with the first inner transition region (116) and the second inner transition region (118). [2] The tripod assembly of claim 1, wherein the first outer transition region (112) and the second outer transition region (114) have an elliptical, non-circular contour. [3] The tripod assembly of claim 1, wherein the first inner transition region (116) and the second inner transition region (118) have an elliptical, non-circular contour. [4] The tripod assembly of claim 1, wherein the first outer transition region (112) and the second outer transition region (114) have an outer elliptical, non-circular contour and the first inner transition region (116) and the second inner transition region (118) have an inner elliptical, non-circular contour. [5] A tripod assembly according to claim 4, wherein the outer elliptical non-circular contour and the inner elliptical non-circular contour are different from each other. [6] The tripod assembly of claim 5, wherein the outer elliptical non-circular contour has an outer major diameter (OMD) and the inner elliptical non-circular contour has an inner major diameter (IMD), the outer major diameter (OMD) being larger than the inner major diameter (IMD). [7] The tripod assembly of claim 6, wherein the outer major diameter (OMD) and the inner major diameter (IMD) extend generally transverse to the first wall (70) and the second wall (72). [8] The tripod assembly of claim 1, wherein the convex inner surface (126) of the outer roller member (81) and the convex outer surface (124) of the outer roller member (81) have a constant radius (R). [9] Tripod housing (20), comprising: a housing body (30) extending along a longitudinal axis (22) between a first housing end (36) and a second housing end (38), wherein the housing body (30) defines a plurality of guide channels (52) extending from the first housing end (36) to the second housing end (38), each of the guide channels (52) having a first wall (70), a second wall (72), and a third wall (74) extending between the first wall (70) and the second wall (72), a first outer transition region (112) extending between the first wall (70) and the third wall (74), and a second outer transition region (114) extending between the second wall (72) and the third wall (74), a first inner transition region (116) extending between the first wall (70) and a first separating element (50), and a second inner transition region (118) extending between the second wall (72) and a second separating element (50),and at least one of the first outer transition region (112), the second outer transition region (114), the first inner transition region (116) and the second inner transition region (118) has an elliptical, non-circular contour., [10] The tripod housing (20) of claim 9, wherein the first outer transition region (112) and the second outer transition region (114) have an elliptical, non-circular contour. [11] The tripod housing (20) of claim 9, wherein the first inner transition region (116) and the second inner transition region (118) have an elliptical, non-circular contour. [12] The tripod housing (20) of claim 9, wherein the first outer transition region (112) and the second outer transition region (114) have an outer elliptical, non-circular contour and the first inner transition region (116) and the second inner transition region (118) have an inner elliptical, non-circular contour. [13] Tripod housing (20) according to claim 12, wherein the outer elliptical non-circular contour has an outer major diameter (OMD) and the inner elliptical non-circular contour has an inner major diameter (IMD), the outer major diameter (OMD) and the inner major diameter (IMD) being different from each other. [14] Tripod housing (20) according to claim 13, wherein the outer major diameter (OMD) is larger than the inner major diameter (IMD). [15] The tripod housing (20) of claim 14, wherein the outer major diameter (OMD) and the inner major diameter (IMD) extend generally transverse to the first wall (70) and the second wall (70). [16] A method for manufacturing a tripod housing (20), comprising: Providing a housing body (30) extending along an axis (22) between a first housing end (36) and a second housing end (38); Forming a plurality of guide channels extending from the first housing end (36) to the second housing end (38), the plurality of guide channels (52) being radially spaced from one another around the longitudinal axis (22) by separating elements (50); Forming each of the guide channels (52) having a first wall (70), a second wall (72), and a third wall (74) extending between the first housing end (36) and the second housing end (38), with a first outer transition region (112) extending between the first wall (70) and the third wall (74), and a second outer transition region (114) extending between the second wall (70) and the third wall (74), and with a first inner transition region (116) extending between the first wall (70) and one of the separating elements (50), and a second inner transition region (118) extending between the second wall (72) and another of the separating elements (50); and Forming at least one of the first outer transition region (112), the second outer transition region (114), the first inner transition region (116) and the second inner transition region (116) in such a way that it has an elliptical, non-circular contour. [17] The method of claim 16, further comprising forming each of the first outer transition region (112), the second outer transition region (114), the first inner transition region (116), and the second inner transition region (116) to have an elliptical, non-circular contour. [18] The method of claim 17, further comprising forming the first outer transition region (112) and the second outer transition region (114) to have an outer elliptical, non-circular contour with an outer major diameter (OMD), and forming the first inner transition region (116) and the second inner transition region (118) to have an inner elliptical, non-circular contour with an inner major diameter (IMD), wherein the outer major diameter (OMD) is different from the inner major diameter (IMD). [19] The method of claim 18, further comprising forming the outer major diameter (OMD) such that it is larger than the inner major diameter (IMD). [20] The method of claim 18, further comprising forming the outer major diameter (OMD) and the inner major diameter (IMD) to extend generally transversely to the first wall (70) and the second wall (72).

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