Prosthesis Connection System

Abstract

A prosthesis includes first and second prosthetic limb components and a fastening system for fastening the first and second prosthetic limb components. The first prosthetic limb component is provided with a first coupling member and the second prosthetic limb component is provided with a second coupling member. The first coupling member includes a cylindric member having an axis, with a wall generally parallel with said axis. The second coupling member includes a circular skirt element having an axis and having corresponding external dimensions to the cylindric member whereby to permit a sliding fit therewith in an unfastened state. The fastening system includes a resilient member having an axis and is operable to expand laterally, orthogonally with respect to the axis such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member.

Claims

1. A prosthesis including a first prosthetic limb component, a second prosthetic limb component, and a fastening system for fastening the first prosthetic limb component to the second prosthetic limb component, wherein the first prosthetic limb component is provided with a first coupling member and the second prosthetic limb component is provided with a second coupling member; wherein the first coupling member comprising a cylindric member having an axis, with a wall generally parallel with said axis; wherein the second coupling member comprises a circular skirt element having an axis and having corresponding external dimensions to the cylindric member whereby to permit a sliding fit therewith in an unfastened state; wherein the fastening system, associated with one of the first or second coupling members comprises a resilient member having an axis and wherein the resilient member is operable to expand laterally, orthogonally with respect to the axis, said axis being coaxial with the axis of the coupling member with which it is attached; wherein the fastening system is arranged such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member; whereby the first and second coupling members couple together with associated first and second prosthetic limb components.

2. A prosthesis according to claim 1, wherein the first prosthetic limb component is a prosthetic limb and comprises one of a shin member, lower arm, foot or a hand, and the second prosthetic limb component is operable to receive a stump of a patient.

3. A prosthesis according to claim 1, wherein the first prosthetic limb component is a prosthetic limb and comprises one of a shin member, lower arm, foot or a hand, and the second prosthetic limb component comprises a coupling member having an additional coupling component and is operable to couple with a known coupling element associated with a prosthetic limb component is operable to receive a stump of a patient.

4. A prosthesis according to claim 1, wherein the first prosthetic limb component is operable to receive a stump of a patient, and the second prosthetic limb component is a prosthetic limb and comprises one of a shin member, lower arm, foot or a hand.

5. A prosthesis according to claim 1, wherein the first prosthetic limb component comprises a coupling member having an additional coupling component operable to couple with a known coupling element associated with a prosthetic limb component is operable to receive a stump of a patient and the second prosthetic limb component comprises a prosthetic limb being one of a shin member, lower arm, foot or a hand.

6. A coupling unit for use in a prosthesis according to claim 1, comprising: a first prosthetic limb component having a first coupling feature, the first coupling feature comprising a cylindric member having an axis, with a wall generally parallel with said axis; a second coupling member comprises a circular skirt element having an axis and having corresponding external dimensions to the cylindric member whereby to permit a sliding fit therewith in an unfastened state; wherein a fastening system, associated with one of the first or second coupling members comprises a resilient member having an axis and wherein the resilient member is operable to expand laterally, orthogonally with respect to the axis, said axis being coaxial with the axis of the coupling member with which it is attached; wherein the fastening system is arranged such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member; wherein the second coupling member is operably associated with one of a stump, a patient's limb receiving prosthesis, an intermediate coupling part and a second prosthetic limb; whereby the first prosthetic limb component and second coupling member couple together.

7. A coupling unit for use in a prosthesis according to claim 1, comprising: a first prosthetic limb component having a first coupling feature, the first coupling feature comprising a cylindric member having an axis, with a wall generally parallel with said axis; a second coupling member comprises a circular skirt element having an axis and having corresponding external dimensions to the cylindric member whereby to permit a sliding fit therewith in an unfastened state; wherein a fastening system, associated with one of the first or second coupling members comprises a resilient member having an axis and wherein the resilient member is operable to expand laterally, orthogonally with respect to the axis, said axis being coaxial with the axis of the coupling member with which it is attached; wherein the fastening system is arranged such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member; wherein the second coupling member is operably associated with one of a stump, a patient's limb receiving prosthesis, an intermediate coupling part and a second prosthetic limb; wherein the first prosthetic limb component is a prosthetic limb and comprises one of a shin member, lower arm, foot or a hand, and the second prosthetic limb component is operable to receive a stump of a patient.

8. A coupling unit for use in a prosthesis according to claim 1, comprising: a first prosthetic limb component having a first coupling feature, the first coupling feature comprising a cylindric member having an axis, with a wall generally parallel with said axis; a second coupling member comprises a circular skirt element having an axis and having corresponding external dimensions to the cylindric member whereby to permit a sliding fit therewith in an unfastened state; wherein a fastening system, associated with one of the first or second coupling members comprises a resilient member having an axis and wherein the resilient member is operable to expand laterally, orthogonally with respect to the axis, said axis being coaxial with the axis of the coupling member with which it is attached; wherein the fastening system is arranged such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member; wherein the second coupling member is operably associated with one of a stump, a patient's limb receiving prosthesis, an intermediate coupling part and a second prosthetic limb; wherein the second coupling member is operable to couple with said one of a stump, a patient's limb receiving prosthesis, or a second prosthetic limb via an intermediate coupling component.

9. A coupling unit for use in a prosthesis according to claim 1, comprising: a first prosthetic limb component having a first coupling feature, the first coupling member comprises a circular skirt element having an axis and having a defined radial dimensions; the second coupling feature comprising a cylindric member having an axis, with a wall generally parallel with said axis, the internal dimensions corresponding to the external dimensions to the skirt member whereby to permit a sliding fit therewith in an uncoupled state; wherein a fastening system, associated with one of the first or second coupling members comprises a resilient member having an axis and wherein the resilient member is operable to expand laterally, orthogonally with respect to the axis, said axis being coaxial with the axis of the coupling member with which it is attached; wherein the fastening system is arranged such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member; wherein the second coupling member is operably associated with one of a stump, a patients limb receiving prosthesis, an intermediate coupling part and a second prosthetic limb; whereby the first prosthetic limb component and second coupling member couple together.

10. A coupling unit for use in a prosthesis according to claim 1, comprising: a first prosthetic limb component having a first coupling feature, the first coupling member comprises a circular skirt element having an axis and having a defined radial dimensions; the second coupling feature comprising a cylindric member having an axis, with a wall generally parallel with said axis, the internal dimensions corresponding to the external dimensions to the skirt member whereby to permit a sliding fit therewith in an uncoupled state; wherein a fastening system, associated with one of the first or second coupling members comprises a resilient member having an axis and wherein the resilient member is operable to expand laterally, orthogonally with respect to the axis, said axis being coaxial with the axis of the coupling member with which it is attached; wherein the fastening system is arranged such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member; wherein the second coupling member is operably associated with one of a stump, a patients limb receiving prosthesis, an intermediate coupling part and a second prosthetic limb; wherein the first prosthetic limb component is a prosthetic limb and comprises one of a shin member, lower arm, foot or a hand, and the second prosthetic limb component is operable to receive a stump of a patient.

11. A coupling unit for use in a prosthesis according to claim 1, comprising: a first prosthetic limb component having a first coupling feature, the first coupling member comprises a circular skirt element having an axis and having a defined radial dimensions; the second coupling feature comprising a cylindric member having an axis, with a wall generally parallel with said axis, the internal dimensions corresponding to the external dimensions to the skirt member whereby to permit a sliding fit therewith in an uncoupled state; wherein a fastening system, associated with one of the first or second coupling members comprises a resilient member having an axis and wherein the resilient member is operable to expand laterally, orthogonally with respect to the axis, said axis being coaxial with the axis of the coupling member with which it is attached; wherein the fastening system is arranged such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member; wherein the second coupling member is operably associated with one of a stump, a patient's limb receiving prosthesis, an intermediate coupling part and a second prosthetic limb; wherein the second coupling member is operable to couple with said one of a stump, a patient's limb receiving prosthesis, or a second prosthetic limb via an intermediate coupling component.

12. A coupling unit for use in a prosthesis according to claim 1, comprising: a first prosthetic limb component having a first coupling feature, the first coupling member comprises a circular skirt element having an axis and having a defined radial dimensions; the second coupling feature comprising a cylindric member having an axis, with a wall generally parallel with said axis, the internal dimensions corresponding to the external dimensions to the skirt member whereby to permit a sliding fit therewith in an uncoupled state; wherein a fastening system, associated with one of the first or second coupling members comprises a resilient member having an axis and wherein the resilient member is operable to expand laterally, orthogonally with respect to the axis, said axis being coaxial with the axis of the coupling member with which it is attached; wherein the fastening system is arranged such that, in a coupling mode of operation, the lateral expansion operates to cause the skirt to be in a state of frictional engagement with the other coupling member; wherein the second coupling member is operably associated with one of a stump, a patients limb receiving prosthesis, an intermediate coupling part and a second prosthetic limb; wherein the coupling feature comprising a cylindric member having an axis, with a wall generally parallel with said axis.

13. A prosthesis according to claim 1, wherein the cylindric member is a recess and upon placement of the skirt about the laterally expanding fixing means, the exterior of the skirt frictionally engages with inside walls of the recess.

14. A prosthesis according to claim 1, wherein the laterally expanding fixing means is selected from one of the following a star nut; an elastomeric material which can be compressed in a direction perpendicular to an axis of the cylindric member.

15. A prosthesis according to claim 1, wherein the inside wall surfaces of the skirt are tapered.

16. A prosthesis according to claim 1, wherein the laterally expanding fixing means is selected from one of the following a star nut; an elastomeric material which can be compressed in a direction perpendicular to an axis of the cylindric member.

17. A prosthesis according to claim 1, wherein one or both of the cylindric and skirt members has a castellation.

18. A prosthesis according to claim 1, wherein the cylindric and skirt members have cooperating lip and grooves.

19. A prosthesis according to claim 1, wherein the cylindric and skirt members are generally circular in plan.

20. A prosthesis according to claim 1, wherein the cylindric and skirt members comprise corresponding polygons in plan.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] An illustrative embodiment of the present invention is explained in more detail below with reference to a drawing, in which:

[0022] FIG. 1A shows a simple limb receiving element to foot prosthesis;

[0023] FIG. 1B shows a simple alternative fastening between a limb receiving element and prosthetic leg;

[0024] FIGS. 2A and 2B show a second simple limb receiving element with a distal stump connector in detail;

[0025] FIGS. 3A, 3B and 3C show a third simple limb receiving element with a distal stump connector in cross-section and a plan view of the inside of the limb receiving element;

[0026] FIGS. 4A and 4B show two slidable socket systems used as a connector between a limb receiving element and a prosthesis;

[0027] FIG. 4D shows the forces arising in attachment in a connector per FIG. 4B;

[0028] FIG. 4C shows a second inverted pyramid connector;

[0029] FIG. 5 show a known compression ring fastener;

[0030] FIG. 6A show a first embodiment of a lower leg prosthesis in accordance in the invention;

[0031] FIG. 6B shows a lower leg prosthesis per FIG. 6 in cross-section, ready to accept a coupling member;

[0032] FIGS. 6C & 6D show two forms of cupped discs for resilient retention;

[0033] FIGS. 6E & 6F show two further embodiments of the invention in section;

[0034] FIGS. 7A and 7B show a prosthesis with an inverted pyramid connector in place, in perspective and cross-section respectively;

[0035] FIGS. 8A-8E show a number of non-limiting examples of radial expanders that are suitable for the present invention;

[0036] FIGS. 9A and 9B show an alternative embodiment;

[0037] FIGS. 10-12 show further variations of a connector where the proximal interface is an industry standard screw thread/tubular interface, as detailed in FIG. 11A;

[0038] FIGS. 13A, 13B, 14A, 14B, 15A, and 15B show how the forces of attachment associated with an circumferentially expanding element are distributed; and,

[0039] FIGS. 16A-17B show how forces are applied in relation to prior clamping arrangements.

DETAILED DESCRIPTION

[0040] There will now be described, by way of example only, the best mode contemplated by the inventor for carrying out the present invention. In the following description, numerous specific details are set out in order to provide a complete understanding to the present invention. It will be apparent to those skilled in the art, that the present invention may be put into practice with variations of the specific.

[0041] FIG. 6A shows an example of a prosthetic socket system 60 according to a first illustrative embodiment of the present invention. The prosthesis socket system is defined at a proximal portion of a tibial prosthesis, with a hydraulic control element not present, to permit a clear view of a cylindrical recess 61 of depth d and a diameter D. The axis of the recess corresponds with an axis of insertion associated with a connection system, with respect to the other prosthetic part to which the system is connected. As will be appreciated, socket systems of this general type are known from the prior art. The prosthesis socket, into the proximal end of which an amputation stump of the patient is inserted when the patient is putting on a prosthesis equipped with such a socket system is not shown. In order to ensure an optimal hold and fit, and in order to cushion the sometimes sensitive amputation stump, a liner made of an elastic material, for example silicone, can first of all be pulled over the amputation stump. The amputation stump thus provided with the liner is then inserted into the prosthesis socket. An index line 62 may be formed, such that it is one of either raised or indented with respect to the recess surface, with corresponding opposite features associated with the outside surface of the skirt whereby the skirt—and the socket—will not be capable of rotating whilst in position, with the compression fitting in operation.

[0042] FIG. 6B shows a section through the prosthesis of FIG. 6A, prior to insertion of a connector element such as shown in FIG. 11A, below, that can mate with a corresponding connector of a proximal stump receiving element. The recess member 61 provides a generally circular recess, with upstanding walls, which depend and come together to include a sub-housing 65 operable to retain a screw-threaded bolt 64 which acts to push circumferential expansion member 63: Circumferentially expandable member 63 lies within cylindrical recess/receiving socket 61, which in an unexpanded state defines a circular channel for reception of a skirt element 62 associated with a connector (See FIG. 7). In such an unexpanded state, the skirt element 62 is of a general sliding/interference fit with respect to the inside of the recess 61. The compression fastener element 63, is urged to expand upon the driving of screw-threaded fastener 65, so that the cupped element is caused to have of a reduced axial length. FIG. 6C show a first cupped element 63, which can be made from aluminium for low weight or from thin walled titanium. FIG. 6D provides a first alternative; there is provided a number of slots 66 radially extending from part-circular apertures 67, set a distance from a central aperture 68, which is used to engage with a screwthreaded device 64 or similar; said slots produce anisotropic radial stiffness and circumferential compliance upon fastening.

[0043] With reference to this figure and FIG. 13A, below, it can be seen that the screw-threaded bolt 64 is arranged to push the centre of the disc (compression fastener element) 63 away from the prosthesis and therefore abuts against the frame 65 of the connector housing. It will be appreciated that the circumferentially/radially expanding element can be caused to expand in a number of different ways, especially as indicated in FIGS. 8C-8E. The outside edge of the fastener element 63 radially expands, in turn causing the skirt 62 to be pushed outwardly, in turn fixing the skirt 62 axially relative to the recess 61. With reference to FIG. 6, an index line (not shown) may be formed, such that it is one of either raised or indented with respect to the recess surface, with corresponding opposite features associated with the outside surface of the skirt whereby the skirt—and the socket—will not be capable of rotating whilst in position, with the compression fitting in operation. In another embodiment, as shown in FIG. 6E, where the clamped aspect of connector 62A is castellated such that the connector 62 terminates with multiple prongs to allow radial securement and high axial rigidity, such to allow an undercut fitment with the base part of the connector 65. This benefits the system in that a resilient bias can be provided to provide a click press fit allows connector 62 to resiliently clip in the undercut 65A in base part of the connector 65. Tightening jack fastener 64, acting on laterally expanding member 63 secures the prongs of connector 62A to be lodged securely in undercut face 65A set). The axial weight of the patient is carried by the preload of the Jack fastener (2), and rotation is prevented by the friction. Continuous force lines are indicated 67A. In yet another embodiment, per FIG. 6F, the undercut is a grooved face 65B whereby the body weight is supported by the exterior face of component 62, whereby inherently preventing a helical freedom of movement.

[0044] FIG. 7A shows the prosthetic socket system of FIG. 6A with an inverted pyramidal connector 120 in place, the pyramid element being received by a flange plane 61A, with a depending circular skirt, indicated by 62 which has a diameter in correspondence with diameter D, the skirt being of a sliding/interference fit with respect to the inside of the recess 61. A compression fastener element 63, is urged to expand upon the driving of screw-threaded fastener 65 with respect to a part 65 fixed relative to the prosthesis. The outside edge of the fastener element 63 radially expands, in turn causing the skirt 62 to be pushed outwardly, in turn fixing the skirt 62 axially relative to the recess 61. FIG. 7B shows the view of the inverted pyramid connector 120 in cross-section.

[0045] An expandable bushing assembly is conveniently associated with a screw-threaded shank of an ordinary bolt creates an expandable diameter bolt that installs freely and easily in a hole. When the nut is tightened against the bushing assembly it becomes radially tight in the hole. Such expandable diameter bolts are as easy to remove as an ordinary nut and bolt and every bit as reusable. Expandable diameter bolts provide the capability of fabricating demountable rigid (minimum deflection under load) structural joints. The extremely tight radial fit of these fasteners makes them ideal for resisting shock loads, severe vibration, reversing, and cyclic shear fatigue type loading.

[0046] With reference to FIGS. 8A through to 8E, there are shown a number of simple expansion members. FIGS. 8A and 8B comprise, respectively double and single star nuts, or tube connectors, known for example, from bicycle headsets; the edges grip an inside of the tube and as a bolt is tightened, the star shape seeks to adopt a generally flatter profile, with the result that the grip of the star nut against the inside edge of the recess is increased. FIGS. 8C and 8D show, respectively expansion bolts having access at each end and a blind access expansion bolt, both types compressing a resilient element outwardly upon fastening of the respective fastener. FIG. 8E shows a cam-operated radial expansion bolt: —the blind bolt is modified to utilize a cam type handle to expand the bushings rather than a threaded nut, you have a quick-acting expandable diameter pin. Such a pin can be tightened or relaxed by one simple, quick, easy motion, yet when installed, provides a tight radial fit in the hole. When relaxed, there is ample clearance for installation and removal. This type fastener offers high shear capability and, being radially tight in the hole, is applicable in areas of high shear fatigue, high shock loading, and difficult vibration.

[0047] FIG. 9A shows another embodiment, wherein the prosthesis 90 is equipped with a connector 91 adapted to cooperate with a four hole connector system of the corresponding to-be-directly-mated-with another prosthetic part, in this case a limb receiving element, not shown. However, FIG. 9B shows a lower leg prosthesis 90. Coupling connector 91 is provided with four bolt-holes to permit four fastening bolts to said other prosthetic part. FIGS. 10 and 11A show an example upper part of a lower leg prosthesis 60 with a tapered receiving connector 101 and a tube extension connector 111. Referring to FIG. 11B, the proximal interface, or the ‘balance of the adapter’ is indicated and is machined to represent any proximal interface that has the MTI machine taper interface in association with the disclosure, with reference numeral 60 indicating the distal balance of the limb system.

[0048] FIG. 12 shows a still further embodiment wherein there is a provided a prosthesis 60 having a socket, with a first axis and an inverted pyramid coupling element 120, associated with the skirt element of the coupler, save that an axis associated with the inverted pyramid is inclined with respect to the first axis, as indicated by the balance of the adapter (BoA). The machined taper interface (MTI) and interlocking interface 61 are also shown in this figure. In this example jacking element 122, driven via a recessed head 64, such as an Allen fastener head is screwthreadedly mounted with respect to the connector 65, which is located within element 124, which depends from socket/recess element 61.

[0049] Turning now to FIG. 13A, there is shown in cross-section (through the middle of the circular member) radially expanding member 63 or wedge suitable for fitment in the prosthesis of FIG. 6 in a relaxed state n1 with relaxed diameter Dn; FIG. 13B is a corresponding force diagram with the compression force or jacking force Fj, with a net resultant radial forces Fr. Given that the compression force is zero, per Fr+Fj/sin A1°, the radial forces are also zero. It is to be noted that angle A1 is the simplified representative angle of the wedge where the wedge has an advantageous radius R, to facilitate a rolling action on the machine taper face of restraining prosthetic adapter in turn also restrained in the housing 61, per FIG. 15A and as indicated by reference numeral 2 in FIG. 13B. FIG. 13A also represents the radially expanding member 63 in a loaded state dl with a correspondingly deformed shape with an increased diameter Dd. Per FIG. 13B, the axial compression force or jacking force, per Fr+Fj/sin A1°, the radial forces define a functional load.

[0050] FIG. 13A represents the rolling action on the restraining outer contour of the wedge as a pivot h1, and the flexural aspect of the centre of the wedge as a hinge (h2). The goniometry predicts the maximum force amplification Fr=Fj/sin(A1), where Fr>>Fj. In the event that angle A1<30°, then 1/sin A1>2−i.e. realizing an amplification factor of more than 2. Advantageously, angle A1 as chosen to be nearer to 10°, meaning 1/sin A1=5.7, a maximum amplification factor of 5.7. Naturally the representation of the pivot points h1, h2 are not ideal, and the angle A1 will not be precisely known; moreover, the flexural properties of the resilient member will affect the pivot properties. Nonetheless, it will be understood that Fr>Fj, and therefore the jacking force Fj will be amplified by a factor greater than one. It has been found helpful for the inside surface of skirt to be tapered, such that the outside edge surface of a cupped resilient element having thickness T, will permit the direction of the force vector Fr to approach the axis of a normal vector arising from the tapered contact surface. It will be appreciated that the taper angle will be within a range of 1-15° with respect to the axis of the skirt element.

[0051] The present invention, nonetheless also benefits in that a second mechanism assists in amplification. With reference to FIG. 14A, the resilient element n1 is modelled as a rigid body, and the low friction between the resilient element wedge n1 and the restraining wall of coupling recess 2 against the self releasing machine taper angle A2 of the resilient element n1 per FIG. 14B is modelled substantially as a roller-sliding interface. In fact the use of a lubricant can be advantageously applied. The geometry of the situation is such that force Fj to generates reaction force Ft to equate at best to Ft=Fj/sin (A2), where Ft>>Fj. This again means an amplification of the force, where the amplification factor is between 1/sin A2>1 depending on the friction conditions, noting that if A2<30 degrees, then 1/sin A1>2 meaning that the amplification is greater than 2. Advantageously A2 as chosen to be nearer to 10°, meaning 1/sin A1=5.7, a maximum amplification factor of 5.7.

[0052] On account of both of these mechanisms, the axial force generated by a tightening the screwbolt/jacking fastener, is redirected and amplified by the wedge n1, subject to the wedge being such that upon deflection, the outer diameter increases—akin to the fashion in which a disk spring operates and that the wedge acts on a mating machine taper where the angle of the machine taper causes a radial reaction force that is always larger than the axial driving force on the wedge.

[0053] Due to the advantageous and ideal round circumferential shape of the resilient wedge contacting the mating adapter part, with reference to FIGS. 15A & B, a uniform force Ft+Fr is transferred through the clamped aspects of the adapter to create a friction force Ffr=(Ft+Fr)*C, where C is the friction coefficient between the contacting materials.

[0054] An analysis of a prior art clamping system with an external pinchbolt is effectively a lever arm system, where the pinching force Fpinch in FIGS. 16A & 16B is effectively hinged by balancing force Fhoop on the opposite side of the clamp. Depending on the friction coefficient between the clamp and restraining clamped-on mating part, an effective lever system is formed where at best the pinch force is amplified to a factor of 2 when the friction coefficient is zero, and the internal mating parts act in accordance with liquid and hoop stress phenomena. That is to say, the pinch and hoop forces add to a sum of forces Fpinch+Fhoop, such that, in lubricated conditions, will give an equally distributed load on the clamped part.

[0055] When the friction coefficient is ‘infinite’ (i.e. bonded), no slippage can occur between mating parts and the force equation is graphically displayed in FIGS. 17A & 17B, where the friction force ff opposes Fpinch (fp). This shows that the amplification of the clamping force theoretically achieves a maximum with a factor equal to 2 in a lubricated condition, and a minimum of 1 in a bonded condition. This means, in effect that in the case of severe friction all pinch forces Fpinch are balanced by a local reaction force F in the clamped part 56, meaning that the friction reduces the ability to transfer load evenly, needing more material in the clamped part to withstand material stress.