Abstract:
A flexible coupling comprising: a flexible torque disc comprising a plurality of mount points; a first coupling member mounted to a first subset of said mount points; one or more second coupling members each mounted to a second subset of said mount points; wherein said first coupling member comprises one or more flanges substantially parallel with said torque disc; and wherein said one or more second coupling members each has a groove engaging with said one or more flanges so as to limit the displacement of said first coupling member with respect to said second coupling member. This coupling arrangement is easy to assemble, repair and maintain in confined spaces, while also being lightweight and using a small number of components.

Description:
FOREIGN PRIORITY 
       [0001]    This application claims priority to Great Britain Patent Application No. 1414801.9 filed Aug. 20, 2014, the entire contents of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    This disclosure relates to a flexible coupling, in particular to a flexible coupling for joining two shafts and transmitting torque there between. 
       BACKGROUND 
       [0003]    Flexible couplings are used for example in drive trains where a certain amount of flex needs to be accommodated. For example in the drive trains for flaps and slats on aircraft wings, a certain amount of flex in the wings needs to be accommodated. 
         [0004]    One type of flexible coupling that has been extensively used is a barreled spline joint. The barreling on this joint allows angular movement of one shaft with respect to the other. However, excessive angular deviation can lead to compromise of the environmental seal of the joint. In tests on one barreled spline joint a torque of less than 50 Nm was required before the joint reached an angle of 7 degrees at which point the seal is considered compromised. At a bending moment of less than 100 Nm, the barreled spline joint showed visible damage. 
         [0005]    Another type of flexible coupling in regular use is the diaphragm coupling which transmits torque through a diaphragm of a different diameter to the shafts so that a certain amount of angular or axial displacement can be accommodated. 
         [0006]    Flexible torque discs are one type of flexible coupling that can be used to connect two shafts together, transmitting torque from one shaft to the other while allowing small angular and axial displacements of one shaft relative to the other to be accommodated. Typically one shaft is fixed to one side of the torque disc and the other shaft is fixed to the opposite side of the torque disc such that movements (axial or angular) result in deformation of the torque disc. 
         [0007]    For reduced weight, torque discs may be formed from composite materials such as carbon fibre reinforced polymer (CFRP). Such composite materials can be designed to have good torque transmission characteristics (i.e. high rigidity) in the rotation direction (i.e. circumferentially) while having a degree of compliance under bending moments (i.e. out of plane loads). WO 2013/064807 describes such a torque disc. 
         [0008]    One problem with composite materials is that too high a bending force may cause delamination of layers within the material or may cause small stress fractures that can weaken the disc. Moreover such defects may be difficult to detect (e.g. not visible on visual inspection) and may cause an unacceptable risk of failure. 
         [0009]    The bending force that occurs during normal use can be calculated in advance and the joint designed appropriately. However, higher bending moments can occur during installation, maintenance and repair. For example, a shaft on one side of the joint may be displaced by an engineer when effecting a repair or trying to gain access in a confined space. The bending induced by such movement may result in a higher bending moment than the joint was designed for and may result in damage as discussed above. 
         [0010]    One way to prevent excessive bending through a coupling is to use hollow shafts for both the driving and driven shafts and to provide a pin across the coupling that passes from the inside of one shaft to the inside of the other shaft. The length and diameter of the pin relative to the internal dimensions of the shafts determines how much angular deviation can take place before the pin engages with the inside walls of the hollow shafts and prevents further bending. However the pin has to span the joint which can make assembly of the joint difficult, particularly in confined spaces. 
       SUMMARY 
       [0011]    According to one aspect of this disclosure there is provided a flexible coupling comprising: a flexible torque disc comprising a plurality of mount points; a first coupling member mounted to a first subset of said mount points; one or more second coupling members each mounted to a second subset of said mount points; wherein said first coupling member comprises one or more flanges substantially parallel with said torque disc; and wherein said one or more second coupling members each has a groove engaging with said one or more flanges so as to limit the displacement of said first coupling member with respect to said second coupling member. 
         [0012]    In use the first coupling member may be attached to or integrally formed with a rotatable member such as a first shaft. The second coupling members are preferably attachable to a second rotatable member such as a second shaft. The interaction between the flange and the groove(s) serves to restrict angular deflection of the first rotatable member with respect to the second rotatable member. The flange and the groove do not need to be in contact with one another. In fact, in normal operation they will not be in contact. However, they cooperate so as to prevent one rotatable member from bending off-axis to a degree that would risk damage to the torque disc. As the groove essentially surrounds or encompasses the flange, movement of the flange (either due to axial movement of the first coupling member or due to off-axis deflection of the first coupling member or a shaft attached thereto) causes the flange to come into contact with one or other side of the groove, restricting further movement and thus restricting the bending moment applied to the torque disc. 
         [0013]    Particularly advantageously, the flange(s) are integrally formed with the first coupling member which is directly fixed to the torque disc. This reduces the number of parts that need to be manufactured and reduces the number of parts that need to be aligned and assembled together when connecting the joint. 
         [0014]    The flange(s) preferably extend partially around the second coupling member(s). For example where the second coupling members are generally circular (in cross section in the plane of the torque disc), the flange extends in an arc around the second coupling member within the groove. The flange preferably then overlaps in the radial direction with the second coupling member so that if the disc fails for any reason, rotational movement can still be transmitted through the coupling by transmitting torque directly through the interface between first and second coupling members (i.e. by the flange pressing on and driving the second coupling member or vice versa). With the flange overlapping radially with the second coupling member on both sides of the coupling member, torque can be transmitted in this manner in either rotation direction. Thus preferably the or each flange has a concave arc shape that mates or engages with the groove around the second coupling member. The concave flange may face radially outwards, i.e. the first coupling member presenting its flange(s) in the radially outward direction. The arc may be a full semicircle in extent or it may be less than a semicircle. 
         [0015]    The flange may be thinner than the rest of the first coupling member. As the flange must fit inside the groove, a thinner flange allows a thinner groove which in turn allows a lower profile second coupling member to be used, thus saving weight. 
         [0016]    The relative dimensions of the flange and the groove will affect the amount of movement (translational or bending) that can be accommodated before contact occurs and further movement is restricted. 
         [0017]    In some preferred examples, the coupling permits at least 0.5 degrees of angular movement, more preferably at least 1 degree of angular movement. The coupling may restrict the angular movement to be not more than 5 degrees, more preferably not more than 3 degrees. 
         [0018]    In some examples, the coupling permits at least 1 mm of axial movement, more preferably at least 2 mm of axial movement. The coupling may restrict axial movement to be not more than 4 mm of axial movement, more preferably not more than 3 mm. 
         [0019]    The above ranges of angular and axial movement may apply in certain examples for use in aircraft drive systems for slats and flaps on aircraft wings. They may also apply to numerous other drive trains such as the drive trains of motor vehicles. In larger implementations e.g. for large generators or wind turbines, the axial and/or angular movement ranges permitted by the coupling may be significantly greater. 
         [0020]    Preferably a plurality of second coupling members are provided. In some examples two, three or more than three second coupling members may be provided. 
         [0021]    The or each second coupling member may comprise a hollow shaft that extends through its mount point in the torque disc. The hollow shaft permits a fixing member to be passed there through for affixing a second rotatable member to the opposite side of the torque disc from the first coupling member (and first rotatable member). The or each second coupling member may be secured to the torque disc by an interference fit in said mount point, optionally with additional adhesive to bond the second coupling member to the torque disc. It is desirable for the first coupling member and the second coupling member(s) to be permanently affixed to the torque disc so that a maintenance or repair engineer cannot later make any attempt to undo or remove them. As the movement restrictions of the joint are defined by the relationship between these components, when they are all permanently fixed in place that relationship is fixed and cannot accidentally be altered which might compromise the joint and the machinery in which it is installed. 
         [0022]    Preferably the or each second coupling member has a captive nut removably held therein for removable attachment to a rotatable member. The second coupling member is to be attached in use to a second rotatable element that is on the opposite side of the torque disc from a first rotatable element. The captive nut may be removably held within a parallel sided slot in the second coupling member. The slot opening may face radially outwardly towards the edge of the torque disc so that the nut may be easily inserted or removed from side access. The nut may align with the hollow shaft so that it can receive a fixing means passed through the shaft from the opposite side of the torque disc. 
         [0023]    The or each second coupling member may further comprise a second groove and a retaining clip removably mounted in the second groove and which holds the captive nut in place. To facilitate installation, maintenance and repair, particularly in restricted spaces, the nut is preferably held in alignment with the hollow shaft by the clip so that the engineer does not risk losing the bolt while trying to assemble or disassemble the joint. With the captive nuts held in place, the fixing means (e.g. bolts) that hold the second rotatable member onto the second coupling members can be removed while leaving the nut in place. The captive nuts do need to be replaced periodically and so it is preferred that they are not permanently affixed to the second coupling members. The retaining clip may be sprung so that it can be removed from its groove by resiliently deforming it to increase its radius. By removing the clip, the captive nut can be removed from the slot in the coupling member and replaced. The retaining clip is then remounted in the groove to hold the new nut in place for re-assembly. 
         [0024]    The retaining clip may comprise a projection that extends across an opening of the slot in which the captive nut is positioned to obstruct removal of said captive nut from said slot. Because the second coupling member has a groove for engagement with a flange of the first coupling member, the location of the groove for the retaining clip is restricted. For optimum positioning of the main groove, the groove for the retaining clip may be positioned further from the torque disc. This may place the retaining clip above the height of an average sized nut. Rather than increase the size of nut (which would add more weight), the retaining clip with a projection can block the slot opening and hold the nut captive within its slot. 
         [0025]    The or each retaining clip may fully encircle its respective second coupling member and may cross over itself, each end of the clip comprising a projection that extends across the opening. The projections together may act as handles to aid removal of the retaining clip from its groove. The fact that the clip crosses over itself means that the clip may be removed by squeezing the projections together. This is in contrast to most circlips which are removed using special circlip pliers. The circlip pliers are used to expand the circlip by bending it to increase its radius. However the degree of force applied to the circlip is up to the user and too much force can often be applied, permanently deforming or breaking the circlip. With the crossed formation described above, the amount of deformation applied to the circlip is limited by the distance between the projections. This can be designed in advance so that it is sufficient to remove the clip, but not so much that permanent deformation occurs. Thus damage to the clip is prevented. Many circlips are also thickened in the area that experiences most stress under deformation so as to mitigate the risk of damage. The crossed formation described above leads to a predictable maximum force which does not require additional thickening of the clip. 
         [0026]    The torque disc may be made from any suitable material and the material can be selected depending on the intended use. However, in many preferred examples, the torque disc is made from a composite material such as carbon fibre reinforced polymer (CFRP) as this has a very good circumferential strength for transmission of torque, but is also adequately compliant to allow a certain bending moment across it. 
         [0027]    Different sizes of torque disc may be used in different applications. In some examples, e.g. for use in aircraft for operating flaps and slats or in the drive trains of motor vehicles (including cars, trains and boats), the torque disc may be up to about 10 cm in diameter. In other examples, e.g. for wind turbines or other large generators, the torque disc may be significantly larger, e.g. greater than about 50 cm or greater than 1 metre in diameter. 
         [0028]    According to other aspects of this disclosure there is provided an aircraft, a land or sea based motor vehicle or a wind turbine comprising a torque disc as described above, optionally including any or all of the optional features also described above. 
         [0029]    According to another aspect of this disclosure there is provided a method of assembling a flexible coupling comprising: attaching a first coupling member to a first set of mount points on a flexible torque disc; attaching one or more second coupling members to a second set of mount points on said flexible torque disc; and positioning one or more flanges on said first coupling member in a groove on the or each second coupling member such that displacement of said first coupling member is limited with respect to the or each second coupling member. 
         [0030]    The second coupling members may be installed first with the first coupling member then positioned with its flange(s) in the grooves before being fixed in place. However, especially where the flanges have a significant extent (and thus a significant overlap with the second coupling members, and particularly when two or more second coupling members are used, it will be difficult to locate the flanges in the respective grooves once the second coupling members have been installed. Therefore the step of positioning flanges in grooves may be conducted before the steps of attaching first and second coupling members. Thus all of the first and second coupling members are positioned in the appropriate configuration before all of them are mounted to the torque disc. The step of attaching the first coupling member may be carried out simultaneously with the step of attaching the second coupling member(s), i.e. all of the coupling members may be simultaneously inserted into the mount points on the torque disc and affixed thereto while maintaining their configuration with the flanges in their respective grooves. This mounting technique allows the flanges to be made integrally on a single first mounting member that is also fixed to the torque disc. The reduction in components made possible by this arrangement reduces size and weight as well as reducing the manufacturing cost by reducing the number of components that need to be made. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]    One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which: 
           [0032]      FIG. 1  shows an assembled joint; 
           [0033]      FIG. 2  shows a first coupling member in isolation; 
           [0034]      FIG. 3  shows a second coupling member in isolation; and 
           [0035]      FIG. 4  shows a partial cross section illustrating operation of the joint. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]      FIG. 1  shows a carbon fibre reinforced polymer torque disc  10  (although it should be noted that other materials may be used). The torque disc  10  has a generally hexagonal shape and has six mount points  11   a - f  in the vicinity of its vertices. The torque disc  10  is slightly thicker in the region of each mount point  11   a - f  due to the particular construction of this disc which provides additional strength around the mount points which are otherwise weaker points of the structure. However it should be noted that this thickening is not a limiting feature and may not be required on other torque discs. 
         [0037]    A first coupling member  12  is mounted to one side of the torque disc  10  via three of the mount points  11   a,    11   c,    11   e . The first coupling member  12  is shown in more detail in  FIG. 2 . The first coupling member  12  has a tripod shape with its three fixing points  13   a,    13   c,    13   e  disposed symmetrically around the axis of rotation. A crown spline  14  is formed on the other end of first coupling member  12  for attachment to a drive shaft in use. Interleaved between the three fixing points  13   a ,  13   c,    13   e  are three flanges  15   b,    15   d,    15   f.  Each flange  15   b,    15   d,    15   f  is in the form of an arc, being a concave opening in the first coupling member  12 . In this example, each flange  15   b,    15   d,    15   f  is a semicircle, but shorter arc lengths may be used in other examples. As shown in  FIG. 1 , each fixing point  13   a,    13   c,    13   e  is fixed to a mount point  11   a ,  11   c,    11   e  on the torque disc  10  by a shear bolt  16 . Shear bolts  16  are threaded fasteners where the tightening mechanism (e.g. hexagonal head) shears off when the fastener is tightened to the desired torque. Subsequent removal of these fasteners is difficult as there is no drive means by which to loosen the fastener. Shear bolts  16  are used so as to avoid any risk of the first coupling member  12  being removed from the torque disc during subsequent maintenance operations, thus avoiding any risk of the angle limiting feature being disturbed or altered such that damage to the torque disc  10  might occur. 
         [0038]      FIG. 1  also shows that each flange  15   b,    15   d,    15   f  engages with and partially surrounds a second coupling member  17   b,    17   d,    17   f . A single second coupling member  17  is shown in more detail in  FIG. 3 . The second coupling member  17  has a generally cylindrical head part  18  and a cylindrical shaft  19  extending therefrom. The cylindrical shaft  19  is hollow, having a central bore through which a fastener may be inserted for mounting as described further below. The cylindrical shaft  19  is mounted to a mount point  11   b ,  11   d,    11   f  of the torque disc  10  using an interference fit with additional adhesive to ensure that it remains firmly mounted to the disc  10 . 
         [0039]    The head part  18  of second coupling member  17  has a circumferential groove  20  around its perimeter which is designed to engage with the flanges  15   b ,  15   d,    15   f  of the first coupling member  12 . This interaction will be described in more detail with reference to  FIG. 4  below. 
         [0040]    Head part  18  also has a slot  21  open to one side so as to permit insertion and removal of a captive nut  22 . Slot  21  has a wider part at the bottom to receive a lower flange  23  of nut  22 . The upper part of slot  21  has a width matching the size of the hexagonal head  24  of nut  22 , i.e. with a width just larger than the distance between opposite faces of the hexagonal head. The slot  21  thus holds nut  22  captive such that it cannot rotate relative to coupling member  17  when mounted in the slot  21 . 
         [0041]    To prevent nut  22  from sliding back out of slot  21  (e.g. under gravity during installation or maintenance), nut  22  is held in place by retaining clip  25 . Retaining clip  25  fully encircles head part  18  and is seated in a second groove  26  in head part  25  located above the main angle limiting groove  20  (i.e. further from the torque disc  10 ). As the second groove  26  is spaced relatively far from torque disc  10  while nut  22  is relatively close to torque disc  10 , retaining clip  25  is provided with two projections  27 , one at each end of the clip  25 . Each projection  27  is formed simply be bending retaining clip  25  at approximately  90  degrees to the rest of clip  25  such that the clip  25  can advantageously be made from a suitably sized and sprung piece of wire. Projections  27  each extend across slot  21  so as to prevent nut  22  from sliding out of slot  21 . Projections  27  extend sufficiently far that they engage with the flange  23  of nut  22  rather than with the hexagonal head  24 . The flange  23  is circular and therefore the engagement of projections  27  and flange  23  defines a stable position of nut  22  regardless of the relative positions of nut  22  and retaining clip  25 . If the projections  27  merely engaged with the head  24  of nut  22 , the nut would be retained, but slight variations in position would affect the point of engagement between the projections  27  and the faces of head  24  resulting in a slight play of nut  22  within groove  21  and thus a potential slight and inconvenient misalignment between the thread of nut  22  and the bore of cylindrical part  19 . 
         [0042]    Retaining clip  25  has a D shape with a flat edge on which the projections  27  are formed. This D shape ensures approximate alignment of the projections  27  with the slot  21  so that they cannot twist out of position in use. As shown in  FIG. 3 , the retaining clip  25  is wound more than a complete turn around the part  18  such that it crosses over itself With this arrangement, projections  27  can be used as handles and squeezed together in order to expand the diameter of retaining clip  25  so that it can be easily removed from its groove  26 . The distance between projections  27  (i.e. the overlap distance) can be selected so as to ensure that the clip  25  can be removed, but also so that it is not expanded so much during removal that permanent deformation occurs which could destroy the clip  25 . No additional tools (such as circlip pliers) are required as the projections  27  can be readily squeezed either by fingers or with ordinary pliers. 
         [0043]    As can be seen in  FIG. 1 , when mounted to torque disc  10 , each second coupling member  17   b,    17   d,    17   f  is arranged with its slot  21  facing radially outwards, i.e. towards the perimeter of torque disc  10  so as to provide best access to the captive nuts  22  and retaining clips  25  for installation, maintenance and repair. 
         [0044]    To complete the joint, as shown in  FIG. 1  a shaft  30  is connected to an intermediate connector  31  which is in turn attached to each of the three second coupling members  17   b,    17   d,    17   f  by bolts  32  inserted through the central bore of cylindrical shaft  19  and screwed into captive nut  22  until a sufficiently tight and rigid connection has been made. The joint can be separated by reversing this process to remove the bolts  32  from nuts  22 . The intermediate connector  31  is another tripod shaped member with three arms arranged for connection to the second coupling members  17   b,    17   d,    17   f  and with a crown spline for attachment to a shaft. It will be appreciated that instead of using an intermediate connector  31 , the tripod could be formed integrally on the end of shaft  30 . Similarly, the first coupling member  12  may be formed integrally on the end of a shaft (not shown), thus avoiding the need for the crown spline connection  14 . 
         [0045]      FIG. 4  shows an enlarged and partially cross-sectioned view of the interaction between a flange  15  on the first coupling member  12  and a groove  20  in a second coupling member  17 . Flange  15  is thinner groove  20  (in the dimension perpendicular to the plane of the torque disc  10 ) and in normal use (as shown in  FIG. 4 ) it does not come into contact with the walls of groove  20 , but rather sits in spaced relationship with it. In normal use, torque applied to one shaft is transmitted from first coupling member to second coupling members (or vice versa) via torque disc  10  and thereby to the opposite shaft (e.g. from crown spline  14  to shaft  30  in  FIG. 1 ). Small angular deviations between the input and output shaft can be accommodated by slight bending of the torque disc  10 . Similarly slight axial shifts of the input and output shafts can be accommodated by deflections in the torque disc  10 . However, as the angular deviation increases, the flange  15  becomes twisted and rotated within groove  20  as shown by arrow  40  until eventually flange  15  contacts a wall of groove  20  and further movement is hindered. If further angular force is applied a small additional deflection may occur until one or more of the other flanges  15  and grooves  20  contact on their opposite walls. For example with reference to  FIG. 1 , flange  15   b  may contact groove  20   b  on the groove face closest to shaft  30  while the flanges  15   d  and  15   f  may contact the grooves  20   d  and  20   f  on the groove faces closest to spline  14 . At this point, no further rotation may be applied without breaking or denting one of the first coupling member  12  or second coupling members  17 . As these are all formed from a tough and rigid material (typically a metal), further angular deflection is essentially prevented. 
         [0046]    Note that in the case of axial movement rather than angular movement, the same principles apply except that the flanges  15  will all contact walls of the respective grooves  20  on the same side, i.e. all the walls closest to the torque disc  10  or all the walls furthest from the torque disc  10 . 
         [0047]    As can be seen from  FIG. 4 , the relative dimensions of the flange  15  and the groove  20  define the limits of movement that are permitted. By making the flange  15  thicker or the groove  20  narrower, movement can be restricted more. Equally by making flange  15  thinner and/or groove  20  wider, more movement can be permitted. The ratio of permitted angular movement to permitted axial movement can also be controlled to a certain extent by adjusting the radial position at which the flange engages the groove (radially with respect to the torque disc). For example by moving this contact point radially inwards (e.g. by using a smaller arc length for flange  15 ), the same axial limitations (defined by the width of groove  20 ) will give rise to a slightly larger permitted angular deviation. By careful selection of these parameters, the permitted deflections of the input and output shafts can be chosen so as to allow the movement required for operation while preventing excessive movement that might damage the torque disc  10  and lead to drive failure. 
         [0048]    It can be seen from  FIG. 1  that an advantage of this arrangement is that the flanges  15  surround the second coupling members  17  to such an extent that even if torque disc  10  were to completely fail, rotation of one shaft (on one side of the joint) will still induce rotation of the other shaft (on the other side of the joint) through contact between the flanges  15  and the grooves  20 . Torque can be transmitted in either rotational direction in this manner. Although this will induce wear and noise and inefficiencies, it can be highly beneficial to still have some drive capability in the event of a torque disc fail. For example when used in the drive trains for flaps and slats on aircraft wings, loss of the whole drive train could result in loss of control of a whole wing. In this scenario it is preferable for the drive train to continue to function (sub-optimally) to retain some control until the aircraft can land and undergo repair. 
         [0049]    Assembly of the coupling is best accomplished by mounting the three second coupling members  17   b,    17   d,    17   f  on their respective flanges  15   b,    15   d,    15   f  and then simultaneously inserting the cylindrical shafts  19  of all of the second coupling members  17   b,    17   d,    17   f  into the mount points  11   b,    11   d,    11   f  of the torque disc  10 . These may be retained simply by an interference fit or adhesive may be applied prior to the insertion for a more permanent attachment. Shear bolts  16  are then used to attach fixing points  13   a,    13   c,    13   e  of first coupling member  12  to mount points  11   a ,  11   c,    11   e  of torque disc  10 . The flexible joint may then be completed by attaching a shaft or another intermediate connection part onto the opposite side of the torque disc  10  by passing bolts  32  through the hollow cylindrical parts  19  of second coupling members  17   b,    17   d,    17   f  and fixing them securely in captive nuts  22 .