Abstract:
A fastening device has: a mounting member attached to a first component, a screw-threaded assembly rotatable in the mounting member and with a screw-threaded portion for engaging with a screw-threaded portion of a second component to secure the components together; and an actuator member mounted on the screw-threaded assembly. The device has, in itself, locked and unlocked states. A torque-transmitting arrangement is arranged such that: when in the unlocked state, torque can be transmitted from the actuator member to the screw-threaded assembly to tighten the screw-threaded portions until a predetermined torque is reached, whereupon the device changes to its locked state; and when in the locked state, the actuator member is rotationally locked to the screw-threaded assembly. A locking arrangement is arranged such that, when in the unlocked and locked states, the actuator member can be rotated and is rotationally locked, respectively, relative to the mounting member.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to fastening devices for screw-threadedly securing together two components.  
         [0003]     The invention was conceived for use in fastening together avionics modules having complementary mating electrical connectors, but it can also be used to secure other components together.  
         [0004]     2. Description of Related Art  
         [0005]     Two avionics modules could be secured together by a simple rugged fastening device in the form of a screw-threaded bolt that passes through a hole in one module and engages a screw-threaded hole in the other module, or by a nut and bolt that act between the two modules. However, for reasons of reliability, safety, ease of maintenance and ease of inspection, in some applications requirements are imposed that the fastening device can easily be done up to a predetermined torque by hand without the use of tools, that it can be seen from visual inspection whether or not the fastening device has been done up properly, that the device will not loosen in service, that it can easily be undone by hand without the use of tools, and yet that the fastening device will remain rugged.  
         [0006]     Various proposals have been made in the past in an attempt to satisfy some of these requirements. For example, patent document EP-0105609-B1 describes a manually-operable fastening device having a torque-limiting arrangement acting between an actuating knob and a main screw-threaded member and provided by balls seated in indents having inclined surfaces and to which spring loading is applied. When the limiting torque is reached, the balls ride out of the indents to that the knob turns without turning the screw-threaded member. In the specific embodiment described in EP-0105609-B1, it is possible to tell from visual inspection whether the device has been done up to some extent, but not whether it has been done up to the limiting torque. Furthermore, reliance is made purely on friction to prevent the device undoing itself in service.  
         [0007]     Patent document EP-0304237-B1 describes developments to the device described in EP-0105609-B1. One development is to provide a ratchet arrangement between the actuating knob and a mounting member of the device so as to provide positive locking of the device once it has been done up. Once side effect of this is that the ratchet wears and makes an annoying click when the knob is being rotated to tighten the fastener. Another side effect is that, to undo the fastener, it is necessary to pull the knob axially all the time that it is being rotated in the undoing direction, and this causes operator wrist fatigue and is very annoying. Another development in EP-0304237-B1 is to provide a leaf spring or semaphore arm on the knob that changes its position to indicate when the device is done up. However, the arrangement described in EP-0304237-B1 only goes so far as indicating that the avionics modules have mated, and not that a predetermined force or torque has been reached. Also, the leaf spring or semaphore flag are prone to damage and can injure the operator&#39;s hand or at least make the knob uncomfortable to handle.  
         [0008]     Patent document EP-0414835-B1 describes another development to the device described in EP-0304237-B1. In addition to having the torque limiting arrangement between the actuating knob and the screw-threaded member, it also has a force detecting arrangement, in the form of Belleville disc spring washers, in the indicating system so that the semaphore arm indicates that the device is done up only if both the modules are mated and at least a predetermined axial force is being applied by the screw-threaded member. That predetermined force is described as being less than the force that arises when the torque-limiting arrangement operates, and so the semaphore arm still does not indicate that the required torque has been reached. Furthermore, the Belleville washers react the whole of the axial force in the screw-threaded member. Therefore, if, in time, the Belleville washers relax and give, the force with which the avionics modules are held together decreases. Vibration can then arise between the electrical connectors in the avionics modules, leading to wear, arcing and system failure.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     An aim of the present invention, or at least specific embodiments of it, is to provide a fastening device that satisfies at least some of the requirements set out above, but which does not suffer from some or all of the disadvantages of the prior art as described above.  
         [0010]     In common with the device of patent document EP-0414835-B1, the present invention provides a fastening device for screw-threadedly securing together first and second components. The fastening device comprises a mounting member or assembly (hereinafter “the mounting assembly”) arranged to be mounted on the first component; a screw-threaded member or assembly (hereinafter “the screw-threaded assembly”) rotatably mounted relative to the mounting assembly and having a screw-threaded portion arranged to be engaged with a complementary screw-threaded portion of the second component in order to secure the first and second components together; and an actuator member or assembly (hereinafter “the actuator assembly”) mounted relative to the mounting and screw-threaded assemblies. A torque-transmitting arrangement is provided between the screw-threaded and actuator assemblies, and a locking arrangement is provided between the actuator and mounting assemblies.  
         [0011]     By contrast to the fastening device of patent document EP-0414835-B1, the device of the present invention has, in itself, distinct locked and unlocked states. The torque-transmitting arrangement is such that: when the fastening device is in its unlocked state, torque can be transmitted from the actuator assembly to the screw-threaded assembly in a direction to tighten the screw-threaded portions until a predetermined torque is reached, whereupon the fastening device changes to its locked state; and when the fastening device is in its locked state, the actuator assembly is rotationally locked to the screw-threaded assembly. The locking arrangement is such that: when the fastening device is in its unlocked state, the actuator assembly can be rotated relative to the mounting assembly; and when the fastening device is in its locked state, the actuator assembly is rotationally locked to the mounting assembly.  
         [0012]     By comparison, the device of EP-0414835-B1 does not, in itself, have such locked and unlocked states. Instead, it has a single state in which the actuator member can always be turned in the tightening direction but cannot be turned in the loosening direction unless the actuator member is also pulled outwardly. Accordingly, there is no suggestion in EP-0414835-B1 of changing from an unlocked state to a locked state once a predetermined torque has been reached.  
         [0013]     In order to allow the fastening device of the invention to be undone, preferably, when the fastening device is in its unlocked state, torque can be transmitted from the actuator assembly to the screw-threaded assembly in a direction to loosen the screw-threaded portions.  
         [0014]     In a preferred embodiment, when the fastening device is in its unlocked state, the actuator assembly has a first longitudinal position, or range of positions, relative to the screw-threaded and mounting assemblies, and, when the fastening device is in its locked state, the actuator assembly has a second, different longitudinal position relative to the screw-threaded and mounting assemblies. In this case, the device is preferably arranged such that it can be changed from the locked state to the unlocked state by manually moving the actuator assembly in the longitudinal direction from the second position whilst rotating the actuator assembly in the direction to loosen the screw-threaded portions. Also, the locking arrangement preferably comprises a dog clutch, or the like, which engages upon movement of the actuator assembly from the first longitudinal position, or range of positions, to the second longitudinal position relative to the mounting assembly (i.e. upon changing of the device to the locked state). Furthermore, the torque-transmitting arrangement preferably comprises an arrangement of spring, cam and follower acting between the screw-threaded and actuator assemblies, such that, when the fastening device is in its unlocked state and torque is transmitted from the actuator assembly to the screw-threaded assembly in a direction to tighten the screw-threaded portions and the predetermined torque is neared, the follower rides up the cam against the action of the spring, and, once the predetermined torque is reached, the follower rides over the cam and the actuator assembly then moves under the action of the spring to the second longitudinal position relative to the screw-threaded assembly.  
         [0015]     Preferably, the state of the fastening device can be detected by external visual inspection of it. In the case where the actuator has different longitudinal positions in the different states, the actuator and screw-threaded assemblies preferably have respective externally-visible surfaces that are flush with each other only when the actuator assembly is in the second longitudinal position relative to the screw-threaded assembly. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0016]      FIG. 1  is a sectioned side view of a first embodiment of fastening device in its locked state;  
         [0017]      FIG. 2  is a sectioned side view of the fastening device in its unlocked state;  
         [0018]      FIG. 3  is an end view (in the direction A shown in  FIG. 1 ) of a mounting member of the fastening device;  
         [0019]      FIG. 4  is an end view (in the direction A shown in  FIG. 1 ) of a screw-threaded member of the fastening device;  
         [0020]      FIG. 5  is an isometric view of the screw-threaded member;  
         [0021]      FIG. 6  is a development of as cam profile on the screw-threaded member;  
         [0022]      FIG. 7  is an isometric view of a knob of the fastening device;  
         [0023]      FIG. 8  is a sectioned side view of a second embodiment of fastening device;  
         [0024]      FIG. 9  is an isometric view of a cam used in the fastening device of  FIG. 8 ; and  
         [0025]      FIG. 10  is an isometric view, on a smaller scale, of the fastening device of  FIG. 8 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Referring in particular to  FIG. 1  of the drawings, the first embodiment of fastening device  10  comprises a mounting member  12 , a screw-threaded member  14 , an external circlip  16  for holding the screw-threaded member  14  captive with the mounting member  12 , an actuator knob  18 , a compression coil spring  20  and a spring retaining cover  22 . The fastening device  10  is shown connecting a first component  24  having plain hole  26  to a second component  28  having a screw-threaded hole  30 .  
         [0027]     Referring in particular to  FIGS. 1, 4  and  5 , the screw-threaded member  14  has a shaft formed with an male screw-thread  32  at one end to complement the thread of the hole  30  in the second component  28 . The other end of the shaft of the screw-threaded member  14  has a screw-threaded hole  34  that receives a threaded boss  36  on the inside of the spring retaining cover  22 . Partway along the shaft of the screw-threaded member  14 , a shoulder  38  is formed. The shoulder  38  has a thrust face  40  facing in the direction towards the end of the shaft with the male screw-thread  32 , and a cam face  42  facing in the direction towards the end of the shaft with the threaded hole  34 . Between the thrust face  40  and the end of the shaft with the male screw-thread  32 , a groove  44  is formed to receive the circlip  16 .  
         [0028]     Referring in particular to  FIGS. 1 and 3 , the mounting member  12  is of generally circular cross-section and has a central hole  46  that receives the shaft of the screw-threaded member  14 . A shallow counterbore  48  is formed in one end face of the mounting member  12  to accommodate the circlip  16 . A pair of screw-threaded holes  50  are also formed in that end face, by which the mounting member  12  is permanently attached to the first component  24  using a pair of screws. A large, deep counterbore  52  is formed in the other end face of the mounting member  12  to receive the shoulder  38  of the screw-threaded member  14 . The bottom of the counterbore  52  forms a thrust face  54  that is engaged by the thrust face  40  of the shoulder  38 . The remaining periphery of the other end face of the mounting member  12  has a series of equally spaced notches  56  (for example twenty-four notches  56 ) cut into it so as to form a series of dogs  58  each between an adjacent pair of the notches  56 .  
         [0029]     Referring in particular to  FIGS. 1 and 7 , the actuator knob  18  is of generally circular cross-section and has a central hole  60  that receives the shaft of the screw-threaded member  14 . A large, deep counterbore  62  is formed in one end face of the knob  18  to house the spring  20 . The other end face of the knob  18  is milled or otherwise machine to provide a recess  64 , with two diametrically-opposed cam followers  66  and four equally-spaced dogs  68  (only two of which can be seen in  FIG. 7 ) projecting from the floor of the recess  64 . The pitch circle diameter of the dogs  68  is generally equal to the pitch circle diameter of the dogs  58  and notches  56  on the mounting member  12 , and the width of each dog  68  on the knob  18  is slightly less than the width of the notches  56  in the mounting member  12 , so that the dogs  68  on the knob  18  can be interengaged with the dogs  58  on the mounting member  12  in any of twenty-four different angular positions. The cam followers  66  are arranged to engage with the cam face  42  on the screw-threaded member  14  as will now be described more detail with reference to FIGS.  3  to  7 .  
         [0030]     The cam followers  66  are arranged rotationally symmetrically through 180° about the axis of the knob  18 , and the cam face  42  has two portions  42 A,B (see  FIG. 6 ) that are rotationally symmetrical through 180° about the axis of the screw-threaded member  14 . Referring in particular to  FIG. 6 , starting at a datum position (0°) and moving anticlockwise around the axis of the screw-threaded member  14  (as viewed from the end having the screw-threaded hole  34 ), the cam face portion  42 A has: a fairly steep, downwardly ramped portion  70 ; then a plateau portion  72  having generally the same width as each cam follower  66 ; then an upward step to a plateau portion  74 ; then a downward step to a plateau portion  76  having generally the same width as each cam follower  66 ; then a small step to a fairly gentle, upwardly ramped portion  78 ; and then a plateau portion  80  to the 180° position. The other cam face portion  42 B then repeats the identical pattern.  
         [0031]     As will be described in more detail below, when the cam followers  66  engage the plateau portions  76 , the fastening device  10  is in its locked state, and when the cam followers  66  engage the plateau portions  72 , the fastening device  10  is in its unlocked state. The difference D ( FIG. 6 ) in height between the plateau portions  72 , 76  is greater than the amount by which the dogs  58 , 68  on the mounting member  12  and the knob  18  interengage.  
         [0032]     The fastening device  10  is assembled by fitting the screw-threaded member  14  into the mounting member  12  and fitting the circlip  16  into the groove  44 . The knob  18  is then fitted to and screw-threaded member  14  with the cam followers  66  engaging the plateau portions  72  (as shown in  FIG. 2 ). The spring  20  is then fitted into the knob  18 , and the retaining cover  22  is fitted to the screw-threaded member  14  and done up tightly. As seen in  FIG. 2 , when the fastening device  10  is in its unlocked state, the outer face of the spring retaining cover  22  is recessed substantially into the knob  18 .  
         [0033]     The assembled fastening device  10  is mounted on the first component by inserting the end of the screw-threaded member  14  into the hole  26  in the first component  24  and by fitting the mounting screws into the holes  50  in the mounting member  12 .  
         [0034]     In order to connect the first component  24  to the second component  28 , the screw-thread  32  on the screw-threaded member  14  is offered up to the screw-threaded hole  30  in the second component  28 , and the knob  18  is rotated manually in the clockwise direction. In the unlocked state of the fastening device  10 , the dogs  58 , 68  are not interengaged so that the knob  18  can be turned relative to the mounting member  12 . Also, the cam followers  66  are pressed by the spring  20  onto the plateau portion  72  so that rotation of the knob  18  is transmitted to the screw-threaded member  14 , and the screw-thread  32  enters the hole  30 . Once the fastening device  10  starts to press the first component  24  against the second component  28 , the torque required to turn the knob  18  increases and the cam followers  66  eventually begin to ride up the ramped portions  70  of the cam face  42 . With increasing applied torque, the cam followers  66  eventually reach the tops of the ramped portions  70 , slide over the plateau portions  80 , slide down ramped portions  78  with the assistance of the force of the spring  20 , and drop onto the plateau portions  76 , as shown in  FIG. 1 . The torque at which this happens is predetermined and is dependent, in the main, generally on the compressive force in the spring  20  as the followers  66  reach the tops of the ramped portions  70 , the inclination of the ramped portions  70 , the coefficient of friction between the followers  66  and the ramped portions  70 , and the general pitch circle diameter of the followers  66  and ramped portions  70 . When the followers  66  drop onto the plateau portions  76 , the four dogs  68  on the knob  18  interengage between respective pairs of the dogs  58  on the mounting member  12 , and so the knob  18  therefore becomes rotationally locked to the mounting member  12 . Furthermore, the followers  66  are trapped between the steps to either side the plateau portions  76 , and therefore the knob  18  becomes rotationally locked to the screw-threaded member  14 . Accordingly, the whole fastening device  10  is rotationally locked with a predetermined torque having been applied.  
         [0035]     As seen in  FIG. 1 , when the fastening device  10  is in its locked state, the outer face of the spring retaining cover  22  is flush with the surrounding portion of the knob  18 . It is therefore possible to detect, merely from visual inspection, whether the fastening device  10  is in its locked state. Without considerable abuse of the fastening device, it is impossible for it to take on its locked state unless the screw-threaded member  14  has been tightened to the required predetermined torque. Therefore, the flushness of the spring retaining cover  22  with the surrounding portion of the knob  18  is an almost foolproof indication that the screw-threaded member  14  has been tightened to the required predetermined torque.  
         [0036]     In order to disconnect the first component  24  from the second component  28 , the knob  18  is grasped and pulled outwardly against the action of the spring  20  while twisting anticlockwise. This action lifts the cam followers  66  off the plateau portions  76 , and they ride up the gently-inclined ramped portions  78 . With further twisting of the knob  18 , the followers  66  ride over the plateau portions  80  and then fall down the ramped portions  70 , under the action of the spring  20 , and rest on the plateau portions  72 . The fastening device  10  therefore attains its unlocked state. With further turning of the knob  18 , the sides of the followers bear against the steps between the plateau portions  72  and the plateau portions  74 , and so torque can be transmitted from the knob  18  to the screw-threaded member  14  in the direction to undo the screw-thread  32  without the need for continued outward pulling of the knob  18 .  
         [0037]     FIGS.  8  to  10  show a second embodiment of the fastening device  10  that employs the same principles as the first embodiment, but which has a number of modifications. For example, the screw-threaded member  14  is held captive in the mounting member  12  by a sleeve  82  that is screw-threaded into the mounting member  12  and acts through a compression coil spring  84  on a sleeve  86  that is fixed to the screw-threaded member  14  by a pin  88 . The dogs  58  of the mounting member  12  are provided by a separate annular element  90  that is fixed to the mounting element  12 . The cam face  42  is provided on a separate cam element  92  ( FIG. 9 ) that is keyed to the screw-threaded member  14  and held captive between a shoulder  94  and circlip  96  on the screw-threaded member  14 . The cam followers  66  are provided by separate elements that are fixed to the knob  18 . The cam followers  66  are formed by cylindrical pins with rounded ends. The dogs  68  of the knob  18  are provided on a separate annular element  98  that is fixed to the knob  18 . A pair of springs  20 , 20 A are provided acting between the knob  18  and the spring retaining cover  22 . The knob  18  is covered with an ‘easy-grip’ sleeve  100 .  
         [0038]     It will be appreciated that many modifications and developments may be made to the fastening devices  10  described above. For example, the cam face  42  may be provided on the knob  18  and the follower or followers  66  may be provided on the screw-threaded member  14 . The screw-threaded member  14  may alternatively have a female-threaded hole to engage a male-threaded projection on the second component  28 .  
         [0039]     It should be noted that the embodiments of the invention has been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.