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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     The present invention relates to a hinge mechanism, more particularly, but not exclusively, to a hinge mechanism for a portable radio communication apparatus. 
     Certain designs of portable radio communication apparatus, such as radio telephones, are provided in two pivoted parts consisting of a main body and a cover member (or flip) which is usually hinged at the top or bottom end of the body. The cover member can be moved between a closed position overlaying a portion of the face of the body, typically the keypad, and an open position pivoted away from the body, exposing the keypad and ready for use. The main body houses the majority of the electronic components of the radiotelephone, whilst the flip sometimes houses an earpiece, or microphone or LCD display. Such radio telephones are generally known as flip or folding phones, and as well as offering protection for the keypad from inadvertent activation, can also make for a more compact design of phone. 
     A wide variety of folding phones are currently available, employing many different hinge solutions. Known hinge mechanisms range from relatively simple pin/recess arrangements to more sophisticated assemblies comprising springs, cams, followers and latching mechanisms. 
     SUMMARY OF THE INVENTION 
     The challenge faced by the present inventors was to design an alternative improved hinge mechanism which met the main criteria of: 
     reliability, in terms of the number of opening/closing operations, 
     smooth motion and good action, offering a slight resistance to movement, 
     providing detent bias in the closed and open positions, and 
     a positive overideable flip stop in the open position 
     ease of assembly. 
     Accordingly, in one aspect the present invention provides a hinge mechanism for pivotally connecting first and second housings of a portable electronic device for movement between closed and open positions, the mechanism comprising a hinge shaft mounted for rotation with the first housing and providing a camming surface, and resilient means mounted for rotation with the second housing and arranged to cooperate with the camming surface so as respectively to load the resilient means corresponding to different orientations with respect to the camming surface for relative rotation therebetween 
     By means of the invention, the hinge mechanism provides a varying torque between the resilient means and the camming surface which causes relative rotation of the resilient means and the camming surface. 
     In a preferred embodiment, when the first and second housings are in the open position they are mutually biased towards the open position, and when the first and second housings are in the closed position they are mutually biased towards the closed position. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Thus, the hinge mechanism provides a detent torque for the housings in the closed and open positions, and also offers some resistance to rotational movement which enhances the feel of the opening and closing motion. 
     It is preferred that the camming surface is shaped to define stable and unstable orientations for the resilient means, the resilient means tending towards the stable orientation in said open and closed positions. In this way, the resilient means indexes around the camming surface. 
     In a preferred embodiment the resilient means comprises a pair of spring arms acting on opposite sides of the camming surface. 
     Advantageously, the resilient means is carried in a retainer and the retainer is coupled to the second housing. This affords the hinge mechanism a modular design. The retainer conveniently comprises a longitudinally split tube, each half of the tube carrying respective resilient means. 
     It is preferred that the camming surface comprises opposing planar surfaces and that the planes defined by the planar surfaces are offset from the vertical axis of the hinge shaft. This conveniently provides for the detent torque. 
     In an advantageous embodiment, the hinge shaft carries a projection adapted to abut a corresponding projection provided on the second housing, whereby the abutment of the projections supports the second housing in relation to the first housing in said open position. Optionally, said projections are adapted to traverse one another if the housings are extended beyond said open position. 
     The invention extends to a radio telephone having first and second housings and including a hinge mechanism as hereinbefore defined for pivotally connecting said housings. 
    
    
     The invention will now be described by way of example with reference to the following drawings in which: 
     FIG. 1 is a front perspective view of a radiotelephone in a closed position and incorporating a hinge mechanism in accordance with the present invention; 
     FIG. 2 is front perspective view of a radiotelephone in an open position and incorporating a hinge mechanism in accordance with the present invention; 
     FIG. 3 a  is an exploded isometric view of a hinge module of a preferred embodiment of the present invention; 
     FIG. 3 b  is an isometric view an assembled hinge module of FIG. 3 a;    
     FIG. 4 is a schematic cross sectional view of a camming profile of the preferred embodiment; 
     FIGS. 5 a  to  5   d  are schematic cross sectional views of the interaction of the camming surface of FIG. 4 with a pair of leaf springs; 
     FIG. 6 a  is an exploded isometric view of the hinge module of FIG. 3 b  in a housing of the radiotelephone of FIGS. 1 and 2; 
     FIG. 6 b  is a perspective view of a hinge knuckle of the preferred embodiment; 
     FIGS. 7 a  and  7   b  are schematic cross sectional views of the hinge with the housing of FIG. 6 in respective closed and open positions; and 
     FIG. 8 is an isometric view of the safety-stop support feature of the preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to FIGS. 1 and 2, there is shown a portable radiotelephone  10  comprising a main body  12  pivotally connected to a flip  14  by a hinge  16  having an axis of rotation indicated by the line AB. FIGS. 1 and 2 show the flip  14  in respective closed and open positions in relation to the main body  12 . 
     The main body  12  comprises an upper casing  18  and lower casing  20  which together house the main internal control circuitry of the radiotelephone  10 . The upper casing  18  carries a keypad  22  and power button  24 , and forms, towards the front end, a chin  26 , below which is housed a microphone. The rear end of the upper casing upsweeps to a pair of knuckles  28  of the hinge  16 . A slidable antenna  30  is provided at the side of the main body  12 . 
     The flip  14  likewise comprises an upper casing  32  and the lower casing  34  which join together to house further electronic components such as an LCD display  36 , an earpiece  38 , and associated circuitry. The foot of the flip  14  is formed into a tube  39  and encases part of the hinge module. 
     The microphone and the earpiece  38  are positioned at opposite ends of the two parts of the radiotelephone to maximise the distance between them, such that this distance approximates the distance between the ear and the mouth of the user. 
     With the flip  14  in the closed position of FIG. 1 it covers the keypad  22  and its upper edge skirts the chin  26 . In the open position of the FIG. 2, the flip  14  is pivotally spaced from the main body  12  and the radiotelephone  10  is ready for use. It is notable that in the open position with the radiotelephone held up to a user&#39;s ear, the flip  14  at least partially screens the antenna  30  and this provides useful blocking against radio magnetic waves. 
     Turning to FIG. 3 a,  a hinge module  40  is shown in the exploded isometric view as comprising a central hinge shaft  42  in part sandwiched by a pair of leaf springs  44  which in turn are encased by respective halves of a spring retainer tube  46 . 
     The hinge shaft  42  is a one-piece integral unit consisting of several distinguishable sections: a shank  48  and a cam section  50  separated by a raised collar  52 , the two ends of the shaft being provided with rectangular locating bosses  54  for mounting in the hinge knuckles  28  (to be described later).The shank  48  has a generally circular cross section and leads to the raised collar  52  which carries a radial lug  56  forming part of the stop feature of the hinge mechanism. Inward of the collar  52  is the cam section  50  which consists of two spaced apart camming surfaces  58 . Each camming surface  58  is formed from opposite pairs of regions where the surface of the hinge shaft  42  is substantially flat. A cross section of a camming surface  58  is illustrated in FIG.  4  and comprises two opposite substantially parallel flat major faces  60  connected by two opposite rounded minor faces  62 . Here, it should be explained that the planes of the flat major faces  60  are not parallel to the vertical axes of the locating bosses  54 . Rather, the planes of the flat major faces are offset by 10° from the vertical defined by the vertical axes of locating bosses  54  for a purpose which will become apparent later. The camming surfaces  58  are closed at their outer ends by a pair of shoulders formed by the flange walls of the distal locating boss  54  and the collar  52 . 
     Turning to the pair of leaf springs  44 , since these are identical, for clarity the description will relate to one of the pair. The leaf spring  44  is elongate and generally planar although it has a convex bow along its length. The spring  44  has a stem  64  joining its ends which are spatulate  66  and having a width greater than that of the stem  64 . The width of the spatulate ends  66  is selected to match or be slightly wider than the width of the major faces  60  of the camming surfaces  58  of the hinge shaft  42  so as to apply a more even force over the camming surfaces  58 . In a natural unstressed state the spring  44  has a bowed height of typically 1.85 mm. 
     Moving on to the spring retainer tube  46 , this consists of two halves of a longitudinally split cylindrical tube, each halve holding one of the pair of leaf spring  44 . Formed in the side wall of a retainer halve  46  is a pair of jaws  68  into which the leaf spring  44  is pressed clipped into position. Each of the retainer halves  46  is provided with corresponding lugs  70  and catches  72  so that the two halves can be latched together. It is advantageous that the spring retainer halves are “handed” in that the correspondence of the lugs  70  and catches  72  is such that any two halves can be assembled despite being mutually opposite; this obviates the need for separate left and right handed retainer halves. Whilst the side walls  74  of the retainer tubes are generally curved, the upper and lower walls are flat  76 . 
     In order to assemble the hinge module, the leaf springs  44  are first clip locked into respective halves of the retainers  46 . Next the spring retainer halves  46  are brought together lengthwise around the camming section  50  of the hinge shaft  42  and are latched to one another. This causes loading of the leaf springs  44  which press against camming surfaces  58  and flex outwards against the shoulders formed by the flange walls of the distal locating boss  54  and the collar  52 . The bowed height of the springs in this loaded state is approximately 1.7 mm. An assembled hinge module is shown in FIG. 3 b.    
     It is notable that it is only possible to latch the spring retainers  46  together once they are correctly aligned in relation to the hinge shaft  42 , because at other locations along the shaft  42  its diameter is too large for the retainer halves to fit together. 
     The hinge mechanism  40  operates as follows. Because in its natural condition the bow of the leaf springs  44  is greater than the space between the retainer  46  and the hinge shaft  42  camming surface, the leaf springs  44  are compressed slightly and pre-loads are introduced in the springs which give rise to spring forces on the camming surfaces. The spatulate ends  66  of the leaf springs  44  contact the camming surfaces  58  at tangents, thus the spring forces on the camming surfaces  58  are directed radially inwards. As the relative orientation of the leaf springs  44  with respect to the shaft  42  changes, so they adopt successive stable and unstable positions. At orientations when the spatulate ends  66  lie flat against the major faces  60  of the camming surface as in FIG. 5 a,  the springs  44  are in a stable position and there is no net torque being applied to the hinge shaft  42 . In the orientation shown in FIG. 5 b,  the springs have moved through 90° and the spatulate ends  66  press on the opposing tips of the minor faces  62  of the camming surface  58  and momentarily reach an equilibrium. 
     It will be understood with reference to FIGS. 5 c  and  5   d  that at other orientations around the camming surface  58 , ie. partway between the major faces  60  and the tips of the minor faces  62 , the leaf springs  44  are unstable inasmuch as the spatulate ends  66  apply spring forces against the camming surfaces  58  in such a manner as to induce a torque which causes the leaf springs  44  to bias towards the flat major faces  60 . If from this stable position, there is relative rotation between the leaf springs  44  and the shaft  42 , the springs  44  apply an opposing torque which acts to restore the leaf springs  44  to stable major faces. With appropriate spring loading and cam dimensions this action can be a snap action. Furthermore, it can seen that relative rotation of the leaf springs  44  and the camming surface  58  represents an indexing action. 
     Referring now to FIG. 6 a,  the assembled hinge module is disposed in the hinge tube  39  of the flip  14  as illustrated. Inside the tube  39  the flat sides of the spring retainer are shaped to mate with opposing flat beds  78  provided by crosswise rails in the tube  39  and in this way prevent relative rotation between the hinge module  40  and the flip, effectively rotatably keying the two together. The step of assembling the hinge module  40  in the tube  39  requires that the hinge module  40  is inserted longitudinally from one end of the flip tube  39  having a square opening, until the inward edge of the spring retainer  46  abuts the final rail to stop it moving in any further. 
     With the hinge module inserted in the flip tube  39 , the flip  14  is joined to the upper casing  18  of the main body  12 . Circumferential projections  80  extending laterally from the ends of the flip tubes mate with corresponding recessed slots  82  in the inside edges of the knuckles  28  of the upper casing  18  (see FIG. 6 b ). This provides for the correct locating of the flip  14  and also ensures continuity of the hinge connection. The rectangular locating bosses  54  of the hinge shaft  42  are received in vertical slots  82  provided in each of the knuckles  28  of the upper casing  18 . Accordingly, the shaft  42  is fixed in relation to the knuckle  28  and the reaction force to the turning force of the hinge shaft  42  is provided by the knuckle  28  in the upper casing  18 . 
     The operation of the hinge and the pivoting of the flip relative to the main body is of course based on the mechanism of the hinge module already described above with reference to FIGS. 5 a  and  5   b . The flip  14  having keyed into it the hinge module  40  containing leaf springs pivots  44  around the hinge shaft  42  which remains relatively static in the main body  12 . In the angular orientation associated with the flip  14  being in the closed position the configuration of the spatulate ends  66  of the leaf springs  44  in relation to the camming surfaces  58  is as shown in FIG. 7 a . Rather than lying squarely on major faces  60  of the camming surfaces  58 , the spatulate ends  66  press against opposite shoulders of the camming surface  58 . This is due to the offset of the planes of the major faces  60  from the vertical. Accordingly, the leaf springs  44  bias towards the major faces  60  and as a result together give rise to a net clockwise turning force which is transmitted to the flip  14  causing it to be urged towards the main body  12 . Since the upper edge of the flip  14  abuts the chin  26  of the radiotelephone  10  at a horizontal, a standing torque or detent torque is set up on the flip  14 . In this way, in the closed position the flip  14  offers a certain amount of in-built resistance to being opened and prevents it from having a sloppy feel. 
     Conversely, with the flip  14  in the open position the configuration of the leaf spring  44  and the hinge shaft  42  is as illustrated in FIG. 7 b . Opening of the flip  14  is facilitated by formations on the side edges and the rear of the flip. In this open position the flip  14  has travelled through 160° and the spatulate ends  66  of the leaf springs  44  have reversed orientation and press against the other pair of shoulders of the camming surface  58 . Again, the spatulate ends  66  do not lie on the major faces  60  and accordingly the leaf springs  44  now transmit an anti-clockwise turning force on the flip. The flip  14  however is prevented from moving to adopt this position by a stop feature  52 / 84 . 
     The stop feature is shown in FIG.  8  and comprises the raised lug  56  on the collar  52  of the hinge shaft which engages a short axial lip  84  spanning two adjacent rails extending from the internal wall of the flip hinge tube  39 . When the flip is moved to the open position, the axial lip  84  approaches the lug  56  and at 160° opening catches against it. Due to the in-built detent torque of the springs  44  and camming surface  58  in this position, the flip  14  is biased in the open position. The stop feature  52 / 84  is designed to be positive enough to provide sufficient support for the flip  14  when it is pressed against a user&#39;s ear during a call. 
     However, a combination of the tolerances of the components and the inherent elasticity of the hinge materials allows the lug  56  to ride over the lip  84  if excessive force is applied to the flip. For example, if the flip  14  is in the open condition and someone inadvertently sits on the phone  10 , the lug  56  overrides the axial lip  84  and the flip  14  can travel passed the 160° opening angle without breaking the flip  14 , hinge  40  or stop feature  52 / 84 . In this event, the leaf spring  44  indexes rounds to the stable major face  60 . To return the flip, the user holding the main body and flip manually snaps the lug  56  back over the axial lip  84 . As is apparent, this override property provides a beneficial safety feature for the radiotelephone  10  against damage. 
     It will be readily understood that alternative arrangements to those described above with reference to the specific embodiment can be made within the inventive concept as defined in the appended claims. For example the resilient means, instead of being a leaf spring could be an elastomeric composite. 
     Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. 
     The appended abstract as filed herewith is included in the specification by reference.

Summary:
A hinge mechanism for pivotally connecting first and second housings of a portable electronic device for movement between closed and open positions, the mechanism comprising a hinge shaft mounted for rotation with the first housing and providing a camming surface, and resilient means mounted for rotation with the second housing and arranged to cooperate with the camming surface so as to vary loading of the resilient means on the camming surface for relative rotation therebetween.