Watch having a glass retaining strap
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a watch casing of the so-called &#34;one piece&#34; type, that is to say of the type in which the back of the casing and its body or middle part are made in one piece. 
     (b) Description of the Prior Art 
     Such watch casings, which are known per se, possess the drawback that, if the glass is snap fitted on the casing body or back-bezel, it is not easy to remove it since it is not possible to act behind the glass for exerting thereon a pressure to extract it. 
     In the case of a domed glass, of synthetic glass and which is resiliently deformable, it is possible to use apparatus which surrounds the glass and deforms it so as to permit it to be extracted. Such a solution is however not possible with glasses made in extra hard material, such as hardened mineral glass or sapphire, which are practically undeformable. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide means for securing a glass of a watch casing the back and the body of which are made in one piece which allows the glass easily to be detached while providing safe attachment of the glass. 
     To this end, a watch casing according to the invention has the glass thereof frictionally engaged into the middle part of the casing and is retained by a diametral strap which bears thereon and which is on the other hand secured, by its ends, to the back/middle part of the casing. 
    
    
     The various features of the invention will be apparent from the following description, drawings and claims, the scope of the invention not being limited to the drawings themselves as the drawings are only for the purpose of illustrating ways in which the principles of the invention can be applied. Other embodiments of the invention utilising the same or equivalent principles may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention and the purview of the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a wrist-watch, the casing of which represents a preferred embodiment of the invention; 
     FIG. 2 is a side view thereof; 
     FIG. 3 is a partial sectional view on the line III--III of FIG. 1 to a larger scale; 
     FIG. 4 is a sectional view on the line IV--IV of FIG. 1 to the scale of FIG. 3, and 
     FIG. 5 is a plan view of a detail, in partial section, to the scale of FIGS. 3 and 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The clockwork of the illustrated wristwatch, designated by reference 1 and provided with its dial designated by reference 2, is located in a waterproof casing of the so-called &#34;one piece&#34; type, that is to say in which the body or middle part of which, designated by reference 3, is made of one piece with the back, designated by reference 3a. The clockwork is secured to this back/body 3-3a by screws 4 threaded therein. 
     The glass of the casing, preferably made of an extra hard material such as hardened mineral glass or sapphire, designated by reference 5, is engaged with light friction in the casing body of the middle part 3 with the interposition of a sealing gasket 6 made of flexible and compressible material. The glass 5 bears on a spacer ring 7 surrounding the clockwork 1. 
     The glass 5 is retained in place in the casing body 3 and is even pressed axially against the front axial face of the ring 7, by a diametral tensioning strap 8 comprising an annular part 8a bearing on the glass and two pairs of arms 8b which are diametrically opposed. The ends of the arm pairs 8b engage on both sides of each of two hinge lugs presented by the casing body or middle part 3, and which serve for the attachment of a bracelet designated by reference 9. 
     The connection between the two ends of the arms 8b and each of the hinge lugs 3b is obtained owing to a stepped small rod 10 the inner end portion of which, which is the thinnest and designated by reference 10a, is threaded and receives, screwed thereon, a threaded sleeve 11 the diameter of which corresponds to the portion of largest diameter, designated by reference 10b, of the small rod 10. The diameter of the part 10b of the small rod 10 and of the sleeve 11 corresponds to the diameter of the bores, designated by reference 12, provided in the arms 8b of the diametral strap 8 retaining the glass 5 in place, while the central part of the small rod 10, designated by reference 10c, has a diameter which corresponds to that of a through bore designated by reference 13 in each of the hinge lugs 3b. The parts 10b and 10c of the small rod 10 are interconnected through a chamfer 14, while the front end of the sleeve 11 is provided with a chamfer 15. 
     The arrangement is such that, when the glass 5 is in place in the middle part 3 with the diametral strap for retaining it applied thereon, it is necessary to exert a slight force on the ends of the arms 8b to bring the bores 12 of these arms opposite the respective bores 13 of the hinge lugs 3b. The small rods 10 are then fitted and the sleeves 11 respectively screwed on to them. The chamfers 14 and 15, co-operating with conical entrances 16 of the bores 12 provide guidance to assist the insertion of the small rods 10 and sleeves 11 if the bores 12 are not perfectly aligned with the bores 13. The small rods exert then a pre-tensioning force on the diametral strap 8 which, hence, exerts a resilient axial force on the glass 5, applying it axially against the ring 7. In this way, the glass 5 is firmly retained in place without any risk, even should its friction on the sealing gasket 6 be insignificant, of untimely separation from the casing body. 
     The parts of the small rod 10 and of the sleeve 11 which extend beyond the arms 8b of the strap once the mounting is effected constitute securing means for the bracelet 9. 
     The extraction of the glass, once the diametral securing strap 8 is detached, is effected easily, since the casing body has, opposite one of its two hinge lugs 3b, a lip or file-mark 17 which permits the introduction of a blade in an annular groove 18 presented for this purpose by the glass 5. 
     It is to be noted that the resetting, even the winding of the main spring if the clockwork is mechanical, is effected by means of a half circular stirrup 19 (FIG. 4), situated outside the back 3a, articulated on a circular member 20 rotatably mounted on the back. This stirrup can occupy two positions, one being an operative position, in which it is perpendicular to the plane of the casing and thus can be gripped for the purpose of driving of the member 20, and the other one being a rest position, illustrated in the drawings, in which it is recessed into the plane of the casing. 
     As a modification, the two hinge lugs 3b for attachment of the bracelet and of the diametral strap may be replaced by two bows. In this case, the arms 8b will be engaged between the two bows, the bracelet itself being engaged between the said arms. 
     Each pair of arms 8b may be replaced by a sole arm terminating in a fork-shaped portion the two branches of which will in each case engage the corresponding hinge lug of the casing or between the corresponding bows thereof. 
     As a further modification, the diametral strap may be provided, for an aesthetic purpose, with two supplementary radial arms extending perpendicularly to its main part; these arms could also be secured, by their ends, to the body of the casing. 
     The attachment of the strap to the casing body can be different from that which has been described and illustrated and, especially, could be independent from the attachment means of the bracelet. 
     Finally, the annular central part 8a of the strap may carry the hours division.