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TECHNICAL FIELD 
     The present invention relates to an access control device including an obstacle—such as a bollard—that is retractable into the ground. 
     The invention applies in particular to the retractable bollards that are used to allow or prevent vehicles accessing a residential, service-sector, or urban zone. 
     STATE OF THE ART 
     The invention applies in particular to bollards comprising a housing for burying in the ground, a structure that acts as an obstacle and that is mounted to move in (vertical) translation within the housing, a rotary electric motor, and a deformable link—such as a cable, a strap, a belt, or a roller chain—connecting the structure that acts as an obstacle to the motor so as to enable said structure to be moved in translation under drive from rotation of the motor. 
     Such bollards are described in particular in patent EP 0 627 527 in which the deformable link is driven by a sprocket or by a drum driven by the motor via a reversible speed-reducing gearbox, such that in the event of the power supply to the motor being interrupted, the structure that acts as an obstacle moves down under gravity towards the bottom of the housing. 
     In such a device where the motor forms part of movable equipment that moves in vertical translation and that includes the bollard that acts as an obstacle, moving and stopping the bollard give rise to impacts and to vibration that are applied to the motor and that accelerate its aging. 
     Furthermore, when the bollard is deployed so as to project above the ground, and when an impact is applied thereto, a fraction of the energy of the impact is transmitted to the casing, to the motor, and to the other electromechanical components for driving the bollard, as well as to the associated sensors, and that can damage the motor, those electromechanical components, and/or the sensors. 
     Other bollards driven by means of a strap that is wound onto a drum are described in patents EP 0 945 550, FR 2 869 629, and WO 2006/024787. In an embodiment described in patent EP 0 945 550 as being advantageous for solid or small-diameter bollards and obstacles, the motor-and-gearbox unit is placed in a housing external to the housing for the bollard. 
     Patent AU 2004/100095 describes a retractable bollard that is not motor driven, and having a housing made up of three compartments: a central compartment that receives the element acting as the retractable obstacle, and two side compartments, each receiving a counterweight. 
     Since the combined weight of the counterweights balances the weight of the element acting as an obstacle, little force is required on the element acting as an obstacle in order to deploy it or retract it. 
     One drawback of manually-driven bollards is that they cannot be remotely controlled; another drawback is that they can easily be retracted by an ill-intentioned person. 
     A drawback of bollards driven by a rotary electrical actuator is that they are fragile and require frequent and expensive maintenance. 
     A drawback of retractable bollards is that they cannot withstand impacts of energy exceeding a few tens of thousands of joules (J). Unfortunately, certain “security” applications require a retractable bollard to withstand an impact of energy that may reach or exceed 10 5  J or 10 6  J. 
     Another drawback of retractable bollards is that in order to renovate the coating of the structure that acts as an obstacle, which coating is subjected to various kinds of attack (moisture, friction, impacts, etc.), it is generally necessary for the structure to be disassembled and extracted, and then subjected to lengthy and expensive treatment in a workshop specialized for that purpose. 
     In particular when the coating is paint and when the wall of the structure, which is generally tubular and made of cast iron or steel, is also thick, the surface appearance of the structure is not very attractive, in particular because of irregularities (projections or indentations) of said surface that result from the method used for fabricating the structure. That is why such structures are generally machined in order to improve their surface state, thereby giving rise to significant extra cost. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to propose an access control device including a retractable obstacle that withstands high levels of impact. 
     An object of the invention is to propose an access control device including a retractable obstacle and for which maintenance is reduced, made easier, and/or inexpensive. 
     An object of the invention is to propose an access control device including a retractable obstacle, which device is improved and/or remedies, at least in part, the shortcomings or drawbacks of known access control devices. 
     According to an aspect of the invention, there is provided an access control device comprising a first housing, an obstacle mounted to move in translation inside the first housing, a second housing remote from the first housing and receiving an electrical motor-and-gearbox unit, (at least) one deformable link connected to the obstacle and co-operating (in particular by meshing or by winding) with the motor-and-gearbox unit, and a mechanical return device for returning the obstacle towards a position projecting out from the first housing, which return device is connected to the deformable link and is arranged in the second housing, such that the deformable link(s) transmit(s) the return force exerted by the mechanical return device to the obstacle, together with the drive force exerted by the motor-and-gearbox unit. 
     In other words, and according to another aspect of the invention, there is provided an access control device comprising a first housing for burying in the ground, an obstacle slidably mounted in the first housing, a second housing for burying in the ground, a rotary electric motor fitted with a speed-reducing gearbox and arranged in the second housing, a deformable link connected to the obstacle and driven by the motor-and-gearbox unit, a hollow structure extending between the first and second housings and having a portion of the deformable link extending therein, and a mechanical return device for returning the obstacle towards a position projecting out from the first housing, which return device is connected to the deformable link and is arranged in the second housing. 
     According to another aspect of the invention, there is provided an access control device comprising a first buried housing, a bollard slidably mounted in the first housing to slide along the substantially vertical longitudinal axis of said housing, a second buried housing separated from the first housing by a dissipative structure—such as a layer of concrete—serving to diminish the transmission to the second housing of the energy of an impact against the bollard acting as an obstacle, a rotary electric motor fitted with a reversible speed-reducing gearbox and arranged in the second housing, a deformable link connected to the obstacle and driven by the motor-and-gearbox unit, a hollow structure interconnecting the respective top portions of the first and second housings and having a (central) portion of the deformable link extending therein—and capable of moving therein—, a mechanical return device for returning the obstacle into or towards a position projecting out from the first housing, which return device is connected to the deformable link and is arranged in the second housing, and a power supply for powering the electric motor, the power supply including an electrical energy storage device such as a battery. 
     In the event of an impact against the projecting portion of the obstacle, the invention serves to limit or to avoid the energy of the impact being transmitted to the second housing and to the members and equipment contained therein. 
     The invention makes it easier to install, maintain, and repair the components of the access control device. 
     The assistance given by the mechanical return device to sliding the obstacle makes it possible to use a motor of lower power, while still enabling the bollard to be moved quickly. 
     The invention also makes it possible to continue deploying and retracting the bollard after the power supply to the access control device has been cut in the event of the device incorporating an electrical energy storage device, thereby making it possible, until the energy stored in the storage device has been used up, to hold the obstacle in a predetermined position (deployed or retracted). 
     In embodiments of the invention:
         the access control device may comprise two deformable links, in particular two roller chains, each fastened to the slidable obstacle at a first one of its two ends and fastened to the mechanical return device at a second one of its two ends, and each engaging a drive member—such as a sprocket—that is driven in rotation by the motor-and-gearbox unit;   when the slidable obstacle comprises a cylindrical body of circular section, the two deformable links may be fastened to respective points or regions thereof that are diametrically opposite;   the slidable obstacle may comprise a tubular body (a cylindrical body of circular section) provided with at least one piece of reinforcement projecting from the outside face of the tubular body and extending over (at least) a substantial fraction of a circumference of the tubular body, in particular reinforcement in the form of a flange, a rim, or a ring surrounding the tubular body and extending to a short distance from the inside face of the first housing; such that in the event of an impact against the portion of the obstacle that projects out from the housing, the reinforcement can bear against the inside face of the housing so that the housing and the material surrounding the housing contribute to withstanding the impact; for this purpose, and in a particular embodiment, the tubular body of the obstacle may be provided with a first annular piece of reinforcement arranged at the bottom end of the tubular body and with a second annular piece of reinforcement arranged above the first piece of reinforcement at a distance therefrom that may be not less than about half the diameter of the tubular body, e.g. a distance equal to about the diameter of the tubular body;   the mechanical return device for returning the obstacle into (or towards) a position projecting out from the first housing may comprise a heavy structure serving as a counterweight for balancing at least a fraction of the weight of the slidable obstacle; the heavy structure may present a weight that is greater than that of the slidable obstacle, e.g. by at least 5% or 10%, or on the contrary a weight that is less than the weight of the slidable obstacle, e.g. by at least 5% or 10%; the heavy structure may comprise two heavy sub-structures of unequal respective weights that are connected together by an electromagnetic connection member such as an electromagnet;   the mechanical return device for returning the obstacle into (or towards) a position for projecting out from the first housing may comprise, in addition to or as a replacement for the heavy structure(s), one or more springs, in particular one or more helical springs each having a first end fastened to the wall of the second housing and a second end fastened to an end of a respectively deformable link;   the slidable obstacle may comprise a tubular body and an end wall—or cap—closing the top end of the tubular body and releasably fastened to the tubular body, thereby making the first housing easier to clean; and   each of the housings may be fitted with jacks in its bottom portion so as to make it easier to put the top ends of the housings in a horizontal position.       

     According to another aspect of the invention, there is provided an access control device comprising a housing, an obstacle mounted to move in translation in the housing, an actuator such as an electric motor-and-gearbox unit, a deformable link connected to the obstacle and co-operating with the actuator to move the obstacle in the housing, and a translucent film covering at least a (substantial) portion of the outside surface of the obstacle and secured to the obstacle in reversible manner. The film may be secured to the tubular outside surface of the obstacle by adhesion, in particular by adhesive. The face of the film that is secured to the obstacle may be provided with patterns that can be seen visually. For this purpose, the patterns may be obtained by silkscreen printing. The film may be essentially constituted by polyester; it may present thickness lying in the range going from approximately 50 micrometers (μm) to approximately 100 μm or 200 μm. 
     Other aspects, characteristics, and advantages of the invention appear from the following description that refers to the accompanying figures that show preferred embodiments of the invention, in non-limiting manner. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a cutaway diagrammatic perspective view showing two housings of an access control device. 
         FIG. 2  is a cutaway diagrammatic perspective view seen from a different viewing angle showing the two housings of an access control device, this figure showing two tensioned springs of a return device provided in one of the housings. 
         FIG. 3  is a diagrammatic perspective view seen from the same viewing angle as  FIG. 2 , showing the two housings of the  FIG. 2  device in a configuration corresponding to the obstacle being deployed out from the first housing, in which position the springs of the return device are shortened (“relaxed”). 
         FIGS. 4 to 6  are diagrammatic perspective views from the same viewing angle and cut away to show the components within the housings, these views showing the housings and the deformable links of an access control device in three distinct configurations of the access control device: in  FIG. 4  the obstacle is retracted inside the first housing; in  FIG. 5  the obstacle is fully deployed outside the housing; and in  FIG. 6  there can be seen a configuration that is intermediate between the configurations of  FIGS. 4 and 5 . 
         FIG. 7  is a cutaway side view of an access control device in a maximally-deployed configuration of the obstacle. 
         FIG. 8  is a perspective view of the housings of an access control device in a retracted configuration of the obstacle. 
         FIG. 9  is a perspective view from a viewing angle similar to that of  FIG. 5  and on a larger scale, showing the arrangement of the main components of the system for driving and balancing the movable obstacle. 
         FIG. 10  is a diagrammatic plan view of a buried access control device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Unless indicated explicitly or implicitly to the contrary, elements or members that are structurally or functionally identical or similar are designated by identical references in the various figures. 
     With reference to  FIG. 10  in particular, the access control device  20  comprises a first housing  21  of tubular shape with a circular cross-section and having its longitudinal axis  22  arranged substantially vertically. 
     The device  20  also has a second housing  23  of tubular shape, of rectangular or square cross-section, with its longitudinal axis  24  arranged substantially vertically. 
     The housing  21  contains an obstacle  25  in the form of a bollard that is mounted to slide along the axis  22 , while the housing  23  contains a brushless electric motor  26  fitted with a speed-reducing gearbox and with a sensor responsive to the absolute angular position of the shaft of the motor or of the outlet shaft of the gearbox. 
     The motor  26 , the angular position sensor that is associated therewith, and an electronic power supply module connected to the motor and to the sensor together form a servo-motor making it possible at all times to know the position of the movable obstacle within the housing  21 , thus making it possible, where appropriate, to avoid using additional sensors in order to monitor the movement of the obstacle. 
     The device  20  also includes two buried ducts  31  that extend between the two housings. Each duct  31  serves as a protective covering for a force-transmission chain connecting the bollard to the motor, and it extends from an opening provided in the wall of the housing  21  to an opening provided in the wall of the housing  23 . In  FIG. 1 , it can be seen that each covering duct  31  opens out substantially tangentially into the housing  21 . 
     The housings  21  and  23  are buried in the ground so that their respective top ends are substantially flush with the surface of the ground, and so that they are spaced apart by a distance  34 , which distance may be of the order of 10 centimeters (cm) or 20 cm, for example. 
     To this end, a pit  29  is dug in the ground, and a mud slab made of concrete may be cast in the bottom of the pit. The housings may be placed on the slab, and their respective longitudinal axes may then be made vertical by acting on screw jacks—or other legs of adjustable length—such as those referenced  35  in  FIGS. 1 to 3 . This also makes it possible to ensure that the parts covering the top ends of the housings are horizontal (which parts are referenced  48  and  49  in  FIG. 8 ). 
     A cage  30  of metal reinforcing bars surrounding the housings may be put into the pit prior to the pit being filled with concrete  33 . The layer of concrete that then extends in the space between the two housings forms a structure capable of damping impacts applied to the bollard and transmitted by the bollard to the housing  21 , and by the housing  21  to this layer of concrete, such that the access control device is capable of withstanding considerable impacts without damaging the members situated in the second housing  23 . 
     The device  20  also includes a cabinet  27  housing the members necessary for powering the motor electrically and for controlling the movement of the retractable bollard  25  as a function of signals delivered by the angular position sensor fitted to the motor-and-gearbox unit, the cabinet  27  being connected to the motor  26  by means of cables  28 . 
     The cabinet  27  includes in particular a backup power supply  32  comprising a battery and an inverter. 
     In the embodiments of the access control device shown in  FIGS. 2 and 3 , the mechanical return device for returning the obstacle to a position projecting out from the first housing is constituted by two helical springs  36 , whereas in the embodiments shown in  FIGS. 4 to 7  and  9 , the mechanical return device is constituted by two heavy structures  37  and  38  in the form of rectangular blocks. 
     With reference to  FIG. 8  in particular, the top end of the housing  23  is closed by a cover  48  giving access to the members contained in this housing. 
     A ring  49  is releasably fastened to the top end of the housing  21  and defines a circular opening  51  allowing the cylindrical body of the retractable obstacle  25 ,  50  to pass through with a small amount of clearance. 
     The covering ducts  31  and the spacers  52  connecting the housings  21  and  23  together enable the two housings to be handled simultaneously in order to place them in a pit. 
     A T-shaped duct  53  interconnects the bottom ends of the housings and enables them to be connected to a pipe (not shown) for draining away any liquid that might penetrate into the housings. 
     With reference to  FIGS. 4 to 6  in particular, the retractable bollard  25  comprises a cylindrical body  47  of circular section and of longitudinal axis that substantially coincides with the axis  22  of the housing  21 , with the bollard sliding along this axis in the housing. 
     The body  47  is closed at its top end by a disk-shaped end wall  50 , which wall is fastened to the body  47  in removable manner as shown diagrammatically in  FIG. 1 . The edge face of the disk  50  may be provided with a groove that receives a flexible printed circuit fitted with light-emitting diodes (LEDs) (not shown) for providing light to indicate the presence of the bollard. 
     These LEDs are powered by the cabinet  27  ( FIG. 10 ) by means of a cable (not shown) that is suspended from the end wall  50  and that runs along a duct having three portions that are connected together end to end: i) a rigid rectilinear portion  54  ( FIGS. 1 ,  7 , and  9 ) extending vertically from the bottom end  57  of the housing  21 ; ii) a rectilinear portion  55  ( FIG. 7 ) with a bend at each end and extending beneath the housings  21  and  23 ; and iii) another rigid rectilinear portion  56  ( FIGS. 6 ,  7 , and  9 ) extending vertically from the bottom end  58  of the housing  23  to the top portion of the housing that houses the bollard drive mechanism. 
     With reference to  FIGS. 4 to 6  and  9  in particular, the bottom end of the body  47  of the bollard  25  is surrounded by a flange  46  serving to reinforce the tube  47  and to guide its movement in the housing  21 . For this purpose, the outside diameter of the flange  46  is slightly less than the inside diameter of the housing  21 , such that the flange  46  slides together with the body  47  while leaving small clearance relative to the housing  21 . 
     The bollard  25  has a second flange  45  similar to the first flange  46 , located above the flange  46 , and carrying or including an anti-friction member or coating on its outer edge face, e.g. constituted by polytetrafluoroethylene. 
     The flanges  45  and  46  thus serve to guide the bollard as it slides inside the housing  21 , and also to transmit forces that need to be withstood by the body of the bollard in the event of an impact against it, the forces being transmitted from the body of the bollard to the wall of the housing  21 . 
     Generally, the housings  21  and  23  and the elements  45  to  47  and  50  of the movable bollard  25  are made of metal, with the wall thickness of the housings being less than the wall thickness of the bollard body. 
     As can be seen in  FIG. 1 , the inside face of the bollard body is provided with two longitudinal fins  59  that extend close to and on either side of the duct  54  so that the duct prevents the bollard  25  from turning inside the casing  21 . 
     Furthermore, as can be seen in  FIGS. 4 to 6 , the duct  56  also serves to guide blocks  37  and  38  that are mounted to move in translation inside the housing  23 . 
     In the embodiments of the access control device shown in  FIGS. 2 to 7 , in particular, the force transmission system connecting the movable obstacle  25  to the mechanical return device  36  to  38  and to the motor  26  is constituted by two identical chains  39  and  40  that extend substantially in two respective vertical planes that are parallel to the plane containing the longitudinal axes  22  and  24  of the housings  21  and  23 . 
     Each of these chains, such as the chain  40 , is fastened at one of its ends  401  to an attachment part  41  secured to the block  37 , and is engaged with the teeth of a drive sprocket  42 , and with the teeth of a deflector sprocket  43 . The chain  40  is also fastened via its second end  402  to an attachment part  44  secured to the flange  45  surrounding the body  47  of the movable bollard. 
     Each of the two deflector sprockets  43  is arranged in the space defined by the wall of the housing  21  and by the body  47  of the bollard. The two sprockets  43  are fastened to the housing  21  and they are mounted to rotate freely relative to the housing about a common axis of rotation that is horizontal and perpendicular to the plane containing the axes  22  and  24  of the housings  21  and  23 . 
     As shown in particular in  FIG. 9 , the two drive sprockets  42  are mounted at respective ends of a shaft  60  that is supported on a plate  61  by two bearings  62 . 
     The axis of rotation of the shaft  60 , and of the sprockets  42 , is parallel to the common axis of rotation of the deflector sprockets  43 . 
     The shaft  60  is driven in rotation by the motor  26  via a speed-reducing gearbox  63  coupled to the outlet shaft of the motor, via a sprocket fitted to the outlet shaft of the gearbox  63 , and via a chain  64  engaged with the teeth of said sprocket and with the teeth of another sprocket mounted on the shaft  60 . 
     Thus, rotation of the motor  26  in a first direction of rotation, starting from the retracted position of the bollard as shown in  FIG. 4 , in particular, causes the bollard  25  to be raised out from the housing  21 , as shown in  FIG. 6 , until it reaches its maximally-deployed position as shown in  FIG. 5 , in which position the top flange  45  of the bollard is close to the top end of the housing  21 . 
     Conversely, rotation of the motor  26  in a second direction of rotation opposite to the first causes the bollard  25  to be retracted progressively into the housing  21 . 
     With reference to  FIGS. 2 and 3 , the mechanical return device for returning the obstacle comprises two identical helical springs  36  extending substantially vertically beneath the plate supporting the drive means for driving the force transmission chains. 
     A first end of each spring  36  is fastened to the end wall  58  of the second housing  23 , and a second end of each spring is fastened to one end  401  of a respective one of the chains  39  and  40 . 
     The presence of the counterweight and/or spring return device enables a low-power motor  26  to be used for moving the bollard  25 , which may present a weight that is as great as or greater than 100 kilograms (kg) to 200 kg. 
     Positioning the drive and transmission means separately, in the independent housing  23  that is reinforced by a concrete structure makes it possible to protect the entire system effectively against any type of impact that may be suffered by the bollard  25 . The separate positioning also presents the advantage of enabling a large majority of the components of the device to be acted on without extracting the movable bollard  25  from the housing  21 . 
     The absence of any wear parts or parts exposed to impacts in and on the movable bollard  25  greatly minimizes the amount of preventative and corrective maintenance that is needed. 
     The reinforcing flanges  45  and  46  situated around the bollard  25  and sliding flush with the wall of the housing  21  make it possible for the forces that are caused by impacts produced by vehicles to be dissipated effectively within the concrete structure  33  surrounding the housings  21  and  23 . 
     As described above, the bollard  25  is extended and retracted by a transmission having two chains  39  and  40 , with each chain being suspended by two sprockets  42  and  43 . One end of the chains may be fastened to the movable bollard  25 , and the other to a load  37 ,  38  of weight that may vary automatically as a function of electrical signals powering a connection member that operates electromagnetically and that connects together the two loads  37  and  38 , thereby making it possible to ensure reference positioning as selected by the operator (bollard raised or bollard lowered). 
     The use of chains  39 ,  40 ,  64  for transmitting forces for raising the bollard and for driving the movement of the bollard provides efficient transmission and a high level of mechanical strength. The use of low maintenance or “maintenance-free” chains such as those available from the supplier Sedis (France) under the name “Chaîne verte®)” can make greasing unnecessary. 
     The use of a servo-motor  26  fitted with an absolute coder (referenced  70  in  FIG. 9 ) enables accurate positioning for the body of the bollard in the housing  21  to be transmitted to a bollard control system, and makes it possible to move the bollard in application of a trajectory (which may be defined in terms of a speed, an acceleration, and a “target” point) that is programmed or recorded in an electronic card for controlling the servo-motor. 
     The movement of the bollard may be stopped so as to position it at a predefined point, so that there is no need to use additional movement sensors. This also makes it possible to provide an access control device in which the maximum height of the projecting portion of the bollard is variable. 
     The bollard may be locked in position by maintaining motor torque that may increase or decrease automatically as a function of the force applied to the bollard in the event of an unauthorized attempt at making it retract. Thus, there is no need to use an electrically operated brake or other locking device. 
     The speed of rotation of the servo-motor may reach 6000 revolutions per minute (rpm) with accelerations that are high, so as to guarantee that the bollard  25  moves quickly, e.g. in order to block access in the event of an alarm being triggered. 
     The counterweight  37 ,  38  may be made up of two loads that are connected together by an electromagnet that either passes or does not pass electric current. Thus, depending on the desired application, it is possible to vary the weight of the counterweight. 
     Using a counterweight that is heavier than the movable bollard enables the bollard to be raised and held in its high position without requiring electrical energy to be expended. 
     Using a counterweight enables the bollard to be accompanied while it is descending, avoiding the bollard dropping suddenly, thereby making it possible to reduce the power of the motor  26  and to reduce its energy consumption, and it also makes it easier to use an electrical power supply for the motor based on an inverter and capable of ensuring uninterrupted service in the event of a power cut (and for a duration that varies as a function of the capacity of the battery of the inverter), capable of blocking the bollard in its high position, and capable of retracting the bollard into its low position and keeping it in that position. 
     The surface of the cylindrical body of the retractable bollard  25  may be protected and decorated by installing a sticky translucent film (not shown) on the outside face thereof. 
     The nature of the film may be selected to be capable of withstanding attack from the environment of the bollard (moisture, sand, impacts, etc.). 
     The inside face of the film may be silkscreen-printed and may include “trompe l&#39;oeil” patterns for modifying the visual appearance of the body of the bollard (i.e. modifying its surface appearance and shape), e.g. giving it the appearance of a (“Roman”) sculptured column with a granite or marble look. 
     The film may be coated in adhesive and may benefit from anti-scratch treatment. 
     The film may be a flexible transparent polyester film coated in acrylic adhesive. 
     In addition to withstanding abrasives, ultraviolet radiation, bad weather, and chemicals, such films are capable of adhering in particular on a bollard body made of steel, regardless of whether or not it is painted, and they enable the bollard body to be decorated and to be cleaned easily (in particular for graffiti). 
     The possibility of unsticking such a film once it has deteriorated and of replacing it with an identical new film makes the body of the bollard easier to maintain.

Summary:
The invention relates to an access control device ( 20 ) comprising a first housing ( 21 ), an obstacle ( 25 ) mounted to move in translation in the first housing, a second housing ( 23 ) remote ( 34 ) from the first housing and receiving an electric motor-and-gearbox unit ( 26 ), a deformable link connected to the obstacle and co-operating with the unit, and a mechanical return device for returning the obstacle towards a position projecting out from the first housing, which return device is connected to the deformable link and is arranged in the second housing.