Abstract:
The invention relates to a device for locking a cover ( 10 ) of a manhole top to the frame thereof, said device comprising at least one locking means ( 16′, 18′ ) which is provided on the cover ( 10 ) and comprises at least two interspaced, adjacent spring arms ( 18 ) that are respectively provided with an engaging section ( 18   a′ ). Said sections ( 18   a ) can be engaged with a part ( 6 ) of the frame and held on the spring arms ( 18 ) in an elastic manner in such a way that they can be displaced in relation to the adjacent section of the periphery of the cover ( 10 ) at an angle smaller than a right angle. The invention is characterised in that a blocking device ( 40 ) is used to block the displacement of the engaging sections ( 18   a ) as required.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a device for locking a cover of a manhole top to the frame thereof, said device comprising at least one locking means provided on the cover which has at least two interspaced, adjacent spring arms that are each provided with an engaging section, whereby the engaging sections can be engaged with a portion of a frame and held on the spring arms in an elastic manner in such a way that the engaging sections are movable relative to the adjacent section of the circumference of the cover at an angle that is smaller than a right angle. The invention further relates to a manhole top comprising a cover, a frame for accommodating the cover, and such a device. 
         [0002]    It is noted that in the present context the term “manhole” is understood to mean all types of shafts, ducts, channels, and other cavities to be covered, such as manhole shafts leading to sewer conduits, pipeline and cable ducts, and drainage channels, so that the term “manhole top” includes all types of tops having a frame and a cover sealing same for all kinds of shafts, ducts, channels, and other cavities such as manhole shafts leading to sewer conduits, pipeline and cable ducts, and drainage channels. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    A locking device of the aforementioned type is disclosed, for exarnple, in DE 202 04 108.5 U1 and WO 03/076729, from which the present invention proceeds. As described therein, the two spring arms of each locking means lie in a plane that extends parallel to the cover, so that the engaging sections of the spring arms are movable in a spring-elastic manner essentially parallel to the adjacent section of the circumference. 
         [0004]    FR 2717840 A1 discloses a cover having locking means which likewise comprise two parallel spring arms. However, the two spring arms of each locking means are situated at right angles to this known cover, and therefore also extend at right angles to the adjacent section of the circumference of the cover, and are designed to be movable at right angles thereto, and thus in the radial direction, in a spring-elastic manner. 
         [0005]    Manhole tops having a lock which typically engages in the manner of an arresting or catching means are known and used in various forms. 
         [0006]    Reference is also made by way of example to WO 86/04624, which discloses a self-locking cover for a manhole top in which, however, each locking means comprises a single elastic arm which is integrally molded to a reinforcing rib at the underside of the cover and extends essentially over a portion of the circumference of the cover and parallel thereto. 
         [0007]    In this regard, further reference is made by way of example to EP 0 694 654 A, which discloses a cover having two radial spring arms which are diametrically opposed by 180° and which have at their free ends an engaging section that may be engaged with a portion of a frame. 
         [0008]    For a device of the aforementioned type the present invention additionally provides a blocking device which blocks the movability of the engaging sections as needed. 
         [0009]    The device having a lock of the aforementioned design has proven to be useful in practice, but since the lock is achieved by means of a catch with the assistance of elastically flexible spring arms, there is still the risk of unauthorized opening of the cover by overcoming the elastic forces on the spring arms which create the lock, in particular by use of a suitable tool by which the cover is pried from the frame. By use of the invention this may be prevented in a particularly effective manner by simply blocking the movability of the engaging sections on the spring arms by means of the blocking device provided according to the invention. Of course, the blocking device is only able to achieve its blocking effect when the cover is in its closed position and the engaging sections lock the cover to the frame. 
         [0010]    As a result of this measure according to the invention, the cover may preferably be used without problems in security-relevant areas. In particular, in such cases temporary welding of the cover to the frame is unnecessary, as has often been practiced heretofore, particularly in a makeshift manner, which, however, not only has entailed significantly greater effort, but has also resulted in damage to the cover and frame during subsequent separation of the weld seam. 
         [0011]    Alternatively, the blocking device may be situated on the cover, preferably on the underside thereof, or on the frame of the manhole top. 
         [0012]    The blocking device preferably has a blocking element which for blocking the movability of the engaging sections may be engaged with two adjacent spring arms and/or the engaging sections thereof. It is practical to design the blocking element in such a way that it may be easily inserted between two adjacent spring arms and/or the engaging sections thereof, and preferably may be brought into contact with the spring arms and/or the engaging sections thereof. The blocking element should have a wedge-shaped design to facilitate engagement. 
         [0013]    One particularly preferred embodiment of the invention is characterized in that the blocking element is movably supported between a blocked position, in which it blocks the movability of the engaging sections, and a rest position in which it enables the movability of the engaging sections. It is practical for the blocking element in its rest position to be located outside the spring arms so as not to impede the elastic movability of the engaging sections and the elastic effect of the spring arms during opening and closing of the cover. 
         [0014]    It is understood that the blocking element is not brought into its blocked position unless the cover is closed and the engaging sections provided on the spring arms are locked on the associated parts of the frame, thus locking the cover to the frame. In contrast, the blocking element must assume its rest position during any opening and closing of the cover so that it is possible for the spring arms to achieve their elastic effect and for the engaging sections to be correspondingly moved. 
         [0015]    By means of the particularly simple design for supporting the blocking element, the blocking element may be pivotably supported between its blocked position and its rest position. It is practical for the blocking element to be pivotably supported about its swivel axis, which is essentially parallel to the cover. Alternatively, the blocking element may also be supported so as to be linearly movable between its blocked position and its rest position. 
         [0016]    For automatic actuation of the blocking device a drive device should be provided which correspondingly moves the blocking element. 
         [0017]    For remote control of the blocking device a remote control device may be provided which preferably is coupled to the blocking device via a cable or wireless connection. It is thus possible to block the lock from a remote location such as a control room, and in the case of a plurality of such covers the control of the blocking devices thereof may preferably be centralized. If the blocking device has a blocking element which is movable by a drive device, it is practical for the drive device to be controlled by the remote control device. 
         [0018]    In addition, a monitoring device may be provided for monitoring the operating state of the locking means. It is thus possible, in particular from a central location, to advantageously check whether the cover is locked on the frame. It is practical for the monitoring device to monitor the operating state of the blocking device and preferably the position of the blocking element in order to easily determine whether the locking means in its closed position is actually blocked from opening. 
         [0019]    The monitoring may preferably be centrally positioned, in particular when multiple manhole tops are to be monitored. In particular, for this purpose the remote control device may include or contain the monitoring device. 
         [0020]    If the remote control device and/or the monitoring device is connected to the blocking device via a cable, it is preferable to select the aforementioned placement of the blocking device on the frame which, compared to a placement on the cover, allows the connecting cable to be installed in a stationary and therefore uncomplicated manner. 
         [0021]    In particular, the engaging section of the spring arm is movable approximately parallel to the adjacent section of the circumference of the cover. 
         [0022]    The engaging section of the spring arm preferably extends at an angle with respect to the adjacent section of the cover. 
         [0023]    In a mutual refinement of the previously mentioned embodiments, the engaging section of the spring arm extends approximately at a right angle with respect to the adjacent section of the cover. For a substantially circular cover, the engaging section of the spring arm may extend radially, and in particular the spring arm may be essentially radially positioned. 
         [0024]    To enhance the elastic properties the spring arm may preferably have a curved section. The curved section may be situated between the engaging section and a further section of the spring arm, with the two sections approximately flush with one another. 
         [0025]    One particularly preferred embodiment is characterized in that the locking means has two interspaced spring arms which preferably extend in parallel, at least in parts, thereby allowing the locking effect of the locking means to be enhanced. When a curved section is provided for both spring arms, said section should be outwardly curved, thus enabling the distance between the two spring arms, and thus the design of the locking means, to be kept as small as possible in this embodiment. 
         [0026]    It is practical for the engaging section to have a wedge-shaped catch section, so that the engagement occurs as a locking catch. The catch section is accordingly provided with at least one surface which is inclined in the engaging direction. However, for an easier release it is also possible to likewise provide a surface on the catch section that is inclined in the opposite direction. Alternatively or additionally, the corresponding engaging surfaces on the frame part may also be designed as wedge-shaped or sloped inclined surfaces. 
         [0027]    The spring arm is preferably attached to the cover at its end facing away from the engaging section. 
         [0028]    The locking means is usually situated on the underside of the cover. 
         [0029]    If the cover is provided with reinforcing ribs, the locking means should be situated on the cover at a distance from the reinforcing ribs, resulting in a particularly compact design of the locking means. 
         [0030]    In a further embodiment in which the cover has an engaging means for a tool, this engaging means is situated at a distance from the locking means, and is designed such that when the tool engages with this engaging means the tool may simultaneously be engaged with a portion of the frame in order to apply leverage to the cover and thus to release the cover from the frame, against the elastic force of the spring arm. This design thus allows the lid to be pried from the frame by use of the tool without the tool acting on the locking means. The engaging means preferably has a recess through which the tool may be inserted. 
         [0031]    Lastly, it is noted that two locking means are usually provided on the cover which are situated opposite from one another, and which for a circular cover are radially opposite. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0032]    Preferred exemplary embodiments of the invention are explained in greater detail below with reference to the accompanying figures, which show the following: 
           [0033]      FIG. 1  shows a piecewise top view of a frame of a manhole top; 
           [0034]      FIG. 2  shows a cross section of the frame from  FIG. 1 , along the dash-dotted line II-II in  FIG. 1 ; 
           [0035]      FIG. 3  shows an enlarged detail view of a section of the frame from  FIG. 1  having a locking recess, in the view indicated by arrow III in  FIG. 1 ; 
           [0036]      FIG. 4  shows a view of the underside of a cover for the manhole top, having a lock in a first embodiment, in this case an additionally blocking device provided for blocking the lock being omitted for simplicity; 
           [0037]      FIG. 5  shows a cross section of the cover from  FIG. 4 , along the dash-dotted line V-V shown in  FIG. 4 , here as well the blocking device for blocking the lock being omitted for simplicity; 
           [0038]      FIG. 6  shows a sectional partial view of the cover from  FIG. 4 , along the dash-dotted line VI-VI shown in  FIG. 4 ; 
           [0039]      FIG. 7  shows a view of the underside of a cover for the manhole top, having a lock in a second embodiment, in this case an additionally provided blocking device for blocking the lock being omitted for simplicity; 
           [0040]      FIG. 8  shows a sectional partial view of the cover from  FIG. 7 , along the dash-dotted line VIII-VIII shown in  FIG. 7 ; 
           [0041]      FIG. 9  shows a cross section of the circumferential section of the cover and frame arrangement, together with a tool for prying the cover from the frame; 
           [0042]      FIG. 10  shows a perspective lower view of the cover from  FIGS. 7 and 8  together with the blocking device provided on the underside of the cover, the blocking device being in a rest position; 
           [0043]      FIG. 10   a  shows an enlarged sectional partial view of the section of  FIG. 10  marked by a circle; 
           [0044]      FIG. 11  shows a perspective lower view of the cover from  FIG. 7 , a blocking device being provided on the underside of the cover according to a first preferred embodiment, and the blocking device being in a locked position; 
           [0045]      FIG. 11   a  shows an enlarged sectional partial view of the section of  FIG. 11  marked by a circle; 
           [0046]      FIG. 12  shows a lower view of the cover from  FIG. 7 , together with the blocking device provided on the underside of the cover according to the first embodiment; 
           [0047]      FIG. 13  shows a sectional partial view of the arrangement from  FIG. 12 , along line XIII-XIII shown in  FIG. 12 ; 
           [0048]      FIG. 14  shows a perspective view of the top side of a manhole top having an opened cover designed according to  FIGS. 7 and 8 , and accordingly having a lock according to the second embodiment and a frame on which a blocking device is provided according to a second embodiment; 
           [0049]      FIG. 15  shows an enlarged sectional perspective view of the underside of the manhole top from  FIG. 14  having a closed cover, a locking bar which forms a portion of the blocking device being illustrated in its closed position; 
           [0050]      FIG. 16  shows a top view of the opened manhole top from  FIG. 14 ; 
           [0051]      FIG. 17  shows a sectional side view of the opened manhole top according to  FIGS. 14 and 16  along line XVII-XVII shown in  FIG. 16 , the locking bar for the blocking device being illustrated in its retracted lower rest position; and 
           [0052]      FIG. 18  shows the same sectional side view of the manhole top as in  FIG. 17 , except with a closed cover, the locking bar for the blocking device being illustrated in its upper blocked position, and also showing in a block diagram the connection of a remote control device and a monitoring device to the blocking device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0053]    The manhole top described below comprises an annular frame  1  and a disk-shaped cover  10  which fits same. The frame  1  is illustrated in  FIGS. 1 through 3 , and the cover  10  together with a lock is illustrated in a first embodiment in  FIGS. 4 through 6  and in a second embodiment in  FIGS. 7 and 8 . 
         [0054]    The frame  1  shown in  FIGS. 1 and 2  has on in its inner side an inwardly projecting circumferential ridge  2 , the upper side  2   a  of which forms a support surface for the cover  10  and which lies in a plane extending essentially horizontally in the installed state of the frame  1 . An inwardly projecting circumferential, flange-like projection  4  is mounted on the ridge  2 , which in the illustrated exemplary embodiment is interrupted by a recess  6  at two oppositely situated locations. As shown in  FIG. 3 , each of these recesses  6  is adjoined on both sides of the flange-like projection  4  by upper and lower inclined surfaces  6   a  and  6   b  which extend outwardly toward one another at an acute angle. 
         [0055]    On its underside the cover  10  has a ridge  12  running along the circumference (see in particular  FIGS. 4 ,  5 , and  7 ), by means of which the cover  10  upon insertion in the frame  1  rests on the top side  2   a  of the circumferential ridge  2  on the frame  1  which serves as a support surface. The cover  10  is also provided on its underside with radially extending reinforcing ribs  14 . 
         [0056]    At one location on the underside of the cover  10 , at a distance from the ribs  14 , a block  16  is provided onto which two spring arms  18  are integrally molded. The spring arms  18  are positioned on the block  16  in a free-floating manner, and extend radially outward in the direction of the circumference  10   a  of the cover  10 . The spring arms  18  extend approximately in parallel to one another and to the cover  10 , at a distance from the underside of the cover. The two spring arms  18  thus form an interspace between one another, as shown in  FIG. 4 , and in the installed state of the manhole top, with the cover  10  inserted in the frame  1 , the spring arms lie approximately in a horizontal plane running in parallel and at a distance from the plane defined by the cover  10 . 
         [0057]    The arrangement comprising the block  16  and the pair of spring arms  18  mounted thereon forms a locking means for locking the cover  10  to the frame  1 . For this purpose the spring arms  18  can be moved toward one another at their free ends  18   a,  and thus in the direction of the circumference  10   a  of the cover  10 , against the effect of an intrinsic elastic restoring force, and are used as catch elements for engaging with the recess  6  in the frame  1 . 
         [0058]    In the exemplary embodiment illustrated in  FIGS. 4 and 6 , the spring arms  18  have a linear design and are provided with a wedge-shaped exterior that is formed by a sloping upper inclined surface  18   b  and a sloping lower inclined surface  18   c,  the two sloping inclined surfaces  18   b  and  18   c  extending outwardly toward one another at an acute angle, as shown in  FIG. 6 . 
         [0059]    The cover  10  is thus placed on the frame  1  in such a way that the free ends  18   a  of the spring arms  18  are aligned with the recesses  6 . The free ends  18   a  of the spring arms  18  initially come into contact at their lower inclined surfaces  18   c  with the upper inclined surfaces  6   a  of the recess  6  in the frame  1 . As the cover  10  continues to move onto the frame  1 , the upper inclined surfaces  6   a  of the recess  6  cause the spring arms  18  to be compressed at their free ends  18   a  until the free ends pass through the narrowing in the recess  6  formed by the two inclined surfaces  6   a,    6   b.  The clearance of the narrowing in the recess  6  formed by the inclined surfaces  6   a,    6   b  is therefore slightly larger than the total width of the free ends  18   a  of the spring arms  18 , viewed in the direction of the circumference  10   a  of the cover  10 . The spring arms  18  are thus compressed against the action of their intrinsic elastic restoring force until they pass through the narrowing in the recess  6 . After passing through the narrowing in the recess  6 , upon continued motion of the cover  10  onto the frame  1  the intrinsic elastic force restores the spring arms to the position shown in  FIGS. 4 and 6 , causing the free ends  18   a  of the spring arms  18  to then come into contact at their upper inclined surfaces  18   b  with the lower inclined surfaces  6   b  of the recess  6 , causing the sections of the flange-like projection  4  of the frame  1  adjacent to the recess  6  to engage from below. In the locked state the free ends  18   a  of the spring arms  18  are similarly separated at a distance from one another, as shown in  FIG. 4 , so that an interspace results between the free ends  18   a  of the spring arms  18  in the relaxed rest state as well as in the locked state. 
         [0060]    Thus, an arresting or catching lock is formed by the spring arms  18  at their free ends  18   a  at the cover  10 , and by the recesses  6  at the frame  1 . 
         [0061]      FIGS. 7 and 8  illustrate a second embodiment of the lock provided on the underside of the cover  10 . Although the spring arms  18 ′ in this second embodiment are integrally molded onto the underside of the cover  10  via a block  16 ′, similarly as for the first embodiment shown in  FIGS. 4 and 6 , and are similarly provided on their exterior with upper and lower inclined surfaces  18   b′  and  18   c′,  the second embodiment differs from the first in that the spring arms  18 ′ have an outwardly curved section  18   d′  between the free end  18   a′  and a section  18   e′  adjacent to the block  16 ′, whereby the curved sections  18   d′  of the two spring arms  18 ′ in each pair of spring arms  18 ′ have a symmetrical design in the second exemplary embodiment illustrated, and the sections  18   a′  and  18   e′  are in flush alignment. The curved sections  18   d′  of the spring arms  18 ′ are used for increasing the restoring elastic force. With regard to the remaining features and in particular the arresting or catching lock effect, reference is made to the previous description of the first embodiment according to  FIGS. 4 and 6 . 
         [0062]    To release the cover  10  which has been placed on the frame  1  and locked, a pin-shaped tool  32  is inserted through a recess  30  provided on the circumference  10   a  of the cover  10 , and is engaged with the section of the frame  1  situated thereunder such that the tool produces a lever effect which allows the cover  10  to be lifted from the frame  1 , as shown in  FIG. 9 . This lever effect must be of such a magnitude that the restoring elastic force of the spring arms  18  or  18 ′ is overcome, and that by lifting the cover  10  from the frame  1  the free ends  18   a  or  18   a′  of the spring arms  18  or  18 ′ can be moved along the lower inclined surfaces  6   b  of the recess  6 , in the direction of the narrowing thereof. This compresses the spring arms  18  or  18 ′ once again until they pass through the narrowing in the recess  6  as the cover continues to move away from the frame  1 . 
         [0063]    Furthermore, a blocking device is provided in order to block the movability of the spring arms  18  or  18 ′ in the locked state, thereby preventing unintentional opening of the cover  10  from the frame  1  (for example, by use of the pin-shaped tool  32  previously described with reference to  FIG. 9 ). In  FIGS. 4 ,  5 , and  7  the illustration of the blocking device has been omitted for simplicity in order to better illustrate and describe the configuration and design of the spring arms  18  or  18 ′. 
         [0064]      FIGS. 10 ,  10   a,    11 ,  11   a,    12 , and  13  illustrate the configuration and design of such a blocking device  40  on the underside of the cover  10  according to a first embodiment. In this regard it is further noted that  FIGS. 10 ,  10   a,    11 ,  11   a,    12 , and  13  show additional details in the circumferential edge section of the cover  10  which have been omitted in  FIGS. 4 ,  5 , and  7  for simplicity. The cover illustrated in  FIGS. 10 ,  11   a,    11 ,  11   a,    12 , and  13  has a lock, as has been previously described as a second embodiment with reference to  FIGS. 7 and 8 . However, the following description of the structure of the blocking device  40  and its function with regard to the lock also similarly applies to a lock as has been previously described as a first embodiment with reference to  FIGS. 4 through 6 . 
         [0065]    As shown in particular in  FIGS. 10 ,  11 , and  12 , the blocking device  40  in the illustrated exemplary embodiment according to the first embodiment has an approximately cuboidal housing  41  situated vertically and with its longitudinal extension approximately in the radial direction. The housing  41  is attached at its inner end to a section of the ribs  14  and to an additional pedestal  42  which is situated in the vicinity of the center of the cover  10  and which, similarly as for the ribs  14 , projects from the underside of the cover  10 . In the housing  41  a locking bar  44  is pivotably supported about a rotational axis  46  oriented parallel to the cover  10  and thus, in the installed position of the cover  10 , approximately horizontally, as shown in particular in  FIGS. 10   a  and  11   a.  Similarly as for the housing  41 , the locking bar  44  supported in the housing also extends radially in the direction of the circumference  10   a  of the cover  10 . At its free end adjacent to the circumference  10   a  of the cover  10  and located at a distance from the rotational axis  46 , the locking bar  44  projects through an opening  48  provided in the housing  41 , and has a nose  44   a  which points toward the cover  10 , as shown in particular in  FIGS. 11   a  and  13 . 
         [0066]    The blocking device  40  is situated adjoining the underside of the cover  10  and at a distance from the spring arms  18 ′ in such a way that the housing  41  has the same orientation as the spring arms  18 ′, and in addition the nose  44   a  of the lock  40  is in flush alignment with the interspace  49  formed between the free ends  18   a′  of the spring arms  18 ′. 
         [0067]    The locking bar  44  is able to swivel between a rest position and a blocked position. In the rest position, as shown in  FIGS. 10 and 10   a  and readily identified in particular in  FIG. 10   a,  the nose  44   a  is located outside the spring arms  18 ′, thus preventing any contact of the locking bar  44  with the spring arms  18 ′. In this rest position of the locking bar  44  the spring arms  18 ′ are freely movable, as the result of which the cover  10  may be easily opened or closed in the previously described manner by disengagement or engagement of the free ends  18   a′  of the spring arms  18 ′ respectively out of or into the recess  6  in the frame  1  ( FIGS. 1 and 3 ). 
         [0068]    If the cover  10  is placed on the frame  1  and locked thereto by engaging the free ends  18   a′  of the spring arms  18 ′ behind the sections of the flange-like projection  6  of the frame  1  which adjoin the recess  6 , it is possible to block the lock by use of the blocking device  40  to prevent unintentional opening of the cover  10 . To this end, the locking bar  44  is swiveled in the direction of the cover  10  into a blocked position, causing the projecting nose  44   a  of the locking bar to engage with the interspace  49  between the free ends  18   a′  of the spring arms  18 ′. This blocked position is shown in  FIGS. 11 ,  11   a,  and  13 , and is readily identified in particular in  FIG. 13 . In the blocked position the nose  44   a  of the locking bar  44  then extends into the interspace  49 , and thus lies between the free ends  18   a′  of the spring arms  18 ′ which are separated from one another by the interspace  49 . In this manner a motion of the free ends  18   a′  of the spring arms  18 ′ toward one another is prevented, since such a motion would result in constriction of the interspace  49  between the free ends  18   a′  of the spring arms  18 ′, which at that time is occupied by the projecting nose  44   a  of the locking bar  44 . In its blocked position between the free ends  18   a′  of the spring arms  18 ′, the projecting nose  44   a  of the locking bar  44  thus acts as a type of wedge which prevents motion of the spring arms  18 ′ toward one another. The manner in which the nose  44   a  of the locking bar  44  in its blocked position extends into the interspace  49  between the free ends  18   a′  is readily seen in particular in  FIG. 13 , which shows a cross section of a section of the cover  10 , the nose  44   a  of the locking bar  44 , and the section of the frame  1  containing the recess  6 . In the illustration of  FIG. 13  the cover  10  thus rests on the frame  1  and is locked thereto by the fact that the free ends  18   a′  of the spring arms  18 ′ contact at their outer upper inclined surfaces the lower inclined surfaces  6   b  of the recess  6  (also see  FIG. 3  in this regard), thereby forming a catching lock. 
         [0069]    To facilitate insertion of the nose  44   a  of the locking bar  44  into the interspace  49  when the locking bar  44  is swiveled into the blocked position, in the illustrated exemplary embodiment the nose  44   a  has the shape of a wedge, as likewise shown in  FIG. 13 . In addition, in the illustrated exemplary embodiment the nose  44   a  of the locking bar  44  is designed so that when it contacts the mutually facing side surfaces of the free ends  18   a′  of the spring arms  18 ′ it arrives at the blocked position, as shown in particular in  FIG. 13 . 
         [0070]    To allow the locking bar  44  to be swiveled between the rest position and the blocked position in the manner previously described, the opening  48  in the housing  41  must have a corresponding opening width, as shown in particular in  FIG. 11   a.    
         [0071]    The previously described swivel motion of the locking bar  44  is preferably achieved by a drive, not shown in  FIGS. 10 through 13  but located in the housing  41 . It is practical for such a drive to be electrically driven, and to have a servomotor or electromagnet, for example. To allow control of such a drive and thus of the swivel motion of the locking bar  44  between the rest position and blocked position thereof from a remote location, remote control systems (likewise not illustrated in  FIGS. 1 through 13 ) should also be provided. To this end, the remote control device may be connected to the blocking device  40  by means of a connecting cable (likewise not illustrated), or alternatively, coupled to the blocking device by wireless link, whereby in the latter case the associated receiver may likewise be accommodated in the housing  41 . When multiple covers are involved, it is also possible to achieve the blocking of the lock simultaneously and/or optionally from a remote location. 
         [0072]    Compared to  FIGS. 4 and 7 ,  FIGS. 10 ,  11 , and  12  also show a pair of interspaced recesses  50  provided at the circumference  10   a  of the cover  10 , specifically, at a location diametrically opposite from the spring arms  18 ′ and the blocking device  40 , and in the exemplary embodiment extending radially and at the same time at right angles to a plane defined by the cover  10 . A pin  52  likewise projects into the two recesses  50 , and together with corresponding bearing elements (not illustrated in  FIGS. 1 and 2 ) provided on the frame mutually form a hinge for pivotable bearing of the cover  10  on the frame  1  in order to swing the illustrated cover  10  up and down for opening and closing. In addition, this hinge should be designed so that the cover  10  may optionally be removed from the frame  1 . 
         [0073]    As an alternative to the hinge described above, other measures may be provided for locking and supporting the cover  10  on the frame  1 . 
         [0074]    Although the previously described cover  10  has only a single pair of spring arms  18  or  18 ′ for locking, as an alternative it is also possible, of course, to provide a plurality of such spring arm pairs, and also to provide a corresponding number of recesses on the frame  1 . 
         [0075]      FIGS. 14 through 18  illustrate the previously described manhole top having a frame  1  and cover  10 , the cover  10  being shown in its open position.  FIGS. 14 through 18  also show the design and configuration of a blocking device  40 ′ on the frame  1 . In this regard, it is further noted that  FIGS. 14 through 18  show additional details of the frame  1  and cover  10  which have been omitted in the previously described figures for simplicity. The cover  10  illustrated in  FIGS. 14 through 18  also has a lock, as has been previously described as a second embodiment with reference to  FIGS. 7 and 8 . The following description of the blocking device  40 ′ and its function with regard to the lock also similarly applies to a lock as has been previously described as a first embodiment with reference to  FIGS. 4 through 6 . 
         [0076]    As shown in  FIGS. 14 through 18 , in a departure from the blocking device  40  according to the first embodiment illustrated in  FIGS. 10 through 13  the blocking device  40 ′ according to the second embodiment has an approximately cylindrical housing which is vertical and which in its longitudinal extension in the installed position of the illustrated manhole top is oriented approximately in the vertical direction. In contrast to the first embodiment according to  FIGS. 10 through 13 , the blocking device  40 ′ according to the second embodiment is attached to the inner side  1   a  of the frame  1 . For this purpose, in the illustrated exemplary embodiment a stationary retaining clip  42 ′ is provided at the inner side  1   a  of the frame  1 , and in the installed position of the manhole top projects vertically downward from the underside of the frame  1 , as shown in particular in  FIGS. 17 and 18  and in conjunction with  FIG. 15 . In the illustrated exemplary embodiment as shown in  FIG. 15 , the cylindrical housing for the blocking device  40 ′ is attached to this retaining clip  42 ′ by use of a bracket  43 ′. 
         [0077]    In the illustrated exemplary embodiment, the cylindrical housing for the blocking device  40 ′ forms the housing of an electric motor  46 ′, which is a linear motor. The electric motor  46 ′ has a rod  46   a′  which is extended or retracted as a function of the energization of the electric motor  46 ′, and which thus performs a reciprocal linear motion in the longitudinal direction and which at its exterior free end projects from an opening, not shown in greater detail in  FIGS. 14 through 18 , provided in the cylindrical housing. Similarly as for the first embodiment according to  FIGS. 10 through 13 , the blocking device  40 ′ according to the second embodiment likewise has a locking bar  44 ′ which rests on the exterior free end of the rod  46   a′  for the electric motor  46 ′. In contrast to the first embodiment according to  FIGS. 10 through 13 , the locking bar  44 ′ is not able to swivel, but in the illustrated second embodiment according to  FIGS. 14 through 18  is instead supported in a linearly movable manner as a result of the previously described configuration and action of the electric motor  46 ′ designed as a linear motor, and specifically is located between a lower rest position, shown in  FIG. 17 , in which it is fully disengaged from the free ends  18   a′  of the spring arms  18 ′ and which is the only position in which the cover  10  can be opened and closed, and an upper blocked position, shown in  FIGS. 15 and 18 , in which the locking bar engages with the interspace  49  between the free ends  18   a′  of the spring arms  18 ′ in a similar manner as illustrated in  FIG. 13 . 
         [0078]    Instead of the described electric motor  46 ′ designed as a linear motor, the drive provided for such a reciprocal linear motion of the locking bar  44 ′ may also have any other conceivable design that allows a linear motion of the locking bar  44 ′. To this end, for example, an electromagnet or a rotary motor having a transmission which converts the rotary motion to a linear motion could also be considered. 
         [0079]    As an alternative, however, a pivotable bearing of the locking bar  44 ′, similarly as for the first embodiment according to  FIGS. 10 through 13 , is also possible in principle. For this purpose, the locking bar  44 ′ must be swiveled far enough into a (lower) rest position until it is fully disengaged from the spring arms. For a lock according to the second embodiment shown in  FIGS. 7 and 8  and  10  through  13 , a short swivel path would be sufficient, since the locking bar  44 ′ is already disengaged from the free ends  18 ′ of the springs  18 ′ when it is located in the expanded interspace between the outwardly curved sections  18   d′  (see  FIG. 15  in conjunction with  FIG. 7 ). In contrast, for a lock having linear spring arms  18 , as has previously been described as a first embodiment with reference to  FIGS. 4 through 6 , the swivel path must be considerably longer, since the locking bar  44  in the absence of an expanded interspace between the linear spring arms  18  must be swiveled completely away from same. 
         [0080]    For further particulars regarding the design and function of the blocking device  40 ′ according to the second embodiment, reference is made to the previous description of the blocking device  40  according to the first embodiment with reference to  FIGS. 10 through 13 . 
         [0081]      FIG. 18  also shows in a block diagram illustration a remote control system whose function has previously been described in conjunction with the first embodiment of the blocking device  40 . The remote control system schematically illustrated in  FIG. 18  has a remote control device  60  which is coupled via a connection  62  to the blocking device  40 ′, whereby this connection  62  may be designed in the form of a cable, as illustrated by way of example in  FIG. 15 , or also as a wireless link. For connecting the remote control device  60  via a permanently installed connecting cable the second embodiment of the blocking device  40 ′ is particularly suitable, since it is attached to the frame  1  and is thus positioned in a stationary manner, i.e., is not movable or contains no movable parts, thus allowing the connecting cable to be easily installed in a stationary manner. For the case that the connection  62  is designed as a wireless link, the receiver should preferably be accommodated in the blocking device  40 ′. 
         [0082]    It is practical for the [remote] control device  60  to influence the electric motor  46 ′ for the blocking device  40 ′, and thus to optionally control the extension of the locking bar  44 ′ into its blocked position, or the retraction of the locking bar  44 ′ into its rest position. 
         [0083]    In addition, according to  FIG. 18  a monitoring device  64  is provided, which in the illustrated exemplary embodiment is contained in the remote control device  60 . This monitoring device  64  is used for checking the position of the locking bar  44 ′. It is thus possible to determine, in particular from a remote location, whether the locking bar  44 ′ is in its blocked position, and thus, whether the cover  10  in its closed position is blocked from opening. To this end, a sensor (not illustrated in the figures) is provided in the blocking device  40 ′ which detects the position of the locking bar  44 ′. The output signals from this sensor are then likewise transmitted via the connection  62  to the remote control device  60 , and from there to the monitoring device  64 . Furthermore, among other possibilities the position of the cover  10  may also be checked, in particular to obtain additional assurance of the results of the monitoring when blocking by the locking bar  44 ′ is determined. For this purpose an additional sensor (likewise not illustrated in the figures) is required, which in particular should be provided in the region of the hinge for the cover  10 . The monitoring device  64 ′ should preferably have a monitor or screen on which all relevant data may be displayed in a particularly advantageous manner. 
         [0084]    In particular for the use of a plurality of covers, by use of the previously described remote control device  60  and monitoring device  64  it is possible to perform blocking of the lock and monitoring of the locked state of all covers simultaneously and from a remote location such as a control panel, for example. 
         [0085]    Lastly, it is noted that the remote control device  60  and monitoring device  64  previously described with reference to  FIG. 18  may be connected similarly as for the first embodiment of the blocking device  40  previously described with reference to  FIGS. 10 through 13 .