Patent Publication Number: US-7896574-B2

Title: Self-locking manhole cover

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/043,418, filed on Apr. 9, 2008 and entitled “Self-Locking Manhole Cover.” The entire contents of said Application Ser. No. 61/043,418 is incorporated herein by this reference. This application also relates to copending application Ser. No. 11/736,623, filed on Apr. 18, 2007 and entitled “Manhole Access Opening Security Device.” This application also relates to copending Application Ser. No. 11/736,634, filed on Apr. 18, 2007 and entitled “Security Key Tool For Manhole Access Opening Security Device”. The entire contents of said Application Ser. No. 11/736,623 and said Application Ser. No. 11/736,634 are hereby incorporated herein by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to lock systems for securing access to manhole openings. 
     2. Description of the Prior Art 
     By way of background, standard manhole covers are designed to be easily removed from manhole openings to allow access to underground facilities such as sewers, electrical and communication equipment vaults, and other infrastructure. This presents a security risk by allowing vandals, terrorists and others to gain unauthorized access to important assets, or to move about undetected via underground passageways. Standard manhole covers are also attractive targets for thieves who sell the covers for their scrap metal value. 
     Various manhole opening locking schemes have been proposed to address such security concerns. One technique is to simply bolt the manhole cover to the underlying manhole frame structure. Although very effective, this method either involves retrofitting existing manhole covers and frames by drilling and tapping bolt holes, or requires that existing covers and frames be replaced with units having preformed bolt holes. Both alternatives are labor intensive and may be prohibitively expensive if the number of manhole locations is large. 
     Another manhole security technique involves the use of a lockable pan unit situated below a standard manhole cover. The pan unit is used to block the manhole opening, which means that the manhole cover itself does not require locking and does not have to retrofitted or replaced. The pan unit is secured to the manhole frame by resting it on the same support surface that supports the manhole cover (typically a ring flange), and then locking the unit to the manhole frame. A disadvantage of such systems is the requirement for a separate pan that must be separately removed after the manhole cover is removed. Moreover, this solution does not prevent manhole cover theft. 
     Another manhole security technique involves providing a lock system on the manhole cover itself. A typical lock system includes a pair of retractable lock rods or bars that extend horizontally to engage the side-wall of the manhole frame or the underside of the ring flange or other support surface that supports the manhole cover. A rotatable key is used to rotate a locking apparatus or actuator that actuates the rods or bars into and out of locking engagement. By way of example, U.S. Pat. No. 4,964,755 discloses a manhole cover wherein a lock apparatus is turned by a key to operate a pair of lock rods. However, the lock rods are not self-locking and the key must be used to return the rods to their locked position once the manhole cover is in place. Moreover, the position of the lock rods in the locked position is fixed. Due to dimensional tolerances and differences between manhole frame designs, the lock rods may not firmly engage some manhole frames or may be overly tight in other manhole frames. U.S. Pat. No. 5,082,392 overcomes this problem by spring-biasing a pair of locking bars to their locked position. The locking bars affirmatively engage the manhole frame under the force of the biasing springs. A specially configured portion of a key mates with a vent hole in the manhole cover when the locking bars are in their unlocked position. This allows the locking bars to be held in the unlocked position during opening and closing of the manhole opening. However, the key must remain engaged with the manhole cover at all times when the cover is not covering the manhole, which may be inconvenient. 
     It is to improvements in manhole opening security systems that the present invention is directed. In particular, what is needed is a security device that improves upon previous designs by reducing the effort required to lock and unlock the device, that provides robust locking capability, and which utilizes an uncomplicated design that is easy to manufacture. 
     SUMMARY OF THE INVENTION 
     An advance in the art is obtained by a manhole cover having an integrated locking system that is self-locking and easy to open. The device includes a cover plate adapted to rest on a manhole cover support surface of a manhole frame so as to be substantially flush with a top portion of the manhole frame and a surrounding surface in which the manhole frame is situated. An anchor on the cover plate is adapted to engage the manhole frame at a first location in a manner that resists lifting of the cover plate proximate to the first location. A locking member on the cover plate is movable between a locked position and an unlocked position. In the locked position, the locking member is adapted to engage the manhole frame at a second location in a manner that resists lifting of the cover plate proximate to the second location. In the unlocked position, the locking member is disengaged from the manhole frame. 
     An example disclosed embodiment illustrates additional optional features. For example, the support surface may comprise an upper surface of an inwardly extending flange on the manhole frame and the anchor may be adapted to engage a lower surface of the flange. The anchor may comprise a rigid bracket. More than one anchor may be provided depending on design requirements. Similarly, more than one locking member may be used. The locking member may comprise a slideable locking pin or the like. A biasing mechanism may be provided on the cover plate to bias the locking member to the locked position. A rotatable locking mechanism may also be provided on the cover plate to engage the locking member. The locking mechanism may have a locking rotational position wherein the locking member is in the locked position and an unlocking rotational position wherein the locking member is in an unlocked position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages of the invention will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying Drawings, in which: 
         FIG. 1  is a partial perspective view showing a manhole cover having an integrated locking system securing the manhole cover to a manhole frame; 
         FIG. 2  is a partial perspective view of the underside of the manhole cover of  FIG. 1  showing a fixed anchor and locking components; 
         FIG. 3  is an enlarged partial perspective view showing the locking components of  FIG. 2  in a locked position; 
         FIG. 4  is an enlarged partial perspective view showing the locking components of  FIG. 2  in an unlocked position; 
         FIG. 5  is an enlarged partially cut-away perspective view showing another view of the locking components of  FIG. 2 ; 
         FIG. 6  is an enlarged partially cut-away perspective view showing another view of the locking components of  FIG. 2 ; 
         FIG. 7  is an enlarged partially cut-away perspective view showing an alternative embodiment of the locking components of  FIG. 2 ; 
         FIG. 8  is an enlarged partially cut-away perspective view showing another view of the locking components of  FIG. 2 ; 
         FIG. 9  is an enlarged partially cut-away perspective view showing an anti-retraction device that may be provided as part of the locking components of  FIG. 2 ; 
         FIG. 10  is an enlarged partially cut-away perspective view showing another view of the anti-retraction device of  FIG. 2 ; 
         FIG. 11  is an enlarged partially cut-away perspective view showing another view of the anti-retraction device of  FIG. 2 ; 
         FIG. 12  is an enlarged partially cut-away perspective view showing another view of the anti-retraction device of  FIG. 2 ; 
         FIG. 13  is a perspective view showing a modification of the manhole cover of  FIG. 1  that includes a slideable anchor member in a locked position; and 
         FIG. 14  is a perspective view showing the modified manhole cover of  FIG. 13  with the slideable anchor member in an unlocked position. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Turning now to  FIG. 1 , a security manhole  2  includes a manhole frame  4  and a manhole cover plate  6  constructed in accordance with the present disclosure. The cover plate is generally flat and can be made out of any suitable material that is of sufficient strength for the intended application. Examples include, but are not limited to, a durable metal such as steel or a polymer-based composite material. Unless otherwise indicated, the remaining components of the manhole cover (to be described in more detail below) may be formed from stainless steel or any other high strength metal that is resistant to corrosion and other types of environmental degradation. Other materials may also be used, depending on design preferences. 
     The cover plate  6  is adapted to rest on a manhole cover support surface  8  (typically a ring flange of the manhole frame  4 ). In this position, the cover plate  6  is preferably substantially flush with a top portion  10  of the manhole frame and a surrounding surface (not shown) in which the manhole frame is situated (e.g., a roadway, walkway, parking lot, etc.). As shown in  FIG. 2 , a fixed anchor  12  on the cover plate  6  is adapted to engage the manhole frame  4  at a first location  14  in a manner that resists lifting of the cover plate proximate to the first location. The anchor  12  can be constructed in many ways, including as a rigid bracket that mounts to the underside  16  of the cover plate  6 . It will be seen in the example installation of  FIG. 2  that the anchor  6  engages a lip  18  on the underside of a flange  20  whose upper surface provides the manhole cover support surface  8 . To accommodate this manhole frame configuration, the anchor  12  may be generally U-shaped. In particular, the anchor may have a first leg  22  attached to the cover plate  6 , a second leg  24  extending away from the first leg, and a third leg  26  that is spaced from and generally parallel to the first leg. The first leg  22  is long enough to facilitate attachment to the cover plate  6  using bolts, rivets or other suitable fasteners. The second leg  24  is preferably long enough to span the thickness of the flange  20 . The third leg  26  is parallel to the lip  18  because the lip orientation is parallel to the surface  8 . In some manhole frames, the lip  18  may angle downwardly toward its point of attachment to the remainder of the manhole frame  4  (such that the lip is not sharply defined). In that case, the third leg  26  could be optionally formed so match the lip&#39;s downward angle. Alternatively, the parallel third leg configuration of  FIG. 2  could be used, with the third leg  26  engaging the lip  18  by point contact on the latter&#39;s sloping surface. An optional strut  28  may be provided between the first leg  22  and the third leg  26  to improve the latter&#39;s resistance to bending in the event that an attempt is made to remove the cover plate by unauthorized means. The anchor  12  can be made of a durable metal such as steel. The anchor  12  may also be integrally formed as part of the cover plate  6 . In an alternative construction, the anchor  12  could be a non-fixed movable structure, such as a slideable anchor pin (see  FIGS. 13-14  below). If desired, there can be more than one anchor  12  provided at different locations on the cover plate. 
     Returning now to  FIG. 1 , a lock aperture  30  is formed at an off-center location on the cover plate  6 . A central location could potentially also be used. Seated in the lock aperture  30  is a lock housing  32  that retains a security lock  34 . Using a security key tool (not shown) to engage and rotate the security lock  34 , the manhole cover  6  can be unlocked when desired and removed from the manhole frame  4  to allow access to the manhole access opening within. An access hole  36  may also be disposed on the cover plate  6  adjacent to the lock aperture  30 . The access hole  36  is provided for releasing a latch (not shown in  FIG. 1 ) that maintains the cover plate  6  in an unlocked position when the cover plate is removed from the manhole frame  4  (as described in more detail below). 
     As can be seen in  FIGS. 2-4 , a locking member  38  on the cover plate  6  is movable between a locked position ( FIGS. 2-3 ) and an unlocked position ( FIG. 4 ). In the locked position, the locking member  38  is adapted to engage the manhole frame  4  at a second location  40  in a manner that resists lifting of the cover plate  6  proximate to the second location. In the unlocked position, the locking member  38  is disengaged from the manhole frame  38 . Although not more than one locking member  38  should be required, plural locking members could be provided if desired. The locking member  38  may comprise a locking pin or the like that is slideably mounted to a locking member mount frame  42  (e.g., a sheet metal frame) mounted on the lower side  16  of the cover plate. If formed as a locking pin, the locking member  38  may have a generally tubular lock shaft  44  with a hardened point  46  at one end for engaging the inside wall of the manhole frame  4  below the lip  18 . Other types of locking member  38  may also be used in lieu of a locking pin. A biasing mechanism  48  is provided on the cover plate  6  to bias the locking member  38  to the locked position. A rotatable main locking mechanism  50  is also provided on the cover plate  6  to actuate the locking member  38  against the force of the biasing mechanism  48 . The locking mechanism  50  has a locking rotational position ( FIGS. 2-3 ) wherein the locking member  38  is in the locked position and an unlocking rotational position ( FIG. 4 ) wherein the locking member is in the unlocked position. 
     Turning now to  FIG. 5 , the locking mechanism  50  includes a rotatable lock bolt  52  whose exposed face is configured to provide the security lock  34  ( FIG. 1 ). The rotatable lock bolt  52  is received in a fitting  54  that provides the lock housing  32  ( FIG. 1 ). The lock housing fitting  54  is formed with an upper head  56  and a downwardly-extending lower stem  58 . The stem  58  of the lock housing fitting  54  is received in a narrowed portion  60  the cover plate&#39;s lock aperture  30 . The lock housing fitting  54  is also formed with a stepped bore  62  that extends through the head  56  and the stem  58  in order to receive the rotatable lock bolt. A bushing  64  made from brass or the like may be inserted in the bore to facilitate rotation of the rotatable lock bolt  52 . The rotatable lock bolt  52  includes an upper head  66  and a downwardly-extending lower stem  68  (note that  FIG. 5  shows an upside down view). The top face of the head  66  provides the security lock  34 . As shown in  FIG. 1 , the head  66  may be formed with an undulating curvilinear groove or other security lock pattern  68 . The security pattern  68  is configured to receive a mating curvilinear ridge or other security key pattern formed on a security key (not shown). 
     The stem  68  of the rotatable lock bolt  52  mounts a drive plate  70  and a latch plate  72 , both of which have a hub that is formed with a mounting aperture  70   a  and  72   a , respectively. The drive plate&#39;s mounting aperture  70   a  is a key-way that fits onto a key-shaped rotational drive boss  74  (e.g., of non-circular shape) formed on the stem  68  of the rotatable lock bolt  52 . The latch plate&#39;s mounting aperture  72   a  is round. It receives a short bushing  76  integrally formed on a lock nut  78  that threads onto the end of the rotatable lock bolt stem  68 . The lock nut bushing  76  extends through the latch plate&#39;s mounting aperture  72   a  and the terminal end of the bushing is received in a counterbore  80  formed at the lower end of the drive plate&#39;s key-way aperture  70   a . During assembly, the lock nut  78  is threaded onto the rotatable lock bolt stem  68  until the lock nut bushing  76  bottoms out in the drive-plate&#39;s counterbore  80 . The lock nut bushing  76  secures the drive plate  70  in position on a shoulder  82  formed on the rotatable lock bolt stem  68 . The head of the lock nut  78  also retains the latch plate  72  on the rotatable lock bolt stem. However, for reasons described in more detail below, the lock nut bushing  76  is long enough so that there will always be a small gap between the latch plate  72  and the lock nut&#39;s head. This gap allows the latch plate  72  to flex up and down on the lock nut bushing  76 . A washer  83  may be disposed between the drive plate  70  and the bottom of the bushing  64 . 
     With additional reference back to  FIGS. 3 and 4 , the latch plate  72  includes a latch arm  84  that is adapted to engage a latch  86  provided on the bottom  16  of the cover plate  6 . The latch arm  84  extends from the hub of the latch plate  72  and has a small tab  88  at its free end that carries a latch nut  90 . The latch nut  90  is adapted to be received in a latch aperture  92  formed in a latch fitting  94  that provides the latch  86 . The latch aperture  92  is aligned with the access hole  36  formed in the cover plate  6 . A stem portion  96  of the latch fitting defines part of the latch aperture  92 . The stem  96  is seated in the access hole  36  from the underside  16  of the cover plate  6 . As will be discussed in more detail below in connection with  FIG. 6 , the latch fitting  94  is also formed with a ramp  98  that the latch nut  90  engages as it approaches the latch aperture  92 . The ramp  98  guides the latch nut  90  into the latch aperture  92 , flexing the latch arm  72  as it does so. 
     With continued reference to  FIGS. 3-5 , the drive plate  70  has a drive arm  100  that extends from the hub of the drive plate  70  to a location that is adjacent to the lock member  38 . With its drive arm  100 , the drive plate  70  forms a locking member actuator  70 / 100  that pivots to actuate the locking member  38  when the rotatable lock bolt  52  is rotated. For strength and stiffness, the latch plate  72  is formed with its own drive arm  102  that extends partially along the drive plate drive arm  100 . The latch plate drive arm  102  is secured to the drive plate drive arm  100  using appropriate fasteners. If desired, the fasteners may also secure a spacer plate  104  (best shown in  FIGS. 3-4 ) to the opposite side of the latch plate drive arm  102  that does not engage the drive plate drive arm  100 . The free end of the drive plate drive arm  100  has a thickened portion  106  that may be integrally formed on the drive plate drive arm or mounted thereto as a separate piece. In the region of the thickened portion  106 , the drive plate drive arm  100  is formed with an open slot  108  ( FIGS. 4-5 ) that is adapted to engage a drive pin  110  ( FIG. 5 ) that connects transversely to the locking member  38 . The drive pin  110  extends through the drive member&#39;s lock shaft  44  and also through a locking spring collar  112  that surrounds the lock shaft  44 . The free end  114  of the drive pin  110  that extends beyond the locking spring collar  112  is engaged by the open slot  108  of the drive plate drive arm  100  as it pivots. Pivoting of the drive plate drive arm  100  (representing a component of the locking member actuator  70 / 100 ) is thus associated with sliding movement of the locking member  38  through the locking member mount frame  42 . 
     In an alternative embodiment shown in  FIG. 7 , the latch plate drive arm  102  also participates in driving the locking member  38 . In particular, the latch plate drive arm  102  includes an extension  102   a  that veers away from a modified drive plate drive arm  100   a  to so that the free ends of the two drive arms are spaced from each other, creating a fork assembly  103 . The fork assembly  103  is coupled to a drive pin  116  that connects transversely to the locking member  38 . The transverse drive pin connection is provided by a shaft collar  118  that is secured to the locking member&#39;s lock shaft  44  (using a set screw or the like). The drive pin  116  extends through slotted holes  120  (only one is shown) in the two drive arms  100   a  and  102   a . Although not shown, the drive pin  116  may be formed with an annular groove that receives a retainer clip to secure the drive pin to the fork assembly  103 . The drive plate drive arm  100   a  is positioned to engage one end of the drive pin  116  and the latch plate drive arm  102   a  is positioned to engage a medial portion of the drive pin. Collectively, the twin drive plate and latch plate drive arms  100   a  and  102   a  provide a locking member actuator  100   a / 102   a  that pivots to actuate the locking member  38  when the rotatable lock bolt  52  is rotated. Due to the fork assembly design of this actuator, normal operational forces imparted by the drive pin  116  will not twist or otherwise deform the two drive arms  100   a  and  102   a . Pivoting of the twin drive arms  100   a  and  102   a  is associated with sliding movement of the locking member  38  through the locking member mount frame  42 . It will also be seen  FIG. 7  that a flexible metal tab  122  may be mounted to the locking member mount frame  42  to resiliently engage the drive plate drive arm  100  as it retracts the locking member  38 . The metal tab  122  functions as a torque adder that increases the unlocking force required to retract the locking member  38  by resisting the drive plate drive arm  100   a  as it rotates. 
     As previously mentioned, a biasing mechanism  48  (shown in both embodiments) serves to bias the locking member  38  to its extended locked position. The biasing mechanism  48  can be implemented using a coil spring  124  that is disposed on the locking member  38 , which extends axially therethrough. One end of the coil spring  124  bears against the locking spring collar  112  mounted on the locking member&#39;s lock shaft  44  (or the shaft collar  118  of the alternative drive arm embodiment of  FIG. 7 ). The other end of each coil spring  124  bears against an end wall  126  of the lock member mount frame  42 . The coil spring  124  is in a minimally compressed condition when the locking member  38  is extended to its locked position ( FIGS. 2-3 ). The coil spring  124  becomes more substantially compressed when the locking member  38  is retracted to its unlocked position ( FIGS. 4-7 ). The locking member mount frame  42  may further include a protruding base plate  128  that can be affixed via anchor bolts  130  (or by other means) to the bottom  16  of the cover plate  6 . 
     When the manhole cover  6  is secured to the manhole frame  4 , the locking member  38  is driven by the biasing mechanism  48  to its locked position. This pivots the locking member actuator  70 / 100  (or  100   a / 102   a  in  FIG. 7 ) due to the coupling provided by the drive pin  110  (or  116  in  FIG. 7 ). The locking mechanism  50  is thereby rotated to its locking position. The latch plate&#39;s latch arm  84  will also be operatively driven to an unlatched position. In this position, the latch nut  90  is disengaged from the latch aperture. As can be seen in  FIG. 6 , an arc-shaped latch recess channel  132  may be formed in the base plate  128  of the locking member mount frame  42  to accommodate the sweep of the latch nut  90  as the latch arm  84  rotates. 
     When it is desired to disengage the manhole cover  6  from the manhole frame  4 , the locking mechanism  50  is rotated. Rotation of the locking mechanism  50  from its locking position is effected by turning a security key (not shown) while it engages the security lock  34  on the head of the rotatable lock bolt  52 . The unlocking direction is preferably counterclockwise when looking down on the cover plate. The security key rotates the rotatable lock bolt  52 , which in turn pivots the locking member actuator  70 / 100  (or  100   a / 102   a  in  FIG. 7 ). The drive plate drive arm  100  of the locking member actuator  70 / 100  (or the fork assembly  103  of the locking member actuator  100   a / 102   a  in  FIG. 7 ) actuates the locking member via the drive pin  110  (or  116  in  FIG. 7 ), causing the locking member  38  to retract against the spring force of the biasing mechanism  48 . As the coil spring  124  of the biasing mechanism  48  is deformed, the person operating the security key tool will feel an increasing unlocking force. 
     Counterclockwise rotation of the locking mechanism  50  also results in the latch arm  84  being pivoted toward the latch  86 . As can be seen in  FIG. 8 , the latch arm  84  and the latch tab  88  are formed such that the latch nut  90  is positioned in a horizontal plane that intersects the surface of the latch ramp  98 . As the latch arm  84  pivots, the latch nut  90  moves horizontally toward the ramp  98 . When the latch nut  90  engages the ramp  98 , the latch arm  84  will bend elastically, causing its fee end portion to displace downwardly as the latch nut  90  rides up the surface of the ramp. This flexes the latch arm  84  and exerts a bending force on the latch plate  72 . As described above, there is a slight gap between the hub of the latch plate  72  and the head of the lock nut  78  ( FIG. 5 ). This gap accommodates the latch plate flex displacement and prevents stress risers that could be formed if the lock nut  78  was snugged onto the latch plate  78 . Instead, the bending forces on the latch plate  78  are reacted at the spacer plate  104 , which is adequately sized to reduce the imparted bending stresses (e.g., it spans the width of the latch plate). As the latch arm  84  continues to rotate, the latch nut  90  eventually clears the ramp  98  and snaps into locking engagement with the latch aperture  92  due to the latch arm returning to its undeformed position. In this configuration, the latch  86  retains the latch arm  84  against counter-rotation, which in turn maintains the locking mechanism  50  in the locking position. This means that the security key can be disengaged from the security lock and the manhole cover  6  can be removed from the manhole frame  4  and placed on the ground or other nearby surface. 
     The latch  86  is designed with a quick release feature that allows the latch arm  84  to be released once the cover plate  6  is ready to be re-secured to the manhole frame  4 . In particular, the access hole  36  ( FIG. 1 ) in the cover plate  6  accommodates a small diameter tool that can be used to contact the latch nut  90  and downwardly deflect the latch arm  84 , thereby popping the latch nut  90  out of engagement with the latch aperture  92 . Due to the relatively large spring biasing force imparted by the biasing mechanism  48  when the locking mechanism  50  is in the unlocking position, the locking mechanism  50  will snap back to its default locking position as soon as the latch nut  90  clears the latch aperture  92 . The locking member  38  will also forcefully spring to its locked position. In order to protect the locking member  38  shaft collar  112  from slamming into the outboard end  134  of the locking member mount frame, a resilient bumper  136  may be mounted on the lock shaft  44  (as can be seen in  FIGS. 3-8 ). 
     Advantageously, a security key tool as disclosed in copending application Ser. Nos. 11/736,623 and 11/736,634,  116  may be may be used to both unlock and lock the cover plate  6 . Rotation of the security lock  34  for approximately one-quarter of a turn (90°) should be sufficient to unlock the cover plate  6  and engage the latch nut  90  in the latch aperture. At this point, the security key can be disengaged from the security lock  34 . The tool portion of the disclosed security key tool may be used to lift the cover plate  6  away from manhole frame  4  by virtue of providing the cover plate access opening  36  with threads that can be engaged by the tool. Advantageously, this threaded engagement of the tool cannot result in the latch arm  84  being inadvertently released from the latch. This is because the threaded portion of the tool is not long enough to reach the latch nut  90 . 
     When it is desired to replace the cover plate  6  on the manhole frame, the tool may be used to slide the cover plate into engagement with the manhole frame  4 . During this procedure, when the cover plate  6  nearly covers the manhole opening, the anchor  12  will be maneuvered into engagement with the lip  18 . Referring back to  FIG. 2 , this can be accomplished by pulling up on the tool to slightly pivot the cover plate. As can be seen in  FIGS. 1 and 2 , the access opening  36  that threadably engages the tool is located approximately 180 degrees from the anchor  12 . Thus, pulling up on the tool will dip the anchor  12  until the third leg  26  thereof drops below the lip. The cover plate  6  can then be advanced over the remainder of the manhole opening as the third leg  26  slides under the lip  18 . The cover plate  6  may then be dropped into fully-seated engagement with the manhole cover support surface  8 . The locking mechanism  50  may then be released to secure the lock member  38  to the manhole frame  4 . 
     If desired, an optional anti-retraction device may be provided to reduce the likelihood of inadvertent retraction of the locking member  38  due to an attempt to pry the cover plate  6  out of the manhole frame  4  with a pry bar or other unauthorized tool. Turning now to  FIGS. 9-10 , one way that this feature can be provided is with an anti-retraction security bolt  138  that is slideably mounted in a blind bore  140  formed in the locking spring collar  112  that surrounds the locking member  38 . A side wall  141  of the locking member mount frame  42  may then include one or more holes  142  and/or slots  144  that are sized to receive the anti-retraction bolt  138  when the locking member is in various states of extension, including the fully extended position. It will be appreciated that the anti-retraction bolt  138 , when so engaged, prevents retraction of the locking member  38 . The use of plural holes  142  and/or slots  144  in the locking member mount frame side wall  141  provides redundant security. Thus, if the locking member  38  is able to be retracted past one hole  142  or slot  144 , the anti-retraction bolt  138  has an opportunity to engage a subsequent hole or slot and thereby arrest the retraction of the locking member. It may be desirable to have the anti-retraction bolt  138  engage a slot  144  instead of a hole  142  when the locking member  38  is in the fully extended position. This will allow the locking member  38  to retract slightly before the anti-retraction bolt  138  engages the locking member mount frame side wall  141 , thereby facilitating self-adjustability of the locking member  38  to accommodate manhole frames of different configuration. 
     As shown in  FIGS. 11-12 , the anti-retraction bolt  138  can be biased toward engagement with the locking member mount frame  42  by a small coil spring  146  located at the base of the blind bore  140  in which the anti-retraction bolt  138  is seated. The anti-retraction bolt  138  can be withdrawn from engagement with the locking member mount frame side wall  141  when the locking member actuator  70 / 100  (or  100   a / 102   a  in  FIG. 7 ) is pivoted. In particular, as can be seen in each of  FIGS. 9-12 , the anti-retraction bolt  138  may include a peg  148  that is engaged by a cam portion  150  of the drive plate drive arm  100  as the latter pivots. The cam portion  150  forms one side of the slot  108  that engages the drive pin free end  114 . As can also be seen in  FIGS. 9 and 10 , the peg  148  extends transversely from the anti-retraction bolt  138  through a slot  152  in the locking spring collar  112 . The slot  152  is open to the blind bore  140  and thus forms a transverse passage therefrom. The slot  152  is angled when its open end is viewed in plan view orientation (looking down on the locking spring collar  112  from the underside  16  of the cover plate  6 ) to provide a camming surface  154  for the peg  148 . As the cam portion  150  of the drive plate drive arm  100  engages the peg  148 , the latter swivels into contact with the camming surface  154  (while rotating the security pin  138 ). Continued swiveling of the peg  148  by the cam portion  150  of the drive plate drive arm  100  causes the camming surface  154  to force the peg  148  (and the anti-retraction bolt  138 ) to translate toward the base of the blind bore  140 . The anti-retraction bolt  138  is thereby driven to overcome the biasing force of the small coil spring  146 , with the result that the anti-retraction bolt  138  withdraws from engagement with the locking member mount frame side wall  141  ( FIG. 10 ). The anti-retraction bolt  138  will remain disengaged from the locking member mount frame side wall  141  for as long as the drive plate drive arm  100  maintains the bolt&#39;s peg  148  in a sufficiently pivoted position. 
     Turning now to  FIGS. 13-14 , a modification of the manhole cover plate  6  is shown in which the fixed anchor  12  ( FIG. 2 ) is replaced with a slideable anchor pin  12 A. The anchor pin  12 A is part of a slave locking mechanism  160  that is operatively connected to the master locking mechanism  50 . For example, the anchor pin  12 A may be connected by a cable  162  to the latch arm  84 . The cable  162  may be routed through a cable guide  163  that can be provided in the mount frame  48  used for the main locking pin  38 . As the latch arm  84  is rotated from its unlatched position ( FIG. 13 ) to its latched position ( FIG. 14 ), the cable  162  retracts the anchor pin  12 A from a locked position ( FIG. 13 ) to an unlocked position ( FIG. 14 ). The anchor pin  12 A can be slideably mounted within its own mount frame  164 . An anchor pin biasing mechanism  168  may be used to bias the anchor pin  12 A to its locked position. When the latch arm  84  is released from its latched position ( FIG. 14 ) to its unlatched position ( FIG. 13 ), the anchor pin  12 A will be driven from its unlocked position ( FIG. 14 ) back to its locked position ( FIG. 13 ). If desired, the mount frame  164  and the biasing mechanism  168  for the anchor pin  12 A may be constructed similarly to the mount frame  42  and the biasing mechanism  48  used for the locking pin  38 . Note that in  FIGS. 13-14 , the configuration of the mount frame  42  is somewhat different than its configuration in other figures, thus demonstrating the wide variety of design alternatives that may be implemented in accordance with the disclosure herein without departing from the spirit and scope of the invention as set forth in the claims. Modified configurations for the latch fitting  94  and the latch ramp  98  are also shown in  FIGS. 13-14 . 
     Accordingly, a self-locking manhole cover for securing a manhole access opening has been disclosed. While exemplary embodiments have been shown and described, it should be apparent that many variations and alternative embodiments could be implemented in accordance with the teachings herein. For example, the disclosed embodiments feature a latching configuration wherein the locking mechanism  50  is axially fixed relative to the cover plate  6  and the latch arm  84  is deflected out of engagement with the latch  86 . In an alternative embodiment, the latch arm  84  could be disengaged from the latch  86  without having to deflect if the entire locking mechanism  50  was downwardly positionable relative to the cover plate  6 . In that case, the locking mechanism  50  could be urged downwardly (e.g., against a biasing force) in order to disengage the latch arm  84  from the latch  86 . It is understood, therefore, that the invention is not to be in any way limited except in accordance with the spirit of the appended claims and their equivalents.