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RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 62/322,344, filed Apr. 14, 2016, and U.S. Provisional Patent Application No. 62/381,387, filed Aug. 30, 2016, which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to electromagnetic door locks for securing a door to a door frame in a closed position; particularly to an electromagnetic door lock having a strike plate and an associated electromagnet wherein the strike plate is held in contact with the electromagnet when the electromagnet is energized; and more particularly, wherein said strike plate is resiliently mounted to the door so that a controlled amount of door movement in the opening direction is permitted while the strike plate remains in contact with the energized electromagnet. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electromagnetic door locks are widely used in diverse electronic door applications. These locks typically use electromagnets attached to the door frame in conjunction with a ferromagnetic strike plate attached to the door, to hold the door firmly closed. 
         [0004]    In many current designs, means are provided in the electromagnetic door lock to permit a controlled amount of door movement in the opening direction while the armature or strike plate of the lock remains in contact with an energized electromagnet, to improve the ability of a door equipped with a magnetic lock to withstand a physical blow. A coil spring disposed with the door allows for some relative movement between the door and strike plate. This design feature of the electromagnetic strike is referred to herein as an “Energy Absorbing” design feature. The means provides linear elasticity to the door by absorbing some of the kinetic energy of the blow upon compression of the spring, thus lowering the peak force experienced to separate the strike plate from the electromagnet during a physical attack against the door and allowing for a lower powered electromagnet to be used. 
         [0005]    In current electromagnetic door lock designs, there may also exist a means that momentarily delays de-energizing of the electromagnet after a force to open the door is applied. This design feature is often associated with exit doors in commercial buildings or restaurants that permit emergency egress through doors normally locked. In a delayed magnetic lock (:“De-Mag” design feature), if an opening force is applied to a locked door continuously through a first predetermined period of time (the “delay period”), the electromagnet will be de-energized, allowing the door to be opened. If the opening force applied to the door is terminated within a second predetermined period of time (the “nuisance delay period”) wherein the second predetermined period of time is less than the first predetermined period of time, the electromagnet will remain energized and the door will remain locked. Typically, an audible signal will be sounded during the first predetermined period of time providing an alarm that an attempt is being made to exit through the locked door. 
         [0006]    In current electromagnetic door lock designs, there may also exist a power savings design feature (Eco-Mag design feature). By the Eco-Mag design feature, the electromagnet has a resting state wherein only enough power is supplied to the electromagnet to keep the door in a locked state when subjected to only environmental stimuli such as a gust of wind. Then, should a more forceful attempt be made to open the door (i.e., an unauthorized attempt to enter), power to the electromagnet is increased to keep the door locked against the unauthorized attempt to open the door. The Eco-Mag design feature also requires a controlled amount of door movement in the door-opening direction, while the strike plate remains in contact with the energized electromagnet, in order for a door position sensor to sense when an unauthorized attempt to enter is being made. 
         [0007]    In each of the three design features (Energy Absorbing, De-Mag or Eco-Mag), the electromagnetic door lock provides for a strike plate mounting bolt assembly whereby, while the strike plate remains in contact with an energized electromagnet, the door moves slightly away from the door frame when a force to open the door is applied to the door. In the prior art, the strike plate mounting bolt assembly includes a relatively large coil spring resiliently mounted in a through bore in the door to provide for relative movement between the door and strike plate. The prior art mounting bolt assembly required a large diameter hole to be bored through the door in order to receive the coil spring. Further, in the prior art, with the use of a coil spring as the resilient member, the dynamics of allowable door movement, that is, the door opening force and amount of door movement needed to compress the spring were not readily adjustable to accommodate the varied requirements of the above mentioned design features. 
         [0008]    What is needed in the art is a strike plate mounting bolt assembly used in an electromagnetic door lock that provides for a more compact and robust electromagnetic door lock. 
         [0009]    What is also needed in the art is a strike plate mounting bolt assembly used in an electromagnetic door lock that may be conveniently and selectively adjusted in the field to accommodate various needs of the associated electromagnetic door lock. 
         [0010]    It is the principal object of the present invention to provide these and other needs. 
       SUMMARY OF THE INVENTION 
       [0011]    Briefly described, the present invention is directed toward a strike plate mounting bolt assembly of an electromagnetic door lock wherein the strike plate mounting bolt assembly resiliently mounts the strike plate of the door lock to the associated door. 
         [0012]    The strike plate mounting bolt assembly includes a bolt, a post and at least one Belleville washer. The bolt includes a shaft and a head wherein, when assembled to the door, the head abuts a rear face of the door. A through bore formed in the door is sized to receive an outer diameter of the shaft. 
         [0013]    The post includes a head end and a shaft end wherein the head end is larger in diameter than the shaft end. Male threads formed in the shaft end are configured for engagement with female threads formed in the bolt at its shaft end. The strike plate includes a first bore and a second bore concentric with and larger in diameter than the first bore. The first bore is sized to receive the shaft end of the post. 
         [0014]    The at least one Belleville washer has an outer diameter smaller than the diameter of the second bore so that the at least one Belleville washer can be received within the second bore. The at least one Belleville washer also includes a center hole larger than an outer diameter of the shaft end of the post so that the shaft end of the post can pass through the center hole. 
         [0015]    In a further aspect of the invention, an assembly sequence to complete the assembly of the armature portion of the electromagnetic door lock is provided. First, the bolt is inserted into the door through bore. After inserting the shaft end of the post through the center hole of the at least one Belleville washer, the shaft end of the post is inserted through the first and second bores of the strike plate and the at least one Belleville washer is secured within a cavity in the strike plate. Male threads of the shaft end are then threaded into female threads formed in the bolt. The post is then tightened into the bolt. 
         [0016]    In yet another aspect of the invention, the strike plate mounting bolt assembly may further include a post bushing having a through bore for receiving the shaft end and an outer diameter configured for being received by the second bore. When assembled, the post bushing is disposed between the underside of the head of the post and the at least one Belleville washer to provide a load bearing surface between the post and the at least one Belleville washer. 
         [0017]    In a further aspect of the invention, a conical surface may be formed in the bushing to receive a similarly contoured conical surface formed in the underside of the head end of the post. In yet a further aspect of the invention, the mating surfaces between the post bushing and the underside of the head end of the post may be formed in a ball and socket arrangement. 
         [0018]    In a further aspect of the invention, the selective stacking of two or more Belleville washers to form a Belleville washer pack may be used. By selecting the number of washers and the relative orientations of the selected washers in the pack, a force/deflection characteristic of the collection of Belleville washers can be varied to suit a variety of electromagnetic door lock features in the field. 
         [0019]    In yet a further aspect of the invention, a method for tuning the force/deflection characteristics of a strike plate mounting bolt assembly to suit a particular electromagnet door lock may be include the steps of:
       1. providing a collection of Belleville washers, each having a certain force/deflection characteristic;   2. determining the force/deflection characteristic needed for a particular feature design;   3. selecting a Belleville washer stack comprising one or more Belleville washers from the collection of Belleville washers in accordance with the determined force/deflection characteristics;   4. assembling the strike plate mounting bolt assembly using the selected stack to achieve the force/deflection characteristics needed.       
 
         [0024]    Numerous applications, some of which are exemplarily described below, may be implemented using the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  shows a typical electromagnetic door lock installation; 
           [0026]      FIG. 2  is a cross section view of a prior art electromagnetic door lock, including a prior art strike plate mounting bolt assembly; 
           [0027]      FIG. 3  is an exploded, sectional view of the prior art strike plate mounting bolt assembly; 
           [0028]      FIG. 4  is a sectional view of the strike plate portion of the electromagnetic door lock assembly in accordance with the invention, including the strike plate mounting bolt assembly wherein the door is in contact with the door frame; 
           [0029]      FIG. 5  is a sectional view of the strike portion shown in  FIG. 4  wherein the Belleville washers are compressed; and 
           [0030]      FIG. 6  is a diametrically sectioned view of a Belleville washer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]      FIG. 1  shows a typical electronic door lock installation. In a typical installation, an electromagnet  22  is secured to a door frame  24 . A ferromagnetic armature or strike plate  26  is mounted on door  28 . When door  28  is closed and electromagnet  22  is energized, electromagnet  22  exerts a magnetic force against strike plate  26  to hold door  28  in a closed and magnetically locked position. 
         [0032]      FIG. 2  depicts the construction details of an electronic door lock of the prior art as disclosed in U.S. Pat. No. 5,758,913, wherein an amount of door movement in the opening direction is permitted while the armature of the lock remains in contact with an energized electromagnet. As shown, electromagnet  22  is mounted onto a door frame  24  via electromagnet mounting bolts  25  or other mounting means. Armature or strike plate  26  is mounted onto door  28  via strike plate mounting bolt assembly  30 . One or more flexible washers  32  allow strike plate  26  to move to a degree so that strike plate  26  can abut electromagnet  22  in full contact for maximum hold force when door  28  is shut and electromagnet  22  is energized. 
         [0033]    Referring now to both  FIGS. 2 and 3 , a strike plate mounting bolt assembly  30  includes a bolt housing  36 , plunger  60 , compression spring  66  and caps  33 ,  68 . Bolt housing  36  includes a flange or head  38  which abuts the rear face  39  of door  28 , and a shaft  34 . Bolt housing  36  may be made tamper-resistant from its exposed end. Inside housing  36  is fitted plunger  60  having spring engagement or flanged portion  64  and having female threads  62 . Spring  66  is also fitted inside housing  36 . Plunger  60  and spring  66  are retained within cavity  70  of housing  36  by seal cap  68 . Seal cap  68  is an annular member having inner threads  69  for engaging corresponding threads  37  on housing shaft  34 , and having outer threads  71 . The foregoing components are held in place within door  28  by post installation cap  33 , having female threads  35  that engage corresponding outer threads  71  of seal cap  68 . Male threads on attaching bolt  31  engage female threads  62  within plunger  60  to fasten plunger  60  to strike plate  26 . Thus, plunger  60  serves to couple strike plate  26  to spring  66 . Spring  66 , as shown, may have a compression force approximately equal to or slightly less than the hold strength of electromagnet  20 , when spring  66  is compressed a predetermined maximum allowable travel distance. 
         [0034]    Thus, in the case of an electromagnetic door lock having the Energy Absorbing feature, while strike plate  26  is magnetically attracted to electromagnet  22  by energizing electromagnet  22 , and when an impact force is applied to door  28  in direction  72  ( FIG. 2 ), spring  66  is compressed within cavity  70 , thereby absorbing some of the impact energy that would otherwise be imparted on the lock mechanism to separate strike plate  26  from an energized electromagnet  22 . This allows for a lower power electromagnet to be used to effectively hold the door in a locked stated when an unauthorized attempt is made to open the door. 
         [0035]    In the case of an electromagnetic door lock having the De-Mag feature, compression of spring  66  allows door  28  to move away from door frame  24  a distance approximately equal to the installed height  67  of spring  66 , as shown in  FIG. 2 , minus the solid height of the spring when fully compressed. Movement of the door through this distance of approximately ¾ of an inch allows the delay function of the lock to operate. 
         [0036]    Finally, in the case of an electromagnetic door lock having an Eco-Mag feature, the movement of the door through the distance of approximately ¾ of an inch allows a door position sensor, or the like, to sense that an unauthorized entry is being attempted and for the circuitry controlling the locking function to apply full power to electromagnet  22  in order to maintain the door in a locked state. 
         [0037]    It is important to note that, in the prior art mechanism just described, the active height, and outer diameter of spring  66  must be selected to meet the force/deflection performance requirements of the particular design feature, whether the lock incorporates the Energy Absorbing, De-Mag or Eco-Mag features or any combination thereof. The resulting active spring height, solid height and outer diameter of the spring, whatever it is, must fit within the dimensions of bolt housing  36  since bolt housing  36  envelops the spring body. Accordingly, the size of cavity  70  must be large enough to receive the outer diameter of spring  66  (and the diameter of flange portion  64  of plunger  60 ), and the hole formed in the door must be large enough to receive the outer diameter  65  of the bolt housing, which may be as large as one inch in diameter. 
         [0038]    Referring now to  FIG. 4 , in accordance with the invention, a cross-section taken through strike bolt mounting bolt assembly  130  is shown. A complementary electromagnet (shown generally as feature  22  in  FIG. 1 ) is mounted to a door frame as known in the art. Strike plate  126  is movably mounted to door  128  via strike plate mounting bolt assembly  130 . One or more flexible washers  132  may be optionally included to allow strike plate  126  to move, to a degree, so that strike plate  126  can abut the electromagnet in full contact for maximum hold force when door  128  is shut and the electromagnet is energized. Guide pins (not shown) in the strike plate that slideably mate with corresponding holes in the door keep the strike plate in proper alignment with the door through the strike plate&#39;s movement relative to the door. 
         [0039]    Strike plate mounting bolt assembly  130  includes bolt  136 , post  160  and at least one Belleville washer  166 . Bolt  136  includes a flange or head  138  which abuts the rear face  139  of door  128 , and a shaft  134 . Through bore  135 , formed in door  128 , is sized to receive an outer diameter  131  of shaft  134 . Head  138  of bolt  136  may be configured to be tamper-resistant from its exposed end. 
         [0040]    Post  160  includes head end  156  and shaft end  158  wherein the head end is larger in diameter than the shaft end. Male threads  159  formed in shaft end  158  are configured for engagement with female threads  157  of bolt  136 . Strike plate  126  includes first bore  142  and second bore  144  larger in diameter than first bore  142 . First bore  142  is sized to loosely receive shaft end  158  of post  160 . Second bore  144  forms a spring cavity for receiving the one or more Belleville washers  166 . 
         [0041]    Included in bolt assembly  130  is a resilient member such as at least one Belleville washer  166  having an outer diameter  170  smaller than a diameter of second bore  144  and a center hole  172  larger in diameter than an outer diameter of shaft end  158  of post  160 . 
         [0042]    To complete the assembly of the armature portion of an electromagnetic door lock assembly, bolt  136  is inserted into through bore  135 . After inserting the shaft end  158  of post  160  through hole  172  of the at least one Belleville washer, preferably with convex side  174  ( FIG. 6 ) of the Belleville washer facing head end  156  of post  160 , shaft end  158  is inserted through first and second bores  142 ,  144  of strike plate  126 . Male threads  159  of shaft end  158  are then threaded into female threads  157  of bolt  136 . Post  160  is then tightened into bolt  136  until opposing surfaces of the at least one Belleville washer are in contact with second bore surface  145  and underside  155  of head end  156  of post  160  and, preferable, until post head end  156  is flush or below an outer surface  127  of the strike plate. 
         [0043]    In one aspect of the invention, bolt assembly  130  may further include post bushing  133  having through bore  137  for receiving shaft end  158 , and an outer diameter  140  configured for being received by second bore  144 . When assembled, post bushing  133  is disposed between the underside of head  156  and the at least one Belleville washer to provide a load bearing surface between post  160  and the at least one Belleville washer. In a further aspect of the invention, a conical surface  141  may be formed in bushing  133  to receive a similarly contoured conical surface formed in the underside of head end  156 . In yet a further aspect of the invention, the mating surfaces between post bushing  133  and the underside of the head end of bolt  136  may be formed in a ball and socket arrangement. 
         [0044]    As shown in  FIG. 4 , dimension  180  of the assembled strike plate mounting bolt assembly represents a first gap  180  between strike plate  126  and door  128  when door  128  is seated within a corresponding door frame. In  FIG. 5 , gap  184  represents a second gap between strike  126  and door  128  when an opening force is applied to the door in direction  182  while the strike remains engaged with the associated energized electromagnet. The difference between the two gaps (gap  184  minus gap  180 ) is associated with the amount of door movement (“initial door movement”) provided by the strike plate mounting bolt assembly to accommodate the requirements of the Energy Absorbing, De-Mag or Eco-Mag design features. In one example in accordance with the invention, the initial door movement provided by strike plate mounting bolt assembly  130  was measured to be ⅛ inch, as compared to an initial door movement in accordance with the prior art of ¾ inch. From a pleasability standpoint, this reduction in initial door movement is a marked improvement. When in use, the reduced initial door movement gives a desirable perception or feel to the operator that the door remains secure. 
         [0045]    In a further aspect of the invention, the selective stacking of two or more Belleville washers (Belleville washer pack  177 ) may be utilized to tune the force/deflection characteristics needed for the particular application. A single Belleville washer exhibits certain load/deflection characteristics based upon its thickness, material, shape, etc. A Belleville washer is generally conical in cross-section ( FIG. 6 ). When two washers are stacked so that their convex surfaces  174  are facing in the same direction, the force (load) doubles with no increase in deflection. When two washers are stacked so that their convex surfaces are facing in opposite directions (e.g., facing each other), deflection is doubled with no increase in force (load). Thus, by selecting the number of washers and the relative orientations of the selected washers, the force/deflection characteristic of the collection of Belleville washers can be varied to suit the application. 
         [0046]    For example, assume a single Belleville washer requires an axial force of 500 pounds to be compressed 0.02 inches. Assume also that the particular feature design (Energy Absorbing, De-Mag or Eco-Mag) needs an axial force of 500 pounds to be developed by the Belleville washer through 0.04 inches of travel. A Belleville washer pack  117   a  consisting of two Belleville washers stacked so that concave surfaces  174  are facing in opposite directions may be selected to meet the design requirement. If the feature design requires an axial force of 1000 pounds to be developed through 0.02 inches of travel, a Belleville washer pack  117   b  consisting of two Belleville washers stacked so that concave surfaces  174  are facing in the same direction may be selected to meet the design requirement. In a final example of selective use of the washers, if the feature design requires an axial force of 1000 pounds to be developed through 0.12 inches of travel, a Belleville washer pack  117   c  consisting of four Belleville washers stacked in two pairs wherein each pair of washers are stacked so that the concave surfaces  174  are facing in opposite directions. From these examples, it can be seen that a Belleville washer stack  117   x  can be built to in a number of different combinations (washer thickness, number of washers and washer orientation) to provide flexibility of use of the strike plate mounting bolt assembly  130 . Thus, a method of tuning the force/deflection characteristics of strike plate mounting bolt assembly may include the steps of:
       1. providing a collection of Belleville washers, each having a certain force/deflection characteristic;   2. determining the force/deflection characteristic needed for a particular feature design;   3. selecting a Belleville washer stack comprising one or more Belleville washers from the collection of Belleville washers in accordance with the determined force/deflection characteristics;   4. assembling the strike plate mounting bolt assembly using the selected stack to achieve the force/deflection characteristics needed.       
 
         [0051]    The strike plate mounting bolt assembly  130 , in accordance with the invention, provides a more compact and robust electromagnetic door locks that may be conveniently and selectively adjusted in the field to accommodate various needs of the associated electromagnetic door lock 
         [0052]    Although the present invention has thus been described in detail with regard to the preferred embodiments and drawings thereof, it should be apparent to those skilled in the art that various adaptations and modifications of the present invention may be accomplished without departing from the spirit and the scope of the invention. Accordingly, it is to be understood that the detailed description and the accompanying drawings as set forth hereinabove are not intended to limit the breadth of the present invention, which should be inferred only from the following claims and their appropriately construed legal equivalents.

Summary:
A strike plate mounting bolt assembly for resiliently mounting a strike plate to a door. The strike plate mounting bolt assembly includes a bolt and a post wherein the bolt is securably engageable with the post. The bolt is inserted through a bore in the door and the post is inserted through a bore in the strike plate. A resilient member such as at least one Belleville washer is disposed between a head of the post and a cavity in the strike plate wherein the at least one Belleville washer is compressed between the post head and cavity upon securing the bolt and post together to provide a resilient mount between the door and strike plate. The at least one Belleville washer may be a number of Belleville washers selectable stacked to form a pack of Belleville washers. By varying the number and orientation of the Belleville washers, the force/deflection characteristics of the pack may be tuned to accommodate the needs of a variety of electromagnetic lock design features.