Patent Publication Number: US-8978305-B2

Title: Apparatus and method for electromechanically retracting a door latch

Description:
BACKGROUND 
     This application is related to U.S. provisional application No. 61/716,274, filed Oct. 19, 2012, entitled “APPARATUS FOR ELECTRICALLY RETRACTING A DOOR LATCH”, naming Scott G. Morstatt, Jeffrey Sharps, and Anthony Benitez as the inventors. The contents of the provisional application are incorporated herein by reference in their entirety, and the benefit of the filing date of the provisional application is hereby claimed for all purposes that are legally served by such claim for the benefit of the filing date. 
     An apparatus is shown and described for electromechanical retraction of a door latch, including a dogging mechanism for holding the door latch in a retracted position and, more particularly, an apparatus for electromechanical door latch retraction and dogging which uses less power in the retracted and dogged state. 
     An exit device for a door generally includes a frame or housing secured across a door face and substantially spanning the width of the door. A touch bar, sometimes referred to as a “panic bar” or “push bar”, is movably mounted to the frame. The touch bar is mechanically linked to a latch mechanism, including a door latch which is movably mounted on the frame adjacent to a free edge of the door. Depressing the touch bar in the frame and toward the door translates the mechanical linkage for actuating the latch mechanism in order to retract the door latch, which may be a rim latch with a latch bolt or vertical rods with ceiling and floor latches, so that the door can be opened. 
     It is known to fix or “dog” an exit device in order to maintain an unlocked condition. Typically, dogging requires that the touch bar be held in its depressed or actuated position and prevented from returning to its outward position such that the door latch remains in a retracted position. Dogging mechanisms are useful to facilitate entry and exit through the door during heavy usage times, such as at the start and end of a work shift and during emergencies, or when there is no latch actuator on the outside of the door to gain entry. 
     Manually or electrically operated dogging mechanisms are available for holding the touch bar in the depressed position or keeping the latch bolt retracted. For example, an electrically operated exit device may use a solenoid to dog the touch bar in its depressed position. The plunger of the solenoid may also be operatively coupled to the door latch for moving the latch bolt from its projected position to its retracted position and holding the latch bolt in its retracted position, whether or not the touch bar is electrically dogged. In either type of exit device, the solenoid must provide a substantial force to retract and hold the touch bar or door latch, especially to overcome initial friction. Consequently, a relatively high operating current is required to reliably retract the touch bar or the latch bolt, and to dog the touch bar or latch bolt in the retracted position. 
     For the foregoing reasons, there is a need for a new apparatus for use in an electromechanically operated exit device, including a solenoid for retracting a latch bolt, and which will require significantly less power for maintaining the solenoid plunger, and therefore the latch bolt, in the retracted position until the latch bolt is released. The new apparatus should include a dogging assembly for an exit device for holding the latch bolt in the retracted position. 
     SUMMARY 
     An exit device is described for use with a door pivotally mounted along one edge for movement about an axis. The exit device comprises a housing having a longitudinal axis, the housing adapted to be secured to a surface of the door. A latch mechanism includes a latch bolt disposed at one end of the housing adapted to be adjacent an edge of the door, the latch bolt movable relative to the housing between a projected position extending outwardly of the housing for latching the door in a closed position and a retracted position where the latch bolt is inside the housing for allowing the door to be opened. An actuator is movably mounted on the housing for movement relative to the housing between a first position and a second position, the actuator operably connected to the latch mechanism for moving the latch bolt to the retracted position when the actuator is moved to the second position in response to application of manual force on the actuator. An electrically energizable retracting mechanism is mounted to the housing, the retracting mechanism including a reciprocating plunger and selectively connected to a source of electrical power for moving the plunger from an extended position to a retracted position when the retracting mechanism is energized and for releasing the plunger from the retracted position when the retracting mechanism is de-energized. A retractor element disposed in the housing includes a rigid slide connected between the latch bolt and the solenoid plunger for movement along the longitudinal axis of the housing between a first position and a second position, the slide including a pin extending transversely of the direction of movement of the slide and operably connected to plunger, a pivoting link defining a slot having an inner end portion extending transversely to the longitudinal axis of the housing, the slot configured for receiving the pin for pivoting the link when the slide moves between the first position and the second position of the slide, and a spring for biasing the pivoting link away from the plunger. A controller adapted to control the operation of the retracting mechanism, wherein when the retracting mechanism is energized the plunger moves to the second position of the plunger causing movement of the slide to the second position of slide and the pin to enter the transverse end of the slot in the pivoting link against the bias of the spring where a bearing surface defining at least a portion of the end of the slot intersects the path of movement of the pin such that significantly less current is required for retaining the plunger armature in the retracted position than to retract the plunger for blocking the return of the latch bolt to the projected position. Upon de-energizing the retracting mechanism allows the pivoting link to move to the first position under force of the spring and the plunger to move to the extended position such that the latch moves to the projected position. 
     An apparatus for use in an exit device for holding a latch mechanism including a latch bolt in the retracted position comprises a retractor element disposed in the housing. The retractor element includes a rigid slide connected between the latch bolt and the solenoid plunger for movement along the longitudinal axis of the housing between a first position and a second position, the slide including a pin extending transversely of the direction of movement of the slide and operably connected to plunger, a pivoting link defining a slot having an inner end portion extending transversely to the longitudinal axis of the housing, the slot configured for receiving the pin for pivoting the link when the slide moves between the first position and the second position of the slide, and a spring for biasing the pivoting link away from the plunger. A controller is adapted to control the operation of the retracting mechanism, wherein when the retracting mechanism is energized the plunger moves to the second position of the plunger causing movement of the slide to the second position of slide and the pin to enter the transverse end of the slot in the pivoting link against the bias of the spring where a bearing surface defining at least a portion of the end of the slot intersects the path of movement of the pin such that significantly less current is required for retaining the plunger armature in the retracted position than to retract the plunger for blocking the return of the latch bolt to the projected position. Upon de-energizing the retracting mechanism allows the pivoting link to move to the first position under force of the spring and the plunger to move to the extended position such that the latch moves to the projected position. 
     Also described, in combination, is a door frame, a door hinged along one edge to the door frame, and an exit device. The exit device comprises a housing having a longitudinal axis, the housing adapted to be secured to a surface of the door. A latch mechanism includes a latch bolt disposed at one end of the housing adapted to be adjacent an edge of the door, the latch bolt movable relative to the housing between a projected position extending outwardly of the housing for latching the door in a closed position and a retracted position where the latch bolt is inside the housing for allowing the door to be opened. An actuator is movably mounted on the housing for movement relative to the housing between a first position and a second position, the actuator operably connected to the latch mechanism for moving the latch bolt to the retracted position when the actuator is moved to the second position in response to application of manual force on the actuator. An electrically energizable retracting mechanism is mounted to the housing, the retracting mechanism including a reciprocating plunger and selectively connected to a source of electrical power for moving the plunger from an extended position to a retracted position when the retracting mechanism is energized and for releasing the plunger from the retracted position when the retracting mechanism is de-energized. A retractor element disposed in the housing includes a rigid slide connected between the latch bolt and the solenoid plunger for movement along the longitudinal axis of the housing between a first position and a second position, the slide including a pin extending transversely of the direction of movement of the slide and operably connected to plunger, a pivoting link defining a slot having an inner end portion extending transversely to the longitudinal axis of the housing, the slot configured for receiving the pin for pivoting the link when the slide moves between the first position and the second position of the slide, and a spring for biasing the pivoting link away from the plunger. A controller adapted to control the operation of the retracting mechanism, wherein when the retracting mechanism is energized the plunger moves to the second position of the plunger causing movement of the slide to the second position of slide and the pin to enter the transverse end of the slot in the pivoting link against the bias of the spring where a bearing surface defining at least a portion of the end of the slot intersects the path of movement of the pin such that significantly less current is required for retaining the plunger armature in the retracted position than to retract the plunger for blocking the return of the latch bolt to the projected position. Upon de-energizing the retracting mechanism allows the pivoting link to move to the first position under force of the spring and the plunger to move to the extended position such that the latch moves to the projected position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference should now be had to the embodiments shown in the accompanying drawings and described below. In the drawings: 
         FIG. 1  is a perspective view of an embodiment of an exit device on a door. 
         FIG. 2  is a top plan view of an embodiment of a latch assembly including a door latch for use in the exit device as shown in  FIG. 1 . 
         FIG. 3  is a perspective view of a portion of the exit device as shown in  FIG. 1  with a cover plate and shield removed and showing an embodiment of an apparatus for electromechanically retracting the door latch. 
         FIG. 4  is an exploded perspective view of the exit device and door latch retracting apparatus as shown in  FIG. 3 . 
         FIG. 5  is a perspective view of a portion of the exit device partially cut-away and the door latch retracting apparatus as shown in  FIG. 3 . 
         FIG. 6  is a perspective view of the latch retracting apparatus as shown in  FIG. 3 . 
         FIG. 7  is an exploded perspective view of the latch retracting apparatus as shown in  FIG. 6 . 
         FIG. 8  is a perspective view of an embodiment of a dogging lever as shown in  FIG. 7 . 
         FIG. 9A  is a partial perspective view of a solenoid assembly as shown in  FIG. 6  in a first position. 
         FIG. 9B  is a bottom plan view of an embodiment of a slide assembly as shown in  FIG. 6  in a first position. 
         FIG. 10A  is a partial perspective view of the solenoid assembly as shown in  FIG. 9A  in a second position. 
         FIG. 10B  is a bottom plan view of an embodiment of a slide assembly as shown in  FIG. 9B  in a second position. 
     
    
    
     DESCRIPTION 
     An apparatus for electromechanically retracting a door latch is shown and described in combination with an exit device including a door latch. The electromechanical latch actuator includes a solenoid for retracting the door latch and holding the door latch in the retracted position. The electromechanical latch actuator is described herein in combination with portions of an exit device, which is generally described in the U.S. Pat. No. 7,469,942, the contents of which are hereby incorporated by reference. It is understood that the electromechanical latch actuator may be used in any conventional touch bar exit device such as, for example, the exit devices described by U.S. Pat. Nos. 4,167,280; 4,796,931; 5,605,362; 5,823,582 and 6,104,594, the contents of all of which are hereby incorporated by reference in their entirety. Accordingly, detailed explanations of the functioning of the entire exit device components are deemed unnecessary for understanding the present invention by one of ordinary skill in the art. However, it should be understood that the present invention has other applications and is not limited to combination with the exit devices disclosed in the patents listed herein. 
     Certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention. For example, words such as “upper,” “lower,” “left,” “right,” “horizontal,” “vertical,” “upward,” and “downward” merely describe the configuration shown in the Figures. Indeed, the components may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise. 
     Referring now to the drawings, wherein like reference numerals designate corresponding or similar elements throughout several views,  FIG. 1  shows an exit device incorporating an embodiment of an electromechanical latch actuator and generally designated by the reference numeral  20 . The exit device  20  includes an elongated housing  22  that is mounted at a horizontal position across the interior surface of an outwardly opening door  23  to be secured. The housing  22  comprises a touch bar  24 , a latch housing  26  at one end and a cover plate  28  having an end cap  30  at the other end. The touch bar  24  longitudinally spans a substantial portion of the housing  22  and defines a face for receiving a pushing force exerted toward the door by a person attempting to egress through the door. A U-shaped shield  32  comprising side rails  36  and a base plate  38  fits around and slides over a frame  34  (not seen in  FIG. 1 ). The shield  32  is adapted to be mounted flat against the surface of the door  23 . The base plate  38  is generally channel-shaped and is secured to the door by screws or other fasteners (not shown). For purposes of the description herein, the base plate defines a central longitudinal axis which extends parallel to the surface of the door and a transverse axis which extends perpendicularly to the longitudinal axis of the base plate  38 . 
       FIGS. 3 and 4  show the exit device  20  with the cover plate  28  and shield  32  removed. A pair of spaced touch bar mounts  40  is seen secured to the base plate  38 . The sidewalls  42  of the touch bar mounts  40  extend transversely from the base plate  38  and are slotted in an arc  44 . A pair of pins  46  is rigidly mounted between the side walls of the touch bar  24  and pass through the slots  44  in the touch bar mounts  40 . An L-shaped lever arm  48  is pivotally attached at its vertex to each of the touch bar mounts  40 . A first leg of each lever arm  48  is connected at its distal end to the touch bar pin  46  that travels in the associated slot  44  for drivingly connecting the touch bar  24  to the lever arms  48 . 
     The distal end of the second leg of each lever arm  48  is pivotally connected to an elongated slide assembly  50 . The slide assembly  50  is a substantially flat bracket piece disposed adjacent the base plate  38 . The slide  50  assembly is attached at each end to the second legs of the lever arms  48 . The slide assembly  50  is movably mounted relative to the baseplate  38  so that movement of the touch bar  24  transversely inwardly toward the door will move the slide assembly  50  longitudinally of the baseplate  38 . 
     A latch mechanism  60 , shown in the  FIG. 2 , is secured adjacent the outer end of the base plate  38 . The latch mechanism  60  includes a latch bolt  62  mounted in a latch housing  64  for reciprocal axial movement from a first, projected position beyond the latch housing  64  to a second, retracted position. The latch bolt  62  depicted in  FIG. 2 , along with the latch mechanism  60 , moves linearly and is described in the &#39;362 patent cited above. A spring  66  acts to bias the latch bolt  62  toward its projected position. It is understood that the latch bolt may be mounted for pivotal movement between projected and retracted positions, as in a latch mechanism according to a conventional rim device, a mortise device, or floor and ceiling latches as in a concealed vertical rod latch assembly, or a combination of the above. Moreover, in each of the embodiments described herein, the exit device depicted is constructed and functions like those well known in the art and as disclosed in the &#39;362 and &#39;931 patents. 
     Retraction of the latch bolt  62  from inside of the door is achieved by pushing the touch bar  24  inwardly toward the door as is conventional. Transverse movement of the touch bar  24  pivots the touch bar mounts  40  through an arc motion, which is translated into a generally longitudinal motion at the bottom of the pivoting lever arm legs  48 . The connection between the lever arms  48  and slide assembly  50  causes the slide assembly  50  to reciprocate linearly inwardly in the frame toward the hinged edge of the door. This movement also causes inward movement of a latch slide, which swings a pin in a latch actuator tongue inward to actuate the latch mechanism to retract the latch bolt  62  allowing the door to be opened. When the touch bar  24  is subsequently released from the manual pressure, the touch bar returns to the position shown in  FIG. 1  due to the force of return springs connected between the touch bar mounts  40  and the base plate  38 . Many different known mechanisms can be used to cause the latch bolt  62  to retract as the touch bar  24  is depressed. It is understood that each of these known mechanisms can be utilized in different embodiments of exit device  20  as described herein. 
     Lock trim (not shown), such as a handle and a rim cylinder, may be installed on the opposite side of the door and operatively connected with the latch mechanism so that the latch mechanism can be operated from the opposite side of the door. The connection between the outside cylinder and the projection is described in the &#39;931 patent cited above. 
     Referring to  FIGS. 3-5 , an electromechanical latch actuator is shown for retracting and holding the latch bolt  62  in the retracted position and is generally designated at  80 . As best seen in  FIGS. 6 and 7 , the actuator assembly  80  comprises a solenoid assembly  82 , including a solenoid body  84  having an electrically actuated magnetic coil. The solenoid body  84  defines an axial bore  86  for slidably receiving a cylindrical ferrous metal plunger  88 . The plunger  88  is rectilinearly moveable in the bore  86  relative to the solenoid body  84  in the direction of its axis between a first extended position and a second retracted position. A linkage assembly  90  operatively connects the movable plunger  88  and the latch assembly  60  for manipulating the latch bolt  62 . The extended position of the plunger  88  corresponds to the projected position of the latch bolt  62  and the retracted position of the plunger  88  corresponds to the retracted position of the latch bolt  62 . 
     The solenoid assembly  82  is a conventional, solenoid of the type which requires more current for retracting the plunger against a load than to hold the plunger in the retracted position against the urging of the load. The solenoid body  84  is attached to a substantially flat bracket  92  having a front portion  94  and a rear portion  96  and attached to the base plate  38 . The front portion  94  of the bracket  92  includes a pair of opposed legs  98  extending transversely to the longitudinal axis of the bracket  92 . A T-shaped flange  100  extends transversely from intermediate the bracket  92 . When assembled, a central, cylindrical threaded shank  102  extending forwardly from a proximal end of the solenoid body  84  is received between the legs  98  of the bracket  92 . The flange  100  fits in a corresponding slot in the solenoid body  84 . A hex nut  104  is threaded onto the shank  102  and tightened for pinning the legs  98  against the end of the solenoid body  84  such that the solenoid is fixedly attached to the bracket  92 . 
     The linkage assembly  90  comprises a bracket slide  110 , a latch slide  112  and a connecting rod  114  fastened between the bracket slide  110  and the latch slide  112 . The bracket slide  110  is a planar rectangular member defining a longitudinal slot  111  intermediate along its length. A link pin  116  extends perpendicularly from the upper surface of the bracket slide  110  adjacent an inner end. The bracket slide  110  is slidingly fixed to the bracket  92  by a guide pin  118  which passes through the slot  111  and into a threaded opening in the outer end of the bracket  92 . The bracket slide  110  is able to slide relative to the bracket  92  and guide pin  118  a distance equal to the length of the slot  111  for guided longitudinal motion of the bracket slide  110 . 
     The latch slide  112  is an elongated planar T-shaped member defining a longitudinal slot  113  extending from adjacent an outer end. The latch slide  112  is secured to the base plate  38  by a guide pin  120  which passes through the slot  113  and into a threaded opening in the base plate  38 . The latch slide  112  is able to slide relative to the base plate  38  and the guide pin  113  a distance equal to the length of the slot  113  for guided longitudinal motion of the latch slide  112 . 
     The link pin  116  of the bracket slide  110  extends through a slot  122  defined by the outer end of the plunger  88  forming two transversely spaced arms  126 . A transverse pin  124  through the arms  126  secures the link pin  116  in the slot  122  for coupling longitudinal motion of the plunger  88  to the latch bolt  62 . Inward longitudinal motion of the plunger  88  and the connected bracket slide  110  and latch slide  112  via the connecting rod  114  causes inward movement of the latch bolt  62 . 
     A dogging lever  130  ( FIG. 8 ) is pivotally connected by a pin  132  to the outer end of the bracket  92  for pivotal movement about the pin. The dogging lever  130  is moveable between a first position out of alignment with the linear path of the plunger  88  and a second position substantially parallel to the path of the plunger  88 . The dogging lever  130  has a leg  132  extending transversely from an edge of the dogging lever. The leg  132  is operably positioned adjacent the plunger  88  when the dogging lever  130  is in the second position. Two longitudinally spaced posts  134  extend transversely from an edge the dogging lever  130  opposite the leg  132 . A torsion spring  136  biases the dogging lever  130  toward the first position. The dogging lever  130  defines a longitudinal J-shaped slot  138 . The link pin  116  extends through the slot  138  for coupling longitudinal motion of the plunger  88  to pivotal movement of the dogging lever  130 . The inner end of the slot  138  has a bearing surface  140  comprising a proximal edge of the slot  138 . The bearing surface  140  intersects the path of the link pin  116  when the dogging lever  130  is in the second position. The dogging lever  130  thus cooperates with the link pin  116  to dog the plunger  88  in the retracted position. 
     The slot  111  in the bracket slide  110  provides a lost motion connection between the latch bolt  62  and linkage assembly  90  and the solenoid assembly  82 . The lost motion connection allows manual depression of the touch bar  24  or contact by the latch bolt  62  with a strike (not shown) to move the latch bolt  62  inwardly to the retracted position causing longitudinal motion of the linkage assembly  90  without retraction of the plunger  88 . Accordingly, the plunger  88  and the dogging lever  130  do not move from their first positions corresponding to the projected position of the latch bolt  62 . Assuming the solenoid is de-energized, after return of the touch bar  24  to its normal, outward position, or cessation of contact between the latch bolt  62  and the strike, the spring-biased mechanism of the exit device  20  returns the latch bolt  62  to the projected position. 
     Power is supplied to the exit device from a remote power source over lines in a conventional manner. Wires to the power source extend into the door, for example, through electrical hinges in a conventional manner. A controller can also be utilized to control operation of the exit device, and in different embodiments the controller can be remote or local to the exit device. The controller can communicate with the exit device using many different “hard-wire” and wireless communication links. Energization of the solenoid may be a result of a control signal sent by the controller. Exit devices are readily adaptable for communication with a remote control or security system. The remote security system can be used to issue commands to the exit device to remotely unlatch the door, and also to maintain the door in an unlatched state. The computer may itself be coupled to a smoke detector or other alarm system to activate the latch actuator under emergency conditions. By integrating a building security system with exit devices including electromagnetic latch retractors, it is possible to effectuate the latching and unlatching of exit bars remotely and/or automatically. 
     In the embodiment shown, a PCB  140  provides a control circuit and is mounted to the bracket  92 . The PCB  140  includes a connector through which power is supplied. In one embodiment, the connector includes a power connection and a control connection. Power is continuously supplied to the power connection and a switch is connected to the control connection. The switch may be a remotely actuated or part of an electrical control system, such as a fire control system or a security system. 
     In use, to dog the latch bolt  62  in the retracted position, the solenoid is energized so that the plunger  88  is moved to the retracted position into the solenoid body  84  by the magnetic field created, as shown in  FIG. 10A . Due to the interconnection of the link pin  116  in the slot  122  of the plunger  88 , the longitudinal motion of the plunger  88  causes the linkage assembly  90  to be drawn longitudinally inwardly which, in turn, moves the latch bolt  62  to its retracted position ( FIG. 10B ). The inward movement of the plunger  88  causes the link pin  116  to follow the path of the slot  138  causing pivotal motion of the dogging lever  130 . As seen in  FIG. 10A , the link pin  116  seats in the end of the slot  138  where the pin engages the bearing surface  139  to block the return of the plunger  88  to its projected position thereby holding the latch bolt  62  in a retracted position. The dogging lever  130  is held adjacent the front end of the plunger  88  by magnetic attraction to the plunger  88  against the force of the spring  136 . In the “dogged” condition, the door is free to be pulled open from the outside or pushed open from the inside, with or without depressing the touch bar  24 . The dogged condition may be utilized during heavy usage times of the day, for example, at the start and close of a work shift to minimize wear on the moving parts of the exit device  20 , and also during emergency conditions. 
     The PCB includes two incoming power wires and two outgoing wires to the solenoid. When power is applied to the circuit board, the circuit board applies full power to the solenoid to retract the latch. After a predetermined time, the circuit switches over to a low PWM (pulse width modulation) for “dogging”, which provides the magnetic field for holding the dogging lever  130  adjacent the solenoid and the plunger  88  in the retracted position. When the power to the solenoid is removed, either by the controller or by loss of power to the controller, the magnetic field collapses and the plunger  88  is released. The latch bolt  62  is biased to its projected position by the springs in the exit device  20 , drawing the plunger  88  to its first position. Without the magnetic field, the dogging lever  130  is pivoted outwardly under the force of the spring  136  to the first position, clearing the path of the plunger  88  and the link pin  116  to move forwardly in the slot  138 , as shown in  FIG. 9A . The linkage assembly  90  returns to the first position under the influence of the biasing springs of the latch assembly  60  returning the latch bolt  62  to its extended position ( FIG. 9B ). It is understood that another spring configuration is suitable. The torsion spring  136  is fixed to the bracket  92  or a bracket fastener is suitable. 
     The apparatus for retracting a door latch has many advantages, including providing a low power, electrically operated exit device  20  and electromechanical latch actuator  80  for an exit device. The power required to drive the plunger  88  to retract the latch bolt  62  against the bias of the various springs and associated frictional interfaces is relatively large compared to the power required to generate a magnetic field to hold the dogging lever  130  in place for blocking the plunger  88  from returning to the projected position. Once the plunger moves to the retracted position and engages the bearing surface  139 , the dogging lever  130  holds the plunger  88  in the retracted position and allows the power to be reduced while still keeping the exit device  20  in the unlatched condition. The only power needed is to energize the solenoid sufficiently to hold the dogging lever  130  against the force of the spring  136 , which is much less than that consumed by the solenoid to retract the latch bolt  62 . This arrangement realizes significant reduction in power consumption compared to a similar exit device  20 . The apparatus can remain in the holding state with the solenoid energized, while drawing very little power and producing very little heat. In addition, the electromechanical latch actuator  80  as described herein may be retrofit into an existing exit device. Moreover, the electromechanical latch actuator is adaptable to exit devices having operating mechanisms other than touch bars. 
     Although the apparatus for electromechanically retracting a door latch has been shown and described in considerable detail with respect to only a few exemplary embodiments thereof, it should be understood by those skilled in the art that we do not intend to be limited to the embodiments since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the apparatus, particularly in light of the foregoing teachings. Accordingly, we intend to cover all such modifications, omission, additions and equivalents as may be included within the spirit and scope of the following claims. In the claims, means-plus-function clause(s) are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.