Patent Publication Number: US-8523248-B2

Title: Latch assembly with a safety device for a fireproof door lock

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a latch assembly for a fireproof door lock and, more particularly, to a latch assembly with a safety device for a fireproof door lock for a door mounted in a passageway or panic exit, with top and bottom latches of the fireproof door lock retained in the latching positions during a fire to avoid expansion of the fire and smoke by avoiding opening of the door. 
     Fireproof door locks are generally mounted to fireproof doors in passageways or panic exits and generally include horizontal type latches and vertical type latches. Vertical type latches include a top latch and a bottom latch. The fireproof lock door should be remained in the closed position during a fire to avoid expansion of the fire or smoke. However, the parts of the fireproof door lock may deform due to the high temperature of the fire, such that the door can not be retained in the closed position. Specifically, the latch bolts of the top and bottom latches may be moved to the retracted, unlatching position due to deformation resulting from the heat of the fire. As a result, there is a risk of expansion of the fire and smoke if the door is opened. 
     Thus, a need exists for a safety device for a fireproof door lock in which the top and bottom latches of the fireproof door lock can be reliably retained in the latching positions during a fire. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention solves this need and other problems in the field of fireproof door locks by providing a latch assembly including a base having first and second sidewalls and a main section extending between the first and second sidewalls. Each of the first and second sidewalls includes a positioning hole. The main section is adapted to be mounted to a door. A latch bolt is movably received between the first and second sidewalls of the base. The latch bolt includes a latch having a first pivotal portion pivotably connected to the first and second sidewalls of the base and a second pivotal portion. The latch is movable between a latching position outside the base and an unlatching position inside the base. A link includes a first end pivotably connected to the second pivotal portion of the latch and a second end. The second end of the link is slideably engaged with the first and second sidewalls of the base. Movement of the link along a first axis causes movement of the latch between the latching position and the unlatching position. A follower plate is movable along the first axis and includes first and second sidewalls spaced along a second axis perpendicular to the first axis. Each of the first and second sidewalls of the follower plate includes first and second ends. An engagement hole is formed in each of the first and second sidewalls of the follower plate and located between the first and second ends of the first and second sidewalls of the follower plate. The first and second sidewalls of the follower plate are movably received between the first and second sidewalls of the base. The first ends of the first and second sidewalls of the follower plate are pivotably connected to the second end of the link, allowing pivotal movement of the follower plate and the link relative to the base, and allowing relative pivotal movement between the follower plate and the link to permit movement of the follower along the first axis. The engagement holes of the follower plate are aligned with the positioning holes of the base when the latch is in the latching position. The engagement holes of the follower plate are not aligned with the positioning holes of the base when the latch is in the unlatching position. A linking rod includes a first end coupled with the second ends of the first and second sidewalls of the follower plate to move therewith. The linking rod further includes a second end adapted to be connected to a vertical rod of a door lock mounted to the door to move therewith. 
     The latch assembly further includes a first sleeve having first and second ends spaced along the second axis. A receiving hole extends from the first end through the second end of the first sleeve. The receiving hole includes a smaller section extending from the first end towards but spaced from the second end of the first sleeve and a larger section extending from the second end through the smaller section and having a diameter larger than the smaller section. The first end of the first sleeve is received in the engagement hole of the first sidewall of the follower plate. A second sleeve includes first and second ends spaced along the second axis. A mounting hole extends from the first end through the second end of the second sleeve. The mounting hole includes a first hole section extending through the first end of the second sleeve and a second hole section extending through the second end of the second sleeve. The second end of the second sleeve is engaged with the second end of the first sleeve. The first end of the second sleeve is received in the engagement hole of the second sidewall of the follower plate. The first and second sleeves are movable together with the follower plate. A first stop is received in the larger section of the receiving hole of the first sleeve and has a melting point lower than the first and second sleeves. A second stop is received in the mounting hole of the second sleeve and has a melting point lower than the first and second sleeves. 
     The latch assembly further includes a first safety pin received in the receiving hole of the first sleeve. The first safety pin includes a first shank having a first distal end. A flange is formed on an end of the first shank opposite to the first distal end and has a diameter larger than a diameter of the first shank. The first shank of the first safety pin extends through the first stop into the smaller section but not extends beyond the first end of the first sleeve. The first stop is located between the first flange of the first safety pin and the smaller section of the first sleeve. A second safety pin is received in the mounting hole of the second sleeve. The second safety pin includes a second shank having a second distal end. A second flange is formed on an end of the second shank opposite to the second distal end and has a diameter larger than a diameter of the second shank. The second shank of the second safety pin extends through the second stop into the first hole section but not extending beyond the first end of the second sleeve. The second stop is located between the second flange of the second safety pin and the first hole section of the second sleeve. A spring is mounted in the larger section of the receiving hole of the first sleeve and includes a first end pressing against the first flange of the first safety pin and a second end pressing against the second flange of the second safety pin. The spring biases the first and second safety pins towards the first and second sidewalls of the base. 
     When the latch is in the latching position and when the first and second stops melt by heat such as during a fire, the spring moves the first distal end of the first safety pin into the positioning hole of the first sidewall of the base and moves the second distal end of the second safety pin into the positioning hole of the second sidewall of the base, preventing the follower plate from moving relative to the base along the first axis. Thus, the latch is retained in the latching position during the fire. 
     The present invention will become clearer in light of the following detailed description of an illustrative embodiment of this invention described in connection with the drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The illustrative embodiment may best be described by reference to the accompanying drawings where: 
         FIG. 1  shows a diagrammatic side view of a door lock mounted to a fireproof door and utilizing latch assemblies with safety devices according to the present invention. 
         FIG. 2  shows an exploded perspective view of a bottom latch of the door lock of  FIG. 1 . 
         FIG. 3  shows an exploded, perspective view of the safety device. 
         FIG. 4  shows a partial, cross-sectional view of the fireproof door of  FIG. 1 , with a latch bolt of the bottom latch in a latching position. 
         FIG. 5  shows a cross sectional view taken along section line  5 - 5  of  FIG. 4 . 
         FIG. 6  shows a view similar to  FIG. 4 , with the latch bolt of the bottom latch in an unlatching position. 
         FIG. 7  shows a view similar to  FIG. 5 , with a safety pin of the safety device for the bottom latch in an extended position. 
         FIG. 8  shows an exploded, perspective view of a top latch of the door lock of  FIG. 1 . 
         FIG. 9  shows a partial, cross-sectional view of the door of  FIG. 1 , with a latch bolt of the top latch in a latching position. 
         FIG. 10  shows a cross sectional view taken along section line  10 - 10  of  FIG. 9 . 
         FIG. 11  shows a view similar to  FIG. 9 , with the latch bolt of the top latch in an unlatching position, and with the door opened. 
         FIG. 12  shows a view similar to  FIG. 11 , with the latch bolt of the top latch being about to move from the unlatching position to the latching position. 
         FIG. 13  shows a view similar to  FIG. 10 , with a safety pin of the safety device for the top latch in an extended position. 
     
    
    
     All figures are drawn for ease of explanation of the basic teachings of the present invention only; the extensions of the Figures with respect to number, position, relationship, and dimensions of the parts to form the preferred embodiment will be explained or will be within the skill of the art after the following teachings of the present invention have been read and understood. Further, the exact dimensions and dimensional proportions to conform to specific force, weight, strength, and similar requirements will likewise be within the skill of the art after the following teachings of the present invention have been read and understood. 
     Where used in the various figures of the drawings, the same numerals designate the same or similar parts. Furthermore, when the terms “first”, “second”, “third”, “lower”, “upper”, “top”, “bottom”, “inner”, “outer”, “end”, “portion”, “section”, “longitudinal”, “vertical”, “length”, and similar terms are used herein, it should be understood that these terms have reference only to the structure shown in the drawings as it would appear to a person viewing the drawings and are utilized only to facilitate describing the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a fireproof door lock  2  mounted to a fireproof door  1  in a panic exit or the like. In the form shown, door lock  2  includes a housing  21  is mounted to door  1 , a touch bar  22  mounted outside housing  21  for manual operation, and a transmission mechanism (not shown) mounted in an end  211  of housing  21 . Door lock  2  further includes a top latch  24  and an upper vertical rod  231  having a lower end coupled with the transmission mechanism and an upper end  234  coupled with top latch  24 . Door lock  2  further includes a bottom latch  25  and a lower vertical rod  232  having an upper end coupled with the transmission mechanism and a lower end  233  coupled with bottom latch  25 . When touch bar  22  is pressed, upper and lower vertical rods  231  and  232  are moved toward each other in a vertical direction to unlatch top and bottom latches  24  and  25 . The transmission mechanism can be of any desired form as conventional including but not limited to of a commercially available type. 
     In the form shown, bottom latch  25  is comprised of basic parts of top latch  24 . According to the form shown, bottom latch  25  includes a base  3 , a latch bolt  4 , a follower plate  5 , and a linking rod  6 . Base  3  is substantially U-shaped and includes first and second sidewalls  32  and a main section  31  extending between first and second sidewalls  32 . Main section  31  includes first and second ends  37  and  38  spaced along a first axis X. Main section  31  includes first and second holes  311  and  312  spaced along first axis X. A protrusion  314  is formed between first and second holes  311  and  312 . First and second sidewalls  32  are spaced along a second axis Y perpendicular to first axis X and each include first and second ends  33  and  34  spaced along first axis X. First and second sidewalls  32  include aligned first sliding slots  321  adjacent first ends  33  and aligned second sliding slots  322  adjacent second ends  34 . Each of first and second sliding slots  321  and  322  has a length extending along first axis X. First and second sidewalls  32  further include aligned first guiding slots  323  and aligned second guiding slots  324 . First guiding slot  323  on each of first and second sidewalls  32  is located between first sliding slot  321  and second sliding slot  322  along first axis X. Furthermore, first guiding slot  323  includes a vertical section  327  extending in a direction parallel to first sliding slot  321  and an inclined section  328  at an obtuse angle (about 130° in the form shown) to vertical section  327 . Second guiding slot  324  on each of first and second sidewalls  32  is located between first guiding slot  323  and first sliding slot  321  along first axis X. Second guiding slot  324  has a length extending along a third axis X perpendicular to first and second axes X and Y (i.e., perpendicular to first sliding slot  321 ). First and second sidewalls  32  further include aligned pin holes  329  at first ends  33  thereof. First and second sidewalls  32  further include aligned positioning holes  325 . Positioning hole  325  on each of first and second sidewalls  32  is spaced from first guiding slot  323  along third axis Z. It can be appreciated that first and second guiding slots  323  and  324  of base  3  are not utilized or even can be omitted when utilizing the universal assembly of  FIG. 2  as bottom latch  25 . In assembly, a screw  35  (the lower one in  FIG. 4 ) is extended through first hole  311  of base  3  into a side  11  of door  1 , and another screw  35  (the upper one in  FIG. 4 ) is extended through second hole  312  into side  11  of door  1 . Base  3  is, thus, fixedly mounted to door  1 . 
     According to the form shown, latch bolt  4  is mounted between first and second sidewalls  32  of base  3  and includes a latch  41  and a link  42 . Latch  41  includes a first pivotal portion  411  on an end thereof and a second pivotal portion  412  between two ends of the latch  41 . A pin  43  is extended through pin holes  329  of first and second sidewalls  32  of base  3  and first pivotal portion  411 . Thus, the other end of latch  41  is pivotable between a latching position ( FIG. 4 ) outside base  3  and an unlatching position ( FIG. 6 ) inside base  3  about a pivot axis parallel to second axis Y. Link  42  includes a first end  421  pivotally connected to second pivotal portion  412  and a second end  422 . Link  42  is substantially U-shaped and includes two sidewalls  426  spaced along second axis Y and an intermediate portion  424  extending between sidewalls  426 . Sidewalls  426  include aligned holes  425  in first ends thereof and aligned holes  423  in second ends thereof. A pin  44  is extended through holes  425  of link  42  and second pivotal portion  412  of latch  41 , allowing second end  422  of link  42  to pivot about a pivot axis defined by pin  44  that is parallel to the pivot axis of pin  43 . A pin  55  is extended through first sliding slots  321  of base  3  and holes  423  of link  42 . When pin  55  slides along first sliding slots  321 , link  42  moves along first axis X and causes latch  41  to move between the latching position and the unlatching position. 
     According to the form shown, follower plate  5  is mounted between first and second sidewalls  32  of base  3 . Follower plate  5  is substantially U-shaped and includes first and second sidewalls  52  spaced along second axis Y and an intermediate portion  51  extending between first and second sidewalls  52 . Latch bolt  4  is movably received between first and second sidewalls  52  of follower plate  5 . A tab or projection  511  is formed on intermediate portion  51 . A spring  57  is attached between projection  511  of follower plate  5  and protrusion  314  of base  3  to bias follower plate  5 . First and second sidewalls  52  include aligned holes  521  in first ends  53  thereof and aligned slots  522  in second ends  54  thereof. Each of first and second sidewalls  52  further includes an engaging notch  524  adjacent first end  53  thereof. Each of first and second sidewalls  52  further includes an inclined slot  523  between hole  521  and slot  522  and at an acute angle to an extending direction of slot  522 . Each of first and second sidewalls  52  further includes an engagement hole  525  between slot  522  and engaging notch  524 . Engagement hole  525  is spaced from inclined slot  523  along third axis Z. Pin  55  is parallel to pins  43  and  44  and extended through first sliding slots  321  of base  3 , holes  521  of follower plate  5 , and holes  423  of link  42 . Thus, an end of the follower plate  5  is pivotably connected to second end  422  of link  42 . Note that pin  43  is extended through pin holes  329  of first and second sidewalls  32  of base  3  and first pivotal portion  411  and that pin  44  is extended through holes  425  of link  42  and second pivotal portion  412  of latch  41 . It can be appreciated that inclined slots  523  and engaging notches  524  of follower plate  5  are not utilized or even can be omitted when utilizing the latch assembly of  FIG. 2  as bottom latch  25 . 
     According to the form shown, linking rod  6  is in the form of a screw rod and includes a first end  61  without threading and a second end  62 . First end  61  has a transverse hole  611  perpendicular to a longitudinal axis of linking rod  6 . Second end  62  of linking rod  6  is connected to lower end  233  of lower vertical rod  232  to move therewith. It can be appreciated that second end  62  of linking rod  6  can be integrally formed with lower end  233  of lower vertical rod  232 . A pin  56  parallel to pins  43 ,  44 , and  55  is extended through second sliding slots  322  of base  3 , slots  522  of follower plate  5 , and transverse hole  611  of first end  61  of linking rod  6 . Thus, follower plate  5  can be moved in the vertical direction together with linking rod  6  to move latch  41  between the unlatching position and the latching position when touch bar  22  is pressed. When latch  41  is in the latching position ( FIG. 4 ), engagement holes  525  of follower plate  5  are aligned with positioning holes  325  of base  3  ( FIG. 5 ). On the other hand, when latch  41  is in the unlatching position ( FIG. 6 ), engagement holes  525  of follower plate  5  are not aligned with positioning holes  325  of base  3 . 
     Bottom latch  25  utilizing the latch assembly of  FIG. 2  is shown in  FIGS. 1 and 4 . When latch  41  is in the latching position outside base  3  ( FIG. 4 ), latch  41  is stopped by a strike  13  on the ground  12 . Door  1  is, thus, locked. When touch bar  22  is pressed, lower vertical rod  232  and linking rod  6  are moved upward. After pin  56  reaches upper ends of slots  522 , follower plate  5  moves upward together with linking rod  6 , which in turn, causes upward pivotal movement of link  42  about a pivot axis defined by pin  44 . Thus, follower plate  5  pivots and compresses spring  57 . Latch  41  is moved to the unlatching position ( FIG. 6 ) in base  3  and disengaged from strike  13 . As mentioned above, first and second guiding slots  323  and  324  of base  3  as well as inclined slots  523  and engaging notches  524  of follower plate  5  are not utilized or can be omitted when utilizing the latch assembly of  FIG. 2  as bottom latch  25 . When linking rod  6  is released due to release of touch bar  22 , spring  57  moves follower plate  5  away from base  3  and, thus, returns latch  41  to the latching position. It can be appreciated that idle movement of pin  56  in slots  522  of follower plate  5  provides a burglar-proof effect. Particularly, when latch  41  of door lock  2  is picked to pivot for causing link  42  and follower plate  5  to move along first axis X, the small displacement of the link  42  and follower plate  5  is smaller than or equal to the length of slot  522  along first axis X such that unlatching of the latch  41  would not occur. 
     According to the form shown, the latch assembly further includes a first safety device  801 A ( FIGS. 2 and 3 ) mounted in follower plate  5  of bottom latch  25 . First safety device  801 A includes first and second sleeves  803  and  817  engaged with each other. First sleeve  803  includes a first end  805  and a second end  807  spaced from first end  805  along second axis Y and having a diameter smaller than that of first end  805 . A receiving hole  809  extends from first end  805  through second end  807 . Receiving hole  809  includes a smaller section  813  extending from first end  805  towards but spaced from through second end  807  and a larger section  811  extending from second end  807  through smaller section  813  and having a diameter larger than smaller section  813 , with a shoulder  815  formed in an intersection of larger and smaller sections  811  and  813 . Second sleeve  817  includes a first end  819  and a second end  831  spaced from first end  819  along second axis Y. A mounting hole  833  extends from first end  819  through second end  831 . Mounting hole  833  includes a first hole section  835  extending from first end  819  towards but spaced from second end  831 , a second hole section  837  extending from second end  831  towards but spaced from first end  819 , and an intermediate hole section  839  between first and second hole sections  835  and  837 , with a first abutment face  851  formed at an intersection of first hole section  835  and intermediate hole section  839 , and with a second abutment face  853  formed at an intersection of intermediate hole section  839  and second hole section  837 . Second hole section  837  includes an inner threading  855  spaced from second abutment face  853 . 
     Second end  807  of first sleeve  803  of first safety device  801 A is threaded into second hole section  837  of second sleeve  817  and engaged with inner threading  855 . An end face of second end  807  of first sleeve  803  abuts second abutment face  853  of second sleeve  817  ( FIG. 5 ). First end  805  of first sleeve  803  and first end  819  of second sleeve  817  respectively extend into engagement holes  525  of follower plate  5 . Thus, first and second sleeves  803  and  817  can move jointly with follower plate  5 . 
     According to the form shown, first safety device  801 A further includes first and second stops  877 A and  877 B respectively mounted in first and second sleeves  803  and  817 . First and second stops  877 A and  877 B are made of a material having a melting point lower than firs and second sleeves  803  and  817 , such as plastic. Each of first and second stops  877 A and  877 B has two ends  879  and a through-hole  891  extending from an end  879  through the other end  879 . First stop  877 A has an outer diameter slightly smaller than the inner diameter of larger section  811  of first sleeve  803 . First stop  877 A is received in larger section  811  of first stop  877 A, with an end  879  abutting shoulder  815 . Second stop  877 B has an outer diameter smaller than the inner diameter of intermediate hole section  839  of second sleeve  817 . Second stop  877 B is received in intermediate hole section  839  of second sleeve  817 , with an end  879  abutting first abutment face  851 . A length of second stop  877 B along second axis Y is equal to a length of intermediate hole section  839  of second sleeve  817 . The melting point of each of first and second safety pins  857 A and  857 B are higher than that of first and second stops  877 A and  877 B. 
     According to the form shown, first safety device  801 A further includes first and second safety pins  857 A and  857 B respectively mounted in first and second sleeves  803  and  817 . Each of first and second safety pins  857 A and  857 B has a cylindrical shank  859  and a flange  871  on an end of shank  859  and having an end face  875 . Shank  859  further has a distal end  873  away from flange  871 . 
     First safety pin  857 A is mounted in receiving hole  809  of first sleeve  803 , with shank  859  of first safety pin  857 A extending through through-hole  891  of first stop  877 A. A length of shank  859  along second axis is equal to a sum of a length of first stop  877 A and a length of smaller section  813  of receiving hole  809  along second axis Y. An outer diameter of shank  859  of first safety pin  857 A is slightly smaller than the inner diameter of smaller section  813  of receiving hole  809  of first sleeve  803  and slightly smaller than through-hole  891  of first stop  877 A. An outer diameter of flange  871  of first safety pin  857 A is slightly smaller than the inner diameter of larger section  811  of receiving hole  809  of first sleeve  803  but larger than the inner diameter of through-hole  891  of first stop  877 A, such that flange  871  of first safety pin  857 A abuts an end  879  of first stop  877 A distant to shoulder  815 . Furthermore, distal end  873  of shank  859  is extended through smaller section  813  of first sleeve  803 , and an end face of distal end  873  of shank  859  of first safety pin  857 A is flush with the end face of first end  805  of first sleeve  803 . 
     Second safety pin  857 B is mounted in mounting hole  833  of second sleeve  817 . A length of shank  859  of second safety pin  857 B along second axis Y is equal to the sum of a length of second stop  877 B and a length of first hole section  835  of mounting hole  833  along second axis Y. An outer diameter of shank  859  of second safety pin  857 B is slightly smaller than the inner diameter of first hole section  835  of mounting hole  833  of second sleeve  817  and slightly smaller than the inner diameter of through-hole  891  of second stop  877 B. An outer diameter of flange  871  of second safety pin  857 B is slightly smaller than the inner diameter of second hole section  837  of mounting hole  833  of second sleeve  817  but larger than the inner diameter of through-hole  891  of second stop  877 B, such that flange  871  of second safety pin  857 B abuts an end  879  of second stop  877 B distant to first abutment face  851 . Furthermore, distal end  873  of shank  859  of second safety pin  857 B is extended through first hole section  835  of second sleeve  817 , and an end face of distal end  873  of shank  859  of second safety pin  857 B is flush with the end face of first end  819  of second sleeve  817 . According to the form shown, first safety device  801 A further includes a spring  893  mounted in larger section  811  of first sleeve  803  and having first and second ends  895  and  897 . First end  895  of spring  893  presses against end face  875  of first safety pin  857 A, and second end  897  of spring  893  presses against end face  875  of second safety pin  857 B ( FIG. 5 ). Thus, spring  893  biases first safety pin  857 A towards first sidewall  32  of base  3  and biases second safety pin  857 B towards second sidewall  32  of base  3 . 
     However, first and second safety pins  857 A and  857 B are still stopped by first and second stops  877 A and  877 B, such that distal ends  873  of first and second safety pins  857 A and  857 B are in retracted positions in first and second sleeves  803  and  817  and, thus, can not extend beyond first and second sleeves  803  and  817 . Namely, follower plate  5  can move along first axis X. 
     Base  3  of bottom latch  25  shown in  FIGS. 1 and 4  is of surfaced type. However, base  3  of bottom latch  25  can be mounted in an interior of door  1  such that the whole bottom latch  25  becomes a concealed-type bottom latch. 
     After adding some parts, bottom latch  25  can form a surfaced-type top latch  24  for door lock  2 . According to the form shown in  FIG. 8 , surfaced-type top latch  24  includes base  3 , latch bolt  4 , follower plate  5 , and linking rod  6  that form bottom latch  25  but arranged in an upside-down manner, wherein like reference numerals designate like elements. Detailed description of these elements is not set forth to avoid redundancy. However, it is noted that second end  62  of linking rod  6  of top latch  24  is connected to upper end  234  of upper vertical rod  231 . Top latch  24  further includes a locking device  7  and a guard plate  8 . Locking device  7  is mounted on base  3  to lock latch  41  in the latching position or the unlatching position. Due to provision of locking device  7 , top latch  24  and bottom latch  25  can be unlatched only when touch bar  22  is pressed. A deadlocking effect is, thus, provided. Furthermore, the shape of engagement holes  525  of follower plate  5  of top latch  24  is in the form of such as a tear drop and includes a reduced end  527  and an enlarged end  529 . 
     According to the form shown, locking device  7  includes a safety plate  71 , a positioning pin  72 , and a spring  74  in the form shown as a tension spring. Safety plate  71  is movably received between first and second sidewalls  52  of follower plate  5  and includes two sidewalls  712  spaced along second axis Y and interconnected by an interconnecting section  711 . Each sidewall  712  includes first and second ends  713  and  714 . Aligned holes  717  are defined in second ends  714  of sidewalls  712 . Sidewalls  712  further include aligned holes  716  between first and second ends  713  and  714  thereof. Pin  56  is extended through second sliding slots  322  of base  3 , slots  522  of follower plate  5 , holes  717  of safety plate  71 , and transverse hole  611  of first end  61  of linking rod  6 . Thus, follower plate  5  can be moved along first axis X together with linking rod  6  to move latch  41  between the unlatching position and the latching position. Furthermore, safety plate  71  can move along first axis X together with linking rod  6 . A pin  73  is extended through first guiding slots  323  of base  3 , inclined slots  523  of follower plate  5 , and holes  716  of safety plate  71 . When pin  73  is moving between two ends of inclined slots  523  and along vertical sections  327  and inclined sections  328  of first guiding slots  323 , safety plate  71  pivots about a pivot axis defined by pin  56  while moving upward or downward along first axis X. Pivotal movement of safety plate  71  is limited due to limitation to first end  713  of safety plate  71  by inclined slots  523  of follower plate  5  and first guiding slots  323  of base  3 . 
     According to the form shown, each sidewall  712  of the safety plate  71  further includes an engaging notch  715  and a lug  718  between engaging notch  715  and second end  714 . Lugs  718  are aligned along second axis Y and include aligned holes  719 . Engaging notches  715  can engage with positioning pin  72  ( FIG. 9 ) or disengage from positioning pin  72  ( FIG. 11 ) through pivotal movement of safety plate  71 . Positioning pin  72  is extended through second guiding slots  324  of base  3  and movable between two ends of each second guiding slot  324 . Spring  74  includes a first end  741  attached to positioning pin  72  and a second end  742  attached to pin  73  to bias positioning pin  72  and pin  73  towards each other. Thus, the follower plate  5  and the latch bolt  4  are biased upward. 
     According to the form shown, a second safety device  801 B is mounted to safety plate  71  to move therewith. Second safety device  801 B is identical to first safety device  801 A, and like reference numerals designate like reference numerals. First end  805  of first sleeve  803  and first end  819  of second sleeve  817  of second safety device  801 B respectively extend through holes  719  of lugs  718  of safety plate  71  into engagement holes  525  of follower plate  5  of top latch  24 . Nevertheless, end faces of first ends  805  and  819  of first and second sleeves  803  and  817  of second safety device  801 B are located between first and second sidewalls  32  of base  3  of top latch  24  ( FIG. 10 ). Thus, first and second sleeves  803  and  817  of second safety device  801 B move jointly with safety plate  71 . Each of first ends  805  and  819  of first and second sleeves  803  and  817  of second safety device  801 B is movable between reduced end  527  and enlarged end  529  of a corresponding engagement hole  525  of follower plate  5  of top latch  24 . When follower plate  5  moves, safety plate  71  and first and second sleeves  803  and  817  of second safety device  801 B move jointly with follower plate  5 . 
     According to the form shown, guard plate  8  is mounted between first ends  33  of first and second sidewalls  32  of base  3 . Guard plate  8  includes two sidewalls  85  spaced align second axis Y and interconnected by an interconnecting section  86  therebetween. 
     Each sidewall  85  includes first and second ends  81  and  82  and aligned holes  87  between first and second ends  81  and  82 , providing a pivotal portion  84  between first and second ends  81  and  82  of each sidewall  85 . Pin  43  is extended through pin holes  329  of base  3 , first pivotal portion  411  of latch bolt  4 , and holes  87  of guard plate  8 , allowing first ends  81  of sidewalls  85  and interconnecting section  86  interconnected between first ends  81  to pivot outside base  3 . Second end  82  of each sidewall  85  has a notch  83  that can be in contact with and movable together with positioning pin  72  of locking device  7 . 
     With reference to  FIG. 9 , when latch  41  of latch bolt  4  is in the latching position outside base  3 , latch  41  of top latch  24  is stopped by a strike  15  on an upper beam  14  of a door frame. Door  1  is, thus, locked. In this case, first and second safety pins  857 A and  857 B of second safety device  801 B are aligned with positioning holes  325  of base  3  of top latch  24 , and positioning pin  72  is engaged with engaging notches  715  of safety plate  71  to lock latch  41  in the latching position. When latch  41  of top latch  24  is pivoted through a small angle due to picking, link  42 , follower plate  5 , and safety plate  71  jointly move downward through a small travel along first axis X. Since positioning pin  72  is engaged in engaging notches  715  of safety plate  71 , the travel of follower plate  5  along first axis X is insufficient to move latch  41  from the latching position to the unlatching position, providing anti-picking function. Note that spring  74  biases positioning pin  72  and pin  73  toward each other to retain positioning pin  72  in engaging notches  715  of safety plate  71 . Furthermore, since bottom latch  25  must move jointly with top latch  24 , latch  41  of bottom latch  25  remains in the latching position as long as latch  41  of top latch  24  is prevented from being picked. 
     When the touch bar  22  is pressed to move upper vertical rod  231  downward and to move lower vertical rod  232  upward along first axis X, linking rod  6  carries safety plate  71  downward along first axis X, such that first end  713  of safety plate  71  is actuated by pin  73  to cause engaging notches  715  to pivot away from positioning pin  72 . When pin  73  is moved to an intersection of vertical section  327  and inclined section  328  of each first guiding slot  323  of base  3 , pin  73  is located in an end of inclined slot  523  of follower plate  5 , and engaging notches  715  of safety plate  71  are completely disengaged from positioning pin  72 . Thus, pin  73  can move further downward along vertical sections  327  of first guiding slots  323  of base  3 , causing latch  41  to move from the latching position to the unlatching position through transmission by link  42 . When pin  73  is in distal ends of vertical sections  327  of first guiding slots  323 , latch  41  is in the unlatching position, and first and second safety pins  857 A and  857 B of second safety device  801 B are not aligned with positioning holes  325  of base  3  of top latch  24  ( FIG. 11 ). With reference to  FIG. 11 , when latch  41  of top latch  24  is in the unlatching position, positioning pin  72  is engaged in engaging notches  524  of follower plate  5 , preventing follower plate  5  from moving upward along first axis X. Thus, latch  41  is retained in the unlatching position so that the latch  41  does not extend beyond the base  3  to interrupt closing of the door  1 . 
     When closing door  1  in a state shown in  FIG. 11 , strike  15  presses against interconnecting section  86  between first ends  81  of sidewalls  85  of guard plate  8  (FIG.  12 ). Guard plate  8  pushes positioning pin  72 , disengaging positioning pin  72  from engaging notches  524  of follower plate  5 . Specifically, when first ends  81  of sidewalls  85  and/or interconnecting section  86  of guide plate  8  is impinged by strike  15  while closing door  1 , second ends  82  of sidewalls  85  of guard plate  8  pivot towards positioning pin  72  and, thus, disengage positioning pin  72  from engaging notches  524  of follower plate  5 . Follower plate  5  is biased by spring  57  to move upward along first axis X. At the same time, spring  74  pulls pin  73  to the upper ends of inclined slots  523  of follower plate  5 . Then, follower plate  5  is moved upward along first axis X, moving latch  41  from the unlatching position to the latching position. Pin  73  moves upward along inclined slots  523  of follower plate  5  under action of spring  74 , causing upward movement of safety plate  71 . Furthermore, safety plate  71  returns to and is retained in the position shown in  FIG. 9  under the action of spring  74  and guidance by inclined slots  523  of follower plate  5  and first guiding slots  323  of base  3 . This is because positioning pin  72  is engaged in engaging notches  715  of safety plate  71 . 
     Now that the basic construction of door lock  2  including top and bottom latches  24  and  25  of the present invention has been explained, the operation and some of the advantages of top and bottom latches  24  and  25  can be set forth and appreciated. In particular, for the sake of explanation, it will be assumed that latches  41  of top and bottom latches  24  and  25  are in their latching positions ( FIGS. 4 and 9 ), first and second safety pins  857 A and  857 B of first and second safety devices  801 A and  801 B are in their retracted positions in first and second sleeves  803  and  817  ( FIGS. 5 and 10 ). Smaller section  813  of first sleeve  803  and first hole section  835  of second sleeve  817  of first safety device  801 A are aligned with positioning holes  325  of base  3  of bottom latch  25 . Smaller section  813  of first sleeve  803  and first hole section  835  of second sleeve  817  of second safety device  801 B are aligned with positioning holes  325  of base  3  of top latch  24 . First end  805  of first sleeve  803  of first sleeve  803  and first end  819  of second sleeve  817  of second safety device  801 B are received in reduced ends  527  of engagement holes  525  of follower plate  5  of top latch  24  ( FIG. 9 ). Since first and second safety pins  857 A and  857 B of first and second safety devices  801 A and  801 B are in the retracted positions, latches  41  of top and bottom latches  24  and  25  can be moved from the latching positions ( FIGS. 4 and 9 ) to the unlatching positions ( FIGS. 6 and 11 ) by operating touch bar  22 . 
     When latch  41  of top latch  24  moves from the latching position to the unlatching position, second safety device  801 B moves jointly with safety plate  71 , such that first end  805  of first sleeve  803  of first sleeve  803  and first end  819  of second sleeve  817  of second safety device  801 B are received in enlarged end  529  of engagement holes  525  of follower plate  5  of top latch  24  ( FIG. 11 ). 
     In a case that a fire occurs while latches  41  of top and bottom latches  24  and  25  are in the latching positions ( FIGS. 4 and 9 ), first and second stops  877 A and  877 B of first and second safety devices  801 A and  801 B made of plastic melt due to the heat. Each first safety pin  857 A of first and second safety devices  801 A and  801 B is moved from the retracted position to an extended position into one of positioning holes  325  of base  3  of top or bottom latch  24  or  25  under the action of spring  893 . Likewise, each second safety pin  857 B of first and second safety devices  801 A and  801 B is moved from the retracted position to an extended position into the other positioning hole  325  of base  3  of top or bottom latch  24  or  25  under the action of spring  893 . Thus, follower plate  5  of bottom latch  25  is positioned by first and second safety pins  857 A and  857 B of first safety device  801 A. Follower plate  5  and safety plate  71  of top latch  24  are positioned by first and second safety pins  857 A and  857 B of second safety device  801 B. As a result, top and bottom latches  24  and  25  can not be unlatched by operating touch bar  22  during the fire. Thus, latches  41  of top and bottom latches  24  and  25  can effectively be retained in the latching positions during the fire, avoiding expansion of the fire by avoiding opening of door  1 . 
     Now that the basic teachings of the present invention have been explained, many extensions and variations will be obvious to one having ordinary skill in the art. For example, the length of shank  859  of first safety pin  857 A of each of first and second safety devices  801 A and  801 B can be slightly smaller than the sum of the length of the first stop  877 A and the length of smaller section  813  along second axis Y, such that distal end  873  of first safety pin  857 A is received in smaller section  813  of first sleeve  803 . Likewise, the length of shank  859  of second safety pin  857 B of each of first and second safety devices  801 A and  801 B can be slightly smaller than the sum of the length of the second stop  877 B and the length of first hole section  835  along second axis Y, such that distal end  873  of second safety pin  857 B is received in first hole section  835  of second sleeve  817 . Furthermore, first and second stops  877 A and  877 B of first and second safety devices  801 A and  801 B can have cross sections other than the cylindrical shape. Further, first and second stops  877 A and  877 B can be of other forms. As an example, each of first and second stops  877 A and  877 B can be comprised of two arcuate plates or a plurality of cylindrical members sandwiched between the inner periphery of large section  811  and shank  859  of first safety pin  857 A or between the inner periphery of intermediate hole section  839  and shank  859  of second safety pin  857 B. Further, mounting hole  833  of second sleeve  817  of one or both of first and second safety devices  801 A and  801 B does not have to include intermediate hole section  839  and second abutment face  853 . In this case, second hole section  837  extends to first hole section  835 , but second end  807  of first sleeve  803  is adjacent to first abutment face  851  of second sleeve  817 . Further, the inner diameter of large section  811  is smaller than the inner diameter of first hole section  835 , such that an end  879  of second stop  877 B abuts first abutment face  851  to keep second safety pin  857 B in the retracted position. 
     Furthermore, only first or second safety device  801 A,  801 B can be utilized in top latch  24  or bottom latch  25 . Further, engagement of only one of first and second safety pin  857 A and  857 B with base  3  of top latch  24  or bottom latch  25  is sufficient to retain latch  41  thereof in position. 
     Thus since the invention disclosed herein may be embodied in other specific forms without departing from the spirit or general characteristics thereof, some of which forms have been indicated, the embodiments described herein are to be considered in all respects illustrative and not restrictive. The scope of the invention is to be indicated by the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.