Door latch

A door latch mechanism mounts on a door for selectively engaging a door strike. A latch bolt moves along a line of action to disengage the strike and allow the door to be opened. A dead lock prevents unwanted opening of the door and includes a blocking element to prevent movement of the latch bolt along its line of action. A release enables the latch bolt to move along its line of action and includes a bell crank for displacing the blocking element whereby the latch bolt can disengage the strike.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to door latches and more particularly, to a door 
latch mechanism for emergency exit doors. 
Applicant is aware of the following U.S. patents, the disclosures of which 
are incorporated by reference herein. 
U.S. Pat. No. 1,203,116 
U.S. Pat. No. 1,430,905 
U.S. Pat. No. 1,706,486 
U.S. Pat. No. 3,325,203 
U.S. Pat. No. 3,189,142 
U.S. Pat. No. 3,705,379 
U.S. Pat. No. 3,819,213 
U.S. Pat. No. 3,869,159 
U.S. Pat. No. 4,007,954 
U.S. Pat. No. 4,015,869 
U.S. Pat. No. 4,083,590 
U.S. Pat. No. 4,311,329 
U.S. Pat. No. 1,253,545 
U.S. Pat. No. 1,612,480 
U.S. Pat. No. 2,824,440 
U.S. Pat. No. 3,335,509 
U.S. Pat. No. 3,477,260 
U.S. Pat. No. 3,811,717 
U.S. Pat. No. 3,854,763 
U.S. Pat. No. 3,877,262 
U.S. Pat. No. 4,012,066 
U.S. Pat. No. 4,074,548 
U.S. Pat. No. 4,183,565 
U.S. Pat. No. 4,384,738 
Emergency exit doors, as typically known in the art, serve the function of 
providing access from a building or area only under certain extraordinary 
situations. Most of the time these doors remain closed and it is a 
particular requirement of such doors that they be tamperproof, i.e. not 
openable by unauthorized persons using a credit card, pry bar or other 
tool. While the need for their tamperproof feature or "dead lock" is 
apparent, at the same time, this feature should not unduly complicate the 
latching and unlatching mechanism or else the possibility of this 
mechanism jamming or becoming otherwise inoperative increases. And, 
despite the importance of foregoing, it is also important to avoid 
impractical and costly alternatives such as having the the deadlock 
mechanism separate from the latching mechanism. 
Certain doors, such as those in schools and other public buildings, must 
remain locked during a fire in order to prevent its spread. This means the 
latching mechanism must include a fire or heat responsive element, a 
fusible material, which renders the latching mechanism inoperative in the 
presence of a fire or intense heat. The material, or a portion of the 
mechanism using it, must be easily activated in the event of a fire to 
make the door unoperable while, under more usual circumstances, not 
interferring with normal functioning of the latch mechanism. 
Finally, the latching mechanism itself should be simple so the deadlock and 
fusible element integrate easily with it. The mechanism should be readily 
installed on a door, and once installed, should require little or no 
maintenance for proper operation. 
The present invention is advantageous in that it provides a latching 
mechanism for emergency exit doors that is readily installed on the door. 
The mechanism incorporates a latch bolt that moves along a line of action 
to engage or disengage a door strike, then allowing the door to be closed 
and opened. The mechanism also incorporates a dead lock feature that 
blocks movement of the latch bolt along its line of action to provide a 
relatively simple, yet effective, dead lock that prevents tampering. The 
latch also includes a release mechanism which disengages the dead lock so 
the door can be opened at the necessary or proper times. This release 
mechanism also restores the dead lock when it is intended for the door not 
to be operable. 
In addition to the above, the latch has a fusible lock feature that 
operates in the presence of fire or intense heat to make the release 
mechanism inoperable. Consequently the door cannot be opened in these 
situations. At all other times, however, the fusible lock does not 
interfere with operation of the release and opening or closing of the 
door. 
The latch preferably has an upper and a lower latch bolt which engage 
respective upper and lower door strikes. Each latch bolt has an associated 
dead lock and the release mechanism is designed to operate both dead locks 
simultaneously to enable the door to be opened and to restore them 
simultaneously when it is intended that the door not be opened.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to the drawings, a door latch mechanism 1 of the present 
invention is indicated generally in FIG. 1. The mechanism 1 is designed 
for installation on a door 3 which may, for example, be an emergency exit 
door. As such, it is intended that door 3 be normally closed but quickly 
openable in the event of an emergency. Since the door is meant to normally 
closed, it should not be openable by someone tampering with it or with the 
latch mechanism. Door 3 has an associated first door strike 5 mounted to 
the door frame or the like adjacent to the top of the door. The strike 5 
has a base plate 7 which is affixed to the door frame by screws 9. The 
strike 5 is generally triangular in shape with a vertically depending flat 
forward face 11, which is offset from the front face 13 of door 3. Strike 
5 has a sloping rear face 15. 
The door 3 also has an associated second door strike 17 mounted on the 
floor 18 at the foot of the door. Strike 17 is similar to strike 5 in that 
it has a bare plate 19 which is affixed to the floor by screws. The strike 
17 is again generally triangular in shape with a vertically extending flat 
forward face 21 and a sloping rear face 23. 
Latch mechanism 1 selectively engages the door strikes to keep door 3 
closed; and for this purpose the mechanism includes a latch bolt means 25. 
Means 25 includes a base plate 27 secured to the upper end of door face 13 
by screws 29. Side plates 31A and 31B extend outwardly from the opposite 
sides of the base plate 27 to form a U-shaped frame 33, the channel formed 
by the base plate 27 and side plates 31A and 31B being vertically oriented 
when the assembly is mounted on the door 3. A cover 34 fits over the frame 
33. 
A latch bolt 35 is mounted between the side plates 31 at the upper end of 
the frame 33. A pivot pin 37 extends between the side plates 31A and 31B 
with the respective ends of the pin 37 projecting through corresponding 
holes in the upper end of the respective side plates 31A and 31B. The ends 
of the pin 37 have retaining rings or other means to secure the pin 37 to 
the frame 33. The latch bolt 35 is of segmental shape with an arcuate face 
39 abutting face 15 of strike 5. With the latch bolt 35 in its position 
engaging the strike 5, the latch bolt 35 has an exposed, flat, outer face 
41. It is possible that someone wishing to tamper with the door 3 may try 
to open door 3 by pushing against face 41 with a credit card or pry tool. 
To prevent unwanted opening of door 3, mechanism 1 includes a deadlock 
means 43. Means 43 includes an arm 45 attached to the latch bolt 35 and 
extending from the latch bolt 35 along its line of action. Arm 45 
comprises two parallel plates 45A and 45B which abut each other and have 
rounded upper ends and tapered lower ends, as shown. Latch bolt 35 has a 
central web of material 47 on its underside and the upper ends of plates 
45A and 45B are attached to opposite sides of the web 47 by a pin 49. 
Dead lock means 43 further includes a pin 51 extending transversely of the 
line of action of the latch bolt 35. Pin 51 is attached to side plates 31A 
and 31B and the distal end of arm 45 bears against the pin 51 when the 
latch bolt 35 is in its strike engaging or extending position. 
Consequently, when someone tries to tamper with the latch bolt 35 by 
pushing against face 41, the arm 45 bearing against pin 51 prevents inward 
movement of the latch bolt 35. 
To allow door 3 to be opened, mechanism 1 also includes a release means 53. 
Means 53 enables latch bolt 35 to move inwardly along its line of action 
away from strike 5. For this purpose, the release means includes a bell 
crank 55 which pivots about pin 51. Arm 45 is attached, at its lower end, 
to the bell crank 55 by a pin 57. Thus, when the bell crank 55 is rotated 
about pin 51, in a manner to be described, arm 45 is displaced from its 
position abutting pin 51 and latch bolt 35 is freed to move along its line 
of action and disengage strike 5. 
The release means 53 includes a manually operable bar 59 which is mounted 
to door 3 as shown in FIG. 1. Operation of bar 59 is well known in the art 
and will not be described in detail. As shown in FIG. 2, a frame assembly 
61 is secured to door 3 at approximately waist height (see FIG. 1). A 
cover 63 fits over the frame and bar 59 protrudes through an opening in 
the cover 63. A U-shaped bracket 65 is attached to base plate 67 of the 
frame assembly 61 and a bell crank 69 is rotatably secured to one arm of 
the bracket 65 by a pin 71. An arm 73 of bell crank 69 has an extension 74 
connected to bar 59. When the bar 59 is moved to the right, as shown in 
FIG. 2, the bell crank 69 is caused to rotate counterclockwise (CCW). 
Release means 53 includes a linkage means 75 comprising an upper linkage 
rod 77 connected to a central link 79. Link 79 has a middle section 81 
which includes a vertical slot 83. A pin 85 extending outwardly from 
bracket 65 fits in slot 83 to limit the vertical movement of the linkage 
means 75. Link 79 has a shoulder 87 formed at the upper end of section 81 
and an arm 89 of bell crank 69 bears against this shoulder 87. At the 
position shown in FIG. 2, pin 85 is at the upper end of slot 83. 
Consequently, when bar 59 is pushed inwardly, the resultant CCW rotation 
of bell crank 69 causes linkage means 75 to be moved upwardly by the force 
exerted on shoulder 87 by crank arm 89. The extent of this movement is 
limited by the length of slot 83. 
A yoke shaped link 91 has its stem end secured between plates 55A and 55B 
of bell crank 55 by a pin 93. Plates 31A and 31B have corresponding 
vertical slots 95A and 95B formed at their lower end and a pin 97 extends 
between the plates 31A and 31B and fits through the respective slots 95A 
and 95B. The bifurcated arms of link 91 have holes through which pin 97 
fits. The upper end of rod 77 is internally threaded and an adjustment 
plug 99 is screwed into this end. Plug 99 has a cross bore 101 through 
which pin 97 also fits. Consequently, upward movement of rod 77 produces 
concomitant movement of link 91 which, in turn, produces CCW rotation of 
bell crank 55 (as seen in FIGS. 2 and 3). Since arm 45 is pinned to the 
bell crank 55, rotation of the bell crank 55 pulls the arm 45 away from 
pin 51, thereby displacing the element (arm 45) blocking movement of latch 
bolt 35 along its line of action. The above described action produces 
inward movement of the latch bolt 35 and it disengages strike 5. 
As this movement occurs, a latch lever 103 is moved from its retracted 
position shown in FIG. 2 to its extended position shown in FIG. 3. Lever 
103 has a base plate 105 abutting face 11 of strike 5 when latch bolt 35 
is in its position engaging the strike 5. The lever 103 has side rails 
107A and 107B. These side rails 107A and 107B have corresponding openings 
through which pin 51 is inserted. Thus, lever 103 pivots about pin 51. 
Each side rail 107A and 107B has an outwardly extending inverted L-shaped 
arm, 109A and 109B respectively, the base legs of which meet so to form a 
generally inverted U-shaped member when viewed as shown in FIG. 4. 
A coil spring 111, having legs 111A and 111B, is mounted on pin 37. The 
legs 111A and 111B of the spring 111 bear against the respective arms 109A 
and 109B. The center of spring 111 is positioned on the underside of face 
41 of latch bolt 35 and is coiled around pin 37. As the latch bolt 35 
disengages strikes 5, the inward movement of the latch bolt 35 causes the 
underside of face 41 to bear against an end 111B of spring 111. This, in 
turn, causes the end 111A to urge lever 103 in a CCW direction by exerting 
a force on lever arms 109A and 109B. Each of side rails 107A and 107B of 
the lever 103 has detents 113 and 114. As latch bolt 35 moves inwardly 
along its line of action disengaging strike 5, the simultaneous CCW 
movement of lever 103 causes the detents 113 or 114 to capture pin 49. 
When this occurs, latch bolt 35 is completely withdrawn into frame 33 and 
is clear of strike 5. Thus, door 3 can be opened. 
The latch bolt 35/lever 103 arrangement is held in a retracted position at 
this point. That is, latch bolt 35 cannot move because pin 49 is held in 
the detents 113 or 114 in the side rails 107A and 107B and lever 103 
cannot be further extended for the same reason. 
When door 3 is reclosed, base plate 105 of lever 103 strikes face 11 of 
strike 5 thus moving lever 103 and disengaging pin 49 allowing latch bolt 
35 to move outward along its line of action due to spring 111 and gravity. 
This simultaneously allows the linkage means 75 to return to position and 
causes tapered end of arm 45 to sit atop of pin 51, as shown in FIG. 2. 
The latch bolt 35 re-engages strike 5. This restores mechanism 1 to its 
original state with door 3 closed. 
With the lock mechanism 1 restored to its original position, arm 45 again 
blocks movement of latch bolt 35 along its line of action. 
As previously described, a second door strike 117 may be secured to the 
floor at the base of door 3. Lock mechanism 1 preferably includes a second 
latch bolt means 115 movable along a line of action concurrently with 
first latch bolt means 25 to simultaneously disengage strike 17, when 
strike 5 is disengaged, permitting door 3 to be opened. Means 115 includes 
a base plate 117 secured to the lower end of door face 13 by screws 119. 
Sides plates 121 (only one of which is shown in FIGS. 2 and 3) extend 
outwardly from opposite sides of plate 117 to form a U-shaped frame 123. 
As with frame 33, the channel formed by the base and side plates is 
vertically oriented when the assembly is mounted to the door. A cover 125 
fits over the frame 123. 
A latch bolt 127 is mounted between the sides plates 121 and is of the same 
construction as bolt 35. The bolt 127 is pivotably secured to the side 
plates 121 by a pin 129. An arcuate face 131 of the latch bolt 127 abuts 
face 23 of strike 17. Latch bolt 127 has a spring 132 coiled around pin 
129 and anchored to side plate 121, as shown. Spring 132 urges latch bolt 
127 outwardly into a strike engaging position. 
Lock mechanism 1 includes a second dead lock means 133 for blocking 
movement of latch bolt 127 along its line of action to prevent tampering. 
Means 133 includes an arm 135 comprised of two parallel plates 137, only 
one of which is shown in the drawings. Arm 135 is attached to latch bolt 
127 by a pin 139. The second dead lock means also includes a pin 141 which 
extends transversely of the line of action of latch bolt 127 and is 
attached to the side plates 121. The tapered end of arm 135 presses 
against pin 141 and blocks inward movement of the latch bolt 127. 
To allow door 3 to be opened, mechanism 1 includes a second release means 
143. Means 143 includes a bell crank 145 which pivots about pin 141. Arm 
135 is atached, at its tapered end, to the bell crank 145 by a pin 147. As 
described previously, rotation of the bell crank 145 about its pin 147 
causes the arm 135 to be displaced away from the pin 147 in a pivoting 
action, thus unblocking the latch bolt 127 for inward movement. 
To achieve simultaneous operation of the upper and lower latch bolts 35 and 
127. Linkage means 75 includes a lower linkage rod 149 connected to the 
lower end of central link 79. A lower link 151 is connected to bell crank 
145 by a pin 153. The lower end of rod 149 is internally threaded and an 
adjustment plug 155 is screwed into this end. Plug 155 has a cross bore 
157 and a pin 159 fits through this bore and an opening in a bifurcated 
arm 161 of link 151. Side plates 121 each have a vertical slot 163 through 
which the respective ends of pin 159 extend. 
In operation, upward movement of the linkage means 79, when bar 59 is 
depressed, moved to the right in FIG. 2, results in rod 149 being lifted 
and bell crank 145 rotated clockwise. This action moves arm 135 to the 
left, as shown in FIG. 2, off of pin 141, allowing latch bolt 127 to move 
inwardly along its line of action and disengage strike 17. 
When the door is closed, downward movement of rod 149 rotates bell crank 
145 CCW, moving arm 135 to the right as shown in FIG. 2. This produces 
outward movement of the latch bolt to re-engage strike 17 and the end of 
arm 135 again bears against pin 141 to block inward movement of the latch 
bolt 127. 
Sometimes, for example in the presence of fire or intense heat, it is 
important to render lock mechanism 1 inoperable. This is to prevent door 3 
from opening and providing a path for the fire or its associated heat and 
smoke. For this purpose, lock mechanism 1 includes a heat sensitive means 
165 for inhibiting operation of the release means. Means 165 includes a 
stop 167 of generally rectangular shape with a central rectangular opening 
169. 
Stop 167 is mounted to base plate 27 and held in place by a fusible 
material 170, such as a fusible rivet or grommet. In the presence of fire 
or intense heat, the fusible material 170 melts and the stop 167 falls, by 
gravity, down the channel formed by plate 27 and side walls 31A and 31B. 
Tabs, not shown, extending outwardly from base plate 27 may be provided to 
guide movement of stop 167. 
The stop is initially located on plate 27 so to be immediately behind bell 
crank 55 as shown in FIG. 4. An L-shaped dog 173 may be formed at the 
upper end of the stop 167 and projects outwardly into the channel. As the 
stop falls, this dog wedges itself against the upper end of link 91. Now, 
if someone tries to open the door, the dog 173 prevents movement of the 
linkage by preventing movement of link 91. Thus, the latch bolts 35 and 
127 can't be moved to disengage the strikes and the door remains closed. 
As shown in FIG. 2, dog 173 may be omitted from stop 167A. When the 
fusible material 170 melts, stop 167A falls, by gravity, and wedges bell 
crank 145 to prevent clockwise rotation thus blocking inward movement of 
latch bolts 35 and 127. 
It will be appreciated by those skilled in the art that various changes and 
modifications may be made to the invention disclosed herein without 
departing from the scope of the invention. The invention is not to be 
limited by the specific embodiments given herein for purposes of 
illustration, but only by the scope of the appended claims and their 
equivalents.