BACKGROUND OF THE INVENTION Many locking devices are known that may be used to lock doors by protruding from an escutcheon into a recess provided in a door frame and the like. A simple device is a door latch, which may be spring-loaded and actuated by a door knob or cylinder lock. Other locking devices that provide better locking security include, for example, dead bolts, which generally protrude straight into a recess, and drop bolts, which generally hook into a recess by rotary motion. However, the known drop bolt mechanisms lack the ability to geometrically lock at the locked position.

SUMMARY OF THE INVENTION The present invention seeks to provide an improved locking mechanism for geometrically locking a drop bolt, as described more in detail hereinbelow.

There is thus provided in accordance with an embodiment of the present invention a locking mechanism comprising a drop bolt geometrically lockable at a limit of travel, and a linkage device linked to the drop bolt and connectable to a door lock bolt, the linkage device being arranged for movement that causes the drop bolt to be moved to and geometrically locked at the limit of travel.

The locking mechanism can include one or more of the following features. For example, the drop bolt may be arranged to move from a first limit of linear travel to a second limit of linear travel. The drop bolt may be rotatable about a drop bolt pivot at the second limit of linear travel. A portion of the linkage device may be arranged to move linearly in a first direction as the drop bolt moves linearly from the first limit of linear travel to the second limit of linear travel, and the portion of the linkage device may move into a geometrically locked position as the drop bolt rotates about the drop bolt pivot at the second limit of linear travel. The linkage device may cause the drop bolt to rotate about the drop bolt pivot at the second limit of linear travel as the linkage device moves into a geometrically locked position. The linkage device may be arranged to move linearly in a second direction different from the first direction as the linkage device moves into the geometrically locked position. The linkage device may comprise a first linkage element that is engageable with a door lock bolt, the first linkage element being rotatably mounted about a first pivot, a second linkage element pivotally linked to the first linkage element, and a third linkage element pivotally linked at a first portion thereof to the second linkage element and pivotally linked at a second portion thereof to the drop bolt. The drop bolt may be arranged to move linearly from a first limit of linear travel to a second limit of linear travel and is rotatable about a drop bolt pivot at the second limit of linear travel, and wherein the third linkage element may be arranged to move linearly in a first direction as the drop bolt moves linearly from the first limit of linear travel to the second limit of linear travel, and the third linkage element may move into a geometrically locked position as the drop bolt rotates about the drop bolt pivot at the second limit of linear travel. The first and second portions of the third linkage element may be constrained to move linearly in first and second tracks, respectively. The second track may comprise an initial portion generally parallel to the first track and an extension portion angled with respect to the initial portion, wherein movement of the third linkage element to a limit of travel in the extension portion geometrically locks the linkage device and the drop bolt. The first linkage element may comprise a hook portion that is engageable with a door lock bolt. The first linkage element may be formed with a notch engageable with a pin when the drop bolt is in a retracted position. An escutcheon may be provided, wherein the drop bolt protrudes from the escutcheon when geometrically locked at the limit of travel, and wherein the drop bolt does not protrude from the escutcheon when in a retracted position. The locking mechanism may include a drop bolt geometrically lockable at a limit of travel, and a linear motion element linked to the drop bolt by a linkage device, the linear motion element being arranged for linear movement that causes the drop bolt to be moved to and geometrically locked at the limit of travel.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: Figs. 1A and 1B are simplified plan view and perspective illustrations, respectively, of a locking mechanism, constructed and operative in accordance with an embodiment of the present invention, wherein the locking mechanism is in a retracted position; Figs. 2A and 2B are simplified plan view and perspective illustrations, respectively, of the locking mechanism of Figs. 1A and IB, wherein the loclcing mechanism is progressively moved from a first limit of travel to a second limit of travel; and Figs. 3A and 3B are simplified plan view and perspective illustrations, respectively, of the locking mechanism of Figs. 1A and 1B, wherein the locking mechanism is geometrically locked at a limit of travel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Reference is now made to Figs. 1A and 1B, which illustrate a locking mechanism 10, constructed and operative in accordance with an embodiment of the present invention.

Locking mechanism 10 may comprise a drop bolt 12, and a linkage device 14 linked to drop bolt 12 and connectable to a door lock bolt (not shown). The linkage device 14 may be arranged for movement that causes drop bolt 12 to be moved to and geometrically locked at a limit of travel, as is described further in detail hereinbelow.

In the illustrated embodiment, linkage device 14 may comprise a first linkage element 16 that is engageable with the door lock bolt, such as by means of a hook portion 18. First linkage element 16 may be rotatably mounted about a first pivot 20, and may be formed with a notch 22 engageable with a pin 24 when drop bolt 12 is in a retracted position, wherein drop bolt 12 does not protrude from an escutcheon 26, as shown in Figs. 1A and 1B.

A second linkage element 28 may be pivotally linked to first linkage element 16 at a pivot 30. A third linkage element 32 may be pivotally linked at a first portion 34 thereof to second linkage element 28 and pivotally linked at a second portion 36 thereof to drop bolt 12.

The first and second portions 34 and 36 of third linkage element 32 may be constrained to move linearly in first and second tracks 38 and 40, respectively. The second track 40 may comprise an initial portion 42 generally parallel to the first track 38, and an extension portion 44 angled with respect to initial portion 42.

Drop bolt 12 may include a hooked end 46 and may be constrained to move linearly by a pin 48 sliding in a track 50. Drop bolt 12 may pivot about pin 48, and pin 48 is accordingly referred to as drop bolt pivot 48.

It is emphasized that the illustrated embodiment is just one example of carrying out the invention, and the invention is not limited to the illustrated linkage device.

Figs. 1A and 1B illustrate drop bolt 12 at a first linear limit of travel, wherein the pinned connections of drop bolt 12 and third linkage element 32 are at the ends of their respective tracks furthest from the escutcheon 26 and drop bolt 12 does not protrude from escutcheon 26.

Reference is now made to Figs. 2A and 2B, which illustrate the locking mechanism 10 being progressively moved from the first limit of travel towards a second limit of travel.

Actuation of the locking mechanism 10 may be initiated by rotating first linkage element 16 about pivot 20 in the direction of an arrow 52. This may be accomplished by the door lock bolt (not shown) lifting first linkage element 16 in the direction of an arrow 54. The door lock bolt may be thrown by a door lock or lock cylinder (not shown), as is known in the art.

The rotation of first linkage element 16 pushes third linkage element 32, via second linkage element 28, and slides third linkage element 32 linearly in a first direction indicated by arrow 56 along first track 38. The movement of third linkage element 32 causes drop bolt 12 to move from its first limit of linear travel (Figs. 1A and 1B) to a second limit of linear travel at the opposite end of tracks 40 and 50 (Figs. 2A and 2B). At this point, drop bolt 12 linearly protrudes from escutcheon 26.

Reference is now made to Figs. 3A and 3B. Further rotation of first linkage element 16 about pivot 20 in the direction of arrow 52 moves second portion 36 of third linkage element 32 along extension portion 44 in a second direction, indicated by an arrow 58, different from first direction 56. This causes drop bolt 12 to pivot about drop bolt pivot 48 as indicated by an arrow 60, this pivoting occurring at the second limit of linear travel of drop bolt 12. Once third linkage element 32 is fully received in extension portion 44, third linkage element 32 is geometrically locked in extension portion 44, thereby also geometrically locking drop bolt 12 at its second limit of linear travel. Thus, linkage device 14 causes drop bolt 12 to be moved to and geometrically locked at a limit of travel.

It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof which would occur to a person of skill in the art upon reading the foregoing description and which are not in the prior art.