A self-locking hinge for use as a pivotal connector between two pivoted members and adapted to lock the members in a fixed angular relation. The hinge comprises a pair of circular plates that are rigidly affixed to opposite faces of one member and rotatably connected to the other member. The circular plates are used to laterally support both of the members and to guide the other member as it is rotated from a folded position to a fully extended position. The hinge is locked in the fully extended position by means of a locking ring that is slidably engaged with the other member. When such other member is fully extended, the ring engages corresponding locking indentations at the periphery of the circular plates and thereby blocks the movement of the other member in one angular direction. The movement of the other member is blocked in the opposite angular direction due to the abutting of the hinge ends of the members. The members may be returned to the folded position by releasing the locking ring from its engagement with the indentations in the circular plates. The operation of the self-locking hinge of the invention is particularly described with respect to a door security apparatus adopted to engage a door and a floor to resist the opening of the door. A further embodiment uses the self-locking hinge of the invention to provide a means to convert a stepladder to a straight ladder of increased height.

BACKGROUND OF THE INVENTION 
The invention relates to a self-locking hinge and, more particularly, to 
such a hinge including means to provide support for axial members attached 
to the hinge and to lock the hinge in a position that resists damage to 
the hinge when force is applied to the axial members. 
Prior art hinges have not combined the simplicity of operation, ability to 
withstand stress, and locking capabilities of the present invention. The 
patent to McGee, No. 1,673,577, is an example of a prior art circular 
plate locking hinge that is not resistant to stress. The circular plate 
hinge of McGee pivots at a single point and uses a sliding ring with a 
locking tab to engage a notch in the plate to lock the hinge in position. 
The use of a single pivotal point renders the McGee hinge susceptible to 
damage when force is applied since the force is concentrated at a single 
point rather than distributed over the hinge. In addition, the McGee 
locking tab is a structurally weak means to lock the hinge since the tab 
itself must resist any angular force that is applied to the hinge. 
Accordingly, it is an object of the invention to provide a simple and 
effective means to distribute the forces applied to a hinge to minimize 
the force-induced strain on the components of the hinge. 
A further object of the invention is to provide a hinge locking means that 
is not dependent upon the strength of a single locking component. 
Another object of the invention is to provide a hinge that will laterally 
support extending hinge members and will not pivot the members about a 
single stress-sensitive pivot point. 
A further object of the invention is to provide a more durable hinge that 
includes stationary hinge guide plates rather than movable plates that are 
more susceptible to damage. 
A further object of the invention is to provide locking hinge embodiments 
that utilize a pivotal connection lockable in a force resistant position. 
These and other objects of the invention will become apparent from a review 
of the detailed specification which follows and a consideration of the 
accompanying drawings. 
BRIEF SUMMARY OF THE INVENTION 
In order to achieve the objects of the invention and to overcome the 
problems of the prior art, the self-locking hinge, according to the 
present invention, includes a pair of circular hinge plates that are 
rigidly affixed to a support hinge member and rotatably connected to a 
pivotal hinge member. The stationary plates and the separate pivotal point 
for the pivotal member reduce the effect of stress on the hinge when force 
is applied to the members, since the resultant force is not applied to a 
single pivotal point but is distributed over the surface of the circular 
plates. 
A locking ring is slidably engaged with the pivotal member and is adapted 
to slide into corresponding indentations of the circular hinge plates when 
the pivotal member is fully extended at an angular relation of 180 degrees 
with respect to the support member. The corresponding identations of the 
discs have a single vertical wall that blocks the movement of the pivotal 
member in one direction when the ring is engaged with the indentations. 
Since the body of the ring is engaged with the indentation, movement of 
the pivotal member is blocked by the inherent strength of the ring itself. 
In addition, the strength of the ring is enhanced due to its slidable 
connection with the pivotal member. 
The movement of the pivotal member is blocked in an opposite direction due 
to the abutting of the ends of the support member and the pivotal member 
when the pivotal member is fully extended. Thus, the engaged ring and the 
abutted end surfaces completely block the movement of the pivotal member. 
The hinge area of the pivotal member and the support member are laterally 
supported by the circular plates, and any forces that are applied to the 
hinge members are distributed over the rigid members themselves or the 
sturdy circular discs. 
An embodiment of a door security device that includes the self-locking 
hinge of the invention is also disclosed. The upper member of the door 
security hinge is adapted to adjustably extend in an axial direction to 
engage the doorknob of a closed door, and the lower support member of the 
hinge is adapted to axially extend to engage the floor. When the door 
security device is locked in the extended position, it provides a means to 
resist the application of an opening force to the door. 
Thus, if an opening force is applied to the door, the force is transmitted 
from the inside doorknob through the upper and lower hinge members to the 
floor. As is apparent from the above discussion, the structure of the 
hinge is adapted to withstand large forces applied to the members and 
therefore strongly resist any opening force that is applied to the door. 
In addition, unlike the more bulky and complicated prior art door security 
devices, the door security device of the present invention can be easily 
and quickly removed from its engagement with the door to allow an 
emergency exit, if necessary. Furthermore, a rubberized foot member of the 
device provides an exceptional frictional engagement with the floor when 
the device is engaged with the door and also ensures that the floor is not 
marred when the device is in use. Finally, due to the pivoting of the 
hinge, the door security device can be easily folded up when not in use 
and, in the folded position, it can easily be stored for later use or can 
be carried in a small suitcase to be used to secure hotel room doors, for 
example, when one is traveling. 
The self-locking hinge of the invention is also included in an embodiment 
of an improved collapsible stepladder that may be converted to a straight 
ladder of increased length. The ladder employs self-locking hinges to 
pivotally connect a step side and a support side of the ladder in a normal 
stepladder position. The support side of the ladder is provided with steps 
that are not used when the ladder is in the normal position but that are 
used when the support side is pivoted and locked at an angle of 180 
degrees with respect to the step side. 
The self-locking hinges are attached to the pivotal ends of corresponding 
opposite step rails and support rails to provide a pivotal connection for 
folding the ladder, and for defining a normal or extended stepladder 
position. In the extended straight ladder position, locking rings block 
the angular movement of the pivotal support rails of the ladder in a 
folding direction and the abutting ends of corresponding opposite step and 
support rails of the ladder block the angular movement of the support 
rails in an opposite direction. Thus, the locked hinges provide rigid 
connections that are resistant to stress on the pivotal and support 
members of the ladder while the ladder is in the extended position. 
It should be readily apparent from the above discussion that the 
self-locking hinge of the invention is not limited to the described 
embodiments. Indeed, the hinge is well adapted for use whenever a strong, 
force-resistant, pivotal connection is required and is particularly suited 
to applications that require a locking engagement of axial hinged members 
at 180 degrees. For instance, the hinge could be easily used by those 
skilled in the art to construct an extendable tree trimming pole or to 
provide a simple and reliable means to lock an extending antenna in an 
upright direction. In addition, the hinge could be used to provide sturdy 
and rigid joint connections of the type used in scaffolding.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The remaining portion of this specification will describe preferred 
embodiments of the invention when read in conjunction with the attached 
drawings, in which like reference characters identify identical apparatus. 
FIG. 6 illustrates the construction of a self-locking hinge in accord with 
the present invention. As shown in FIG. 6, a lower support tube 1 is 
connected to an upper pivotal tube 2 by the interconnection of a mounting 
block 3 and circular guide plates 4. 
The lower support tube 1 is rigidly affixed to the circular guide plates 4 
by means of support hinge bolts 5. The mounting block 3 is attached to a 
lateral face of the upper pivotal tube 2 by means of mounting block bolts 
6, and the mounting block is pivotally connected to guide plates 4 by 
means of a pivotal hinge bolt 7. The upper and lower tubes are positioned 
with respect to the guide plates 4 so that when the pivotal tube 2 is 
fully extended at an angle of substantially 180 degrees with respect to 
the lower support tube 1, the respective ends of the tubes abut one 
another. Thus, the pivotal tube 2 is adapted to pivot from a fully folded 
position to a fully extended position that is defined by the abutting of 
the ends of the tubes. 
The relative movement of the components of the hinge and the locking action 
of the hinge is shown in the illustrations of a door security embodiment 
in FIGS. 1-5. In FIG. 4, the upper pivotal tube 2 is connected to the 
mounting block 3 as explained above, and is telescopically engaged with an 
extendable door engaging member 8. The door engaging member 8 includes an 
upper rectangular bar 21 that is dimensioned so that it can easily slide 
within tube 2. Holes 9 are drilled at intervals along the bar 21 and a 
hole 10 is drilled near the top end of the tube 2. When member 8 is 
slidably engaged with tube 2, the extension of member 8 may be adjusted by 
aligning the hole 10 with a particular hole 9 and inserting a locking pin 
19, as shown in FIG. 2, through the aligned holes to fix the position of 
the member 8. Of course, the adjustable extension means disclosed herein 
is for illustrative purposes only and other extension means known to the 
art could be used to accomplish the same purpose. 
As shown in FIG. 4, a locking ring 11 encloses tube 2 and is adapted to 
slide along the axis of tube 2 to engage corresponding locking 
indentations 12 in the guide plates 4 when the pivotal tube 2 is locked in 
the fully extended position. 
A tubular handle 13 is attached at a point above the mounting block 3 to 
the upper pivotal tube 2, for instance by welding, and provides a means to 
force the door security device into a locked position. The handle 13 and 
the top of the mounting block 3 define a sliding area for the locking ring 
11. 
The bar portion 22 of a lower extendable foot member 14 is dimensioned to 
slide inside of the lower support tube 1 and to thereby provide an 
adjustable extension means to engage a floor. Holes 15 are provided in the 
bar portion 22 and a hole 16 is provided near the end of the tube 1. The 
extension of the foot member 14 is adjusted by aligning a particular hole 
15 with the hole 16 and inserting a locking pin 20 through the aligned 
holes to fix the extended foot member 14 in position. 
A pivotal foot 17 is pivotally attached to the free end of the bar portion 
22 of the extendable foot member 14 so that the foot 17 may frictionally 
engage the floor when the door security device is locked in position. The 
pivotal attachment point is closer to the front of the foot in order to 
permit the large surface area at the rear of the foot to frictionally 
contact the floor when the door security device is positioned. A 
rubberized or other nonslip material is adhered to the bottom of the foot 
in order to provide a strong frictional contact between the foot and the 
floor. 
A tubular U-shaped yoke 18 is attached to the top bar portion 21 of the 
door engaging member 8, for instance by welding, to enable the member 8 to 
engage a doorknob on a closed door when the door security device is in a 
locked relation with respect to the door. 
In operation, the hinge of the door security device provides for a pivotal 
movement of the tube 2 from a folded position, as illustrated in FIG. 3, 
to a fully extended locked position as illustrated in FIGS. 1 and 5. As 
shown in FIG. 3, in the fully folded position the tube 2 is pivoted about 
the pivotal bolt 7 until the mounting block 3 contacts the surface of the 
lower support tube 1. The separation of the upper tube 2 from the lower 
tube 1 by the mounting block 3 in the fully folded position facilitates 
the handling of the unit when it is collapsed. 
When the tube 2 is pivoted from the fully folded position to the fully 
extended position, the ring 11 slides along the outer periphery of the 
plates 4 until the tube 2 is fully extended and the ring 11 is engaged 
with the indentations 12. It is noted that at all times during the 
rotation of the upper pivotal tube 2, a portion of the lower support tube 
1 and the pivotal tube 2 remains within the confines of both circular 
guide plates 4, thereby ensuring that both tubes are laterally supported 
by the guide members 4 so that the strain on the hinges is minimized. 
As shown in FIG. 1, in the locked position the body of the ring 11 fully 
engages the indentations 12 to prevent angular movement of the pivotal 
tube 2 in the folding direction. The movement of the pivotal tube 2 in an 
opposite angular direction is blocked by the abutting of the hinge ends of 
the tube 2 and the tube 1. Thus, in the locked position, the tubes are 
rigidly held in place by the inherent strength of the ring 11, the large 
indentations 12, and pressure on the abutting tubes themselves. In 
addition, it is noted that the overlap of the plates 4 with the upper tube 
2 and the lower tube 1 and the skewing of the pivotal point of tube 2 from 
the stationary attachment points of tube 1 contribute to the dissipation 
of forces applied to tubes 2 and 1 and thereby ensure added durability of 
the hinge of the invention. 
FIG. 5 illustrates the operation of the door security device. The device is 
initially positioned at an angular relation intermediate the fully folded 
and locked positions by adjusting the extension of the door engaging 
member 8 and the foot member 14 so that the yoke 18 is secured under a 
doorknob of a closed door and the pivotal foot 17 is contacting the 
surface of the floor. A downward force is then applied to the handle 13 to 
move the pivotal tube 2 from its intermediate angular position to a fully 
extended locked position. 
In the locked position, the distance between the yoke 18 and the pivotal 
foot 17 is maximized and the counterforce that is applied through the 
tubes 2 and 1 to the door is likewise maximized. Once the door security 
device is locked in position, it is apparent that the device will resist 
the opening of the door by providing a counterforce to any force that is 
applied to open the door. As is apparent from the above discussion, the 
door security device, according to the present invention, will counter the 
door opening force without unduly straining the hinge components of the 
device. In particular, it is noted that the abutting ends of the upper 
tube 2 and the lower tube 1 provide a means to transfer the applied door 
opening force directly through the tubes and to thereby avoid placing any 
portion of the force on the components of the hinge. Thus, the resistance 
of the door security device is maximized and the problem of hinge failure 
due to excessive door opening force is avoided. In addition, the device 
may be easily and quickly disengaged from the door by pressing on the 
handle 13 and disengaging the locking ring 11 from the indentations 12. 
It is noted that prior art hinges do not provide such a means to eliminate 
strain on the hinge and thus cannot maximize the ability of a door prop 
device to resist the application of a door opening force. In addition, 
prior art devices cannot be so easily and quickly disengaged from the 
door. 
A second embodiment of the present invention is illustrated in FIGS. 7-9. 
FIG. 7 illustrates the use of the hinge of the invention in a stepladder 
that is adapted to be converted into a straight ladder of increased 
height. As shown in FIG. 8, a hinge assembly 29 is provided for each of 
the stationary step rails 23 that make up the step side 24 of the ladder. 
The rails 23 are affixed to their respective circular guide plates 4 by 
the associated bolts 5. As shown in FIG. 7, the corresponding opposite 
pivotal support rails 27 on the support side 28 of the ladder are attached 
to their respective mounting blocks 3. The mounting blocks 3 are pivotally 
connected to their respective guide plates 4 by means of associated 
pivotal hinge bolts 7. Thus, each hinge assembly 29 connects a particular 
stationary rail 23 on the step side 24 of the ladder to an opposite 
pivotal support rail 27 on the support side 28 of the ladder. 
As shown in FIG. 8, the inside guide plates of the hinge assemblies are 
connected by a hinge step 25 that may be affixed to each plate by welding. 
In addition, it is noted that the support rails 27 are cross-connected by 
support steps 32 that are oriented in an upside-down direction when the 
ladder is in a normal stepladder position, as illustrated in FIG. 7. In 
the normal ladder position, the steps 33 on the step side 24 are, of 
course, oriented in their normal upright step position. 
The ladder is maintained in the normal step position by cross-support bars 
30 that each connect a particular stationary rail 23 with an opposite 
support rail 27. The cross-support bars 30 are adapted to extend to define 
a normal ladder position and to pivotally collapse to allow folding of the 
ladder. 
A locking ring 11 is slidably engaged with each of the support rails 27 
between the top step of the support rails and the hinge assembly. Since 
the inside diameter of the ring is slightly larger than the outside 
diameter of the support rail, the ring is able to freely slide within the 
area defined by the top step of the support rail and the hinge assembly. 
Thus, in the normal stepladder position the rings 11 are disengaged from 
their associated guide plates 4, each stationary rail 23 is at an 
intermediate angular relation with its associated opposite support rail 
27, and the extended cross-support bars 30 fix the ladder in position. It 
is obvious from the above that when the ladder is in the normal stepladder 
position, the steps 33 on the step side 24 are used to climb the ladder. 
FIG. 9 illustrates the relation of the components of the improved ladder 
when the ladder is in the extended straight ladder position. The ladder is 
moved to the straight ladder position by disengaging the cross-support 
bars 30 from their respective stationary rails 23 and support rails 27. 
The engagement or disengagement of the cross-support bars with the rails 
of the ladder can be accomplished by means well known to the art. For 
instance, the ends of the cross-support bars could be bolted to the 
appropriate rails in the normal stepladder position and the bolts could be 
removed to disengage the cross-support bars in the extended straight 
ladder position. 
When the cross-support bars 30 have been disengaged, the pivotal support 
rails 27 are rotated about the pivotal bolts 7 of their respective hinge 
assemblies 29. As the pivotal support rails are rotated, the rings 11 ride 
on the outer periphery of their respective guide plates 4 and, when the 
support rails 27 are fully extended and the angular relation between the 
support rails 27 and their associated step rails 23 is substantially 180 
degrees, the locking rings 11 drop into engagement with their respective 
guide plate indentations 12. 
As shown in FIG. 9, the engagement of the rings 11 with the indentations 12 
blocks the angular movement of the pivotal support rails 27 in a folding 
direction. The movement of the support rails 27 in an opposite direction 
is blocked by the abutting of the hinge ends of the step rails 23 with 
their respective opposite support rails 27. Thus, the locking rings and 
the abutting ends of the rails completely block the angular movement of 
the pivotal support rails 27. As shown in FIG. 9, when the support rails 
27 are in the extended and locked straight ladder position, the support 
steps 32 are properly oriented to allow climbing of the upper support 
section of the extended ladder. 
Surface engaging feet 26 are pivotally connected to the free ends of the 
stationary rails 23 and the support rails 27. In the extended locked 
position, as shown in FIG. 9, the feet on the stationary rails 23 are 
adapted to frictionally engage the ground and the feet on the support 
rails 27 are adapted to frictionally engage the surface against which the 
extended ladder is leaning. FIG. 7 illustrates the ground engagement of 
all of the pivotal feet when the ladder is in the normal stepladder 
position. 
Thus, as explained above, the self-locking hinge of the invention provides 
a means to convert a stepladder to an extended straight ladder of 
increased height and, more importantly, provides a hinge means that is 
resistant to the damaging effects of stress in a locked position. 
The invention may be embodied in other specific forms without departing 
from its spirit or essential characteristics. The present embodiments are, 
therefore, to be considered in all respects as illustrative and not 
restrictive, the scope of the invention being indicated by the claims 
rather than by the foregoing description, and all changes which come 
within the meaning and range of the equivalents of the claims are 
therefore intended to be embraced therein.