Overload detection apparatus

Apparatus for monitoring structures and structural members for overloads has an elongated frangible closed ended tube containing a dense, visible liquid. The tube includes a central reduced cross-sectional area for forming a weak portion thereof. The tube is enclosed in a transparent shield and the tube and shield resiliently attached to the structure to be monitored. The frangible tube is designed to cause the weakened area to fracture when the tube and structure are exposed to a preselected level of an acceleration force, causing the liquid to be deposited in the shield as an indication of an overload.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates generally to the field of apparatus for 
detecting when a structure or structural member has been overloaded. This 
field is of particular interest to the aircraft industry, among others. 
SUMMARY OF INVENTION 
The present invention is an inexpensive and accurate apparatus for 
detecting when a structure or structural member has been overloaded. It 
comprises an overload detection member, typically elongated, that is made 
so it will fracture upon overload, that is, when the structure or 
structural member of interest is overloaded. The overload detection member 
can be provided with a weak or narrowed section to facilitate or control 
the fracturing. 
The occurrence of an overload can be detected by visual inspection of the 
member for fracturing or by other means, some of which are described 
herein by way of example. To keep the piece or pieces of the overload 
detection member from being scattered about upon fracturing, or for other 
purposes herein described, the overload detection member may be enclosed 
in a tube or shield. 
The overload detection member, with or without an enclosing tube or shield, 
typically is attached at one or both ends to the structure or structural 
member of interest. This is accomplished by conventional means. For 
example, the structure or structural member may be fitted with a bracket 
to hold the overload detection member and to accept a new member upon 
fracture of the original member. 
This invention is particularly useful in applications where overload of a 
structure or structural member could result in catastrophic consequences, 
such as loss of human life. The aircraft industry is an area of prime 
usefulness for the invention. The structural members of an aircraft 
experience loading during turning, banking, or changing the altitude of 
the aircraft during flight, and can be overloaded during these maneuvers. 
Repeated overloading can result in sudden failure of key structural 
members and subsequent destruction of the aircraft. The apparatus of the 
invention is typically installed at the center of gravity of the aircraft. 
However, it also can be installed on a specific structural member of the 
aircraft to monitor the loading on that particular member. 
A preferred embodiment of the invention comprises a closed ended frangible 
inner tube containing a highly visible dense liquid, an open ended outer 
tube disposed about the frangible inner tube, and a pair of end closure 
members engaging respective ends of the two tubes. The end closure members 
also detachably engage a bracket member connected to the structure or 
structural member being monitored for overloading. 
Preferably, the dense liquid filling the close ended frangible inner tube 
is mercury mixed with cinnabar. The cinnabar makes the mercury highly 
visible. 
The frangible inner tube is preferably made of a temperature resistant 
material that will not be affected by rapid temperature changes or 
extremes in temperature. The frangible inner tube has a diameter at one 
end that narrows to a second diameter and then returns to the first 
diameter of the tube. Typically, the second diameter is approximately one 
half of the first diameter. The point where the tube narrows to the second 
diameter is the weak point of the tube. Although typically the second 
diameter is one half of the first diameter, all that is necessary is that 
the diameters are set so that the tube will fracture at stress equivalent 
to overload. 
When an overload condition is experienced by the apparatus, the frangible 
inner tube will fracture at the weak point. When this happens, the highly 
visible dense liquid within the frangible inner tube is deposited in the 
liquid-tight enclosure about the inner tube formed by the outer tube and 
the end closure members. If when the frangible inner tube is inspected, 
the inner tube is broken, the inspector knows that the structure or 
structural member to which the apparatus of the invention is attached has 
experienced overload. If, on the other hand, the frangible inner tube is 
not broken, the inspector knows overload was not experienced. 
The frangible inner tube, once broken, can be quickly replaced without 
replacing the remainder of the apparatus. First, the apparatus is removed 
from the bracket. Once removed, the end closure structures are pulled from 
the respective ends of the inner and outer tubes. After removal of the end 
closure members the dense liquid is poured from the outer tube and the 
fractured frangible inner tube is removed from within the outer tube. 
After removal of the fractured frangible inner tube, a new frangible inner 
tube and the old outer tube are again engaged with the end closure 
members. This assembly is then replaced in the bracket member. 
An object of the invention is to provide an inexpensive and accurate 
apparatus for detecting when a structure or structural member has been 
overloaded. 
Another object of the invention is to provide an apparatus in which 
overloading can be easily detected upon visual inspection or other simple 
means. 
These and other objects of the invention will be described in detail in the 
remaining portions of the disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, generally at 1, a partially exploded view of the 
preferred embodiment of the overload detection apparatus of the invention 
is shown. The apparatus of the invention has close ended frangible inner 
tube 2 filled with dense liquid 4, an open ended outer tube 6 disposed 
about and spaced away from tube 2, end closure members 8 and 12 engaging 
the respective ends of the tubes 2 and 6 for maintaining the tubes in 
their spaced apart relationship, and bracket member 16 detachably fixed to 
the end closure members for connecting the two tubes and the end closure 
members to a structure or structural member of, for example, an aircraft 
being monitored for overloading. 
Frangible tube 2 in the preferred embodiment is close ended and filled with 
a dense liquid, as described. However, frangible tube 2 can be replaced by 
any elongated frangible member which has a weak point and will fracture at 
that weak point at overload, e.g., a solid glass rod with a defined weak 
point. 
Preferably, dense liquid 4 is mercury colored with cinnabar. The mixture of 
mercury and cinnabar provides a highly visible material that is easy to 
see when the apparatus is inspected to determine if the inner tube has 
been fractured and hence that overloading has been experienced by an 
aircraft. Although in the preferred embodiment the liquid is highly 
visible, it does not have to be so. The dense liquid is highly visible 
merely to aid the inspector to determine if tube 2 has fractured. Also, 
the liquid does not have to be dense. Although a dense liquid like mercury 
is preferred, a liquid of less density can be used. However, the density 
of the liquid will affect the dimensions of the frangible tube. 
Frangible inner tube 2 is preferably constructed of a heat resistant 
material. This is because the apparatus, when used on, for example, an 
aircraft is subjected to extreme temperatures during a normal flight. When 
the aircraft is at high altitudes, the apparatus is subjected to very low 
temperatures, and likewise, if the aircraft is on the ground, it can be 
subjected to high temperatures. By making the tube out of temperature 
resistant material, the frangible inner tube will not fracture when 
experiencing these temperature extremes or rapid temperature changes. 
Frangible inner tube 2 narrows, along incline section 7, from its first 
diameter to the second diameter at 3. From the second diameter at 3, the 
tube widens, along incline section 5, back to the first diameter. The 
second diameter at 3 is approximately one-half the first diameter of tube 
2. The point along tube 2 where it has the second diameter is the weak 
point of the tube. 
Frangible tube 2 is the portion of the apparatus of the invention that 
responds to the loading experienced by the structure or structural member 
to which it is attached. When frangible tube 2 fractures, the dense liquid 
is emptied from frangible tube 2. It is the fracturing of tube 2 that 
evidences overloading of the structure or structural member. Therefore, 
the invention can be practiced with an overload detection apparatus 
comprising nothing more than a frangible member, such as frangible tube 2, 
which is connected to the structure or structural member being monitored 
for overloading. 
Outer tube 6 is disposed about and spaced away from frangible inner tube 2. 
Tube 6 has a diameter greater than the first diameter of tube 2. Tube 6 is 
open ended and preferably made of a transparent, strong, durable material 
so it will generally protect the fragile frangible inner tube 2 and 
contain it and its contents upon fracture. Although outer tube 6 is 
concentrically disposed in the preferred embodiment, this is not 
necessary. Any disposition of the outer tube about inner tube 2 is 
acceptable if it, along with the end closure members 8 and 12, form a 
liquid-tight enclosure around the inner tube. 
Bracket member 16 has a top member 18 with holes 20 and 22 defined therein. 
Holes 20 and 22 are for connecting the bracket member to the structure or 
structural member being monitored for overloading. Holes 20 and 22 are for 
disposition of connecting means, such as screws. However, other means, 
such as adhesives, can be used for connecting the bracket member to the 
structure or structural member being monitored for overloading. 
Disposed from the ends of top member 18 are downward extending side members 
24 and 32, respectively. Side member 24 has disposed from its distal end 
upwardly extending circular shaped cut out 26. Circular shaped cut out 26 
is disposed from the opening in the distal end of side member 24 between 
points 28 and 30. An extension of the distal end between points 28 and 30 
across the opening defines the bottom of cut out 26 and forms a chord less 
than the diameter of circular shaped cut out 26. 
Side member 32 has disposed from its distal end upwardly extending circular 
shaped cut out 34. Cut out 34 is substantially the same as the circular 
cut out 26 in side member 24. Circular shaped cut out 34 is disposed from 
the opening in the distal end of side member 32 between points 36 and 38. 
An extension of the distal end between points 36 and 38 across the opening 
defines the bottom of the cut out and forms a chord less than a diameter 
of circular shaped cut out 34. 
Bracket member 16 is the preferred member for attaching tubes 2 and 6, and 
end closure members 8 and 12 to the structure or structural member being 
monitored for overloading. It is within the scope of the invention that 
other means can be used to attach the apparatus of the invention to the 
structure or structural member being monitored for overloading, as long as 
these means will not prevent the fracturing of tube 2 at overload. 
End closure members 8 and 12 have continuous circumferentially disposed 
grooves 10 and 14 disposed around their edges, respectively. Grooves 10 
and 14 are cut to provide a circular core member (FIG. 2) within each 
groove. The circular core members have a diameter slightly less than the 
diameter of the respective circular shaped cut outs 26 and 34 in sides 
members 24 and 32 of bracket 16. Since the end closure members 8 and 12 
are constructed of rubber, tubes 2 and 6 with the engaged end closure 
members 8 and 12 are moved in a direction opposite the direction C such 
that the circular shaped cut outs 26 and 34 of sides members 24 and 32 are 
disposed in grooves 10 and 14, respectively, around the circular core 
members (FIG. 2). lt is necessary to depress the thickness of the circular 
core members in order for edges 28 and 30 of side member 24, and edges 36 
and 38 of side member 32, to move past the respective circular core 
members. Once the cut outs are disposed around the core members, the two 
tubes and the end closure members 8 and 12 wil remain attached to bracket 
member 16 until physically removed. 
Referring to FIG. 2, generally at 50, the longitudinal cross-sectional view 
of the preferred embodiment of the apparatus of the invention in its 
assembled condition is shown. As shown, circular cut out 26 (FIG. 1) of 
side member 24 is disposed in groove 10 about circular core member 52, and 
circular cut out 34 (FIG. 1) of side member 32 is disposed in groove 14 
about circular core member 54. The inside surface of end closure 8 engages 
first closed end 15 of tube 2 and first open end 11 of tube 6 in a 
detachably sealable relationship. The inside surface of end closure 12 
engages second closed end 9 of tube 2 and second open end 13 of outer tube 
6 in a detachably sealable relationship. The end closure structures 8 and 
12 maintain tubes 2 and 6 in their spaced apart relationship. 
Referring to FIGS. 2 and 3, the method by which the apparatus of the 
invention is used to detect overloading will be described. 
Prior to experiencing overloading, the apparatus attached to a structure or 
structural member being monitored for overloading is as shown in FIG. 2. 
When overloading is experienced, tube 2 will fracture. The amount of 
loading that will fracture tube 2 can be adjusted in a number of ways, for 
example, by varying the length or diameter of the frangible inner tube or 
the formation of the weak point. 
As will be understood, rubber closure members 12 tend to isolate inner tube 
2 from the structural member to which the device 50 is mounted. When 
acceleration (G forces) which occur which would overload the structural 
member, the same forces will be applied to inner tube 2. Such forces cause 
the masses formed by liquid 4 and the glass walls of inner tube 2 on 
either side of the central weakened area 3 to tend to force to force tube 
2 in the direction of the force. Since the ends 9 and 15 of tube 2 are 
constrained from moving by bracket member 16, a moment is generated about 
the central weakened area 3. From a knowledge of the masses of the inner 
tube 2 and the characteristics of the material, such as glass, from which 
frangible tube 2 is made, the device may be designed such that tube 2 will 
fracture at weak point 3 at a preselected G force. Therefore, When 
overloading is experienced by the apparatus of the invention, frangible 
inner tube 2 will fracture at the weak point at 3, as shown in FIG. 3. 
When tube 2 fractures, dense liquid 4, which is preferably highly visible, 
is deposited in the enclosure formed by tube 6 and end closures 8 and 12. 
Since the enclosure is liquid-tight, it will receive and hold liquid 4. 
After the flight when the inspector inspects the aircraft structure for 
overloading, he can easily see that the frangible inner tube has been 
fractured by the dense liquid 4 being disposed in the enclosure. 
To place the apparatus back in its original unfractured form, as shown in 
FIG. 2, end closure members 8 and 12, outer tube 6, and fractured 
frangible tube 2 are moved in direction D out of attachment with bracket 
member 16. Once removed from bracket member 16, end closure member 8 is 
moved in direction A out of engagement with the ends of tubes 2 and 6, and 
end closure member 12 is moved in direction B out of engagement with the 
opposite ends of tubes 2 and 6. After end closure members 8 and 12 are 
removed, the fractured frangible inner tube 2 is removed from within in 
tube 6 and the liquid is poured from within the outer tube. In this 
disassembled condition, outer tube 6 and the end closure members are 
appropriately cleaned. After cleaning, the apparatus is reassembled with a 
new nonfractured frangible inner tube 2 and moved in direction E so that 
it is again detachably fixed to bracket member 16 attached to the 
structure or structural member being monitored for overloading. 
A second embodiment of the apparatus of invention is configured essentially 
the same as the preferred embodiment of the apparatus shown in FIGS. 1 and 
2, except that a different method is used to provide a visual indication 
when frangible inner tube 2 has fractured due to overloading. In this 
second embodiment, dense liquid 4 within frangible tube 2 is not highly 
visible. Disposed within the enclosure formed by outer tube 6 and end 
closure members 8 and 12 is a material that is chemically reactive with 
the dense liquid disposed within frangible inner tube 2. This material is 
also not highly visible. However, upon fracturing of tube 2, the dense 
liquid mixes with the material disposed in the enclosure. When they mix, a 
highly visible material is produced. The material disposed in the 
enclosure can be a gas, liquid or solid, as long as it will allow for 
proper fracturing of frangible inner tube 2. 
When it is desired to replace the frangible inner tube 2 subsequent to 
overload detection, the apparatus is disassembled as described for the 
preferred embodiment and reassembled with a new frangible inner tube 2. 
The material chemically reactive with the dense liquid is disposed during 
assembly in the enclosure formed by outer tube 6 and end closure members 8 
and 12. 
Referring to FIG. 4, generally at 70, and FIG. 5, generally at 120, a third 
embodiment of the overload detection apparatus of the invention will be 
described. Referring to FIG. 4, the third embodiment of the invention has 
bracket 72 having top 78 and side members 74 and 76 disposed downwardly 
from respective ends of top member 78. Top member 78 is connected by 
conventional means to the structure or structural member being monitored 
for overload detection. Side members 74 and 76 of bracket member 72 are 
similar to those shown for the preferred embodiment of the invention in 
FIG. 1. The respective cut outs in the side members 74 and 76 are disposed 
in grooves 100 and 106 around core members 98 and 104 of end closure 
members 96 and 102, respectively. Frangible inner tube 82 has its ends 92 
and 94 engaged by end closure members 96 and 102, respectively. Frangible 
inner tube 82 narrows, along incline 84, to a second diameter at 86, and 
widens from the second diameter, along incline 88, to the original 
diameter. The second diameter at 86 is the weak point of the frangible 
inner tube. 
Frangible inner tube 82 is preferably constructed of a temperature 
resistant material. Dense liquid 90 disposed in frangible inner tube 82 is 
preferably both highly visible and electrically conductive. The highly 
visible electrically conductive dense liquid is preferably mercury colored 
with cinnabar. 
Disposed about but spaced away from frangible inner tube 82 is outer tube 
80. Ends 81 and 83 of tube 80 are engaged in a detachably sealable 
relationship with end closure members 96 and 102, respectively. Outer tube 
80 and end closure members 96 and 102 form a liquid-tight enclosure around 
frangible inner tube 82. The outer tube 80 is preferably made of a 
transparent, strong, durable material that is not electrically conductive. 
Disposed in the enclosure formed by outer tube 80 and end closure members 
96 and 102 are two spaced apart connection pads 112 and 114. Indicator 110 
is connected on one side to connection pad 112 and is connected on a 
second side to power source 108. Power source 108 on a second side is 
connected to connection pad 114 and ground. The circuit including 
indicator 110 is open before fracture of inner tube 82, since there is no 
connection between connection pads 112 and 114. Indicator 110 can be any 
conventional electrically activated device such as a light or a buzzer. 
Referring to FIGS. 4 and 5, the method by which an overload is indicated by 
the apparatus disclosed in the third embodiment will be described. 
When the apparatus of the third embodiment experiences overload, frangible 
inner tube 82 will fracture at the weak point at 86. When tube 82 
fractures, electrically conductive dense liquid 90 is deposited in the 
enclosure surrounding frangible tube 82 formed by outer tube 80 and end 
closure members 96 and 102. When the electrically conductive dense liquid 
90 is deposited in the enclosure, an electrical path is formed between 
pads 112 and 114 in the enclosure, the electrical circuit is completed, 
and indicator 110 is activated. 
Indicator 110 is generally disposed in the cockpit and will indicate to the 
pilot when an overload condition has been experienced by the aircraft. By 
any conventional means, such as an on/off switch attached to indicator 
110, the pilot can turn off the indicator once it has made its indication 
that an overload condition has been experienced. After the aircraft has 
landed, the inspector can inspect the overload detection apparatus to 
ensure that there was a proper indication to the pilot that an overload 
condition was experienced. If frangible inner tube 82 is not fractured, 
the inspector knows that there was a false indication of an overload 
condition. However, if frangible inner tube 82 is fractured and the highly 
visible electrically conductive dense liquid 90 is deposited in the 
enclosure, the inspector knows the aircraft did experience overloading. 
When it is desired to replace frangible inner tube 82, the end closure 
members, and tubes 80 and 82 are moved in direction F out of engagement 
with bracket member 72. Once moved in direction F, the end closure members 
96 and 102 are moved in directions H and J, respectively, to remove them 
from the ends of the two tubes. Once one of the end closure members have 
been removed, electrically conductive dense liquid 90 is poured from tube 
80. Fractured frangible inner tube 82 is then removed, and a new frangible 
inner tube replaces the old fractured one. New frangible inner tube 82 and 
outer tube 80 have their ends engaged by the two end closure members, then 
this assembly is moved in direction G, so that the openings in side 
members 74 and 76 of bracket member 72 are disposed around core members 98 
and 104 of end closures 96 and 102, respectively. Once in this position, 
indicator 110 has its on/off switch placed in an on mode, and the 
apparatus is again prepared to monitor the aircraft for overloading. 
Although the above described embodiment of the invention are described for 
use in determining an overload condition for the structure or structural 
members of an aircraft, it is within the scope of the invention that the 
overload detection apparatus of the invention can be used to detect 
overloading of any structure or structural member which is subjected to 
loading because of movement of the structure or structural member. 
The terms and expressions that are employed here are used as terms of 
description and not of limitation. And there is no intention, in the use 
of such terms and expressions, of excluding the equivalence of the 
features shown, and described, or portions thereof, it being recognized 
that various modifications are possible in the scope of the invention as 
claimed.