Aneroid bellows assembly

Aneroid bellows assembly utilized in the emergency oxygen supply systems of aircraft includes a leaf spring mounted at its center to an adjustably positioned end fitting located on the axis of an evacuated bellows spring. The outer edges of the leaf spring are engaged with an adjustable ring threadedly mounted on the outer rim of a tubular housing member in which the bellows spring is mounted. The adjustable end fitting and ring permit elements having a substantial size tolerance to be assembled together and readily adjusted to achieve precise calibration.

BACKGROUND OF THE INVENTION 
The invention relates to aneroid bellows assemblies of the type used in 
emergency oxygen systems in aircraft. In commercial aircraft, it is 
required that crew members wear oxygen masks at appropriate times and that 
oxygen be supplied in increasing amounts if the cabin pressure is reduced 
below the normally maintained pressure of 11.8 psia associated with an 
altitude of 6,000 feet. It is also required that the rate of flow of 
oxygen increase substantially at pressures of less than about 7.24 psia, 
which corresponds to an altitude of 18,000 feet. 
Aneroid assemblies capable of controlling the flow of oxygen in the above 
manner have been sold for many years by Scott Aviation Corp. of Lancaster, 
New York. Typically, such devices have included an evacuated bellows 
spring mounted internally of an open ended tubular housing and affixed at 
one of its ends to the closed end of the housing. The bellows expands 
axially as ambient pressure decreases. An end plate attached to the free 
end of the bellows spring overlies the open end rim of the housing and 
extends radially outwardly of the housing where it is engaged by a coil 
spring which encircles the housing. It is important that the coil spring 
the deflected a precise distance in order to insure that the movable tip 
end of the bellows assembly will move as necessary to control the flow of 
oxygen in the manner desired. Usual manufacturing tolerances in the coil 
spring, housing and bellows portions of the assembly are much greater than 
those needed to assure proper functioning of the various parts after 
assembly and thus, it has been necessary to sort the parts by size and 
selectively fit them to each other or, alternatively, to remove material 
from one of them. Such operations are very time consuming and expensive. 
SUMMARY OF THE INVENTION 
It is among the objects of the present invention to provide an aneroid 
bellows assembly which is much easier to assemble and calibrate than 
previously available units and which is also slightly more compact and 
lighter in weight. In the assembly of the invention, the housing end 
plates and the large coil spring of the prior art assembly are eliminated 
and replaced by a small, lightweight leaf spring and a threaded housing 
end adjustment ring. The adjustment ring cooperates with an adjustable 
leaf spring mount on the free end of the bellows spring to accommodate a 
large range of manufacturing tolerances in the respective parts and permit 
rapid and simple calibration.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, my improved aneroid bellows assembly is shown in 
is finally assembled and adjusted sea level position in FIG. 1 where the 
parts are shown in partial axial cross-section and in FIG. 3 where they 
are shown in perspective. Referring particularly to FIG. 1, the aneroid 
assembly 10 can be seen as including a tubular can or housing member 12 
having an outer threaded end portion 14 at its upper end and an axially 
aligned opening 16 at its lower end. A bellows assembly indicated 
generally at 20 is mounted within the housing 12 and the opening 16. The 
bellows assembly 20 includes a bellows spring member 22 which is evacuated 
and closed at its upper end by an upper end plate 24 and at its lower end 
by a lower end plate 26. The lower end plate includes a lower abutment 
portion 28 which extends into the hollow interior chamber 29 of the 
bellows and limits the extent to which the bellows 22 may be compressed 
when it is evacuated. A threaded retaining portion 30 extends downwardly 
from the lower end plate 26 and has an evacuation tube 31 passing through 
its center. The lower end (not shown) of the evacuation tube 31 is 
compressed, cut off and soldered closed during the course of the chamber 
29. A retaining ring member 32 anchors the threaded portion 30 of the 
bellows assembly 20 to the housing 12. After the ring 32 is assembled, an 
adjustable mounting shaft 34 is screwed onto the threaded portion 30 and 
into firm engagement with the housing 12. An adjustment slot 36, which is 
adapted to receive a screw driver blade is formed at the lower end of the 
threaded shaft 34 and permits the entire assembly 10 to be vertically, 
adjustably positioned in an external mounting plate (not shown). Extending 
upwardly from the upper end plate 24 of the bellows assembly 20 is a 
bellows adjustment screw 40 having an actuator tip portion 42 which 
controls the flow of oxygen in a device (not shown) with which the 
assembly 10 is used. A bellows nut assembly 46 is threaded onto the 
adjustment screw 40 and comprises a generally flat leaf spring portion 48 
which has a recessed center portion 50 and bent end portions 52. A flat 
end plate member 54 is mounted in the recessed area 50 and brazed thereto 
as well as to an adjustment nut 56 which has threads which are 
complementary to those on bellows adjustment screw 40. The end plate 54 is 
shown as resting on the housing 12 so that the bottom end portions of the 
end plate 54 overlie and contact the upper end surface of the housing 12. 
The bent ends 52 of the leaf spring 48 are shown as resting on a rotatable 
adjustment ring 60 which is threadedly engaged with the threaded end 
portion 14 of the housing 12. Since the assembly 10 is shown in its sea 
level position, one can readily appreciate that the ambient pressure in 
combination with the vacuum within the chamber 29 is exerting a downward 
force on the adjustment screw 40 and the nut assembly 46 carried thereby 
so that the end plate 54 will bear on the housing 12 with a predetermined 
amount of preload force. This preload force can be increased or decreased 
by rotating the nut 56 so as to reduce or enlarge the space between the 
central portion of the leaf spring 48 and the upper portion of the end 
plate 24, respectively. An additional preload is placed on the bellows 
assembly 20 by the leaf spring 48 which is shown as being biased to a 
loaded position by the adjustment ring 60. 
Before proceeding further to describe the manner in which the assembly 10 
is calibrated, it would be well to briefly describe the use of the device. 
This can be best done in connection with FIG. 8 wherein a graph is 
illustrated plotting the inches of aneroid travel versus altitude as 
expressed in absolute pressure. It is well known that the absolute 
pressure of the atmosphere varies from 14.7 psia. at sea level to about 
3.98 at 32,000 feet. It is also known that no supplemental oxygen is 
required at an altitude of 6,000 feet where the pressure is 11.8 psia. but 
that a steadily increasing amount of oxygen is required as one goes up in 
the atmosphere from 6,000 feet to about 18,000 feet where the pressure is 
7.24 psia. As one goes above 18,000 feet, the rate at which oxygen must be 
supplied is much higher than at lower altitudes. In attempting to closely 
approximate the non-linear manner (not shown) in which oxygen should 
ideally be supplied, it has been determined that the straight line curves 
A-B and B-C in FIG. 8 are quite satisfactory and can be achieved by 
combining two springs which have different spring rates. It has been found 
that the use of a bellows spring 22 having a spring rate of 10.1 pounds 
per inch and a leaf spring having a spring rate of 34 pounds per inch will 
provide satisfactory results when the device is correctly assembled and 
preloaded. To correlate FIGS. 1 and 8, it can be noted that the aneroid 
tip 42 will move outwardly relative to the housing 12 by the approximately 
0.030" distance between points A and B as a lowering of the ambient 
pressure from 11.8 to 7.24 psia. permits the bellows 22 to expand and 
remove the preloading of the leaf spring 48. Once the effect of the leaf 
spring is removed, the bellows 22 can expand at a much more rapid rate so 
that tip 42 will move the approximately 0.070" distance between points B 
and C as the ambient pressure decreases from 7.24 to 3.98 psia. The 
configuration of the assembly at 3.98 psia. is shown in FIG. 7. 
To properly calibrate the bellows assembly 10 to produce the travel shown 
in FIG. 8, the assembly is placed in a test chamber having a pressure of 
7.24 psia., as symbolized by the dotted lines in FIG. 4. The bellows nut 
assembly 46 is screwed along the shaft 40 until end plate 54 is 0.030" 
above the end surface of the housing 12. The adjusting ring 60 is backed 
off, as shown, so that when the device is removed from the vacuum it will 
assume the configuration shown in FIG. 5. The ring 60 is then threaded up 
to the FIG. 6 position where it exactly meets the ends 52 of the leaf 
spring 48. The ring 60 is then threaded upwardly another 0.030" to preload 
the spring 48 to the configuration shown in FIG. 1. 
From the preceding discussion, the advantages of my improved aneroid 
bellows assembly over prior art devices such as that shown in FIG. 2 will 
be readily evident. The latter embodiment has elements 112, 120, 134, 140, 
142, 148, 154 and 156 which correspond to similar elements 12, 20, 34, 40, 
42, 48, 54 and 56 in FIG. 1. As seen in FIG. 2, element 164 is a retaining 
plate which functions as a radial extension of the bottom of the housing 
112 and cooperates with the plate 154 affixed to nut 156 to compress the 
coil spring 148. As previously noted, the usual manufacturing tolerances 
for the spring 148, the housing 112 and the bellows 120 are much greater 
than those needed to assure proper functioning of the parts after 
assembly, thus making it necessary to spend much time and money to sort 
the parts by size and selectively fit them to reach each other or, 
alternatively, to remove material from one of them.