HIGH PRESSURE BELLOWS CAPSULE AND METHOD OF MAKING SAME

Abstract: A bellows capsule (10) for use in applications where the pressure on the outside of the bellows capsule is substantially higher than that on the inside is formed with spacer rings (42) positioned between the diaphragms (286, 326) forming adjacent bellows convolutions at the inner diameter of the bellows. The total thickness of each diaphragms pair and the intervening ring is greater than the diameter of the bead (34) at the outer diameter or rim of each bellows convolution. When the bellows is urged to its fully compressed position, its diaphragms are able to nest so that the wall portions having a large pressure differential across them are supported by an adjacent diaphragm and a load bearing column is provided at the inner diameter of the bellows thereby minimizing diaphragm fatigue caused by repetitive high pressure stresses on the bellows when the bellows is cycled to its fully compressed position.

Description: HIGH PRESSURE P T.TCWS CAPSULE AND MEHTOD OF MAKING SAME Background of the Invention This invention relates to a bellows capsule. It relates more particularly to a nesting metal bellows capsule for use in applications where the pressure on the outside of the bellows is sometimes substantially higher than the pressure at the inside thereof. Bellows capsules are used in a wide variety of applications as pressure transducers, linear actuators, temperature and pressure responsive devices, etc. In many cases, the bellows capsule is composed of a series of annular rippled diaphragms whose alternate inner and outer edges are welded, brazed or otherwise secured to the corresponding edges of adjacent diaphragms to form a series of bellows convolutions. In the application of particular interest here, the bellows capsule is situated inside a pressure vessel. One end of the capsule is closed while its other end is connected to a header in the vessel to isolate the spaces inside the pressure vessel at the inside and outside of the bellows. Further, fluid communication is provided ' through the header into the bellows and through the wall of the pressure vessel to the space inside the vessel but outside the bellows. When fluid under high pressure is injected into the pressure vessel, it tends to compress or collapse the bellows capsule so that the rippled diaphragms nest against one another, the fluid inside the bellows then being expelled through the header. Conversely, when the pressure in the vessel is reduced relative to the pressure inside the capsule, the capsule tends to extend, drawing fluid into the capsule. It has been found that when a conventionally fabricated, nesting welded bellows is subjected to repeated high pressure stress upon assuming its fully nested position, diaphragm fatigue occurs within a relatively few pressure cycles. It is believed that this diaphragm fatigue stems from excessive strain of small unsupported annular portions of the bellows diaphragms forming each bellows convolution near the inner edges of the diaphragms. These diaphragm portions are unsupported because the widths of the joints, i.e. the weld beads or spillover , between the bellows diaphragms at the inner and outer diameters of the capsule are wider than twice the diaphragm thickness. Consequently when the bellows is compressed, adjacent beads contact one another preventing the bellows diaphragms from nesting completely at those portions and leaving a small space in each convolution. Since these diaphragm portions "see" high pressure on one side and low pressure on the other, they are bowed or bent toward each other into the intervening space thus producing pressure cycling fatigue. In actual practice, this fatigue occurs within only a relatively few pressure cycles on the capsule. Consequently, such capsules used in high pressure applications have a relatively short life. Many of the capsules with which we are particularly concerned here are used in aircraft and aerospace applications. Also, they are often made of expensive metals, such as stainless steel, titanium, etc. Thus it is important that they be made more reliable and long-lived both for cost and safety reasons. It is possible to avoid the spillover problem by connecting the diaphragm edges together by brazing them. However, this involves inserting a thin ring of different metal between the diaphragms which melts forming the bond. In many applications, the use of this different brazing metal creates manufacturing problems. Also corrosion occurs because of electrolytic action. OMPI Consequently in such applications, it has been found necessary to weld the diaphragms together. Also it is known that weld beads such as those found on conventional bellows can be removed by coining. This entails smashing the bead between a hammer and an anvil to flatten the bead. This can be accomplished relatively easily at the joints between the diaphragm pairs at the inside diameter of the bellows. This is because each such diaphragm pair is a separate unit so that there is ready access to both sides of the weld bead. However, it is very difficult and not economically feasible to coin the weld beads at the outside diameter of the bellows because the bellows is already formed at this point and the individual weld beads cannot easily be segregated from the adjacent beads for the coining operation. Summary of the Invention Accordingly, the present invention aims to provide a bellows capsule which is particularly effective in high pressure applications. Another object is to provide a bellows capsule whose adjacent convolutions are able to nest completely. A further object of the invention is to provide a nesting, rippled, welded diaphragm bellows which is able to withstand repetitive high pressure stresses for a prolonged period. A further object of the invention is to provide a method of making a metal bellows capsule having one or more of the aforesaid advantages. Other objects will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the article possessing the features, properties and relation of elements which are exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims. Briefly, the present bellows capsule is able to withstand repeated cyclings to a fully compressed position in a high pressure environment because it is fabricated so as to leave no unsupported diaphragm portions that are exposed to high pressure when the capsule is fully nested. This is accomplished by sandwiching a flat spacer ring between the inner edges of the bellows diaphragms forming adjacent bellows convolutions. For purposes of this disclosure, a convolution is defined as a segment of the bellows comprising a pair of annular diaphragms joined together at their rims. Each ring is TIG welded in place at the time the inner edges of the diaphragms of adjacent bellows convolutions are joined. The thickness of the spacer ring is selected to be substantially the same as the added thickness of the weld beads or spillover at the rims of the adjacent bellows convolutions. Consequently, when the bellows is in its fully compressed position, the diaphragms forming each bellows convolution are able to nest against one another providing mutual support across their entire widths. Furthermore, the inner margins of the diaphragms and the spacer rings form a continuous load bearing column that supports the stack of bellows convolutions so that the weld beads at their rims barely contact one another when the bellows is fully nested. With this arrangement, there are no unsupported portions of the bellows diaphragms subjected to a high pressure differential that are not also fully supported by an adjacent diaphragm portion when the bellows is in OMPI its fully compressed position. Resultantly the diaphragms suffer minimum stress when the capsule is subjected to repeated pressure cycling and therefore the capsule should have a long, useful life. Yet the implementation of the present improvement does not result in any material increase in the overall cost of such bellows because the only added components are a plurality of identical flat rings which are easily formed by conventional stamping techniques. Brief Description of the Drawing For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken in connection with the accompanying drawing, in which: FIG. 1 is a perspective view of a rippled diaphragm bellows capsule made in accordance with this invention, shown in a high pressure environment; FIG. 2 is a perspective view of the spacer ring incorporated into the FIG. 1 capsule; FIG. 3 is a fragmentary sectional view of the FIG. 1 capsule on a larger scale showing the capsule in greater detail; and FIG. 4 is a similar view showing the conventional bellows capsule construction. Description of the Preferred Embodiments Referring to FIG. 1 of the drawing, the subject bellows capsule shown generally at 10 is positioned inside a pressure vessel indicated in dotted lines at 12. The inner end of the capsule is closed by a circular disk 14. The outer end of the capsule has a flange 10b_ that is welded or otherwise secured to an end wall or header 12a. of vessel 12. Thus the capsule 10 divides the volume of vessel 12 into two isolated spaces, namely, a space 16 inside the capsule and a space 18 outside the capsule. In use, fluid communication from without is established to the two spaces 16 and 18 via conduits extending through the walls of vessel 12 as shown in dotted lines at 22 and 24 respectively. Referring now to FIGS. 1 and 3, the bellows capsule 10 comprises a series of identical convolutions shown generally at 26. Each convolution comprises a female diaphragm 28 and male diaphragm 32. Typically, bellows diaphragms are made of stainless steel, titanium or other rugged metal and they are rippled so that they flex readily with minimum stress as the capsule is cycled between its fully extended and fully compressed positions. FIG. 3 shows the relative positions of adjacent bellows convolutions 26 when the capsule 10 is in its fully nested or compressed position. FIG. 4 illustrates the comparable positions of adjacent convolutions 26a. of a prior conventional capsule 10a_. Refer, for a moment to FIG. 4. In prior bellows of this general type, each bellows convolution 26a consists of a pair of nesting, rippled female and male diaphragms 28 and 32. Each convolution is formed by joining the outer edges or rims of diaphragms 28 and 32 usually by TIG welding those edges together. Invariably, this process results in the formation of an annular bead 34 due to spillover of molten metal during the welding process. In a typical case, each weld bead 34 has a cross section which is roughly three times the width of the diaphragm thickness. Thus, each weld bead 34 has a portion 34sι. that projects out beyond the surface of each diaphragm at the joint in the direction of the bellows axis A, a distance roughly equal to one-half the diaphragm thickness. Similar weld beads 38 are formed at the inner diameter of the bellows when the diaphragms of adjacent bellows convolutions are joined together in the same manner. These inner weld beads 38 also have portions 38a_ which project out beyond the surfaces of the diaphragms as shown. Still referring to FIG. 4, when pessure is applied to the outside of capsule 10, the bellows is prevented from nesting completely because the adjacent weld beads 34 and the adjacent weld beads 38 contact one another at the inner and outer diameters of the capsule. The stacking of the weld beads 38 at the inner diameter of the bellows leaves annular diaphragm portions 28a_ and 32a_ adjacent those weld beads unsupported. Moreover, the space between those diaphragm portions corresponds to the space 16 (FIG. 1) inside the capsule which is maintained at relatively low pressure. On the other hand, the opposite sides of those diaphragm portions 28a_ and 32a_ are exposed to the relatively high pressure in the space 18 at the outside of the bellows capsule. Resultantly, when the capsule lOei is fully nested, the pressure differential across those diaphragm portions causes those portions to bulge toward one another into the gap between them as shown in dotted lines at P in FIG. 4. As a consequence, the diaphragms suffer fatigue distress at those diaphragm portions 28_a and 32a. after the bellows capsule 10a_ has undergone only a relatively few pressure cycles. Turning now to FIGS. 2 and 3 of the drawing, the present long-lived bellows capsule 10 is somewhat similar to the prior one in that it is composed of a stack of bellows convolutions formed by connecting rippled female and male diaphragms 28 and 32 alternately at their inner and outer edges. Actually, the joints at the outer rims of the diaphragms are more or less the same as those described above. That is, they are characterized by weld beads 34, having portions 34a_ projecting out from the surfaces of the diaphragms along the bellows axis A. The capsule 10 does differ substantially from the prior one, however, in its joints or connections between the inner edges of the diaphragms of adjacent convolutions. In lieu of the usual inner connections that form weld beads 38 (FIG. 4), the present capsule 10 has flat spacer rings 42 sandwiched between the diaphragms 28 and 32 of adjacent convolutions 26. The inner diameter of each ring 42 is substantially the same - s the diaphragm inner diameter. The ring outer diameter is not particularly critical. As best seen in FIG. 3, when forming the diaphragms 28 and 32, their inner edge margins 28b_, 32b are flattened so that when the rings 42 are in position, they lie flush against those edge margins as shown. Each ring 42 is more or less coextensive with those edge margins 28b_, 32b_ and the thickness of each ring is approximately equal to twice the thickness of the weld bead portion 34a_. Each pair of diaphragm margins 28b_, 32b_ and the intervening spacer ring 42 are joined by TIG welding as indicated at 46 in FIG. 3. It should be understood that the process of welding the inner joints 46 between the bellows convolutions does itself result in some spillover. However, the extra thickness provided by the ring 42 accommodates most of the molten metal flow as the joint is formed. Resultantly, there is no great flow of metal beyond the edges of the sandwich comprising each inner joint so that the resultant weld bead is relatively small. In any OM7I event, the weld beads at the inner joints are easily removed by coining as described above. With this arrangement, the diaphragm portions 28_a and 32a_ of each convolution 26 inboard of the rings support one another and there is no gap or void between them. In other words, the diaphragms 28, 32 forming each convolution 26 are able to nest completely along their entire width. Consequently, when the space 18 between the bellows convolutions 26 is exposed to high pressure and the space 16 inside the bellows is exposed to a lower pressure, those engaging diaphragm portions 28a_ and 32a_ provide mutual support for one another. In fact, there are no portions of the bellows diaphragms that are exposed to a pressure differential that are not also supported by an adjacent diaphragm. Further as shown in FIG. 3, the thickness of each ring 42 is such that when the bellows is fully compressed, the diaphragm margins 28b, 32b_ and rings 42 form a load bearing column that permits the weld beads 34 at the rims of the convolutions to barely contact one another so that there is little or no distortion at the outer edge margins of the diaphragms. Resultantly, the subject capsule 10 can undergo numerous pressure cycles without suffering fatigue distress. The spacer rings 42 being simple stamped metal parts add little to the overall cost of the bellows capsule 10. Accordingly, a high pressure capsule of superior performance and long life can be fabricated at a cost not appreciably more than that of prior comparable capsules having lesser performance characteristics. It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and, since certain changes may be made in carrying out the above method and in the construction set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described.

Claims: 1. A metal bellows capsule comprising A. a stack of annular metal diaphragms, B. means connecting the rims of pairs of adjacent diaphragms directly to one another so as to form a series of convolutions, and C. means for joining the inner edges of the diaphragms in adjacent convolutions, each said joining means including: (1) a metal spacer ring sandwiched between said inner edges, and (2) means for connecting said inner edges to opposite surfaces of said spacer ring to form continuous seams all around the ring, whereby the opposing surfaces of the diaphragms in each convolution are able to contact one another over their entire areas when the bellows is fully compressed thereby providing mutual support over their entire areas enabling the capsule to withstand relatively high external pressures. 2. The bellows capsule defined in claim 1 wherein the diaphragms are of rippled nesting type construction. 3. The bellows capsule defined in claim 1 wherein the connecting means includes a weld. 12 4. The bellows capsule defined in claim 1 wherein A. the inner edge margin of each diaphragm is substantially flat, and B. each said spacer ring has substantially flat upper and lower surfaces coextensive with said inner edge margins. 5. The bellows capsule defined in claim 1 wherein each said spacer ring has a thickness such that when the capsule is fully compressed, said inner edges and said rings form a continuous load bearing column that maintains adjacent bellows convolutions in spaced-apart relation. 6. The bellows capsule defined in claim 5 wherein the inner edge of each ring coincides with said inner edges of the diaphragms. 7. The method of forming a metal bellows capsule comprising the steps of A. forming a plurality of similar annular metal diaphragms, B. forming a plurality of metal spacer rings whose inner edge has substantially the same diameter as the inner edges of said diaphragms, C. securing the rings between the inner edges of pairs of said diaphragms, D. superimposing said diaphragm pairs, and E. joining the outer rims of the adjacent diaphragms from different pairs flush to one another to form a series of convolutions so that when the capsule is fully compressed, the diaphragms in each convolution contact one another over their entire areas to provide mutual support over their entire areas whereby the capsule is able to tolerate relatively high external pressures.

Inventors: SHAMIE RAYMOND, SHAMIE, RAYMOND
Applicants: METAL BELLOWS CO, METAL BELLOWS CORPORATION
Application year: 1982
Main classification: F16J3
IPC codes: B21C37, F16J3