US3897942A - Compressible and expansible chambers - Google Patents

Abstract

The disclosure relates to compressible metallic bellows of the type which comprise a plurality of axially aligned, concave sheet metal annuli joined to one another at their inner and outer margins respectively so that the concavities of the annuli are alternately facing towards and away from one another, in which the radial cross-sectional shape of each annulus is such as to afford high volumetric compression ratios, impactless closing of such bellows and freedom from resonance effects when in dynamic use.

Description

United States Patent McNamee COMPRESSIBLE AND EXPANSIBLE CHAMBERS Aug. 5, 1975 3,224,344 12/1965 Baumann et a1 .1 267/162 Primary E.rumilierJames B. Marbert Attorney, Agent, or Firm-Allison C. Collard [57] ABSTRACT The disclosure relates to compressible metallic bellows of the type which comprise a plurality of axially aligned, concave sheet metal annuli joined to one an other at their inner and outer margins respectively so that the concavities of the annuli are alternately facing towards and away from one another, in which the radial crosssectional shape of each annulus is such as to afford high volumetric compression ratios, impactless closing of such bellows and freedom from resonance effects when in dynamic use.

5 Claims, 15 Drawing Figures PATENTEB AUG 1 75 SHEET PATENTED 55375 3' 897' 942 SHEET 4 COMPRESSIBLE AND EXPANSIBLE CHAMBERS This invention relates to compressible metallic bellows, to methods of manufacturing such bellows and to machines such as, for example, compressors, engines, actuators and the like, in which such bellows replace and perform the duties of a cylinder and piston. in particular the invention relates to metallic bellows of the type which comprise a number of axially aligned, concave, sheet metal annuli joined one to another at their outer and inner margins respectively so that the concavities of the annuli are alternately facing towards and away from one another.

Such bellows are primarily intended for use in compressors, although are not restricted to such, and must therefore satisfy a number of requirements which are:

l. Able to withstand long-term continuous mechanical flexure.

2. Capable of withstanding high internal (or external) pressures.

3. Permit high volumetric efficiencies and high compression ratios.

4. Be free from natural resonance effects.

5. Be free from impact upon closure, i.e., free from high velocity contact between adjacent portions of the bellows during any part of the bellows stroke.

Compressors which employ metallic bellows have been described in British Patent Specification No. 438,404, in which the bellows are compressible and which comprise a series of concave annular discs of the type described above, the shape of which is not defined but which are illustrated as conically dished discs. There is no teaching in the specification as to how the stringent requirements of a compressor may be met by the bellows, or as to how the shape of the disc concav ity may be varied in order to modify a bellows performance towards meeting the stated requirements.

Bellows which possess some of the desired characteristics are described in British Patent Specification No. 474,753 and which comprise a series of thin, flat, annular metal discs alternately welded together at their in nermost and outermost margins. Such bellows are nor mally closed (i.e., the surfaces of adjacent discs are fully contiguous) and in use are extended, rather than compressed as are bellows of the invention. However, the volumetric efficiency of such bellows would appear to be high when internal pressures are low but, as FIG. 8 of the specification shows, it would appear difficult to obtain contiguity of adjacent internal disc surfaces at high pressures, furthermore, considerable stress would also appear to be generated in the outermost margins of the discs, leading to metal fatigue and eventual rupture of the bellows.

British Patent Specification No. 759,849 describes compressor bellows similar to those of British Patent Specification No. 474,753 except that these are compressed by fluid pressure acting upon and externally thereto, thus reducing the pressure difference across the bellows to negligible proportions. Accordingly, the bellows described can be made from thin, freely bendable material such as for example light gauge copper.

Compressible bellows comprising a series of conically dished annular discs have been proposed, such as in previously mentioned British Patent Specification No. 438,404, which bellows suffer from several practical disadvantages, namely:

I. When a conical annular disc, i.e. like a Belleville washer, is axially compressed until the inner and outer perimeters lie in the same plane, the disc itself does not assume a planar condition, as may be imagined, but in radial cross-section adopts a wavelike curve. Bellows comprising such conical discs can only be compressed sufficiently for complete radial contiguity of the internal surfaces, by the application of a large axially directed pressure acting over the whole external surface of the two ends of the bellows. Furthermore, the stresses raised in each disc when repeatedly so flattened would be sufficient to ensure rapid failure due to fatigue.

2. Such bellows have a substantially constant spring rate and accordingly have a natural resonance which can limit the operation speeds of a compressor using the bellows.

3. As the bellows are compressed, flexural motion is abruptly arrested as maximum compression is achieved, i.e., as the adjacent discs come together, albeit in distorted form.

It is such disadvantages that the invention seeks to overcome by the expedient of providing bellows in which the radial cross-sectional shape of each concave sheet metal annulus in the unstressed condition is such that when the inner and outer perimetric margins are moved axially relative to one another so that they lie in the same plane, the adjacent surfaces of any two adjacent annuli are contiguous between the two margins and substantially flat.

In practice, where adjacent annuli are interattached at their margins, the joint is made over a radial margin length substantially greater than the combined thickness of the two annuli. In order to facilitate joining, the inner and outer margins are flat, lying in planes parallel with each other, and may be joined one to another by brazing, soldering, welding and the like.

In such an arrangement, when a bellows is compressed, the joined margins, although moving axially, remain flat and parallel with one another.

It has been found that, if each annulus of such a bellows is pre'formed to a shape identical with that shape which would be generated in a similar but flat annulus when the two margins thereof are axially and undeformably displaced whilst remaining parallel to one another, the bellows which result exhibit advantageous characteristics.

For example, as an annulus so shaped is axially compressed, the curve of its shape becomes progressively shallower until, when the two margins are in the same plane, the annulus is flat. Accordingly, a bellows fabricated from such annuli may be fully compressed, all fluid being expelled from between the annuli, thus affording high compression ratios. Furthermore, increasing internal pressure modifies the action of the bellows causing rolling to occur between the external surfaces of adjacent annuli, commencing at the inner margin and moving radially outwards until, at full compression, complete contiguity of the surface results. An immediate effect of this rolling action is to cause the stiffness of the bellows to increase as the axial length reduces, thus obviating natural resonance effects. However, when no pressure difference exists across the bellows, the stiffness is substantially constant.

Also, it can be shown that the circumferential and radial membrane stresses, and circumferential bending stresses of each fully compressed annulus are negligible, unlike in other known bellow configurations, the dominant stress being the radial bending stress which is greatest adjacent the inner margin and is conveniently of such sign as to oppose and tend to cancel internal pressure-induced stresses. The bellows can therefore withstand higher internal pressures than comparable known bellows.

Although bellows fabricated from annuli shaped in accordance with a naturally deformed flat annulus as described, which shape is hereinafter referred to as the natural shape," have improved characteristics and are embraced by the invention, t has been found that simple modifications to the natural shape afford further improvements and enable a particular bellows characteristic to be predetermined. For example, increasing the curvature in the region of the outer margin of each annulus affords (a) a greater bellows stroke, (ba radial rolling action from inside to outside of adjacent annuli, even in the zero pressure difference condition, with resulting change of bellow stiffness with axial length, and (c) an increase in radial bending stress towards the outer margin of each annulus when compressed (with out appreciably increasing the maximum stress of the inner margin) thus permitting higher internal pressures which tend to be cancelled by such bending stress. The modification to shape is, of course, not so great that the outermost radial bending stresses exceed the permissible fatigue stress of the bellows material.

The effect of decreasing the curvature of the annular region adjacent the outer margin reduces the radial bending stress adjacent the outer margin of each annulus and thus permits bellows so modified to withstand higher external pressures and, furthermore, results in a rolling action between adjacent internal annuli surfaces commencing at the outer margin and moving radially inwards, even in the zero pressure difference condition, until complete contiguity of the surfaces occurs. Again, the stiffness of bellows so modified varies with axial length but, the stroke of the bellows is reduced by an amount related to the decrease in curvature.

Accordingly the invention provides compressible metallic bellows of the type specified in which each annulus is, in its neutral condition, in radial cross-sectional shape curved between a flat outer margin which lies in one plane and a flat inner margin lying in a plane parallel to that of the other margin, the curve being such as to resemble that natural curve, which would be generated if the inner and outer margins of a dimensionally similar flat annulus of the same metallic material were axially and undeformably displaced whilst remaining parallel with one another.

In one aspect of the invention the radial crosssectional shape of each annulus corresponds with that of a flat annulus deformed as described. This natural shape in cross-section is ogival, sinusoidal or like an elongated S, having a positive curvature between the outer margin and a point somewhere between the two margins at which point the curvature has reduced to zero, and continuing with negative curvature from that point towards the inner margin, each margin being tangential to its respective curve. The choice of terms positive" and negative" curvature being quite arbitrary and chosen to assist in the definition of the modifications to the natural annular shape yet to be described.

Further according to the invention each annulus of the bellows may be shaped so as to correspond with the natural shape with the exception of that annular region of positive curvature, the curvature of which region is increased throughout its radial length.

Also in accordance with the invention each annulus may be shaped so as to correspond with the natural shape with the exception of the positively curved region, the curvature of which region is decreased throughout its radial length.

The invention also provides for a method of manufacturing compressible metallic bellows of the type specified and which comprises the following steps:

1. Forming the annuli to the desired shape with concurrent or subsequent heat treatment thereof.

2. Joining together the inner and outer margins of adjacent annuli by electron-beam welding, the beam being directed so as to impinge radially upon the abutting edges of the respective margins whilst rotating the annuli relative to the beam, the beam being of such dimensions and intensity as to produce a seam of radial depth preferably greater than the combined thickness of the abutting margins.

Bellows according to the invention may be adapted for actuation by external fluid pressure by the provision of annular spacers interposed between and joined with the inner margins of adjacent pairs of annuli cojoined at their outer margins. The use of such spacers provides access for pressurised external fluid to the outer surfaces of the annuli at all stages of bellows compression, thus affording faster closing of bellows so adapted. The provision of a longer junk piston within the bore of such a bellows is desirable, to provide the desired overall compression ratio, due to the increase in length and resulting increase in free volume of the bellows when fully compressed, following the use of spacer rings.

A similar arrangement of spacer rings joining with the outer margins of adjacent annuli pairs cojoined at their inner margins is also embraced by the invention.

Further according to the invention, compressors are provided which include a bellows as described herein.

Examples of the invention will now be described with reference to the accompanying drawings which are as follows:

FIG. 1 shows in cross-section a bellows of the kind specified; 1

FIG. 2 illustrates in part-section a portion ofsuch bellows constructed from naturally shaped annuli;

FIGS. 3 and 4 show what is meant by the term naturally shaped" annulus and how such an annulus may be formed;

FIG. 5 illustrates in part the bellows of FIG. 2 when partially compressed,

FIG. 6 shows the same bellows fully compressed;

FIG. 7 depicts a naturally shaped annulus and also shows one in dotted outline which has increased positive curvature;

FIG. 8 again shows the naturally shaped annulus of FIG. 7, but the dotted outline in this instance depicts an annulus having decreased positive curvature;

FIG. 9 shows a partially compressed bellows in which the annuli have increased positive curvature;

FIG. 10 is of a similarly compressed bellows having annuli of decreased positive curvature;

FIGS. 11 and I2 illustrate bellows in which cojoined pairs of annuli are spaced apart by rings;

FIGS. 13 and I4 illustrate diagrammatically a compressor in which the bellows of FIG. II are used to compress a fluid; and

FIG. shows another compressor driven by a cam shaft and which employs bellows having increased positive curvature.

Referring to FIGS. 1 and 2, a bellows of the kind specified comprises a series of concave sheet metal annuli 1 arranged in axial series as shown and welded together at their respective inner and outer margins, 4 and 5 respectively, these inner and outer margins being flat and lying in parallel planes. The radial length of each weld 4A and 5A being substantially that of the width of its associated margin 4 and 5. Such depth of weld may be obtained by use of electron beam welding.

In order to define the shape of each annulus l, we now refer to FIGS. 3 and 4. FIG. 3 shows a flat annular disc of sheet metal 6 clamped at its outer margin be tween the two halves of a rigid ring 7 and clamped at its inner margin between the two halves of a rigid cylindrical clamp 8. When clamp 8 is deflected axially relative to ring 7 by an amount d, whilst maintaining parallelity between the inner and outer margins of disc 6 without exceeding the elastic and fatigue limits of the material, the disc in radial cross-section assumes a shape defined hereinbefore as the natural shape." If the deflection d is now maintained and thus deformed disc 6 together with clamps 7 and 8 heated to the region of the stress-relieving temperature of the disc material, upon subsequent cooling, the disc 6 will have permanently assumed the natural shape.

In designing bellows which use such permanently formed discs 6, deflection d is made of such magnitude that, when a disc is fully compressed, i.e., until it is flat, the permissible fatigue stress of the disc material is preferably not exceeded. The maximum stress thus created, a radial bending stress, resides adjacent the inner margin of the disc.

The bellows illustrated in part in FIGS. 2, 5 and 6 comprises discs or annuli 1 formed with natural shape, which shape permits such bellows to be fully compressed so that the facing internal surfaces 2 and facing external surfaces 3 of the annuli are flat and contiguous. FIG. 5 illustrates such bellows partially compressed and, as can be seen, the annuli l are approaching a flat condition without buckling or otherwise becoming deformed as would occur had the annuli not possessed the natural shape. FIG. 6 shows the same bellows in fully compressed condition.

It should be mentioned that the shape of each annulus l as shown in FIG. 5 is that which would be assumed thereby in the absence of internal pressure within the bellows, and that in such absence the bellows formed from naturally shaped annuli possess a constant spring rate or stiffness.

The effect of internal pressure within the bellows as illustrated in FIGS. 2, 5 and 6, as would be the case if such bellows were employed to compress a fluid, would be to squeeze together adjacent annuli so that the facing external surfaces 3 thereof become progressively contiguous, a rolling action occurring between the surfaces 3 commencing adjacent margin 4 and moving towards margin 5 (FIG. 9 illustrates a similar effect but which is due to the modified shape). Upon full compression, contiguity is once more achieved as shown in FIG. 6.

The effect of the internal pressure is to generally increase the stress within the annuli, but as compression of the bellows increases, the ensuing outward rolling action provides natural support between annuli so that stresses, where contiguity of surfaces 3 has occurred, do not exceed the normal radial bending stresses. However, the pressure stress under such circumstances can greatly exceed the radial bending stress adjacent the outer margin 5 and limit the usefulness of the bellows.

Since this outer radial bending stress is of opposite sign to that of internal pressure stress, by deliberately increasing the bending stress adjacent outer margin 5 of each annulus I when the bellows are partially or wholly compressed, it becomes possible to tolerate higher internal pressures. In order to achieve such increase in outer radial bending stress the curvature of each annulus is modified as will now be described with reference to FIG. 7.

The cross-sectional radial shape of a naturally shaped annulus I is illustrated in solid outline in FIG. 7, and which is composed of flat outer margin 5 tangential to the curve which commences at A, moving towards inner margin 4, and continuing with decreasing positive curvature until a point C is reached where the curvature has reduced to zero, at which point the curvature changes sign and now continues towards the inner margin 4 with increasing negative curvature until point B is reached, at the boundary of margin 4, where the inner margin is tangential to the curve. Point C is not necessarily mid-way between A and B, but displaced somewhat towards B since the curve is that of an annular disc rather than that of a linear beam.

A line drawn through C, normal to the annulus surface at that point would cut two parallel lines which respectively pass through points A and b, at some angle When the positive curvature, i.e., that between points A and C, is generally increased without substantially changing that curvature lying between C and B, the shape of the annulus assumes that illustrated by dotted line A C and the solid line C B. Point C' is move somewhat nearer B, and the normal line passing through C now cuts the two mentioned parallel lines with increased angle 0'.

A bellows made from annuli having such a modified shape would, during compression, possess a rolling action from inside to outside between surfaces 3, and FIG. 9 illustrates such a bellows when partially compressed, with or without internal pressure, although the presence of internal pressure exagerates the action.

A greater radial bending stress is produced at point A (FIG. 7) (this is not strictly a point but a circumferential line) when such bellows are compressed, the modification to shape being such that the maximum permissible fatigue stress is not exceeded. Conversely, if the region of positive curvature in each annulus l is generally decreased, as illustrated by the dotted line A" C" in FIG. 8, angle 0 now being decreased to 9", a rolling action from outside to inside between facing interval surfaces 2 of adjacent annuli I occurs (see FIG. 10) thus squeezing internal fluid radially inwards, and the radial bending stress at point A is decreased. Since stresses induced in such bellows by external pressure are of the opposite sign at the inner margin as the bending stresses, increased bending stress will permit higher external pressures to be achieved. Annuli of decreased positive curvature are therefore employed in bellows which are compressed by the action of external pressure as in the example of compressor to be described in relation to FIGS. 13 and I4.

The rolling action induced by this modification to shape results in improved inward scavenging of gases and (b) mutual support between contiguous surfaces so that pressure induced stresses are not domi nant.

Where bellows are to be compressed by the action of external pressure. as the bellows approaches the fully closed condition, the compressing action becomes less effective and therefore slower as access of the pressurising fluid to external surfaces 3 of annuli l is restricted. A modification to such bellows is shown in FIG. 11, in which cojoined pairs of annuli are spaced apart by rings II) which are joined to the respective inner margins 4 of adjacent cojoined pairs. The closing action of the individual pairs of annuli remain unchanged, but the overall length of bellows is increased.

Similarly, where a compressed bellows is to be extended by the use of internal pressure, such as in an actuator for example, spacer rings such as rings II of FIG. 12, may be used to separate pairs of annuli I, each cojoined at their inner margins 4, the rings being inserted between the outer margins 5 of adjacent pairs.

A compressor, such as for compressing air will now be described with reference to FIGS. 13 and 14, and which comprises a chamber inside which a bellows 26 is joined to one end thereof, and which chamber is provided with a cylindrical portion 21 in which slides a piston 22. The free end of bellows 26 is closed by means of a circular plate 28 to which it is welded and which plate carries a junk piston 29 on its surface within and concentric with the bellows 26. The bellows 26 is similar to that illustrated in FIG. II, and has spacer rings 27 corresponding to rings I0 of FIG. 11. The axial length ofjunk piston 29 is approximately that of the three rings 27 combined plus the total thickness of annuli, and of slightly smaller overall diameter than the internal diameter of the rings. Air inlet and outlet valves, diagrammatically illustrated by discs 30 and 31 co-operating with respective ports 32 and 33 in the chamber 20 wall, are provided for admission of air into, and expulsion of air from, the internal volume of bel lows 26.

The space defined by the piston 22, cylinder 2I, chamber 20, bellows 26 and endplate 28 is filled with a fluid 25 such as oil so that, as piston 22 is caused to reciprocate within cylinder 21 by means of crank 23 and connecting rod 24, the bellows 26 are alternately compressed and expanded by the action of fluid 25 thereupon.

FIG. I shows the bellows 26 in the fully compressed condition, and illustrates how the provision ofjunk piston 29 serves to reduce the free volume of the compressed bellows, and thus affords, together with the complete closure of each pair of bellow annuli, high compression ratios.

Bellows contemplated by the invention are made from quite thick sheet material compared with bellows of similar dimensions intended for similar use, for example an external annulus diameter of 8 inches and internal diameter of 4 inches, the thickness of each annulus is between 0.032 and 0.036 inches for martensitic stainless steel, permitting internal pressures of ISO to 500 lbs/ins to be obtained in compression without exceeding the fatigue stress limit, and permitting greater than [0 flexural cycles to be obtained. For bellows compressed by external fluid pressure, it is believed that internal pressures of the order of 3,000 atmospheres may be tolerated whilst nondynamic bellows are expected to tolerate even higher pressures. It is anticipated that somewhat lower pressures may increase still further the fatigue. Such stiffness can be utilized to provide the restoring force which extends the bellows after compression, and FIG. 15 illustrates diagrammatically a compressor in which a face-cam is used to compress the bellows in one direction, the bellows being free to extend as a result of their stiffness when so per mitted by the cam. A face-cam 45 is driven to rotate by a shaft 47, any axial thrust on the cam 45 being carried by thrust-bearing 46 and casing 48. A roller 43, attached eccentrically to a pillar 42 which is aligned with the axis of shaft 47, and free to slide axially in guide 44, bears upon cam 45 and follows the contour thereof as the cam rotates. The roller 43 accordingly imparts a reciprocating motion to pillar 42 which in turn alternately compresses a bellows 40, to which it is attached via bellows end-plate 41, and permits it to extend. the spring force of the bellows 40 maintaining the roller 43 thrust against the face-cam 45.

Where reference is made in this specification to increasing or decreasing the positive curvature of an annulus radial cross-section, it is understood that this modification is relative to that section having negative curvature and that similar results may be obtained by changing the degree of negative curvature relative to that which is positive.

Furthermore, although reference is made throughout the specification to circular annuli, the invention embraces bellows having elements of different shape such as for example oval. Also, annuli comprising two or more thicknesses of sheet metal, i.e., for bellows known as multiply bellows, are included in the scope of the invention.

In conclusion the readers attention is directed to U.S. Pat. No. 3,090,043 wherein bellows, such as are known to have been used in cryogenic gas expansion engines, are described having such shape as to permit complete bellows closure, but which rely upon thin, multiply discs together with a tortuous cross-sectional shape to avoid destructive stress and undue buckling of the annular elements when fully compressed. US. Pat. No. 3,090,403 clearly indicates some of the problems which the invention seeks to conquer.

I claim:

1. Compressible metallic bellows comprising a plurality of concave, sheet metal annuli axially aligned and joined one to another at their inner and outer margins respectively so that the concavities of the annuli are alternately facing towards and away from one another, in which each annulus in its neutral condition comprises:

at least one flat outer margin lying in a first plane;

at least one flat inner margin lying in another plane parallel with said first plane;

a curved portion disposed between and continuous with said flat inner and outer margins;

the shape of the curved portion being such that over its length between said inner and outer margins, a sinusoidal curve is generated so that when the bellows is axially displaced, the shape of the curved portion conforms to a sinusoidal curve over at least a part of its length, and when said bellows are compressed, the curved portions are flattened with respect to each other.

10 said margin.

5. Bellows according to claim 4 wherein the election beam is directed so as to impinge radially upon the abutting edges of the respective margins while the annuli are rotated relative to the beam, the beam having an intensity and dimension so as to produce a seam of radial depth greater than the combined thickness of the abutting margins.

Claims (5)

1. Compressible metallic bellows comprising a plurality of concave, sheet metal annuli axially aligned and joined one to another at their inner and outer margins respectively so that the concavities of the annuli are alternately facing towards and away from one another, in which each annulus in its neutral condition comprises: at least one flat outer margin lying in a first plane; at least one flat inner margin lying in another plane parallel with said first plane; a curved portion disposed betweeN and continuous with said flat inner and outer margins; the shape of the curved portion being such that over its length between said inner and outer margins, a sinusoidal curve is generated so that when the bellows is axially displaced, the shape of the curved portion conforms to a sinusoidal curve over at least a part of its length, and when said bellows are compressed, the curved portions are flattened with respect to each other.

2. Bellows according to claim 1 in which adjacent inner margins are axially separated by and joined with an annular spacer.

3. Bellows according to claim 1 in which adjacent outer margins are axially separated by and joined with an annular spacer.

4. Bellows according to claim 1 in which said inner and outer margins are joined together by an electron beam weld, the depth of each weld in radial direction being substantially the radial length of the associated said margin.

5. Bellows according to claim 4 wherein the election beam is directed so as to impinge radially upon the abutting edges of the respective margins while the annuli are rotated relative to the beam, the beam having an intensity and dimension so as to produce a seam of radial depth greater than the combined thickness of the abutting margins.

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