GB2246414A - Springs - Google Patents

Abstract

A stack of washers in a Belleville spring is arranged in a sequence of shorter sub-tacks (12) each of which is "containerised" in an open-ended tubular can (13) that fits slidingly into the next can (13) and acts upon the Belleville washers (11) therein, the allowable range of movement of one can (12) into the next being in practice insufficient to cause any of the washers (11) in the latter to be distorted beyond their elastic limit. In this way the total force generated in one can (13) is transmitted to the next via the can (13) itself rather than via the Belleville units (11), and the risk of stack buckling and frictional binding is reduced because the stacks (12) are short. <IMAGE>

Description

Springs This invention concerns springs, and relates in particular to that type of spring made up of a plurality of those units known as "Belville" springs or washers.

It is common in the engineering field for there to be a requirement for two parts to be sprung one relative to the other. Sometimes the two are sprung together, sometimes they are sprung apart, but in either caste the equipment can usually be designed to make use of a compression spring - that is, a spring that is intended in use to be compressed, and that when so compressed generates a restoring force acting to oppose the compressing forces.

Of the various types of compression spring available, helical coil springs are particularly useful, and a typical example of such a spring is the valve spring as used in an internal combustion engine to keep the inlet and exhaust valves shut unless forced open by the cams on the camshaft.

Coil springs are particular cases of torsion springs - a torsion spring is in principle a rod of springy material Ca metal like spring steel, say) that when twisted around its long axis resists the twist, attempting to untwist itself, and so generating restoring forces opposing the applied twisting forces.

To obtain a high restoring force one naturally needs a long rod, and it is convenient to form the rod into the generally tubular form of a helical coil Cwith sufficient space between adjacent turns) and then apply the "twisting" forces in the form of a compression force across the opposed faces of the coil/tube.However, if extremely high restoring forces are required - if the spring is to be "hard" - and at the same time the relative movement of the end faces - the longitudinal displacement of the spring - is to be small, then the "rod" from which the spring is formed must be of a substantial diameter (the large the diameter, the greater the torsional forces developed), and it may well be that, with the available materials, it is not possible, or it is not convenient, to make the spring so large; thus, for example, the space into which the spring will fit may simply be too small.One possible solution is to change to a different type of spring altogether - and one variety of alternative spring for use where the space constraints dictate the employment of a spring having a generally tubular form is that using o stack cf co-calld "Bclvl1c" washers, the whole then being referred to as a 'iBelville4' spring.

A Belville washer is, simply, a slightly dished washer - a ring or annul us - of springy material. It will be appreciated that if placed between two flat surfaces a dished washer will separate the two in a compressible way - that is, that if the two surfaces are forced towards each other the dish in which the washer is formed will begin to distort, and the elastic nature of the washer material will generate forces that act to try to prevent this distortion, to cause the washer to "undistort" itself, and so push the two surfaces apart, once the applied compressive forces are relaxed/removed.

Clearly, provided the distortion of the washer has not gone beyond the elastic limit (or provided, for any other reason, the washer is not prevented from returning to its undistorted state), the washer is acting as a spring - indeed, rather like one turn of a conventional helical coil spring.

Of course, in practice one Belville washer, like one turn of a coil spring, is not particularly useful.

Though the restoring forces it generates are extremely high, the permitted travel (the distance moved as one face is compressed towards the other) is very small, especially if the washer is not to be distorted too close to its elastic limit, and therefore in use it is common to employ a whole series of washers, arranged in face-to-face pairs (each rather like a couple of saucers arranged open-face-to-open-face, as opposed to nested one in the other). Thus, a Belville spring is constructed from a plurality of units each of which is a pair of face-to-face Belville washers.

A Belville spring has, in certain circumstances, considerable advantages over a conventional helical spring. Specifically, a relatively flimsy structure, occupying little space (especially laterally), can generate very high restoring forces for a relstively small longitudinal displacement. There are, however, certain drawbacks which render Belville springs less attractive than they might otherwise be.Firstly, when using Belville springs - long stacks of the Belville washer pair units - it is a fact that, for reasons that are presently unclear, the applied compression forces are not absorbed uniformly - linearly - along the length of the stack, but on the contrary tend to be taken up at one or other end of the stack (usually at the end that moves), so that there is a significant possibility that several of the units at the one end will be completely flattened - and thus pushed beyond the elastic limit, and so be unable to recover when the compression forces are removed - while at the other end the units are hardly compressed at all.Secondly, as might perhaps be expected, it is difficult to keep a long stack exactly straight, and linearly aligned, and so, as the stack "buckles" under increasing load, so the units bind press in a high friction manner - against either the inner or outer guides that constrain the stack. Not only does this binding reduce the efficiency of the stack as a spring (and contribute to the non-linear response of the stack, as described above but in addition the hardened edges of the units can dig into the surfaces of the inner and/or outer guides, doing serious damage thereto (and, of course, further increasing the possibility of non-linear action on some subsequent compression).

The present invention seeks to deal with both these problems by the very simple expedient of breaking the stack into a sequence of shorter sub-stacks each of which is "containerised In an open-ended tubular can that fits slidingly into the next can and acts upon the Belville units therein, the allowable range of movement of one can into the next being in practice insufficient to cause any of the washers in the latter to be distorted beyond their elastic limit. In this way the effect of any significant non-linearity in absorption of applied compressive force is overcome, because the total force generated in one can is transmitted to the next via the can itself rather than via the Belville units, and the risk of stack buckling and frictional binding is reduced because the stacks are short.

In one aspect, therefore, the invention provides an elongate spring member comprising a sequential assembly of a plurality of similar open-ended tubular holders springingly nested one into the next, each holder being stepped to have a long wide portion of one external diameter and a short narrower portion of a (lesser) external diameter corresponding to the internal diameter of its long wide portion, whereby in the assembly each holder's short narrower portion fits slidingly into the next holder's long wide portion, the space within the long wide portion of each holder being filled with Belville spring pair units, the length of each short narrower portion being such that when one holder is fitted slidingly into the next the distance it can travel thereinto while compressing the Belville units is not more than 60% of the distance that would result in all the units being compressed flat.

The spring member of the invention can be for any use where there might be employed an ordinary Belville spring - and indeed, where there might be utilised a conventional helical coil spring. One particular use is in drill stem test tools as employed at the bottom of pipes let down into wells (specifically oil wells) for testing the well, where it is commonly required to open and close valves to control the flow of fluid along the pipe, and it is desired to use spring-loaded valves movable in one direction by an applied force (a pulse of annulus pressure, say) and then automaticall1' movabJe ; the other when the force 1s r-rnt} d.

There may ba am many tubular holders - cans - as required to make up the necessary length of the spring (though obviously the number of holders depends upon how many Belville units each holds. A typical spring might be 20 units long, there being 5 units to each of 4 cans.

The actual length of each holder will again depend upon how many Belville spring units it is to contain, Cand, of course, upon the dished depth of each unit). A common size for a single Belville washer is 7.7cm (3in) inner diameter and 9.7cm C3.8in) outer, with a dished depth of 0.5cm (0.2in). A pair of such washers face to face in a unit will thus be lcm (0. 4in) deep, so that a holder containing 5 units in its wide portion will be about 11.3cm (4.4in) long plus the length of the narrower portion (about 4cm 1. 61no; see below).

Apart from the first in the stack, which may have a Belville-unit-retaining lip at the wider end, each holder will be like all the others, and have a long wide end and a short narrower end, the narrower end' 5 external dimensions being such that it fits slidingly inside the next unit's wide end to press upon the top Belville unit therein. The actual, and absolute, sizes will again depend upon the particular circumstances (and upon the size of each Belville unit), but a typical holder might be a circular section tube 10. 4cm (4. lin) outer diameter and 9. 8cm (3.9in) inner diameter at its wider end, and 9.7cm (3.8in) outer and 7.6cm (3.Oin) at its narrower end.

Each holder contains sufficient Belville units to fill its long, wide portion. It will be clear that the actual number of units depends upon the length of that portion and upon the depth of each unit, but a typical holder will contain about 10 units.

The length of the holder's narrower portion is such that when slid into the wide portion of the next holder it will not be able to move therein a distance greater than is likely to cause any one washer therein to distort beyond its elastic limit - which as a practical rule of thumb means that it cannot move more than 60D of the distance that would flatten all the units (and preferably cannot move more than 50%, and especially 40%, of that distance). Exactly what that distance is depends upon the nature of each Belville washer - how dished it is, what it is made of (and what its elastic limit is), and what its "flat" depth is.However, a common Belville washer has a dished depth of 0. scum (0. 2in) and a "flattened" depth of 0.3cm (0.12in), and it can notionally be squashed by 0.2cm C0.08in). Each holder will contain 20 of them (in 10 unit pairs), so that the sub-stack in the holder could notionally be flattened by 2cm (0.8in). If so, then the narrower portion of each holder will be no more than 1.2cm (.5in) long (60% of the "flattenable" distance), else the likelihood of one or more units actually being flattened in use is too great.

As will have been seen, dividing a long stack of Belville units into several shorter sub-stacks, and holding each in a can that is able to slide into the next can in the sequence, to bear upon the sub-stack in that can, significantly improves the performance of a Belville spring, by reducing the chance of nonrecoverably distorting the units and of jamming them against the sides of the equipment in which they are used.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying Drawings in which: Figures 1A. B & C show respectively perspective, sectional and plan views of a single Belville washer; Figure 2 shows a stack of the Figure 1 washers, forming a Belville spring; and Figures 3A & B show a Belville spring constructed of canned sub-stacks . n accordanc wjth the invention.

The Belville washer ( shown in Figure 1 is a plane circular washer slightly dished to assume a frustoconical shape. A stack of eight of these, in four face-to-face pairs (as lit lib), is shown in Figure 2.

The Belville spring construction of Figure 3 (3A shows the spring in its natural state, while 3B shows it compressed) contains 24 washers 11 arranged in 12 pairs and sub-divided into three sub-stacks (generally top sub-stack 12t, middle sub-stack 12m, and bottom substack 12b). Each sub-stack is loosely contained within a tubular holder, or can (as 13), and each can 13 has a long wide portion (as 14) and a short narrower portion (as 15), the dimensions of these being such that each can's narrower portion 15 fits slidingly into the open end of the next can's wide portion 14, resting against the uppermost washer 11 therein.

Resting on the uppermost washer 11 of the top substack 12t is a sleeve piston C16). If this is urged downwardly (as viewed) while the lowermost sub-stack can 13 is prevented from moving, then, as the applied force F increases, so the washers 11 in the top can 13 are gradually compressed. Of course, the applied force is shared with the washers 11 in the middle can 13, for the lower end of the upper can bears directly upon them.

Moreover, the force is also shared with the washers in the lower can 13, for the middle can's lower end bears on them. Accordingly, as the force F increases so all the washers in the complete stack gradually distort to absorb the load. As they do so the upper can's narrower lower end 15 slides further and further into the middle can's wide upper end 14, and at the same time the middle can's narrower lower end 15 slides into the bottom can's wide upper end 14. This "telescoping" of the cans continues until each narrower portion 15 is fully within the appropriate wide portion 14, and is able to slide no further - a situation shown in Figure 3B. The lengths of each short narrower portion 15 is arranged to be no more than 60% of the distance by which each sub-stack would be reduced if the washers therein were completely flattened.

Claims (6)

1. An elongate spring member comprising a sequential assembly of a plurality of similar open-ended tubular holders springingly nested one into the next, each holder being stepped to have a long wide portion of one external diameter and a short narrower portion of a Clesser) external diameter corresponding to the internal diameter of its long wide portion, whereby in the assembly each holder's short narrower portion fits slidingly into the next holder's long wide portion, the space within the long wide portion of each holder being filled with Belville spring pair units, the length of each short narrower portion being such that when one holder is fitted slidingly into the next the distance it can travel thereinto while compressing the Belville units is not more than 60% of the distance that would result in all the units being compressed flat.

2. A spring member as claimed in Claim 1, wherein there are 20 Belville units disposed in 4 cans, 5 units in each can.

3. A spring member as claimed in either of the preceding Claims, wherein the length of each holder is about 15.5 cm C6 in).

4. A spring member as claimed in any of the preceding Claims, wherein each holder is a circular section tube 10. 4cm (4. liy) outer diameter and 9.8cm (3. 9in) inner diameter at its wider end, and 9.7cm C3.8in) outer and 7.6cm C3.Oin) at its narrower end.

5. A spring member as claimed in any of the preceding Claims, wherein the length of the holder's narrower portion is such that when slid into the wide portion of the next holder it will not be able to move therein a distance greater than 40% of the distance that would flatten all the units.

6. An elongate spring member as claimed in any of the preceding Claims and substantially as described hereinbefore.