WO2018055516A1

WO2018055516A1 - Disc spring assembly for vibration reduction

Info

Publication number: WO2018055516A1
Application number: PCT/IB2017/055685
Authority: WO
Inventors: Josef GIVLI; Itamar BENICHOU
Original assignee: Technion Research And Development Foundation Limited
Priority date: 2016-09-22
Filing date: 2017-09-20
Publication date: 2018-03-29
Also published as: IL247992A0

Abstract

New spring systems which prevent the transfer of forces above a predefined level, for isolating a body from high acceleration vibrations. The systems use a stack of shell elements which have a bistable elastic response, and are connected serially in the stack, with the elements co-aligned relative to the axis of the stack. The connections between neighboring elements are executed by sets of support connections at two or more locations, e.g. the inner and outer diameters of the elements. A simple way to materialize such a stack is by using dished washers or other shell-like elements that feature mechanical bistability. The bistable elements are spaced sufficiently far apart from each other to enable them to reach all three branches (elastic states) of positive, negative and positive stiffness, characteristic of bistable mechanical elements.

Description

DISC SPRING ASSEMBLY FOR VIBRATION REDUCTION

FIELD OF THE INVENTION

The present invention relates to the field of spring devices incorporating filter functionality which limits the accelerative force which the spring can transmit, especially those involving stacks of connected shell elements or concave-shaped discs having bistable mechanical behavior.

BACKGROUND

In a conventional shock absorber, comprising a spring and a damper, the damper absorbs kinetic energy, generally by converting it to heat energy, in a manner proportional to the relative speed of the motion imparted to the two ends of the shock absorber. While this is useful in absorbing the energy of shocks applied through the spring-damper system from the surroundings to the target object it is intended to protect, it does not limit the maximum accelerations, and hence the maximum forces that can be transferred from the surroundings to the protected target object.

There are many situations in which the target object must be protected from the application of excessive force from the surroundings in which it is mounted, this being equivalent to limiting the accelerations which can be applied to it from the surroundings in which it is mounted. Such a need also arises, inter alia, in the automotive industry, where the limitations of conventional shock absorbers are noticeable in poor surface terrain.

There have been described semi-active shock absorbers, which can vary the characteristics of the variable damper, and thus are able to limit the acceleration forces transmitted to the vehicle body, have become available, such as are described in US Patent No. 5,396,973 for "Variable shock absorber with integrated controller, actuator and sensors" to L.J. Schwemmer et al. The shock absorber dampers are fitted with a microprocessor controlled device which varies the level of damping according to the acceleration input to the damper, as detected by a sensor. This is typically achieved by providing the fluid dampers with a controllable valve to control the fluid flow between the shock absorber chambers, and by providing a feedback loop which provides the signal to the motive power element that varies the valve opening, according to the acceleration detected. In this way it becomes possible to limit the acceleration forces transmitted by the shock absorber to the car body.

However, such dynamically variable shock absorbers are complex and costly, and there therefore exists a need for a simpler mechanical device, which generates a limiting action to the accelerative force which can be applied through the device to the target body from the dynamic surround environment.

Mechanical devices and systems which show the use of beveled or dished washers have been described in the prior art, but such devices are often of complex construction and may not be selective of the accelerative forces transmitted. Other spring applications using such dished washers are also well known. The following list represents prior art relating to that field: US 3,735,952, US 5,263,694, US 3,682,466, US 2007/0138720, US 2010/0030335, US 3,744,814, US 5,423,400, US 2,708,110, US 6,705,813, US 5,628,388, US 3,871,193, US 7,169,190, US 8,029,574, US 3,743,266, US 6,695,294, EP 0516 874A1, US 4,267,648, EP 1 507 464 and US 2003/0056396.

There therefore exists a need for a spring assembly which passively limits the forces that can be transmitted through the assembly, and hence also the acceleration level of the motion transferred through the assembly, which overcomes at least some of the disadvantages of prior art systems and methods.

The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.

SUMMARY

The present disclosure describes new exemplary spring systems which prevent the transfer of forces or accelerations above a predefined level. Such a spring system can therefore be used to isolate or protect a body from high accelerations or forces generated in vibratory surroundings. Such spring assemblies combine acceleration limiting, and hence force limiting capabilities with efficient energy dissipation in a simple and passive device. The spring systems comprise a stack of shell structured elements which have a bistable elastic mechanical response, the shell structure having its conventional mechanical engineering meaning of a three dimensional, curved solid element having a thickness which is small compared with its other dimensions. A shell structure, as used in the present application, is understood to include a structure which may have openings or finger protrusions, or the like, so long as the thickness of the material making up the shell is substantially less than the lateral dimensions of the shell element. The bistable shell elements are connected serially in the stack, with the elements co-aligned relative to the axis of the stack. In the simplest configuration, the connections between neighboring bistable elements in the stack are generally executed by means of two sets of connections for each of the elements of the stack, one set being connected at a first location in the inner region of the elements, and the second set at a second location in the peripheral region of the elements. However, more complex connection schemes are possible, in which the bistable shell elements are connected to their immediate neighbor on one side by means of two or more support points, with the counter- directed support from the immediate neighbor on the opposite side being achieved at a point located radially between the above two-mentioned points. Various other configurations are also feasible, and the novel structures of the present disclosure are not intended to be limited by the form of connection between neighboring bistable shell elements. In any of these configurations, the connections must have sufficient longitudinal dimensions to enable the bistable shell elements to extend sufficiently that they can switch between their two bistable states.

A simple way to materialize such a stack is by using dished washers or Bellville springs as they are called in common use, and this configuration will be used for illustrating and describing the spring assemblies of the present disclosure and their operational functionality, though it is to be understood that other shell-like elements that feature mechanical bistability may also be used in such spring assemblies. For simple Bellville springs, for example, bistability requires a height to thickness ratio of h/t > 1.414, where h is the height of the Belleville washer when stress-free and t is the thickness of the washer material itself. Stacks of Belleville springs are known, such as in some of the above referenced patent documents, but these do not exploit the bistabililty features of the elements, and are used as simple nonlinear springs, which are not intended to enter the region of strain where the elements have negative stiffness. Also, for the purpose of energy absorption or for suspension devices, the Belleville springs which only take the role of nonlinear springs, are used in conjunction with a designated damper, usually a hydraulic or pneumatic one. It should be noted that not all Belleville springs imply a bistable elastic behavior. On the contrary, the vast majority of Belleville washers that are regularly manufactured for general use, do not have bistable behavior. As noted above, a Belleville washer may possess bistable behavior, on condition that the height to thickness ratio h/t > 1.414. Such a disc has a nonstandard geometry with substantially thinner material than conventionally used discs, and results in very high stresses as the spring is compressed, often beyond the yield stress of the metal used in the manufacture. Thus, even if the Belleville washers was designed to possess a bistable behavior, this behavior cannot generally be exploited due to the impracticality of using a spring with such a geometry. However, since the bistable behavior of a Belleville spring washer is simple to visualize, and to explain the operation of such complete spring assemblies, the example of Belleville discs is used where necessary to describe the structure and operation of the more generic devices of the present disclosure.

Such a stack of dished washers may be arranged around a central axis to maintain their mutual axial alignment, with alternative washers connected to their immediate neighboring washers alternately at their outer edges and their inner edges. A number of different configurations may be used, with the elements arranged either in the same direction or in alternatively opposite directions, and different configurations can provide either compression spring assemblies or assemblies for extension, or assemblies for both compression and extension operation.

The boundary conditions under which the rims of the discs are held, has a significant effect on the elastic performance of the stack. Unlike a disc which is not constrained radially, if a disc has rigid boundary conditions in the radial directions, both outwardly and inwardly, such as by the use of a disc holding element having clamping lips which limit the ability of the disc to extend radially, both inwards and outwards, the effective stiffness of the disc increases substantially, such that the disk stack can withstand larger loads. In other words, the force at which a transition occurs from one of the element' s bistable states to the other, is substantially increased, the slopes of the stiffness curve becoming steeper. This enables achievement either of an assembly able to withstand higher applied forces, or alternately, construction of a more compact spring assembly yet still able to withstand the same forces as those of a system without the limiting boundary conditions. One important feature of all of the spring assemblies described in this disclosure, is that the elements must be spaced longitudinally sufficiently far apart, such as by use of spacers at both the inner and the peripheral locations where the elements are connected or supported, such that the elements when compressed or extended, can experience a bistable response, namely can explore the two branches of positive stiffness that are separated by a negative stiffness "spinodal" branch. It is important to note that this is one important feature by which the stack of bistable elements of the present devices differ from prior art stacks of disc washers, which are well known in the art, but which have been used only in order to apply linear or even nonlinear forces, but without fully exploring the negative stiffness.

A spring assembly comprising a stack of bistable mechanical element, provides smooth operation, since, as will be shown below in the detailed description, the larger the number of elements in the stack, the larger the number of saw-tooth perturbations on the force/strain response curve, yielding less abrupt loading and unloading plateaus, and quieter and smoother operation. However, such spring systems can also be implemented with only a single bistable mechanical element, so long as the element is mounted between the source of the vibration and the item to be isolated in such a manner that, when compressed or extended, it can fully experience its bistable response, namely to reach both branches of positive stiffness that are separated by the negative stiffness "spinodal" branch.

Since a spring system constructed of separate dished washers and spacers involves a large number of parts assembled together, other manufacturing techniques may be used to reduce the number of parts, e.g. by 3-D printing of the entire spring system in one body, or in a few separate parts, or by rolling, or by any other manufacturing technique which can generate the necessary structure, specifically with the necessary stiffness properties of the bistable spring element components of the system.

Shell-like elements possess a bistable mechanical behavior if they have the correct dimensions and stiffness properties. Mechanical bistable elements have a non-monotonous force-strain relationship, where the strain, defined as the change of "length" of the element/device normalized by its original, stress-free, length. Such elements have, as the strain of the element increases, a first region with positive stiffness, followed by a second "spinodal" region with negative stiffness, and finally a third region again with positive stiffness. In order to achieve such a mechanical response, the dimensions and hence stiffness of the washers must be correctly chosen, and the washers must be sufficiently spaced apart longitudinally along the stack axis that they can cover the whole of their elastic range and switch freely from one equilibrium state to the other.

When loaded with small forces, a spring system composed of bistable elements connected in series has a relatively high stiffness, until reaching a critical force F_max at which the elements flip to their alternative equilibrium state, implying a dramatically reduced stiffness, represented by a plateau in the force transmitted as the displacement increases. It is noted again that a smooth plateau occurs only with a very large number of elements. Otherwise, a saw tooth pattern is obtained, which still limits the forces. When reducing the loading, hysteresis is observed, with the force reducing as the displacement is reduced until reaching a second and lower plateau at F_min<F_max, when the elements flip back to their first equilibrium state. This nonlinear force-strain relation leads to the following features of the bistable spring system:

(a) As long as the loading forces applied on the system are smaller than Fmax, the system behaves like a standard spring with relatively high stiffness.

(b) The plateau at Fmax, limits the magnitude of force and hence acceleration transmitted from the surrounding to the target body.

(c) The existence of a different unloading plateau, with F_min < F_max is an indication of hysteresis, and hence of energy absorption or dissipation of unwanted energy input from the surroundings.

(d) Unlike other energy absorbers which are often limited to one cycle, the system described in the present application operates for large number of cycles.

If constructed with the correctly selected stiffness and dimensions, the system operates in the region of the hysteresis, and the maximum displacements incurred should be such that the system maintains its operating point within the hysteresis region, such that it absorbs the energy imparted and limits the maximum force that can be transferred.

The system can therefore fulfill one or both of the following two functions:

(i) it can limit the force that can be transferred from the surroundings to the object to be protected, and (ii) it can dissipate the energy being transmitted from the surroundings towards the object to be protected.

Prior art spring assemblies comprising a stack of bistable spring elements, are generally constructed of a stack of the spring elements mounted on the outer surface of a mandrel or enclosed within an outer cylinder, which constitutes the base end element, with the force applied (in the case of a compression spring) to the stack by means of a second activating end element, which applies pressure to the stack of spring elements, forcing them down the mandrel or cylinder. One effect of this is that the mandrel or cylinder has to be substantially longer than the compressed length of the spring, since the spring elements have to be mounted on either the static base element, or the activating element pushing down on the mandrel or cylinder. This can be best illustrated by the common construction of an outer cylinder sliding on an inner mandrel, such as is shown in the exemplary spring assembly described in US patent application publication number US 2007/0138720 for "Belleville spring guide system" to Robert W Evans, which shows a cylinder sliding on an inner mandrel or two cylinders with one sliding over the other. Since the inner and outer sliding parts must have different diameters, the disc springs must all be mounted on one of those two sliding parts. This means that the total uncompressed length of the spring assembly device must be substantially longer than the compressed spring length, by an amount depending on the compression range of the spring stack, or expressed conversely, compression of the spring is not accompanied by an overall reduction of the length of the spring assembly. The devices described in the present disclosure provide a construction of such a spring assembly, in which as the spring is compressed, the total length of the spring assembly also shortens accordingly. This is achieved by utilizing a pair of cylinders for supporting the disc spring stack, with one of the springs sliding over the other, but incorporating a spiral spring which fills the necessary radial gap between the two sliding surfaces as they mutually slide. Hence, the spiral spring functions as a self-adjustable spacer, which passively adjusts its length while maintaining a constant diameter. Use of a spring assembly with this feature can significantly simplify mechanical constructions in which such a spring assembly is incorporated. This disclosure thus describes acceleration- sensitive, vibration-limiting suspension systems, which are simple in construction, are completely passive in operation, and should be designed such that the elastic limit of the bistable spring elements is not exceeded. Such systems are thus useful for automotive suspensions, whether for cars, trucks, motorcycles, bicycles, or trains, for the isolation of industrial machinery from their environment, for absorbing the recoil of firearms, in highly vibrating and tools such as hammer drills and jack hammers, to protect sensitive electronic circuits from vibration, such as in rocket propelled vehicles, and even in such domestic products as orthopedic shoes.

There is thus provided in accordance with an exemplary implementation of the devices described in this disclosure, a spring assembly comprising a stack of shell structured elements, at least some of the elements being bistable elements having bistable elastic characteristics when compressed or extended along the axial direction of the stack, wherein the bistable elastic characteristics comprise, in order of increasing strain of the element,

(i) a first state of elasticity having positive stiffness,

(ii) a second state of elasticity having negative stiffness, and

(iii) a third state of elasticity having positive stiffness,

and wherein at least some of the stacked bistable elements are spaced sufficiently far apart to enable them, when force is applied to the assembly, to reach the third state of elasticity, having positive stiffness.

In such a spring assembly, at least some of the bistable elements may be shaped in the form of any one of dished washers, disc springs or Belleville springs. Alternatively, at least some of the bistable elements may be annular discs having a concave shaped profile.

In a further implementation of any of the above described spring assemblies, at least one of the bistable elements may be connected to its immediate neighboring bistable elements at radially separated locations, the locations being such that a force applied between the radially separated locations is adapted to cause the bistable element to switch from its first state of elasticity to its third state of elasticity. In such a situation, the radially separated locations are two locations, one located at an inner peripheral region of the bistable element, and the other in the outer peripheral region of the bistable element. Alternatively, the radially separated locations may be two locations, one located at a radially inner or a radially outer region of the bistable element, and the other at a radially intermediate region of the bistable element. In yet another implementation, the radially separated locations may be three locations, the first two at radially inner and radially outer regions of the bistable element, and the third in a region radially intermediate between the first two locations on the bistable element. In the latter case, the bistable element may be connected at the first two radially separated locations to one of its immediate neighbors through a rigid annular support spacer, and at the third intermediate radial location to its other immediate neighbor through a rigid, elastic, or viscoelastic connection element. In that case, the bistable element and one of its immediate neighbors may have apexed profiles facing each other, and the rigid, elastic, or viscoelastic connection element is attached between the bistable element and the facing immediate neighbor at the third intermediate radial location in the region of the apexes.

Yet further implementations of any of the above described spring assemblies may comprise rigid annular support spacers having at their inner and peripheral regions, mechanical features adapted to restrain radial compression and/or radial extension, respectively, of the bistable elements.

In other implementations of such spring assemblies, at least some of the bistable elements may have an annular shape.

In any of the above described implementations, the stack is adapted to be connected at one end to a vibrating environment, and at the other end to a body, the spring assembly being intended to limit the accelerative forces transmitted to the body from the vibrating environment.

According to yet further implementations of the above described spring assemblies, the stack may be manufactured in the form of a single body. Alternatively, it can be constructed of individual shell structured elements connected at their central regions and at their peripheral regions.

In any of the above described spring assemblies, at least one of the bistable elements may be an annular disc having a convex profile between its inner and outer edges. In such a case, the bistable element may be supported such that a force may be applied to it between the apex region of the convex profile and two points located at radially opposite locations of the at least one bistable element.

In yet another exemplary implementation of such spring assemblies, the bistable elements may be mounted on an inner core comprising an inner cylindrical tube adapted to slide within an outer cylindrical tube. The inner and outer cylindrical tubes may be connected respectively to first and second end elements adapted to transfer an applied external force to the bistable elements of said spring assembly. Such implementations may further comprise a spiral spring located within a radial space formed between the inner edge of said stack of bistable elements and the outer surface of said inner cylindrical tube.

Finally, any of the above described spring assemblies may be used in a method of limiting transmission from surroundings to a body, of forces arising from motion having accelerations greater than a predetermined level, by attaching the spring assembly between the surroundings and the body.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig. l illustrates schematically the force-strain relation of a single bistable element;

Fig. 2a which shows the theoretical force-strain equilibrium curves of a bistable element chain, while Fig. 2b (solid thick line) shows the effect of gradual increase followed by a gradual decrease of the overall length of the chain;

Fig. 3a shows a representation of the theoretical curve shown in Fig. 2b, for a chain with a large number of bistable elements, while Fig. 3b shows the force-displacement curve of a double bistable element chain enabling operation for compression and tension forces;

Fig. 4 is a representation of a Belleville washer such as could be used to construct a bistable element chain acting as a vibration absorbing spring assembly;

Figs 5a and 5b show an exemplary implementation of a complete disc spring assembly using a stack of Belleville type of washers as the bi-stable spring elements, Fig. 5a showing the spring assembly at rest, and Fig. 5b showing the spring assembly in a compressed state; Figs 6a and 6b show another exemplary implementation of a complete disc spring assembly, similar in structure to that of Figs. 5a and 5b, but fabricated from a single piece of material;

Figs. 7a and 7b illustrate an alternative compression spring assembly configuration to that of Figs 5a and 5b, in which all of the bistable washers are all aligned in the same direction;

Fig. 8 illustrates schematically a configuration of stacked dished washers, which enables the construction of a spring assembly which can operate in tension and compression;

Fig. 9 illustrates schematically a further configuration of a dished washer spring assembly which can operate in tension, and which uses alternately aligned washers;

Fig. 10 schematically illustrates the vibration reducing operation of the disc spring assembly devices of the present disclosure;

Fig. 11a shows a typical Belleville washer structure for use in the assemblies shown in the previous drawings, while Figs. 1 lb to 1 lj show a number of different disc configurations which enable application of a substantially higher force than can be applied than to a Belleville washer structure of the same overall dimensions;

Fig. 12 illustrates schematically a complete spring assembly constructed using a stack of bistable washer elements having a convex profile; and

Fig. 13 is a blown-up view of a section A- A of Fig. 12, so that the details of the construction can be clearly shown.

DETAILED DESCRIPTION

Reference is first made to Fig. 1, which illustrates schematically a graph of the force-strain relation of a single bistable element, as indicated by the dashed curve. For simplicity in presenting calculations and in order to understand the formalism for the multiple element case of a chain or stack of bistable elements connected in series, and without loss of generality, this relation can be approximated by a trilinear relation, shown as a solid line in Fig. 1. This relation includes two branches with positive stiffness, k¹ > 0 and k¹¹ > 0 separated by a "spinodal" branch with negative stiffness, k^* < 0.

Considering now a chain made from N identical bistable elements connected in series, where each element has a tri-linear elastic response. The chain is fixed at one end and is extended by AX at the other. The strain of the i-th element is then s_i = Ax_{ I a , where At. is the elongation of the element and a is its undeformed length. Correspondingly, the overall strain of the system is given by:

AX 1 "

ε≡ =— 2^ ^εί

N i=i _{( 1 )}

Where = Na is the overall length of the unloaded chain. In the initial stages of extension, all the elements are in "Phase-I", and as the chain is stretched all elements have identical lengths, since mechanical equilibrium requires identical force to be applied on all of the elements. This case persists as long as the force is smaller than F_max. The overall stiffness, in terms of strain, of N s rings connected in series is given by:

where k_i = dF I ds_i is the stiffness of the i-th spring. Therefore, during this stage, where all elements are in Phase-I, the overall stiffness of the chain is K = k' . Extending the chain beyond this point requires that one of the elements be in the spinodal region, while all the other elements remain in Phase-I.

Applying similar considerations to those above, it can be shown that the overall stiffness in this case is given b

which can be either positive or negative, depending on the number of elements in the chain. If N is large enough, K is positive and this configuration is unstable; thus the system switches to the next configuration, where one element is in Phase II while all other elements are in Phase I. On the other hand, if N is small enough, K is negative and this configuration is stable. Thus, this stage (with one element in the spinodal region) continues until the force is reduced to F_min. Extending the chain beyond this point leads to the next configuration where one element is in Phase-II while all other elements are in Phase-I. The overall stiffness in this new configuration is positive with

₍₄₎ and the force increases until it reaches F_max. The process described above then repeats itself until all elements are in Phase-II.

Reference is now made to Fig. 2a, which shows the resulting force-strain equilibrium curve of a bistable element chain. The solid lines show stable equilibrium configurations with no elements in the spinodal region, while the dashed lines correspond to unstable configurations that involve one element in the spinodal region.

Reference is now made to Fig. 2b, which shows the effect of gradual increase followed by a gradual decrease of the overall length of the chain. The slow increase in the overall length of the chain, as indicated by the right-pointing arrow, results in a saw-tooth pattern associated with Fmax , while unloading the chain, by slowly decreasing the overall length, results in a saw-tooth pattern associated with F_min , as indicated by the left-pointing arrow. The greater the number of elements in the chain, the narrower and smaller the "teeth" of the force- strain graph.

Reference is now made to Fig. 3a which shows a more life-like representation of the theoretical curve shown in Fig. 2b, for a chain with a large number of bistable elements. In Fig. 3a, curve 1 shows the non-monotonous force-strain relation of a single bistable element, while curve 2 shows the saw-tooth pattern resulting from a chain of 15 elements connected in series. If the number of bistable elements is high enough, as in curve 3, the saw-tooth pattern will smoothen, yielding the above described loading and unloading plateaus, with hysteresis effects. A more rigorous theoretical treatment of bistable chains and their applications has been given by the inventors of the present application in the article entitled "Structures Undergoing Discrete Phase Transformation" published in Journal of the Mechanics and Physics of Solids, Vol. 61, pages 94-113 (2013), with specific reference to structures ranging from proteins and sub-cellular components in biological systems to microscale structures of standard materials.

The above description has been presented in terms of a system operating in extension, thus force and strain are shown in the positive quadrant, although the mathematical description is equally relevant for a system operating in compression. If two such systems are assembled, one for compressive forces and the other for tension, the two-directional force-displacement curve of a double bistable element chain described in Fig. 3b is obtained, such that the spring system can limit the transfer of acceleration forces and absorb the energy for vibrational forces operating in both directions. System that can operate in compression and in tension can also be envisaged, as will be described hereinbelow in connection with the implementations shown in Fig. 8 as an example of such a realization.

Turning now to practical designs for the above described stacks or chains of bistable elements, a convenient method of visualizing the operation of such chains is by using the simplest type of dished washers, also known as Belleville washers or spring washers, to act as the bistable spring elements, and to assemble them in a stack in series, as described in the summary section, and separated by spacers. The spacers fulfil an essential function in the systems of this application, that being to allow the bistable washers to explore the necessary range of displacements required for the full non-monotonous force-displacement behavior range. Without such spacer elements, the bistable washer elements, when extended, would encounter mechanical stops at the edges of their mounting structure, that will limit their motion to a linear, or, at most, a non-linear elastic characteristic. Prior art stacks of such dished washers generally omit or are oblivious of this condition.

If necessary, concentric alignment can be maintained with a cylindrical guide, or by means of a rod through the central apertures of the washers. In particular, the inner and outer boundary components locating the bistable disc elements should be connected, so that the whole stack maintains longitudinal stability, and the discs maintain a prescribed or preferred alignment. Failure to connect the edges accordingly may result in degradation of the boundary conditions, and undesirable performance of the stack. In order to avoid this, the boundary conditions must be maintained over the working range of the stack. In order to achieve this, it may be required in some implementations to use a connecting bridge. This will be further discussed in relationship to the implementations shown in Fig. 1 1 and Fig. 12.

The dimensions of the elements used must be such to enable a non-monotonous force-strain relation. For simple Belleville springs, such behavior is achieved for ratios of h/t > 1.414, where, from the representation shown in Fig. 4, the thickness of the washer material is shown as t, and the height of the unloaded washer is shown as h. If the Belleville spring is not simply supported at its inner and outer diameters, or if the spring element used has a geometry different than that of a conventional Belleville spring, as will be shown in Figs, l ib to l lj herein below, the ratio of h/t that results in bistable behavior may be significantly different.

Reference is now made to Figs. 5a and 5b which shows an exemplary implementation of a complete disc spring assembly using a stack of Belleville type of washers as the bistable spring elements. In Fig. 5a, there is shown a stack of spring washers, 51, 52, stacked with successive washers being aligned in opposite directions. Neighboring pairs of washers are rigidly connected at their outer peripheries by means of annular spacers 53, and at their inner boundaries by annular spacers 54. The complete series stack, made up of repeated pairs of neighboring washers, is guided by a central rod 55 having a step 57, to butt against the stack and to compress it as the rod is inserted relative to the outer guide tube 56. The outer guide tube 56 may also be used to maintain concentric alignment of the disc washers, either together with the central rod 55 or in place of the central rod 55, and to provide environmental protection to the chain of bistable elements. Alternatively neither the central rod nor the outer guide tube may be necessary for the purpose of maintaining alignment, as will be shown in the exemplary implementations shown in Figs. 6a, 6b, 8 and 9. Fig. 5b shows the spring assembly of Fig. 5a, when the assembly is compressed, which requires the application of a force F, showing how the bistable elements (the Belleville washers) have reversed their state, and yet in which the inner and outer spacers are of sufficient length to prevent the washers from bottoming against each other under full compression. The space between the outer edges of neighboring discs closes up on compression, as shown in Fig. 5b, but does not go to zero, which would imply failure to ensure the necessary condition of ensuring the full travel of the bistable elements into its third elastic state.

One feature of the spring assembly shown in Figs. 5a and 5b is that unlike a conventional spring which shortens as it is compressed, compression of the spring assembly of Figs. 5a and 5b is not accompanied by an overall reduction of the length of the spring assembly. Thus, when uncompressed, the assembly has a length LI + L2, where LI is the length of the guide tube and the spring stack within it, and L2 is the protruding length of the central rod 55 for applying the load force F on the spring stack. As the rod 55 compresses the spring stack, it shortens within the guide tube, but the central rod protrudes from the other end of the guide tube, such that the overall length remains unchanged. This may complicate the construction of any mechanism incorporating such a spring assembly, and a novel type of such a spring assembly is presented in Figs. 12 and 13 hereinbelow, which overcomes this problem, in that the overall length shortens as the spring is compressed.

Reference is now made to Figs. 6a and 6b, which show respectively, an alternative exemplary method of manufacturing the disc spring assembly of Figs. 5a and 5b, but manufactured from a single piece of material, whether rolled, 3-D printing, or produced by some other method. The operation of the device is similar to that shown in Figs. 5a and 5b, showing the four components having critical design features, namely the two bistable elements, 61, 62, and the outer and inner spacers, 63 and 64, respectively. The device can be manufactured of any suitable material, with the bistable elements themselves having the specific stiffness, material thicknesses and dimensions to ensure correct and full range of operation of the disc spring assembly.

The disc spring assemblies shown in Figs. 5a, 5b, 6a, and 6b are designed for use in a compressive force application. Reference is now made to Figs. 7a and 7b, which illustrate an alternative configuration of a compression spring assembly, in which all of the bistable washers 71 are all aligned in the same direction. The peripheral outer rim of each washer is connected to the inner border of its neighboring washers by the use of spacers - spacer 72 being connected to the outer rim of the washers and the other spacer 73 being connected to the inner border of the washers. The spacers are shown connected to each other by means of a circular conventional connection bridging disc 74, though any other connection method may be used. In effect, the bridging disc 74 may incorporate the spacers 72 and 73 into one disc shaped element having oppositely directed lips 72, 73 at its outer and inner diameters. The implications of this geometry will be further considered in relation to Figs 12 and 13 hereinbelow. Fig. 7b shows the device under compression, and it is observed that as a result of the compressive force F, the bistable discs 71 have flipped from one stable phase to the other stable phase, the spacers being sufficiently long as to provide sufficient room to enable this action to be executed.

Reference is now made to Fig. 8 which illustrates schematically a configuration of stacked dished washers, which enables the construction of a spring assembly which can operate in tension and compression, whose characteristic elastic behavior was previously shown in Fig. 3b. In this implementation, the washers are all aligned in the same direction, and are alternatively used for implementing the tension or the compression phase of the forces applied thereto. The washers are labeled separately 81, 82, even though they may be identical in physical properties, because one set of the washers is operable when the spring is in tension, and the other is operable when the spring is compressed. Referring to Fig. 8, the spring washers 81 on the upper side (relative to the drawing) of each joined pair of bistable elements, are operable when the spring is compressed, and the spring washers 82 on the lower side of each joined pair of bistable elements, are operable when the spring is extended. Outer spacers 83 and inner spacers 84 ensure that each bistable spring washer has sufficient longitudinal room to cover its entire range of motion, including all three stiffness phases. In operation, when the spring assembly is compressed, the applied force can cause the upper bistable washers 81 to flip to their third state of elasticity, namely the second region of positive stiffness, denoted as Phase II in Fig. 2. Under compression, the direction of application of the compressive force is such that the lower bistable washers 82 act as a conventional spring and simply tilt further in the direction in which they are disposed, without undergoing any bistable switching. Likewise, under tension, the direction of application of the extensive force is such that the upper bistable washers 81 act as conventional springs and simply tilt further in the direction in which it is disposed, without undergoing any bistable switching.

Reference is now made to Fig. 9 which illustrates a further configuration of a dished washer spring assembly which can operate in tension, and which uses alternately aligned washers. The spring washers 91 on the upper side (relative to the drawing) of each joined pair of bistable elements, are operable bistably when the spring is extended, together with the spring washers 92 on the lower side of each joined pair of bistable elements. Outer spacers 93 and inner spacers 94 ensure that each bistable spring washer has sufficient longitudinal room to cover its entire range of motion, including all three stiffness phases.

Reference is now made to Fig. 10 which is composed of three drawings, schematically illustrating the operation of the disc spring assembly devices of the present disclosure. Referring to the left hand drawing of Fig. 10, there is shown the vibrating surroundings at the bottom of the device with the spring device attached thereto and enclosed within a flexible covering, such as for environmental protection, with the body which is intended to isolate from the vibrating surroundings. The purpose of the device is to filter out high accelerations or forces from passing from the surroundings to the target. This will limit the accelerations experienced by the target, thus providing means of mechanical protection or isolation of the target. The graph next to the drawing element representing the surroundings, shows a schematic representation of a typical acceleration as a function of time which the surroundings would impart to the target body if no vibration filtering device were used. The graph next to the drawing element representing the target, shows how the use of the device of the present application has filtered out accelerations over a certain predetermined level, thereby providing a level of isolation of the target from those vibrations of the surroundings which are above a predetermined higher level.

As previously explained, because of the geometrical limitations required of dished washers to ensure that they have bistable mechanical behavior, the material of the disc can easily reach the plastic limit, and fail in fulfilling its purpose. Therefore, for practical use, an element shaped such that it has higher intrinsic mechanical strength is needed. Reference is now made to Figs. 1 la to 1 lj, showing a number of different disc configurations in order to illustrate practical solutions to this problem.

Reference is first made to Fig. 1 1a, which illustrates schematically a conventional dished washer (a simple Belleville spring) of the type discussed hereinabove, showing the support points, one at its outer periphery, and the other, being the point at which the load is applied, at its radially inner boundary. In order to increase the strength of such a shell structure, a number of different configurations are shown in Figs, l ib to l lj. All of these configurations are in the form of annular washers having an open central region. A common feature of all of these configurations is that they show a convex shaped profile, with one location at which the compressive or tensile force is applied to the structure being at or near the apex of this convex-shaped profile. The counter-directed support location or locations may be located in the region or regions of either the inner and outer peripheries of the washer, or at an intermediate position, depending on the location of the apex of the disc profile. Various optional designs are shown in Fig.1 lb-j, and also summarized in Table 1 , which shows a number of possible configurations of the support and load locations on such annular disc implementations, and the drawings in Figs, l ib to l lj where some of these implementations are illustrated. By having a support location at or near the apex of a convex-shaped shell structure, the ability of the shell structure to withstand higher applied forces is greatly increased. The geometry of each of these convex-shaped shell structures must be calculated such that the shell structure shows bistable elastic behavior. The term geometry is used to include, for any chosen material of which the washer is constructed, both the thickness of that material, and the spatial profile of the washer.

TABLE 1

Fig. l ib illustrates a configuration in which the support locations are respectively at (or near to) the apex of the convex shape of the washer and at the outer edge of the washer. Fig. 11c on the other hand, shows a configuration in which the support locations are respectively at (or near to) the apex of the convex shape of the washer and at the inner locations r of the washer. Fig. 1 Id, on the other hand shows an element which is supported at both peripheral locations, inner and outer, with the counter- directed support applied at the washer apex between those two supports. This configuration enables a higher force to be applied, for an equivalent sized washer, than any of the other configurations of Figs. 11a to l lj, and a practical example of a complete spring assembly constructed using this configuration, is shown in Figs. 12 and 13 below. It is noted the abovementioned arrangements and those described in Table 1 and Fig. 1 1 of the locations of the supports and force, present some typical examples, yet other arrangements may be used. In all of the above mentioned Figs. 1 la to 1 Id, one support location is called and shown as a base or support location or locations, and is shown with the triangle marks in the lower side of the disc in the drawings, and the other as a location at which the force or load is applied, and is marked by arrows on the higher side of the disc in the drawings. It is to be understood, however, that in the mechanical formalism, there is no difference between the differently used nomenclature for these two support locations, since the force is applied between them, regardless of nomenclature.

Reference is now made to Figs. 12 and 13, which illustrate an exemplary complete spring assembly 120 constructed using a stack of oppositely facing bistable washer elements 121 having a convex profile, with support locations at the inner and outer boundaries of the washers, and the intermediately positioned support location being in the region of the apex of the washers. This configuration provides high force capacity in minimum dimensions. Fig. 12 shows an overall representation of a complete spring assembly 120, while the detailed view of the section labelled B in Fig.12, is shown in a blown-up view in Fig. 13, so that the details of the construction can be clearly shown and described. The stack of bistable washer elements 121 are mounted on a central guide, which is made up of two concentric tubes, an inner tube 124 sliding within an outer tube 125. The load F is applied between the inner 124 and the outer 125 tubes, and is transferred to the ends of the stack of bistable washer elements 121 by means of end plates 128 and 129, which are attached to their respective tubes. A top view on the end plate 129 is shown at the top of the drawing. The oppositely facing bistable elements 121, are separated by an elastic or a visco- elastic or a rigid connection element 123, functional to transfer the forces from one bistable washer element to its neighbor, the force transfer taking place near the apex of the convex profile of the elements.

Reference is now made to Fig. 13, which shows an enlarged view of a section of the bistable stack, so that the detailed construction and operation of the stack can be described. The bistable elements 121 are arranged in pairs with the apex of pairs facing each other. Each bistable element is held within an annular support spacer 122, which has shoulders at its inner and outer diameters for maintaining the positions of the inner and outer boundaries of the bistable elements within the annular support spacer 122. In the space between the apex regions of the bistable elements, an intermediate connection element 123 is located, whose function is to transfer the force between the apex regions of adjacent bistable elements. This is therefore what was called the intermediate support location shown in Fig. l id, located between the peripheral support locations at the inner and outer diameters of the annular support spacer 122. These three support locations, two peripheral and one intermediate in the apex region, therefore define the manner in which each bistable element experiences the application of the transmitted force down the stack of bistable elements, being supported by the peripheral support points and being subject to the force exerted in the apex region by the intermediate connection element 123. The empty space on the concave side of the profiled bistable element 121, opposite the convex apex, it is essential to enable the bistable element to switch to its second phase of positive stiffness. The intermediate connection element 123, can be rigid or compliant, and can be made of a rigid material, or an elastic material or a viscoelastic material.

The intermediate connection element 123 has a number of functions, besides acting as the spacer between the bistable disc elements. Firstly, the outer "limbs" of the intermediate connection element 123 are lodged in the corner of the outer lip of the annular support spacer 122. This corner therefore, not only has the function of holding the bistable element 121 firmly in place between the annular support spacer 122 ends, but it also holds the intermediate connection element 123 in place to prevent it from moving laterally should it for instance undergo vibrations or shocks that would tend to make it move laterally. In addition, these outer limbs press against the surface of the bistable element 121, provide a barrier to prevent the entry of dirt and contamination into the spring mechanism itself. Furthermore, the volume radially within the intermediate connection element 123, as defined by the inner limbs of the spacer, define a sealed internal volume, which can be filled with gas or liquid, such as oil, in order to act as a vibration damper to the entire spring mechanism. The inner limbs then also act as an oil seal.

Additionally, if the intermediate connection element 123 has elastic properties, it can be used to control the stiffness of the entire assembly. By amending the stiffness of the elastic spacer element, the stiffness of the whole stacked spring assembly can be increased or decreased. This is then a simpler method of planning the stiffness of the stacked spring assembly. Thirdly, if it is constructed of a viscoelastic material, it is able to absorb the energy of the residual small amplitude vibrations and oscillations which are not filtered by the main disc assembly. The viscoelasticity of the material therefore acts as a shock absorber for residual vibrations or oscillations. The shape of the intermediate connection element 123 can also be adjusted to provide a small preload unloaded, such that the system is always stable even when no load is applied. Furthermore, the central region of the spacer, i.e. the region radially inside of the contact points with the bistable disc element, can be filled with gas or liquid, such as oil, to provide additional damping properties. Finally, because of the surface contact between the bistable element 121 and the intermediate connection element surface 123, which involves not only normal pressure but also slight lateral movement as the disc flexes, if the intermediate connection element were made of viscoelastic material, contact should preferably be made through a metal contact plate embedded into the surface of the viscoelastic intermediate connection element 123. Contact is thus metal to metal, instead of metal to polymer, resulting in a longer lifetime of the system.

Reference is now made to the central region of the device shown in Figs. 12 and 13, with the inner cylindrical tube 124 and outer cylindrical tube 125, these tubes being attached to the two end plates 128, 129 of the spring assembly, for inputting the load force which the spring assembly is intended to handle.

As mentioned previously, the structure shown in the central guide region enables a spring assembly to be constructed in which the bistable elements 121 occupy almost the entire length L of the inner core, thereby generating a shorter overall spring assembly length. The stack of bistable washer elements 121 are mounted on a central guide, which is made up of two concentric tubes, an inner cylindrical tube 124 sliding within an outer cylindrical tube 125. The spring stack is mounted over the outer cylinder 125, with the inner cylinder 124 pushing down the spring stack by means of the attached top plate 128, in order to compress it. In the completely compressed state of the spring stack shown in the example of Figs. 12 and 13 , much of the spring stack may be in contact with the outer cylinder 125. However, in the completely uncompressed state, a significant part of the spring stack will not contact the inner cylinder 124 since there is necessarily a space to allow the outer cylinder 125 to slide past the inner cylinder 124. Such a space may allow some of the bistable elements of the stack to move laterally, into and out of the space, thereby degrading the stability of the complete spring assembly. In the present configuration, this problem is alleviated by use of a spiral spring 126, positioned over the inner cylinder 124, and used as a self-compensating spacer which maintains a support for the inner diameter of the stack when the inner cylinder 125 is in a position which leaves such a space. The material of the spiral spring turns should have a thickness close to the wall thickness of the outer tube 125, so that the outer diameter of the spiral spring 126 is approximately the same as the outer diameter of the outer tube 125. By this means the complete spring assembly can use a stack of springs held positively over the whole of its inner diameter contact length, partly contacting the outer tube 125, and partly contacting the compressed spiral spring 126, without any degradation in performance because of lateral instability. In order to assist the inner and outer cylinders to slide over each other, linear bearings 127 are used, positioned between the two cylinders.

It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

Claims

CLAIMS We claim:

1. A spring assembly comprising:

a stack of shell structured elements, at least some of the elements being bistable elements having bistable elastic characteristics when compressed or extended along the axial direction of the stack,

wherein the bistable elastic characteristics comprise, in order of increasing strain of the element,

(i) a first state of elasticity having positive stiffness,

(ii) a second state of elasticity having negative stiffness, and

(iii) a third state of elasticity having positive stiffness,

and wherein at least some of the stacked bistable elements are spaced sufficiently far apart to enable them, when a force is applied to said assembly, to reach the third state of elasticity, having positive stiffness.

2. A spring assembly according to claim 1, wherein at least some of the bistable elements are shaped in the form of any one of dished washers, disc springs or Belleville springs.

3. A spring assembly according to claim 1, wherein at least some of the bistable elements are annular discs having a concave shaped profile.

4. A spring assembly according to any of the previous claims, wherein at least one of the bistable elements is connected to its immediate neighboring bistable elements at radially separated locations, the locations being such that a force applied between the radially separated locations is adapted to cause the bistable element to switch from its first state of elasticity to its third state of elasticity.

5. A spring assembly according to claim 4, wherein the radially separated locations are two locations, one located at a inner peripheral region of the bistable element, and the other in the outer peripheral region of the bistable element.

6. A spring assembly according to claim 4, wherein the radially separated locations are two locations, one located at a radially inner or a radially outer region of the bistable element, and the other at a radially intermediate region of the bistable element.

7. A spring assembly according to claim 4, wherein the radially separated locations are three locations, the first two at radially inner and radially outer regions of the bistable element, and the third in a region radially intermediate between the first two locations on the bistable element.

8. A spring assembly according to claim 7, wherein the bistable element is connected at the first two radially separated locations to one of its immediate neighbors through a rigid annular support spacer, and at the third intermediate radial location to its other immediate neighbor through a rigid, elastic, or viscoelastic connection element.

9. A spring assembly according to claim 8, wherein the bistable element and one of its immediate neighbors have apexed profiles facing each other, and the rigid, elastic, or viscoelastic connection element is attached between the bistable element and the facing immediate neighbor at the third intermediate radial location in the region of the apexes.

10. A spring assembly according to any of the previous claims, further comprising rigid annular support spacers having mechanical features at their inner and peripheral regions, adapted to restrain radial compression and/or radial extension, respectively, of said bistable elements.

11. A spring assembly according to any of the previous claims, wherein at least some of the bistable elements have an annular shape.

12. A spring assembly according to any of the previous claims, wherein the stack is adapted to be connected at one end to a vibrating environment, and at the other end to a body, the spring assembly being intended to limit the accelerative forces transmitted to the body from the vibrating environment.

13. A spring assembly according to any of the previous claims, wherein the stack is manufactured in the form of a single body.

14. A spring assembly according to any of claims 1 to 12, wherein the stack is constructed of individual shell structured elements connected at their central regions and at their peripheral regions.

15. A spring assembly according to any of the previous claims, wherein at least one of said bistable elements is an annular disc having a convex profile between its inner and outer edges.

16. A spring assembly according to claim 15, wherein said bistable element is supported such that a force may be applied to it between the apex region of said convex profile and two points located at radially opposite locations of said at least one bistable element.

17. A spring assembly according to any of claims 1 to 12 and 14 to 16, wherein the bistable elements are mounted on an inner core comprising an inner cylindrical tube adapted to slide within an outer cylindrical tube.

18. A spring assembly according to claim 17, wherein the inner and outer tubes are connected respectively to first and second end elements adapted to transfer an applied external force to the bistable elements of said spring assembly.

19. A spring assembly according to either of claims 17 and 18, further comprising a spiral spring located within a radial space formed between the inner edge of said stack of bistable elements and the outer surface of said inner cylindrical tube.

20. A method of limiting transmission from surroundings to a body, of forces arising from motion having accelerations greater than a predetermined level, by attaching a spring assembly according to claim 1 between the surroundings and the body.