Abstract:
The present invention is a spring, comprising a support structure and a substantially straight walled, solid elastomeric column. The support structure has a continuous and selectively vented support wall and a base wherein the support wall has different minimum and maximum inside diameters. The elastomeric column has a diameter less than, equal to, or slightly larger than the minimum diameter of the support structure wall and a height greater than the height of the support structure.

Description:
FIELD OF THE INVENTION 
     The present invention is directed toward a rubber spring. More specifically, the present invention is directed toward a tunable rubber spring having a centrally located rubber column and a rigid outer support structure. 
     BACKGROUND OF THE INVENTION 
     Although the concept is obscure outside the discipline of structurally confined rubber spring design, and limited in understanding within the discipline, general applications of such rubber springs have been attempted in the past. Usually, the outer structure is hollow cylindrical, and an inefficient use of at least the lower regions of the internally located rubber results. The springs are not very spring rate tunable when the wall has a purely cylindrical configuration, and the rubber has undefined or unstable coefficient(s) of friction with the support wall, as increased interfacing occurs. 
     Additionally, durability is a problem, due to abrasion at the contact stick and slip surfaces between the outside of the rubber spring and the inside of the support structure. Other prior art, regarding column stability, teaches locating a hole in the cylindrical rubber spring column, and inserting a structural post whose height does not exceed the maximum deflected height of the spring. This gives column stability, but is not as efficient, nor as durable, as the presently disclosed invention, and is non-tunable. 
     Examples of such springs are illustrated by U.S. Pat. Nos. 3,262,985 and 3,037,764. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a compression spring body that has a compact geometry for high volumetric efficiency, yet will accommodate high displacement, high energy storage and long life in compression spring applications. 
     The present invention is a spring comprising a support structure and a straight walled, solid elastomeric column. The support structure has a continuous and vented support wall and a base wherein the support wall has different minimum and maximum inside diameters. The elastomeric column has a diameter less than, equal to, or slightly larger than the minimum diameter of the support structure wall and a height greater than the height of the support structure. 
     In another aspect of the invention, the elastomeric column is positioned with the support structure so that the central axis of the elastomeric column and the support structure are aligned, with the elastomer extending beyond both open ends of the structure. 
     In another aspect of the invention, the elastomeric column of the spring has a height greater than twice the diameter of the column. The height of the column may be up to eight times the diameter of the column and even greater depending upon the desired spring characteristics and application contact conditions. 
     In another aspect of the invention, the column is fixedly secured to the base of the support structure. The column may be fixedly secured to the support structure by either adhesively securing the column to the base of the support structure or by compression fitment between the column and the support structure. 
     In another aspect of the present invention, the support structure walls have a configuration set from among the group of bellows, repeating bellows, helical twists, hourglass, repeating hourglass. The outside surface of the support structure wall is either a mirror image, or a structurally complimentary configuration of the inside wall, which when combined are adequate to support the stresses imposed by the sliding and compressive rubber impingement on said inner wall. 
     In another aspect of the invention, either the outer surface of the elastomeric column and/or the inside surface of the support structure are coated with a reduced friction coating to yield low friction surfaces. Alternatively, the elastomeric material forming the column may contain a friction reducing material to produce a self-lubricating reduced friction surface to the column. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1   a  is a cross-sectional view of the spring body under no compression; 
         FIG. 1   b  is a cross-sectional view of the spring body under partial load displacement; 
         FIG. 1   c  is a cross-sectional view of the spring body under maximum load displacement; 
         FIG. 2  is an alternative construction for the outer rigid support structure; 
         FIGS. 3   a  and  3   b  are alternative cross-sectional views of the structure of  FIG. 2 ; and 
         FIG. 4  is a cross-sectional view of another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now to the drawings, and in particular to  FIGS. 1   a  to  1   c,  a compression spring embodying the principles and concepts of the present invention will be described. 
     Within the spring element and structure system, the spring  10  has a rigid tubular support structure  12  and a solid rubber column  14 . The support structure  12  is constructed from a rigid material such as metal, fiber reinforced thermoplastics, fiber glass molding compounds, and other similar metallic or non-metallic materials. The strength and durability of the structural component of the system-should be sufficient to withstand the environment into which the spring  10  will be employed, and withstand the forces to which the spring will be subjected during operation. The strength and durability should be adequately defined in a finite element or other equivalent analysis of the material, as prescribed at the limits of load, deflection, and cyclical life estimates, and factors of safety. At a minimum, the material should have tensile strength of 100 MPa, and have a flex strength of at least 200 MPa. The structure  12  may be produced in a number of economical ways, including, but not limited to, hydroforming and spinning in the metal composed variant. The structure  12  has a wall  16  and a base  18 . To reduce or eliminate the entrapment of air, during loading, or creation of a vacuum, during unloading, between the structure  12  and the column  14 , the structure  12  has strategically positioned vent holes  20 . 
     The wall  16  has a bellows-type configuration, preferably terminating in an upper lip  22 . The structure  12  is illustrated with two bellow sections, creating an inside minimum diameter A. The wall  16  can have any type of wave-like, or non-straight, configuration having differing minimum and maximum inside diameters. Another wall configuration is illustrated in FIG.  2 . The walls  16 ′ have a helical configuration. The inside surface  24  of the wall  16 ′ can be either a mirror image, see  FIG. 3   a,  or a complimentary configuration, see  FIG. 3   b.  By varying the wall configuration and inside surface configuration, the spring  10  can be tuned for specific applications. The structure  12  can also have a simple hourglass configuration. 
     The structure  12 , and the accompanying column  14  as discussed below, are illustrated as having an overall circular configuration. However, depending on the required performance requirements for the spring  10 , the structure  12  and the column  14 , they may have a non-circular configuration, such as elliptical, requiring appropriate rotational alignment of the two components  12 ,  14  about the compression axis, prior to compressive load application, in order to optimize performance. 
     Set inside the structure  12  is the straight walled, solid rubber column  14 . The column  14  is set within the structure  12  so that the central axis of the structure  12  and the column  14  are aligned, but not necessarily coincident. The outer surface  26  corresponds in configuration to the configuration of the minimum diameter A of the structure  12  and, in this embodiment, has a diameter B less than the minimum diameter A. In other words, for the illustrated circular structure  12 , the column  14  is a smooth walled circular column, and if the structure  12  has an overall oval configuration, the column  14  would have an oval configuration. Thus, prior to compression and when the axis of the column  14  and the structure  12  are coincident, a loosely defined constant distance x is maintained between the structure minimum diameter A and the column diameter B, in its minimum size configuration. The spring rate of column  14  in the radial direction is significantly lower than the compression spring rate, such that the distance x, in any radial direction, may vary prior to any compression load application. 
     The column  14  is preferably securely fixed within the structure  12  at the base, in the preferred embodiment, so that, prior to compression or assembly in a larger system, the components  12 ,  14  do not inadvertently separate. The column  12  may be secured by compression fit at the base  18 , as illustrated, or it may be bonded, adhesively secured, or attached by the use of any other securing means known as applicable between the two types of materials used for the structure  12  and the column  14 . If the securing means limits the compression and expansion capability of the column  14 , only the lowermost portion of the column  14  should be so limited. 
     The column  14  has an uncompressed height, C 1 , greater than the height D of the structure  12 . The height C 1  is also relative to the maximum diameter B, and is at least twice the diameter B, thus the column has a height to diameter ratio of at least 2:1. The column has a preferred height to diameter ratio of 2:1 to 8:1; however, unique applications requiring high displacements or deflections may have column height to diameter ratios greater than 8:1. The top of the column  14  may be secured to a connection structure  30  through which a compressive load is applied to the spring  10 ;  FIGS. 1   a - 1   c  show such a secured configuration achieved by compression fit, but which may be achieved in any conventional manner similar to those possible for securing the base  18 . 
     The column  14  is formed from rubber. The rubber has the following properties: compression modulus range of 150 to 1500 psi, bulk or volumetric modulus range of 120,000 to 240,000 psi and a tensile strength of 1,500 to 4,000 psi. In the preferred embodiments, damping or energy absorption properties are not a significant characteristic affecting the performance of the spring  10 . Examples of suitable rubbers include, but are not limited to, natural rubber, polybutadiene, chloroprene, nitrite, butyl, styrene butadiene, silicone, and combinations of any of the these rubbers, some of which would provide useful damping and energy absorption. 
     A friction reducing material such as a PTFE impregnated coating may be applied to the inside surface  24  of the structure  12  or the outer surface  26  of the column  14 . Alternatively, a friction reducing material may be incorporated into the rubber of the column  14 , resulting in a self-lubricating interface between structure  12  and column  14 . 
     For the system of  FIG. 1   a,  prior to compression, the rubber column  14  has a height C 1  at least three times greater than the diameter B of the column  14 , and greater than the height D of the structure  12 . Except at the base of the spring  10  where the column  14  is held into the structure  12  by compression fit or by other means, the rubber column  14 , in the smallest diameter configuration, is distanced from the inside diameter A of the structure  12  by a distance x. 
     As a load is applied to the top of the column  14 , as seen in  FIG. 1   b,  the rubber column  14  begins to compress and buckle or bend, with initial contact being made between the outer surface  26  of the column  14  and the minimum diameter locations  28  on the inside of the structure  12 . Thereafter, in all embodiments, the rubber column  14 , between the minimum diameter locations  28 , begins to bulge toward the remainder of the structure  12 . As the load continues to be applied to the spring  10 , the contact surface area between the structure  12  and the column  14  increases. 
     At full compression, see  FIG. 1   c,  full contact is made between the outer surface  26  of the column  14  and the inside surface  24  of the structure  12 . The portion of the column  14  that is not contained within the structure  12  rests on the lip  22 , providing an additional cushioning effect. 
       FIG. 4  illustrates an alternative embodiment of the invention, with aspects that are also applicable to previously discussed embodiments. In the illustrated spring  32 , the structure  34  is open at both ends and is shown with a bellows type configuration, having a minimum inner diameter A. The rubber column  36  has a substantially constant diameter B, with a single portion  38  having an increased diameter X such that the diameter X is at least 1.1 times the diameter A. The maximum dimension of X is, of course, the maximum inside diameter of the structure  34 . The presence of the portion  38  is twofold: it permits some tunability of the spring  32  and permits the use of a rubber column  36  that is not previously secured at either end to a fixed structure. As with the previous embodiment, the column  36  has a height greater than height C 4  of the structure  34 . 
     All of the variations already disclosed as permissible with the spring  10  are applicable herein for the spring  32 . 
     To assemble the spring  32 , the rubber column  36  may be pushed in or pulled into the structure  34 . In either method, once the increased diameter portion  38  has passed the first minimum diameter location  40  of the structure  34 , the column  36  snaps into place in the structure  34 . When a load is applied to the spring, the column  36  responds similarly to column  14 ; gradually making contact with the inner surface of the structure  34  with structure  34  supporting the buckling and bending of the column  36  until full contact along the inside of the structure  34  is achieved. 
     For each particular application wherein the spring  10  or  32  is to be used, the spring  10  or  32  is designed using a finite element analysis and using appropriate rubber materials characterization properties, or alternatively, other lesser or equivalent predictive methodologies, which indicate the change in outside rubber surface shape and size versus displacement or deflection. Thus, in designing a spring for a particular application, the contact profile and rate of increased surface contact can be predicted. 
     The efficient utilization of all the rubber volume, above the base connection, is what essentially allows for the greater displacements and energy storage capability within the relatively tall and narrow, or small foot print, spring geometry. 
     The spring assembly may be used in applications having both static and dynamic compressive (and, when specifically allowed, intermittent dynamic tensile) loads and displacements, such as encountered in shock and vibration isolator applications, or energy storage and recovery devices. Two assemblies may also be used in a compressively preloaded condition, with the central axes aligned with each other and the compression directions 180 degrees opposed to each other, providing additive beneficial energy storage capacity. The above described novel spring assemblies can be used in any parallel axes and some multiple orientation, non-parallel axes groupings, increasing the load capacity and spring rate of a system, and improving the multi-directional performance of systems containing such assemblies. 
     Conventionally, a solid rubber cylinder having a height to diameter ratio of 3:1 or greater would be unstable in compression. Upon application of a compressive force or load, the cylinder would attempt to buckle. However, with the disclosed spring, the cylinder is increasingly supported by the interior walls of the rigid structure. As the compressive force increases, the load contact surface of the cylinder continues to deflect in the direction of shortening the height of the column and increasing the outside diameter of the cylinder. Thus, after initial contact of the sides of the cylinder and the structure, the surface area of rubber-to-structure contact increases, eventually making near full or full perimeter contact. As this occurs, and because of the material properties of rubber, where the compression modulus is significantly lower than the bulk or “volumetric” modulus, the spring rate increases significantly, while at the same time, the below described novel structural support geometry allows greater column stable deflection than otherwise possible in an equivalent or shorter unsupported rubber spring. 
     Full utilization of all of the volume of rubber above the base connection, over the entire displacement range, is realized in a compact cross-sectional area, thus reducing the undesirable high strain density concentrations, when compared to less fully utilized designs. Likewise, average regional strains are reduced, for a given displacement versus size, when compared to prior unconstrained or circumferentially constrained designs. Prior art rubber springs, having rubber bulge surface contacts, have “dead spots” or volumetric sections of rubber that are not contributing to the performance of the product in any meaningful way. Because of the combined lower peak and lower average strain versus displacement phenomena contained in the novel concept, displacement or stroke capacity is increased and durability and life are extended. 
     Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims.