Abstract:
A bowed snap-disc includes inner and outer perimeters that provide the disc with operating characteristics not found in comparably sized discs. At certain points along its circumference, the inner perimeter extends farther from the disc&#39;s center than certain other points of the disc&#39;s outer perimeter. This provides the disc with a unique combination of spring constant, compressive force, deflection and coefficient of compliance. The disc is particularly useful as a small, flat compression spring; a shaft or bar locking element; tightness indicator for a threaded fastener; or a lock washer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to a snap disc device, and more specifically pertains a snap disc device whose particular geometry provides exceptional operating characteristics. 
     2. Description of Related Art 
     The use of conventional compression springs can be limited by their physical size, as such springs are usually much longer than other types of springs. So, in applications where space is limited, other types of springs are often used, such as Bellville washers, curved disc springs, wave disc springs, and finger disc springs. 
     Belleville washers are resiliently compressible conical washers that provide a spring effect. For a given length, Belleville washers typically have much higher spring rates and significantly less travel than compression springs. This limits the use of Bellville washers to applications requiring relatively high forces and little travel. Belleville washers can be stacked back-to-back to provide lower spring rates and more travel, but a stack of washers will of course consume more space. 
     Curved disc springs have the shape of a flat washer that has been bent or bowed about a line parallel to the face of the washer. For a given size, a single curved disc springs may provide a lower spring rate than that of a Bellville washer. However, stacking curved disc springs to achieve even lower spring rates can be difficult to accomplish. Stacking the springs peak-to-peak is difficult to maintain, as the discs are normally free to rotate to a more stable arrangement of peak-to-valley. 
     Wave disc springs are similar to curved disc springs, but with more bends to create a wavy shape. Just as with curved disc springs, it can be difficult to maintain a stack of wave disc springs in a peak-to-peak arrangement. For a given size, wave disc springs tend to have less travel than curved disc springs. 
     Finger disc springs comprise an annular disc whose outer perimeter includes several fingers that are bent out of coplanar alignment with the rest of the disc. The fingers can resiliently deflect to create a spring-like effect. The fingers, however, may also interfere with being able to effectively stack finger disc springs with predictable results. 
     The physical structure of conventional disc springs limits their application. Current disc springs have limited use as springs and are not readily adapted for other uses such as gripping a square key. 
     Snap disc devices, invented by Pierre Schwab and disclosed in U.S. Pat. Nos. 4,822,959 and 5,269,499 have clover leaf shapes to create a bi-stable snap-action. However, the physical structure, operating characteristics, and/or method of pre-stressing such snap discs limits their usefulness. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations of current disc springs and snap disc devices, an object of some embodiments of the invention is to provide an elastic disc that serves as a fastener by gripping the four sides of a square shaft. 
     Another object of some embodiments of the invention is to provide a fastener with a particular disc geometry that provides the fastener with a surprisingly high coefficient of compliance, and yet the fastener is readily stackable to lower or increase its spring rate. 
     Another object of some embodiments is to provide a fastener with a threaded member, wherein the fastener indicates the degree of tightness to which the threaded member compresses a bowed disc a predetermined amount of deflection against a standoff element. 
     Another object of some embodiments is to provide a disc-like fastener that helps inhibit a threaded fastener from unscrewing under vibration. 
     Another object of some embodiments is to provide a fastener with a threaded member, wherein the fastener indicates the degree of tightness to which the threaded member compresses a bowed disc a predetermined amount of deflection against a standoff element, and wherein the standoff element is a simple unitary ring. 
     Another object of some embodiments of the invention is to provide a fastener with a particular disc geometry that provides the fastener with a lower spring rate and more travel than a Belleville washer of similar material, thickness and diameter. 
     Another object of some embodiments of the invention is to provide a fastener with a particular disc geometry that provides the fastener with a higher coefficient of compliance than a Belleville washer of similar material, thickness and diameter. 
     Another object of some embodiments of the invention is to provide a fastener with a particular disc geometry that provides the fastener with spring characteristics that are generally between that of a Belleville washer and a compression spring. 
     Another object of the invention is to provide a cloverleaf shaped disc having a bowed shaped when it its normally unstressed position. 
     Another object of some embodiments is to provide a cloverleaf disc with radial protrusions around its outer perimeter that provide the disc with more freedom to deflect. 
     Another object is to provide a disc fastener that is radially symmetrical so it can be installed alone or in a stacked arrangement regardless of its rotational orientation. 
     Another object of some embodiments is to provide a disc fastener with ample travel and a significant spring rate even though the disk has a rather large diameter to thickness ratio. 
     Another object of some embodiments is to provide a disc fastener whose material thickness is less than 5% of its diameter, thereby making the disc especially useful where axial space is limited. 
     These and other objects of the invention are provided by a disc fastener having an outer edge and an inner edge, wherein portions of the inner edge extend radially further out than some portions of the outer edge. The disc&#39;s geometry provides a coefficient of compliance greater than 10, wherein the coefficient of compliance is defined as the disc&#39;s thickness cubed divided by a product of the disc&#39;s spring constant at 75% compression times the disc&#39;s effective outer diameter squared, wherein the thickness is expressed in mils, the spring constant is expressed in pounds per inch and the effective outer diameter is expressed in inches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a disc fastener according to one embodiment of the invention. 
     FIG. 2 is a side view of the disc in FIG.  1 . 
     FIG. 3 is similar to FIG. 1, but of another embodiment. 
     FIG. 4 is a cross-sectional view taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is similar to FIGS. 1 and 3, but of yet another embodiment. 
     FIG. 6 is similar to FIG. 4, but of another embodiment. 
     FIG. 7 is a chart comparing various characteristics of the present invention, Bellville washers, curved washers, wave washers and finger washers. 
     FIG. 8 is similar to FIG. 3, but showing the disc gripping a round rod. 
     FIG. 9 is a front view of FIG.  8 . 
     FIG. 10 is similar to FIG. 8, but showing the disc of FIG. 1 gripping a square bar. 
     FIG. 11 is a front view of FIG.  10 . 
     FIG. 12 is a top view of another embodiment. 
     FIG. 13 is a cross-sectional view taken along line  13 — 13  of FIG.  12 . 
     FIG. 14 is similar to FIG. 13, but with the disc compressed 75%. 
     FIG. 15 is similar to FIG. 14, but with the disc at another position. 
     FIG. 16 is similar to FIG. 13, but of another embodiment. 
     FIG. 17 is similar to FIG. 16, but with the disc compressed 75%. 
     FIG. 18 is similar to FIG. 17, but with the disc compressed 100%. 
     FIG. 19 is similar to FIG. 16, but showing a different threaded fastener and showing a set of discs stacked in such a way as to provide a greater spring rate. 
     FIG. 20 is similar to FIG. 19, but with a set of discs stacked in such a way as to provide a lower spring rate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A fastener shown in FIGS. 1 and 2 includes a disc  10  with various design features that provide the fastener with the versatility to perform a variety of functions. Disc  10  (as shown or with some modification) can selectively serve as a variety devices including, but not limited to, a compression spring, a shaft or bar locking element, or tightness indicator for a threaded fastener. 
     Disc  10  is preformed so that its face surface  12  assumes a bowed or conical shape when in its unstressed position of zero percent (i.e., the disc&#39;s natural relaxed state, as shown by various discs in FIGS. 1,  2 ,  3 ,  4 ,  5 ,  12 ,  13  and  16 ). A fully stressed position of 100% is where a disc is completely flattened out, as shown in FIG.  18 . An intermediate position is when a disc is compressed to a position between its unstressed position of zero percent and its fully stressed position of 100%. For example, FIGS. 14,  15  and  17  show a disc compressed to an intermediate position of 75%, wherein the disc has been compressed 75% of its fill travel distance toward its fully stressed position of 100% (e.g., if the disc&#39;s fill travel is 0.100 inches, the disc is compressed 0.075 inches to reach an intermediate position of 75%). 
     Referring to FIGS. 1 and 2, disc  10  includes a curved outer edge  14  and an inner edge  16 . A minimum radial point  18  on outer edge  14  is at least as close to the disc&#39;s center of gravity  20  as is a maximum radial point  22  on inner edge  16 . In some embodiments, minimum radial point  18  is preferably closer to the disc&#39;s center of gravity  20  than is maximum radial point  22  on inner edge  16 . Likewise, disc  24  of FIGS. 3 and 4 includes a curved outer edge  26  and an inner edge  28 . A minimum radial point  30  on outer edge  26  is at least as close to the disc&#39;s center of gravity  32  as is a maximum radial point  34  on inner edge  28 . Referring to FIG. 5, disc  36  also includes a curved outer edge  38  and an inner edge  40 . A minimum radial point  42  on outer edge  38  is closer to the disc&#39;s center of gravity  44  than is a maximum radial point  46  on inner edge  40 . 
     Discs  10 ,  24  and  36  can be made of a variety of materials including, but not limited to carbon steel alloys, stainless steel alloys, copper alloys, inconel, monel, plastics and temperature responsive materials. Disc  48  of FIG. 6, for example, is made of bimetal where two intimately joined layers of material  50  and  52  have different coefficients of thermal expansion, so that disc  48  deflects as its temperature changes. Such a disc may be useful as a temperature sensor. 
     To create operating characteristics not available with existing fasteners, discs  10 ,  24  and  36  are provided with a thickness  54 , an effective outer diameter  56 , an effective inner diameter  58 , and a 75% compression stroke  60  that produces a coefficient of compliance  62  in the range of ten to fifteen with an unusual spring rate  64  (i.e., axial compression force  66  divided by deflection  60 , as shown in FIG.  7 . Such characteristics can be achieved when the disc is made of an iron or iron alloy (e.g., steel, stainless steel, etc.) having a tensile strength of 60 to 250 psi and/or a modulus of elasticity of 25×10 6  to 35×10 6  psi. Disc  24  of FIGS. 3 and 4, for example, has an outer diameter  68  of 0.400 inches, an inner diameter  70  of 0.156 inches, a material thickness  72  of 11 mils (i.e., 0.011 inches), and a 75% deflection stroke of 0.022 inches when subjected to a compressive force of 10.4 pounds, thereby providing disc  24  with a coefficient of compliance of 13.7 (13.7=11 3 /(608×0.4 2 )). The “coefficient of compliance” pertains to a spring&#39;s degree of compliance and is defined herein as a ratio of a disc&#39;s thickness cubed (in units of cubic mils) divided by the product of the disc&#39;s effective diameter squared (in units of square inches) times the disc&#39;s spring constant (in units of pounds-force per inch of compression at the disc&#39;s intermediate position of 75%). 
     The “effective diameter” of a disc is defined as the diameter of the smallest circle in which the outer edge of the disc can be inscribed. Disc  24  has an effective diameter  68 , as shown in FIG. 3, and disc  36  has an effective diameter  76 , as shown in FIG.  5 . Disc  36  includes a plurality of protrusions  78  extending radially outward from the disc&#39;s outer edge  38 , whereby a distal edge  80  of each protrusion  78  defines effective diameter  76 . Protrusions  78  provide disc  36  with discrete points of contact around the disc&#39;s outer perimeter. In some applications, such points of contact allow disc  36  to flex more freely without inhibiting the disc&#39;s outer perimeter from flexing. 
     Returning back to the chart of FIG. 7, various embodiments of the current invention, e.g., discs  10 ,  24 ,  36  and another similar disc  82 , have operating characteristics that are not available with other comparably sized devices. For example, an average coefficient of compliance  84  of discs  10 ,  24 ,  36  and  82  is 12.3 with a range of 10.3 to 13.7. Similar embodiments can provide a coefficient of compliance ranging from 10 to 15. However, some Bellville washers  86  may provide an average coefficient of compliance  88  of 4.2 with a range of 4.0 to 4.4; some curved washers  90  may provide an average coefficient of compliance  92  of 21.3 with a range of 17 to 23; some wave washers  94  may provide an average coefficient of compliance  96  of 3.5 with a range of 3 to 5, and some finger washers  98  may provide an average coefficient of compliance  100  of 4.8 with a range of 2 to 8.5. 
     Besides the coefficient of compliance, other characteristics of discs  10 ,  24 ,  36  and  82  distinguish them from comparably sized Bellville washers, curved washers, wave washers and finger washers. Generally speaking, discs  10 ,  24 ,  36  and  82  have significantly greater deflection than Bellville washers  86 , they have a much lower spring rate than Bellville washers  86 , they resist a greater force of deflection than curved washers  90 , they have greater deflection than wave washers  94 , and they have a higher spring rate than finger washers  98 . It should be noted that FIG. 7 is for general comparison purposes wherein discs  10 ,  24 ,  36 ,  82 ,  86 ,  90 ,  94  and  98  are of a generally similar material, i.e., made of an iron or iron alloy, and/or made of a material having a tensile strength of 60 to 250 psi and/or a modulus of elasticity of 25×10 6  to 35×10 6  psi. 
     Such unique operating characteristics enable various embodiments of the invention to perform functions that are not readily achieved by other known devices. For instance, disc  24  can serve as an effective rod-clamping device, as shown in FIGS. 8 and 9. Here, disc  24  can be forced over a generally smooth round rod  102 , so inner edge  28  of disc  24  can grip rod  102  without rod  102  having to include an additional holding feature, such as a groove or shoulder. Two discs  24  facing in opposite directions can hold one or more members  104  at a generally fixed location along rod  102 . 
     In another embodiment, similar to disc  24 , disc  10  is provided with an inner edge  16  having four linear edges  106  that are able to grip four faces  108  of a square bar  110 , as shown in FIGS. 1,  2 ,  10  and  11 . Two opposite facing discs  10  gripping bar  110  are able to hold bar  110  fixed relative to one or more members  112 . Disc  10 , in this case, has an inner diameter  114  defined by the largest circle  116  that can be inscribed within the inner edge  16  of disc  10 . 
     Referring to FIGS. 12-14, in some cases, a standoff element, such as a ring  116 , may be attached or simply placed adjacent to disc  24  to inhibit the disc from deflecting completely to its fully stressed or flat position. Here, disc  24  can be compressed between a first surface  118  (e.g., underneath an internally threaded member, such as a nut  120 ) and a second surface  122 , thereby compressing disc  24  from its unstressed position of FIG. 13 to an intermediate position of FIG.  14 . Alternatively, disc  24  may be compressed between a first surface  126  (underneath the head of an externally threaded member, such as a bolt  124 , screw, etc.) and a second surface  128 . 
     In some cases, the standoff element can be an integral part of the threaded member that compresses the disc. In FIGS. 16 and 17, for example, a shoulder  130  on threaded member  132  provides a standoff that inhibits disc  24  from being compressed beyond its intermediate position of FIG.  17 . Once nut  120  is tightened against shoulder  130 , further compression of disc  24  is inhibited. Of course, if shoulder  130  does not extend beyond the total thickness of members  134  and  136 , then disc  24  could be compressed to its fully stressed position of 100%, as shown in FIG.  18 . 
     The radial symmetry of disc  24  allows two or more discs to be stacked, as shown in FIG.  19 . The expressions, “radial symmetry” and “radially symmetrical” describe a shape, wherein the entire shape can be divided into substantially identical pie pieces. Stacking discs  24  as shown in FIG. 19 allows the discs to resist an overall greater compressive force for a given amount of deflection. To achieve greater deflection for a given amount of compressive force, discs  24  can be stacked as shown in FIG.  20 . 
     Although the invention is described with reference to a preferred embodiment, it should be appreciated by those skilled in the art that various modifications are well within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the claims that follow.