Process for improving fatigue life in spring-cushioned shoes
Kind Code:

A process for reducing heat transfer within a spring in a spring cushioned shoe, said process comprises the steps of applying a residual compressive stress to said spring, for example by shot peening, and then mounting the spring between an inner sole and an outer sole of the shoe.

Krafsur, David S. (Loveland, CO, US)
Levert, Francis E. (Knoxville, TN, US)
Application Number:
Publication Date:
Filing Date:
Primary Class:
International Classes:
A43B13/18; C21D7/06; C21D9/02; (IPC1-7): C21D9/02
View Patent Images:

Primary Examiner:
Attorney, Agent or Firm:
PITTS & LAKE P C (P O BOX 51295, KNOXVILLE, TN, 37950-1295, US)
1. A process for reducing heat transfer within a spring in a spring cushioned shoe, said process comprising the steps of: applying a residual compressive stress to said spring; and mounting said spring between an inner sole and an outer sole of said shoe.

2. The process of claim 1 wherein said step of applying a residual compressive stress is selected from a group comprising shot peening, roll burnishing, knurling, compression over a mandrel, heat treating and magnaforming.

3. The process of claim 1 wherein said spring comprises a multi-turn crest-to-crest wave spring.

4. The process of claim 1 and further comprising the step of baking said spring after applying a residual compressive stress to said spring.

5. The process of claim 1 wherein the compressive stress applied to said spring is about 50% of the tensile strength of said spring.

6. The process of claim 1 wherein said step of applying a residual compressive stress comprises shot peening.

7. The process of claim 6 wherein said shot peening step creates dimples over 10 to 200% of the surface of said spring.

8. The process of claim 6 wherein said shot peening step creates dimples over 50 to 100% of the surface of said spring.

9. The process of claim 6 wherein said spring is stretched during said shot peening to expose additional surfaces for shot peening.



Pursuant to 35 USC Section 119, this application claims the benefit of priority from Provisional Application Ser. No. 60/355,485 with a filing date of Feb. 8, 2002, and Non-provisional Application Ser. No. 10/358,514 with a filing date of Feb. 5, 2003.


Not Applicable


1. Field of Invention

This invention relates to the field of shoes, and more particularly to spring cushioned shoes.

In most running, walking, and jumping activities, the return force resulting from foot strikes causes great shock to the body. Repeated foot strikes stress joints and bones, and can lead to injuries to the lower back and the rotating joints of the legs.

To minimize injury to the body resulting from repeated foot strikes, and also to 10 improve athletic performance, shoe engineers have added springs to the soles of shoes. The springs in spring-cushioned shoes are designed to reduce shock to the body during a foot strike, and also to recover and return impact energy to the user. Various spring-cushioned shoe designs are described in U.S. Pat. No. 6,282,814 to Krafsur et al., pending U.S. patent application Ser. No. 09/982,520 to LeVert et al., and U.S. Pat. No. 5,743,028 to Lombardino, all of which are incorporated herein by reference.

Shoes incorporating metal springs, however, have had two distinct disadvantages: (1) they are heavier than traditional shoes; and (2) the springs often set or fail prematurely. Prior solutions to these two disadvantages are conflicting. Making shoes lighter by, e.g., reducing the size of the spring's coil, causes the springs to fail earlier, and using sturdier springs makes the shoes heavier.


An object of the present invention is to produce a spring-cushioned shoe in which the springs do not fail prematurely, but the shoe is not undesirably heavy.

Another object of the present invention is to reduce the transfer of heat to the heat-sensitive portions of the shoe that are in contact with the springs.

As used herein, a material's “tensile strength” is the stress point at which a material will either break or deform beyond usefulness.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and in the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.


The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawing in which:

The FIGURE is a cross-sectional view of a wire experiencing a load.


In most spring-cushioned shoes, metal compression springs are embedded in the sole of the shoe to provide cushioning and energy return. The springs are generally formed from one or more wires coiled into a particular shape. The springs can be, e.g., disk springs, cone springs, Belleville springs, or, as described recently in U.S. Pat. No. 6,282,814, wave springs, such as multi-turn crest-to-crest wave springs.

When a spring is placed under a load, the metal wire-forming the spring is subject to certain stresses. Consider, for example, a rectangular metal wire subject to a bending stress. Referring to FIG. 1, a wire 10 is subject to a load (force arrows F) that bends the wire. Under load F, an outer section 12 of the wire 10 is stretched, while an inner section 14 is compressed. The outer section 12 is therefore subject to tension, or tensile stress, while the inner section 14 is subject to compressive stress. If the wire fails (e.g., cracks), it will fail on the tensile stressed side 12, not the compressed side 14.

When a person wearing a spring-cushioned shoe stands, walks, or runs in the shoe, the springs in the shoe are subject to loading, which stresses the metal in the spring. Some portions of the spring are subject to tensile stress, or “tensile loading,” while others are subject to compressive stress. As discussed above, tensile stress, not compressive stress, potentially causes failure. Thus, the portions of the spring subject to failure are the portions that experience tensile loading. Over time, cyclical tensile loading fatigues the metal in the spring, and can cause the spring to set (i.e., fail to return to its original state after removal of a load) or the metal to crack and fail.

Spring failure from cyclical tensile loading can be delayed or avoided by, e.g., increasing the thickness of the spring wire. As discussed above, however, this makes the spring-cushioned shoe heavier, which is undesirable.

The present invention relates to pre-treating springs used in spring-cushioned shoes to impart a residual compressive stress by, e.g., a process known as “shot peening.” The shot peening process imparts a permanent compressive stress on the spring, which counters the tensile loading that results from standing, walking, or running in the shoe. The shot peening process therefore enhances the spring's ability to withstand cyclical tensile loading, and increases the useful life of a spring in a spring-cushioned shoe, without increasing the shoe's weight.

The conditioning of the surface 16 of the spring 10 by the process of cold working or hammering the surface 16 with small spheres of steel, ceramic or glass media propelled against the surface 16 of the spring 10 puts the upper layers of the material into compression and helps to prevent failure in the material as stated hereinabove. The physical change in the volume of the spring 10 from the compression, though small, causes a significant positive change in the magnitude of the electrical conductivity of the spring material near its surface 16. It is known that the resistance of a material is inversely related to its electrical conductivity. Because both heat energy and electrical energy are carried by free electrons in a metal, a good electrical conductor is generally a good heat conductor. Conversely, a poor electrical conductor is generally a poor heat conductor.

Mechanical energy is converted into thermal energy during the constant flexing of the spring 10 when it is mounted in a shoe sole. The repetitive flexing and expansion of the spring 10 causes the temperature of the spring 10 to rise above the ambient temperature. In accordance with the present invention, shot peening is used to increase the surface compressive stress of the spring 10 and thereby reduce the heat conductivity of the spring 10. Accordingly, there is a reduction in the amount of thermal energy conducted through the spring 10 to the more temperature sensitive materials of the shoe that contact the spring 10. Instead, the thermal energy developed in the spring 10 is more uniformly dispersed through the shoe by the convection heat transfer of the air or other fluid media surrounding the spring 10.

Shot Peening: In shot peening, a substrate surface, usually metal, is bombarded with small media called “shot.” Shot pieces are usually spherical in shape, and harder than the substrate they strike. When a piece of shot strikes the substrate, it creates a small dimple in the surface of the substrate. The metal grains displaced by the shot strike impart a compressive force onto the sides of the dimple, trying to restore the surface of the metal to its original shape. If the shot peening process covers the surface of the substrate with overlapping dimples, then the entire surface will have a uniform, permanent, residual compressive stress at and near its surface. The residual compressive stress imparted to the surface of the substrate by shot peening is generally about half the tensile strength of the substrate material. Just below the surface the residual compressive stress imparted is greater, e.g., about 60% of the tensile strength. (See Metal Improvement Company, Inc. Shot Peening Applications 6-7 (8th ed. 2001).

Wave Spring Embodiment: In one embodiment, the crest-to-crest, multi-turn wave springs used in the spring- cushioned shoes of U.S. Pat. No. 6,282,814 are pre-treated with shot peening. Prior to placing the springs within the heel and ball vacuities of the shoe sole, both springs are shot peened for, e.g., about 10-15 minutes with 0.023 inch diameter spherical shot. All exposed surfaces of the spring, including inner and outer surfaces, are bombarded with shot. After the 10-15 minutes of shot peening, the entire exposed surface is covered with dimples, such that the dimples overlap. Each dimple is, e.g., up to 0.0004152 square inches in area, and there are, e.g., at least 2400 dimples per square inch of surface area. The springs are made from, e.g., 1075 carbon steel or 17-7 PH stainless steel with a Rockwell hardness of 53. The shot has a hardness greater than the hardness of the springs. After the shot peening process is complete, the springs are inserted into the heel and ball vacuities of the shoe, as shown in U.S. Pat. No. 6,282,814, which is incorporated herein.

The shot peening process imparts a permanent compressive stress to the surface of the spring equal to, e.g., about 50% of the spring's tensile strength. Just below the surface, the dimples impart maximum compressive stresses of up to, e.g., about 60% of the tensile strength. This residual compressive stress allows the spring to more easily withstand tensile loading, and therefore improves the fatigue life of the spring, and the useful life of the spring-cushioned shoe.

Comparison Example: In this Example, we compare two spring-cushioned shoes, one with shot peened springs and one with non-shot peened springs. We demonstrate that the shot peened springs can be made thinner and lighter, and still achieve a satisfactory fatigue life.

Both spring-cushioned shoes have substantially the structure shown in U.S. Pat. No. 6,282,814. The springs are crest-to-crest, multi-turn wave springs made from flat wire steel having a tensile strength of 211 ksi (where 1 ksi=1000 psi). The outer diameter (O.D.) of the wire is 2.5 inches, and the inner diameter (I.D.) is 2.0 inches. The spring coil has 3.5 waves per turn. Below, we demonstrate that the shot-peened spring can have a wire thickness about 31% less than the non-shot peened spring, and will therefore be 31% lighter.

Consider first the non-shot peened spring, which has no residual compressive stress. According to the Engineering and Parts Catalog of Smalley Steel Ring Company (a manufacturer of wave springs), a crest-to-crest multi-turn wave spring experiences a bending stress, or tensile stress, according to the following equation: (1) S=(3πPDm)÷(4bt2N2) where S is tensile stress in psi, P is load in pounds, Dm is the spring's mean diameter [(O.D+I.D.)÷2] in inches, t is the thickness of the wire in inches, and N is the number of waves per turn. According to the Smalley Catalog, for the wave spring to endure one million load cycles without failure, the spring should be operated in stress range no greater than 50% of the tensile strength. If the shoe is worn by an average-sized man of 160 pounds, then an average cycle (e.g., a step) will impart a load P of about 160 pounds.

If we let S=105,500 psi and P=160 in equation (1) and solve for the spring thickness t, we see that t must be at least 0.051 inches in the non-shot peened spring to last one million cycles.

In the shot-peened spring (shot peened as described above), the spring has a residual compressive stress near its surface equal to 60% of the tensile strength, or 126.6 ksi. Thus, in equation (1), we let S=105,500÷126,600=232,100 psi. Solving for t, we find that the thickness can be 0.035 inches, and still withstand one million cycles.

By using a shot peened spring, therefore, the thickness of the spring can be reduced by about 31%, which translates to a 31% reduction in the weight of the spring. For the wave springs described in U.S. Pat. No. 6,282,814, shot peening allows the weight of each spring to be reduced from, e.g., about 2.0 ounces to approximately 1.4 ounces. Since each shoe in this embodiment includes two wave springs, shot peening reduces the weight of each shoe by, e.g., about 1.2 ounces.

In fact, however, shot peening allows the weight to be reduced even more than 31% for each spring. If the springs are shot peened, it is possible to use a considerably more brittle (i.e., less ductile) metal material, without fear of early failure. For example, instead of using a metal with a tensile strength of 211 ksi, it is possible to use harder metal, with a tensile strength of about 275 ksi. Using a shot peened 275 ksi metal, the spring has a residual compressive stress of about 165 ksi, and can withstand regular stress of 137.5+165=302.5 ksi and still survive one million cycles. If we then let S=302,500 in equation (1), and solve for t, we find that the wire can thickness can be 0.030 inches. This translates to a weight reduction of 41% compared to a non shot-peened shoe, and a reduction of about 1.64 ounces per shoe. In, e.g., a typical running shoe, a reduction in weight by 1.64 ounces can be the difference between a satisfactory and unsatisfactory weight.

In preliminary tests, we found that running shoes with shot peened, crest-to-crest wave springs last at least ten times longer than shoes with comparable non-shot peened crest-to-crest wave springs. For example, in one test, non-shot peened springs failed after approximately 50 miles of running, while shot-peened springs remained functional after 500 miles.

Other Embodiments: Other embodiments are within the spirit and scope of the invention. For example, the shot peening process can be modified. In the above described embodiment, the springs were shot peened for 10-15 minutes, until 100% of the surface was covered (i.e., the dimples 5 overlapped). Alternatively, the shot peening can continue for twice the amount of time needed for 100% coverage, such that the surface is “200%” covered with dimples. In addition, less of the surface can be shot peened, e.g., 10%-50%, 50%-100% or 100%-200%. Different sized and different shaped shot can also be used.

After shot peening, the springs can be baked at, e.g., 205 degrees Celsius for, e.g., about 30 minutes, to reduce the likelihood of setting. Other times and temperatures are possible, so long as the temperature is not so high that it relieves the residual compressive stress imparted by the shot peening. (See Metal Improvement Company, Inc., Shot Peening Applications 25 (8th ed. 2001).

In the above described embodiment, all exposed surfaces of the crest-to-crest wave springs are shot peened. It is also possible to shot peen the non-exposed surfaces where crests from different turns contact each other. This can be done by stretching the spring to pull the different turns apart during the shot peening process, to expose the crest surfaces which contact each other when the spring is relaxed.

Springs other than wave springs can be shot peened and placed within shoes. For example, it is possible to shot peen coil springs, disk springs, Belleville springs, spiral springs, cone springs, or other types of springs, and then locate them in the sole of a shoe. The shot peened springs can be placed within heel and ball vacuities of the shoe between an inner sole and an outer sole, as described in U.S. Pat. No. 6,282,814, or can be placed only within the heel area, or in other areas of the shoe sole. Alternatively, the spring may be mounted in a shoe having an inner sole and an outer sole, but no side walls.

The springs can be treated with other processes which impart a residual compressive stress instead of, or in addition to, shot peening. For example, the springs can be treated with roll burnishing, knurling, compression over a mandrel, heat treating, and “magnaforming” (large magnets impart compressive stress).

While the present invention has been illustrated by description of several embodiments and while the illustrative embodiment has been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.

Having thus described the aforementioned invention, we claim: