U.S. patent application number 11/185158 was filed with the patent office on 2005-11-10 for process for improving fatigue life in spring-cushioned shoes.
Invention is credited to Krafsur, David S., Levert, Francis E..
Application Number | 20050247385 11/185158 |
Document ID | / |
Family ID | 35238359 |
Filed Date | 2005-11-10 |
United States Patent
Application |
20050247385 |
Kind Code |
A1 |
Krafsur, David S. ; et
al. |
November 10, 2005 |
Process for improving fatigue life in spring-cushioned shoes
Abstract
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.
Inventors: |
Krafsur, David S.;
(Loveland, CO) ; Levert, Francis E.; (Knoxville,
TN) |
Correspondence
Address: |
PITTS AND BRITTIAN P C
P O BOX 51295
KNOXVILLE
TN
37950-1295
US
|
Family ID: |
35238359 |
Appl. No.: |
11/185158 |
Filed: |
July 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11185158 |
Jul 20, 2005 |
|
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10358514 |
Feb 5, 2003 |
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60355485 |
Feb 8, 2002 |
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Current U.S.
Class: |
148/580 |
Current CPC
Class: |
A43B 13/182 20130101;
C21D 7/06 20130101; C21D 9/02 20130101; A43B 13/183 20130101 |
Class at
Publication: |
148/580 |
International
Class: |
C21D 009/02 |
Claims
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.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] 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.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] 1. Field of Invention
[0004] This invention relates to the field of shoes, and more
particularly to spring cushioned shoes.
[0005] 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.
[0006] 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.
[0007] 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.
BRIEF SUMMARY OF THE INVENTION
[0008] 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.
[0009] 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.
[0010] As used herein, a material's "tensile strength" is the
stress point at which a material will either break or deform beyond
usefulness.
[0011] 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.
BRIEF DESCRIPTION OF THE DRAWING
[0012] 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:
[0013] The FIGURE is a cross-sectional view of a wire experiencing
a load.
DETAILED DESCRIPTION OF THE INVENTION
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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).
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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=(3.pi.PD.sub.m).div.(4bt.sup.2N.sup.2) where S is
tensile stress in psi, P is load in pounds, Dm is the spring's mean
diameter [(O.D+I.D.).div.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.
[0027] 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.
[0028] 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.div.126,600=232,100 psi. Solving for t, we
find that the thickness can be 0.035 inches, and still withstand
one million cycles.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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).
[0034] 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.
[0035] 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.
[0036] 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).
[0037] 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.
[0038] Having thus described the aforementioned invention, we
claim:
* * * * *