Spring devices – Coil – Circular
Reexamination Certificate
2003-10-27
2004-11-02
Siconolfi, Robert A. (Department: 3683)
Spring devices
Coil
Circular
C029S090700
Reexamination Certificate
active
06811149
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to springs. More particularly, the invention relates to metallic coil springs subject to coil binding.
BACKGROUND
Metallic coil springs are generally subject to fatigue failure after many displacement cycles. Fatigue results from the cyclic tensile and compressive stresses induced in active portions of the coil spring wire (i.e., those portions that resist bending during spring displacements). In their resistance to bending forces, these portions of the coil spring wire are analogous to the flanges of an I-beam. And as the web of an I beam comprises an area between the flanges (the neutral plane) that typically experiences little or none of the cyclic stress in the flanges, the inner portion of a coil spring wire has an analogous (generally curved) neutral plane.
During normal displacement cycles of a coil spring, linear displacement occurs substantially parallel to the spring's longitudinal coil axis, which is the axis about which the spring wire is coiled. Such cyclic coil spring displacement tends to alternately increase and decrease the distance between adjacent coils. And under certain dynamic conditions (e.g., when standing and/or traveling waves in the spring have sufficient magnitude) the distance between adjacent coils may be reduced to zero (i.e., adjacent spring coils may come into contact).
To increase the tolerance of a metal structure to fatigue and thus lengthen service life, special techniques have been developed to reduce or eliminate cyclic tensile stresses in certain portions of the structure that would otherwise be subject to them. Techniques to thus extend service life include the introduction of residual compressive stresses within and near metallic surfaces of the structure by cold working the surfaces using, for example, shot peening, laser shock peening or burnishing. The residual compressive stresses thus created tend to add algebraically to tensile stresses created during force cycling, with the (ideal) result that the structure surface experiences little net cyclic tensile stress. Under these conditions the structure becomes relatively tolerant of cyclic force applications that would otherwise predispose it to early fatigue failure at a surface. Instead, the presence of relatively high residual compressive stress levels at and near a structure's surface tends to shift the likely origin of fatigue failure from the surface (which is prone to fatigue damage due to surface roughness, discontinuities, micro-cracks, etc.) toward the (relatively smoothly continuous) central portion of the structure that can better tolerate tensile stress. See, for example, U.S. Pat. Nos. 6,200,689; 6,449,998 and 6,551,064, incorporated herein by reference.
The above techniques have been used to improve fatigue tolerance in springs operating in demanding environments. Specifically, different peening techniques (e.g., variations of two-stage or double shot-peening) have been developed to extend the depth of residual compressive stress under a surface while retaining relatively high residual stress at and near the spring surface. See, for example, U.S. Pat. Nos. 3,073,022; 6,346,157 and 6,544,360, incorporated herein by reference. Note however that when a spring is at rest, the aggregate total of residual compressive stress induced by working the spring wire surface must be balanced by tensile forces in the portions (core area) of the spring wire that are not under residual compressive stress.
Consequently, while some of the fatigue-related effects of high cyclic tensile forces can be reduced by inducing residual compressive stress at and near a spring wire surface, consideration must also be given to the potentially deleterious effects of the balancing tensile stress thereby required in the wire core area (near what would otherwise be the wire's neutral plane). Particular care must be taken to avoid tensile stress levels in the core that are high-enough to cause plastic deformation of the spring metal.
Thus, proper depth and magnitude of induced residual compressive stress are important parameters in metallic coil spring design, and improvements in spring performance are consequently limited by several factors. Such factors include the method(s) of inducing residual compressive stress at and near the spring wire's surface, as well as the magnitude and distribution of balancing tensile stress present in and near the core area. Since balancing tensile stress can not be allowed to exceed a predetermined maximum value, depending on a spring wire's core material, there is an effective limitation on the aggregate total of residual compressive stress that can safely be created at and near the spring wire surface.
In practice, peak residual stress values near the surface of the spring wire may be so limited that surface tensile stress occurring during spring cycling will overwhelm residual compressive stress to place portions of the spring wire surface in net tension during part of a displacement cycle. If sufficiently large, this net tensile stress can significantly increase the spring's susceptibility to fatigue failure at the surface. Note that this tendency for the spring wire surface to experience net tensile stress is further exacerbated by the fact that the theoretical maximum achievable amount of residual compressive stress is actually lower in the thin metal surface layer of spring wire than in the layer of metal located immediately below the wire surface (hereinafter the subsurface metal layer).
The limitations described above on the depth, distribution, peak values and aggregate total of residual compressive stress that may be induced in coil spring wire impose particularly significant manufacturing constraints on springs comprising relatively small wire. Additional limitations are manifest in springs that operate at high speeds that are associated with adverse dynamic effects which further increase the likelihood of fatigue failure. One such adverse dynamic effect is coil binding.
Coil binding occurs when adjacent active coils in a spring come into momentary contact during rapid cycling. Each such coil binding contact creates impact forces that can raise surface tensile stress levels above the values of residual compressive stress that may have been previously established to resist fatigue failure. Under coil-binding conditions then, springs may fail prematurely even though they have been conventionally treated to increase fatigue tolerance by inducing sufficient residual compressive stress for applications that are not likely to cause coil binding. Further, such premature spring failure is likely to be initiated at the spring wire surface near areas of momentary coil binding contact. The present invention relates to methods and apparatus for reducing the likelihood of such premature spring failure.
SUMMARY OF THE INVENTION
The present invention relates to metallic coil springs having non-uniform residual compressive stress distributions induced in the spring wire that tend to reduce the likelihood of premature failure due to combined effects of fatigue and coil binding impacts. The invention includes methods and apparatus for making such springs, including methods for inducing desired residual compressive stress distributions in the spring wire.
Representations of residual compressive stress in cross-sections of coil spring wire of the present invention reveal non-uniform distributions having magnitudes that vary in characteristic ways along one or more spring wire axes. For example, a representation of residual compressive stress in a transverse cross-section of coil spring wire of the present invention exhibits substantial symmetry about a transverse axis connecting potential or actual opposing coil binding contact points on the spring wire surface (hereinafter an opposing contact axis). Further, residual compressive stress in coil spring wire of the present invention also varies in a predetermined manner when measured along the longitudinal axis of the wire. This stems from the fact
Siconolfi Robert A.
Torres Melanie
LandOfFree
Fatigue and damage tolerant coil spring does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fatigue and damage tolerant coil spring, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fatigue and damage tolerant coil spring will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3308639