Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2002-03-19
2003-12-09
Osele, Mark A. (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S222000, C156S273700, C156S274800, C156S275500, C156S307700, C264S045300, C072S386000, C072S412000, C029S896910
Reexamination Certificate
active
06660114
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to vehicle suspension systems that employ leaf springs, and more particularly to leaf springs incorporating layers of composite material and methods for fabricating said springs.
BACKGROUND OF THE INVENTION
Known leaf springs are constructed from several elongated strips or leaves of metal stacked one-on-top-of-the-other and clamped together in a substantially parallel relationship. Typically, these springs are employed in vehicle suspension systems in one of two different load carrying configurations, cantilevered, or three-point-bending; the latter being the more common method of use. A cantilevered leaf spring is one where the leaf spring is fixed or supported at one end to the frame of a vehicle and coupled to an axle at its other end. Alternatively, a leaf spring mounted in three-point-bending, is supported or fixed at one end to the vehicle with the other end supported in a manner that allows for relative movement of the spring. A load is carried by the spring between the two ends. The use of leaf springs mounted in three point bending is so widespread that the Society of Automotive Engineers (SAE) has developed a formal leaf spring design and use procedure.
Metal leaf springs constructed in the manner described above are incorporated into a variety of different vehicle suspensions including, automobiles, light to heavy trucks, trailers, construction equipment, locomotives, and railroad cars. They are also employed in recreational vehicles, such as bicycles, snowmobiles, and ATV's (all terrain vehicles). The leaf springs improve the quality or smoothness of the vehicle's ride by absorbing and storing energy for later release in response to bending and/or impact loads imposed on the vehicle resulting from such things as encountering obstructions in a road during the vehicle's operation.
The mechanical properties defining a vehicle suspension system, particularly the spring rate and static deflection of the leaf springs, directly influence the smoothness of the vehicle's ride. Generally, a smooth ride requires the leaf springs to have large static deflections. The smoothness of the ride is also influenced by the vibration damping characteristics of the leaf springs. Damping is a parameter that quantifies the ability of the leaf spring to dissipate vibratory energy. Therefore, a high degree of damping is desirable in leaf springs used in automobiles to minimize the vibratory amplitudes transferred to the passenger area.
The ability to accurately determine the mechanical properties and performance characteristics of a leaf spring is critical to the proper design of vehicle suspension systems. One of the problems resulting from the construction of conventional leaf springs is that the variable lengths of the stack of individual leaves creates a stepped spring construction that only approximates constant stress, the steps tend to create localized areas of high stress known as stress concentrations which detrimentally affect the load carrying capability and useful life of the leaf spring. In addition, the fact that the springs are composed of lengths of metal stacked one-on-top-of-the-other causes them to be quite heavy; this additional weight causes a concomitant reduction in fuel economy.
Moreover, because it is impossible to predict the exact conditions and stresses that a leaf spring will be subjected to, the fatigue life of the spring is generally limited. This problem is further exacerbated by the build-up of foreign material on and between the individual leaves. Not only does this cause corrosion, thereby weakening the leaf spring and making it more susceptible to fatigue failure, but it also affects the stiffness of the leaf spring and hence the smoothness of the ride of the vehicle in which the spring is employed. Generally the magnitude of the contribution made to the strength of a particular leaf spring due to inter-leaf friction is determined empirically. When foreign material gets between the leaves it can dramatically increase, in the case of particulate matter, or decrease, in the case of oil, the friction between the leaves, thereby altering the original mechanical properties of the spring. In addition, the shear conductivity between the leaves, which is a measure of the amount of shear stress transferred from leaf-to-leaf, is typically low in conventional leaf springs because the individual leaves are only clamped at the ends. Therefore, the stress transfer capability along the length of the spring is dependent on the aforementioned inter-leaf friction.
In many applications, leaf springs are loaded not only by vertical forces but also by horizontal forces and torques in the longitudinal vertical and transverse vertical planes. These forces are typically generated when the brakes on the vehicle incorporating the leaf spring are applied. The aforementioned horizontal forces and torques cause the leaf spring to assume an “S” shaped configuration, a phenomena referred to as “S-ing” or wrap-up. The stresses induced in the spring when this occurs can be quite high. In order to minimize S-ing in a leaf spring, the stiffness of the spring must be increased; however, this can detrimentally affect the smoothness of a vehicle's ride.
In order to address the above-described problems, those skilled in the art have attempted to fabricate purely composite leaf springs, wherein the individual leaves are formed from a composite material of the type consisting of a plurality of fibers embedded in a polymeric matrix. However, while these springs offered significant reductions in weight, as well as increased fatigue life and damping, their cost was prohibitive. In addition, these composite springs are difficult to fabricate and attach to the frame of a vehicle and required the use of special adapters. A hybrid leaf spring having a metal primary leaf with one or more layers of composite material bonded thereto has been proposed in U.S. patent application Ser. No. 08/906,747 to Meatto, Pilpel, Gordon and Gordon entitled “Hybrid Leaf Spring And Suspension System For Supporting An Axle On A Vehicle”, filed on Aug. 6, 1997, the disclosure of which is incorporated herein by reference. The metal primary leaf also defined the means, for example, an aperture extending through each end of the leaf, to mount the spring to the vehicle.
Composite components usually comprise multiple individual layers of material juxtaposed, one on top of the other with adhesive material located between successive layers of the composite, thereby forming a laminate. As used herein, the term “composite material” should be construed to mean a fiber or particle reinforced polymeric material. To bond the layers of composite material together, the adhesive must be cured unless a thermoplastic adhesive is used which requires only melting and fusing. Curing is usually accomplished by heating the composite layers under pressure in a mold to a known curing temperature and then maintaining that temperature for a predetermined period of time.
A difficulty often encountered with producing laminated composite components in this manner is that the individual layers of composite material act as insulators. Therefore, to completely cure a multiple layer laminated composite part, long heating periods are required to allow the adhesive between the inner-most layers to reach curing temperature. This results in decreased productivity, increased energy consumption, wear on the mold, and higher overall cost. These problems are further exacerbated with respect to the above-described hybrid leaf spring because the metal primary leaf acts as a heat sink, drawing thermal energy away from the adhesive material.
Another difficulty encountered with producing hybrid leaf springs is that for applications with high spring rate camber designs, molded-in bond line shear stress between the primary leaf and composite layers can be relatively high so as to reduce fatigue life when the hybrid leaf spring is fully deflected under a full load.
Based on the forego
Gordon D. Michael
Gordon, Jr. David C.
Meatto Frank
Pilpel Edward
McCormick Paulding & Huber LLP
Osele Mark A.
Pacific Coast Composites
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