Field of Invention. This invention relates to elastomeric compression springs. Th preferred embodiment of our invention is primarily directed to a compression spring for th bumper assembly of an automobile.

The automotive and insurance industries have long sought a bumper assembly tha would absorb the energy of a collision at speeds of 5 miles an hour or less. Th acceptability of these units is evaluated by two different criteria. The first is the "no visibl damage" criteria. On belief, existing hydraulic bumper assemblies generally meet thi criteria, Le., they protect the vehicle from visible damage. The second is the "no dolla damage" criteria. On belief, the presently used hydraulic units cannot consistently meet the latter criteria. Either the bumper assembly itself or other parts of the automobile suffer functional damage from collisions at 5 mph. Moreover, such bumper assemblies are comprised of a plurality of parts that add undesirable weight, impose assembly costs, require separate, preload springs to minimize vibration, and do not provide a generally constant spring rate.

Related Art. Numerous efforts have been made to design a bumper spring for automobiles that would meet the industry goals and criteria. These efforts include U.S. Patent No. 4,893,857 which issued to General Motors Corporation on an application o Bobinger, et. aL This device of General Motors includes a cylindrical tube containing a first energy absorbing medium, a preload unit comprised of a compression spring and disks and a telescoping piston tube that extends to the bumper. One energy absorbing medium described by General Motors is a thermoplastic material such as Hytrel ^® , a copolymer elastomer manufactured by E.I. duPont de Nemoirs which has a very high compression set characteristic and which, in part, necessitates the preload assembly to insure that the bumper assembly returns to its original position after a collision.

Another prior effort to design a bumper spring is illustrated by U.S. Patent 4,624 which issued to Chrysler Motors Corporation on an application of Hillebrand, et. al the General Motors patent, the Chrysler patent also includes a plurality of parts, Le., a l bearing tube member with an abutment, a reaction coil spring for preload and an en absorbing capsule containing the Hytrel ^® copolymer elastomer of E.I. duPont de Nem Additional prior art related to the present invention is U.S. Patent No. 4,566, which issued to Miner Enterprises, Inc. on an application of David G. Anderson. patent focuses on the thermoplastic Hytrel ^® . It explains a method of producing a hol spring of Hytrel ^® and of eliminating the compression set problems of this thermopla While this patent effectively overcomes the compression set characteristics of Hytrel ^® provides a very effective spring for absorbing substantial energy in rail car applicati modifications are required to apply this concept to the automotive industry in order to m the "no visible damage" and "no dollar damage" tests. A basic modification is the provis of a different force travel curve. And this, in turn, necessitates design modifications enhancements that go beyond the teachings of this Anderson patent.

SUMMARY OF INVENTION Our invention is a compression spring for absorbing substantial amounts of ene at a constant maximum rate over a limited displacement. It is an elongated hollow b formed of an elastomer whose ratio of plastic deformation to its elastic deformatio greater than 1.5 to 1. Preferably, the elastomer is a copolyester polymer elastomer such that manufactured and sold by E.I. duPont de Nemoirs under the trademark Hytrel ^® . the preferred embodiment, the hollow body is designed for installation into the bum assembly of an automobile and the material is provided with a shape and orientation t permit the unit to absorb the forces resulting from a collision without visible or dol damage to the vehicle.

Accordingly, the objectives of this invention are to provide, inter alia,

1) a unique elastomeric compression spring shaped to provide a substantially const spring rate capable of absorbing the collision energy of an automobile traveling 5 mph without either visible damage or dollar damage to the vehicle;

2) a simple, one piece bumper compression spring that eliminates the costs preload components and their assembly, and minimizes the costs of installation up new and existing automobiles;

3) an elastomer spring formed of an elastomer that is very durable, inert to reacti with highway grime, salt and corrosive fluids, not subject to tear propagation, a that has tensile characteristics such that the ratio of plastic strain to elastic strain greater than 1.5 to 1; and

4) an elastomeric bumper compression spring of relatively low costs and light weig for improving the Corporate Average Fuel Economy of the automobi manufacturers.

DESCRIPTION OF THE DRAWINGS

The manner in which these objectives and other desirable characteristics can obtained from our invention is explained in the following specification and attach drawings in which:

Figure 1 is an illustrative side elevation view of an automobile with portions brok away to depict our invention and its application to the bumper assembly of an automobil

Figure 2 is a plan view of a preform used in the manufacture of the preferr embodiment of our invention;

Figure 3 is a sectional side elevation view of a preform used in the manufacture the preferred embodiment, the view being taken along the lines 3-3 of figure 2;

Figure 4 is a side elevation view of a preferred embodiment of a preform disclosing the apparatus for used to precompress and orient the preform to obtain the preferred embodiment of our invention;

Figure 5 is a side elevation view of a preferred embodiment of a preform disclosing the compression step in the manufacture of a preferred embodiment of our invention;

Figure 6 is a side elevation view of a preferred embodiment of our invention; and Figure 7 is an illustrative displacement graph depicting the force-displacement curve of our compression spring.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of our invention is illustrated in one intended environment in Figure 1 in which the front of an automobile is depicted at 10. Two laterally spaced and similar supports 12 extend from and are rigidly attached to the frame of the automobile (not shown). One end of our compression springs 14 are affixed each of these supports and extend forward to be affixed to and support the bumper 16. Over this bumper 16 is mounted a plastic facia 18 which is designed to provide an attractive appearance to the automobile 10.

The shape and configuration of the preferred embodiment of our compression spring 14 is illustrated in figure 6. It comprises a central spherical segment 22 whose ends integrally join cone shaped frustums 24 having an acute angle which is generally tangent to the ends of the spherical segment. The frustums 24 terminate in flanges 26 which have apertures 28 for receiving bolts (not shown) or other means for attaching the spring 14 to the vehicle and to the bumper.

When made according to the process described below, this compression spring provides a force-displacement curve similar to that illustrated in figure 7. The vertical axis on which the curve is plotted represents the force of a collision in thousands of pounds. The horizontal axis represents the bumper displacement in inches resulting from a collision at

speeds up to 5 mph. As shown, the force upon collision is limited to some 7,000 p (14,000 pounds for two springs) by the spring rate of the spring and the displacement bumper 16. Moreover, the force quickly increases towards this maximum rate and re substantially constant over a substantial portion of the displacement of the bumper limiting this maximum spring rate and by providing sufficient displacement of the s which will absorb the collision energy, visual and dollar damage to the vehicle is avo Moreover, the simplicity of our one piece compression spring design and the use elastomer that is resistant to tear propagation avoids damage to the bumper spring i

On belief, the configuration of our spring enables our invention to obtai desirable spring rate of the curve of figure 7. Important to this configuration is the frustum 24 which is believed to serve as a transition segment permitting the spri maintain the maximum desired force for a greater percentage of its available defle Such differs from the Anderson patent (mentioned above) in which a straight cylin body is compressed to form a generally spherical compression spring. The spring of patent permits the applied force to continually rise to a peak. An increasing, rather a constant, incremental force is absorbed with each increment of spring displacement a result, the teachings of the Anderson patent and similar teachings would permit the applied to exceed the maximum and result in visual or dollar damage to the automo

Manufacture of our invention begins with the molding of the integral preform 32 depicted in figures 2 and 3. This preform takes the shape of a hollow cylindrical 34 having a cone shaped frustums 24 joined to each end of the cylinder. Joined to t frustums 24 are flanges 26 which serve as means for attachment of the spring to the ch and to the bumper. Apertures 28 in these flanges or other affixing devices may be us affix the spring to the chassis and to the bumper.

This preform, according to our invention, is formed of an elastomer having te characteristics such that the ratio of plastic strain to elastic strain is greater than 1.5

I One such elastomer is a copolyester polymer elastomer manufactured and sold by E.I. d Pont de Nemoirs under the trademark Hytrel ^® . It is reasonably inert and significantly, i is quite durable. Moreover, this elastomer is not subject to tearing or to crack propagatio even when made in relatively thin cross sections. We prefer to use du Pont's Hytrel composition no. 5556. (For a more complete description of this elastomer, see Anderso patent no. 4,198,037 and the references cited therein). Normally, the selecte I elastomer is purchased in pellet form, and, according to our invention, is injected o I extruded into a mold to form the preform 32. Various plastic molding techniques such a melt casting, injection molding, rotational molding, etc. may be used to make this preform. The preform 32 is not, without additional manufacturing steps, adequate to serve as a compression spring. This is primarily due to the fact that Hytrel ^® and similar elastomers take a compression set— and upon the first compression, will not return to their original length. In addition, orientation of the molecular structure of the elastomer is needed to provide the desirable spring characteristics. This inadequacy of such elastomers are demonstrated by the General Motors and Chrysler Corporation patents referenced above. Each calls for additional preload devices that are normally essential to overcome the compression set characteristic of the elastomer. The additional manufacturing step needed to overcome this compression set characteristic of Hytrel ^® includes precompression and orientation of the molecular structure. Such may be accomplished in the manner depicted in figures 4 and 5. These figures include a precompression press 40 which has a bottom fixed plate 42, a top movable plate 44, and a rod 46 that is vertically reciprocated by hydraulic or 1 mechanical means. In the manufacture of our invention, the preform 32 is inserted into this W press as depicted in figure 4 and, preferably, it is fully compressed as illustrated in figure 5 1 by reciprocating the plate 44 downward until the entire unit is pressed flat. When the i downward force of rod 46 is removed and the plate 44 is raised, the compression spring of

our invention partially recovers to its original height and takes a new shape as shown in t preferred embodiment depicted figure 6. As depicted, the transition section or cone shap frustums 24, after compression and orientation, form a more acute angle with the flan 26, and the cylindrical section 34 has been reshaped into a spherical section 22.

After the precompression step and partial recovery of the preform, our compressi spring is free of the compression set problem and, upon subsequent compressions, the spri will return or spring back to the height reflected in figure 6. In part, the "spring bac characteristics, as well as the spring rate characteristics of our invention are the result orientation of the molecules of the Hytrel ^® that resulted from the precompression step figures 4 and 5.

In as much as various models of automobiles have different weights and differe desired ride characteristics, there is no one manufacturing design for our invention whi will accommodate all vehicle models and some experimentation for each potent application will be required. Nevertheless, a primary, important design element necess to obtain the desired constant spring rate curve is the transition section or cone shap frustums 24 which, upon the precompression step, form a more acute angle with the flang 26 and which are oriented to provided the desired spring effects. In designing a compressi spring according to our invention for a new application, the preferred, direct procedure to make two or more springs, correlate their dimensions to their resulting spring rate a then interpolate or modify the dimensions until the desired spring rate is obtained.

Persons skilled in the art of plastic forming and compression spring design w discover that many modifications of our invention will produce satisfactory resul Elastomers other than Hytrel ^® may be acceptable for some applications. With respect the shape, only one transition section 24, may be used adjacent one of the flanges Moreover, it may take various shapes. Similarly, the angle of the transition section wi respect to the flanges 26 and the spherical sections 22 might be reversed.

The process of molding the compression spring of our invention can also incl various modifications. Extrusion blow molding would yield acceptable springs. These a other variations, which will be appreciated by those skilled in the art, are within t intended scope of our invention as claimed below.