SPECIFICATIONFiber-reinforced coil springField of the inventionThis invention relates to fibre-reinforced coil springs and more particularly this invention relates to improved fiber-reinforced coil springs especially useful in applications wherein the spring is subjected to only tension or only compression loads.
Background of the inventionThe dependence of the automotive industry on oil as a fuel source is well known. Government, industry and consumer interests, in increasing fuel efficiency of automotive vehicles to offset the escalating costs of such fuels while concomitantly conserving oilresources, has led to increased searches for newlight-weight automotive components which will lead to increased vehicle efficiency. One such application for light-weight, high-strength composite structures in motor vehicle applications is in coil springs for use in suspension systems and in valve lifters and the like.
Prior artIn U.S. Patent 2,852,424, a method-for producing a reinforced plastic coil spring is described which involves pulling a length of glass roving through a liquid resin bath into a flexible tube. The tube is then wound helically around a mandrel and cured thereon. Subsequently, the mandrel is removed as well as the exterior tubing material, thereby providing a solid glass reinforced plastic spring in which the glass is aligned substantially at 0 with respect to the center line of the spring wire.
In U.S. Patent 3,378,426, an apparatus is disclosed for producing a glass fiber-reinforced chemical coil by introducing glass fibers and resin into a rotating drum having an endless mold member on the drum so that the glass resin can be superimposed and molded on the drum and separated from the drum during drum rotation.
Another technique for making a helical spring is disclosed in U.S. Patent 3,728,189. As with the preceding techniques, a spring is produced in which the spring wire is essentially solid and in which the dominant direction of the glass fibers in the spring wire is substantially parallel to the center line of the spring wire.
Summary of the inventionBriefly stated, the present invention contemplates a coil spring formed of a continuous unidirectional fiber-reinforced tubular resin member in which substantially all of the continuous fibers are oriented substantially at the same predetermined angle of orientation with respect to the center line of the tubular member when coiled or helically wound. In general, the continuous unidirectional fibers are oriented substantially at an angle ranging from between 150 to 750 with respect to the center line of the helically wound tubular member; however, in a preferred embodiment of the present invention, the fibers are oriented at from 300 to 600 with respect tothe center line of the tubular member of the spring.
Most preferably, the continuous unidirectional fibersare oriented at between 430 and 470 with respect tothe center line of the helically wound tubularmember.
The present invention also contemplates an improved method of preparing a tubular coil springfrom a fiber-reinforced resin sheet material.
These and other embodiments of the presentinvention will become apparent upon further reading of the specification in conjunction with theaccompanying drawings.
Brief description of the drawingsFigure 1 is a side elevation partly cut awayshowing the angle of orientation of the fiber in the fiber-reinforced helical spring of the present invention.
Figures 2 3 and 4 illustrate a preliminary step in the method of the invention wherein oblong blanks of fiber-reinforced sheet material having unidirectional fibers are cut in a predetermined pattern to berolled upon a mandrel to form a tubular member for use in formation of tubular coil springs in accordance with the present invention.
Figure 5 is an isometric drawing partly in perspective and partly cut away illustrating the technique of forming a fiber-reinforced resin wire useful in form ing the tubular helical spring of the present invention.
Figure 6 is a cross-sectional view of the preferred number of wrappings of fiber-reinforced sheet material around a mandrel when producing a wire for tubular spring fabrication in accordance with the present invention.
Figure 7 is a diagrammatic illustration of the wrapping of a wire around a mandrel to form the spring of the present invention.
Figure 8 is a cross-section along lines 8-8 of Figure 7.
Detailed description of the inventionReferring now to the drawings, it should be noted that like reference characters designate corresponding parts throughout the several drawings and views.
The coil spring 10 of the present invention is formed from a continuous unidirectional fiberreinforced tubular resin member in which substantally all of the continuous fibers 11 are oriented at the same or substantially the same predetermined angle of orientation, 0, with respect to the center line 12 of the tubular member.
In the practice of the present invention, the unidirectional continuous fibers are selected from typical fiber-reinforcing materials, such as boron, carbon, graphite, glass, polyaramids and mixtures thereof. Preferably, however, the fibers are selected from carbon and graphite fibers, and more particularly carbon and graphite fibers having a Youngs modulus of about 32 x 106 psi and a tensile strength of about 400,000 psi or greater.
As indicated herein, the continuous unidirectional fibers are embedded in a resin matrix. In general, any resin may be employed although it is preferred that the resin matrix be a thermosetting resin.
Suitablethermosetting resin materials include epoxy and polyester resins.
The epoxy resins are polyepoxides, which are well known condensation products or compounds containing oxirane rings with compounds containing hydroxyl groups or active hydrogen atoms such as amines, acids and aldehydes. The most common epoxy resin compounds are those of epichlorohydrin and bis-phenol and its homologs.
The polyester resins are polycondensation products of polybasic acids with polyhydric alcohols.
Typical polyesters include polyterephthalates, such as polyethylene terephthalate.
The amount of carbon fiber in the resin is generally in the range of from about 50 to about 65 volume % of fibers in the resin matrix, and preferably between about 60 to 65 volume % of fibers in an epoxy resin matrix.
The magnitude and direction of the angle of orientation, H, of the unidirectional continuous fibers 11 in coil spring 10 depends upon a number of factors including the use to which the coil spring 10 is to be put, the pitch of the coil, the direction of the pitch of the coil, the mean spring diameter, the diameter of the tubular member, and the like. Suffice itto say that when the spring is to be used in compression applications such as in automotive suspension systems, the fibers are directionally oriented so that the shear load on the spring places the fibers in tension. For applications where the spring is to be stretched, i.e. loaded in tension, the fibers are oriented so that the shear load on the spring places the fibers in tension.In general, the magnitude of the angle of orientation of the continuous unidirectional fibers 11 is between 150 to 750 and preferably between about 300 and 600 with respect to the center line of the helically wound tubular member. Most preferably, in springs for automotive suspension systems, the continuous unidirectional fibers will be oriented, for example, at between 430 and 470 with respect to the center line of the helically wound tubular member.
In the practice of the present invention, coil spring 10 is made from sheets of unidirectional carbon or graphite fibers impregnated with a thermosetting resin. A plurality of such sheets of such unidirectional fiber impregnated resin sheet material is first cut into the shape of a predetermined pattern. Typically, three patterns of material go into forming the tubular coil spring of the present invention. Consequently, the description which follows will make specific reference to three layers or sheets of resin impregnated fibers; however, it should be appreciated that more or less than this number may be employed.
Each of the three layers of resin impregnated continuous unidirectional fibers are cut in the shape generally of a rectangle. As is shown in Figures 2 to 4,the length ofthe rectangle for each ofthethree layers of resin impregnated fiber sheet material is the same. Indeed, the length of the rectangular sheet material will be at least as long as that required for formation into a coil spring of the requisite length.
Typically-the width of each sheet is different. Thus, as can be seen in Figures 2 to 4, the width, W1, of sheet 14 is narrower than the width, W2, of sheet 15 which in turn is narrower again than the width of sheet 16.
Each sheet 14, 15 and 16 will have continuous unidirectional fibers 11 oriented at a specific angle with respect to the longitudinal axis of the rectangu lar sheet material.
In fabricating the coil spring, the tubular member is first formed by successively and circumferentially wrapping the layers of resin impregnated fibers 14, 15 and 16 on a mandrel 17 having a resilient tubing 18, such as a rubbertubing, surrounding the mandrel 17. The diameter of the mandrel 17 with its resilient tubing 18 is selected to provide the requisite diameter of the tubular memberforthe coil spring.
Since the flat patterns 14, 15 and 16 are going to be successively wound around the mandrel 17 with its rubber sleeve 18, it is desirable that the widths W1,W2 and W3 be sufficient to provide at least a single convolution of material around the mandrel having the preceding layer applied thereto. Thus, W1 of sheet 14 desirably is at least sufficiently wide to accommodate at least one complete turn around mandrel 17 and rubber sleeve 18. Sheet 15 has a sufficient width W2to provide at least one complete turn about the mandrel containing sheet 14. Similarly, W3 of sheet 16 is sufficientto provide at least one complete turn around the mandrel containing its layers of sheets 14 and 15. Preferably the width of each sheet is chosen to provide substantially the same number of a plurality of complete turns around the mandrel.
Wrapping of the sheet materials around the mandrel with its rubber sheath is accomplished very simply by placing the mandrel 17 with its rubber sheath 18 along the lengthwise bottom edge of sheet 14 and thereafter rolling the mandrel and the sheet material in an upwardly direction such as shown by arrow 19 in Figure 5. Each one ofthe layers is so successively wrapped around the mandrel.
After the three layers of sheet material are wrapped around the rubber sleeve, the steel mandrel 17 is removed, thereby leaving fiber-reinforced tubular resin member having a rubber tubing in the center thereof. This tubular member with its rubber insert constitutes the spring wire used in fabricating the coil spring of the present invention. This spring wire 20 is helically wrapped around a mandrel such as mandrel 21 of Figure 7 which has helical grooves 22 in the circumference thereof of the requisite width r and depth to accommodate the diameter of spring wire 20. The pitch for the helical grooves 22 will depend upon the desired spring pitch. Similarly, the diameter of the mandrel 21 will depend upon the desired main spring diameter.
In any event, spring wire 20 is wrapped in helical fashion around mandrel 21. During wrapping of the spring wire 20 around mandrel 21, sufficient twist is given so that the fibers 11 in spring wire 20 will all be oriented at the predetermined desired angle 8 with respect to the center line of the helically wound tubular member. The direction and the amount of twist required will depend, of course, on the angle of orientation of the unidirectional fibers in the flat patterns 14, 15 and 16 used in forming spring wire 20. Obviously the direction of twist is chosen to provide the desired predetermined fiber orientation in the coil spring with minimum displacement of the fibers from their orientation in the spring wire to their orientation in the coil.
As was indicated with respect to Figures 2, 3 and 4, the flat patterns 14,15 and 16 are each cut so as to have continuous unidirectional fibers 11 which are oriented at specific angles of orientation 6i, 62 and 63, respectively, of the longitudinal or lengthwise axis of the flat pattern. 1, 62 and 63 are chosen such that if sheets 14,15 and 16 are formed into a suitable wire 20, upon twisting wire 20 on mandrel 21 the unidirectional continuous fibers in each of the respective layers 14, 15 and 16 will be oriented at the desired angle 6 or the helical spring.Thus, for example, in a particularly preferred embodiment of the present invention, coil spring is provided having a unidirectional fiber-reinforced tubular resin member in which the continuous fibers are oriented substantially at 45 with respect to the center line of the helically wound tubular member. In such instance, the unidirectional fibers 11 of flat sheet 14 will have an angle of orientation of 40 with respect to the lengthwise axis of layer 14. The preferred angle of orientation in that instance for fibers 11 of sheet 15 will be 36 with respect to the lengthwise axis of sheet 15. Finally, the angle of orientation of fibers 11 in flat sheet 16 will be 33 with respect to the lengthwise axis of sheet 16.
As will be readily appreciated, the angles 6i, 62 and 63 will be chosen depending upon the desired angle that the fibers have in the helical coil and the degree of twist that is to be applied during wrapping of the tubular wire in the form of a coil. More specifically, the change in the angle of orientation of the fibers in each layer of the wire when the spring wire is twisted once per coil is given by the equation: A6 = are tan r/R where: A6 = change in angle;r = radius of the wire;R = radius of the coil.
From the foregoing equation, it can be seen that twisting the wire causes a larger change in the angle of orientation of the fibers in each layer, the larger the distance of that layer in the wire from the center thereof. For example, assuming a coil radius of 3 inches and a radius of .5 inches from the center to the outer layer of the spring wire, then the change in the angle of orientation for the fibers in that outer layer from their original position to their final position upon one twist per coil is 9.5 since A6 = arc tan .5/3 = 9.5 . If the radius of the spring wire to some given inner layer is 0.25, then the fibers in the inner layer under the same circumstances will be displaced 4.8 since A6 = arc tan .25/3 = 4.8 .
Returning to the process of the present invention, it can be seen, for example, in Figure 7 that top layer 16 of wire 20 has fibers 11 that, prior to twisting, are oriented at an angle 63 with respect to the center line of the wire 20. Hence, the wire 20 is twisted, for example, in the direction shown by arrow 25 as it is wrapped upon mandrel 21. All of the fibers 11 throughout the wire 20 are substantially oriented at the preferred angle 6.
After wrapping the wire 20 around the mandrel, the wire 20 can be held in place by any suitable mold means well known in the art. For example, the wire can be held in place on the mandrel 21 by an appropriately shaped tool or by wrapping of cellulose acetate tape or sheet material (not shown) which serves in effect as a mold. The rubber tube inside the spring wire 20 is pressurized in an amount sufficient to facilitate the resin to flow and to compact during curing. Generally, the pressure will range between 35 to 85 psi and preferably about 65 to 75 psi. The entire assembly is then heated so as to cure the resin. The temperature at which the assembly is heated, of course, depends upon a number of factors, including the resin which is used to impregnate the fibers. These temperatures are well known.Typically, for epoxy resin impregnated fibers, the temperature will be in the range of from about 1 00'C to about 180 C, and preferably at about 120 C. Similarly, the time for heating will depend on the curing temperatures as well as the resin employed.
After heating the assembly, the assembly is next allowed to cool to room temperature, and the external wrapping of cellulose acetate tape and rubber tubing are removed as well as mandrel 21.
The end edges 31 and 32 of the wire may be chamfered, if desired, by cutting.
To further illustrate the present invention, reference is now made herein to a typical suspension coil spring for one mode of application. In such an application, the spring 10 consists of six coils providing a free spring height of 18 inches. The coil spring 20 will have a spring diameter of 6.15 inches.
The tubular member comprising the helical coil 20 will have an outer diameter of one inch and an inner diameter of one-half inch. The angle of orientation 6 of the continuous carbon fibers will be in the range of from about 43 to 470 and preferably 45".