WO1988007927A1

WO1988007927A1 - Fiber reinforced plastic springs and method of manufacturing the same

Info

Publication number: WO1988007927A1
Application number: PCT/DK1988/000069
Authority: WO
Inventors: Mogens Nielsen
Original assignee: Bms Kunstfiber A/S
Priority date: 1987-04-15
Filing date: 1988-04-15
Publication date: 1988-10-20
Also published as: DK200987A; DK200987D0; AU1689088A; KR890700461A

Abstract

A spring made of fiber reinforced plastic material may be manufactured without requiring any expensive manufacturing tools in that the fiber reinforced plastic material is provided in the form of tapes (2), which are precured while under tension and which have the fiber material arranged in the longitudinal direction. The tapes (2) used are placed on a mandrel (3), the outer dimension of which determines the inner dimension for the final spring, and the tapes (2) are secured to the mandrel by gripping means (4). The tapes are provided with a glue material, which can glue together a number of corresponding tapes in order to make a laminate with the dimensions required for the spring (1). The laminate is glued together on the mandrel, as the glue material is at least partially cured so that the glued form stable spring may be removed from the mandrel and subsequently the chemical process may continue at room temperature in order to obtain maximum strength of the spring. By this method it is possible to manufacture springs with very different characteristics solely with one manufacturing tool as the characteristics of the spring made on the same mandrel may be variated by changing the pitch, the width of the tapes, the thickness of the tapes and the number of the tapes.

Description

Fiber reinforced plastic springs and method of manufacturing the same Background of the invention

The present invention relates to a fiber reinforced plastic spring and a method of manufacturing the same.

In many applications where metal springs are used a destruction of the springs or a deterioration of the spring properties occur. Moreover, it may be desirable to use lighter materials in order to obtain weight saving benefits compared to the weight of the steel springs.

A metal spring may be destructed in a corrosive environment and, accord¬ ingly, a steel spring applied in a saline environment like e.g. seawa- ter, is thus exposed to an accelerating corrosion with an ensuring deterioration of the spring properties. Moreover, in springs applied in an environment with temperatures below freezing point brittleness phenomena may occur, and consequently there is a risk of the spring going to pieces when exposed to load. Another drawback of metal springs, is the occurence of fatigue fractures, which come about after the spring has been exposed to a certain number of stresses, even though each stress is below the tensile strength capacity of the mate¬ rial .

In an attempt to solve the above mentioned drawbacks, experiments have been conducted for several years to make springs from materials not suffering from the same weaknesses, and thus attempts have also been made to construct springs from fiber reinforced plastic materials.

Accordingly, springs made of fiber reinforced plastic materials and methods of manufacturing the same are known. However, as a common fea- ture for these prior art fiber reinforced plastic springs, very great dimensions are required compared to the dimensions of metal springs having corresponding properties. This contributes to making the manu¬ facturing costs for such springs excessive and moreover, structural reasons may render the use of extremely voluminous springs impossible or inconvenient.

Complicated manufacturing tools in the form of moulds and mandrels are required for the manufacture of known fiber reinforced plactic springs. Such manufacturing tools contribute to making the manufacturing costs excessive, and the manufacture of springs in such moulds is often pro-" tracted due to a long cycle time. Thus, only a limited number of springs per mould is manufactured within a specific period of time.

Moreover, according to prior art methods using such manufacturing tools, it is possible only to produce one specific embodiment for a spring with each manufacturing tool. Accordingly, the production of a wide range of different springs presupposes a large number of manu¬ facturing tools.

In accordance with the prior art methods, the uncured fiber reinforced plastic is placed in the manufacturing tool, and the spring is subse¬ quently cured. After the curing of the spring it is separated from the manufacturing tool. In such springs the strength obtained is not as high as in comparably sized steel springs, and the manufacture is time and labour consuming.

It is the object of the invention to remedy the above mentioned draw¬ backs and to provide a method of manufacturing fiber reinforced plas- tic springs, which method does not require the use of expensive manu¬ facturing tools and which is simple to conduct. At the same time it i rendered possible to make a wide range of springs with one manufac¬ turing tool .

According to the present invention this object is achieved with a me¬ thod, which is characterized in that the fiber reinforced plastic ma¬ terial is provided in the form of tapes cured under tension with the fiber material arranged in the longitudinal direction, that at least one tape is placed on a mandrel having a configuration depending on the configuration of the spring to be manufactured, which tape is placed with its longitudinal direction orientated substantially along the intended final configuration of the spring, that the tape is se¬ cured to the mandrel since the ends thereof are placed in gripping means, that the tape is provided with a glue material permitting it to be glued together with one or more substantially corresponding tapes, which are secured in the gripping means, too, that a number of tapes are used in order to provide a laminate with the number of layers and the dimensions desired, that the laminate having substantially the desired final configuration of the spring is glued together on the mandrel as the glue material undergoes a chemical process during which gluing occurs and that the glued form stable spring is removed from the mandrel and the chemical process may continue in order to obtain the maximum strength in the gluing.

Tape as defined in the present specification and claims means an elon¬ gated element, the cross section of which has a width to height ratio within the range of 5 to 100 or even higher and which has a substan¬ tially infinite longitudinal extension compared to the width of the element.

Since the tapes to be used are precured under tension a material is obtained having properties that are comparable with the properties obtained in prestressed concrete and laminated wood. Accordingly, a starting material of very high strength is obtained. This starting material consisting of cured tapes is used for the manufacture of the laminate. Between the layers of the laminate a specific glue material is applied, which glue material allows a small mutual displacement of the individual layers in case several tapes- are simultaneuosly wound around a mandrel. After having wound the number of tapes required for the manufacture of a laminate of substantially the dimensions desired in the final configuration, the tapes are bound together as the glue material undergoes a chemical process. In this way a laminate is made, the inner dimension and form of which is determined by the mandrel.

When the chemical process has developed sufficiently for the spring to be form stable it is possible to remove the spring from the mandrel, allowing the chemical process to continue with a free after curing to obtain a maximal strength of the gluing. As the spring may be removed from the mandrel, when it has become form stable, it is possible to reduce the cycle time during which the spring is placed on the man¬ drel. Since previously cured tapes are used producing a certain re¬ sistance to bending the ends of the tapes must be placed in gripping means securing the tape(s) on the mandrel. Such gripping means are known in the art and may be provided in the form of a hand vice, said means need not exercise much force since the tape(s) is (are) to be wound while under tension. Accordingly, the gripping means need only prevent the tape(s) from "swishing" away from the mandrel. As the tapes individually, in bundles or as an assembly are wound a- round the mandrel, the inner dimension of the spring is determined by the outer form of the mandrel .

The mandrel may be produced with any desired configuration for the manufacture of coil springs, spiral springs, volute springs etc. Ac¬ cordingly, for the manufacture of a coil spring solely a cylinder with a smooth surface is required and a smooth cone formed mandrel may be used for manufacturing a spatial spiral spring.

To ensure a uniform pitch of the tapes means known per se may be used, e.g. the constant pitch may be ensured by positioning guide pins in the mandrel or by using a spindle governed displacing of the mandrel when laying up the tapes. Hereby, it becomes possible to ensure that all tapes of the laminate obtain the pitch required. Moreover, a spind¬ le governed pitch makes it possible, with one mandrel, to manufacture a large number of springs having different pitches, in that the pitch of the individual spring is defined by a simple regulation following the same principle as the one used with a lathe. According to the pre- sent invention it is also possible to vary the pitch along the length of the spring in order to obtain the spring properties required. This is obtained in regulating the ratio between the wind speed and the displacement of the mandrel when laying up the tapes.

Furthermore, in using tapes of different widths and thicknesses it be¬ comes possible, with one mandrel, to vary the spring characteristic, as it is possible to manufacture springs having a cross section which increases or decreases radially just as the radial dimension may be provided in accordance with the number of tapes used and the thickness of the individual tapes.

It is possible to use tapes having different widths and thicknesses to provide a laminate for the manufacture of a spring. Hereby it is pos¬ sible to make springs with substantially any section required.

In tests conducted with springs made in accordance with the method of the present invention it has been possible to replace a steel spring made of a round steel bar with a diameter of 20 mm by a spring having a cross section of approximately 20 x 20 mm in order to obtain the same spring characteristics. With such a spring considerable weight saving benefits are obtained. Said spring provides a further advantage over a steel spring, namely its being "dead", and consequently the vibrations, "the nod oscillations", transmitted through the spring were reduced as compared with the vibrations occuring in connection with a steel spring.

According to a preferred embodiment the method is characterized in that a number of tapes necessary for the required spring dimensions, and which are cured under tension, are provided with a specific glue material and are put together to produce the laminate, that said la¬ minate in assembled state is placed on the mandrel, whereafter said gluing together is provided as the glue material undergoes a chemical process whereby the gluing together occurs and that the glued together spring is removed from the mandrel.

This embodiment is particularly suitable when a spring is to be manu¬ factured under conditions not allowing for the individual winding up of the tape, e.g. due to the configuration of the mandrel, or when it is desirable to lay up a laminate comprising all tapes at a time to obtain advantages of production, e.g. in the form of a quicker laying up.

Thus, this method is especially advantageous when a spiral spring is to be manufactured, in the case of a flat spiral spring as well as in the case of a spatial or conical spiral spring. Thus, for the manufac¬ ture of the flat spiral spring a mandrel or a mould is used filling up the space between the coils of the spiral spring, preferably provided in the form of an insertion, which is wound together with the laminate having tapes between which the glue material is provided, said glue material not having undergone a chemical process at the time of the winding. Thus the individual tapes will be able to conduct a mutual displacement during the winding, and the forces required for the wind¬ ing will not be inconveniently great.

For the manufacturing of a spatial spiral spring or an evolute spring a mandrel or a mould is used, which, too, fills up the space between successive coils. When manufacturing such springs the laminate is wound on a mandrel in order to obtain the inner diameter and subsequently the laminate is wound together with an insertion placed between succes¬ sive coils, during which winding an axial displacement of the mandrel, on which the first coil has been laid, is effected. This axial dis¬ placement is adapted to the winding speed, so as to provide a spring with the desired pitch. In this embodiment, as in the above embodi¬ ment, the mandrel and/or the insertion placed between the coils of the spring may be removed when the chemical process has progressed suffi¬ ciently for the spring to be form stable. Thereafter the chemical pro¬ cess may continue, and simultaneously the mandrel and the insertion may be used for the manufacture of a new spring.

When adapting the method according to the present invention to spring manufacture on an industrial scale, a number of mandrels may advan¬ tageously be placed in a production unit, e.g. on a round table, the cycle time of which corresponds to the time required for the laying up of a spring plus the mould time of the spring on the mandrel plus the time required for removing the spring. Accordingly, it is possible to perform the method continously in successive stations and, thus, to obtain an optimum utilization of the mandrels.

According to a further embodiment the chemical process is a curing process and the glue material undergoes a heat curing while placed on the mandrel thus allowing for a substantial reduction of the mould time of the spring on the mandrel. After heat curing on the mandrel the spring is removed and aftercured at room temperature.

According to a further embodiment the fiber reinforced plastic materi¬ al comprises an epoxy resin, and the glue material used is based on an epoxy resin, too. Tests with epoxy resins have proved it possible to obtain satisfactory results with a fiber content as high as 80%. How¬ ever, it is also possible to use polycarbonates for the fiber rein¬ forced plastic material as well as for the glue material.

In tests springs have been made of tapes each having a width of 20 mm and a thickness of mm, and which are provided in a number of 40 to obtain a substantially square section of 20x20 mm and coil springs were wound with an inner diameter of approximately 100 mm. When all tapes were glued and placed on the mandrel the assembly was heated to a temperature of about 140°C, and after about 3% minutes it was pos- sible to separate the spring from the mandrel as the spring was now form stable. Then the spring aftercured for 48 hours at room tempera¬ ture to obtain maximum strength. In this test epoxy resin a fiber con¬ tent of 80% was used and the glue material used was based on the same epoxy resin.

Since, as mentioned above, the spring may be manufactured by means of a mandrel without any grooves or tracks it is possible to provide manu¬ facturing tools at very low costs. Thus, when manufacturing a coil spring it suffices to use a tool in the form of a round steel bar hav¬ ing an outer diameter corresponding to the inner diameter of the spring plus the tapes and glue material required. This single tool permits the manufacture of a large number of coil springs having dif¬ ferent pitches and having varying spring characteristics since the laminate may be provided with larger or smaller number of tapes, ac¬ cordingl tape^'s of different sizes may be used, too.

Even though according to the present invention it is possible to manu¬ facture a spring by means of a mandrel without any grooves wherein the spring is laid, in some situations, it might be desirable to use a mandrel comprising a groove wherein the laminate is placed, e.g. due to demands on the surface of the spring. After the initial chemical process, which makes the spring form stable, it is possible to "un¬ screw" the spring from the mandrel.

However, a spring for most of the intended fields of the application may be manufactured with sufficient accuracy on a mandrel provided with guides or spacers at spaced apart positions on the mandrel. Thus, a test made with a coil spring manufactured with an inner diameter of 100 mm has shown that said inner diameter after removal was increased by a magnitude of a few hundredth of a millimeter only.

When manufacturing springs the following ratios between the different dimensions may be used cross section of spring: 40x40 mm, inner diameter of spring: 300 mm, to this spring tapes each having a width of 40 mm and having a thick¬ ness of between about \ mm to 2 mm may be used. As a general rule, the greater the inner diameter of the spring, the greater the thicknesses of the individual tapes. The same rule applies the other way around, however, in practice, a lower limit for the thickness of the tapes exists, being at about 0.2 mm. A thickness below this limit will cause productional difficulties which substantially obstruct the use of a smaller thickness. :

Description of the drawings

The invention will now be further explained with reference to the accom¬ panying drawing, wherein

Ffg. 1 is a diagrammatic side view of a tool for use in the 0: manufacturing of a coil spring and

Ffg. 2 an end view of the tool shown in Fig. 1.

Ffg. 1 illustrates diagrammatically a tool for use in the manufactur¬ ing of a coil spring 1 according to the present invention. As illu- 5Ξ strated, and as it is especially shown in Fig. 2, the coil spring 1 is manufactured of a number of elongated tapes 2, precured while under tension and having the fiber material arranged in the longitudinal direction of the tapes. In the illustrated embodiment the elongated tapes 2 are wound simultaneously around a cylindrical mandrel 3, which 0 may optionally be hollow or solid. The exemplary mandrel is provided with gripping means 4 consisting of a plate 5, two thread bars 6, two nuts 7 and two guides 8, mounted on the mandrel 3 either by welding or by other suitable securing means.

5- Fn the illustrated embodiment the nuts 7 are provided in the form of wτng nuts, as they may hereby be tightened by hand. The wing nuts are secured to the thread bars 6. The guides 8 are at their end parts re¬ mote from the mandrel 3 bent down, and threaded holes are provided in the bended parts 8', which holes are intended for engagement with the D^" thread of the thread bars 6 to press the tapes 2 positioned below the plate into contact with the mandrel 3 when the thread bars are screwed against the plate 5. Hereby it is ensured that one free end of the tapes 2 is kept in contact with the mandrel. In the exemplary situa¬ tion the thread bars 6 are shown as substantially screwed home. 5

The gripping device 4 is only intended to hold the tapes in order for them not to "swish" away from the mandrel, thus said gripping device does not need to exercise great squeezing force, the tension occuring in the longitudinal direction of the tapes being rather small. The mandrel 3 is provided with spacers 9 placed diametrically opposed on the mandrel 3. In the embodiment illustrated the spacers 9 are mounted on the outer surface of the mandrel. However, it is possible to provide spacers, which may be retracted into a hollow or partly hollow mandrel 3, which facilitates the removel of the form stable spring 1 from the mandrel. Moreover, the spacers 9 may be replaced by detachable spacer blocks or optionally pins for guiding the pitch of the spring 1 on the mandrel 3.

Through an axle neck 10 the mandrel is rotatable in a support 11, which support may be provided with drive means (not shown) enabling rotation of the mandrel when laying the tapes 2.

At the other end (not shown) the mandrel 3 is also supported so as to permit rotation. The mandrel 3 may, as known, be dismantled from the supports 11 when the spring 1 is to be removed.

The illustrated coil, spring 1 is manufactured by the tapes 2, precured under tension with the fiber material arranged in the longitudina-1 direction, being placed simultaneously on the mandrel 3, said tapes being previously provided with a heat curable glue material. One end of the surmounted tapes 2 is placed in the gripping device 4 and is secured between the surface of the mandrel and the plate 5, which is screwed against the mandrel. Subsequently, the glue provided tapes are wound around the mandrel, which is simultaneously rotated slowly by the drive means, not shown, during this rotation it is solely neces¬ sary to exert a slight traction in the direction of the arrow 12, and during this laying the tapes may perform a mutual displacement in or¬ der to adapt itself to the different winding diameters.

In case tapes 2 having equal lengths are used in the manufacturing of the coil spring 1 these tapes will produce different extensions at the end of the coil spring as illustrated in Fig. 2. Alternatively, tapes 2 of different lengths may be used to ensure that the tapes have sub- stantially the same extension at the opposite end of the coil spring, however, this does not cause any technical problems. The ends of the spring may be cut according the intended application.

When all the tapes are wound for the forming of the helical form wanted a gripping device (not shown) ensuring that the tapes will re¬ main in their position following the circumference of the mandrel 3 is also placed at the opposite end of the spring. Subsequently, the spring 1 is heated by heating means (not shown) for a period permit- ting at least a partial heat curing of the glue material, and subse¬ quently the spring may be removed from the mandrel 3 and aftercured at room temperature.

The heating means used may be provided in the form of heating means positioned either inside or outside the mandrel, e.g. in the form of an outer heating tunnel which, following the positioning of the spring on the mandrel, is moved over the mandrel and over the spring placed thereon and is held in this position for the period wherein the heat curing of the glue material is desired.

According to this method of manufacturing the fiber reinforced coil spring 1 it is possible to use a very simple manufacturing tool and, as illustrated- in the drawing, it is possible in principle to use only one smooth cylinder for the manufacturing of coil springs with diffe- rent characteristics. As said smooth cylinder determines the inner diameter of the coil spring it is possible to vary the widths of the tapes, the thicknesses of the tapes, the number of tapes, and the pitch may thus be determined by detachable spacer blocks placed be¬ tween the coils of the coil spring.

It is obvious that the method as described above in connection with a cylindrical mandrel may also be used in connection with other mandrel forms, e.g. a conical mandrel for the manufacture of a spiral spring.

Claims

C l a i s

1. Method of manufacturing a fiber reinforced plastic spring, c h a ¬ r a c t e r i z e d in that the fiber reinforced plastic material is provided in the form of tapes cured while under tension with the fiber material arranged in the longitudinal direction, that at least one tape is positioned on a mandrel, having a configuration according to the configuration of the spring to be manufactured, which tape is placed with its longitudinal direction orientated substantially along the intended final configuration of the spring, that the tape is se¬ cured to the mandrel by its ends being placed in gripping means, that the tape is provided with a glue material so that it may be glued to¬ gether with one or more substantially corresponding tapes, which are secured in the gripping means, too, that a number of tapes are used to provide a laminate with a number of layers and with dimensions as de¬ sired, that the laminate having substantially the desired final con¬ figuration of the spring is glued together on the mandrel as the glue material undergoes a chemical process during which a gluing takes place and that the glued form stable spring is removed from the man- drel and the chemical process may continue to obtain maximum gluing strength.

2. Method according to claim 1, c h a r a c t e r i z e d in that a number of tapes cured while under tension, which tapes are necessary for obtaining the required spring dimensions, are provided with a glue material and are put together in order to produce the laminate, that this laminate as an assembly is placed on the mandrel, whereupon said gluing is provided as the glue material undergoes a chemical process whereby gluing takes place, and that the glued spring is removed from the mandrel .

3. Method according to claim 1 and 2, c h a r a c t e r i z e d in that the chemical process is a curing and that the laminate subsequent to an at least a partial heat curing of the glue material is removed from the mandrel and is aftercured at room temperature.

4. Method according to any of the beforegoing claims, c h a r a c t e ¬ r i z e d in that the fiber reinforced plastic material comprises an epoxy resin or polycarbonates and that glue material used also is based on an epoxy resin or polycarbonates.

5. Method according to any of the beforegoing claims, c h a r a c t e ¬ r i z e d in that the fiber reinforced plastic contains up to 80% fi- bers.

6. Method according to any of the beforegoing claims, c h a r a c t e - r i z e d in that tapes are used each having a minimum thickness of 0.2 mm and that the maximum thickness is deter inded by the diameter of the mandrel .

7. Method according to any of the beforegoing claims, c h a r a c t e ¬ r i z e d in that a mandrel is used, which is free of any kind of tracks or grooves and which has a smooth surface allowing for the manu- facturing of springs having different pitches.

8. Method according to any of the beforegoing claims, c h a r a c t e ¬ r i z e d in that regulation means are used during the placing of the tapes on the mandrel, which means make it possible to determine the pitch of the spring.

9. Spring made of a plastic material, wherein fibers are used as rein¬ forcing means, c-h a r a c t e r i z e d in that the spring comprises a number of tapes, having the fiber material arranged in the longi- tudinal direction and being placed with the longitudinal direction orientated substantially along the intended final configuration of the spring and which is glued together in order to make a laminate, that the individual tapes are cured before the winding of the spring and that the glue between the tapes is cured after the winding of the spring.

10. Spring according to claim 9, c h a r a c t e r i z e d in that the fiber reinforced plastic material comprises an epoxy resin or polycar¬ bonates, that the applied glue is based on an epoxy resin or polycar- bonates and the fiber reinforced plastic contains up to 80% fibers.