Technical Field

-The invention relates to a leaf spring which consists, at least in essential resilient regions, of a fiber-reinforced duromeric plastic in which high-strength reinforcement fi- bers, possibly consisting of glass fibers, carbon fibers or other suitable fibers, extend at least approximately in the direction of the maximum elongation or compression of the fiber material occurring under spring load, and which is suitable in particular for replacing the steel leaf Springs customary in motor vehicle construction. The invention fur- ther relates to an advantageous process for the production of the plastic leaf spring of the invention.

Background Art

Plastic Springs have been known for many years but have not been accepted by the motor vehicle industry. For example, U.S. patent No. 3,900,357 describes a method for the produc¬ tion of glass fiber-resin Springs ade from a foil of a non- woven glass fiber material and a curable matrix of epoxy resin. Strips of the foil are stacked in several layers in a mold which are then heated at elevated temperatures un¬ der pressure to eure the matrix resin and form the finished leaf spring. Due to the fact that the foil layers are only approximately 0.01 inches thick, 67 layers of foil are nee- ded to obtain a spring with a thickness of three quarters of an inch and the hardening ti e required for such a spring is on the order of 45 minutes. The insertion of this many foil layers requires too uch labor to be economical and

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the prolonged curing times are also too expensive.

Disclosure of Invention

The first object of the invention is a new plastic leaf spring which, as it is known per se, consists at least in essential resilient regions of a fiber-reinforced duromeric plastic in which high-strength reinforcement fibers possib- ly consisting of glass.fibers, carbon fibers or other suitab¬ le fibers, extend at least approximately in the direction of the maximum elongation or co pression of the fiber material occuring under spring load, but which, according to the in¬ vention, may contain a higher volume of reinforcement fibers than it was possible with known plastic leaf Springs.

The second object of the invention is to provide an improved process for the preparation of plastic leaf Springs with less raanual labor and shorter manufacturing ti e.

The novel plastic leaf spring, object of the invention, is

_^ that characterized £nγttϊese resilient regions consisting of fiber- reinforced cured duromeric plastic consists at least in part of cured fiber-reinforced plastic laminates, having advan- tageously a thickness of at least 0.5 mm, but preferably of at least 0.8 mm, which via bonding agent layers are bonded to each other to form one or more composite parts or re¬ gions serving to compose the leaf spring, or to the leaf spring itself. Leaf Springs with a thickness variable in longitudinal direction preferably are formed by a relisient composite part or region or contain such a one, in which laminates of different lengths are joined togehter.

According to an advantageous embodiment of the invention

the leaf spring comprises at least one resilient composite part or region in which the greater part of the laminates has reinforcement fibers extending exclusively in lengthwise di¬ rection of the leaf spring and being optionally pre-stressed.

According to a further advantageous embodiment of the ^" in¬ vention the leaf spring comprises at least one resilient composite part or region in which a smaller part of the laminates has reinforcement fibers, the axes of which in- clude with the lengthwise direction of the leaf spring an angle of between 5 and 90 , but more particularly of about 90°.

According to another advantageous embodiment of the inven¬ tion the leaf spring has two resilient regions which on two opposite sides of a core of non-resilient material are joined to the two sides of the core via bonding agent layers. Thereby the core preferably consists at least in part of an elastomeric plastic and/or fibrous material is embedded in the material of the core.

According to further advantageous embodiments of the inven- tion the bonding agent layers of the leaf spring contain a duromeric bonding agent and/or in the bonding agent layers fibrous material is embedded. Thereby the duromeric bonding agent is advantageously a cured duromeric or thermosetting adhesive on the same base as the matrix resin of the cured laminates bonded together to form composite parts or to form the leaf spring.

The novel process for the preparation of plastic leaf spring object or the invention is characterized in that for the pro- duction of fiber-reinforced duromeric plastic laminate Strips

in a continuous process first a rope is formed by impregna- ting a fibrous support material with a curable resin mixtu- re, which rope is transformed by curing into a ribbon shaped laminate, which is optionally ground and cut to lengths of Strips, that several of these laminate Strips after coating with a duromeric bonding agent or with insertion of adhesive sheets,optionally containing a fibrous support material and a duromeric bonding agent, are bonded to each other in a mold and to any additional components of the leaf spring that may be provided, with activation and curing of the bon¬ ding agent to form a resilient composite part serving for the construction of the leaf spring or to form the leaf spring or a leaf spring profile body. Thereby the activating and cu¬ ring of the bonding agent is effected at elevated temperatu- res, preferably temperatures above 100 C.

According to an advantageous embodiment of the process ac¬ cording to the invention the heating up for the purpose of activating and curing the bonding agent is effected in an electromagnetic high-frequency field.

According to a further advantageous embodiment of the pro¬ cess of the invention the resilient composite part or the leaf spring or leaf spring profile body is removed from the mold already after partial curing of the bonding agent and only then is fully cured at elevated temperature.

According to another advantageous embodiment of the invention the process for the preparation of the leaf spring is charac- terized in that first, by bonding together several fiber-re- inforced duromeric plastic laminates, a basic spring compo¬ site part is formed, in which at least for a part of the la-

minates of the composite part their lengths decrease with formation of a stepped surface that these steps are removed by mechanical machining to form an arcuate smooth surface of the basic spring composite part, preferably are ground off and that onto this arcuately smooth surface one or, in stacked arrangement, several fiber-reinforced cured durome¬ ric plastic laminates are glued which form an additional re¬ silient composite region.

According to a last advantageous embodiment of the process of the invention the leaf spring profile body produced is divided into Single leaf Springs by saw cuts.

Brief Description of Drawings

The figures of the drawing show schematically in side ele- vation and cross sectional views respectively*

Fig. 1 a laminate structure stacked for the production of a resilient composite part;

Fig. 2 a press with HF capacitor field heating with the in- serted laminate structure per Fig. 1 as material be¬ ing pressed; Fig. 3 a resilient composite part produced from the lami¬ nate structure as blank for a leaf spring to be produced$

Fig. 4 a finished leaf spring profile body made with the use of the resilient composite part per Fig. 3; Fig. 5 a spring eye to be mounted at the end of the manu- factured leaf spring; and Fig. 6 a leaf spring with rubber-elastic core.

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In Fig. 1 to 4 and 6, the thicknesses of the laminate struc- tures,of the resilient composite parts, of the leaf Springs and of the leaf spring profile bodies are shown exaggerated for better illustration.

Fig. 1 shows a plurality of cured laminate Strips 1 cut in appropriate lengths from continuously produced endless glass fiber epoxy resin laminates with glass fiber reinforcements of the roving type or of the fabric type and - after having been coated on both sides with a bonding agent - are stacked on a flat support 3 provided with a Separation layer 2 in order to form a laminate structure 5 manageable as a whole and stepped at surface 4.

For the production of a resilient composite part of this laminate structure 5 there serves a press 6 provided with a HF heating equipment (see Fig. 2), with a press mold whose mold halves 7 and 8 have electrically conducting coverings 9 which are connected as HF capacitor field electrodes to a HF generator 10.

Fig. 2 shows the press 6 before its closing showing thereby the laminate structure 5 placed on the lower mold half 7 bet¬ ween two pressure compensating mats 12 of soft-elastic mate¬ rial each covered with Separation foils 2.

Fig. 3 shows a resilient composite part 15 produced from the laminate structure 5 by hot pressing in press 6 and by grinding the resulting part on its stepped surface to form a smooth surface 14 with arcuate contour.

Fig. 4 shows a leaf spring profile body 16, which is pro¬ duced from the resilient composite part 15 by joining it to a second resilient composite region 17.

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Fig. 5 shows an advantageous embodiment for the realization of the introduction of the forces at the ends 18 of a leaf spring of the invention by spring eyes. The spring end 18 is fastened in a slot 19 of the extension 20 of a spring eye 21 by screws 22 which pass through corresponding bores in the spring end 18, as well as by gluing.

In Fig. 6 is shown a leaf spring 23 with a core 24 consi¬ sting of non-resilient material, which is bonded at its top and bottom sides to a laminate packet, which packets form resilient composite regions 25.

Modes for Carrying Out the Invention

For the construction of the leaf Springs described below, glass fiber-reinforced epoxy resin laminates are used, which are advantageously produced in a manner known per se in a continuous process by impregnating glass fiber rovings or a ribbon of a glass fiber fabric with a resin-hardener mix- ture, by calibrating the wet rope thus produced, and by sub- sequent curing the rope at elevated temperature to form an endless ribbon shaped laminate. Due to the fact that in this process the glass fiber rovings or the glass fiber fabric is under tensile stress after it has been drawn off the cali¬ brating device, one obtains in the fully cured laminate a mechanical prε-stress of the reinforcement fibers extending lengthwise of the laminate. The resin-hardener mixture be- ing used consists e.g. of an epoxy resin on a base of Bis¬ phenol A and a cycloaliphatic diaminic hardener. The resin content of the laminates is about 25 wt % (solid substance), corresponding to a volume percentage of about 41%. To im- prove the bondability, the produced laminate ribbon is ground on both sides.

For the leaf Springs according to the invention described below, glass fiber roving type laminates of a thickness of 1.0 mm and glass fiber fabric type laminates of a thickness of 0.4 mm are employed, the latter serving to obtain a suf- ficient transverse strength of the leaf Springs to be pro¬ duced.

With reference to Figures 1 to 4, an advantageous form of realization for the production of the leaf Springs according to the invention will now be described, which is composed of cured laminate Strips of different lengths.

The laminates of the roving type and laminates of the fabric type used, which in the production example to be described have a width of 195 mm each, are made available as endless laminates wound on rolls, and the,jιeeded laminate Strips are drawn off the rolls in the order in which they are sub- sequently employed in the leaf spring manufacture and are cut off in the needed different lengths.

These laminate Strips then pass successibely through a glue applicator, in which they are provided on both sides with a

2 bpnding agent application of 80 g/m (solid substance); the binder is a resin-hardener mixture of the sa e composition as used in the preparation of the laminates.

For the preparation of a laminate structure (Fig. 1) on the flat support 3 provided with a Separation layer 2 the said laminate Strips are stacked, whereby for the example to be described four roving type Strips, one fabric type strip and four additional roving type Strips follow each other -upwardly - in lengths of 1550 mm and thereafter 19 lami¬ nate strips in lengths decreasing from 1200 to 200 mm, in

the latter a fabric type strip being followed by six or seven roving type Strips. The laminate Strips thus stacked form a - laminate structure 5 manageable as a whole and being stepped at a surface 4.

As preparation for the pressing Operation, the concave press surface 11 of the lower mold half 7 is covered with a pres- sure-compensating mat 12 of soft-elastic material and a Se¬ paration foil 2 contiguous thereto. On it the laminate struc¬ ture 5 is placed with its stepped surface down. Because of the high flexibility of the cured laminate Strips 1, the la¬ minate structure 5 becomes deformed as a whole and hugs the Separation foil 2. The laminate structure 5 is then covered on its smooth surface with an additional Separation foil 2, followed by an additional pressure-compensating mat 13. Fig. 2 shows this phase of the manufacturing process.

For the hot-pressing Operation the mold is now closed by lo- wering the upper mold half 8, a pressing pressure of about

2 40 N/cm being applied on the material to be pressed, and the

HF generator 10 operating with a frequency of 27.12 MHz, be- ing switched on. In the electromagnetic HF field both the ma¬ terial of the cured laminate Strips 1 and the bonding agent layers absorb dissipated HF energy, whereby the laminate struc ture 5 is heated quickly and uniformly. After the pressed ma¬ terial has reached a teπrperature of 140 , the HF energy supply is stopped or throttled to the extent that this temperature is essentially maintained during the remaining pressing ti e _• An important advantage here is that, because of the relati- vely small percentage of uncured, that is. still reactive resins in the total resin mass of the pressed material - which percentage is limited to the bonding agent - local

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temperature increases which are difficult to control and would be caused by the actually exothermic curing reaction need not. be feared.

During the pressing Operation, the bonding agent in the glue joints of the laminate structure 5 becomes relatively fluid at the pressing temperature of 140 C, so that it flows well under the action of the pressing pressure applied. The excess bon¬ ding agent is squeezed laterally out of the glue joints and with it any air inclusions existing inside the glue joints are removed. The bonding agent, which fills the glue joints comp- letely and uniformly, subsequently eures quickly, whereby the laminate structure 5 is transformed into a resilient composite part. At the end of the pressing period, which in the given example is 5 minutes, the mold is opened and the composite part removed while still hot,allowed to cool, and then ground on its stepped surface caused by the differently long laminate strips 1 for the formation of a smooth surface 14 with arcuate contour. The composite part 15 then has approximately a form as shown in Fig. 3. To process the resilient composite part 15 thus produced to a leaf spring profile body in a second process step part 15 is bonded on its convex surface 14, which had just been ground smooth to several stacked laminate strips 1. To this end, four roving type laminate strips, one fabric laminate strip and four additional roving laminate strips of a width of 195 mm and a length of 1560 mm which - as in the production of the first laminate structure 5 - have been provided on both sides

2 with a bonding agent application of 80 g/m (solid substance), are layered one over the other to produce a second laminate structure on the flat support 3 provided with the Separation layer 2.

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This second laminate structure is then - similarly as des¬ cribed in connection with Fig. 2 - inserted on the concave pressing surface, covered with a pressure-compensating mat and a Separation foil, of the lower mold half of a HF energy- heated press. The composite part 15, which had been coated on its convex surface 14 with bonding agent also, is now placed with this surface 14 down onto the second laminate structure already introduced in the press, the composite part surface 14 applying snugly on the laminate structure. After a Separation foil and another pressure-compensating mat have been placed on, the mold is closed, and by HF heating of the pressed material to 140 C at a pressing pressure of 40 N/cm during a pressing time of 5 minutes the individual pressed material components are bonded togehter to a leaf spring pro- file body.

The mold is theh opened and the finished leaf spring profile body 16 shown in Fig. 4, which now consists of the resilient composite part 15 and a second resilient composite region 17, is taken out while still hot. After cooling, the leaf spring profile body is divided by saw cuts into three identical leaf Springs ready for use, each 60 mm wide.

In the center the leaf spring has a thickness of 34 mm and at its ends thicknesses of 17 mm. The height h of the spring cen¬ ter over the spring ends is 130 mm. It has a spring constant of 60 N/mm and is rated for a maximum deflection in the cen¬ ter of 200 mm. In endurance load alternation tests the des¬ cribed leaf spring withstood more than double the number of load cycles at which a similar steel leaf spring became de- fective in comparison tests.

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The above described leaf spring produced by a two-step manu- facturing process has the advantage that the edge regions at . its top and bottom sides, which are under increased stress when the spring is loaded, are formed by laminate strips which extend with continuous fibers of the glass fiber reinforcement over the entire spring length.

Should it ^" be desirable in the above de cribed process to shor- ten the pressing times in the HF-heated press, then - as a rao- dification of the above described process - the resilient com- posite part or the leaf spring profile body can be taken out of the mold already after partial curing of the bonding agent and only then completely cured in a furnace at elevated tem- perature.

According to a further variant for the production of leaf Springs - instead of applying the binder by means of a glue applicator - between two adjacent components to be bonded an adhesive sheet activatable at elevated temperature is inserted, which may advantageously be a glass fiber fabric impregnated with an epoxy resin-hardener mixture and driεd (glass fiber prepreg). The bonding is then effected preferably again simi- larly as described with reference to Fig. 2 in a press with HF capacitor field heating of the pressed material. However - because of the inhibition of free flow of bonding agent in the glue Joint - the temperatures required for the bonding will generally be higher when such adhesives are used than with the use of bonding agent layers applied on the laminate strips e.g. by means of a glue applicator. When employing adhesive sheets containing such a glass fiber fabric, the spring body may in many cases be composed exclusively of glass fiber la- minate strips"of the roving type, because the glass fiber fab¬ ric reinforcement in the bonding agent joints usually already JΕEA

produces a sufficient transverse tensile strength of the spring body.

The introduction of force at the spring ends takεs place in -the dεscribed leaf Springs of the invention by spring eyεs as already describεd in connεction with thε dεscription of Fig. 5.

In thε lεaf Springs according to thε invεntion, mainly only thε laminatεs arrangεd in thε uppεr and lowεr εdgε rεgions of the spring contribute to the attainable spring forcε. Be- sides, shearing strεssεs to bε absorbεd by thε bonding agεnt joints arε always grεatεst in thε cεntεr of a spring cross- sεction. These facts are taken into account in a further ad¬ vantageous design of thε lεaf spring 23 according to thε in¬ vεntion (Fig. 6) where in thε centεr of thε lεaf spring a corε 24 of non-rεsiliεnt atεrial is disposεd, which is bon- dεd at its top and bottom sidεs to a laminatε packεt, which packεts form rεsiliεnt compositε rεgions 25. Thε core 24, whic may advantageously consist . of a fiber-reinforced εlastomεric or thεrmoplastic plastic, is usually producεd in a sεparatε procεss, coatεd on both sidεs with bonding agεnd, and thεn, contiguous to its top and bottom sides, joinεd to a lami¬ natε structure consisting of sevεral laminatεs strips of εqual lεngth - analogously as described in connection with Fig. 2 - bondεd in a heated press to form a leaf spring 23 or leaf spring profile body. It should be noted hεrε that thε corε 24, tεrminating in wεdgε form at its two εnds, should not εxtend up to the ends of the leaf spring 23. Because of the rubber-εlastic dεformability of the core material, upon deformation of thε leaf spring 23 under load no appreciable shearing stressεs can build up in core 24.

Industrial Applicability

Thε plastic lεaf Springs according to thε invεntion can be used in motor vehicle construction in a similar manner as thε known steel leaf Springs. Advantages of thε said pla¬ stic leaf Springs over the usual steεl lεaf Springs par¬ ticularly consist in thεir low wεight and in thε fact that they are corrosionproof.

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