Patent Description:
Springs are typically used to store, and subsequently release, energy, to absorb shock, or to maintain a force between contacting surfaces. They deform under an applied load and return to their natural length when unloaded. There is a wide range of springs, including compression coil springs designed to resist being compressed, tension/extension coil springs designed to resist stretching, torsion coil springs designed to resist twisting actions, leaf springs commonly used for the suspension in wheeled vehicles, conical spiral springs, Belleville springs etc..

When used in the automotive industry, springs are commonly made of steel and designed to meet target mechanical stress and fatigue characteristics. Efforts are directed to reducing the weight of the springs while increasing resistance to fatigue or maximum loads, by selecting the material of the springs, the geometry of the wire used and/or of the spring, the manufacturing method etc..

Composite materials have recently increasingly been used in the automotive industry, for example in relation to leaf springs and drive shafts.

European Patent Application Publication number <CIT>, discloses a carbon fiber reinforced resin coil spring for compression or tension, in which the carbon fibers are oriented at an angle of ±<NUM>~±<NUM>° relative to the axis of the cord of the spring. Further, the amount ratio A/B of the fibers oriented in the compression direction and the fibers oriented in the tension direction lies within the range of: <NUM>. <NUM><A/B<<NUM>.

<CIT>, discloses a torsion spring that may be configured as a torsion bar or a helical spring made of a spring wire made of fiber-composite material. The torsion spring may have a plurality of layers of fiber reinforcement that have been saturated with a matrix material, wherein the layers may have fibers that are tension-loaded and fibers that are compression-loaded. The at least one compression-loaded group may have a lower group stiffness than the tension-loaded group with the highest group stiffness. Methods for designing or making torsion springs made of fiber-composite material are also disclosed.

Publication number <CIT> discloses a method for manufacturing fiber composite-torsion spring of vehicle, involving twisting blank before a curing process.

There is still a need in the art for a coil spring and a method of fabrication thereof.

More specifically, in accordance with the present invention, there is provided a composite spring according to claim <NUM>.

There is further provided a method according to claim <NUM> for fabricating a composite spring.

Other objects, advantages and features of the present disclosure will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.

The present invention is illustrated in further details by the following non-limiting examples.

A coil spring comprises a wire of a longitudinal axis (Y), which is shaped, by winding around an axis (X), into a coil of longitudinal axis (X).

<FIG> shows a wire <NUM> of longitudinal axis (Y) made to curve around the axis (X) in the counterclockwise direction, into a compression coil spring <NUM> having a right winding direction.

<FIG> shows a wire <NUM> made to curve around the axis (X) in the clockwise direction, into a compression coil spring <NUM> with a left winding direction.

As illustrated in <FIG>, the springs <NUM> and <NUM>, which comprise the same number of turns of the wire <NUM> around the axis (X), the same spring mean diameter (ΦD), the same wire cross section diameter (Φd) and the same spring free height (h), are chiral objects : they are distinguishable from their mirror image relative to a plane (O) and they cannot be superposed onto it, due to their different winding directions.

<FIG> show the springs <NUM> (<FIG>) and <FIG> (<FIG>) under application of a same compressive load P along their respective axis (X). Parallelepiped elements <NUM>, <NUM> and <NUM>, <NUM> are pointed out at the surface of the springs to follow the deformation and associated stresses resulting from the compressive load P submitted to the springs. Elements <NUM> on the surface of the wire <NUM> of the spring <NUM> and element <NUM> on the surface of the wire <NUM> of the spring <NUM> are positioned close to the axis (X), i.e. on the inner side of the springs <NUM> and <NUM> respectively, while element <NUM> on the surface of the wire <NUM> of the spring <NUM> and element <NUM> on the surface of the wire <NUM> of the spring <NUM> are positioned away from the axis (X), i.e. on the outer side of the springs <NUM> and <NUM> respectively, as best seen in <FIG>.

<FIG> show the deformation of the elements <NUM>, <NUM> and <NUM>, <NUM> of <FIG> due to the application of the load P on the compression springs <NUM> and <NUM>. The applied load P generates a torsional moment M, as well as a direct shear force V, on the wire section.

As illustrated in <FIG>, the torsional moment, defined as M= P×ΦD/<NUM>, causes a deformation on the circumference of the wire section along a tangential direction relative to the surface of the wire. The direct shear force V causes a deformation along the direction of the applied load P, i.e. along the axis (X) of the spring. Thus, on the inner side of the spring, the two deformations have a same direction, while they have opposite direction on the on the outer side of the spring.

Thus, these two components cause shear stresses that deform the elements <NUM>, <NUM> and <NUM>, <NUM> in a way that their sides are no longer perpendicular, as shown by the dotted (direct shear force) and dashed (torsional moment) lines in <FIG>.

It can be seen that their effects are additive at the inner side of the springs (as exemplified by <NUM> and <NUM> for example) while they are subtracted at the outer side of the springs (as exemplified by <NUM> and <NUM> for example). The overall net effect is represented in <FIG> by bold solid lines.

As clearly seen in <FIG> by comparing inner elements <NUM> and <NUM> and outer elements <NUM> and <NUM> for example, there is a relationship between the winding direction of the spring and the orientation of the shear stresses acting on the material elements in regards with the longitudinal axis (Y) of the wire <NUM>. These shear stresses can also relate to tensile and compressive stress components along the diagonals of the elements, respectively noted T and C in <FIG>. Again there is a direct relationship between the winding direction of the spring and the direction in regard with the longitudinal axis (Y) of the tensile and compressive stress components.

The index of the spring, defined by the ratio of the spring mean diameter (ΦD) over the wire diameter (Φd) as c= ΦD/Φd, also impacts the length of the infinitesimal material elements depending on their position on the wire, which also determines the shear stresses caused on the wire section under loading. The lower the ratio c, i.e. the larger the diameter Φd of the wire, the larger the distances between the inner side (ΦD/<NUM> - Φd/<NUM>) and the outer side ((ΦD/<NUM> + Φd/<NUM>) respectively, and the (X) axis of the spring. Under a torsional moment, the wire thus deforms according to a relative rotation of the sections thereof, in a different way: since the inner circumference of the wire is shorter the effect (shear stresses) increases towards the inner side of the springs (as exemplified by <NUM> and <NUM> for example).

Thus, torsional moment, direct shear and curvature combine and yield a shear stress distribution over the periphery of the wire cross section. The shear stress may be larger by up to <NUM>% on the inner side of the spring compared to on the outer side of the spring in the case of a spring with an index of about <NUM>. In case of an index of <NUM> and lower, this effect may be even higher. Maximum shear stress always occurs at the inner side of the springs. Hence, failure of the springs, in the form of cracks in the wire, generally initiates from the inner side of the spring.

Due to this non-uniform stress distribution over an axisymmetric cross section of the spring wire as discussed hereinabove, generally the energy storing and releasing capacity of a spring with spring wires having an axisymmetric section are limited by the more solicited locations on the wires, while the remaining portions of the spring are underused.

<FIG> illustrates angular positioning of fibers within a composite wire <NUM>, relative to the longitudinal axis (Y) of the wire <NUM>. Fibers <NUM> are oriented at <NUM>° relative to the axis (Y), fibers <NUM> are oriented at <NUM>° relative to the axis (Y), fibers <NUM> are oriented at +θ° relative to the axis (Y) and <NUM> are oriented at -θ° relative to the axis (Y).

<FIG> shows a closed-end compression coil spring with a right winding direction. As shown in cross sections along line A-A of <FIG> in <FIG>, the spring of <FIG> comprises a wire <NUM> comprising a core <NUM> and several layers <NUM> of fibers wound around the core <NUM>. The fibers of the layers <NUM> may be glass fibers, carbon fibers or a combination thereof, and they may be embedded in a resin such as polyester, epoxy or urethane for example. The core <NUM> with the surrounding fibers layers <NUM> may be embedded in a heat-shrink tube <NUM> (see <FIG>) or wrapped in a shrink tape <NUM> (see <FIG>) or a coextruded flexible thermoplastic sheath or covered by a sprayed plastic film for example.

According to an embodiment of an aspect of the present disclosure, the angular positioning of the fibers within the wire of the spring, i.e. the orientation angle of the fibers relative to the direction (Y) of the wire coiled into the spring, and the sequence of the fibers starting from the fiber closest to the center of the section of the wire to the fiber the most remote therefrom, are selected depending on the direction of winding.

<FIG> show wires comprising a core <NUM> and a symmetrical stacking sequence of composite lamina comprising an outermost layer <NUM>, located on the outer surface of the wire, intermediate layers <NUM> and innermost layer <NUM>, located between the intermediate layers and the core <NUM>, made of fibers impregnated in resin; for a given layer, an angular positioning of the fibers relative to the axis (Y) is selected and the layer is wound accordingly relative to the core <NUM> of the wire <NUM>.

In a right winding spring (<FIG>), the stacking sequence may be balanced, i.e. with a same number of layers or a same quantity of fibers oriented at +θ° than at - θ° relative to the axis (Y) of the spring wire; or the stacking sequence may be unbalanced, with a higher number of layers or higher quantity of fibers oriented at -θ° than oriented at +θ° relative to the axis (Y) of the spring wire (see <FIG>).

In a left winding spring (<FIG>), the stacking sequence may be balanced, i.e. with a same number of layers or a same quantity of fibers oriented at +θ° than at -θ° relative to the axis (Y) of the spring wire; or unbalanced, i.e. with a higher number of layers or higher quantity of fibers oriented at +θ° than at -θ° relative to the axis (Y) of the spring wire (see <FIG>).

The angular positioning of the fibers, i.e. the orientation angle of the fibers relative to the axis (Y) of the spring wire, is selected depending on the direction of the spring winding direction about the spring axis (X), according to target characteristics of the spring including: high natural frequency of the spring, resistance to buckling of the spring, and resistance to tensile and compressive stress components induced by application of a compressive load on the spring. Thus, the angular layout of the fibers in a compression coil spring with a right winding or with a left winding direction may be as follows:.

Moreover, the number of fiber layers, i.e. layers of fibers within a thermoplastics or thermoset resin matrix, or the quantity of fibers within the wire <NUM> may be selectively varied along a length of the wire; for example the number of fiber layers or the quantity of fibers within the wire <NUM> may be lower at the ends <NUM>, <NUM> of the spring than along the length thereof in between these ends <NUM>, <NUM>. <FIG> for example shows a compression coil spring with a right winding direction, with tapered terminal ends <NUM>, <NUM>. Alternatively, the coil spring may have thickened ends as shown for example in <FIG>.

Moreover, the number of fiber layers or the quantity of fibers within the wire <NUM> may be selectively adjusted to tailor a cross section of the wire, which may be circular, polygonal such as square or rectangular or hexagonal for example, convex or ovoid or potato-like for example, as will be discussed hereinbelow in relation to <FIG>. The geometry of the cross section of the spring wire may thus be selected based upon the intended use of the spring. Thus, for example, <FIG> shows a wire having a polygonal cross section at an end thereof for stability of the spring, while the remaining cross section thereof is circular,.

Fiber fabrics may be inserted between the layers, to yield non-axisymmetric fiber layer placement around the core, as will be discussed hereinbelow in relation to <FIG>.

The coil spring may have a constant pitch as illustrated for example in <FIG>, or a variable pitch as illustrated for example in <FIG>. The coil spring may have a cylindrical shape as illustrated for example in <FIG> and <FIG>, a conical shape as illustrated for example in <FIG>, a barrel shape as illustrated for example in <FIG>, or an hourglass shape as illustrated for example in <FIG>.

The coil spring may have open ends as illustrated for example in <FIG> or closed ends as can be seen for example in <FIG>, ground ends (<FIG>) or unground ends (<FIG>).

In a method according to an embodiment of an aspect of the present disclosure, a composite, non-axisymmetric or axisymmetric, uncured, i.e. unsolidified hence flexible, preform comprising a core and at least two fiber layers is fabricated.

As schematized in <FIG>, the uncured preform <NUM> may be straight (<FIG>) or non-straight (<FIG>), yet flexible. Then, during a coiling step, the uncured preform <NUM> is selectively oriented into a spring according to target performances of the spring. All these steps may be performed at ambient temperature.

Optionally, an external envelope of a shrinkable plastic material, such as a tubing <NUM> or a tape <NUM> as discussed hereinabove in relation to <FIG> for example, may be positioned around the uncured preform <NUM> before shaping. When manufacturing the preform, such an external envelope may insure a uniform distribution of the resin while it is still in a liquid form, as this external envelop, being impervious, contains the resin and insures a correct wetting of the previously orientated fibers before the solidification of the matrix. It can also insure a correct compression of the wetted fibers as well as insuring a correct final fiber-to-resin ratio. The external envelope may then be removable from the spring once formed, after curing. However, it may be decided to keep it in the final spring as will be discussed hereinbelow.

A non-axisymmetric geometry of the wire may be achieved by at least one of: <NUM>) selecting a core of a non-axisymmetric cross section, <NUM>) selecting a non-axisymmetric placement of the fiber layers about the core, <NUM>) by selectively selecting the ratio between the fiber and the resin, and <NUM>) by lateral displacement of the core <NUM> of the wound preform <NUM> prior to curing (see <FIG>).

Thus, the core may be selected as an ovoid, a prismatic or a potato-like core for example.

Non-axisymmetric fiber layers placement may be achieved by inserting fiber fabrics between the layers of the preform, as shown in <FIG> for example. These fabrics <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be of different compositions: mat, cloth or unidirectional; they may be combined into a specific stacking (<NUM>, <NUM>) of any of these types of fabrics. These inserted fabrics may be layered on only a portion of the periphery of the preform or extent over the whole circumference of the preform. A combination of different fabrics (<NUM>, <NUM>) may be used to cover the whole periphery of the cross section of the preform to achieve different properties in different portions of the wire of the resulting spring.

The resulting preforms <NUM> may thus have a circular, a polygonal such as square or rectangular or hexagonal (<FIG>), convex or ovoid (<FIG>) or potato-like (<FIG>) cross sections, resulting into corresponding cross sections of the wire <NUM>.

By selectively positioning different numbers of fiber layers along the length of the preform and/or by varying the ratio between resin and fiber along the length of the preform, and/or by selecting a core having a varying cross section along its length as shown in <FIG> for example, the preform may be fabricated to provide rigidified or more flexible lengths of the spring wire along the longitudinal axis (Y) of the spring wire, without modifying the composition of the composite material used.

As shown in <FIG>, a further way of achieving non-axisymmetric placement of the fibers relative to the axis (Y) of the wire is to insert a cable within the hollow core <NUM> and then applying a tension on each end of the cable thereby deflecting the core <NUM> along its length towards the inner side of the wound preform while still uncured, before curing.

In <FIG>, the preform <NUM> is shown with a first constant diameter Φd1 over a length L<NUM> thereof, and an increasingly larger diameter along a length L<NUM> on each end thereof (only on end shown), until a diameter of the cross section at the ends thereof Φd2 > Φd1. The resulting spring shown in <FIG> thus comprises thickened ends <NUM>, i.e. of a diameter Φd2 larger than the diameter Φd1 along the remaining part of its length. Such a thickening of the ends of the spring wire <NUM> allows precompression of the spring while minimizing contact between terminal coils of the spring, which might otherwise induce mechanical damages thereto. Providing a transition length L<NUM> of increasing diameter from Φd1 to Φd2 prevents concentration of stresses. The resulting spring is then characterized by a linearly varying spring constant, without dead coils and resistant to mechanical stresses submitted locally.

The preform may be fabricated with a linearly varying diameter of its cross section, with tapered extremities for example, so that the resulting spring has tapered ends (see for example <FIG>), thereby suppressing a grinding step as typically done on the art.

Thus, independently of the winding direction during the coiling step, the cross section of the wire of the resulting spring may be variable according to the position along the length of the spring.

<FIG> shows an hourglass-shape compression spring comprising a last coil of a diameter ΦD<NUM> greater than the mean spring diameter ΦD<NUM>, thereby providing an increased contact surface, which eliminates dead coils or the use of a compensation plate for stability, as typically done in the art.

<FIG> shows another embodiment comprising forming the helicoidal section and then overmolding a plate geometry <NUM> of a selected diameter and thickness at the end coils, thereby ensuring that all coils of the spring are operational. Such overmolding of the end coils may be done directly on the wound preform using a bulk molding compound (BMC) such as a polyester /fiber compound by pressure die-casting for example (see <FIG>).

There is thus provided a composite coil spring fabricated using a straight or non-straight uncured preform, by forming the uncured preform into a coil spring shape.

Depending on the desired spring mean diameter and the external diameter of the preform, the helical forming during the coiling step involves a curvature, i.e. a length difference between the inner and outer sides of the spring, which determines the effective angle at which the fibers are positioned in the resulting spring once the shaping is complete. The angle placement of the fibers used for the realization of the straight preform is thus selected according to the mean diameter of the spring, as well as the position from the center of the preform and the desired angle on the inner side of the spring, where stresses are maximal. Furthermore, for a variable mean diameter spring, i.e. conical, hourglass or barrel geometry for example, the angle placement of the fibers of the preform may be modified along its length considering the final geometry, aiming for an optimal fibers orientation on the inner side of the spring for the whole spring.

The core <NUM> may be a flexible plastic tube in which conductive elements such as metallic filaments, including coated fibers for example, or twisted wires are integrated, i.e. encapsulated, thereby allowing the use of an electrical source to generate the heat required to cure the thermoset resin matrix such as by resistance and induction heating. Alternatively, the outermost layer of the preform may comprise electrical conductive elements such as fibers or filaments to allow curing the resin matrix and thus the solidification of the preform. Solidification may also be achieved through other methods such as through microwave or radiation heating. The flexible tube can be removed after curing, yielding an inner cavity in the cross section of the resulting spring and thus a reduced weight of the spring.

Alternatively, the inner cavity of the core <NUM> may be used to add selected characteristics to the spring by housing a functional element. For instance, it may be filled with a viscoelastic or a thixotropic material allowing for a damping effect. Optic fibers, sensor gauges or thin wire cables may be inserted as sensors for mechanical properties based on inductive effect, deflection, deformations for example.

Alternatively, the core <NUM> may be made of metallic wires in a twisted form, in the way of a steel cable for example. By selecting the twisting direction of the metallic wires, the core <NUM> may be formed with an undulated surface that may improve the adherence with the wounded reinforcing fibers of the preform and yield an increased better overall strength of the coil spring.

As mentioned hereinabove, the wire may be provided with an external envelope such as a heat-shrink tube <NUM> or wrapped in a shrink tape <NUM> or a coextruded flexible thermoplastic sheath or covered by a sprayed plastic film.

Such an external envelope may protect the spring from damages caused by the impacts and wear from projections of rocks, mud, sand or any other debris, to which suspension springs, for instance, are typically exposed, in on-road vehicles, such as cars and trucks, and in off-road vehicles, such as motorcycles, ATVs and snowmobiles for example. Furthermore, the external envelope may protect the spring from stresses due to weather conditions such as rain, or prolonged exposition to humidity or sunlight, or to any chemicals prone to be in contact with the spring in use, such as for example de-icing salts, sea water, etc. The external envelope may also be selected as a colored or luminescent sheath, or as a layer incorporating patterns, drawings, logos or the like, as a substitute for traditional finishing processes such as painting, labelling or decals application. The external envelope may also be selected as a layer providing surface finish (roughness), embossment or texturing for example.

Still alternatively, the external envelope may be selected as a transparent sheath, thereby letting the rough resin color show. This color may be modified by adding colored or luminescent pigment to the resin.

There is thus provided a composite helical spring designed for compressive force, with either a constant pitch or a variable pitch, in a range of shapes such as conical, barrel, hourglass forms etc., a range of wire cross sections, and a range of ends type, i.e. open ends, closed ends, tapered ends, thickened ends, ground, unground ends.

The present composite coil springs may be used in vehicle suspensions systems such as automotive suspensions, snowmobiles suspensions, off-road vehicles suspensions and recreational vehicles suspensions for example.

There is provided a method for fabrication of a composite coil spring, generally comprising fabricating a preform, forming the preform into a coil spring shape and then solidifying it by using a heating source.

As an example, two identical preforms having a same core, same inner layers at same angular positioning of fibers relative to the axis (Y), same amount of fibers and fiber-to-resin ratio were fabricated. Both preforms were coiled with a same number of turns around an axis (X) to yield a same spring mean diameter (ΦD), a same cross section diameter (d) and a same free height (h). The difference was the angular positions of the outermost layer, which were opposite. Both springs were tested under compression until they broke. Table <NUM> below shows the measured results.

Table <NUM> records the deflection of the springs when the springs broke, the corresponding load at break and, in the third column, an estimation of the energy stored. It appears that for a given winding direction of the coil spring, there is a preferential orientation of the outermost layer teter as more energy can be stored in Spring B compared to Spring A.

There is thus provided a method comprising selectively fabricating a preform and combining the winding direction given to a coil spring, the direction of stress when the spring is compressed, and the anisotropic properties of the composite material of the spring wire, according to a target compression spring and optimized stored and restored in energy by the compression spring per unit mass.

Claim 1:
A composite spring (<NUM>; <NUM>) made of a wire (<NUM>) having a longitudinal axis (Y) and curved around a spring axis (X) in a winding direction, wherein said wire (<NUM>) comprises:
a core (<NUM>); and
fiber layers wound around said core (<NUM>);
wherein said fiber layers comprise innermost layers (<NUM>) closest to said core (<NUM>), an outermost layer (<NUM>) and intermediate layers (<NUM>) between said innermost layers (<NUM>) and said outermost layer (<NUM>), an orientation, relative to the spring axis, of each one of said fiber layers being selected, along a length of said core, depending on said winding direction of the wire about the spring axis, to adjust at least one of: high natural frequency of the spring, resistance to buckling and resistance to tensile and compressive stress components induced by a compressive load on the spring,
characterized in that the orientation of the outermost layer (<NUM>) is a negative angle (-Θ), as illustrated in Figure <NUM> of the drawings, in a range between -<NUM>° and -<NUM>° relative to the longitudinal axis (Y) for the given winding direction of the spring (<NUM>) in the case of a right winding direction, or
the orientation of the outermost layer (<NUM>) is a positive angle (+Θ), as illustrated in Figure <NUM> of the drawings, in a range between +<NUM>° and +<NUM>° relative to the longitudinal axis (Y) for the given winding direction of the spring (<NUM>) in the case of a left winding direction.