Patent Description:
Torsional load is usually the primary load component applied to drive shafts. Certain applications of shafts, such as, for example, in helicopters, cause them to experience torque primarily in one direction. Torque generates in-plane shear stresses in the shaft walls, which can be decoupled into tensile stresses in one direction and compressive stresses in another direction. Conventional composite drive shafts have symmetrical layups that are consequently often heavily loaded with respect to one stress path (e.g., compression), and under-loaded with respect to another (e.g., tensile) during ordinary operation. Under such conditions, composite drive shafts can be prone to failures primarily along compressive or tensile load paths, depending on the material properties of the composite shaft itself. <CIT> relates to a drive shaft extending along a central axis and being configured to operate under dominant unidirectional torsional load, the drive shaft comprising: an asymmetrically-structured composite body configured to have a greater torque-carrying capability in a first torsional direction than in a second torsional direction, the second torsional direction being opposite the first torsional direction; wherein the asymmetrically-structured composite body comprises: a first plurality of fibers oriented in a first direction at a first angle with respect to the central axis; and a second plurality of fibers oriented in a second direction at a second angle with respect to the central axis, wherein the first and second directions are different; and wherein the first plurality of fibers and the second plurality of fibers provide a greater torque-carrying capability in the first torsional direction than in the second torsional direction; wherein the first plurality of fibers comprises more fibers than the second plurality of fibers; wherein fibers of the first plurality of fibers are embedded within a polymer matrix and arranged in a first plurality of layers and wherein fibers of the second plurality of fibers are embedded in a polymer matrix and arranged in a second plurality of layers; wherein the first plurality of fibers is substantially oriented in a compressive stress direction when operating under the dominant unidirectional torsional load and wherein the second plurality of fibers is substantially oriented in a tensile stress direction when operating under the dominant unidirectional torsional load.

A drive shaft extends along a central axis and is configured to operate under dominant unidirectional torsional load. The drive shaft is provided in claim <NUM> and comprises an asymmetrically-structured composite body. The asymmetrically-structured composite body is configured to have a greater torque-carrying capability in a first torsional direction than in a second torsional direction opposite the first torsional direction.

A method of manufacturing a composite drive shaft is provided in claim <NUM> and includes orienting a first plurality of layers in a first direction at a first angle with respect to a central axis of the drive shaft and orienting a second plurality of layers in a second direction that is different from the first direction at a second angle with respect to the central axis. The first plurality of layers provides a first total thickness and is comprised of a first plurality of fibers oriented in the first direction. The second plurality of layers provides a second total thickness and is comprised of a second plurality of fibers oriented in the second direction. The first plurality of fibers comprises more fibers than the second plurality of fibers. The first plurality of layers and the second plurality of layers are impregnated with a polymer matrix.

While the above-identified figures set forth embodiments of the present invention, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope of the principles of the invention as set out in the claims. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features, steps and/or components not specifically shown in the drawings.

The present disclosure is directed to a composite drive shaft with an asymmetric fiber layup. Torsional load is a primary load component applied to drive shafts. This torsional load causes in-plane shear stresses in the walls of the shaft. These shear stresses can be decoupled into mutually orthogonal compressive stresses and tensile stresses. There is a broad class of drive shaft applications, for example, in rotorcraft, where orientation of the torque load is consistently applied in one orientation, i.e., these shafts are under a dominant unidirectional torque. For these applications, orientations of the compressive and tensile stresses are also consistent, and this understanding is used for drive shaft enhancement as described below.

In the case of a dominant unidirectional load, the design is driven by corresponding compressive stresses and tensile stresses and their paths or directions, respectively. Fiber-based composites generally have greater tensile strength than compressive strength along the fiber direction. Thus, if a composite drive shaft is consistently under a dominant unidirectional load, more fiber-reinforced layers can be placed in the compressive stress direction than in the tensile stress direction. This asymmetric design gives the shaft a greater torque-bearing capability in one torsional direction than in the opposite torsional direction, meaning that when the shaft is attached to a load, it will be able to bear a greater torsional load in one torsional direction about the central axis than in the opposite torsional direction. Because the shaft is under a dominant unidirectional load, it can be designed to suit this unidirectional load without also needing to bear a similar load in the opposite direction.

By placing more layers in the weakest orientation (for example, along the compressive stresses, if compression strength is lower than tensile strength) and less layers in the strongest orientation (for example, along the tensile stresses, if compression strength is lower than the tensile strength), the asymmetric design can unify the risks of the shaft's structural failures under both compressive and tensile stresses. Such an asymmetric design provides advantages over conventional symmetric designs where the numbers of fiber-reinforced layers in the compressive stress and tensile stress directions are equal. This asymmetric design increases the efficiency of the composite drive shaft through increased bearing ability and/or reduced weight.

<FIG> is a perspective view of composite drive shaft <NUM> aligned along central axis Z and radial line R, and experiencing dominant torque T in first torque direction <NUM> and second torque direction <NUM>. Dominant torque T in first torque direction <NUM> and second torque direction <NUM> generates compressive stresses and tensile stresses along the length of composite drive shaft <NUM>. These compressive stresses are oriented in a consistent alignment along the length of composite shaft <NUM>, defined herein as compressive stress direction <NUM>. The tensile stresses are similarly oriented in a consistent alignment along the length of composite shaft <NUM>, defined herein as tensile stress direction <NUM>. Compressive stress direction <NUM> can be oriented in a direction at an angle of +α=+<NUM> degrees with respect to central axis Z. In other embodiments, the angle +α of compressive stress direction <NUM> can be oriented in a broader range around +<NUM> degrees, for example, between +<NUM> degrees and +<NUM> degrees with respect to central axis Z. Tensile stress direction <NUM> can be oriented in a direction at an angle of -α=-<NUM> degrees with respect to central axis Z. In other embodiments, the angle -α of tensile stress direction <NUM> can be oriented in a broader range around -<NUM> degrees, for example between -<NUM> degrees and -<NUM> degrees with respect to central axis Z. It will be understood by one of ordinary skill in the art that the "+" orientation corresponds to compression and the "-" orientation corresponds to tension under the dominant unidirectional torsional load and that if torque is applied in an opposite direction, the orientations of compression and tension will be mutually shifted.

In one embodiment, composite drive shaft <NUM> is comprised of composite material <NUM>, which is made up of layers <NUM> of fibers <NUM> (shown in <FIG>) and polymer matrix <NUM> (shown in <FIG>). Layers <NUM> include a plurality of first layers <NUM> with fiber orientation along compressive stress direction <NUM> (fiber orientation shown in <FIG>) and a plurality of second layers <NUM> with fiber orientation along tensile stress direction <NUM> (fiber orientation shown in <FIG>). Fibers <NUM> can be impregnated with polymer matrix <NUM> so as to become embedded within it and form layers <NUM>. Fibers <NUM> can be carbon fibers, glass fibers, organic fibers, or combinations thereof, or another suitable material as known in the art. Polymer matrix <NUM> can be a thermoset polymer, a thermoplastic polymer, or another suitable matrix material.

<FIG> illustrates representative first layer <NUM> with fiber orientation along compressive stress direction <NUM> within composite drive shaft <NUM> of <FIG>. In this embodiment, fibers <NUM> in each first layer <NUM> are oriented at +<NUM> degrees with respect to central axis Z and arranged within polymer matrix <NUM>.

<FIG> illustrates representative second layer <NUM> with fiber orientation along tensile stress direction <NUM> within composite drive shaft <NUM> of <FIG>. In this embodiment, fibers <NUM> in each second layer <NUM> are oriented at -<NUM> degrees with respect to central axis Z and arranged within polymer matrix <NUM>.

When composite drive shaft <NUM> experiences dominant torque T in first torque direction <NUM> and second torque direction <NUM>, first layers <NUM> experience compressive stress σc in compressive stress direction <NUM>, and second layers <NUM> experience tensile stress σt in first tensile stress direction <NUM>. In fiber-based composites, fibers <NUM> are generally weaker under compressive stress σc than under tensile stress σt. This causes first layers <NUM> to be weaker than second layers <NUM> in the case of symmetric composite layups, i.e., designs with equal number of first layers <NUM> and second layers <NUM>. In the present invention, drive shaft efficiency is improved by providing different numbers of first layers <NUM> (with fibers substantially oriented along compression stress direction <NUM>) and second layers <NUM> (with fibers substantially oriented along tensile stress direction <NUM>) according to their strengths, respectively. The roles of first layers <NUM> and second layers <NUM> (i.e. compressive vs. tensile) would be flipped if first torque direction <NUM> and second torque direction <NUM> were reversed. Because the expected direction of dominant torque T is known, however, the present structure can produce aggregate strength sufficient to efficiently handle dominant torque T, without needing to also withstand stresses corresponding to the torque in the opposite direction.

<FIG> schematically show stresses in the fiber directions, namely, compressive stresses, σc, and tensile stresses, σt, along with introduced values of failure susceptibility F. The failure susceptibility is defined as a ratio of a stress to a corresponding strength, i.e., as Fc in case of layer-wise compressive strength Sc and Ft in case of layer-wise tensile strength St.

<FIG> schematically depicts stresses and failure susceptibility for a conventional symmetric design, where aggregate numbers of first layers <NUM> and second layers <NUM> are equal. In this case, stresses σc and σt are equal, but failure susceptibilities Fc and Ft are not equal, due to differences in compression and tensile strengths of layers <NUM>. Therefore, for this conventional design, if compressive strength Sc is lower than tensile strength St, compressive failure susceptibility Fc is higher than tensile susceptibility Ft, making first layers <NUM> over-loaded and second layers <NUM> under-loaded.

<FIG> schematically depicts stresses and failure susceptibility for an asymmetric composite drive shaft design, where aggregate numbers of first layers <NUM> and second layers <NUM> are not equal. In this case, stresses σc and σt are not equal, but failure susceptibilities Fc and Ft are equal or almost equal. Therefore, for the asymmetric design, even if compressive strength Sc is lower than the tensile strength St, compressive failure susceptibility Fc and tensile susceptibility Ft are practically the same, making risks of failure in first layers <NUM> and second layers <NUM> very close and ideally equal.

In <FIG>, compressive failure susceptibility Fc in compressive stress direction <NUM> is equal to tensile failure susceptibility Ft in tensile stress direction <NUM> because tensile stress σt has been scaled up by a factor of tensile strength St over compressive strength Sc. This can be accomplished by reducing the total number of second layers <NUM> with fibers oriented in tensile stress direction <NUM> and/or increasing the total number of first layers <NUM> with fibers oriented in compressive stress direction <NUM>, such that either first layers <NUM> outnumber second layers <NUM>, or the total thickness of first layers <NUM> is greater than the total thickness of second layers <NUM>, by a factor of layer-wise tensile strength over layer-wise compressive strength. In other embodiments, the factor of outnumbering of first layers <NUM> in comparison with second layers <NUM> can be defined according to other potential failure mechanisms or their combinations. Among such failure mechanisms, for example, buckling of the shaft wall or buckling of individual layers due to interlaminar debonding can be considered in addition to the strength limits of layers in the fiber direction.

In accordance with the claims, the total number of first layers <NUM> is greater than the total number of second layers <NUM>, or the total thickness of first layers <NUM> is greater than the total thickness of second layers <NUM> by a factor of at least <NUM>. While discussed in terms of the number or thickness of first layers <NUM> and second layers <NUM>, the increase or decrease in the number of first layers <NUM> and second layers <NUM> or total thickness of first layers <NUM> and second layers <NUM> represents an increase or decrease in the number of fibers oriented in compressive stress direction <NUM> and tensile stress direction <NUM>. As such, the disclosed embodiments having more first layers <NUM> than second layers <NUM> or greater thickness of first layers <NUM> than thickness of second layers <NUM> have more fibers arranged in compressive stress direction <NUM> than are arranged in tensile stress direction <NUM>.

<FIG> is an enlarged axial cross-sectional representation of one embodiment of a composite drive shaft where layers Li (i = <NUM>,<NUM>,<NUM>,. , N) make up composite drive shaft <NUM>, taken along radial line R of <FIG>. Layers Li (i = <NUM>,<NUM>,<NUM>,. , N) are comprised of fibers <NUM> and polymer matrix <NUM>. Layers Li (i = <NUM>,<NUM>,<NUM>,. , N) are a combination of first layers <NUM> and second layers <NUM>. For a representative example shown in <FIG>, layers L<NUM>, L<NUM>, and L<NUM> are first layers <NUM> with fiber orientation along compressive stress direction <NUM>, and layer L<NUM> is second layer <NUM> with fiber orientation along tensile stress direction <NUM>. First layers <NUM> experience compressive stress σc in compressive stress direction <NUM>, and second layers <NUM> experience tensile stress σt in tensile stress direction <NUM>.

Because first layers <NUM> with fibers oriented in compressive stress direction <NUM> are weaker than second layers <NUM> with fibers oriented in tensile stress direction <NUM>, there is a greater number of first layers <NUM> than second layers <NUM>. Decreasing the total number of second layers <NUM> increases tensile stress σt on each individual second layer <NUM>.

The arrangement of first layers <NUM> and second layers <NUM> within composite drive shaft <NUM> can be varied. For example, first layers <NUM> and second layers <NUM> could be arranged in a blocked design where some or all of first layers <NUM> are arranged together and some or all of second layers <NUM> are arranged together. Layers L<NUM>-L<NUM> in <FIG> illustrate a simple blocked design where layers L<NUM>, L<NUM>, and L<NUM> are all first layers <NUM> with fibers arranged in compressive stress direction <NUM> and are arranged together separately from L<NUM>, an individual second layer <NUM> with fibers arranged in tensile stress direction <NUM>. Alternatively, second layers <NUM> could be interspersed within first layers <NUM> to create a pattern of alternating first layers <NUM> and second layers <NUM> or alternating blocks of first layers <NUM> and second layers <NUM>. A person of ordinary skill in the art will recognize that designs having differing numbers and groupings of layers Li may be desirable and that varying designs can be used to achieve the same result of making the failure susceptibilities in compressive stress direction <NUM> and tensile stress direction <NUM> equal or nearly equal.

In addition to first layers <NUM> and second layers <NUM>, with fibers oriented in compressive stress direction <NUM> and tensile stress direction <NUM>, respectively, composite drive shaft <NUM> can include additional layers that are designed to handle other stresses within composite drive shaft <NUM> due to additional load components, such as for example, bending or/and axial loads, and/or potential failure mechanisms, such as for example, buckling of shaft walls. This can be done by, for example, orienting fibers <NUM> at <NUM> degrees to minimize the risk of buckling, or <NUM> degrees to limit stresses due to bending or axial loads. Additional layers can also be designed at other angles with respect to central axis Z. Additional layers in these alternative orientations can require that the angles of fibers <NUM> in first layers <NUM> and second layers <NUM> be adjusted to account for reinforcement provided by these additional layers.

A composite drive shaft under unidirectional torque has a load applied in a single known direction. Using an asymmetric layup allows the composite drive shaft to be tailored to differing compressive and tensile stresses it experiences in different directions. This increases the efficiency of the composite drive shaft at bearing expected loads, reducing the required mass by unifying the risks of the shaft's structural failure under both compressive and tensile stresses.

Any relative terms or terms of degree used herein, such as "substantially", "essentially", "generally", "approximately" and the like, should be interpreted in accordance with and subject to any applicable definitions or limits expressly stated herein. In all instances, any relative terms or terms of degree used herein should be interpreted to broadly encompass any relevant disclosed embodiments as well as such ranges or variations as would be understood by a person of ordinary skill in the art in view of the entirety of the present disclosure, such as to encompass ordinary manufacturing tolerance variations, incidental alignment variations, transient alignment or shape variations induced by thermal, rotational or vibrational operational conditions, and the like. Moreover, any relative terms or terms of degree used herein should be interpreted to encompass a range that expressly includes the designated quality, characteristic, parameter or value, without variation, as if no qualifying relative term or term of degree were utilized in the given disclosure or recitation.

The following are non-exclusive descriptions of possible examples of the present invention.

A further example of any of the foregoing drive shafts, wherein the polymer matrix can be comprised of a thermoset polymer or a thermoplastic polymer.

A further example of any of the foregoing drive shafts, wherein the fibers can be fibers selected from a group consisting of carbon fibers, glass fibers, organic fibers, and combinations thereof.

A further example of any of the foregoing drive shafts, wherein the drive shaft can further include layers which are oriented in a third direction at a third angle with respect to the central axis. The third angle can be different than the first and second angles.

A further example of any of the foregoing drive shafts, wherein the first angle can be oriented between +<NUM> degrees and +<NUM> degrees with respect to the central axis, and the second angle can be oriented between -<NUM> degrees and -<NUM> degrees with respect to the central axis.

A further example of any of the foregoing drive shafts, wherein the first angle can be oriented +<NUM> degrees with respect to the central axis, and the second angle can be oriented -<NUM> degrees with respect to the central axis.

A further example of any of the foregoing methods, wherein the polymer matrix can be comprised of a thermoset polymer or a thermoplastic polymer.

A further example of any of the foregoing methods, wherein the fibers can be fibers selected from a group consisting of carbon fibers, glass fibers, organic fibers, and combinations thereof.

A further example of any of the foregoing methods, which can include orienting layers in a third direction and at a third angle with respect to a central axis of the drive shaft. The third angle can be different than the first and second angles.

A further example of any of the foregoing methods, wherein the first angle can be oriented between +<NUM> degrees and +<NUM> degrees with respect to the central axis and the second angle can be oriented between -<NUM> degrees and -<NUM> degrees with respect to the central axis.

A further example of any of the foregoing methods, wherein the first angle can be oriented +<NUM> degrees with respect to the central axis and the second angle can be oriented -<NUM> degrees with respect to the central axis.

Claim 1:
A drive shaft (<NUM>) extending along a central axis and being configured to operate under dominant unidirectional torsional load, the drive shaft comprising:
an asymmetrically-structured composite body configured to have a greater torque-carrying capability in a first torsional direction than in a second torsional direction, the second torsional direction being opposite the first torsional direction;
wherein the asymmetrically-structured composite body comprises:
a first plurality of fibers (<NUM>) oriented in a first direction at a first angle with respect to the central axis; and
a second plurality of fibers (<NUM>) oriented in a second direction at a second angle with respect to the central axis,
wherein the first and second directions are different; and
wherein the first plurality of fibers and the second plurality of fibers provide a greater torque-carrying capability in the first torsional direction than in the second torsional direction;
wherein the first plurality of fibers (<NUM>) comprises more fibers than the second plurality of fibers (<NUM>);
wherein fibers of the first plurality of fibers (<NUM>) are embedded within a polymer matrix and arranged in a first plurality of layers and wherein fibers of the second plurality of fibers (<NUM>) are embedded in a polymer matrix and arranged in a second plurality of layers;
wherein the first plurality of fibers is substantially oriented in a compressive stress direction when operating under the dominant unidirectional torsional load and wherein the second plurality of fibers is substantially oriented in a tensile stress direction when operating under the dominant unidirectional torsional load;
wherein the first plurality of layers outnumbers the second plurality of layers by at least a factor of <NUM>; and/or
wherein a total thickness of the first plurality of layers is greater than a total thickness of the second plurality of layers by at least a factor of <NUM>.