Patent Application
20120067020
Aircraft landing on the deck of an aircraft carrier are brought to a stop by a cable arrestment system. See
The cables are monitored and replaced at the first sign of failure. The cross deck pendant is used in approximately 125 arrestments before it is replaced. The purchase cable is replaced after approximately 1200 arrestments.
In a typical cable arrestment system used on a US Navy aircraft carrier, multiple sheaves hold 2,300-ft reels of purchase cable below the flight deck. The purchase cable is 1.5-inch diameter, highly lubricated plain steel wire rope with a 6×31 lang-lay, Warrington Seale Die Formed strand. The strands surround a polyester core, which is roughly ⅓ the diameter of the cable. This cable weights 3.72 lb/ft and is preloaded to 3,000 lbs. off line. The required measured break strength of the cable is 215 kip, and the maximum service tension is 105 kip during an arrestment. The cable is continually cyclically load tested during the carrier's deployment. The entire 8,600-lb reel is replaced upon observation of any individual steel wire failures.
A composite cable is provided that employs synthetic fibers as a primary strength member and that can be used in bend-over-sheave applications such as a purchase cable in an aircraft arrestment system.
Synthetic fibers have typically not performed well in bend-over-sheave applications, particularly applications involving the high stress levels and cyclic operation of an aircraft arrestment system. In one embodiment, the present cable incorporates wire lay synthetic fibers coaxially wound around a cable core, and an outer jacket of wire lay steel wires coaxially wound over the primary strength member. In several embodiments, the synthetic fibers may include aramid fibers, liquid crystal polymer fibers, high modulus polyethylene fibers, or a combination of one or more of aramid fibers, liquid crystal polymer fibers, and high modulus polyethylene fibers. The synthetic fibers are disposed in layers separated by low friction layers to minimize frictional abrasion between the fibers. The outer jacket is formed of a layer of steel wires, separated from the outermost strength member layer by a low friction layer.
The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:
The disclosures of U.S. Provisional Patent Application No. 61/155,360, filed on Feb. 25, 2009, and U.S. Provisional patent Application No. 61/165,220, filed on Mar. 31, 2009, are incorporated by reference herein.
An aircraft cable arrestment system employs cables that are cyclically subject to high tensile stresses and bend-over-sheave situations. Cables employing synthetic fibers have historically performed poorly in bend-over-sheave applications. Factors that affect the cyclic bend-over-sheave performance of a synthetic rope or cable are the load (as a percentage of the breaking strength of the rope), the sheave design (radius, groove, shape, width, and depth), the cycle rate, the rope or cable construction, and the materials used in the rope or cable.
For an application such as a cable arrestment system on an aircraft carrier, the first two of the above factors—load and sheave design—are fixed. The sheave design affects the local stresses in the section of the cable under load that is being bent. The smaller the ratio of the sheave diameter D to the cable diameter d (D:d), the more the stress level is increased. Typical sheave diameters and cable diameters in an aircraft cable arrestment system result in a relatively low D:d ratio, leading to an environment with increased stress levels in the cables. More particularly, fibers on the side of the cable farthest from the sheave surface experience increased levels of tensile stress, and fibers on the inside of the bend nearest the sheave surface experience increased levels of compressive stresses. To avoid excessive stress becoming concentrated in fewer fibers, causing them to break and setting off a cascade of strand failures, the load in the cable can be redistributed by fiber realignment and load sharing. This movement of fibers inside the cable, however, causes large shearing forces and results in fiber-on-fiber friction and abrasion.
The cycle rate can also be significant for cables constructed with temperature-sensitive fibers (for example, high-modulus polyethylene (HMPE)). Higher cycle rates tend to result in greater heat build-up in the cable, because there is less time for the heat to be dissipated between passes over the sheave.
The present invention relates to a cable incorporating synthetic fibers that is particularly suitable for a purchase cable for use in a cable arrestment system and other applications employing bend-over-sheave situations. A cable 10 having a suitable construction and materials is illustrated schematically in
The primary strength member 13 generally carries most of the tensile load on the cable. Thus, the primary strength member must contain sufficient synthetic fiber material to maintain the integrity of the primary strength member 13, while also including sufficient low friction separation layers to minimize frictional abrasion between the fibers. Frictional abrasion can be detrimental to the synthetic fiber material due to excessive heat and tearing of the fibers. Generally, a fill factor of at least 70% of the cross-sectional area of the cable, which includes the core 12, provides sufficient fiber material to bear the tensile load. That is, at least 70% of the cross-sectional area of the cable is filled with the synthetic fiber material.
The synthetic fiber strength members 14 can suitably be formed of high performance synthetic fibers of aramid or liquid crystal polymers. Suitable aramid fibers include TECHNORA® available from Teijin and KEVLAR® K-49 available from DuPont de Nemours. Suitable liquid crystal polymer fibers include VECTRAN® available from Kuraray Co. Other synthetic fibers may also be suitable, depending on the application requirements and materials properties. For example, in some applications, HMPE or PBO may be suitable.
The use of high performance synthetic fibers to form the strength member layers allows the cable to have a reduced weight compared to the weight of legacy steel purchase cables. Although high performance synthetic fibers offer advantages in their tensile strength to weight ratio compared to steel, they suffer from a shortcoming: reduced life in bend-over-sheave applications. This shortcoming is in some cases related to the anisotropic nature of synthetic fibers (i.e. they are much stronger in the axial direction than the radial direction, unlike steel, which has relatively isotropic properties). In other cases, bend-over-sheave performance degradation is related to creep rupture. Typical failure modes of synthetic fibers in bend-over-sheave applications include abrasion, cutting, bending fatigue, creep rupture, and thermal degradation.
Abrasion on the outer surface of a rope or cable occurs due to rope fibers rubbing on something in the external environment (i.e., the sheave or objects that are in close contact with the rope while it is under tension or moving across the sheaves). Fiber-on-fiber abrasion occurs within the rope due to the relative movement of fibers when the rope passes over a sheave and when the rope contracts as it is placed under tension. This is a concern for ropes made of liquid crystal polymer (LCP, VECTRAN®), PBO (poly(p-phenylene-2,6-benzobisoxazole, ZYLON®) or aramid (TECHNORA®) fibers. HMPE (high modulus polyethylene, DYNEEMA®, SPECTRA®) has a greater abrasion resistance due to its lower coefficient of friction.
Cutting can occur when a sharp object cuts into the rope and damages the outer fibers. Although high performance fibers are generally considered to be more cut resistant than lower strength fibers, this is because the cutting action in various cut tests involves a combination of shear force in the radial direction of the fiber and tensile forces in the axial direction. If a high performance fiber is tightly restrained on both ends while in tension, the fiber is much easier to cut than would be anticipated from standardized cut testing methods. This is because these fibers are not as strong in the radial direction as they are in the axial direction.
Aramid fiber ropes are inherently susceptible to fatigue damage due to the bending of the aramid fibers. LCP and HMPE are not as susceptible to this problem.
Although all high performance fibers exhibit creep under some conditions of loading, HMPE fibers are more susceptible to creep than other high performance fibers. Creep rates are proportional to applied stress levels (as a percentage of the breaking strength of the cable) and temperature. Creep rupture, especially when accelerated by heat generation within the rope during bend-over-sheave operations, is a primary means of failure for HMPE ropes. LCP, aramid, and PBO fiber ropes have a greater resistance to creep than HMPE.
HMPE also differs from other high performance fibers in that it has a relatively low melting point (approximately 147 C). During cyclic bending of the rope and the application of high loads, the temperature of the rope's interior rises due to friction of the fibers moving past one another. The fibers not only move relative to one another in the axial direction because of the bending motion, they also contract in the radial direction due to the application of tension forces on the rope. If internal temperatures in a rope build up until they approach the melting point of the HMPE fiber, individual filaments can soften and fuse together upon cooling. Since the filaments are no longer acting as individual tiny (microns in diameter) strands, they form a rigid body inside the rope, which exacerbates abrasion problems. Also, this thermal transition may cause the HMPE fiber to lose its axially aligned crystalline structure, which is the basis for its extremely high tensile strength. Thus, the mechanical properties of the fused filaments are lower than the non-heat affected filaments, and this becomes an initializing point for failure. LCP, PBO, and aramid ropes do not suffer as much from this problem, because the melting or degradation point of those fibers is greater than 300° C.
In one embodiment, suitable for a purchase cable, the cable is constructed such that each strength member layer 14 is laid up with a wire lay or helical twist. This wire lay construction does not include the “pinch points” that are present in a braided rope construction, in which the fibers cross or intersect. Also, wire lay provides a larger fill factor compared to braided rope. The wire lay of the fibers also allows the cable to stretch when it is bent over a sheave, in contrast to a “straight lay” construction. Additionally, the wire lay construction produces good translational efficiency, that is, the cable strength is a high percentage of the sum of the strength of the individual fibers. The lay angle is selected so that there is adequate constructional stretch to allow the outside of the cable to extend when bent over the sheave, while still optimizing translation efficiency.
The individual synthetic fibers are comprised of fine diameter filaments, on the order of 0.01 inch in diameter or smaller. For example, one suitable VECTRAN® LCP fiber diameter is 0.023 mm. One suitable TECHNORA® aramid fiber diameter is 0.12 mm, and KEVLAR® aramid fiber diameter is 0.018 mm. One suitable HMPE fiber diameter is 0.015 mm. The individual filaments are formed into bundles, which are wound on spools. In fabrication of the cable, a plurality of fiber bundles, provided from the spools, are helically wound about the core 12 to form the first strength member layer 14. A low friction layer is disposed over the first strength member layer. A second strength member layer 14 is then helically wound over the low friction layer in the same manner, by helically winding a plurality of fibers bundles provided from spools over the low friction layer. A further low friction layer is then disposed over the second strength member layer. Subsequent strength member layers 14, with intermediate low friction members, are built up in layers in the same way to form the primary strength member 13. The low friction layers are of minimal thickness compared to the strength members 14.
The core 12 provides a support for the helically wound strength member layers 14 and may also provide some load carrying capacity. The core is formed of a bundle of fibers of a synthetic material, which may and typically do have a helical twist.
In one embodiment, the core 12 is made of high modulus polyethylene (HMPE) fibers. HMPE can be susceptible to creep rupture and so is not believed to be a preferred choice for the strength member layers 14 for a purchase cable, although it may be acceptable in certain applications. However, HMPE is suitable as the central core 12, where the HMPE can “creep to fit.” Thus, the creep of the HMPE can aid in distributing the load to the strength member layers 14 and the outer jacket 16. The high strength of the HMPE can also make some contribution to carrying the load, in addition to forming the support for the strength members.
In an alternative, the core can be formed of rod of a suitable material, such as DACRON® polyester or another plastic material. The rod can be formed by any suitable process, such as an extrusion process.
The outer wire jacket or armor layer 16 is formed of a layer of steel wires 18 helically wound over the outermost strength member. In one embodiment, the outer wire jacket 16 is formed from wires of extra improved plow steel (XIPS), a high carbon steel having a high tensile strength. The plow steel may be galvanized if desired for the application (GXIPS). The low friction layer 22 is disposed between the outermost strength member layer 14 and the outer wire jacket 16. In one embodiment, the low friction layer 22 is formed from a sleeve of extruded fluorinated ethylene propylene (FEP), although other materials can be used.
The lay angle of the wires 18 in the wire jacket 16 is selected so that the load is balanced or shared between the wire jacket 16 and the inner synthetic strength member layers 14. The relatively large diameters of the wires of the wire jacket also reduces the potential for the wires to cut through into the neighboring synthetic fibers, and thus further protects the synthetic strength member layers 14 from abrasion. The wire jacket 16 and the low friction layer 22 also protect the synthetic core 12 and the strength member layers 14 from damage from ultraviolet radiation, which can degrade the properties of synthetic fibers.
Also, the strength of the wire jacket 16 in the radial direction can help to reduce the deformation and compression experienced by the synthetic core and strength members during bending over the sheaves, and thus improve load sharing among the synthetic fibers, decrease inter-fiber abrasion, and increase cable life.
Additionally, the wire jacket may decrease heat buildup in the synthetic core and strength members by providing a heat sink with a high thermal conductivity. The rate of heat transfer via conduction into the steel wires is higher than the rate of heat transfer by convection into the air for an synthetic cable without an outer layer of steel.
Further, the outer wire jacket 16 may maintain the legacy mode of visual observation to detect failure. For example, the cable can be constructed so that the wires 18 fail in a visible manner when the load on the cable reaches a determined percentage, such as 50%, of the cable's breaking strength in a bend-over-sheave situation.
In one lightly-armored embodiment, 36 wires are wound about the outermost strength member. In one more heavily-armored embodiment, 26 wires, each having a larger diameter, are wound about the outermost strength member. The wires are wound with a small gap between the wires (0.017 inch in one embodiment), which provides sufficient room for them to shift when the cable is wrapped around the sheaves and to avoid substantial friction between the wires during cycling.
The following tables specify several exemplary cables that have been computationally analyzed. The break strength (BS) and cable weight per linear foot length are shown for each cable example. The tables also specify the material of each layer, the outer diameter of the cable after each layer, the lay angle and direction of each fiber layer, the modulus of each layer, and the ultimate strength of each layer. The cables are designed to have an outer diameter of 1.5 inch, which is suitable for use in a legacy aircraft cable arrestment system, such as that shown in
This cable example is a lightly-armored liquid crystal polymer (VECTRAN).
This cable example is a lightly-armored aramid (TECHNORA).
This cable example is a lightly-armored aramid (KEVLAR K-49).
This cable example is a lightly-armored HMPE (SK-78).
This cable example is a heavily-armored liquid crystal polymer (VECTRAN).
This cable example is a heavily-armored aramid (TECHNORA).
This cable example is a heavily-armored aramid (KEVLAR K-49).
This cable example is a heavily-armored HMPE (SK-78).
As can be seen in the above tables, the lay angles of the layers increase toward the outside of the cable. If the lay angles decrease toward the outside, the outer layers tend to overload under tension and decrease the overall diameter of the cable more severely. In addition, the outer layers are more influenced by bend fatigue, so it is desirable to have high lay angles in the outer layers to accommodate bend fatigue, but not so high that the layers do not carry sufficient load. Thus, the lay angle is a compromise between proper load sharing among the layers and acceptable bend fatigue performance.
The direction of lay is to the right in the above examples. Contrahelical layups, in which some layers have a lay angle to the left can be used if desired for a particular application. In the aircraft cable arresting application, the uni-lay construction rotates less than a contrahelical construction.
A numerical analysis of the above cable examples was run, with results as indicated in the following table:
A numerical analysis of Cable Example 2 under tension of 100,000 lbs indicates good load sharing among the layers. The torque was −14,497 lb-in. The diameter of the cable reduced from 1.5 in to 1.3939 in under this load. The tensile stress within each layer was relatively well matched:
The torque characteristics under load of the purchase cable 10 match (are equal and opposite) that of the cross deck pendant, which is about 18,000 in-lb under a tension of 100 kips.
The various layers of the cable should be at 50% of the breaking strength when the cable is in a bend-over-sheave situation.
The present cable provides a minimum weight while still meeting the tensile load requirement of a cable arrestment system for an aircraft carrier. The lighter weight aids in increasing the life of the arresting engine, because the cable's weight adds to the total weight that the arresting engine must decelerate. The lighter weight also reduces labor requirements, because the cable is easier to handle. Lighter weight cables can also be used by smaller aircraft such as unmanned aerial vehicles (UAVs) that cannot handle the heavier legacy cables.
Other variations are encompassed. Each strength member layer can be formed from a blend of fibers. Also, low-friction fibers, such as, for example, fibers of ePTFE, can be blended within the high performance synthetic fibers of each strength member layer to provide a solid lubricant to reduce friction between the fibers. The synthetic fibers can also be coated with a low-friction coating, such as, for example, a silicon-based coating such as dimethylsiloxane. Other coatings may be suitable.
In another embodiment, a fail-safe or secondary inner cable can be included coaxially within the cable. The secondary inner cable can be formed, for example, of a fiber-reinforced composite material. For example, E-glass rods embedded in an adiprene urethane matrix can be used. The individual E-glass rods can move relative to each other within the matrix, while remaining consolidated in a specific cross-sectional geometry and protected from environmental elements, such as moisture, ultraviolet radiation, and abrasion.
While example embodiments have been described, those of skill in the art will appreciate that various changes and modifications may be made to the embodiments, for example, as technology develops, and equivalents may be substituted for elements thereof. It will be appreciated that various aspects of each embodiment may be used with other embodiments. Modifications may be made to adapt the teachings to a particular situation.
The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/155,360, filed on Feb. 25, 2009, and U.S. Provisional Patent Application No. 61/165,220, filed on Mar. 31, 2009, the disclosures of which are incorporated by reference herein.
This invention was made under Navy Contract No. N68335-09-C-0021. The Government may have certain rights to this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61155360 | Feb 2009 | US | |
| 61165220 | Mar 2009 | US |
