Patent Publication Number: US-H1647-H

Title: Cylindrical keyed coupling for composite propulsion shafting

Description:
This invention was made with Government support by the Naval Surface Warfare Center. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to coupling propulsion shafting to other drive train components and, more specifically, to couplings used in U.S. Navy propulsion systems with components made of composite materials. 
     2. Description of the Related Art 
     Propulsion shafts, often made of metal in the past, are today increasingly being made of composite materials. The composite materials may be laminated or filament-wound to form shafts. The shafts connect with motors, engines, generators, etc. to deliver torque to other drive-train components. Components are joined to drive shafts and other drive train components, such as flexible couplings, thrust bearings, gearboxes, prime movers, and propulsors to complete the drive train. Many of the components are often fabricated of metal. Thus, a shaft coupling is a necessary interface between two very distinct shaftline components. 
     Composite materials typically used in the fabrication of drive train components include polymer-based resin matrix materials, for example thermosetting epoxies and thermoplastic organic polymers, reinforced with a continuous organic fiber, such as continuous carbon fibers or continuous glass fibers. In an exemplary manufacturing process, a layered, filament-wound product is made by winding the organic fiber, saturated with the matrix material, onto a spinning mandrel to create the desired article. The organic fiber reinforces the layered article, with the reinforcement being contained within each layer. 
     There are only two approved methods for coupling the components of the drive train as currently practiced on U.S. military ships. The first is to provide shafting with flanges forged integrally. At the present time, however, it is not practical to filament wind a composite flange capable of handling ship propulsion loads. The second method is to utilize square keys and keyways to transfer the torsional load, a system which poses particular problems, especially with respect to propulsion systems combining composite and metal component materials. 
     The primary load for a Navy propulsion shaft is torsion. Composite structures, which are anisotropic materials, can be quite intolerant of high stress concentrations such as can occur in the vicinity of holes, fillets, notches, and grooves. U.S. Navy propulsion shafting must be capable of handling high torsional loads, extended bending-fatigue life in a seawater environment, and moderate axial loads to push or pull a vessel through the water. 
     Work in the application of composite propulsion shafting, conducted at the Naval Surface Warfare Center in Annapolis, has resulted in successful demonstrations of composite materials to carry the loads necessary. Filament wound composite tubes have been designed and fabricated to carry all of the above-mentioned combined propulsion loads. The most challenging work involves the design and demonstration of composite joining techniques capable of transferring the primary propulsion loads between the composite shafting and other shaft-line components. 
     Due to their anisotropic nature, fiber-reinforced composites can exhibit stress concentrations as high as 9. (Stress concentrations are unitless factors which are applied to &#34;local&#34; regions of a structure. The &#34;local&#34; regions exhibit an abrupt change in geometry, material or both. Instead of performing a detailed stress analysis for each &#34;local&#34; region, the &#34;average&#34; stress of the structure is multiplied by the stress concentration to give an approximate stress for the &#34;local&#34; region). Typical stress concentrations for isotropic materials range from 2 to 3. These stress concentrations result from holes, cut-outs or other dramatic changes in structural geometry. 
     Shafting generally exhibits a constant geometry along its length until a coupling is employed to couple powertrain components or additional shaft sections together. For example, current practice utilizes a steel shaft coupled to a bronze propulsor. Couplings are always accompanied by stress concentrations, whether it is due to a change in geometry or a change in materials. The problem is magnified with composite materials, due primarily to the extreme stress concentrations associated with these highly anisotropic materials. 
     Couplings have been standardized for conventional, forged steel, Navy propulsion shafting. Standard, metallic keyed couplings, use square keys and keyways mounted along the axis of the shaft. An example of a typical prior art coupling is shown in cross-section in FIG. 1. The keys transfer torsion through shear at the midplane of the key, or at the interface between the shaft and the coupling. The machined, square key systems induce stress concentrations where abrupt changes in geometry occur, typically at the corners of the key. This corresponds to the root or base of the keyway machined in the shaft. 
     A composite shaft comprises layered material. The layers are concentric rings of reinforced material, typically as shown in cross-section in FIG. 2. A composite shaft has no reinforcement, however, in the radial direction. For this reason, a typical mode of failure for a layered composite is delamination between the axially-reinforced layers. Square, machined keys and keyways in a layered, composite propulsion shaft generate peak stresses at the root of the keyway, then immediately drop to zero toward the bore of the shaft and rapidly drop to zero as one moves toward the midplane of the key. The high stress gradient promotes premature interlaminar failure. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems noted above by optimizing the shape of a key and keyway machined in a composite shaft to provide a gradual reduction of bearing pressure, from a maximum at an outer portion of the shaft, to zero or near zero at a base of the keyway in the composite shaft. 
     Specifically, in accordance with the present invention, a key with a round shape is preferably provided, which significantly reduces the stress concentration at the base of the key, as compared to that of a standard &#34;square&#34; or &#34;rectangular&#34; machined keyway. 
     Cylindrical keys and keyways provide a simple and efficient way of mechanically coupling a composite propulsion shaft to hubs made of metal. Stress concentrations in the composite are reduced as compared to a rectangular keyway. The cylindrical keys have a variable bearing load on the machined composite keyway and will not promote delamination loads. The mechanical coupling can be easily machined, assembled and disassembled with standard procedures. 
     In the present invention, either the shaft, the hub or both can be composite material. Preferably, the hub is fabricated out of the same composite material as the shaftline, thereby minimizing stress concentrations due to material changes. Further, the keyways can be machined on a taper, which will result in both a frictional coupling and a keyed coupling. The combination of the two will provide load redundancy which is often utilized in current design practice. 
     The key and keyway of the present invention may also be elliptical. Further, an adhesive can be used to bond an interface of the coupling, for example, between the key and keyway or between the hub and the shaft. The combination will provide another load-handling redundancy in the coupling, thereby further increasing its capabilities. 
     The cylindrical keyway can be optimized for dissimilar materials and the geometries involved. In addition, the key itself can be optimized with respect to materials used and the geometries involved. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional representation of a prior art coupling using a square key and keyway. 
     FIG. 2 is a cross-sectional representation showing the coupling according to a first embodiment of the present invention. 
     FIG. 3 is a partial cross section representing the prior art coupling showing pressure profiles and areas of stress concentration. 
     FIG. 4 is a partial cross section representing the coupling according to the first embodiment of the present invention showing pressure profiles and areas of stress concentration. 
     FIG. 5 is a side view showing two composite shafts coupled together according to the first embodiment of the present invention. 
     FIG. 6 is a partial cross section showing typical dimensions as the first embodiment of the present invention would be applied in a prototype propulsion system. 
     FIG. 7 is a cross section of a second embodiment of the present invention utilizing an elliptical key and keyway. 
     FIG. 8 is a detail view showing a tapered key and keyway according to an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 2 and 5, the key coupling according to a preferred embodiment of the present invention comprises four major components: a metal plug 1, a cylindrical key 2, a hub 3 and a composite shaft 4. A hub keyway 5 is formed in hub 3, and shaft keyways 6 are formed in shaft 4. The components are described more fully as follows: 
     1. Metal Plug in the Bore of the Composite Shaft 
     As shown in FIGS. 5 and 8, metal bore plug 1 is disposed inside bore 7 of hollow composite shaft 4 in substantially axial alignment with key 2. Metal plug 1 restricts radial deformation that may take place in composite shaft 4. When placed under a torsional load, cylindrical key 2 will tend to deform the composite shaft 4 radially. Bore plug 1 restricts this deforming motion. 
     2. Cylindrical Keys 
     Cylindrical key 2 is designed to match a maximum allowable bearing strength of the composite laminate used to form shaft 4. Any difference in stiffness between the composite shaft and the metal hub 3 induces a stress concentration. In order to reduce differences and stress, the key material is designed to be of some intermediate stiffness, thereby helping to control or moderate the stiffness differential between the &#34;soft&#34; or relatively low-modulus composite shaft and the &#34;stiff&#34; or relatively high-modulus metal hub If both materials are characterized by the same stiffness and geometry then the stress concentration due to dissimilar materials would reduce theoretically to zero. 
     3. Hub 
     Hub 3 is considered to be a relatively high modulus metal hub. As noted above, an optimized hub material is the same as that used in the composite shaft. This reduces stress concentrations due to a material change theoretically to zero. Remaining stress concentrations in the coupling are due to geometry changes. 
     As shown in FIGS. 5 and 8, keyways 5, 5c in hub 3 may be open-ended so that hub 3 may slide over keys 2, 2c and shaft 4 once keys 2, 2c are placed in keyways 6, 6c in shaft 4. As with the keyways 6 in the shaft, the cylindrical keyway 5 in metal hub 3 also improves stress concentration from conventional square keyways of the prior art as shown in FIG. 1. Square key 2b fits into a square keyway formed of hub keyway 5b and shaft keyway 6b. As stated above, the highest stress concentrations occur at the root of a square keyway, as shown at areas I and J in FIG. 3, whereas lower bearing reactions that approach zero more gradually at the base of the keyway are associated with the cylindrical key coupling concept. 
     4. Composite shaft 
     Filament-wound composite propulsion shaft 4 has half-round cylindrical keyways 6 machined near the end. The keyways 6 are machined so as to capture keys 2 in closed-end slots. 
     In operation, torque is transferred from the composite shaft 4 to the metal hub 3 via the bearing reaction of the cylindrical keys 2. As shown at areas E and F in FIG. 4, when a torsional force is applied in the direction indicated by arrow D, cylindrical key 2 exerts maximum bearing loads E1 and F1 at a midplane A of the key, which is coincident with a tangent to an outside diameter of the shaft 4. As shown by graphical arrows at areas E and F in the representation of FIG. 4, for a composite cylindrical keyway, the bearing load transfer is a maximum at the composite/metal interface A and gradually reduces to a minimum at the root or depth B and C of the keyway. 
     By comparison, with reference to areas I and J of FIG. 3, the bearing load transfer is maximized at the corners of the square key 2b. Stress is maximized in the hub at points G and in the shaft at points H. 
     FIG. 6 represents the dimensional interface of the cylindrical key with the metal hub and composite shaft of a prototypical coupling. Test components were fabricated and an assembled coupling was tested for demonstration purposes. 
     A shaft diameter of 3,666&#34; was fabricated. A key having a diameter of 0.75&#34; was also formed. As can be seen with reference to FIG. 6, the shaft keyway 6 has an edge-to-base depth P of 0.324&#34; and an edge-to-edge width Q of 0.743&#34;. An arc R representing the circumference of cylindrical key 2 outside shaft keyway 6 is 196 degrees. A hub key and keyway were formed accordingly. 
     A strain field was observed using a photoelastic coating on the outside diameter of the composite shaft adjacent to the cylindrical key. The strain field indicated a peak bearing pressure induced on the composite. For this demonstration, a maximum average bearing pressure based on a projected area was 40,000 psi. Past testing on this composite material indicated a maximum bearing strength of about 65,000 psi. The linear-loading curves that were generated were in a range of &#34;maximum&#34; composite shafting design practice. 
     The example provided substantial evidence of the viability of the cylindrical key concept. By loading cylindrical keys to an average bearing pressure of 40,000 p.s.i., or more than 60% of the composite substrate&#39;s maximum bearing strength, without inducing any indication of delamination, substantial evidence supporting low stress-concentration features of this mechanical coupling concept have been developed. 
     Referring to FIG. 7, an alternative embodiment incorporating an elliptical key 2a is shown. Hub keyway 5a and shaft keyway 6a are formed so as to form an ellipse upon alignment. 
     FIG. 8 shows a tapered key 2c and keyway 6c in yet another embodiment of the present invention. The tapered key provides frictional coupling in addition to the keyed coupling. The two combine for load redundancy as utilized in current design practice. The direction of the taper is chosen to conform with design requirements and constraints. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.