Abstract:
A fiber reinforced plastic propeller shaft has a fiber reinforced plastic pipe, and at least one metal member attached to an end of the pipe. The metal member is provided with a serration having a plurality of teeth having an apex angle. When the metal member is attached to the end of the pipe, each tooth forms on the inner surface of the pipe end a groove extending along the axial direction of the pipe. The apex angle of each tooth is between 45° and 75°.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a fiber reinforced plastic propeller shaft that includes a fiber reinforced plastic pipe and metal members attached to the ends of the pipe, each metal member having serration including a number of teeth that form grooves extending in the axial direction in the inner surface of the ends of the pipe.  
           [0002]    A propeller shaft for transmitting power generated by the engine of an automobile to driven wheels typically includes a metal shaft and yokes welded to the ends of the shaft. The yokes form part of metal universal joints. The universal joints are coupled to a drive shaft and a driven shaft, respectively. This type of propeller shaft is referred to as a metal propeller shaft.  
           [0003]    In recent years, there is a great demand for lighter parts of vehicles to reduce the weight of vehicles. Accordingly, propeller shafts made of fiber-reinforced plastic (FRP) are used to reduce the weight. FIG. 5( a ) shows such a fiber reinforced plastic (FRP) propeller shaft  51 , which is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2000-120649. The propeller shaft  51  has an FRP pipe  52  and metal yokes  53  press fitted to the ends of the pipe  52 . The yokes  53  couple the pipe  52  to a drive shaft and driven shaft (neither is shown).  
           [0004]    Each yoke  53  has a serration  54  formed on a part of the outer surface that contacts the FRP pipe  52 . The outer diameter of the serration  54  is greater than the inner diameter of the FRP pipe  52 . Press fitting the contacting part of the yoke  53  into the FRP pipe  52  causes the teeth of the serration  54  of the yoke  53  to form grooves on the inner surface of the FRP pipe  52 . The engagement of the serration  54  and the FRP pipe  52  ensures a sufficient coupling strength to permit the yoke  53  and the FRP pipe  52  to rotate integrally.  
           [0005]    The apex angle θ of each tooth  54   a  of the serration  54  is approximately 90°. As shown in FIG. 5( b ), the apex angle θ refers to an angle defined by lines Ls representing the sides of the tooth  54   a . A greater apex angle θ requires a greater force to press fit the serration  54  into the FRP pipe  52 . This requires facilities of a greater press force and may break the pipe  52 . The cost is increased accordingly. Further, since it is difficult to point the end of the tooth  54   a , the end of the tooth  54   a  is formed to have a trapezoidal or arcuate cross-section. Therefore, an apex angle of approximately 90° is likely to cause the teeth  54   a  to expand the FRP pipe  52  when the serration  54  is press fitted. In this case, the teeth  54   a  cannot form grooves having a sufficient depth, and the engagement of the teeth  54   a  with the inner surface of the FRP pipe  52  is not sufficient. As a result, the coupling strength of the FRP pipe  52  and the yokes  53  is not satisfactory.  
           [0006]    The engagement portions of the yokes coupled to an FRP pipe must transmit a required torque (torsional torque) and prevent the FRP pipe from receiving excessive force when the yokes are press fitted to the pipe. Therefore, the press fitting force needs to be minimized. However, the torque transmitting capability from the yokes to the FRP pipe does not depend only on the engagement amount of the teeth  54   a  with the FRP pipe  52 , but also on the reactive force, or fastening force, produced when the serration  54  is press fitted to the pipe  52  and expands the pipe  52 . Thus, if the apex angle θ is too small, the pipe  52  will not be sufficiently expanded and there will be no sufficient fastening force. As a result, a sufficient torque transmitting capability will not be obtained. Also, if the apex angle θ is too small, a required strength will not be obtained.  
           [0007]    In recent car designs, a technology to make a propeller shaft to collapse or break in the axial direction for gradually absorbing the great impact of a collision has been proposed. This technology prevents an excessive impact in a collision and thus creates a sufficient time for various safety devices such as air bags to operate. In one of the designs according to the technology, the yokes are pressed further into an FRP pipe than the original positions by the impact force of a collision when the impact force exceeds a predetermined value. This axially collapses or breaks the propeller shaft. In this configuration also, the yokes are preferably press fitted to the FRP pipe with a relatively small force during manufacture.  
         SUMMARY OF THE INVENTION  
         [0008]    Accordingly, it is an objective of the present invention to provide an FRP propeller shaft that permits serrations to be easily press fitted to an FRP pipe and sufficient torsional torque to be transmitted between the FRP pipe and the serrations.  
           [0009]    To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a fiber reinforced plastic propeller shaft is provided. The shaft has a fiber reinforced plastic pipe, and a metal member attached to at least one end of the pipe. The metal member is provided with a serration having a plurality of teeth having an apex angle. When the metal member is attached to the end of the pipe, each tooth forms on the inner surface of the pipe end a groove extending along the axial direction of the pipe. The apex angle of each tooth is between 45° and 75°.  
           [0010]    Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a partial cross-sectional view illustrating an FRP propeller shaft according to one embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a partly cross-sectional view illustrating the yoke of FIG. 1;  
         [0014]    [0014]FIG. 3( a ) is an enlarged partial front view of the serration of the yoke shown in FIG. 2;  
         [0015]    [0015]FIG. 3( b ) is an enlarged partial cross-sectional view showing the engaging portion of the serration and the FRP pipe;  
         [0016]    [0016]FIG. 4 is an enlarged partial front view showing a serration according to another embodiment;  
         [0017]    [0017]FIG. 5( a ) is a cross-sectional view showing a prior art FRP propeller shaft; and  
         [0018]    [0018]FIG. 5( b ) is a schematic view showing the apex angle of a tooth of the serration shown in FIG. 5( a ). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    One embodiment according to the present invention will now be described with reference to FIGS.  1  to  3 . FIG. 1 is a cross-sectional view showing an FRP propeller shaft  11 . FIG. 2 is a side view of a yoke  13  with a half cut away. FIG. 3( a ) is an enlarged partial front view showing a serration  14 . FIG. 3( b ) is an enlarged partial cross-sectional view showing the engaging portions  13   a  of the serration  14  and an FRP pipe  12 .  
         [0020]    As shown in FIG. 1, the FRP propeller shaft  11  includes the FRP pipe  12  and a pair of metal members, which are the metal yokes  13  in this embodiment. Each yoke  13  is press fitted into one end of the pipe  12 . Each yoke  13  includes an engaging portion  13   a  and a joint portion  13   b . The engaging portion  13   a  is press fitted in the corresponding end of the FRP pipe  12 . The joint portion  13   b  is coupled to a universal joint (for example, a cross joint), which is used to couple the propeller shaft  11  with the drive shaft of the vehicle. A hole  13   c  is formed in the joint portion  13   b  (see FIG. 2). The universal joint is engaged with the hole  13   c . The engaging portion  13   a  of each yoke  13  is press fitted to an engaging portion  12   a  located at each end of the FRP pipe  12 . The yokes  13  are thus coupled to the FRP pipe  12 .  
         [0021]    The engaging portions  12   a  of the FRP pipe  12  are thicker than the remainder of the pipe  12 . The FRP pipe  12  is manufactured through the filament winding method (FW method). The reinforcing fibers of the pipe  12  are carbon fibers. The matrix resin is epoxy resin. Fibers are impregnated with resin and are wound about a mandrel. Then, the resin is hardened with heat. Thereafter, the mandrel is removed to form the FRP pipe  12 .  
         [0022]    The serration  14  having axially extending teeth  14   a  is formed on the outer surface of each engaging portion  13   a . The teeth  14   a  form axially extending grooves  12   c  (see FIG. 3( b )) on the end inner surface  12   b  of the FRP pipe  12 . As shown in FIG. 3( a ), the teeth  14   a  are formed at a predetermined pitch P along the circumferential direction. Each tooth  14   a  has a triangular cross-section.  
         [0023]    The apex angle θ of each tooth  14   a  is 60°. The connecting angle φ defined by any adjacent pair of the teeth  14   a  is substantially equal to the apex angle θ. Specifically, the difference between the apex angle θ and the connecting angle φ is from 0° to 5°. In this embodiment, the cross-section of the teeth  14   a  form a saw-tooth pattern.  
         [0024]    The outer diameter of each serration  14  is between 70 mm and 75 mm (in this embodiment, 71 mm). A predetermined number of teeth, which is between 142 and 145 (i.e., 142, 143, 144, or 145), are formed on the serration  14 . In this embodiment, the number of the teeth is  144 . The sides of each tooth  14   a  are represented by lines Ls in FIG. 3( a ). The distance H between the intersection of lines Ls of adjacent teeth  14   a  and the outer circumferential diameter line Ld of the serration  14  is between 0.9 mm and 1.8 mm. In this embodiment, the distance H is 1.25 mm. In this embodiment, the tooth height h is equal to the distance H.  
         [0025]    The distal tooth thickness T of the teeth  14   a  is equal to or less than 0.1 mm and the width W of the proximal end of the teeth  14   a  is 1.5 mm. In this embodiment, the distal tooth thickness T is 0.05 mm. The radial dimension of the portion of each tooth  14   a  that engages with, or digs into, the FRP pipe  12  is equal to or less than one fifth of the tooth height h. In this embodiment, the radial dimension of the digging portion is 0.15 mm. For purposes of illustration, the digging portions are exaggerated in FIG. 3( b ).  
         [0026]    The serration  14  of each yoke  13  is formed, for example, with a topping hob. Unlike a normal hob, the topping hob can machine the distal section of the teeth  14   a  to make the distal end narrow.  
         [0027]    The operations of the yoke  13 , which is constructed as above, will hereafter be described. When coupling the yokes  13  with the FRP pipe  12 , the FRP pipe  12  is fixed with a jig. The pipe  12  and the yoke  13  are aligned and the serration  14  is press fitted in the pipe  12  with a tool. When the serration  14  is press fitted, the teeth  14   a  enter the pipe  12  while forming the grooves  12   c  on the inner surface of the pipe  12 . The teeth  14   a  are firmly engaged with the grooves  12   c , which engages the yoke  13  with the pipe  12  at a high strength. When the yokes  13  are attached to the ends of the FRP pipe  12 , the manufacture of the propeller shaft  11  is completed.  
         [0028]    If the apex angle θ of the serration teeth  14   a  is approximately 90° as in the prior art, a great force is required to press fit the serration  14  into the FRP pipe  12 . However, in the above embodiment, the apex angle θ is 60° and the connecting angle φ (defined by the sides  14   b  of each adjacent pair of the teeth  14   a ) is substantially equal to the apex angle θ. This configuration reduces the force required for press fitting and guarantees the torsional torque transmitting capability between the FRP pipe  12  and the yokes  13 .  
         [0029]    The torsional torque transmitting capability of the FRP pipe  12  and the press fitting force were examined by using the yokes  13  of varied apex angles θ and varied tooth height h of the serration  14 . The examination revealed that in the range of the apex angle θ between 45° and 75°, the press fitting force and the torsional torque transmitting capability are satisfactory. If the apex angle θ is less than 45°, the strength of the teeth  14   a  is not sufficient. If the apex angle θ is greater than 75°, a relatively great press fitting force is required.  
         [0030]    The apex angle θ should be between 45° and 75°, preferably between 50° and 70°, more preferably between 55° and 65°.  
         [0031]    When the apex angle θ is 45°, and the tooth height is 1.7 mm, the width W of the tooth distal end is slightly less than that in a case where the apex angle θ is 60°. When the apex angle θ is 75°, and the tooth height is 0.95 mm, the width W of the tooth distal end is slightly greater than that in a case where the apex angle θ is 60°.  
         [0032]    This embodiment provides the following advantages.  
         [0033]    (1) The propeller shaft  11  includes the FRP pipe  12  and the metal yokes  13  attached to the ends of the pipe  12 . Each yoke  13  has the serration  14  with the teeth  14   a . The teeth  14   a  form the axially extending grooves  12   c  in the corresponding end of the pipe  12 . The apex angle θ of the teeth  14   a  is between 45° and 75°. Therefore, a force required when press fitting the serration  14  of each yoke  13  to an end of the FRP pipe  12  is reduced. Also, the torsional torque transmitting capability of the pipe  12  is improved.  
         [0034]    (2) The apex angle θ of each tooth  14   a  in the serration  14  is between 45° and 75°. The connecting angle φ (defined by an adjacent pair of the teeth  14   a ) is substantially equal to the apex angle θ. Therefore, a force required when press fitting the serration  14  of each yoke  13  to an end of the FRP pipe  12  is reduced. Also, the torsional torque transmitting capability of the pipe  12  is improved.  
         [0035]    (3) The radial dimension of the portion of each tooth  14   a  that digs into the FRP pipe  12  is equal to or less than one fifth of the tooth height h. Therefore, when press fitting the serration  14  of the yoke  13  into the FRP pipe  12 , the FRP pipe  12  does not receive excessive expanding force.  
         [0036]    (4) The outer diameter of the serration  14  is between 70 mm and 75 mm, and the number of the teeth  14   a  is between 142 and 145. Thus, when pressing fitting the serration  14 , the FRP pipe  12  does not receive excessive expanding force.  
         [0037]    (5) The serration  14  is formed such that the distance H between the intersection of adjacent lines Ls representing the sides  14   b  of the teeth  14   a  and the outer diameter line Ld of the serration  14  is between 0.9 mm and 1.8 mm. This configuration facilitates the machining of the serration  14 .  
         [0038]    (6) The serration  14  is formed such that the distal tooth thickness T of the teeth  14   a  is equal to or less than 0.1 mm (In this embodiment, the distal tooth thickness T is 0.05 mm). This configuration requires less press fitting force and makes the digging amount appropriate.  
         [0039]    (7) The serration  14  is formed such that the distal tooth thickness T of the teeth  14   a  is equal to or less than 0.1 mm and the width W of the proximal end of the teeth  14   a  is 1.5 mm. In this embodiment, the distal tooth thickness T is 0.05 mm. Therefore, a force required when press fitting the serration  14  of each yoke  13  to an end of the FRP pipe  12  is reduced. Also, the torsional torque transmitting capability of the pipe  12  is improved.  
         [0040]    It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.  
         [0041]    The proximal ends of an adjacent pair of the teeth  14   a  need not be continuous. As shown in FIG. 4, the proximal ends may be separated by a predetermined distance. The connecting angle φ defined by the sides  14   b  of the adjacent pair of the teeth  14   a  is substantially the same as the apex angle θ. This modification has the same advantages as the case where the teeth  14   a  have a saw-tooth cross-section.  
         [0042]    The sides  14   b  of each tooth  14   a  need not be linear as represented by lines Ls. Lines representing the sides  14   b  may be curved at the proximal end of the tooth  14   a . In other words, the facing sides  14   b  of each adjacent pair of the teeth  14   a  are connected through a curved plane. In this case, the apex angle θ refers to the angle defined by the linear sections of lines Ls.  
         [0043]    The sides  14   b  of the teeth  14   a  represented by lines Ls need not be linear. The entire sides  14   b  may be, for example, involute. If the ratio (h/W) of the tooth height h and the proximal width W is between 0.63 and 1.16, and the distal width is 0.05±0.02 mm, the force required for press fitting the serrations  14  to the ends of the pipe  12  is reduced, and the torsional torque transmitting capability is improved. If the sides  14   b  of each tooth  14   a  is flat and the ratio (h/W) is 1.16, the apex angle of each tooth  14   a  is approximately 45°. If the sides  14   b  of each tooth  14   a  are flat and the ratio (h/W) is 0.63, the apex angle is approximately 75°.  
         [0044]    In FIG. 4, sections of the sides  14   b  of each tooth  14   a  at the proximal end may be arcuate. In other words,. the sides  14   b  of each tooth  14   a  are curved in the vicinity of the proximal end. The connecting angle φ need not be substantially the same as the apex angle θ.  
         [0045]    In the illustrated embodiment, the yoke  13  includes the integrated engaging portion  13   a  and joint portion  13   b . The serration  14  is formed on the engaging portion  13   a . However, the engaging portion  13   a  and the joint portion  13   b  may be separately formed. The joint portion  13   b  may be welded or friction welded to the engaging portion  13   a  on which the serration  14  is machined. In this case, if a component used for conventional propeller shaft may be used as the joint portions  13   b , the manufacturing cost is reduced.  
         [0046]    In the modification where the yoke  13  is formed by welding the joint portion  13   b  to the engaging portion  13   a , the joint portion  13   b  may be welded to the engaging portion  13   a  after the engaging portion  13   a  is press fitted in the FRP pipe  12 .  
         [0047]    The radial dimension of the part of each tooth  14   a  that digs into the pipe  12  may be greater than one fifth of the tooth height. If the apex angle θ is approximately 45°, an amount of the digging portion that is greater than one fifth of the tooth height does not excessively increase the press fitting resistance and guarantees a sufficient torsional torque transmitting capability.  
         [0048]    In the illustrated embodiment, the serration  14  is formed by machining a metal pipe on which the joint portion  13   b  is formed. However, the serration  14  may be formed through cold or hot forging.  
         [0049]    Instead of the yokes  13 , metal shafts on which serration is formed may be press fitted in the FRP pipe  12 . In this case, the metal shafts function as the metal members.  
         [0050]    The FRP pipe  12  need not be entirely cylindrical. However, the FRP pipe  12  may be a polygonal prism with the ends of circular cross-section.  
         [0051]    The FRP pipe  12  may be manufactured through a method other than the filament winding method. For example, the FRP pipe  12  may be formed through sheet winding method. As long as the FRP pipe  12  has the required characteristics as a propeller shaft, the pipe  12  may be manufactured through any method. However, it is preferable that the pipe  12  be manufactured through filament winding.  
         [0052]    The reinforcing fibers and the matrix resin of the FRP pipe  12  need not be carbon fibers and epoxy resin. For example, other types of fibers that have high elasticity and high strength such as aramide fiber and glass fiber may be used as the reinforcing fibers. Thermosetting resin such as unsaturated polyester, phenol resin, and polyimide resin may be used as the matrix resin.  
         [0053]    The matrix resin of the FRP need not be thermosetting. For example, an ultraviolet curing resin or a thermoplastic resin may be used as the matrix resin.  
         [0054]    Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.