Patent Publication Number: US-2013239741-A1

Title: Rocker Arm Shaft with Improved Abrasion Resistance and Rocker Arm Shaft/Bush Assembly Comprising Same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This Application is a Section 371 National Stage Application of International Application No. PCT/KR2011/008760, filed Nov. 16, 2011 and published, not in English, as WO2012/067426 on May 24, 2012. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to a rocker arm shaft having a surface on which minute concave-convex portions are processed in order to improve abrasion resistance. In addition, the present disclosure relates to a rocker arm shaft/bush assembly including a rocker arm shaft having a surface on which minute concave-convex portions are processed, and a rocker arm bush which encloses the rocker arm shaft. 
     BACKGROUND OF THE DISCLOSURE 
     In general, in an engine, while a cam shaft is rotated by rotational force of a crank shaft, and intake and exhaust valves reciprocate up and down at predetermined time intervals by cams formed on the cam shaft, a process is repeatedly performed in which mixed gas of fuel gas and air is injected, compressed, and exploded in a combustion chamber, and power is obtained by explosion pressure. 
     A unit including a series of elements for operating the intake and exhaust valves such as the cam shaft, the cam, a cam follower (for example, a tappet), a push load, a rocker arm, a valve spring, a valve, and the like is called a valve train. 
       FIG. 1  illustrates a valve train according to the related art, a plurality of cams  2  is formed on a cam shaft  1  along an axial line at predetermined intervals, and a cam follower  5  is provided at a lower end portion of a push load  4  which is provided to be slidable up and down in an engine body block  3 . In addition, an upper end portion of the push load  4  is pivotally connected to one side of the rocker arm  6 , the other side of the rocker arm  6  is connected to an intake port or an exhaust port of a cylinder head block  7  to be pivotally connected to an upper end portion of a valve  9  which is elastically supported by a valve spring  8 . 
     A rocker arm shaft  10  and a rocker arm bush  11  are provided at the rocker arm  6  in order to move and support the rocker arm  6 . The rocker arm  6  may be reciprocated by the rocker arm shaft  10  and the rocker arm bush  11 , and here, the rocker arm shaft  10  and the rocker arm bush  11  reciprocate with respect to each other using engine oil. 
     The rocker arm shaft  10  and the rocker arm bush  11  continuously reciprocate by driving the engine, and thus may not be free from abrasion. Particularly, a time point when a motion velocity of the reciprocation of the rocker arm shaft  10  and the rocker arm bush  11  is zero occurs two times per one period, and when the motion velocity becomes zero, a lubricative film is not formed according to a theory of lubrication. Because an oil film is not formed in the aforementioned stopped-acceleration state, particularly serious friction and abrasion occur on the rocker arm shaft and the rocker arm bush. In a case in which a long time operation is performed under this operational condition, a gap between the rocker arm shaft and the rocker arm bush is increased, and accordingly, a change in valve gap which may affect performance of an internal combustion engine occurs and noise and vibration become serious. 
     In the related art, researches have been conducted mainly in a direction of improving materials and ingredients of bushes in order to improve abrasion resistance of the rocker arm shaft and the rocker arm bush. However, the method of improving materials and ingredients causes an increase in manufacturing cost, and particularly, as an operational environment of the engine is recently changed to a direction which causes more abrasion of the rocker arm shaft and the rocker arm bush, there is a limitation in reducing abrasion only by improving materials and ingredients. 
     As such, in order to improve performance of the engine, or the like, it is necessary to reduce abrasion of the rocker arm shaft and the rocker arm bush, and particularly, it is necessary to improve abrasion resistance by reducing abrasion of the rocker arm shaft and the rocker arm bush by using a more economical method in view point of economy than the method of improving materials and ingredients. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
     SUMMARY 
     This summary and the abstract are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. The summary and the abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter. 
     Accordingly, the present inventors have conducted a research regarding friction between two surfaces which are relatively moved using lubricant, and abrasion due to the friction, and a research regarding a method capable of reducing abrasion due to the friction. 
     The present disclosure has been made to provide a method of reducing abrasion of a rocker arm shaft and a rocker arm bush by using a comparatively simple method without causing high costs such as a change in materials. In addition, the present disclosure has been made to provide a rocker arm shaft and an assembly of a rocker arm shaft and a rocker arm bush with improved abrasion resistance, which may be manufactured at a comparatively low manufacturing cost and in a short manufacturing period of time. 
     The present disclosure has been made to provide a rocker arm shaft on which concave-convex portions are formed to reduce abrasion of a rocker arm shaft and a rocker arm bush. 
     In addition, the present disclosure has been made to provide a rocker arm shaft in which a position and a shape of the concave-convex portion are optimized in order to maximize an effect of improving abrasion. 
     The present disclosure provides a rocker arm shaft having a surface on which concave-convex portions are formed to improve abrasion resistance. 
     An exemplary embodiment of the present disclosure provides a rocker arm shaft  10  which is accommodated in a rocker arm bush  11  and configured to be relatively moved with respect to the rocker arm bush  11  using lubricant so as to allow a rocker arm  6  to reciprocate, the rocker arm shaft  10  including: a plurality of concave-convex portions  21  having groove shapes formed on at least a portion of an outer surface  20  of the rocker arm shaft  10 , which is in contact with the rocker arm bush  11 . 
     Meanwhile, the concave-convex portions may be formed on the entire outer surface  20  of the rocker arm shaft  10 , but may be formed only on a part of the outer surface  20  of the rocker arm shaft  10 . That is, the concave-convex portions  21  may not need to be formed on the entire outer surface  20  of the rocker arm shaft  10 , and may be formed only on a portion where the rocker arm shaft  10  and the rocker arm bush  11  frequently come into contact with each other. 
     According to an exemplary embodiment of the present disclosure, a ratio of a region of the outer surface  20  of the rocker arm shaft  10 , where the concave-convex portions  21  are formed, may be 20% to 50% of an area of the entire outer surface  20  of the rocker arm shaft  10 . 
     According to another exemplary embodiment of the present disclosure, as illustrated in  FIG. 3 , the concave-convex portions  21  may be formed on both sides based on a corresponding portion  30  of the rocker arm shaft  10 , where a maximum line contact load is generated with the rocker arm bush  11  when a valve connected through the rocker arm  6  is opened by 50% (see  40 ). Here, a portion on which the concave-convex portions are formed may be formed on a portion corresponding to an arc of a sector of which a central angle is 72° to 180° based on a center of the portion where the maximum line contact load is generated. 
     Meanwhile, according to an exemplary embodiment of the present disclosure, a ratio of a sum of surface areas occupied by the concave-convex portions  21  in a region on which the concave-convex portions  21  are formed may be 5% to 30%, which may be easily understood with reference to  FIG. 6  which illustrates the concave-convex portion having the dashed line. 
     According to an exemplary embodiment of the present disclosure, as illustrated in  FIG. 6 , the concave-convex portion  21  may have a dashed line shape, and a long side of the dashed line may be disposed in parallel to an axial direction of the rocker arm shaft  10 . In the concave-convex portion having the dashed line shape, a depth of the dashed line may be 10 μm to 30 μm, and a length of a long side of the dashed line may be in a range from 100 μm to 500 μm. In this case, a ratio of surface areas occupied by portions of the dashed lines to the region on which the dashed lines are formed may be 5% to 30%. The ratio of surface areas occupied by portions of the dashed lines in the region on which the dashed lines are formed may be called density, and it may be understood that the expression “density” herein refers to “compactness of disposition of dashed lines”. The density may be calculated by the following Formula 1. 
       Density=( La×Lb )/{( La+Lc )×( Lb+Ld )}  &lt;Formula 1&gt;
 
     Here, La refers to a length (thickness) of a short side of the dashed line, Lb refers to a length of a long side of the dashed line, Lc refers to an interval between the dashed lines in a thickness direction, and Ld refers to an interval between the dashed lines in a length direction. 
     In addition, the concave-convex portion  21  according to yet another exemplary embodiment of the present disclosure may be provided as a circular groove. 
     The concave-convex portions  21  may be formed on both sides based on a corresponding portion of the rocker arm shaft  10 , where a maximum line contact load is generated with the rocker arm bush  11  when a valve connected through the rocker arm  6  is opened by 50%, and a portion on which the concave-convex portions  21  are formed may be formed on a portion corresponding to an arc of a sector of which a central angle is 72° to 180° based on the portion  30  where the maximum line contact load is generated. 
     In addition, the concave-convex portion  21  may be provided as the circular groove, and a diameter of the groove may be 100 μm. 
     In addition, the concave-convex portion  21  may be provided as the circular groove, and a depth of the groove may be 10 μm to 20 μm. 
     In addition, the concave-convex portion  21  may be provided as the circular groove, and an interval of the groove may be 350 μm to 450 μm. 
     In addition, the present disclosure provides a rocker arm shaft/bush assembly including the described rocker arm shaft  10  and a rocker arm bush  11  in which the rocker arm shaft  10  is accommodated. 
     In addition, the present disclosure provides a rocker arm including the rocker arm shaft/bush assembly. 
     According to the present disclosure, abrasion between the rocker arm shaft and the rocker arm bush may be remarkably reduced by forming the minute concave-convex portions on a surface of the rocker arm shaft. In the present disclosure, in order to maximize the effect of improving abrasion, a shape, a size, and a position of the minute concave-convex portion are optimized, thereby maximizing the effect of reducing abrasion without consuming excessive manufacturing costs and time. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating an example of a valve train to which a rocker arm shaft  10  and a rocker arm bush  11  according to the present disclosure are applied. 
         FIG. 2  is a view illustrating cross sections of the rocker arm shaft and the rocker arm bush of  FIG. 1 . 
         FIG. 3  a view illustrating a portion  30  of the rocker arm shaft  10 , where a maximum line contact load is generated with the rocker arm bush  11 , when a valve of a cylinder is opened by 50% at a rocker arm  6  to which the rocker arm shaft  10  and the rocker arm bush  11  according to the present disclosure are applied. 
         FIG. 4  is a perspective view illustrating the rocker arm shaft  10  according to an example of the present disclosure. 
         FIG. 5  is a view illustrating a reduction ratio of an amount of abrasion in a case in which concave-convex portions are formed in comparison with a case in which concave-convex portions are not formed on the rocker arm shaft  10 , and illustrates a relationship between a region with the concave-convex portions indicated by an angle and a reduction in amount of abrasion. 
         FIG. 6  relates to an exemplary embodiment of the present disclosure in which the concave-convex portions are formed in a dashed line shape, and is an enlarged view of the concave-convex portions having a dashed line shape. 
         FIG. 7  is an explanatory view regarding a size of a contact region between the rocker arm shaft and the rocker arm bush. 
         FIG. 8  relates to another exemplary embodiment of the present disclosure in which the concave-convex portions are formed in a circular groove shape, and is an enlarged view of the concave-convex portion having a circular groove shape. 
     
    
    
     DESCRIPTION OF MAIN REFERENCE NUMERALS OF DRAWINGS 
       2 : Cam 
       5 : Cam follower 
       6 : Rocker arm 
       10 : Rocker arm shaft 
       11 : Rocker arm bush 
       12 : Friction region on which a minute hole is not formed 
       20 : Outer surface of rocker arm shaft 
       21 : Concave-convex portion 
       30 : Center of direction of resultant force 
       41 : Concave-convex portion forming region 
     D: Gap 
     M: Relative motion of shaft 
     DETAILED DESCRIPTION 
     Hereinafter, the present disclosure will be described in more detail with reference to the drawings. 
     The present disclosure relates to a technology which reduces friction and abrasion by processing minute concave-convex portions on at least one surface among two surfaces which are relatively moved using lubricant. 
     It is a well-known fact from the theory of fluid lubrication that if two surfaces are parallel to each other, fluid dynamic pressure is not generated in the lubricant even though the two surfaces are relatively moved using liquid lubricant. There is an exception, but the fluid dynamic pressure is generally generated when there is a wedge effect in which a thickness of an oil film is reduced along a sliding direction. Taking a dynamic pressure thrust bearing and a journal bearing as examples, the thrust bearing generates the wedge effect by an assembly error, and the journal bearing generates the wedge effect by an eccentricity ratio. 
     However, in general, a mechanical work piece has minute flexure and surface flexure due to surface roughness. Accordingly, a region is present in which a thickness of an oil film is locally reduced along a sliding direction even though two surfaces are relatively moved in parallel to each other, and oil film pressure generated in the region improves lubricity between the two surfaces. However, a region is also present in which a thickness of the oil film is increased along the sliding direction, and pressure in this region is similar to ambient pressure because bubbles are generally generated in this region. Therefore, if a plurality of minute concave-convex portions is processed on at least one surface of two surfaces which are relatively moved, fluid dynamic pressure is generated between the two surfaces even though the two surfaces are relatively moved in parallel to each other, thereby improving lubricity. In addition, the aforementioned surface minute concave-convex portions trap abraded particles or serve as minute oil storages. 
     Accordingly, in the present disclosure, a plurality of concave-convex portions  21  having groove shapes is formed on at least a portion of an outer surface  20  of a rocker arm shaft  10  which is accommodated in a rocker arm bush  11  and relatively moved with respect to the rocker arm bush  11  so as to allow a rocker arm  6  to reciprocate. 
     Meanwhile, in order to more efficiently reduce friction and abrasion, a shape, a size, an arrangement method and the like of the concave-convex portions are also important. A shape, an arrangement and a size of the concave-convex portion, which minimize friction and abrasion, are greatly influenced in accordance with an operating condition such as a contact shape between two surfaces, a load, a sliding velocity, or the like. For example, a shape and an arrangement method of the concave-convex portion, which minimize friction and abrasion, are changed in accordance with whether a shape of a contact portion is a line shape, a spot shape, or a surface shape. 
     Accordingly, in the present disclosure, a shape, a size, an arrangement, and the like of the concave-convex portion are optimized to meet operational properties of the rocker arm shaft  10  and the rocker arm bush  11 . In addition, in the present disclosure, the concave-convex portion is formed only on an optimum region because manufacturing costs are increased when the concave-convex portion is formed on unnecessary portions. 
     As an example of various exemplary embodiments of the present disclosure, in  FIGS. 4 and 6 , the concave-convex portions are formed on a surface of the rocker arm shaft  10  in dashed line shapes parallel in a direction vertical to a friction motion direction of the rocker arm shaft  10 . Here, the direction vertical to the friction motion direction of the rocker arm shaft  10  may be an axial direction of the rocker arm shaft  10 . 
     Meanwhile, the concave-convex portions trap abrasive particles which accelerate abrasion between the rocker arm shaft  10  and the rocker arm bush  11 , serve to supply lubricant in a situation in which the lubricant is insufficient, and serve to increase oil film pressure between the rocker arm shaft  10  and the rocker arm bush  11 , thereby having an advantage of reducing abrasion. 
     As described above, it is important that the concave-convex portions are formed only on optimum regions because manufacturing costs are increased when the concave-convex portions are formed on unnecessary portions. According to the present disclosure, a sufficient effect of reducing abrasion may be achieved even when the concave-convex portions  21  are formed only in a region of about 20 to 50% of the entire region of the outer surface  20  of the rocker arm shaft  10 . Then, a portion where the concave-convex portions are formed is important. 
     In this regard, the rocker arm shaft  10  and the rocker arm bush  11 , which are illustrated in  FIG. 1 , come into contact with each other only at specific regions according to properties thereof. As illustrated in  FIG. 2 , a center of the contact region in a circumferential direction becomes a center of a portion where the rocker arm shaft  10  and the rocker arm bush  11  come into contact with each other along a direction of resultant force of forces applied to the rocker arm. Based on the center, the rocker arm shaft  10  and the rocker arm bush  11  are moved in a direction of the arrow indicated by “M” in  FIG. 2 . 
     Accordingly, in an example of the present disclosure, a center  30  in a direction of resultant force of forces applied to the rocker arm is calculated based on when an opening amount of a valve, which is adjusted by the rocker arm, is 50%, and a processing region of the minute concave-convex portions is determined based on the center, thereby maximizing efficiency of processing of minute concave-convex portions (see  FIG. 3 ). 
     According to an example of the present disclosure, as illustrated in  FIG. 3 , the concave-convex portions may be formed on both sides based on the corresponding portion  30  of the rocker arm shaft  10 , where a maximum line contact load is generated with the rocker arm bush  11  when the valve connected through the rocker arm  6  is opened by 50%. Here, the portion on which the concave-convex portions are formed is formed on a portion  40  corresponding to an arc of a sector of which a central angle is 72° to 180° based on the center portion  30  where the maximum line contact load is generated. 
     Specifically, as illustrated in  FIG. 2 , a gap D is present between the rocker arm shaft  10  and the rocker arm bush  11  because of a difference between an outer diameter of the rocker arm shaft  10  and an inner diameter of the rocker arm bush  11 . A size of the contact region between the rocker arm shaft  10  and the rocker arm bush  11  may be changed in accordance with the gap D. Therefore, the concave-convex portion forming region considering the contact region needs to be calculated in consideration of a load applied between the rocker arm shaft  10  and the rocker arm bush  11 , and elastic deformation of the rocker arm shaft  10  and the rocker arm bush  11 . For example, when the gap D between the rocker arm shaft  10  and the rocker arm bush  11  is 10 μm, the rocker arm shaft  10  and the rocker arm bush  11  come into contact with each other in a region of 180°, and when the gap is 50 μm, the rocker arm shaft  10  and the rocker arm bush  11  come into contact with each other in a region of about 72°. 
     As such, a processing region of the concave-convex portions is determined by analyzing operational properties and physical phenomena of the rocker arm shaft  10  and the rocker arm bush  11 . 
       FIG. 5  illustrates a measurement result of a reduction ratio of an amount of abrasion with respect to an area on which the concave-convex portions are formed in comparison with a case in which the concave-convex portions are not formed on the rocker arm shaft  10 . Here, the angle corresponds to a range of the concave-convex portion forming region  40  formed in an arc shape of a sector when seen from a cross section of the rocker arm shaft  10 , and a structure of the concave-convex portions follows a size of Example 10 disclosed in the following Table 1. Referring to  FIG. 5 , in a case in which the angle of the arc corresponding to the concave-convex portion forming region  40  is smaller than 72°, because an effect of reducing an amount of abrasion is not great, the angle of the forming region of the concave-convex portions  21  is set to be equal to or greater than 72°. Meanwhile, it may be known that the effect of reducing an amount of abrasion is no longer increased even when the angle is greater than 180°, and therefore it is not necessary to consume additional manufacturing costs and time by unnecessarily forming the concave-convex portions up to the range equal to or greater than 180°. 
     In an example of the present disclosure, which is illustrated in  FIG. 6 , the concave-convex portion  21  has a dashed line shape, and a long side of the dashed line is disposed in parallel to an axial direction of the rocker arm shaft  10 . The concave-convex portion having the dashed line shape is easily processed and economical, and has an excellent effect of trapping abrasive particles, supplying lubricant, and increasing oil film pressure. 
     Here, a depth of the dashed line may be 10 to 30 μm, and a length of a long side of the dashed line may be in a range from 100 to 500 μm. In this case, a ratio of surface areas occupied by portions of the dashed lines in the region on which the dashed lines are formed is 5 to 30%. The ratio of surface areas occupied by portions of the dashed lines in the region on which the dashed lines are formed may be also called density, and it is understood that the expression “density” herein refers to “density of disposition of dashed lines”. The density may be calculated by the following Formula 1. 
       Density=( La×Lb )/{( La+Lc )×( Lb+Ld )}  &lt;Formula 1&gt;
 
     Here, La refers to a length (thickness) of a short side of the dashed line, Lb refers to a length of a long side of the dashed line, Lc refers to an interval between the dashed lines in a thickness direction, and Ld refers to an interval between the dashed lines in a length direction. 
     A ratio of a sum of surface areas occupied by the concave-convex portions  21  in the region on which the concave-convex portions  21  are formed may be 5 to 30%. This may be easily understood with reference to  FIG. 6  which illustrates the concave-convex portion having the dashed line shape. For reference, Le of  FIG. 6  refers to a depth of the concave-convex portion. In a case in which the concave-convex portion having the dashed line shape is formed at about the aforementioned depth, there is an excellent effect of trapping abrasive particles, supplying lubricant, and increasing oil film pressure. 
     In the present disclosure, in order to confirm the effect of improving abrasion between the rocker arm shaft  10  and the rocker arm bush  11  by the concave-convex portion having the dashed line, an abrasion test was performed with respect to parameters, as presented in the following Table, by setting as design parameters the length La of the short side of the concave-convex portion having the dashed line shape, the length Lb of the long side, the interval Lc between the dashed lines in a thickness direction, and the interval Ld between the dashed lines in a length direction. As a Comparative Example, a case in which no concave-convex portion is formed was used as an example. 
     Here, the density was calculated according to Formula 1 disclosed above. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Amount of 
               
               
                   
                 Lb (μm) 
                 Le (μm) 
                 Density (%) 
                 abrasion (μm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 100 
                 10 
                 5 
                 3.93 
               
               
                 Example 2 
                 100 
                 20 
                 13 
                 2.86 
               
               
                 Example 3 
                 100 
                 30 
                 21 
                 5.13 
               
               
                 Example 4 
                 300 
                 10 
                 13 
                 3.05 
               
               
                 Example 5 
                 300 
                 20 
                 21 
                 3.33 
               
               
                 Example 6 
                 300 
                 30 
                 5 
                 3.52 
               
               
                 Example 7 
                 500 
                 10 
                 21 
                 3.01 
               
               
                 Example 8 
                 500 
                 20 
                 5 
                 3.74 
               
               
                 Example 9 
                 500 
                 30 
                 13 
                 3.43 
               
               
                 Example10 
                 300 
                 20 
                 13 
                 2.57 
               
               
                 Comparative 
                 — 
                 — 
                 — 
                 7.05 
               
               
                 Example 
               
               
                   
               
            
           
         
       
     
     The test for Table 1 was designed and performed based on an experimental design method, and an amount of abrasion was represented. Here, a processing range of the minute concave-convex portions was 180°. The measurement of an amount of abrasion (Wear) was performed by measuring abrasion depths, which is typically performed. 
     In Table 1, Comparative Example is a test result with respect to a case in which the minute concave-convex portion is not processed, and when comparing Example 10 with Comparative Example, an amount of abrasion in Example 10 was reduced by 60% or more compared to Comparative Example. The effect of improving abrasion in other Examples was also excellent. That is, it was confirmed that when an opening amount of the valve is 50%, and the concave-convex portions having the dashed line groove shape, of which the length of the long side is 0.3 mm, the depth is 0.02 mm, and the density is 13%, are processed on the outer circumferential surface of the rocker arm shaft  10  in a region of 180° based on a direction of force of the rocker arm bush applied to the rocker arm shaft, abrasion may be reduced by 60% or more. 
       FIG. 8  illustrates circular grooves provided on an outer circumferential surface of a rocker arm shaft  10  according to yet another exemplary embodiment of the present disclosure. As illustrated in  FIG. 8 , the minute concave-convex portions according to the present disclosure are circular minute concave-convex portions having a rectangular arrangement on a sliding surface where friction is generated. 
     The groove may be provided to have a diameter of 100 μm to 150 μm, a depth of 10 μm to 20 μm, and an interval of 350 μm to 450 μm. 
     In general, in an internal combustion engine, an operational environment of the rocker arm assembly has a type in which rocker arm bush linearly reciprocates on a sliding surface in a state of being inserted based on a center axis of the rocker arm shaft. Lubricative performance of a lubricative surface formed on the sliding surface, which reciprocates, deteriorates in a region where a direction of motion is changed due to physical properties thereof, and accordingly, generation of abrasion on the lubricative surface is accelerated. 
     However, the rocker arm shaft including circular grooves according to yet another exemplary embodiment of the present disclosure greatly improves lubricative performance of a lubricative surface of the rocker arm assembly, which reciprocates, by the circular grooves, thereby remarkably reducing an amount of generation of abrasion. 
     The present disclosure may be applied to the rocker arm shaft which may obtain an effect of reducing abrasion without consuming excessive manufacturing costs and time by forming minute concave-convex portions on a surface of the rocker arm shaft. 
     Although the present disclosure has been described with reference to exemplary and preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the disclosure.