Abstract:
A shaft coupler to connect two coaxial segments of a drive shaft. Each segment includes a tongue, which tongues complement each other to transmit torque and resist axial separation. A bearing surface on each tongue lies within the locking angle of their material so as to resist lateral separation of the segments.

Description:
FIELD OF THE INVENTION 
   A coupler to join two segments of a shaft to hold them together for mutual rotation and to resist their axial separation. 
   BACKGROUND OF THE INVENTION 
   The joinder of two shaft segments has been the subject of extensive development. The objective is the same in all cases, which is to join two shaft segments so that one can drive the other, and so they can not be pulled apart. The general concept is somehow to hold the segments together by means such as engaging threads, or headed shapes which are somehow engaged. Relative rotational movement of the segments is often attended to by splined engagements. 
   Despite widespread development, problems continue to arise to be solved. Some of these are the problems of wear, fatigue, and stress concentrations in the shafts, all of which should be minimized or avoided entirely. It is an object of this invention to provide a simply constructed coupler devoid of stress concentration points in the assembly, which tends to be self-centering to a stable position, all in a simple construction. 
   BRIEF DESCRIPTIONS OF THE INVENTION 
   This invention is for the coaxial coupling together of a  first shaft segment and a second shaft segment, which when joined are drivingly joined. The segments are complementary to each other. Each has a terminal end and a cylindrical peripheral wall contiguous to the terminal end. When joined, the terminal ends interengage, primarily to resist axial separation. 
   Each peripheral wall includes an axially extending groove into which a respective key is fitted. A coupler sleeve fits over both of the peripheral walls. It includes an axial slot or slots that receives or receive the keys (which project into it or into them), to resist counter-rotation of the segments. 
   Each terminal end has a transverse receiver recess and a tongue. It has a base wall, a bearing wall and a transition wall interconnecting the base wall and the bearing wall as a smooth curve which is tangent to both of them. 
   A tongue wall tangent to the bearing wall extends as a smooth curve toward the end of the tongue. The absence of points of stress concentration such as intersections of planar surfaces at an angle to each other, or line contact between surfaces will be noted. Beyond the bearing wall, the shape of the tongue to its end is arbitrary, except when one considers the situation when the axial force is reversed, and the segments are in compressive load with one another. Then the base wall and the terminal wall of the tongue can be formed to make a surface-to-surface contact with one another.  
   Both bearing walls are flat surfaces which are disposed at equal angles to the central axis. According to a preferred but optional feature of the invention, this angle is within the locking range of the materials and surfaces of which the tongue is formed. 
   The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an axial view partly in cross-section, showing the presently preferred embodiment of the invention; 
       FIG. 2  is a cross-section taken at line  2 — 2  in  FIG. 1 ; 
       FIG. 3  is a side view of one of the segments; 
       FIG. 4  is an exploded view of  FIG. 3 ; 
       FIG. 5  is a right hand view taken at line  5 — 5  in  FIG. 3 ; 
       FIG. 6  is an axial view partly in cross-section, showing a prior art coupler; and 
       FIG. 7  is a view similar to  FIG. 6 , showing another prior art coupler. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A coupled shaft  10  according to this invention is shown in  FIG. 1 . It includes a first shaft segment  11 , a second shaft segment  12  and a sleeve  13 . The segments when assembled are concentric on a central axis  14 . The coupling is intended to  resist axial separation along the central axis, and to provide for rotation around the central axis. 
   A common use for this assembly is when the first shaft segment is the output shaft from a motor, and the second shaft segment is the drive shaft for a turbine rotor. In one example, a separative force of about 4,000 pounds is resisted. While the shafts are supported by adequate bearings, torque loads can vary, and oscillating forces can exist. It is much to be preferred for the coupling to provide substantial bearing areas to transmit axial forces, to resist lateral movement between the terminal ends of the shaft segments, and to provide structure which tends to maintain a specific rotational relationship between the segments. All of this is accomplished by this invention. 
   Shaft segments  11  and  12  are identical in all pertinent features, so only shaft segment  11  will be described in detail. It includes a terminal end  15  and a cylindrical peripheral wall  16 . End  17  may be considered to be either the driven or driving end of the coupled assembly. 
   An axially extending groove  18  is formed in wall  16 . It has a depth  19  and a dimension of width  20 . This groove will accept in close fitting relationship a key  25  with a width about equal to the width of the groove, and a height greater than the depth of the groove so it will project radially above the peripheral wall.  
   Sleeve  13  has an internal cylindrical wall  31  to form a passage  32  from end  33  to end  34  of the sleeve. Axially extending slots  35  and  36  are formed in wall  31  each with a width about equal to the width of the key. The diameter of the passage is about equal to the outer diameter of peripheral wall  16 . Accordingly, the sleeve, the segments, and the keys, make a good fit with each other to restrain the segments against counter rotation. It will be noted that when the identical segments are assembled, the keys will be on opposite sides of the assembly. For this reason two slots are formed, 180 degrees apart. Should only one slot be desired, then the groove in one of the segments must be on its opposite side. 
   It is most convenient for slots  35  and  36  to open at both ends of the sleeve. Then the sleeve is reversible and available for all assortments of shafts and from both ends. However, for any specific arrangement, opening at only one end is acceptable if assembly from only one end is acceptable. Also, more than two slots may be used, perhaps with shorter keys if desired to reduce the load on the individual keys. 
   Restraint  40  comprises a ring groove  41  formed in the outer peripheral wall, and a snap ring  42  in this groove that will hold the sleeve against axial retraction. 
   The terminal end of the shafts is the critical part of this invention. It is formed in a complementary manner so that the  same structures on both segments will engage one another. By “complementary” is meant surface-to-surface contact of similarly shaped surfaces. 
   The terminal end begins with a transverse receiver recess  50 . It starts with a base wall  51  that extends across the central axis, preferably as developed by a straight surface generator that is maintained normal to the center axis. It terminates at a curved transition wall  52  which is tangent to it. It has a radius  53  which generates a smooth transition to a bearing wall  54 , to which it is tangent. The reader will recognize the smooth transition from the base wall into the bearing wall. There will be no abruptly singular line contact with a next surface. 
   Bearing wall  54  is a flat planar surface having a substantial area between its edges  56  and  57 , which are spaced by a dimension  55 . It is disposed at an angle  58 , which is of significant importance to this invention. 
   A tongue wall  60  is a curved structure generated by a straight line generator that is preferably tangent to the bearing surface at edge  57 , and which extends toward the free end of the segment. It need not reach the very end of the segment. 
   The principal resistance to axial separation of the segments is the mutual contact of the bearing walls. These involve substantial areas of full contact at angle  58 . Obviously if this  angle is too great, there will be a lateral separation force component tending to cause lateral movement between the terminal ends. This can exert an enlarging expansive force on the sleeve. 
   However, if angle  58  is kept within the locking range of the materials of construction, the more forceful the axial separation forces, the stronger will be the resistance to lateral separation. This is the consequence of the classical relationship between the coefficient of friction, the angle, and the applied forces. 
   Speaking generally, for bearing surfaces such as would be used in steels, an angle of about 15 degrees or less would be well within the “locking range”. Useful dimensions for bearing wall  54  for a steel segment are about ⅛ inch by 1{fraction (7/16)} inches, with angle  58  about 15 degrees. This readily attends to axial loads of well over 4,000 pounds. 
   With the above in mind, the reader may advantageously study  FIGS. 1–4 , and notice that no twist or pull can develop a localized stress concentration. All surfaces smoothly fair into one another. There is no line contact which could create a local region of high stress. 
   There is an additional advantage to this construction. When the two planar bearing surfaces are rotationally aligned and brought together, they will establish the rotational positions of the segments as well as their axial location. Any rotational  movement from that central orientation would require an axial displacement of the shafts relative to one another. The tendency of the shafts when brought toward one another is to minimize their separation. Then when brought together, counter rotation requires axial respective movement. But this is opposed by the keys, and by the separation force exerted by the user device. In one situation the separation force will be generated by the turbine wheel. 
   In addition, the keys resist counter rotation. In every situation any tendency toward counter rotation is doubly resisted. When that force is removed, such as when the system is shut down, forces during re-start are all channeled toward the stable situation shown. 
   The shape of terminal end  15  from edge  57  of the bearing wall is of lesser importance, because, with proper design it will resist separative forces, and will not bend so as to pull the bearing surface in an axial direction. Preferably it will be designed as shown with a substantial body that resists deformation. 
   Another feature of this construction is abutment surface  65  at the free end  15 . It preferably includes a substantial area that is flat and normal to the central axis.  FIG. 3  schematically shows a dimension  66  that is the consequence of manufacturing tolerances. It is the amount of relative axial  movement of these segments between when they are forced apart and when they move together. The first is when a turbine wheel shows, idles, or stops. 
   When it stops, the segments abut at surfaces  51  and  65 . This is a solid contact. 
     FIGS. 6 and 7  show previous efforts to form a stable coupling. In  FIG. 6  two end caps  70 ,  71  are threaded onto shaft segment  72 ,  73  to hold a sleeve  74  to them through rings  75 ,  76 . Keys  77 ,  78  are keyed to the sleeve. 
     FIG. 7  shows segments  80 ,  81  with heads  82 ,  83  held together by a clip  84 . Keys  85 ,  86  hold the segments to the sleeves. 
     FIGS. 6 and 7 , the parts are more numerous, the shapes more complicated and stressed, and the assemblies are less reliable. 
   This invention is not to be limited by the embodiment shown in the drawings and described in the description, which is given by way of example and not of limitation, but only in accordance with the scope of the appended claims.