Patent Publication Number: US-6905280-B2

Title: Apparatus and method for securing a hub to a rotatable shaft

Description:
TECHNICAL FIELD 
   The present invention relates to apparatus and method for securing a hub to a rotatable shaft; more particularly, to such apparatus and method wherein the hub must first be free to rotate on the shaft to permit rotational indexing of the hub to the shaft and then must be secured to the shaft without altering the indexing; and most particularly, to apparatus and method for securing a throttle position sensor to a throttle shaft after the throttle rotary position and the sensor are independently calibrated and the sensor is thus indexed to the position of the throttle shaft. 
   BACKGROUND OF THE INVENTION 
   In the automotive art, it can be desirable to monitor the rotary position of an engine throttle shaft as an input operating parameter to an engine control module. In the prior art, typically the lever arm of a variable-voltage sensor has a hub having an axial bore including an internal flat that is slipped onto the end of a throttle shaft provided with a mating external flat, such that the hub, once installed onto the shaft, cannot rotate independently of the shaft. Typically, the mating of the hub to the shaft is deliberately slightly loose to facilitate assembly and to allow for stack-up of manufacturing variances in the various components. In modern engine control, however, the resulting rotational play between the hub and the shaft can be unacceptably large, leading to imprecise determination of the actual position of the throttle shaft and hysteresis in the control loop. 
   A further disadvantage of this connecting scheme is that the rotary relationship between the throttle shaft and the sensor is fixed for all throttle shafts and sensors by the design placement of the flats and does not allow for individual, independent calibration of throttle flow and sensor output on any given engine prior to fixing the rotary relationship between the shaft and the sensor. 
   It is highly desirable to be able to set the throttle position at a predetermined air flow and to independently set the sensor at a predetermined output after the sensor hub has been slipped onto the throttle shaft, and then to immobilize the indexed relationship by securing the sensor hub to the throttle shaft. Such immobilization by prior art means can be expensive, as in for example the case of laser welding or a set screw or adhesives, and/or can jeopardize the delicate rotary relationship just established, as in the case of staking or crimping of the hub onto the shaft. 
   Therefore, there is a strong need for a simple, inexpensive, reliable means for immobilizing a hub onto a shaft without inducing rotational forces therebetween, while previously allowing relative rotary motion therebetween as may be necessary to index the hub to the shaft. 
   It is a principal object of this invention to provide an improved apparatus for immobilizing a hub onto a rotatable shaft without inducing rotational forces therebetween, to cause fixed rotation of the hub and shaft together, while allowing relative rotational motion therebetween prior to such immobilizing, as may be necessary to index the hub to the shaft. 
   SUMMARY OF THE INVENTION 
   Briefly described, the present invention is directed to an improved apparatus for rotationally immobilizing a hub on a rotatable shaft. The portion of the shaft for receiving the hub is cylindrical and has a first outer diameter. The hub has an axial, cylindrical bore having a second diameter larger than the diameter of the shaft over an axial width of the hub. An axially-extending keyway is provided in the shaft in the hub-receiving region. When the hub-receiving region of the shaft has been inserted into the hub axial bore, the hub is free to rotate upon the shaft. To immobilize the hub rotationally with respect to the shaft, a tapered key having a height greater than the depth of the keyway is inserted axially into the keyway as a radial jam fit against the hub. Preferably, the key is inserted to a point at which the maximum height of the key is positioned at the axial midpoint of the hub bore. Preferably, the hub is formed of a deformable polymer and the key is formed of a hard substance, such as metal, and is provided with at least one relatively sharp corner such that the key incises the hub bore as it moves along the keyway, thus rigidly locking the hub to the shaft. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention, as well as presently preferred embodiments thereof, will become more apparent from a reading of the following description in connection with the accompanying drawings in which: 
       FIG. 1  is a cross-sectional view of a shaft, hub, and locking key in accordance with the invention; 
       FIG. 2  is a cross-sectional view of the apparatus shown in  FIG. 1 , taken along line  2 — 2  in  FIG. 1 ; 
       FIG. 3  is a view like that shown in  FIG. 2  of an alternative embodiment of the invention; and 
       FIG. 4  is an elevational view of a chain of locking keys in accordance with the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 ,  2 , and  4 , a rotatable shaft  10  has a cylindrical portion  12  having a first diameter  14 . Portion  12  is provided with an axially-extending slot or keyway  16  having preferably a rectangular cross-sectional profile and having a wall depth  18  extending from the surface of portion  12  to the bottom  19  of keyway  16 . 
   Hub  20  is provided with an axial bore  22  having a second diameter  24  greater than first diameter  14  and an axial length  25  less than the axial length of shaft portion  12  such that portion  12  may be inserted into bore  22 . (Note: the difference between diameters  14  and  24  need be only a few thousandths of an inch, and are shown exaggerated for clarity in  FIG. 1 ) Hub  20  is free to rotate about shaft portion  12 . When hub  20  is urged radially against shaft portion  12  at a point  13  on the shaft diametrically opposite from keyway  16 , a maximum radial distance  27  is established between keyway bottom  19  and the adjacent wall portion of bore  22 . 
   Locking key  26  is substantially rectangular in cross-sectional profile and is formed to be substantially full-fitting in keyway  16 . Key  26  is tapered longitudinally from a first portion  28  having a height less than distance  27  to a second portion  30  having a height at high point  31  slightly greater than distance  27 . Either hub  20  or key  26  is formed of a deformable material, such as a polymer. In a currently preferred embodiment, the hub is more deformable than the key. With hub  20  rotatably mounted on shaft portion  12 , key  26  is forced into keyway  16  with high point  31  adjacent hub  20 , thereby causing either hub  20  or key  26  to become slightly deformed to accommodate a jam fit therebetween. All forces therebetween will be seen to be only radial, and therefore there is no tendency to shift the rotational relationship between the hub and the shaft during insertion of key  26  into keyway  16 . Note: During insertion (assembly) of the key into the keyway, the hub and the shaft must be fixed axially so as to prevent any movement of the hub relative to the shaft. Typically, this can be accomplished by having a shoulder  17  on the shaft against which the hub will rest and the shaft will have a tool backup (stop) to maintain its position axially, thus opposing the friction force created when the key is pressed into the keyway. 
   In a currently preferred embodiment, shaft  10  is formed of metal, hub  20  is formed of a deformable composite polymer, and key  26  is formed of metal as by stamping or by molding of other hard material such as glass, ceramic, or mineral-filled polymer composite. The stamping process beneficially provides key  26  with at least one relatively sharp corner at high point  31 , which corner incises the wall portion of bore  22  and thereby serves to rotationally lock hub  20  with respect to key  26 , and therefore, to shaft  10 . Preferably, both corners of high point  31  so incise the hub. Preferably, key  26  is urged into hub  20  until high point  31  coincides with the axial midpoint of bore  22 , thus causing the portion of shaft  12  opposite keyway  16  to be firmly engaged with the wall of bore  22  along the entire axial length of bore  22 . Preferably, hub  20  is formed to be diametrically deformable by insertion of key  26 , such that bore  22  is urged to be slightly out-of-round, thereby holding key  26  firmly in place by radial restoring force of the hub exerted on the key and keyway. 
   Referring to  FIG. 4 , if desired, key  26  may be readily formed as one of a chain  29  of keys which may be sequentially inserted into a plurality of keyways on a manufacturing assembly line and individually severed along break-line  32  from the chain after being inserted. Each severed key is not readily removed from its destination keyway and is thus tamper-proof. 
   Key  26  may be formed in various tapered shapes and preferably in the “gable” shape shown in  FIGS. 2 through 4 , which is a double-ended wedge, such that the key may be inserted from either end of the key and therefore requires no specific orientation. 
   In an examplary automotive application of the invention, shown in  FIG. 1 , shaft  10  is a conventional throttle shaft of a conventional throttle valve. Shaft  10  is provided with a keyway  16 , and the sensing arm  15  of a conventional throttle shaft variable-voltage position sensor (not shown) is provided with a bored hub  20 . During assembly, the hub is installed onto the throttle shaft. The throttle shaft is then rotated on a test bed (not shown) to open the throttle valve until a predetermined calibration airflow through the throttle valve is obtained. Independently, the hub  20  is rotated on the calibrated shaft until a predetermined calibration output is obtained from the position sensor. When both conditions are satisfied, a tapered key  26  is pressed into the keyway in the throttle shaft, thereby rotationally locking the hub to the shaft and associating, unambiguously and without mechanical variance, a rotational position of the throttle shaft with an output signal from the position sensor. In such an application, the key lock has been found to be able to withstand a torque of up to at least 0.5 N-m without either axial or rotational slippage on the shaft. Preferably, the throttle shaft is configured so that in the finished assembly the keyway is disposed in the hub substantially 180° opposite from the attachment point of the sensor arm to the hub, as shown in  FIG. 1 , since the hub is drawn snugly against the shaft at that point by the action of the key in the keyway. 
   Referring to  FIG. 3 , in an alternative embodiment of the invention, a keyway  16 ′ may be provided in the hub  20 ′ instead of in the shaft  12 ′, as in FIG.  1 . In this embodiment, the key is inverted prior to insertion into the keyway and incises the shaft to secure the hub thereto. Note that optimally the key is harder than the shaft. In practice, this may be difficult to provide, since shafts capable of sustaining torque loads typically are formed of hard materials such as steel or aluminum; hence, a suitable key could be quite expensive. The alternative embodiment shown in  FIG. 3  is not presently preferred. 
   While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention include all embodiments falling within the scope and spirit of the appended claims.