Abstract:
An improved flywheel for use in a small gasoline engine of the type utilized in string trimmers, chain saws, lawn mowers and other such devices. The flywheel comprises a flywheel body including a central hub portion and an integral wheel portion. The hub portion of the flywheel body defines an axial bore extending therethrough in which the engine&#39;s drive shaft is received when the flywheel is mounted thereon. An integral key is located in the axial bore for receipt in a corresponding keyway defined in the drive shaft. The key is configured without deep-cut stress reliefs, having opposed side walls extending directly from an inside surface of the through bore.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to magneto flywheels utilized in small gasoline engines. More particularly, the invention relates to a magneto flywheel having an improved key. 
     Small gasoline engines, such as are frequently incorporated into string trimmers, chain saws, lawn mowers and other such devices, typically utilize a magneto ignition system. As is well known, a magneto ignition system will selectively produce a spark at the engine spark plug due to the magnetic cooperation between a stationary coil assembly and a rotating permanent magnet assembly. 
     Typically, the magnet assembly of the magneto ignition system will be carried by the engine&#39;s flywheel. Often, the flywheel will also include cooling vanes to circulate cooling air over the engine as the flywheel is rotated. The flywheel itself is generally mounted to a rotating shaft attached at one end to the engine&#39;s crank arm. The magnet assembly is positioned at a particular angular location about the axis of the shaft to ensure correct ignition timing. 
     In a typical construction, the flywheel is formed of cast aluminum. The hub of the flywheel defines a conical or “tapered” bore which mates with a complementary surface machined on the shaft. The tapered bore of the flywheel&#39;s hub includes a key located in a keyway defined on the tapered portion of the shaft. A securing element, such as a nut, maintains the flywheel in position on the shaft taper. 
     The flywheel is normally attached to the shaft at a level of tightening torque deemed to be optimum in a particular situation. For example, a typical cast aluminum flywheel having a 3.5 inch outer diameter may be attached to the shaft by a tightening torque of about 16 to 18 foot-pounds. Occasionally, however, a flywheel may be subjected to much higher tightening torques (e.g., 50 foot-pounds or more). The higher tightening torques may be due to overtightening at the factory, or may be caused in some cases by the design of downstream components such as clutches. Whatever the cause, severe overtightening of the flywheel has often resulted in flywheel breakage. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes and addresses the foregoing disadvantages, and others of prior art constructions and methods. Accordingly, it is an object of the present invention to provide a novel flywheel for use in a small gasoline engine. 
     It is a particular object of the present invention to provide a flywheel for a small gasoline engine having an improved key. 
     It is a more particular object of the present invention to provide a flywheel for use in a small gasoline engine which is less susceptible to breakage due to overtightening. 
     It is also an object of the present invention to provide a flywheel for use in a small gasoline engine which may be efficiently manufactured. 
     Some of these objects are achieved by a flywheel mountable to a drive shaft of a small internal combustion engine. The flywheel comprises a flywheel body including a central hub portion and an integral wheel portion. The hub portion of the flywheel body defines an axial bore extending therethrough in which the engine&#39;s drive shaft is received when the flywheel is mounted thereon. An integral key is located in the bore for receipt in a corresponding keyway defined in the drive shaft. The key has opposed side walls extending directly from an inside surface of the through bore about a radius located at the respective base thereof. 
     The key defines a top surface extending between the opposed side walls, which may have an arcuate profile in the axial direction of the bore. Preferably, the key in such embodiments will have an axial extent less than that of the axial bore. For example, the arcuate profile of the top surface may terminate at both ends thereof by convergence into the inside surface of the axial bore. The axial bore itself will often be configured having a conical configuration. 
     Other objects of the present invention are achieved by a flywheel mountable to a drive shaft of a small internal combustion engine. The flywheel comprises a flywheel body including a central hub portion and an integral wheel portion. The hub portion of the flywheel body defines an axial bore extending therethrough in which the engine&#39;s drive shaft is received when the flywheel is mounted thereon. 
     The flywheel further comprises an integral key located in the axial bore for receipt in a corresponding keyway defined in the drive shaft. The key has opposed side walls and a top surface extending therebetween. The side walls of the key extend directly from an inside surface of the through bore. In addition, the top surface of the key has an arcuate profile in the axial direction of the axial bore. 
     In addition, the flywheel includes a magneto assembly located adjacent a periphery of the wheel portion of the flywheel body. A plurality of vane members, configured to pass air around the flywheel during operative rotation thereof, are integrally formed on the wheel portion of the flywheel body. At least one counterweight assembly is located adjacent a periphery of the wheel portion so as to balance the magneto assembly during operative rotation of the flywheel. 
     Other objects, features and aspects of the present invention are discussed in greater detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which: 
     FIG. 1 is a perspective view of a string trimmer device in which a flywheel of the present invention may be utilized; 
     FIG. 2 is an enlarged view showing the housing of the string trimmer device of FIG. 1 opened to reveal the flywheel; 
     FIG. 3A is a cross sectional view of the flywheel of FIG. 2; 
     FIG. 3B is a cross sectional view of the flywheel of FIG. 3A as taken along line  3 B— 3 B; 
     FIG. 4A is an enlarged plan view of the hub portion of a prior art flywheel showing the key located in the hub&#39;s tapered bore; 
     FIG. 4B is an enlarged cross sectional view of the key taken along line  4 B— 4 B of FIG. 4A; 
     FIG. 4C is an enlarged cross sectional view of the key taken along line  4 C— 4 C of FIG. 4A; 
     FIG. 5A is an enlarged plan view similar to FIG. 4A showing a key manufactured in accordance with the present invention; 
     FIG. 5B is an enlarged cross sectional view of the key taken along line  5 B— 5 B of FIG. 5A; 
     FIG. 5C is an enlarged cross sectional view of the key taken along line  5 C— 5 C of FIG. 5A; 
     FIG. 6 is an enlarged elevational view of the key shown in FIG. 5A; 
     FIG. 7 is an enlarged fragmentary view of the portion so indicated in FIG.  5 C. 
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     It is understood by one skilled in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary construction. 
     Referring to FIG. 1, a string trimmer  10  is illustrated as being typical of a gasoline engine powered device which may utilize a flywheel constructed in accordance with the present invention. As is well known, string trimmer  10  includes a housing  12  containing a small gasoline engine. The engine drives a cutting spool  14  at the end of an elongate tube  16 . During use, an operator grasps handles  18  and  20  to manipulate string trimmer  10 . The engine&#39;s throttle is controlled by a trigger located at handle  20 . 
     Referring now to FIG. 2, respective portions  22  and  24  of housing  12  are separated to reveal various internal components therein. As illustrated, the engine includes a cylinder head  26 , within which the piston is contained. Because the engine is air-cooled in this case, cylinder head  26  carries thereon a plurality of cooling fins. Fuel for engine operation is stored in gas tank  28 , which is filled by removing gas cap  30 . 
     Operative reciprocation of the piston within cylinder head  26  causes rotation of a drive shaft on which a flywheel  32  is mounted. In this case, flywheel  32  is maintained in position on the drive shaft by a retaining nut  34 . A flexible rod  36 , located inside of tube  16 , is connected at its distal end to spool  14 . The proximal end of rod  36  is connected to nut  34  such that operation of the engine will cause rotation of spool  14 . 
     The requisite ignition spark for providing combustion within cylinder head  26  is generated in this case by a magneto ignition system. The magneto ignition system includes a stator module  38  having a primary coil and a secondary coil wound about a common magnetically permeable core. A time varying flux is generated within the core by magnetic cooperation with a permanent magnet assembly carried by flywheel  32 . Specifically, rotation of flywheel  32  causes pole faces  40  and  42  of the magnet assembly to pass generally complementary pole faces of the stator module&#39;s core. 
     In many embodiments, the magneto ignition system may be of a type referred to as a capacitive discharge (“CD”) ignition. An exemplary CD ignition which may be utilized for this purpose is shown and described in U.S. Pat. No. 4,036,201, issued Jul. 19, 1977 to Burson, incorporated herein by reference. In such an ignition, the time varying magnetic flux functions to charge an appropriate storage capacitor. At a predetermined time, the capacitor will discharge through the primary coil of stator module  38 . A large voltage is thus induced on the secondary coil according to the primary-to-secondary turns ratio. This higher voltage is applied by spark plug wire  44  to the engine&#39;s spark plug  46 . 
     As is common with gasoline engines of this type, the engine of string trimmer  10  is started utilizing a recoil starting mechanism. The starting mechanism includes a starting handle  48  attached to a pull cord. The pull cord is typically wrapped about a pulley which is combined with a rachet wheel. The rachet wheel is engaged during cranking by a pair of pawls (not shown) pivotally mounted to flywheel  32 . In this case, the starting pawls are attached to the “back” of flywheel  32  (i.e., the side juxtaposed by nut  34 ). In other devices, it may be desirable to attach the pawls to bosses on the “finned” side of the flywheel. After the engine is cranked, centrifugal force will pivot the pawls out of engagement with the rachet wheel. 
     Further details regarding the construction of flywheel  32  can be seen in FIGS. 3A and 3B. As shown, flywheel  32  includes a central hub portion  50  integrally extending into a wheel portion  52 . Hub portion  50  defines a tapered bore  53  received on a tapered portion  54  of shaft  56 . Shaft  56  also has a threaded end  58  which is engaged in this case by nut  34  so as to secure flywheel  32 . It can be seen that a configured hole  60  is defined in the end of nut  34  for receipt of a driven element (e.g., flexible rod  36 ). 
     Certain additional details of flywheel  32  can be most easily explained with reference to FIG.  3 B. As can be seen, flywheel  32  includes an integral core carrying portion  62 , in which the permanent magnet assembly is embedded. An integral counterweight portion  64  is also provided to balance core carrying portion  62  during rotation of flywheel  32 . In this case, core carrying portion  62  is mounted at a location diametrically opposite to that of counterweight portion  64 . 
     A plurality of vane members (or “fins”), such as those indicated at  66 , are formed on flywheel  32  to circulate cooling air around the engine. A key  68  is located in tapered bore  53 , as shown. Key  68  is received in a corresponding keyway defined in shaft  56 . 
     In order to place the invention in context, it is helpful to first review certain further aspects of the prior art. Toward this end, FIG. 4A illustrates a typical hub portion  70  as may be found in a prior art flywheel. Hub portion  70  defines a tapered bore  72  extending therethrough for receipt on the tapered portion of the engine&#39;s drive shaft. A key  74  is received in a corresponding keyway defined on the drive shaft. As shown in FIG. 4B, it can be seen that key  74  extends the entire axial length of tapered bore  72 . 
     Referring now to FIG. 4C, deep-cut stress reliefs  76  and  78  are provided along each lateral side of key  74 . As a result, the sidewalls of key  74  do not extend directly up from the inner surface of tapered bore  72 , but extend up from the trough of the corresponding deep-cut stress relief. The conventional wisdom in the art is that stress reliefs of this type are needed to keep the keyway from shearing off the cast-in key. 
     In accordance with the present invention, it is found that breakage due to overtightening has tended to occur in the stress relief area. Specifically, cracks produced in a flywheel due to overtightening have often begun at the deep-cut stress reliefs located adjacent to the flywheel&#39;s key. Thus, a feature believed necessary to prevent damage to the key has itself served to facilitate damage to the overall flywheel. 
     In accordance with the present invention, it has been found that deep-cut stress reliefs undesirably contribute a bending moment to the key and the hub along with the predictable direct shear. The combined stress of the bending moment and the direct shear causes cracks in either the key or hub areas, or both. Furthermore, and surprisingly, it has been found that deep-cut stress reliefs are not necessary to prevent the key from shearing off during tightening of the flywheel or operation of the engine. 
     Referring now to the remaining figures, the present invention provides a key which is configured so as to lessen the occurrence of overtightening breakage that has been attributed to deep-cut stress reliefs. Thus, as shown in FIGS. 5A through 6, key  68  is constructed having sidewalls  80  and  82  extending directly from the inside surface of tapered bore  53 . A top surface  84  extends between sidewalls  80  and  82 , as shown. It can be seen that key  68  does not utilize deep-cut stress reliefs as have been utilized in the past. 
     As can be seen most clearly in FIG. 5B, top surface  84  of key  68  may have an arcuate profile in the axial direction of bore  53 . Preferably, the axial extent of key  68  will be less than the axial extent of bore  53 . For example, ends  86  and  88  of top surface  84  may terminate by convergence into the inside surface of bore  53 . This construction provides a key of “low profile” which serves to favorably locate the angular orientation of flywheel  32 . It will be appreciated that the keyway of drive shaft  56  may also be defined having a “low profile” corresponding to that of key  68 . 
     Key  68  is preferably configured having small radiuses at the corners so as to eliminate straight corners which could be susceptible to shearing. This is illustrated, for example, in FIG. 7 where radiussed corners  90  and  92  can be clearly seen. Thus, the construction of the present invention provides relief from shearing stress, as desired, without contributing an undesirable bending moment as has been seen in the prior art design. 
     While presently preferred embodiments of the invention have been shown and described, it should be understood that various modifications and variations may be made thereto by those of ordinary skill in the art. For example, flywheel  32  is illustrated in FIG. 2 with its “finned” side directed toward the engine. In other embodiments, it may be desirable to reverse the flywheel&#39;s orientation such that the “finned” side is directed away from the engine. 
     In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and it is not intended to be limitative of the spirit and scope of the invention so further set forth in the following claims.