Abstract:
A planet gear includes a toothed portion having a bore extending axially therethrough, a hub disposed within the bore such that an annular space is defined between the hub and the toothed portion, and an elastomeric element injection-molded into the annular space. The toothed portion includes a first groove circumferentially disposed in an inner surface of the toothed portion, and the hub includes a second groove circumferentially disposed in a peripheral surface of the hub. The grooves provide surfaces upon which the injection-molded elastomer can set. Holes may be disposed axially within the elastomeric element to decrease the spring rate of the elastomer, thereby providing additional radial compliancy to the planet gear. A method for manufacturing the gear includes disposing the hub within the bore of the toothed portion, injecting the elastomer in the annulus defined by the hub disposed within the bore, and curing the injected elastomer. A method for manufacturing the gear having axially disposed holes within the elastomer includes disposing a plurality of pins in the annulus prior to injecting the elastomer, then removing the pins once the elastomer has cured.

Description:
TECHNICAL FIELD  
         [0001]    This disclosure relates to planetary gear systems, and, more particularly, to a planet gear having an injection-molded elastomeric damping element that causes vibration generated during the operation of a planetary gear system into which the planet gear is incorporated to be minimized.  
         BACKGROUND  
         [0002]    Planetary gear systems typically comprise a plurality of drivable or idler gears (e.g., planet gears) engaged by a pinion (e.g., a sun gear). Because they share a single load between several meshes of gears, planetary gear systems are generally more compact than parallel shaft drives and offer significant space savings.  
           [0003]    The problem of the audible noise is exacerbated as the result of two conditions that exist within the planetary gear system. The first condition is a function of the material of fabrication of the gears. Typically, at least one of the gears is fabricated from metal. Metal gears provide a much more aggressive impact of the surfaces of the gear teeth when the gears mesh during the operation of the system. Such aggressive impact increases the amount of noise generated. The second condition is a function of the damping aspects of the system. Planet gears of the related art generally include a bushing positioned axially within an outer portion of the gear. Elastomeric O-rings may be disposed between the outer portion of the gear and the bushing to transfer vibration from the teeth of the gear to the hub and the remainder of the system, thereby reducing audible noise. The elastomeric O-rings, however, are typically pre-fabricated and inserted into the gear/bushing assembly mechanically. Because they are mechanically inserted into the assembly, they can oftentimes be easily removed during operation of the gear system if the system experiences continual jarring motion or frequent sudden impacts. Furthermore, the assembly of a gear system in which an elastomeric O-ring is disposed between the outer portion and the bushing affords opportunity for foreign particulate matter or debris to be inadvertently introduced into the gear system. Finally, the assembly of the elastomeric O-ring and the gear components creates an additional manufacturing operation.  
         SUMMARY  
         [0004]    A low noise planet gear for use in a planetary gear system and a method of manufacturing such a gear are described herein. An injection-molding technique is used to dispose an elastomeric element between the structural components of the planet gear. The elastomer used to form the elastomeric element allows for a compliant relationship to be maintained between the toothed portion of the gear and the hub of the gear and is selected on the basis of the desired properties of the cured elastomer and their effect on the gear during its operation. The elastomer, in conjunction with the architecture of the planet gear, provides for a radial spring rate and a radial damping ability that effectively minimizes the amount of vibration transferred to other elements of the planetary gear system.  
           [0005]    The planet gear includes a toothed portion having a bore extending axially therethrough, a hub disposed within the bore such that an annular space is defined between the hub and the toothed portion, and an elastomeric element injection-molded into the annular space. The toothed portion includes a first groove circumferentially disposed in an inner surface of the toothed portion, and the hub includes a second groove circumferentially disposed in a peripheral surface of the hub. In place of the grooves, ridges may be disposed circumferentially on either or both the inner surface of the toothed portion and the peripheral surface of the hub. The grooves or ridges provide surfaces upon which the injection-molded elastomer can set. Holes may be disposed radially in the faces of the planet gear and axially within the elastomeric element to decrease the spring rate thereof, thereby providing additional radial compliancy to the planet gear.  
           [0006]    A method for manufacturing the gear includes disposing the hub within the bore of the toothed portion, injecting the elastomer into the annular space defined by the hub disposed within the bore, and curing the injected elastomer. A method for manufacturing the gear having axially disposed holes within the elastomer includes disposing a plurality of pins in the annulus prior to injecting the elastomer, then removing the pins once the elastomer has cured. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a perspective view of a planetary gear system.  
         [0008]    [0008]FIG. 2 is a front elevation view of a face of a toothed portion of a planet gear.  
         [0009]    [0009]FIG. 3 is a side sectional view of the toothed portion of the planet gear of FIG. 2.  
         [0010]    [0010]FIG. 4 is a front elevation view of a face of a hub of a planet gear.  
         [0011]    [0011]FIG. 5 is a side sectional view of the hub of the planet gear of FIG. 4.  
         [0012]    [0012]FIG. 6 is a front elevation view of a planet gear having an elastomeric element disposed therein.  
         [0013]    [0013]FIG. 7 is a side sectional view of the planet gear of FIG. 6.  
         [0014]    [0014]FIG. 8 is a front elevation view of a planet gear having an elastomeric element having holes disposed therein.  
         [0015]    [0015]FIG. 9 is a side sectional view of the planet gear of FIG. 8. 
     
    
     DETAILED DESCRIPTION  
       [0016]    Referring to FIG. 1, a planetary gear system is shown generally at  10 . Planetary gear system  10  reduces the speed of an input shaft and multiplies its torque. Applications in which planetary gear system  10  may be incorporated include, but are not limited to, various automotive steering and drive systems, aircraft and marine drive systems, and turbine engine reduction gear systems. In particular, planetary gear system  10  may be part of a rear electric steering mechanism for a motor vehicle.  
         [0017]    Planetary gear system  10  comprises a ring gear  12 , a sun gear  14  rotatably positioned within ring gear  12  and driven by an input shaft  16 , and a plurality of planet gears, two of which are shown generally at  18 . Planet gears  18  are configured to be in meshed engagement simultaneously with an outer toothed surface of sun gear  14  and an inner toothed surface of ring gear  12 . Each planet gear  18  is axially and rotatably positioned on a dowel pin  19  mounted to a planet carrier  20 . In one embodiment, each planet gear  18  rotates perimetrically about sun gear  14  and within ring gear  12  to simultaneously apply a load to planet carrier  20 , which rotates to apply a torque to an output shaft  22  depending from planet carrier  20 . In another embodiment (not shown), planet gears rotate on a planet carrier, which remains fixed relative to a sun gear, to apply a load to a ring gear. The ring gear then rotates to apply a torque to an output shaft (not shown) depending from the ring gear. Although planet gear  18  is applicable to either embodiment, only the configuration in which the output shaft depends from the planet carrier is described herein.  
         [0018]    Planet gear  18  comprises a toothed portion, a hub, and an elastomeric element disposed therebetween to provide compliancy during the operation of planetary gear system  10  in which planet gear  18  is incorporated. Referring now to FIGS. 2 and 3, toothed portion, shown generally at  24 , is illustrated in detail. Toothed portion  24  comprises a bore  26  defined by an inner surface  28  extending axially through the geometric center of toothed portion  24 . Gear teeth  30  extend radially outward from an outer surface of the body of toothed portion  24 . Teeth  30  are configured and dimensioned to engage the teeth of both the sun gear and the ring gear and to effectuate the movement of the planet carrier during operation of the planet gear system. A first continuous groove  34  is formed circumferentially within inner surface  28  and extends completely around inner surface  28 . Toothed portion  24  is typically fabricated from a plastic material, which may be a polyamide.  
         [0019]    Referring now to FIGS. 4 and 5, hub, shown generally at  36 , is illustrated in detail. Hub  36  comprises a cylindrical element having a peripheral surface  38  and a bore  40  defined by an inner surface  42  extending axially therethrough. Bore  40  is typically chamfered at its opposing ends in order to facilitate the rotatable mounting of the planet gear on the dowel pin on the planet carrier. The perimetrical dimensions of hub  36  are less than a diameter of the bore extending through the geometric center of the toothed portion. A second continuous groove  44  formed circumferentially within peripheral surface  38  extends completely around hub  36 . Second continuous groove  44  is axially located to correspond with the axial location of the first continuous groove when hub  36  and toothed portion  24  are assembled.  
         [0020]    Hub  36  may be fabricated from a polyamide impregnated with a solid lubricant, or it may be fabricated from a metal such as iron. Alternately, steel or some other type of ferrous material may be used. Iron, however, provides damping characteristics that are superior to those of steel. Additionally, the graphite flake structure of iron imparts an inherent lubricity to a hub made from iron, which causes minimal friction to be realized by the rotation of hub  36  on the dowel pin. Furthermore, the use of iron or a ferrous metal provides an impedance mismatch between the toothed portion and hub  36 , thereby providing a more effective barrier against the transmission of vibration from the planet gear to planetary gear system  10 .  
         [0021]    Referring now to FIGS. 6 and 7, the assembled planet gear  18  is illustrated. The coaxial assembly of toothed portion  24  and hub  36  defines an annulus between peripheral surface  38  of hub  36  and inner surface  28  of toothed portion  24  in which an elastomeric element  46  is accommodated. The annulus is defined by first and second grooves  34 ,  44 , as can be seen in FIG. 7, which are each typically of a rectangular cross sectional shape or a similar geometry. Other geometries in which first and second grooves  34 ,  44  can be configured include, but are not limited to, semi-circular, triangular, or trapezoidal cross sectional shapes.  
         [0022]    First and second grooves  34 ,  44  are dimensioned such that the gap defined therebetween is of a size that provides improved retention of hub  36  within toothed portion  24  when planet gear  18  is properly assembled. In particular, when the elastomer is disposed within the annulus and cured to form elastomeric element  46 , hub  36  is secured into place within tooth portion  24  and is prevented from axial movement relative to tooth portion  24  without first tearing elastomeric element  46 . By selecting an elastomeric material that has known resiliency, hardness, and tear properties, the amount of force required to cause planet gear  18  to fail can be predetermined for a specific application.  
         [0023]    Elastomeric element  46  is disposed in the annulus formed by the coaxial assembly of hub  36  within toothed portion  24  to effectuate a compliant relationship between toothed portion  24  and hub  36 . The annulus is defined as having a cross-shaped cross section. Such a shape maximizes the surface area over which elastomeric element  46  engages toothed portion  24  and hub  36 , thereby enabling elastomeric element  46  to provide improved axial retention of hub  36  within toothed portion  24 . Such a configuration may provide for the torsional retention of hub  36  within toothed portion  24  by providing friction between peripheral surface  38  of hub  36  and elastomeric element  46  and between elastomeric element  46  and inner surface  28  of toothed portion  24 . Furthermore, a mild chemical bond may be effectuated between elastomeric element  46  and peripheral surface  38  of hub  36  to either further or alternately provide torsional resistance between elastomeric element  46  and hub  36 .  
         [0024]    Elastomers used in the formation of elastomeric element  146  include, but are not limited to, thermoset rubbers such as nitrile rubber, natural rubber, polychloroprene rubber, and silicone rubber. The elastomer of choice is determined by the properties of the elastomer, the particular application, and the likelihood that the elastomer will withstand the environmental conditions that the planet gear into which the elastomer is incorporated is subjected to. In addition to thermoset rubbers, various types of thermoplastic materials may be used to form elastomeric element  46 . Thermoplastic materials exhibit rubber-like characteristics but can be processed like plastic, which makes them likely candidates for injection-molding processes. One particular thermoplastic material that can be used is SANTOPRENE®, which is available from Advanced Elastomer Systems, located in Akron, Ohio.  
         [0025]    Referring now to FIGS. 8 and 9, another embodiment of a planet gear is shown generally at  118 . Planet gear  118  comprises a toothed portion  124 , a hub  136  disposed axially within toothed portion  124 , and an elastomeric element  146  disposed therebetween. Planet gear  118  is configured similarly to planet gear  18  as shown in FIGS. 6 and 7. Planet gear  118 , however, includes holes  147  disposed axially through elastomeric element  146  in order to impart compliance to planet gear  118  in directions radial to an axis extending through hub  136  of planet gear  118 .  
         [0026]    Toothed portion  124  comprises a bore defined by an inner surface  128  extending axially through the geometric center of toothed portion  124 . Gear teeth  130  extend radially outward from an outer surface of the body of toothed portion  124  and are configured and dimensioned to engage the teeth of both the sun gear and the ring gear of a planetary gear system into which planet gear  118  is incorporated, thereby effectuating the movement of either the planet carrier or the ring gear during operation of the planet gear system. Toothed portion  124  also includes a continuous ridge  134  disposed circumferentially about inner surface  128 .  
         [0027]    Hub  136  is substantially similar to hub  26  as shown in FIGS. 4 through 7 and comprises a cylindrical element having a peripheral surface  138  and a bore defined by an inner surface  142  extending axially therethrough. A continuous groove  144  is formed circumferentially within peripheral surface  138 . When toothed portion  124  and hub  136  are coaxially assembled, continuous ridge  134  corresponds to the positioning of continuous groove  144 .  
         [0028]    Elastomeric element  146 , which may be a thermoset rubber or thermoplastic material as described above, is disposed in the annulus formed by the coaxial assembly of hub  136  within toothed portion  124 . Elastomeric element  146  is, however, formed to include holes  147  disposed axially therethrough. Holes  147 , which may vary in number, cross sectional shape, and positioning within elastomeric element  146 , provide planet gear  118  with a decreased radial spring rate and impart enhanced radial damping characteristics to elastomeric element  146 .  
         [0029]    Holes  147  are formed by the insertion of pins (not shown) into the annulus formed between toothed portion  124  and hub  136  during the coaxial assembly thereof. The pins may be fixedly disposed on a molding die (not shown) that is used to form planet gear  118 , thereby causing the simultaneous insertion of the pins into the annulus. The pins are dimensioned to extend into the annulus a predetermined distance and may engage continuous ridge  134 . An elastomer is typically injection-molded into the annulus between toothed portion  124  and hub  136  and around the pins. Upon curing of the elastomer, elastomeric element  146  is formed. The pins are removed from elastomeric element  146 , either individually or in unison by the retrieval of the molding die, to leave holes  147 . Holes  147  can also be mechanically formed in a cured elastomer disposed in the annulus using a drilling, boring, or similar technique.  
         [0030]    Regardless of the structural configuration of the radially compliant planet gear, one of the benefits occasioned by the use of an injection-molded elastomer into the annulus formed between the toothed portion and the hub is that of a reduced chance of having foreign particles disposed on the surfaces of the toothed portion and the hub that are engaged by the elastomer during the assembly of the gear. The injection-molding process facilitates the displacement of any contaminating matter disposed on the surfaces of the grooves or ridges disposed in the toothed portions or hubs during the manufacturing of the parts of the planet gear. Furthermore, the injection-molding process eliminates the mechanical disposition of a pre-formed elastomeric element (e.g., an O-ring or a similar element) into the planet gear assembly.  
         [0031]    Another of the benefits occasioned by the use of a single elastomeric element injection-molded into the annulus between the hub and the toothed portion is that in the event of a manufacturing oversight that results in the omission of the elastomeric element, the condition is immediately discovered. Because the elastomeric element provides support for the assembly of the hub within the toothed portion, the absence of the elastomeric element causes the planet gear to fall apart. Such a condition is extremely noticeable during manufacturing procedures. By immediately discovering the absence of the elastomeric element, the cause of the defective planet gear can be corrected, thereby preventing the release of an incomplete final product.  
         [0032]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it should be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.