Abstract:
A planetary gear set includes: a sun gear, a plurality of planetary gears externally meshed with the sun gear, and a carrier that interconnects the plurality of planetary gears such that the plurality of planetary gears revolve around a exterior circumference of the sun gear, and an arrangement of the plurality of planetary gears is rotationally non-symmetrical.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2007-0131657 filed in the Korean Intellectual Property Office on Dec. 14, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    (a) Field of the Invention 
         [0003]    The present invention relates to an automatic transmission. More particularly, the present invention relates to a planetary gear set used in an automatic transmission. 
         [0004]    (b) Description of the Related Art 
         [0005]    A typical automatic transmission employs a planetary gear set. The planetary gear set includes a sun gear, pinion gears, a ring gear, and a carrier that carries the pinion gears. 
         [0006]    While a vehicle is driven, the automatic transmission frequently changes gears, and power interaction between the sun, pinion, and ring gears also frequently changes. When the sun, pinion, and ring gears or an arrangement thereof is not precisely designed, vibration or noise may be increased during shifting of the gears. 
         [0007]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the present invention provide a planetary gear set having advantages of reduced vibration and/or noise. 
         [0009]    An exemplary embodiment of the present invention provides a planetary gear set of a transmission of a vehicle that includes: a sun gear; a first planetary gear that is externally meshed with the sun gear; a second planetary gear that is externally meshed with the sun gear and is away from the first planetary gear by a first angular spacing around the sun gear; a third planetary gear that is externally meshed with the sun gear and is away from the second planetary gear by a second angular spacing larger than the first angular spacing around the sun gear; and a carrier that interconnects the first, second, and the third planetary gears. 
         [0010]    The first angular spacing may be smaller than 120°, and the second angular spacing may be greater than 120°. 
         [0011]    The exemplary planetary gear may further include: a fourth planetary gear that is externally meshed with the first planetary gear; a fifth planetary gear that is externally meshed with the second planetary gear; and a sixth planetary gear that is externally meshed with the third planetary gear, wherein the fourth, fifth, and sixth planetary gears are also interconnected by the carrier. 
         [0012]    The second angular spacing may be 120.05°. 
         [0013]    An exemplary embodiment of the present invention provides a planetary gear set of a transmission of a vehicle that includes: a sun gear; a plurality of planetary gears externally meshed with the sun gear; and a carrier that interconnects the plurality of planetary gears such that the plurality of planetary gears revolve around a exterior circumference of the sun gear, wherein an arrangement of the plurality of planetary gears is arranged rotationally non-symmetrical. 
         [0014]    A plurality of angular differences between adjacent planetary gears may form a plurality of values. 
         [0015]    The plurality of planetary gears may include three planetary gears of first, second, and third planetary gears. 
         [0016]    An angular difference of at least one pair of adjacent planetary gears may be greater than 120°. 
         [0017]    The angular difference of the at least one pair of adjacent planetary gears may be 120.05°. 
         [0018]    An angular difference of at least one pair of adjacent planetary gears may be less than 120°. 
         [0019]    The angular difference of the at least one pair of adjacent planetary gears may be 119.95°. 
         [0020]    A first angular difference between a first pair of adjacent planetary gears may be greater than 120°, and a second angular difference between a second pair of adjacent planetary gears may be less than 120°. 
         [0021]    The first angular difference between the first pair of adjacent planetary gears may be 120.05°. 
         [0022]    The second angular difference between the second pair of adjacent planetary gears maybe 119.95°. 
         [0023]    A third angular difference between a third pair of adjacent planetary gears may be 120°. 
         [0024]    The first angular difference may be 120.05° and the second angular difference may be 119.95°. 
         [0025]    The exemplary planetary gear set may further include a plurality of outer planetary gears that are externally meshed with the plurality of planetary gears respectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic diagram of a planetary gear set according to an exemplary embodiment of the present invention. 
           [0027]      FIG. 2  is a chart showing reduction of noise obtained by a planetary gear set according to an exemplary embodiment of the present invention. 
       
    
    
       [0028]    In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
       REPRESENTATIVE REFERENCE NUMERALS 
       [0029]      100 : sun gear 
         [0030]      105   a ,  105   b ,  105   c : planetary gear 
         [0031]      110   a ,  110   b ,  110   c : planetary gear 
         [0032]      115 : carrier 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0033]    Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. 
         [0034]    Referring to  FIG. 1 , a planetary gear set includes a sun gear  100  and inner planetary gears of a first planetary gear  105   a , a second planetary gear  105   b , and a third planetary gear  105   c . The first, second, and third planetary gears  105   a ,  105   b , and  105   c  are externally meshed with the sun gear  100 , respectively. 
         [0035]    In addition the exemplary planetary gear set includes outer planetary gears of a fourth planetary gear  110   a , a fifth planetary gear  110   b , and a sixth planetary gear  110   c  that are externally meshed with the first, second, and third planetary gears, respectively. The first to sixth planetary gears  105   a ,  105   b ,  105   c ,  110   a ,  110   b , and  110   c  are assembled to a carrier  115  so that they may revolve around the sun gear  100  with a fixed spatial configuration. 
         [0036]    Conventionally, planetary gears are symmetrically disposed. That is, planetary gears are disposed with a uniform angular difference between adjacent planetary gears. 
         [0037]    However, according to an exemplary embodiment, an arrangement of the first to third planetary gears  105   a ,  105   b , and  105   c  is rotationally non-symmetrical. In more detail, a plurality of angular differences between adjacent planetary gears form a plurality of different values. 
         [0038]    For example, the second planetary gear  105   b  is angularly away from the first planetary gear  105   a  by 120° around the sun gear  100 . However, the third planetary gear  105   c  is angularly away from the second planetary gear  105   b  by 120.05° around the sun gear  100 . Thus, the third planetary gear  105   c  is angularly away from the first planetary gear  105   a  by 119.95°. That is, according to an exemplary embodiment of the present invention, three different values of 120°, 120.05°, and 119.95° are formed between adjacent planetary gears. 
         [0039]    When an entire angle of 360° is divided into three uniform angles, three radial reference lines  120   a ,  120   b , and  120   c  shown in  FIG. 1  may be formed with a uniform angular difference of 120° around a rotation center  150  of the sun gear  100 . 
         [0040]    A rotation center of the first planetary gear  105   a  lies on a first reference line  120   a , and a rotation center of the second planetary gear  105   b  lies on a second reference line  120   b.    
         [0041]    However, a rotation center of the third planetary gear  105   c  does not lie on a third reference line  120   c  but lies on a fourth reference line  120   d  that is offset from the third reference line  120   c  by 0.05°. 
         [0042]    Considering that teeth of the sun gear  100  and the planetary gears  105   a ,  105   b , and  105   c  have finite size and the numbers of the teeth thereof are also finite, the sun gear  100  may not always maintain tight contact with all the three planetary gears  105   a ,  105   b , and  105   c  during the operation of the planetary gear set when the planetary gears  105   a ,  105   b , and  105   c  are arranged with exactly the same angular spacing. 
         [0043]    Therefore, by disposing one or more planetary gears slightly offset from a geometrically symmetrical position, the sun gear  100  may better maintain the contact with the planetary gears in the average. That is, if the contact with one planetary gear becomes looser, the contact with another planetary gear may become tighter so as to compensate the loosened contact. In this way, the average contacting strength of all the planetary gears  105   a ,  105   b , and  105   c  with the sun gear  100  may be better maintained, and thus, vibration, noise, and backlash of an automatic transmission may be reduced. 
         [0044]      FIG. 2  illustrates noise reduction obtained by a planetary gear set according to an exemplary embodiment of the present invention. As shown therein, the horizontal axis denotes rotation speed and the vertical axis denotes noise level. Broken line represents noise level of a conventional planetary gear set, and the solid line represents noise level of a planetary gear set according to an exemplary embodiment of the present invention. Thus, as shown in  FIG. 2 , noise is decreased throughout an overall range by altering the planetary gear position on the carrier, by more than 5 dBA for a low speed range of 1,000-2,000 rpm. 
         [0045]    In the above description, a double pinion planetary gear set is taken as an example for an embodiment of the present invention. However, it is obvious that the present invention is not limited thereto, since the spirit of the present invention may easily be applied to a single pinion planetary gear set by a person of ordinary skill in the art. 
         [0046]    Furthermore, in the above description, a simple planetary gear set is taken as an example for an embodiment of the present invention. However, it is obvious that the present invention is not limited thereto, since the spirit of the present invention may easily be applied to a compound pinion planetary gear set by a person of ordinary skill in the art. 
         [0047]    Furthermore, in the above description, a planetary gear set having three inner/outer planetary gears is taken as an example for an embodiment of the present invention. However, it is obvious that the present invention is not limited thereto, since the spirit of the present invention may easily be applied to a planetary gear set having four or more planetary gears by a person of ordinary skill in the art. 
         [0048]    Furthermore, in the above description, angular differences have a deviation of 0.05°. However, it is obvious that the present invention is not limited thereto. A specific value of the deviation may be altered by a person or ordinary skill taking into account of the number of pinion gears, the number of teeth for the sun gear and the pinion gears, etc. While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.