Abstract:
A planetary gear for mounting on an electromotor is proposed, in which higher efficiency is achieved by positioning a sealing element ( 22 ) between a housing ( 2 ) and a sun gear shaft ( 4 ) on the input side, axially outside a receiving area ( 16 ) for the output shaft of the electric motor, in an axial section of the sun gear shaft ( 4 ) with an outer reduced diameter compared to the receiving area ( 16 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a planetary gear for mounting on an electromotor. 
     BACKGROUND OF THE INVENTION 
     Such planetary gears are used for many purposes in automation technology and plant and machinery in general. With such a planetary gear, in which a sun gear can be driven by an output shaft of the electromotor, an annular gear is positioned in the housing and a planetary carrier forms the output, various transmission ratios, typically in the range 4:1 to 10:1, can be produced by varying the geometry of the sun gear and planetary gear wheels and of the planetary carrier. 
     Owing to the high power density involved, even small internal power losses can give rise to undesired high temperatures. Because of the compactness of the structure, the heat generated by these losses often cannot be dissipated to the desired extent. High temperatures affect service life adversely. A large part of the power loss is attributable to the seals and bearings of the rapidly rotating sun gear shaft on the input side. 
     A gear of this type is disclosed for example in DE 198 08 184 C1. To receive an output shaft of the electromotor, the sun gear shaft of this known planetary gear is made hollow in a receiving area of enlarged diameter. The sun gear shaft is sealed with respect to the housing by a radial sealing ring. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to develop further a planetary gear of the type described so as to minimise the power loss. Furthermore, the planetary gear should be of compact structure and economical to manufacture. 
     Thus, in accordance with the invention the sealing element that seals the sun gear shaft on the outside with respect to the housing is arranged axially 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 shows a longitudinal section through a planetary gear according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the single FIGURE attached, the housing is indexed as  2 , the sun gear shaft on the input side as  4  and the output shaft of a planetary gear according to the invention as  6 . The output shaft  6  rotates with a planetary carrier  8 , on which several uniformly distributed planetary gear wheels  10  are mounted and able to rotate. The planetary gear wheels  10  are in simultaneous gear-tooth engagement with a central sun gear  12  that can be driven by the sun gear shaft  4  and with an annular gear  14  fixed in the housing  2 . 
     To receive an output shaft (not shown) of an electromotor, the sun gear shaft  4  has a hollow receiving area  16  that extends axially over the length of a cylindrical bore  18  in the sun gear shaft  4 . The inside space of the housing  2  is filled with lubricant and sealed with respect to the outside by two sealing elements formed as radial sealing rings  20 ,  22 . The radial sealing rings are fixed in the housing  2  and are associated with cylindrical functional surfaces  24 ,  26  of the output shaft and the sun gear shaft respectively. Sliding friction takes place between the radial sealing rings and the said functional surfaces. 
     According to the invention, the radial sealing ring  22  arranged between the housing  2  and the sun gear shaft  4  is positioned outside the receiving area  16  for the output shaft of the electromotor in an axial section of the sun gear shaft whose outer diameter is smaller compared with that of the receiving area. Only very small frictional losses occur on the functional surface  26 , whose diameter is smaller than the diameter of the bore  18 , so that higher efficiency is attained and problems due to high temperatures are avoided. Between the receiving area  16  and the location of the radial sealing ring  22  the sun gear shaft has a diameter step  23 . 
     The inner ring of the bearing  28  for the sun gear shaft  4  is also arranged outside the receiving area  16  for the output shaft of the electromotor in an area with reduced outer diameter, so that a bearing of smaller size can be used. 
     The diameter of the bearing holder on the sun gear shaft is also smaller than the diameter of the bore  18 . 
     The outer bearing ring of the bearing  28  is located in the planetary carrier  8 , radially inside the inner ring of a planetary carrier bearing  30 . The bearing  28  is positioned axially within the structural space occupied by the planetary carrier bearing  30 , and this makes it possible for the axial length of the planetary gear to be short. For the planetary carrier  8  a second bearing  32  is provided which, like the bearing  30 , is in the form of a conical-roller bearing and which forms an X arrangement together with the latter. 
     In the axial space between the conical-roller bearings  30 ,  32  the planetary carrier  8  has on the two sides of each planetary gear wheel  10  bores  34 ,  36  that extend through the carrier. These bores  34 ,  36  each accommodate a planetary bearing pin  38 , on which the planetary gear wheel  10  is mounted by means of cylindrical rollers  40  and can rotate. The end surface of the planetary bearing pin  38  abuts against the inner ring of the planetary carrier bearing  32 ,  34 , so that it is advantageously secured against axial displacement without further measures. On each side of each planetary gear wheel  10  thrust washers in the form of annular discs are arranged on the planetary bearing pin  38 , and these restrict the axial movement of the planetary gear  10 . 
     The bearing  28  for the sun gear shaft  4  is secured in the planetary carrier  8  against axial displacement in one direction by a circlip  46 . To fit the bearing  28 , the circlip  46  can be pressed completely into an annular groove  48  in the planetary carrier, which is axially adjacent to the functional surface  47  that receives the outer ring of the bearing. As soon as the outer ring of the bearing  28  has been pushed past the area of the annular groove  48  during assembly, the circlip  46  snaps together and so secures the outer ring of the bearing against axial displacement.