Abstract:
A method of connecting a rotationally symmetrical part ( 11 ) having precision-machined functional surfaces ( 16 ) to a hub part ( 21 ) by welding is to deliver a distortion-free end product. To this end, the rotationally symmetrical part ( 11 ) and the hub part ( 12 ), in their longitudinal sections, are dimensioned in such a way that, when the rotationally symmetrical part ( 11 ) is shrunk onto or pressed onto the hub part ( 12 ), stresses are produced in the former and said stresses produce deformations which are opposed to the stresses to be expected during the subsequent welding and to deformations caused by said stresses. This is achieved by one of the contact surfaces ( 14; 15 ) being conical or by shaping the rotationally symmetrical part ( 11 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method of connecting a rotationally symmetrical part to a hub part by welding, the contact surfaces to be connected to one another being essentially cylindrical and the rotationally symmetrical part having functional surfaces, the accurate position and/or angle of which is essential to the function. The expression “functional surfaces” refers, for example, to the rolling surfaces of a gear or to the sealing surfaces of a pump rotor. The expression “essentially cylindrical” means that the contact surfaces are not surfaces normal to the axis but exert a certain centering effect. The hub part may also be part of the shaft carrying the rotationally symmetrical part or may be this shaft itself. In the case of gears, consideration is given in particular to the crown wheel of an axle drive for motor vehicles, the shape and position of the functional surfaces of said crown wheel resulting in high susceptibility to angular distortions due to welding stresses, but especially high demands are imposed on the accuracy of the engagement with said crown wheel. 
   Such rotationally fixed connections between shaft and hub or between a hub and a rotationally symmetrical part are normally produced merely by pressing on or by shrinking on, with especially high and fluctuating forces, as occur inter alia, for instance, at the crown wheel in the axle drive of a motor vehicle, by high-strength body-fit bolts. However, the connection by means of body-fit bolts is costly and requires considerable construction space. Welded connections are also conceivable, but are not advisable where there are functional surfaces of high accuracy on account of the welding distortion. 
   SUMMARY OF THE INVENTION 
   According to the invention, a method of reliably connecting such finish-machined, high-precision parts in mass production by welding is provided. In the method of the invention, the parts to be connected, in their longitudinal sections containing the rotation axis, are dimensioned in such a way that stresses are produced in the rotationally symmetrical part when the latter is pressed on or shrunk on, and said stresses produce deformations which are opposed to the stresses to be expected during the subsequent welding and to deformations caused by said stresses. 
   A joining operation therefore precedes the welding. The joining operation is deliberately designed in such a way that the part to be fitted is deformed, to be precise, in direction and magnitude, in opposition to the deformation caused by the welding distortion, which appears after the welding. The joint can be designed according to known methods, in particular using an FE method (FE=finite element). Pressing-on and shrinking-on are equivalent inasmuch as stresses are produced in the contact surfaces in both connections, in the first case by elastic deformation, in the second by thermal expansion. 
   There are two ways to design the joint and dimension the components to be connected, which are each feasible on their own or together. The first way consists in the fact that: 
   the rotationally symmetrical part is pressed or shrunk onto the hub part, at least one of the two contact surfaces having different radii along the axial direction in such a way that, when the rotationally symmetrical part is put on, stresses are produced therein which are higher on the one side than on the other side, and the functional surfaces are displaced in one direction, and 
   the welding is then effected on the one side, the functional surfaces returning again into the original accurate position due to the welding. 
   It is advantageous for tolerance and cost reasons if one of the surfaces to be connected to one another is cylindrical and only the other has different radii in the axial direction, the smaller radius being on the side of the weld in the case of different radii of the outer surface or the larger radius being on the side of the weld in the case of different radii of the inner surface. Considered in the tolerance zone, this means that the diameters of the mating surface overlap on the side of the welding. 
   The different radii can be produced by stepping, which is simpler, but an unsteady stress characteristic over the length can be expected. It is better if the other surface of the surfaces to be connected to one another is conical, the smaller radius of the cone being on the side of the weld in the case of a conical outer surface or the larger radius of the cone being on the side of the weld in the case of a conical inner surface. 
   The second way of actually realizing the invention consists in the fact that: 
   the longitudinal section, containing the rotation axis, of the rotationally symmetrical part, between the contact surface and the functional surface, has a constriction which is offset axially relative to the area center of the sectional plane lying outside the constriction, so that the functional surfaces of the rotationally symmetrical part are displaced in one direction when being pressed on or shrunk on, 
   the welding is then effected, as a result of which the functional surfaces return again into the original accurate position. 
   This way is certainly the more elegant way, since it requires no conical or stepped contact surfaces. However, it is not feasible with all basic forms and loading states. In particular, the resultant of the forces transmitted via the constriction is offset axially relative to the area center of the sectional plane lying outside the constriction. It is especially advantageous if the product thus produced is a crown wheel, since considerable angular changes due to welding distortion may occur on account of its shape and a constriction can also be readily accommodated from the design point of view. 
   Since the invention shows a way of compensating for welding distortions instead of preventing them, it is suitable in principle for most welding processes. Of course, especially good results will be achieved if the welding is effected by means of a high-energy beam, in particular a laser beam. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described and explained below with reference to figures, where: 
       FIG. 1  shows a connection according to the prior art, first phase, 
       FIG. 2  shows the connection as in  FIG. 1 , second phase, 
       FIG. 3  shows the connection as in  FIG. 1 , third phase, 
       FIG. 4  shows a first embodiment of the connection according to the invention, first phase, 
       FIG. 5  shows the connection as in  FIG. 4 , second phase, 
       FIG. 6  shows the connection as in  FIG. 4 , third phase, 
       FIG. 7  shows a second embodiment of the connection according to the invention, first phase, 
       FIG. 8  shows the connection as in  FIG. 7 , second phase, 
       FIG. 9  shows the connection as in  FIG. 7 , third phase, 
       FIG. 10  shows a variant of the first embodiment, 
       FIG. 11  shows a scheme of the stress characteristic with respect to  FIGS. 4 ,  5  and  6 , 
       FIG. 12  shows the corresponding tolerance zones. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a rotationally symmetrical part  1  and a hub part  2  before they are connected. Here, the rotationally symmetrical part  1  is the crown wheel of an axle drive which meshes with a driving pinion (not shown). Its tooth flanks  6  are the functional surfaces, the accuracy of which is very important for the operation. The tip cone of these teeth is designated by  7 , its position, also typical of all the other functionally important dimensions of the functional area, will be considered below. The rotationally symmetrical part  1  has a cylindrical contact surface  4  on its smallest diameter. The hub part  2 , here in one piece with a shaft  3 , has an outer contact surface  5 , which according to the prior art is likewise cylindrical. The tolerance zones of the contact surfaces  4 ,  5  are selected in accordance with a light interference fit. The rotation axis of both parts to be connected is designated by  0 . 
   The interference fit between the two parts  1 ,  2  is produced in  FIG. 2 . The position of the tip cone  7  remains unchanged if the pressure exerted approximately equally over the axial length by the interference fit has not led to any deformation of the rotationally symmetrical part  1 . 
     FIG. 3  shows the rotationally symmetrical part  1 ′ and the hub part  2 ′ after welding has been completed. Due to its shrinkage, the cooled weld  10  exerts shrinkage forces on the two parts  1 ′,  2 ′, these shrinkage forces being indicated by the arrows  8  and leading to a change in shape of the rotationally symmetrical part  1 ′. This can be seen by comparing the contours  7 ,  4  depicted by a broken line with the contours  7 ′,  4 ′ depicted by a solid line. The tip cone  7  has become the tip cone  7 ′. This angular deviation has a very adverse effect on the interaction between crown wheel and driving pinion. So much for the prior art. 
   In  FIG. 4 , the rotationally symmetrical part  11  again has a contact surface  14  and functional surfaces  16  with the tip cone  17 , that is to say it is unchanged compared with that in  FIG. 1 . The contact surface  15  of the hub part  12 , however, is conical. The conicity is optimized for achieving the effect according to the invention, which may be effected by trial and error, but may be effected especially accurately by calculation according to the finite element method. The hub part  12  is pressed into the rotationally symmetrical part  11 . 
     FIG. 5  shows the parts  11 ,  12  thus combined after they have been pressed on and pressed in, respectively. Due to the pressing-in, the rotationally symmetrical part  11  (only depicted by broken lines in  FIG. 5 ) has been deformed to  11 *. Its contact surface  14  (broken line) has been deformed into the slightly conical contact surface  14 *, and its tip cone  17  (broken line) has been deformed into the tip cone  17 *(solid line). This deformation can be attributed to the forces caused by the pressing-in, which are indicated by the arrows  18 * and act radially outward on one side. These forces are of course greatest on the side of the largest diameter of the contact surface  15 , for which reason the arrow  18 * is also on this side. The force introduced on one side results in an angular deviation, indicated by the arrow  13 *, of the tip cone  17  to  17 *. 
     FIG. 6  shows the next stage, after the welding. The weld  20 , shrinking during the cooling, exerts tensile forces  18 ′, indicated by the arrows  18 ′, on the rotationally symmetrical part  11 . Since these tensile forces  18 ′ again act only on the side of the weld  20 , but in the opposite direction, they cause an angular restoration  13 ′. The latter leads to the tip cone being drawn from the position  17 * in  FIG. 5  into the position  17 ′. Given the correct design of the interference fit, the angular restoration  13 ′ is equal to the angular deviation  13 * of  FIG. 5  and the tip cone  17 ′ is again congruent with the original tip cone  17 . Therefore, despite welding on one side, the connection according to the invention has not resulted in any displacement of the high-precision functional surfaces. 
   However, the invention can also be realized in another way, as shown in  FIGS. 7 ,  8  and  9 . In  FIG. 7 , the hub part  22  has a cylindrical contact surface  25 . The rotationally symmetrical part  21  also has a cylindrical contact surface  24 , the tolerance zones of the diameters of the two contact surfaces  24 ,  25  again establishing the interference fit. The effect according to the invention is achieved here by the rotationally symmetrical part  21  having an encircling groove  31  which, in the longitudinal section shown, forms a constriction  33  between the main cross section having an area center  32  and a sleeve part  35 . The determination of the area center  32  is not dealt with in any more detail, since this is done according to the rules of statics. In any case, the constriction  33  is offset relative to the area center  32  in the direction of the zero axis by a distance designated by  34 . This offset is essential. The sleeve part  35  remains inside the groove  31  and participates in the interference fit over its entire axial length. 
     FIG. 8  shows the arrangement of  FIG. 7  after the hub part  22  has been pressed into the rotationally symmetrical part  21 . The pressure, developed in the process and acting outward, between both contact surfaces  24 ,  25  is introduced into the main cross section of the rotationally symmetrical part  21  only in the region of the constriction  33 . This again results in an angular deviation  23 *, which causes a displacement of the tip cone  27  (broken line) to the position  27 * (solid line). This force directed outward is indicated by the arrow  28 *. A weld preparation has deliberately not been depicted here, since it is established in accordance with the respectively selected welding process. The two parts  21 ,  22  are now welded, for example by means of a high-energy beam, in particular by means of a laser. 
     FIG. 9  shows the connected parts after the welding and cooling. The welding  30  has been carried out from the side on which the constriction  33  is also located. The cold weld again exerts a tensile force, indicated by the arrows  28 ′, on the main cross section with the area center  32  of the rotationally symmetrical part  21 , this tensile force again being offset axially relative to the area center  32  by a distance  34 ′. The angular restoration thus brought about, arrow  23 ′, returns the tip cone  27 * and the associated functional surfaces back into the position  27 ′, which, given the correct design, is again equal to the original position  27 . 
   In the variant in  FIG. 10 , the contact surface  33  of the hub part  32  is not conical as in  FIG. 4  but consists of two (or more) stepped cylindrical surfaces  34 ,  35  of different diameters, separated by a conical bevel  36 . 
     FIG. 11  shows the stress characteristic in the contact surfaces  14 ,  15  in the arrangement in  FIG. 4 . There, the line of the contact surface  14  may be regarded as the zero axis, from which the local stresses are plotted, positive toward the top and negative toward the bottom. The curve  40  represents the stress characteristic after the hub part has been pressed in and is approximately a straight line; the curve  41  represents the stress distribution during the welding, that is to say at maximum temperature; and the curve  42  represents the shrinkage stress produced during the cooling of the metal of the weld pool. The curve  43  is then the resultant of the curves  41  and  42 ; the areas  44  and  45 , in each case hatched, are identical. The depth of the weld (not depicted) is designated by  46 . 
     FIG. 12  shows the effects which can be achieved in practice with the method according to the invention. The horizontal axis  50  is a time axis without a scale, from which the positive deviations are depicted toward the top and the negative deviations are depicted toward the bottom. Ranges defined with the brackets  51 ,  52  are the ranges of the permissible positive and negative angular deviations, respectively. The actual dimensions of the finished workpiece are to lie within this range. However, the welding according to the prior art produces an angular deviation which results in a tolerance zone designated with the bracket  53  and displaced toward a positive angular deviation. It can be seen that only a small part lies within the range of the permissible angular deviation. Its average value is at the distance  54  above the zero axis of the tolerance zones. This is remedied by the measures according to the invention, which return the tolerance zone  53 , given the correct design, into the range  55 , which lies exactly symmetrical to the zero axis of the desired tolerance zone.