Patent Publication Number: US-2006005385-A1

Title: Method and assembly system for manufacturing an assembled camshaft

Description:
Priority is claimed to German Patent Application No. DE 10 2004 032 587.1, filed on Jul. 6, 2004, the entire disclosure of which is incorporated by reference herein.  
      The present invention relates to a method for manufacturing an assembled camshaft made up of a metallic shaft including shrunk-on cams. Furthermore, the present invention relates to an assembly system for carrying out this method.  
     BACKGROUND  
      A shrink-on method for manufacturing an assembled camshaft made up of a metallic hollow shaft and multiple cams is described in DE 32 47 636 C2, which is incorporated by reference herein. The cams are heated, threaded onto the shaft using an elevated temperature with respect to the shaft, and brought into the correct position there using a positioning device. A formfitting shrink joint between the cams and the shaft is achieved due to the subsequent temperature equalization between the cams and the metallic shaft.  
      In order to be able to thread the cams onto the shaft and to position them there with high accuracy, the cams must be heated to a temperature which is higher than the tempering temperature of the cam material. This extreme heating results in changes in the material properties of the (already hardened) cams which have an adverse effect on the wear resistance of the cams and is therefore undesirable.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to improve on the known shrink-on method for manufacturing an assembled camshaft in such a way that the material properties of the cams are not affected. Moreover, an assembly system is proposed which enables cost-effective and large scale-capable manufacturing of such cams.  
      According to the present invention, the shaft is cooled down prior to threading of the heated cams. The temperature difference, which is necessary for a non-slip fit of the cams on the shaft, is thus not generated via heating of the cams alone, but via cooling of the shaft paired with heating of the cams. The temperature to which the cams must be heated depends on the cooling temperature of the shaft and may therefore be set in a temperature range which is below the tempering temperature of the cams. In this way, a structural change of the cams may be ruled out so that the wear resistance of the cams remains unchanged during joining with the shaft.  
      This is particularly advantageous in commercial vehicle camshafts having brake cams which are exposed to great forces during operation. In fully hardened cams made of 100Cr6, for example, which are to be shrunk onto a hollow shaft made of St52-3, a definite temperature difference (of at least 150° C.) is necessary between the cams and the shaft in order to enable threading of the cams onto the shaft during manufacturing and to implement a high degree of bite of the cams on the shaft. If the hardened cams are heated to temperatures above 200° C., it results in significant “softening” of the cam hardening. According to the present invention, the shaft is cooled to a low temperature for threading and positioning of the cams onto the shaft, while the hardened cams, depending on the required joint clearance or intended bite, are only heated to temperatures between 150° C. and 200° C. In this way, the required joint clearance may be achieved, optimum bite of the cams on the shaft may be ensured, and structural change of the hardened cams may be effectively avoided at the same time.  
      If the method is used for manufacturing hollow camshafts, it is recommended to cool the shaft with the aid of a cooling lance which is inserted into the interior of the shaft.  
      For manufacturing the camshafts, an assembly system is used which includes rotatable drums for accommodating the shafts to be fitted, the cooling lances, and the cams. The cams are heated and the shafts are cooled down in these drums. The drums are situated with respect to one another in such a way that their rotational axes are parallel; their rotary motions are adjusted to one another in such a way that, at the time of assembly, the axis of the shaft to be fitted, the axes of the cams to be threaded onto this shaft, and the axis of the cooling lance are collinear with one another. The assembly system advantageously includes an axially displaceable counterholder with the aid of which the shaft and the cooling lance inserted into the shaft may be guided with high accuracy during axial displacement of the shaft, in particular during threading of the cams onto the shaft This counterholder may also be accommodated in a rotatable drum whose rotational axis is collinear with the rotational axis of the lance drum. This assembly system makes camshaft manufacturing in a continuous operation possible and is suitable for cost-effective large-scale production; loading, cooling down of the shaft, heating of the cams, assembly, temperature equalization, and unloading of the finished camshafts overlap in time, so that a high camshaft production rate may be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention is subsequently explained in greater detail based on an exemplary embodiment illustrated in the drawings, in which:  
       FIG. 1  shows a schematic representation of a shaft including cams to be shrunk on: 
           FIG. 1   a : with cams and shaft at the same temperature; and      FIG. 1   b : with heated cams and cooled shaft;        
       FIG. 2  shows a schematic representation of selected process steps during manufacturing of an assembled camshaft: 
           FIG. 2   a : insertion of a cooling lance into the shaft;      FIG. 2   b : feed of a counterholder;      FIG. 2   c : insertion of the cooled shaft into the pre-positioned heated cams;      FIG. 2   d : temperature equalization between the cams and the shaft; and      FIG. 2   e : unloading of the finished camshaft:        
       FIG. 3  shows a detailed representation of a contact area between a cooling lance and a counterholder (area III in  FIG. 2   b );  
       FIG. 4  shows a schematic top view on an assembly system for manufacturing composite camshafts;  
       FIG. 5  shows schematic sectional views of the assembly system of  FIG. 4  according to selected sections in  FIG. 4 : 
           FIG. 5   a : section Va-Va (lance drum);      FIG. 5   b : section Vb-Vb (axis drum);      FIG. 5   c : section Vc-Vc (cam drum); and      FIG. 5   d : section Vd-Vd (counterholder drum); and        
       FIG. 6  shows a detailed representation of two cam support discs according to detail VI in  FIG. 4 . 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows a schematic representation of a hardened cam  1  which is to be shrunk onto a hollow shaft  2 . In order to ensure a firm hold of cam  1  on hollow shaft  2 , internal diameter  3  of cam opening  4  at room temperature T R  is smaller then external diameter  6  of shaft  2  by what is called a “bite”  5 . If shaft  2  is cooled down to a temperature T L &lt;T R , its external diameter is reduced to a value  6 ′; if cam  1  is heated to a temperature T H &gt;T R , internal diameter  3 ′ of cam opening  4  increases with the effect that internal diameter  3 ′ of cam opening  4  is larger than external diameter  6 ′ of shaft  2  by what is called a “joint clearance”  7  so that heated cam  1  may be slid onto cooled shaft  2  (arrow  8  in  FIG. 1   b ). During cooling down of cam  1  and heating of shaft  2 , cam  1  is shrunk onto shaft  2  along with the temperature equalization, “bite”  5  preventing cam  1  from slipping on shaft  2 .  
       FIGS. 2   a  through  2   e  show a schematic representation of the process steps involved in carrying out the shrink-on method according to the present invention. An assembled camshaft  9  is manufactured from a hollow shaft  2  made of St52-3 and multiple cams made of 100Cr6. Cams  1  are inserted into cam holders  10  with the aid of which cams  1  are held in the intended relative position and relative angular position with respect to one another and heated in these cam holders. Hollow shaft  2  is aligned with respect to cam holders  10  in such a way that shaft axis  11  is collinear with axes  12  of openings  4  of cams  1  inserted into cam holders  10  (see  FIG. 2   a ). A cooling lance  13 , cooled by a fluid coolant  18 , is inserted in this position into an interior  14  of hollow shaft  2  (arrow  15  in  FIG. 2   b ), thereby cooling down hollow shaft  2  which results in shrinking of shaft  2 . A counterholder  16  is simultaneously pushed through cam openings  4  from the opposite side of cam holders  10  (arrow  17  in  FIG. 2   b ). As is apparent from the detailed representation of  FIG. 3 , counterholder  16  is provided at its end with a location opening  19  which engages in a formfitting manner a projection  20  on the end of cooling lance  13 . Highly accurate positioning of the end of counterholder  16  vis-a-vis the end of cooling lance  20  is hereby achieved, which in turn enables a highly accurate alignment of cooling lance  13  and counterholder  16 . Both interlocking ends  19 ,  20  may be designed in a different way.  
      When shaft  2  is sufficiently cooled down, cooled shaft  2  is inserted into cam holders  10  which contain heated cams  1  (arrow  15  in  FIG. 2   c ). Counterholder  16  retreats in the process (arrow  17 ′) thereby ensuring that shaft axis  11  is accurately aligned vis-a-vis cam axis  12 , so that cooled shaft  2  does not come into contact with heated cams  1 . When shaft  2  has reached the intended position, cams  1  are shrunk onto shaft  2 ; this is initially carried out slowly (based on the cooling down of cams  1  by ambient room temperature T R ) and subsequently faster (based on the temperature equalization between shaft  2  and cams  1  (see  FIG. 2   d )). Cooling lance  13  is pulled out of shaft  2  in the following unloading phase and the completely fitted camshaft is removed from cam holders  10 . ( FIG. 2   e ).  
      In order to ensure economical large-scale production of composite camshafts with the aid of the method according to the present invention, shafts  2  to be fitted and cams  1  to be fitted are kept in rotatable magazines (“drums”) in which they are supplied to the place of assembly. Such an assembly system  34  is shown in  FIG. 4  in a schematic view and in the sectional views of  FIGS. 5   a  through  5   d  using an example of drums for simultaneously accommodating eight shafts  2 :  
      A first drum  21  (axis drum) contains eight tubes  22  for accommodating hollow shafts  2  and is used for accommodating and supplying hollow shafts  2  to the place of assembly which is indicated in  FIG. 4  by shaft axis  11  and cam axis  12 . Hollow shafts  2  are inserted into axis drum  21  in an angular position  21   a  (see the sectional view in  FIG. 5   b ). After further rotation of the axis drum (arrow direction  23 ), cooling lance  13  is inserted into interior  14  of hollow shaft  2  in angular position  21   b  and remains in hollow shaft  2  for the subsequent cooling phase (during which the drum rotates further to angular position  21   c ). In angular position  21   c , the assembly position is reached in which shaft  2  together with cooling lance  13  is pushed out of axis drum  21  through cams  1  (see  FIGS. 2   c  and  2   d ). Temperature equalization subsequently takes place between shaft  2  and cams  1  until cooling lance  13  is retracted from interior space  14  of shaft  2  (see  FIG. 2   e ). Tube  22  may then be provided (in angular position  21   a ) with a new shaft  2 .  
      A lance drum  24  is shown in the sectional view of  FIG. 5   a  and a counterholder drum  25  is shown in the sectional view of  FIG. 5   d . Each of these drums  24 ,  25  contains eight tubes  26 ,  27  for accommodating cooling lances  13  and counterholders  16  and rotates synchronously with axis drum  21  (arrow directions  28 ,  29 ) around a common axis  30 ,  30 ′. In an angular position  24   b , which corresponds to angular position  21   b  of axis drum  21 , cooling lance  13  is inserted into hollow shaft  2  which is held in axis drum  21  (see  FIG. 2   a ). In the assembly position, cooling lance  13  together with shaft  2  is pushed through cams  1  and subsequently retracted from the finished camshaft  9 . In an angular position  25   b  of counterholder drum  25  situated in front of assembly position  25   c , counterholder  16  is extended and docked on tip  20  of cooling lance  13  (see  FIG. 2   b ). Counterholder  16  is retracted after completed assembly in assembly position  25   c.    
      As is apparent from  FIGS. 4 and 5   c , a cam drum  31  is situated axially offset vis-a-vis axis drum  21 , the cam drum containing multiple cam holder discs  32  each having eight cam holders  10  for accommodating cams  1 . Cam drum  31  is rotatable around a rotational axis  35 . The number of cam holder discs  32  corresponds to the number of cams  1  which are to be assembled on shaft  2 . Rotation  33  of cam drum  31  is synchronized with rotation  23  of axis drum  21 . This means that at the time of assembly (when a given shaft  2  is situated in assembly position  21   c  of axis drum  21  and associated cams  1  are situated in an assembly position  31   c  of cam drum  31 ) axes  12  of cams  1  are collinear with shaft axis  11 . In a loading position  31   a  opposite assembly position  31   c , cams  1  are inserted into cam holders  10  of cam holder discs  32  in a predetermined alignment. Cams  1  fixed in cam holders  10  are heated during further rotation of cam drum  31 . In assembly position  31   c , shaft  2  is inserted through cams  1  into cam drum  31  (see  FIGS. 2   c  and  2   d ). During subsequent further rotation of cam drum  31 , the above-described temperature equalization takes place between cams  1  and shaft  2  via which cams  1  are shrunk onto shaft  2 . In unloading position  31   d  of cam drum  31 , completely fitted camshaft  9  is removed from cam drum  31 .  
      An assembly system  34  having eight tubes  22 ,  26 ,  27  and eight cam holders  10  is shown in the exemplary embodiment of  FIGS. 4 and 5 ; the assembly system may, of course, also have a greater or smaller number of tubes and cam holders.  
      In addition to or instead of cams  1 , other elements, e.g., bearing rings, may also be mounted on a hollow shaft using the method according to the present invention.  
      In addition to the described application on hollow camshafts  2 , the method may also be used mounting cams  1  on solid shafts. In this case, however, the shaft cannot be cooled with the aid of a cooling lance  13  which is inserted into interior  14  of shaft  2 .