Patent Publication Number: US-7210440-B2

Title: Camshaft assembly

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority under 35 USC 119 of United Kingdom Patent Application No. 0505296.4 filed Mar. 16, 2006. 
   FIELD OF THE INVENTION 
   The present invention relates to a camshaft assembly comprising an inner shaft, an outer tube surrounding and rotatable relative to the inner shaft, and two groups of cam lobes mounted on the outer tube, the first group of cam lobes being fast in rotation with the outer tube and the second group being rotatably mounted on the outer surface of the tube and connected for rotation with the inner shaft by means of pins that pass with clearance through slots in the outer tube. 
   BACKGROUND OF THE INVENTION 
   An adjustable camshaft assembly as set forth above, herein also termed an SCP (single cam phaser) camshaft, allows variable valve timing to be implemented in engines having different valves operated by lobes on the same camshaft. A phaser mounted on one end of the SCP camshaft allows the inner shaft and/or the outer tube to be rotated relative to a crankshaft driven pulley to permit the timing of at least one of the two groups of cam lobes to be altered in relation to the crankshaft timing. 
   There are numerous known types of phase change mechanisms, or phasers, some of which, for example vane-type phasers, are hydraulically operated. Within such phasers intended for use with a solid camshaft, it is known to incorporate a spring to bias the phaser into an extreme end position, to enable the engine to start and idle correctly while there is still insufficient oil pressure to operate the phaser. An example of such a phaser is described in US 2003/0217718. 
   OBJECT OF THE INVENTION 
   A problem is however encountered in the prior art in finding sufficient space within a phaser to accommodate a spring to bias the inner shaft of an SCP camshaft relative to the outer tube, bearing in mind that there are severe constraints on the overall size of the phaser. 
   SUMMARY OF THE INVENTION 
   With a view to mitigating the foregoing disadvantages, the present invention provides a camshaft assembly comprising an inner shaft, an outer tube surrounding and rotatable relative to the inner shaft, and two groups of cam lobes mounted on the outer tube, the first group of cam lobes being fast in rotation with the outer tube and the second group being rotatably mounted on the outer tube and connected for rotation with the inner shaft by means of pins that pass with clearance through slots in the outer tube, wherein a compliant member is incorporated in the camshaft assembly to bias the inner shaft relative to the outer tube towards one extreme of its angular range. 
   In the invention, the spring biasing the components of an SCP camshaft into a position suitable for starting the engine forms part of the camshaft not the phaser driving the camshaft. As will be clear from the ensuing description, there are numerous suitable locations for such a spring on the camshaft that do not create the packaging problems that occur when attempting to integrate such a spring into the phaser. 
   The compliant member, which is preferably a spring, may suitably be connected to the outer tube via a camshaft bearing, a camshaft lobe or a sensor ring. 
   The compliant member may be connected to the inner shaft via an intermediate component fixed in rotation to the drive shaft, for example a cam lobe or a sensor ring. 
   One or more compliant members may be housed inside one of the camshaft bearings, between two adjacent cam lobes, or in a bore of the outer tube. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a partially exploded perspective view of an SCP camshaft of a first embodiment of the invention, 
       FIG. 2A  side view of the camshaft of  FIG. 1 , 
       FIG. 2B  is an end view of the camshaft of  FIG. 1 , 
       FIG. 3A  is a section on the line A—A in  FIG. 2A , 
       FIG. 3B  is a section on the line B—B in  FIG. 3A , 
       FIG. 4  is a partial perspective view of a second embodiment of the invention, 
       FIG. 5  is an axial section through one end of the camshaft of  FIG. 4 , 
       FIG. 6  is a perspective view of a third embodiment of the invention, 
       FIG. 7  is a side view of the camshaft shown in  FIG. 6 , 
       FIG. 8A  is a section similar to that of  FIG. 5  shown in a fourth embodiment of the invention, taken along the line VIII—VIII in  FIG. 8B , 
       FIG. 8B  is an end view of the camshaft of  FIG. 8A , 
       FIG. 9  shows a method by which a spring may be connected to the inner shaft and outer tube of an SCP camshaft, and 
       FIG. 10  is a view of the front end of a camshaft having an integrated stop to limit the degree of angular movement of the inner shaft relative to the outer tube. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Throughout the drawings, like parts in the different embodiments have been allocated the same reference numerals and modified components serving the same function have been allocated reference numerals differing from one another by multiples of one hundred. 
     FIGS. 1 to 3  show an SCP camshaft  10  connected at its front end to drive sprocket  11  which incorporates a vane-type phaser. The principle of operation of an SCP camshaft and a vane-type phaser are both known and well documented in the prior art and they need not therefore be described herein in detail. It suffices in the present context to understand that the SCP camshaft is formed of an inner shaft  12  and an outer tube  14  that can be rotated relative to one another through a limited angular range by means of the phaser  11 . The outer tube  14  carries two groups of lobes of which the first group of lobes  16  is fast in rotation with the outer tube  14  and the second group  18  can rotate on the outer surface of the outer tube  14  and is connected for rotation with the inner shaft  12  by means of pins  20  that pass with clearance through circumferentially elongated slots in the outer tube  14 . When the inner shaft  12  is rotated relative to the outer tube  14 , the two groups of lobes rotate relative to one another and thereby vary the timing of valve operated by the respective cam lobes. 
   It is desirable in such a camshaft to provide a spring to bias the inner shaft  12  relative to the outer tube  14  towards one angular position establishing suitable valve timing conditions for the engine to start and to idle until sufficient hydraulic pressure has been built up to enable the phaser  11  to function correctly. Hitherto, such a spring has been incorporated into the phaser  11  but, because space within a phaser is at a premium, the present invention incorporates a biasing spring in the camshaft, the different embodiments illustrated in the drawings showing various methods by which this may be achieved. 
   In the embodiment of  FIGS. 1 to 3 , a bearing sleeve  30  having slots  32  to allow it to form part of an angular position sensor is mounted at one end for rotation with the outer tube  14  of the camshaft. A helical torsion spring  38  has its axially outer end bent radially inwards and its opposite end bent to projecting axially from the coil of the spring. The axially projecting end is received in a hole in the sleeve  30  while the radially bent end is received in a radial slot  35  in the end of the inner shaft  12 . An end plate  34  fitted over the end of the inner shaft  12  and retained on it by means of a circlip  36  serves to hold the spring  38  in position within the sleeve  30  and also prevents the sleeve  30  and the outer tube  14  from moving axially and sliding off the end of the inner shaft  12 . 
   The embodiment of  FIGS. 4 and 5  differs from that of the previous figures in that the end plate  134  has a notch  133  engaged by a key  137  inserted into the inner shaft  12  so that the end plate  134  is fast in rotation with the inner shaft. In this case, the spring  138  has two axially projecting ends one engaging as previously in a hole in the sleeve  132  and the other engaging in a hole  140  in the end plate  134 . The end plate  134  also has two diametrically opposed holes  142  to be engaged by an assembly tool. 
   To assemble the camshaft of  FIGS. 4 and 5 , the ends of the spring  138  are engaged in the holes in the sleeve  132  and the end plate  134  while the notch  133  in the end plate  134  is misaligned with the key  137 . The plate  134 , while gripped using the holes  142 , is then turned to align the notch  133  with the key  137  and pretension the spring  138 . After the plate  134  has been engaged over the key  137 , the circlip  136  is fitted into an annular slot in the inner shaft  12  to retain the spring  138  in position and once again prevent the outer tube  14  from moving axially relative to the inner shaft  12 . 
   In the embodiment of  FIGS. 6 and 7 , several springs  238  are used to bias the inner shaft relative to the outer tube. The springs have axially projecting ends that engage directly in holes in the cam lobes of the different groups  16  and  18  that rotate with the outer tube and the inner shift, respectively, of the SCP camshaft. Clearly these springs  238  can individually be more compact and use a smaller wire diameter than an equivalent single spring. 
   In the embodiment shown in  FIGS. 8A and 8B , the spring  338  is located inside the rear of the camshaft outer tube and the length of the inner shaft is reduced in order to make space for the spring. 
     FIG. 9  shows the method by which a return spring may be connected to the inner drive shaft and the camshaft tube. The drive shaft  12  is machined with a slot  435  to engage with the spring  438 , whilst the tube has a slot  450  that acts as a ‘bayonet’ fitting in order to retain the spring in position. 
   All of these return spring embodiments described above require a physical stop to limit the angular motion of the SCP camshaft.  FIG. 10  shows how an angular position stop  501  may be integrated into a bearing sleeve fitted to the front of the camshaft. 
   It is also important in all the above embodiments for the outer tube of the camshaft not to move axially relative to the inner shaft and in addition to the plates that act as end stops it is possible to provide a spring or other compliant member to bias the two apart in an axial direction.