Abstract:
A camshaft assembly is disclosed which comprises 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, the second group being rotatably mounted on the outer surface of the tube and connected for rotation with the inner shaft. The connection between the second group of cam lobes and the inner shaft is effected by means of driving members whose positions are adjustable in order to compensate for significant manufacturing inaccuracies between the inner shaft and its associated group of cam lobes.

Description:
FIELD OF THE INVENTION 
     The 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 while the second group is rotatably mounted on the outer surface of the tube and is connected for rotation with the inner shaft. This type of camshaft assembly is also termed a single cam phaser (SCP) camshaft, because it allows the timing of two groups of cam lobes on the same camshaft to be varied in relation to one another by relative rotation of the outer tube and the inner shaft. 
     BACKGROUND OF THE INVENTION 
     It is well known that an SCP camshaft can be very sensitive to component manufacturing tolerances and that the component parts must be made to an accurate specification in order for the camshaft to function correctly. This has an adverse effect upon the manufacturing costs of the camshaft. 
     In particular, the alignment of the holes in the drive shaft and the cam lobes through which each connecting pin is fitted is critical. If significant misalignment is present, the fitting of the connecting pin will act to align the holes and this will cause the drive shaft to lock in its bearings inside the camshaft tube. Variation in components due to manufacturing tolerances can therefore result in the inner shaft being unable to rotate relative to the outer tube of the camshaft. An example of the current practice for connecting cam lobes to the inner drive shaft is shown in GB-A-2375583. 
     OBJECT OF THE INVENTION 
     The present invention seeks to overcome the effect of manufacturing tolerances by providing a method for connecting the camshaft lobes to the inner drive shaft that allows the shaft to control the angle of the cam lobes, but does not dictate the axis of rotation of the drive shaft. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided 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, the second group being rotatably mounted on the outer surface of the tube and connected for rotation with the inner shaft by means of driving members whose positions are adjustable in order to compensate for significant manufacturing inaccuracies between the inner shaft and its associated group of cam lobes. 
     In one embodiment of the invention, the driving members comprise a drive pin and a drive sleeve, the drive pin being firmly received in a transverse bore in the inner shaft of the camshaft and the drive sleeve being loosely mounted to surround the outer tube of the camshaft, and wherein the drive sleeve is firmly engaged by the drive pin and is coupled to cam lobes that are rotatably mounted on the outer tube by formations that permit the drive sleeve to move transversely to the axis of the drive pin. 
     In an alternative embodiment of the invention, the driving members are constituted by a compound driving pin formed of a plurality of parts having contact surfaces for mating with the inner shaft of the camshaft and the cam lobes on the outer tube, the contact surfaces being movable to allow them to be separately aligned with the inner shaft and the cam lobes during assembly and being lockable in situ to maintain their correct alignment after assembly. 
     As can be seen, the driving members may take on a wide variety of different forms, but the novelty of the invention does not reside in the particular form that the driving members adopt. The invention is predicated on the realisation that the driving members must allow for the fact that the coupling formations, usually holes, in the drive shaft and the associated cam lobes are not always necessarily in perfect alignment with one another and it does not therefore suffice simply to drive a cylindrical pin through such holes. 
     The different embodiments of the invention offer the advantage that components can be manufactured to a lower level of accuracy, resulting in reduced overall system cost. Furthermore, certain embodiments of the invention offer additional possibilities for designing moving cam lobes as a sub-assembly, to simplify the assembly process. 
    
    
     
       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. 1A  is a perspective view of an SCP camshaft of a first embodiment of the invention, 
         FIG. 1B  is a exploded view of the driving connection between the inner shaft and a movable cam lobe in the embodiment of  FIG. 1A , 
         FIG. 2A  is a side view of an SCP camshaft of a second embodiment of the invention, 
         FIG. 2B  is a section along the line B-B in  FIG. 2A , 
         FIG. 2C  is a section along the line C-C in  FIG. 2A , 
         FIG. 2D  is a partially exploded perspective view of the camshaft of  FIG. 2A , 
         FIG. 2E  is a partially cut-away perspective view of the camshaft of  FIG. 2A , 
         FIG. 3A  is section similar to that of  FIG. 2C  showing a modification of the second embodiment of the invention using blind bores in a cam lobe or sensor ring, 
         FIG. 3B  is section similar to that of  FIG. 3A  but showing the position of the components after they have been locked in place, 
         FIG. 4A to 4E  are views corresponding to  FIGS. 2A to 2E  respectively showing a fourth embodiment of the invention, 
         FIG. 5A  shows a perspective view of a multi-part driving pin, 
         FIG. 5B  is an exploded view of the driving pin of  FIG. 5A , 
         FIGS. 6A and 6B  are view similar to  FIGS. 5A and 5B  respectively showing an alternative design of a multi-part driving pin, 
         FIG. 7A to 7E  are views corresponding to  FIGS. 2A to 2E  respectively showing a further embodiment of the invention, and 
         FIG. 7F  shows the part of  FIG. 7B  contained with the circle designated F drawn to an enlarged scale. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The construction and principle of operation of SCP camshafts is well known and will not be described herein in detail. The sections of  FIGS. 2B ,  4 B and  7 B suffice to explain their operation for the present context. Each of these camshafts  10  has an inner shaft  12  surrounded by an outer tube  14 . Selected cam lobes  16  are firmly mounted (such as by heat shrinking) on the outer tube and are fast in rotation with the outer tube  14 . Other cam lobes  18  are journalled to rotate freely about the outer tube  14  and are connected by a driving connection, which is the subject of the present invention, for rotation with the inner shaft  12 . In this way, rotating the inner shaft  12  relative to the outer tube  14  has the effect of altering the phase of the cam lobes  18  relative to the cam lobes  16 . A crankshaft driven phaser (not shown) mounted to one end of the camshaft drives the camshaft  10  and allows the phase of the outer tube  14  and/or the inner shaft  12  to be set as desired relative to the phase of the engine crankshaft. In addition to cam lobes  16  and  18 , the outer tube  14  carries bearing sleeves  20  for rotatably supporting the camshaft in pillar blocks in the engine cylinder block or cylinder head and sensor rings  22  to permit the angular positions of the inner shaft  12  and/or the outer tube  14  to be measured. 
     The problem addressed by the present invention can readily also be understood from  FIG. 2B . The connection between the cam lobes  18  and the inner shaft  12  is conventionally established by inserting a straight pin into aligned holes in the inner shaft and the cam lobes. However, such alignment is subject to manufacturing tolerances and, in the event of a slight inaccuracy, the insertion of the pin can force one or other of the inner shaft and the outer tube off axis with the result that the two are locked and cannot rotate relative to the camshaft tube  14 . 
     To mitigate this problem, in the embodiment of  FIGS. 1A and 1B  a coupling sleeve  30  is loosely fitted over the camshaft tube  14  and is connected for rotation with the inner drive shaft  12  via a connecting pin  32 , which is itself locked in position in the inner shaft  12  by means of a fixing peg  34 . The coupling sleeve has key slots  36  in its two faces that transfer drive to the adjacent cam lobes  18  via dogs  38  or other keying formations protruding from their faces. 
     If the axes of the key slots  36  in the sleeve  30  are perpendicular to the axis of the connecting pin  32 , the axis of rotation of the cam lobes  18  will be completely independent from that of the inner drive shaft  12 . Therefore any manufacturing inaccuracies in the positions of the connecting pin bores will not cause the camshaft to lock. 
     A further advantage offered by this embodiment of the invention is that the moving cam lobe components may all be identical if the angle of the connecting pin bore is chosen carefully. A collar on the sides of the moving cam lobes can prevent them from moving apart, which would cause the keying formations to become disengaged. 
     In the embodiment of the invention shown in  FIG. 2A to 2E , the movable cam lobes  18  are connected to the inner drive shaft  12  via a two-piece connecting pin  50  constructed as a nut  50   a  and a bolt  50   b . The shank of the bolt  50   b  passes with clearance through a hole in the drive shaft  12 , whilst the head of the bolt  50   b  and the nut  50   a  ends are a tight clearance or interference fit in the cam lobe  18 . The nut  50   a  and the bolt  50   b  constituting the connecting pin  50  can be clamped to flat surfaces  12   a  provided on each side of the drive shaft  12  (as best shown in  FIG. 2E ). 
     The angular alignment of the connecting pin  50  is dictated by the flat surfaces  12   a  of the drive shaft  12 , but the position of the connecting pin axis is dictated only by the bore in the moving cam lobe  18 , not the bore through the drive shaft. Hence the bore in the drive shaft can be machined less accurately because any misalignment with respect to the connecting pin bore in the cam lobe will simply result in the connecting pin taking up an eccentric position. 
     It can be seen from the cutaway view of  FIG. 2E  that the inner shaft  12  may be machined with two flats  12   a  along its whole length, which eliminates any angular tolerance between different connecting pins. This is not however a requirement of this design, as it would be alternatively possible to have a counter-bore on each end of the holes through the shaft to provide a seat for the two halves of the connecting pins. 
     The nut  50   a  of the connecting pin  50  is shown with two anti-rotation flats to aid assembly, but there are many alternative designs. All that is required is some method, such as a slot, to prevent the nut  50   a  from rotating as the connecting pin is tightened. 
     In some cases, it is not possible to design sensor rings or cam lobes with through-holes for receiving a connecting pin. As is shown in  FIGS. 3A and 3B , the concept of using a connecting pin designed as a nut and bolt can be adapted to suit such situations by allowing the nut  50   a  to sit captive in a blind bore in the sensor ring  22  (or a cam lobe if necessary). Conventional hollow pins with an expanding peg pushed into their bore could be used in these cases, but they would interfere with dismantling of the camshaft. 
     The section of  FIG. 3A  shows the nut  50   a , as it would be positioned for assembly of the sensor ring on to the outer tube  14 . The section of  FIG. 3B  shows the final assembled arrangement where the bolt  50   b  has drawn the nut  50   a  out of the bore in the sensor ring  22  and clamped it into position on the flat surface of the inner drive shaft  12 . 
     The embodiment of  FIGS. 4A to 4E  uses a connecting pin  60  formed in two halves  60   a  and  60   b , each of which has a tubular section which engages firmly in a bore in the inner shaft  12  and an eccentric head that engages firmly in a hole in the cam lobe  18 . Any variation in manufacturing tolerances will be compensated for by the rotational position taken up the eccentrics. 
     The connecting pin  60  is made up of two identical parts  60   a  and  60   b  that can be assembled into each side of the moving cam lobe  18 . The two parts of the connecting pin  60  are then secured in place by inserting an interference fit peg  62  through the centre. The peg  62  expands the connecting pin  60  to retain it in the inner drive shaft  12 . 
     It should be noted that the eccentrics are not offset along the axis of the camshaft, but rather at an angle of around 45° to the camshaft axis. This configuration is created by machining the bores in the inner drive shaft  12  and the moving cam lobes  18  with a deliberate offset. Variations in manufacturing tolerances will then cause the installed eccentric angle to vary either side of 45°. This approach increases the stiffness of the connecting pins and ensures that the eccentrics will not rotate when torque is applied to the cam lobes  18 . 
     A number of different designs are possible for creating eccentrics on the connecting pin. In  FIGS. 5A and 5B  loose eccentric sleeve components  74   a  and  74   b  are simply retained in position and are free to rotate to the most ‘ideal’ position at all times about the shank  70   a  and  70   b  of the connecting pins. Similarly in  FIGS. 6A and 6B , loose sleeves  84   a  and  84   b  are free to rotate relative to the central shank  80  about the fixing pegs  82   a  and  82   b  serving to retain the central shank  80  in a transverse bore of the inner shaft  12 . 
     The embodiment of  FIGS. 7A to 7F  uses two connecting pins  90  made up of two parts  90   a  and  90   b  with barrelled surfaces in contact with the bores of the inner drive shaft and the moving cam lobes. The barrelling of the pin parts is best shown in  FIG. 7F , where it is much exaggerated for ease of understanding. In reality, the barrelling would be closer to that found on a needle roller element. 
     The barrelling of the pin parts  90   a  and  90   b  allows their position to compensate for any manufacturing tolerances in the inner drive shaft and the cam lobe because the barrelled pins are not constrained to lie on the axis of either bore. 
     Once inserted, the connecting pins are retained by an additional peg  92  pressed through their central bore. If a single peg  92  is used to lock the parts  90   a  and  90   b  of the connecting pin  90  in position, it is possible for final machining (reaming etc) of the central bores of the connecting pins to be carried out after they have been assembled into the camshaft. This will ensure that the peg  92  will lock them in the ideal position when it is inserted and not force them into a new position that could cause the camshaft to jam. 
     It would alternatively be possible to have separate pegs  92 , one in each connecting pin part so that the connecting pin parts could be finish machined before assembly.