Variable phase coupling

Abstract: A variable phase coupling 10 is disclosed for connecting a crankshaft of an engine to a camshaft 16. The coupling 10 comprises a drive member 14, 18 having a first set of grooves 20, a driven member 26 having a second set of grooves 28 and mounted in a fixed axial position relative to the drive member 18 and an intermediate member 30 movable axially in relation to the drive and driven members. Grooves are arranged on the same side of the intermediate member to face the grooves of both the drive member and the driven member. A first set of balls 36 is engaged in the grooves 20 of the drive member 18 and the intermediate member 30 and a second set of balls 34 is engaged in the grooves 28 of the driven member 26 and the intermediate member 30 so as to transmit torque from the drive member to the driven member through the intermediate member. At least some of the grooves are helical grooves so that the relative phase of the drive and driven members varies in dependence of the axial position of the intermediate member 30.

Description: Field of the invention The present invention relates to a variable phase coupling for allowing the phase of a drive member and a driven member to be changed in relation to one another. The invention is particularly applicable to a coupling for varying the phase of a camshaft in relation to the crankshaft of an internal combustion engine. Background of the invention The optimum angles at which the inlet and exhaust valves of an internal combustion engine should open and close, both in relation to one another and in relation to the engine crankshaft, vary with the engine speed and load conditions. In an engine with a fixed valve timing, a compromise setting must be adopted in which different performance parameters are traded off one against the other. To achieve improved performance over a range of engine speeds and loads, it has already been proposed to use a variable phase coupling to vary the phase of a camshaft in relation to the crankshaft and in relation to another camshaft. Several variable phase couplings are known from the prior art, each having its own advantages and disadvantages. Noise and wear are particularly serious common problems that are caused by the fact that camshafts are subjected to torque reversal during operation. While a valve is being opened by a cam on the camshaft, torque has to be applied to the camshaft in one direction to overcome the resistance of the valve spring. On the other hand, while a valve is closing, its spring attempts to accelerate the camshaft and the camshaft experiences a torque reaction from the valve train acting in the opposite direction. To suppress the noise resulting from torque reversals, it is necessary either to make the couplings very accurately or to employ some form of active backlash control. Such active backlash control conventionally contributes to an increase in sliding friction and increases the force required to bring about a change in phase. As a result, it is necessary to resort to a larger actuator and, if a hydraulic actuator is used, this also means a slower response because of the small diameter of the drillings in the camshaft that feed oil to the actuator. A further problem with some known designs is that they cannot be retro-fitted to an existing engine because they require major modification to the engine block, cylinder head or valve train. With a view to mitigating the above problems, the Applicants have earlier proposed in GB-A-2 327 737 a variable phase coupling for connecting a crankshaft of an engine to a camshaft. The coupling comprising a drive member having a first set of grooves and a driven member having a second set of grooves and mounted in a fixed axial position relative to the drive member. An intermediate member movable axially in relation to the drive and driven members has grooves facing the grooves of both the drive member and the driven member. The three members are coupled for rotation with one another by means of a first set of balls engaged in the grooves of the drive member and the intermediate member and a second set of balls engaged in the grooves of the driven member and the intermediate member. At least some of the grooves are helical grooves so that the relative phase of the drive and driven members varies in dependence of the axial position of the intermediate member. Different means are also proposed for eliminating backlash between the drive, intermediate and driven members, In the Applicants' latter earlier proposal, grooves are formed on the radially inner and the radially outer side of the intermediate member and the three members lie radially inside one another thereby requiring the coupling to have a relatively large diameter. This can create packaging difficulties when there is insufficient space to accommodate a coupling of a large diameter. Object of the invention The present invention seeks therefore to provide a variant of the coupling of the Applicants' earlier proposal which is more suitable for engines in which the radial space available to accommodate the variable phase coupling is restricted. Summary of the invention According to the present invention, there is provided a variable phase coupling for connecting a crankshaft of an engine to a camshaft, the coupling comprising a drive member having a first set of grooves, a driven member having a second set of grooves and mounted in a fixed axial position relative to the drive member, an intermediate member movable axially in relation to the drive and driven members, grooves facing the grooves of both the drive member and the driven member, a first set of balls engaged in the grooves of the drive member and the intermediate member and a second set of balls engaged in the grooves of the driven member and the intermediate member so as to transmit torque from the drive member to the driven member through the intermediate member, at least some of the grooves being helical grooves whereby the relative phase of the drive and driven members varies in dependence of the axial position of the intermediate member and means being provided for eliminating backlash between the drive, intermediate and driven members, characterised in that the grooves on the intermediate member that face the grooves of the drive and driven member are formed on the same radial side of the intermediate member and in that a cage is provided between the intermediate member and the drive and driven members to retain the balls axially in relation to one another. The invention differs from the Applicants' earlier proposal in that centres of the balls of the two sets can lie on circles of the same diameter that are axially offset from one another instead of being one inside the other. This allows a coupling to be made of a smaller diameter though the axial length of the coupling will at the same time be greater. As with the earlier proposed coupling, because of the torque reversals to which the coupling is subjected during operation, it is important to take steps to eliminate backlash. In one embodiment of the invention, at least one of the grooves in each of the drive and driven members has a slightly different pitch from the corresponding groove in the intermediate member and means are provided for resiliently urging the drive and driven members axially relative to one another. In an alternative embodiment of the invention, at least one of the grooves in one of the drive member, driven member and intermediate member is formed on a separate component from the respective member and means are provided for spring biasing of the separate component in the axial direction. Conveniently, the outer surface of the intermediate member may serve as one of the sealing surfaces of a hydraulic actuation system of the variable phase coupling. Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through a coupling of the invention in the plane containing the axis of rotation of the coupling, the Figure showing the intermediate member positioned in the middle of its range of adjustment, Figure 2 is a similar section through the coupling of Figure 1 showing the intermediate member at one of the two end positions of its range of adjustment, Figure 3 is an end view of the coupling, Figure 4 is a perspective view of the drive and driven members, Figure 5 is a view similar to that of Figure 3 showing the balls positioned in the grooves, Figure 6 is a view similar to that of Figure 5 showing the cage fitted over the balls, Figure 7 is a view similar to that of Figure 6 showing the intermediate member fitted over the balls and the cage, Figure 8 is a section through the coupling in a plane normal to the axis of rotation, Figure 9 is a projection of the races onto a flat serving to illustrate one method of eliminating backlash, and Figure 10 is a similar view to Figure 9 illustrating an alternative method of eliminating backlash. Detailed description of the preferred embodiments Figure 1 shows a section through a variable phase coupling 10 of the invention passing through the axis of rotation 12 of the coupling. The coupling comprises a drive member which in the illustrated embodiment is a gear 14 driven by an engine crankshaft and the driven member constituted by a camshaft 16 that is only partly shown in the drawings. The gear 14 is rotatably mounted on the camshaft 16 and is formed integrally with a ball race 18 (see Figure 2) having external helical grooves 20. An annular sleeve 22 is secured by means of a bolt 24 to the end of the camshaft 16 and rotates with the camshaft. A second ball race 26 (see Figure 4) having helical grooves 28 is held captive between shoulders on the camshaft 16 and the sleeve 22 and rotates in unison with the camshaft. An intermediate member 30 (see Figure 7) having internal helical grooves 32 surrounds the two ball races 18 and 26 and is coupled for rotation with the two ball races by means of two sets of balls 34, 36. The intermediate member 30 is movable hydraulically in an axial direction relative to the two races 18 and 26 in that it is connected to a piston 38 reciprocable within a working chamber contained within a cylinder 40 that rotates with the drive member and is sealed by means of a rotary seal 44 at its other end relative to a stationary support collar 42 that forms part of the engine cylinder head. An axially extending passage 46 is formed in the support collar 42 to allow oil to flow to the left hand side of the piston 38, as viewed whereas oil reaches the right hand side of the piston through a radial passage 48 in the collar 42, an annular recess 50 and radial bores 52 in the annular sleeve 22, the annular space 54 between the bolt 24 and the sleeve 22 and openings 56 in the camshaft 16. The two sets of balls 34, 36 are received in a cage 60 (see Figure 6) arranged between the intermediate member 30 and the ball races, the cage being retained axially by means of an upturned tongue 62 (see Figure 3) that engages in a recess in the end of the intermediate member 30. A corrugated spring 70 is arranged between the two ball races 18 and 26 to urge them apart and a washer 72 is arranged between the annular sleeve 22 and the ball race 26 to prevent the balls 34 from moving out of their helical grooves. The phase of the gear 14 is adjusted in relation to the camshaft 16 by axial displacement of the intermediate member relative to the two races 18 and 26. Because of the different pitches of the helical grooves 20 and 28 and the corresponding grooves in the inner surface of the intermediate member, axial displacement of the intermediate member will act to rotate the ball races relative to one another and thereby vary the phase of the drive member relative to the driven member. It is important in any mechanism driving a camshaft to eliminate backlash because the reversals of the torque transmitted through the coupling would result in severe noise and wear. In the described variable phase coupling, various means can be employed to eliminate backlash. One possibility, illustrated in Figure 9, is to provide one helical groove 20', 28' on each race 14, 26 that has a slightly different pitch from the corresponding groove 32 in the intermediate member 30. By holding the balls in a cage and resiliently urging the races axially apart or by resiliently biasing the drive and driven members apart, it is possible to take up any backlash. Another possibility is to form the helical grooves in one race with a slightly different pitch from the grooves in the other race and to position two balls within each pair of grooves that are spring biased apart to eliminate backlash. A further possibility is to form the intermediate member as a radially flexible cup whereupon backlash can be eliminated by radial clamping of the balls between the intermediate member and the ball races. In a still further possibility, shown in Figure 10, the inner and outer ball races 14, 26, 30 may be machined with grooves 20, 28, 32 having the same helix angle but one of the grooves 20", 28" in either the inner or the outer race may be formed on a separate component 74. Spring biasing of this separate component 74 in the axial direction will then remove backlash from the set of balls.

Claims: A variable phase coupling for connecting a crankshaft of an engine to a camshaft, the coupling comprising a drive member (14) having a first set of grooves (20), a driven member (26) having a second set of grooves (28) and mounted in a fixed axial position relative to the drive member (14), an intermediate member (30) movable axially in relation to the drive and driven members (14,26), grooves (32) facing the grooves (20,28) of both the drive member (14) and the driven member (26), a first set of balls (36) engaged in the grooves of the drive member (14) and the intermediate member (30) and a second set of balls (34) engaged in the grooves of the driven member (26) and the intermediate member (30) so as to transmit torque from the drive member (14) to the driven member (26) through the intermediate member (30), at least some of the grooves being helical grooves whereby the relative phase of the drive and driven members varies in dependence of the axial position of the intermediate member (30) and means being provided for eliminating backlash between the drive, intermediate and driven members, characterised in that the grooves (32) on the intermediate member (30) that face the grooves of the drive and driven member are formed on the same radial side of the intermediate member and in that a cage (60) is provided between the intermediate member (30) and the drive and driven members (14,26) to retain the balls (34,36) axially in relation to one another. A variable phase coupling as claimed in claim 1, wherein in order to eliminate backlash between the drive, intermediate and driven members, at least one of the grooves (20', 28') in each of the drive and driven members (14,26) has a slightly different pitch from the corresponding groove (32) in the intermediate member (30) and means (70) are provided for resiliently urging the drive and driven members (14,26) axially relative to one another. A variable phase coupling as claimed in claim 1, wherein in order to eliminate backlash between the drive, intermediate and driven members, at least one of the grooves in one of the drive member, driven member and intermediate member is formed on a separate component (74) from the respective member and means are provided for spring biasing of the separate component (74) in the axial direction. A variable phase coupling as claimed in any preceding claim, wherein the outer surface of the intermediate member (30) serves as one of the sealing surfaces of a hydraulic actuation system of the variable phase coupling.

Inventors: METHLEY IAN, METHLEY, IAN
Applicants: MECHADYNE PLC
Application year: 2000
Main classification: F01L1
IPC codes: F01L1