Rolling element phaser

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.

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
 retrofitted 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, a variable phase coupling has
 already been proposed in the Applicants' co-pending International Patent
 Application PCT/GB98/02153, now WO 99/06675, published on Feb. 11, 1999,
 to provide which comprises a drive member for connection to the crankshaft
 having grooves of a first pitch, a driven member for connection to the
 engine camshaft having helical grooves of a different pitch facing towards
 the grooves in the drive member, balls engaged in the two helical grooves
 and serving to couple the drive and driven members for rotation with one
 another, an intermediate member disposed between the drive and driven
 members in contact with the balls, and means for displacing the
 intermediate member relative to the drive and driven members, the
 displacement of the intermediate member serving to move the balls relative
 to the helical grooves in the drive and driven members so as to vary the
 phase between the drive and driven members. In the latter co-pending
 patent application, the intermediate member has grooves on its inner and
 outer surfaces and two sets of balls are provided, the first set engaging
 in the pairs of helical grooves comprising the helical grooves in the
 driven member and the facing grooves on one surface of the intermediate
 member and the second set of balls engaging in the pairs of helical
 grooves that comprise the grooves in the drive member and the facing
 grooves on the other surface of the intermediate member.
 The drive, driven and intermediate members in the latter proposal thus lie
 radially one inside the other 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 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 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,
 wherein a cage is provided between the intermediate member and the drive
 and driven members to retain the balls in relation to one another, and
 wherein 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.
 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. This is achieved in the first aspect of the
 invention by suitable choice of the pitch of the different grooves. Thus,
 by forming one of the grooves in a set with slightly different pitch angle
 from the others the balls have a defined axial position. Biasing of the
 ball in the axial direction will then ensure that the balls remain firmly
 in contact with the tracks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 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 FIG. 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 FIG. 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 FIG. 7) having internal helical grooves 32
 surrounds the two inner ball races 18 and 26 and is coupled for rotation
 with the two inner 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 FIG. 6)
 arranged between the intermediate member 30 and the inner ball races, the
 cage being retained axially by means of an upturned tongue 62 (see FIG. 3)
 that engages in a recess in the end of the intermediate member 30.
 A corrugated spring 70 is arranged between the two inner 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 inner
 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, which serves as an outer ball race, axial
 displacement of the intermediate member will act to rotate the inner 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 FIG. 9, is to provide one helical groove
 20', 28' on each race 18, 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.