Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Reexamination Certificate
2000-02-17
2001-06-26
Lo, Weilun (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
C074S56800M, C464S002000, C464S160000
Reexamination Certificate
active
06250267
ABSTRACT:
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.
REFERENCES:
patent: 3807243 (1974-04-01), Yada
patent: 5078647 (1992-01-01), Hampton
patent: 5172661 (1992-12-01), Brune et al.
patent: 5172662 (1992-12-01), Hampton
patent: 5219313 (1993-06-01), Danieli
patent: 5803030 (1998-09-01), Cole
Lo Weilun
Mechadyne PLC
Smith-Hill and Bedell
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