WO1992003637A1

WO1992003637A1 - Variable valve timing

Info

Publication number: WO1992003637A1
Application number: PCT/GB1991/001360
Authority: WO
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Company Limited; Ford Werke A.G.; Ford France S.A.; Ford Motor Company
Priority date: 1990-08-11
Filing date: 1991-08-09
Publication date: 1992-03-05
Also published as: GB9017636D0; GB2246834A

Abstract

The invention provides a brake for applying a retarding torque to a reaction member (32) rotatable with a camshaft drive pulley (14) to bring about a change in the angular position of the camshaft (10) relative to the drive pulley (14), the brake comprising a locking member (40) acting between a stationary surface fixed relative to the engine and an opposed surface on the reaction member (32), the friction between the locking member (40) and the rotating reaction member acting to move the locking member (40) in a sense to increase the braking force.

Description

Title

VARIABLE VALVE TIMING

Field of the invention

The present invention relates to a variable valve timing system.

Description of known prior art

Various variable valve timing mechanisms have been proposed by the present Applicants which are actuated by mean :f an external brake. Amongst such proposals in that in European Patent Application No. 89310791.2 in which a reaction member is braked relative to a drive member to cause the phase of the camshaft to be varied. For as long as the brake is applied in this case, the phase of the camshaft is advanced or retarded by a suitable mechanism.

In another proposal, described in British Patent Application No. 9008732.1, the camshaft tends to remain in one of two stable positions and is moved out of its stable positions by means of a brake to trigger a change of phase towards the other stable position.

A brake band can be used to effect such braking and has been proposed in the above applications. However, such a brake band is difficult to package within the restricted available space.

Object of the invention

The present invention seeks to provide a brake mechanism which is compact yet capable of applying sufficient braking force to effect or trigger a change in phase of camshaft. Summary of the invention

According to the present invention, a brake is provided for applying a retarding torque to a reaction member rotatable with a camshaft drive pulley to bring about a change in the angular position of the camshaft relative to the drive pulley, the brake comprising a locking member acting between a stationary surface fixed relative to the engine and an opposed surface on the reaction member, the friction between the locking member and the rotating reaction member acting to move the locking member in a sense to increase the braking force.

Because of the positive feedback resulting from the fact that as the friction between the locking member and the reaction member increases, the locking member is dragged by the rotation of the reaction member in a direction to increase the friction still further, the brake is self- jamming and can only be off or fully on. Only a small force is needed to move it to a position where it starts to grip between the stationary surface and the rotating reaction member and thereafter the inertia of the reaction member is used to jam the locking member in its maximum braking position.

Preferred features of the invention

In a preferred embodiment of the invention, the locking member is rotatable between a first position in which a clearance remains between the reaction member and the locking member, and a second position in which the locking member engages the axial end face of the reaction member and the stationary surface simultaneously.

Conveniently, the reaction member may have a discontinuous frictional surface engaged by the locking member whereby, in at least one angular position of the reaction member, even when the locking member is in the second position, the braking force on the reaction member is released and the locking member is freed to move back to its first position.

The locking member can in this case be very compact and can be housed even in a narrow gap between the cam drive pulley and the cylinder head block. The energising system of the brake can be very simple since a brief actuation will resul in the jamming of the locking member between the rotating member and the fixed engine surface. Thereafter, if the locking member is resiliently urged into its first position, then it will automatically release itself as the reaction member reaches the next release position.

It is important to ensure that the reaction member does not come to a full stop before the locking member can be released. This can be achieved by balancing of the braking force exerted in the jammed position against the inertial forces acting on the reaction member.

The braking force can be regulated by providing a spring either between the friction surface of the reaction member and the body of the reaction member or between the locking member and the fixed surface. Such a spring will limit the maximum braking torque that can be applied even with the locking member fully jammed between the two surfaces.

Conveniently, the braking surface on the reaction member can be formed of spring ramps.

Alternatively, the frictional surface on the reaction member may be formed of separate small friction pads spaced from one another by smoothly polished low friction faces so that the braking force is always applied in intermittent bursts and the brake can be released by releasing the force tending to move the locking member into the jamming position. According to a further feature of the invention, the locking member may be designed in a manner analogous to a one-way clutch, being formed of rolling elements received in apertures in a cage and sandwiched between the reaction member surface and ramps formed in the stationary surface.

The cage may be spring biased into a position in which it . keeps the rolling elements away from a jamming position and when the cage is turned in the direction of rotation of the reaction member, it moves the rolling elements into engagement between the reaction member and the stationary ramp surfaces in a self-jamming position.

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 section through a phase change mechanism which can be actuated by a brake, being a section along the line I-I in Figure 2,

Figure 2 is a section along the line II-II in Figure 1,

Figures 3 and 4 schematically illustrate different embodi¬ ments of a brake formed by a rotatable locking member,

Figure 5 is a view similar to that of Figure 1 showing an embodiment in which the brake is constituted by a one-way clutch, the Figure being a section along the line V-V in Figure 6,

Figure 6 is a section on the line VI-VI in Figure 5,

Figures 7 and 8 schematically show the rolling elements of the one-way clutch when in the braking position and the released position, respectively. Detailed description of the preferred embodiments

The phase change mechanism of both of the embodiments to be described below is a modification of that shown in the Applicant's co-pending GB Pat. Appln. No. 90087362.1 filed on 18 April, 1990.

In Figure 1, a camshaft 10 is journalled in the usual manne in a cylinder head block 12 and has a drive pulley generall designated 14 which is driven through a toothed timing belt by the engine crankshaft at half engine speed. The pulley i this case is not a rigid pulley but operates as will now be described to enable the phase of the camshaft 10 to be varied relative to the crankshaft.

The pulley 14 comprises a main body 16 which is secured to the camshaft 10 by a bolt 18. A toothed ring 20 tr en¬ gaged by the toothed belt (not shown) is journalIt out the main body 16 by means of a bearing 22. The connection between the main body 16 and the ring 20 is effected by a spiral spring 24 more clearly shown in Figure 2 which at one end is anchored to the inner surface of the ring 20 and at its other end is anchored to an arcuate projection 16a of the main body 16.

In the vicinity of its central region, the spiral spring 24 is connected to a collar 26 by means of a T-piece 28. If this section of the spring 26 is moved and then held radial¬ ly it will effect an angular displacement between the main body 16 and the ring 20 to bring about the desired phase shift.

Movement of the collar 26 is carried out by an eccentric 30 projecting from a reaction member 32 which is journalled about a central hub section 16b of the main body 16. If the reaction member 32 rotates about the hub 16b it moves the T- piece 28 from an innermost to an outermost position in approximately 180°. These positions correspond respectively to the dead centre position when the lobe of the eccentric is in line with the centre of rotation and the T-piece 28.

In use, the camshaft 10 is subjected to torque fluctuations as a result of the effect of the valve springs on the cams. These fluctuations will tend to exert an oscillating torque on the eccentric 30 in all but the dead centre positions, which are stable. The pulley 14 is designed to take advan- tage of these torque fluctuations to drive the eccentric from one dead centre position to the other and this is achieved by the provision of a one-way or overrunning clutch 34 acting between the reaction member 32 and the ring 20. This one way clutch 34 ensures that only one half of the torque fluctuations is effective in bringing about a phase change and if the eccentric 30 is in some way moved out of a dead centre position then it will be made automatically to rotate towards the other dead centre position by the one-way clutch 34 and will stop on reaching the other stable posi- tion. A spring biased detent 36 cooperating with depres¬ sions in the main body 16 helps to retain the eccentric in its two stable dead centre positions.

The invention is concerned with the manner in which the eccentric is moved out of its stable positions when it is desired to trigger a phase change. It has already been proposed for this purpose to use a brake but in view of the limited space available and the speed with which the brake is expected to respond, the design of the brake calls for special attention.

In the present invention, the braking is effect by placing a releasable locking member in the gap between the pulley 14 and the cylinder head block 12. This makes for a compact arrangement and allows a high braking force to be applied. Furthermore, the locking member is designed to be a self- jamming on/off device deriving most of its power from the rotation of the reaction member which is being braked.

In the embodiments of the Figures 1 to 4, the locking member is an asymmetrical wedge 40 mounted on a shaft 42. The reaction member 32, in the case of Figure 2, has a sprung ramp 44 projecting from its rear surface towards the cylin¬ der head block 12. If rotated counter-clockwise as viewed in Figure 3, the wedge 40 is engaged by the sprung ramp 44 which moves in the direction of the arrow 46. Friction between the ramp 44 and the wedge 40 cause the wedge to rotate into a position where it is jammed still further between the cylinder head block 12 and the ramp 44. The reaction member 32 is braked in this manner to move the eccentric 30 out of its dead centre position and on reaching the end of the ramp 44 the brake is released to allow the wedge 40 to spring back to its rest position but the reac¬ tion member 32 continues to rotate relative to the main body 16 under the action of the torque fluctuations and the one¬ way clutch 34.

The embodiment of Figure 4 differs from Figure 3 in that the sprung ramp 44 is replaced by separate braking pads 50 separated from one another by smooth low friction sections 52 of the reaction member 32. A resilient pad 54 may be provided to ensure that the wedge 40 does not jam too tight and bring the reaction member to a complete stop as this might prove difficult to release.

Where the torque fluctuations are sufficient to move the reaction member effectively, as is the case in an in-line four cylinder engine with twin overhead camshafts, the brake need only be operated momentarily and the mechanism then left to find its own next stable position. Where on the other hand such torque fluctuations are weak, it is possible to apply to the brake for as long as necessary to achieve the desired retardation of the reaction member 32. In the latter case, the braking will be intermittent in that

the wedge will be released at the end of each ramp 44 or friction pad 50 and this provides for reliable control since the wedge can be released at will at any time after the desired amount of movement of the reaction member is sensed to have taken place.

The embodiment of Figure 5 differs from the previously described embodiments only in the design of the brake be¬ tween the reaction member 32 and the cylinder head block 12. The pulley 14 need not therefore be described again.

The brake 60 in this embodiment is a one-way clutch formed of rolling elements 62 trapped between stationary ramps 64 in the cylinder head block 12 and the reaction member 32. the rolling elements 62 are held in apertures in a cage 66 which has a radially projecting arm 70 by means of which it can be manipulated. The rolling elements 62 in the aper¬ tures are biased towards the ramps 64 by springs 68 acting between the cage 66 and the rolling elements 62.

In the normal position shown in Figure 8, the cage 66 abut the rolling elements 62 and keeps them out of contact with the ramps 64. This may be achieved by biasing the cage 66 into this released position be a spring having enough strength to disengage the rolling elements 62 from their ramps 64.

When the reaction member 32 is to be braked, the cage 66 is rotated in the direction of rotation of the pulley 14 against its spring bias into the position shown in Figure 7. The rolling elements 62 can now be jammed between the ramps 64 and the reaction member 32 in the same manner as in a one-way clutch to apply a braking torque to the reaction member. Once again, it is noted that the brake is located in the gap between the pulley 14 and the cylinder head block 12 and is self jamming. In this embodiment, it is desirable to ensure that the ramp is not too shallow and the rolling elements may become s severely wedged in place as not to be released by the sp of the cage 66. If desired, release depressions can be provided in the surface of the reaction member to achiev intermittent release of the braking.

Claims

1. A brake for applying a retarding torque to a reac¬ tion member (44) rotatable with a camshaft drive pulley (14) to bring about a change in the angular position of the camshaft (10) relative to the drive pulley (10) , characte¬ rised in that the brake comprises a locking member (40) acting between a stationary surface fixed relative to the engine (12) and an opposed surface on the reaction member (32) the friction between the locking member (40) and the rotating reaction member (32) acting to move the locking member (40) in a sense to increase the braking force.

2. A brake as claimed in claim 1 , wherein locking member (40) is rotatable between a first position in which a clear¬ ance remains between the reaction member and the locking member, and a second position in which the locking member engages the axial end face of the reaction member and the stationary surface simultaneously.

3. A brake as claimed in claim 2, wherein the reaction member (40) has a discontinuous frictional surface (44) engaged by the locking member whereby, in at least one angular position of the reaction member, even when the locking member is in the second position, the braking force on the reaction member is released and the locking member is freed to move back to its first position.

4. A brake as claimed in claim 3, wherein the braking surface on the reaction member (32; is formed of sprung ramps (44) .

5. A brake as claimed in claim 3, wherein the frictional surface on the reaction member is formed of separate small friction pads (50) spaced from one another by smoothly polished low friction faces (52) so that the braking force is always applied in intermittent bursts.

6. A brake as claimed in any preceding claim, wherein the braking force is regulated by providing a spring betwee the friction surface of the reaction member and the body of the reaction member.

7. A brake as claimed in any of claims 1 to 5, wherein the braking force is regulated by providing a spring between the locking member and the fixed surface.

8. A brake as claimed in claim 1, wherein the locking member is formed of rolling elements (62) received in aper¬ tures in a cage (66) and sandwiched between surface of the reaction member (32) and ramps (64) formed in the stationary surface.

9. A brake as claimed in claim 8, wherein the cage is movable and spring biased into a position in which it keeps the rolling elements away from a jamming position, turning of the cage in the direction of rotation of the reaction member serving to move the rolling elements into engagement between the reaction member and the stationary ramp sur¬ faces.