WO1998016725A1

WO1998016725A1 - Phase change mechanism

Info

Publication number: WO1998016725A1
Application number: PCT/GB1997/002781
Authority: WO
Inventors: Derek Frost; David Crabb; Timothy Lancefield; Mark Eaton
Original assignee: Mechadyne International Plc
Priority date: 1996-10-16
Filing date: 1997-10-09
Publication date: 1998-04-23
Also published as: GB9621561D0; AU4630497A

Abstract

A phase change mechanism (10) is described for coupling a drive member (12) to a driven member (14). The mechanism comprises a cam plate (30) rotatable about the central axis of the mechanism and having cam surfaces (30a) that lie on opposite sides of the central axis and that are oppositely ramped in the axial direction. Cam wedges (54) are mounted for axial movement relative to the cam plate, each cam wedge having a cam surface (54a) engaging a respective one of the cam surfaces (30a) on the cam plate (30). Each wedge (54) is also pivotable at its end remote from the cam surface (54a) about an axis generally parallel to, and radially offset from, the central axis. A hydraulic jack is provided to cause relative axial movement between the cam wedges (54) and the cam plate (30), the engagement of the cam surfaces (54a) of the cam wedges (54) and the cam plate (30) causing the cam plate (30) to rotate about the central axis in response to the relative axis movement, the wedges (54) at the same time pivoting to maintain area contact between the cam surfaces on the wedges (54) and the cam plate (30). The cam wedges (54) or the cam surfaces (30a) on the cam plate (30) are allowed to rock to maintain surface contact.

Description

Phase Change Mechanism

The present invention relates to a phase change mechanism for altering the angular position of a drive member relative to a driven member, and will be described herein, by way of example, with particular reference to a mechanism for varying the phase of a drive sprocket or pulley relative to a camshaft. Such a mechanism may be used to allow the phase of the camshaft of an engine to be changed in relation to the phase of the crankshaft.

Various mechanisms have in the past been proposed to enable the valve timing of an engine to be modified to suit the driving conditions so as to improve efficiency and reduce noxious emissions. Some known mechanisms cannot be retro-fitted to existing engines as they would require major modification of the engine block and cylinder head and can only sensibly be incorporated when designing new engines. The present invention on the other hand is intended in common with some existing phase change mechanisms to be capable of being retro-fitted.

In order to meet this requirement, the phase change mechanism must be capable of being housed within the limited space available for a conventional drive sprocket or pulley and its control mechanism must be capable of being implemented without major change to the engine block or cylinder head. These requirements are particularly onerous because of the reaction forces from the valve springs that have to be tolerated by the phase change mechanism.

Conventionally the drive sprocket is attached to the camshaft by a centre bolt fixing and if retro-fitting is to be achieved without modification to the camshaft then the centre bolt fixing should be retained by a drive sprocket incorporating a phase change mechanism. A phase change mechanism that complies with all these objectives is disclosed in US-A-5, 263, 442. This patent, as summarised in its abstract, describes a valve timing control apparatus including a rotary member drivingly connected to an engine crankshaft for rotation with the engine crankshaft, and a drive mechanism for transmitting rotation of the rotary member to a camshaft. The drive mechanism includes a piston member provided for reciprocation between first and second positions within the rotary member to rotate the camshaft with respect to the rotary member. The piston member defines first and second pressure chambers on the opposite sides thereof along with the rotary member. A valve member is provided to connect one of the two pressure chambers to a pressure source and the other to a drain port to produce a pressure differential between the first and second pressure chambers to move the piston member and thereby bring about a phase change.

In US-A-5, 263, 442 axial movement of the piston is converted into relative angular displacement between the crankshaft and the camshaft by inclined surfaces on wedge- shaped cam members that move with the piston and engage with inclined surfaces that are fast in rotation with the camshaft. It is important that there should be a large area of contact between these surfaces in all positions of the piston in order to avoid excessive pressures and it is difficult to machine these surfaces to the necessary tolerances to achieve smooth and reliable operation of the mechanism.

With a view to mitigating the foregoing disadvantages, the present invention provides a phase change mechanism for coupling a drive member to a driven member, comprising a cam plate rotatable about the central axis of the mechanism and having two or more cam surfaces that are oppositely ramped in the axial direction, two or more cam wedges mounted for axial movement in synchronism with one another relative to the cam plate, each cam wedge having a cam surface engaging a respective one of the cam surfaces on the cam plate and being pivotable at its end remote from the cam surface about an axis generally parallel to, and radially offset from, the central axis, and a hydraulic jack for causing relative axial movement between the cam wedges and the cam plate, the engagement of the cam surfaces of the cam wedges and the cam plate causing the cam plate to rotate about the central axis in response to the relative axis movement, the cam wedges at the same time pivoting to maintain area contact between the cam surfaces on the wedges and the cam plate, wherein means are provided for permitting a limited degree of rotation of one of the cam surfaces on the cam wedge and the cam plate relative to the other about a generally radial axis.

In practice, the wedges can be connected to either the drive or the driven member while the cam plate is connected to the other of the two so that the phase change between the cam plate and the cam wedges causes a phase change between the drive and driven members. Furthermore the piston of the hydraulic jack may be physically connected to move either the cam plate or the cam wedges.

It is preferred for the cam wedges to be mounted on pins that are connected to the piston of the hydraulic jack and are fast in rotation with the camshaft.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a first section through a phase change mechanism of the invention, taken along the section line I-I in Figure 3,

Figure 2 is a second section through the phase change mechanism of Figure 1, taken along the section line II-II in Figure 3, Figure 3 is a section taken on the section line III-III in Figure 1 showing the phase change mechanism in a central position,

Figure 4 is a section similar to that of Figure 3 showing the cam plate after it has been rotated by axial movement of the cam wedges to end position,

Figure 5 is a side view with a portion of the mechanism cut-away to expose the engagement between the cam surfaces of one of the cam wedges and the cam plate, Figure 6 is a view similar to that of Figure 5 showing an alternative embodiment of the invention,

Figure 7 is a view similar to that of Figures 5 and 6 showing a further embodiment of the invention, and

Figure 8 is a detail of the piston of a still further embodiment of the invention.

In the Figures 1 to 5 of the drawings, a phase change mechanism of the invention, generally designated 10 is used to couple a drive sprocket 12 to a camshaft 14. The phase change mechanism 10 comprises a cylindrical housing 16 and a back plate 18 which are formed as a unitary member, and a central hub 20. The housing 16 and the hub 20 are secured by a single central bolt 22 to the camshaft 14 and are aligned relative to the camshaft 14 by a pin 24. The sprocket 12 is journalled on bearings 26 for rotation about the housing 16 and the sprocket 12 has a radial extension 12a that extends through an arcuate slot 16a (see Figure 3) into the interior of the housing 16. Within the interior of the housing 16, the extension 12a of the sprocket 12 is connected by bolts 28 to a cam plate 30.

A guide block 32 is press fitted onto the hub 20 and is prevented by a roll pin from rotating relative to the hub 20. The guide block 32 and the hub 20 are introduced as one piece into the housing 16 and when the hub 20 is tightened by the bolt 22 onto the camshaft, the guide block 32 is received between the back plate 18 and a shoulder 34 on the hub 32.

The guide block 32 serves to seal off one end of a hydraulic jack that has a moving double acting piston 36, two working chambers 38 and 40 on opposite sides of the piston 36 and a closure cover 42 that is retained in the housing 16 by a circlip 44 and seals against both the hub 20 and the housing 16. Hydraulic fluid is supplied to the working chambers 38 and 40 through eccentric axial ducts 46 and 48 (see Figure 2) .

As also shown in Figure 2, two actuating rods 50 are secured by bolts 52 to the piston 36 and pass through seals in the guide block 32. Each of the actuating rods 50 carries a respective cam wedge 54. The two cam wedges 54 are fitted onto reduced diameter portions of the actuating rods 50 and are urged by springs 56 in opposite axial directions. The cam wedges are held for axial movement with the actuating rods 50 by means of collars 58 fitted to the ends of the rods 50 to allow the cam wedges only a limited degree of free play that in use is taken up by the springs 56.

The ends of the cam wedges have oppositely inclined flat cam surfaces 54a that make contact with oppositely inclined flat cam surfaces 30a on the cam plate 30. As the cam wedges 54 are moved axially by the actuating rods 50, the interaction between the cam surfaces 30a and 54a causes the cam plate 30 to rotate relative to the housing 16 and at the same time the cam wedges 54 rotate slightly about the actuating rods 50 to maintain surface contact between the cam surfaces. The surfaces of the wedges 54 remote from the cam surfaces 54a are formed as part-cylindrical support surfaces that rest on correspondingly shaped troughs defined by the guide block 32 to prevent the actuating rods 50 from being distorted by the reaction forces on the cam wedges 54. The surfaces on the cam wedges 54 are also crowned in the axial direction, as shown by the exaggerated curve 60 in Figure 5, so that they should only make contact with the troughs in the guide block 32 in one axial plane at a time. This allows the cam wedges 54 also to rock slightly in the troughs about an axis perpendicular to the plane of the drawing in Figure 5 to accommodate tolerances in the cam surfaces .

As an alternative to putting a crown 60 on the cam wedges 54, it is possible to maintain area contact by allowing movement of the cam surface 30a of the cam plate 30. An embodiment incorporating this feature is shown in Figure 6 in which the cam surfaces of the cam plate 30 are defined by part cylindrical inserts 62 that can rotate relative cam plate 30 to take up misalignment. The embodiment of Figure 7 is similar in principle to that of Figure 6, the difference being that the part-cylindrical insert 64 in this case is built into the cam wedge rather than the cam plate.

Figure 8 is a detail that shows a modification of the piston 36 to prevent it from jamming if the reaction forces on it from the cam wedges 54 are uneven. The outer surface 68 of the piston 36 and the annular inner surface 66 are both crowned to allow the piston 36 to tilts slightly without jamming and while still sealing against the housing 16 and the hub 20.

All the components of the phase change mechanism 10 including the sprocket 12 can be pre-assembled to one another and offered as one unit to the camshaft 14. To assemble the phase change mechanism, the sprocket 12 is fitted loosely over the housing 16 with its extension 12a passing through the arcuate slot 16a. The sprocket is then bolted to its support bearings so that it may be free to rotate on the outside of the housing. In the next step, the cam plate 30 is bolted to the extension 12a of the sprocket 12. Each of the cam wedges 54 premounted on its actuating rod is then positioned with its cam surface 54a engaging the cam surface 30a of the cam plate 30 and its correct alignment can be maintained by a jig having locators passing through openings in the back plate 18. The guide block 32 and the hub 20, which are pre-assembled to one another are then slid as one piece over the two actuating rods 50 until the hub contacts the back plate 18. In the next step, the piston 36 is bolted by the bolts 52 to the actuating rods 50 and finally the cover plate 42 is slipped over the hub 22 and into the housing 16 and retained by insertion of the circlip 44.

The phase change mechanism is assembled onto the camshaft by first correctly orientating the entire assembly to align it with the pin 24. It is then held in position by a single bolt 22. The mounting automatically achieves connection of the hydraulic jack to the oil system of the engine and suitable bleed orifices (not shown) are provided to allow the hydraulic system to be bled of air after the phase change mechanism has been fitted to the camshaft.

If the phase of the drive is to remain constant, then the working chambers on opposite sides of the piston 36 are either both isolated or they are both connected to a high pressure supply at the same time, so that the piston 36 should remain stationary.

When the phase of the drive is to be changed, the hydraulic circuit acts to connect one of the chambers to a pressure supply and the other to a drain to effect movement of the piston 36 in the desired direction. This in turn causes the actuating rods 50 to move axially and with them the cam wedges 54. As the cam wedges 54 move axially, one of them will push the cam plate 30 up on one side while the other is retracted to allow it to move down on the other side, the net effect being to rotate the cam plate 30. The cam wedges will automatically realign themselves by pivoting about the axes of the actuating rods to maintain area contact between the cam surfaces 30a and 54a and any tolerances are taken up at this time by the springs 56 which allow a small axial displacement of the cam wedges 5 . The springs 56 are strong springs so that they should withstand the reaction forces of the valve springs on the camshaft without affecting the valve timing.

The reaction forces of the actuating rods 50 on the piston 36 are not symmetrical and there is a reaction torque applied to the piston 36 tending to make it twist and jam within the cylinder. As an alternative to the crowning of the piston surfaces described with reference to Figure 8, it is possible in a further embodiment of the invention to form the piston 36 with a skirt that extends towards the back plate 18. In this case, it is possible for the guide block to seal against the inner surface of the skirt of the piston instead of sealing against the interior of the housing 16.

As the phase of the mechanism is varied, the different components of the mechanism, that is to say the cam plate 30 and the cam wedges 54, will lie in different positions and this will affect the dynamic balance of the mechanism.

While it would be possible to incorporate a balance weight that moves with the piston 36 to maintain the entire mechanism in balance in all positions, such a solution is believed to be unduly complex and it is preferred to balance the mechanism at high speed and to tolerate the slight imbalance that may be present at low speed. If the low speed imbalance should present a problem then it can be alleviated by appropriate selection of the materials from which the mechanism is made, in particular the cam plate 30 and the cam wedges 55.

Claims

1. A phase change mechanism for coupling a drive member to a driven member, comprising a cam plate (30) rotatable about the central axis of the mechanism and having two or more cam surfaces (30a) that are oppositely ramped in the axial direction, two or more cam wedges (54) mounted for axial movement in synchronism with one another relative to the cam plate (30), each cam wedge (54) having a cam surface (54a) engaging a respective one of the cam surfaces (30a) on the cam plate (30) and being pivotable at its end remote from the cam surface (54a) about an axis generally parallel to, and radially offset from, the central axis, and a hydraulic jack (36) for causing relative axial movement between the cam wedges (54) and the cam plate (30), the engagement of the cam surfaces of the cam wedges and the cam plate causing the cam plate (30) to rotate about the central axis in response to the relative axis movement, the cam wedges (54) at the same time pivoting to maintain area contact between the cam surfaces on the wedges and the cam plate, characterised in that means are provided for permitting a limited degree of rotation of one of the cam surfaces (54a, 30a) on the cam wedge (54) and the cam plate (30) relative to the other about a generally radial axis.

2. A phase change mechanism as claimed in claim 1, wherein in use the wedges (54) are connected to the drive member and the cam plate (30) is connected to the driven member such that the phase change between the cam plate and the cam wedges causes a phase change between the drive and driven members .

3. A phase change mechanism as claimed in claim 1, wherein in use the cam plate (30) is connected to the drive member and the cam wedges (54) are connected to the driven member such that the phase change between the cam plate and the cam wedges causes a phase change between the drive and driven members .

4. A phase change mechanism as claimed in any of claims 1 to 3, wherein the piston (36) of the hydraulic jack is connected to move the cam wedges (54) .

5. A phase change mechanism as claimed in any of claims 1 to 3, wherein the piston of the hydraulic jack is connected to move the cam plate (30) .

6. A phase change mechanism as claimed in any preceding claim, in which the mechanism comprises a hollow cylindrical housing (16) to be secured to the driven member, the driving member (12) being journalled about the outer surface of the housing (16) and being connected to the cam plate (30) by an extension thereof passing through an arcuate slot (16a) in the housing (16) .

7. A phase change mechanism as claimed in claim 6, wherein the hydraulic jack comprises a guide block (32) mounted on a cylindrical hub (20) that is received coaxially within the housing (16), a double acting piston (16) slidable along the hub (20) and a closure cover (42) for sealing against the hub (20) and the housing (16), the jack thereby having two working chambers, the first defined between the guide block (32) and the piston (36) and the second defined between the piston (36) and the closure cover (42) .

8. A phase change mechanism as claimed in claim 7, wherein the piston (36) is connected to two or more axially extending actuating rods (50), each carrying a respective one of the cam wedges (54), the actuating rods (50) passing through and being sealed relative to the guide block (32) .

9. A phase change mechanism as claimed in claim 8, wherein the guide block (32) is formed with troughs that underlie the actuating rods (50) and act as load bearing support surfaces for the ends of the cam wedges (54) remote from the cam surfaces (54a) .

10. A phase change mechanism as claimed in claim 9, wherein the means for permitting a limited degree of rotation of one of the cam surfaces (54a, 30a) on the cam wedge (54) and the cam plate (30) relative to the other about a generally radial axis comprise a crowned surface (60) on the end of the cam wedges (54) remote from the cam surfaces (54a) to permit the cam wedges (54) to rock relative to the guide block (32) .

11. A phase change mechanism as claimed in any preceding claim, wherein a cam surfaces (30a) on the cam plate (30) are defined by part-cylindrical inserts (62) that can rotate relative to the cam plate (30) .

12. A phase change mechanism as claimed in claim 7 or any claim appended thereto, wherein the piston has a skirt to enable the piston to withstand uneven reaction forces from the actuating rods.

13. A phase change mechanism as claimed in claim 7 or any of claims 8 to 11 appended thereto, wherein the sealing surfaces (66,68) of the piston (36) are crowned to enable the piston to tolerate misalignment without jamming.