GB2301165A

GB2301165A - A hydraulic damper having a variable vibration characteristic

Info

Publication number: GB2301165A
Application number: GB9610160A
Authority: GB
Inventors: John Philip West; Peter Michael Trewhell Fursdon
Original assignee: AVON CLEVITE Ltd; Avon Vibration Management Systems Ltd
Current assignee: AVON CLEVITE Ltd; Avon Vibration Management Systems Ltd
Priority date: 1995-05-26
Filing date: 1996-05-15
Publication date: 1996-11-27
Anticipated expiration: 2016-05-15
Also published as: GB9510660D0; GB9610160D0; GB2301165B; DE19620219A1

Description

2301i65- 1 HYDRAULICALLY DAMPED MOUNTING DEVICE The present invention

relates to a hydraulically damped mounting device. Such a device usually has a pair of chambers for hydraulic fluid, connected by suitable passageway, and damping is achieved due to the flow of fluid through that passageway.

EP-A-0115417 and EP-A-0172700 discussed two different types of hydraulically damped mounting devices for damping vibration between two parts of a piece of machinery, e.g. a car engine and a chassis. EP-A-0115417 disclosed various "cup and boss" type of mounting devices, in which a "boss", forming one anchor part to which one of the pieces of machinery was connected, was itself connected via a deformable (normally resilient) wall to the mouth of a "cup", which was attached to the other piece of machinery and formed another anchor part. The cup and the resilient wall then defined a working chamber for hydraulic fluid, which was connected to a compensation chamber by a passageway (usually elongate) which provided the damping orifice. The compensation chamber was separated from the working chamber by a rigid part-itilon, and a flexible diaphragm was in direct contact with the liquid and, together with the partition formed a gas pocket.

In EP-A-0172700 the mounting devices disclosed were of the "bush" type. 1n this type of mounting device, the 2 anchor part for one part of the vibrating machinery is in the form of a hollow sleeve with the other anchor part in the form of a rod or tube extending approximately centrally and coaxially of the sleeve. In EP-A- 0172700 the tubular anchor part was connected to the sleeve by resilient walls, which defined one of the chambers in the sleeve. The chamber was connected via a passageway to a second chamber bounded at least in part by a bellows wall which was effectively freely deformable so that it could compensate for fluid movement through the passageway without itself resisting that fluid movement.

In the hydraulically damped mounting devices disclosed in the specifications discussed above, there was a single passageway. It is also known, from other hydraulically damped mounting devices, to provide a plurality of independent passageways linking the working and compensation chambers for hydraulic fluid.

In EP-A-0115417, there was a single diaphragm, which was configured to give a specific influence on the vibration characteristics of the hydraulically damped mounting device. Those characteristics depended on the stiffness of the diaphragm, by which is meant the change in applied pressure needed to cause unit change in the volume displaced by the diaphragm. Furthermore, the surface of the diaphragm which is in contact with the fluid in the working chamber must be covered by a snubber 3:-."ate, with openings therein for fluid communication therethrough between the upper surface of the diaphragm and the rest of the working chamber, and it has been found that the configuration of those openings also affects the characteristics of the mount.

In EP-A-0115417, those openings were in the form of holes in a relatively thin plate, so that the diameter of the holes was much greater than their axial length. In EP-A-0115417, the intention was that hydraulic fluid would pass relatively freely through those holes, from the working chamber to the diaphragm.

As mentioned above, it has been found that the size of the openings in the snubber plate affects the characteristics of the mount. The applicants have realised that it is therefore possible to vary the characteristics of the mount by varying the ratio of the total cross-sectional area to the total length of the openings.

One way of varying that ratio is to vary the effective aperture of the openings. The effective aperture is the sum of the cross-sectional areas of the openings. The effective aperture may therefore be varied by wholly or partially closing some or all of the openings, or by changing the diameter of some or all of the openings. This variation may be applied to different openings sequentially.

4 Another way of varying the ratio is to vary the length of some or all of the openings. Again, this variation may be applied to different openings sequentially.

In practice, it is not desirable to have a zero effective aperture. Therefore either only some of the apertures are closed, or some or all the apertures are not completely closed (ie the effective aperture is not zero). Furthermore, it is preferable that the ratio should vary so that the maximum value of the ratio is at least twice, preferably at least four times, the minimum value.

At its simplest, the present invention can be achieved by providing a movable plate with holes therein directly adjacent the snubber plate. If the holes in the movable plate are aligned with openings in the snubber plate, then the effective area is maximum. If the position of the movable plate is then changed relative to the snubber plate, so there is imperfect alignment of the holes in the movable plate and the openings in the snubber plate, the effective area of the openings will be reduced, because the openings are then partially blocked.

Sufficient movement of the movable plate will result in the openings in the snubber plate being wholly blocked.

Such an arrangement is simple, but has the disadvantage that the mass of fluid in the openings does not change significantly. Therefore, it is preferable to arrange the movable plate so that its movement blocks or unblocks the openings in a sequential way. Thus in one position of the plate, all but one of the openings may be unblocked, and movement of the plate then successively blocks further ones of the openings, until all of the openings are unblocked. The effective area then varies as each opening is unblocked. Moreover, as the number of blocked and unblocked openings varies, the mass of hydraulic fluid moving through the openings as the mount vibrates also varies.

It is also possible, however, to vary the effective area by using a hydraulic fluid which is electrorheological. Then, if electrodes are positioned at the openings in the snubber plate, the application of suitable voltages to those electrodes will cause the characteristics of the electro-rheological fluid to change at the openings, thereby blocking them.

A further alternative is for each of the openings in the snubber plate to be in the form of the closable valve, and to provide suitable control means for closing some or all of the valves.

If the length of the openings is to be varied, the openings may be formed be passageways in rotatable members, the position of each member determining the length of the opening be varying the length of passageway 6 which forms the corresponding opening.

It should be noted that the present invention is concerned with varying the effect of the openings in the snubber plate which lies between the diaphragm and the rest of the working chamber. Such variation changes the air-spring characteristic of the mount, by varying the linking between the air spring formed by the gas pocket and the working chamber. Thus, the effect is different from arrangements in which the passageway between the working and compensation chambers is valved, which affects the damping characteristics of the mount. The present invention permits tuning of the frequency characteristics of the mount to permit isolation of dominant excitation frequencies by anti-resonance dips. 15 An embodiment of the present invention will now be described in detail, by way of example, with reference to the accompanying drawing in which: Fig. 1 shows an embodiment of a hydraulically damped mounting device incorporating the present invention; 20 Fig. 2 is a graph showing the behaviour of a hydraulically damped mounting device of Fig. 1, arranged to provide a first vibration characteristic; Fig. 3 is a graph showing the behaviour of a hydraulically damped mounting device of Fig 1, arranged to provide a second characteristic.

Refe-rring first to Fig. 1 an embodiment of a 7 hydraulically damped mounting device according to the present invention is shown for damping vibration between two parts of a structure (not shown). The mount has a boss 1 connected via a fixing bolt 2 to one of the parts of the structure, and the other part of the structure is connected to a generally U-shaped cup 4. A resilient spring 5 of e.g. rubber interconnects the boss 1 and the cup 4. A partition 7 is also attached to the cup 4 adjacent a ring 6, and extends across the mouth of the cup 4. Thus, a working chamber 8 is defined within the mount, bounded by the resilient spring 5 and the partition 7.

The interior of the partition 7 defines a convoluted passageway 9 which is connected to the working chamber 8 via an opening (not shown) and is also connected via an opening (not shown)to a compensation chamber 12. Thus, when the boss 1 vibrates relative to the cup 4 (in the vertical direction in Fig. 1), the volume of the working chamber 8 will change, and hydraulic fluid in that working chamber 8 will be forced through the passageway 9 into, or out of, the compensation chamber 12. This fluid movement causes damping. The volume of the compensation chamber 12 needs to change in response to such fluid movement, and therefore the compensation chamber 12 is bounded by a flexible wall 13.

The above structure is generally similar to that 8 described in EP-A-0115417, and the manner of operation is similar. As in EP-A-0115417, the partition supports a diaphragm 20 which acts as a boundary between fluid in the working chamber 8 and a gas pocket 19.

In this embodiment, the partition 7 has a lower snubber plate 21 in the gas pocket 19 and an upper snubber plate 22 which extends over the diaphragm 20. The upper snubber plate has openings 25,26 therein which permit fluid in the working chamber 8 to communicate with the diaphragm 20. Immediately above the upper snubber plate 22 is a rotatable plate 30 which is pivoted about the symmetry axis X-X of the mount, the plate being moved by the action of a bar 31 which is driven to move by a drive 32. The movable plate 30 has holes therein which correspond to the openings 25,26 in the upper snubber plate 22. If all the holes 33 in the plate 30 are aligned with the openings 25,26, the fluid in the working chamber 8 can pass freely to the diaphragm 20. If the drive 32 is operated to move the movable plate 30, however, the alignment of the holes 33 and the openings 25,26 changes. As that alignment changes, the parts of the movable plate 30 adjacent the holes 33 sequentially block the openings 25,26, thereby reducing the effective cross-sectional area of the openings 25,26. This restricts movement of fluid from the working chamber 8 through the openings 25,26 to the diaphragm 20, thereby 9 changing the characteristics of the mount. The plate 30 is preferaby moveable between a position in which all but one of the openings 25,26 are blocked,and a position in which none of the openings 25,26 are blocked, via positions in which some of the holes are blocked and some are not.

Thus, as the drive 32 moves the movable plate 30, a varying number of the openings 25,26 may be blocked by the plate 30, thereby changing the fluid movement from the working chamber 8 to the diaphragm 20, and so modifying the air-spring effect provided by the diaphragm and gas pocket 19.

In an alternative arrangement, the fluid in the working and compensation chambers 8,12 is an electro rheological fluid. Electrodes are then formed adjacent the openings 25,26 and connected to a suitable control.

By applying voltages to those electrodes, the characteristics of the electrorheological fluid in the openings 25,26 may be changed, thereby blocking the holes 25,26. If the electrodes at the holes 25,26 can be controlled independently, then only some of the openings 25,26 need be blocked if desired. Depending on the number of openings that are blocked, the effective aperture between the working chamber 8 and the diaphragm is changed, thereby varying the characteristics of the mount discussed above.

The effect of the present invention will now be discussed with reference to Figs. 2 and 3. The graphs shown in Figs. 2 and 3 illustrate the variation with frequency of the dynamic stiffness of a hydraulically damped mounting device used for damping an engine relative to the chassis of a motor vehicle.

Fig. 2 illustrates the stiffness characteristic of the hydraulically damped mounting device when the rotatable plate 30 is in a first position, and so the effective aperture of the openings 25,26 in the snubber plate 22 has a first value. As can be seen from Fig. 2, as the frequency of vibration increases, there is a dip in the dynamic stiffness at about 100 Hz, shown at A in Fig 2. By adjusting the position of the plate 30 to a second position, in which a different number of the openings 25,26 are blocked, a characeristic corresnonding to that shown in Fig. 3 can be obtained in which the dip in the characteristic is at approximately 150 Hz. This permits an anti-resonance to be created, the frequency of which can be varied in dependence on the vibration frequency of the engine. As a result, the full effect of the engine vibrations is not passed to the vehicle. Since the frequency of the engine vibrations; and hence the desired frequency of the anti-resonance trough will vary with engine speed, so it is therefore preferable for the drive 32 of the rotatable plate 30 to be controlled 11 by a suitable controller (not shown) in dependence on the engine behaviour. As a result, the effective aperture of the openings 25,26 can be varied to vary the position of the trough A in Figs. 2 and 3, and so provide appropriate anti-resonance for the dominant excitation frequencies at any particular engine speed.

12

Claims

1. A hydraulically damped mounting device comprising: first and second anchor parts, a first deformable wall connecting said first and second anchor parts, a working chamber for hydraulic fluid at least partially bounded by the first deformable wall, a compensation chamber for hydraulic fluid at least partially bounded by a second deformable wall, a passageway interconnecting the working and compensation chambers for flow of hydraulic fluid there through, a diaphragm in contact with the hydraulic fluid in the working chamber, the diaphragm separating the working chamber and a gas pocket, and a snubber plate in the working chamber extending over the diaphragm, the snubber plate having at least one opening therein for hydraulic fluid flow therethrough, the ratio of the total effective cross-sectional area to the total effective length of the at least one opening being variable. 2. A hydraulically damped mounting device according to claim 1, having closure means for wholly or partially closing said at least one opening thereby to vary the total cross sectional area of the at least one opening. 3. A hydraulically damped mounting device according to 13 claim 2, having a plurality of openings in the snubber plate, and wherein said closure means is arranged to vary the cross-sectional area of the openings sequentially. 4. A hydraulically damped mounting device according to claim 2 or claim 3, wherein the closure means.is a movable plate mounted on the snubber plate. 5. A hydraulically damped mounting device accord4-ng to claim 4, wherein the movable plate has holes therein alignable with the openings in the snubber plate.

6. A hydraulically damped mounting device according to claim 1, having a plurality of openings in the snubber plate, and wherein said closure means is arranged to vary the cross-sectional area of the openings independently.

7. A hydraulically damped mounting device according to any one of claims 2, 3 or 6, wherein the fluid is electro-rheological, and the closure means comprises electrodes at the at least one opening. 8. A hydraulically damped mounting device according to any one of claims 2, 3 or 6, wherein the closure means comprises a controllable valve in the or each opening9. P hydraulically damped mounting device according to claim 11, having means for varying the total length of the at least one onening.

10. A hydraulically damped mounting device according to any one of the preceding claims, wherein the snubber 14 plate is part of a rigid partition separating the working and compensation chambers, that partition also supporting the diaphragm. 11. A hydraulically damped mounting device substantially as herein described with reference to an as illustrated in the accompanying drawings.