EP0792424A1

EP0792424A1 - Mounting devices

Info

Publication number: EP0792424A1
Application number: EP95938539A
Authority: EP
Inventors: Peter Michael Trewhella Fursdon; John Philip West
Original assignee: Avon Vibration Management Systems Ltd
Current assignee: Avon Vibration Management Systems Ltd
Priority date: 1994-12-02
Filing date: 1995-12-04
Publication date: 1997-09-03
Also published as: WO1996017184A1; JPH10509787A; GB9424416D0

Abstract

A hydraulic mounting device (1) has a primary hydraulic chamber (8) whose deformable side wall (81) lacks load-bearing resilience, connected through an access opening (61) to a secondary hydraulic chamber (9) whose deformable wall (91) may or may not have load-bearing resilience. A control valve (10, 34) is provided to open or close the access opening and thereby adjust the stiffness of the device. In particular the stiffness may be adjusted between an essentially rigid condition in which the mounting device transmits applied vibrations and shocks to an additional shock-absorbing means e.g. a hydraulically-damped mounting device, and a non-stiff condition in which the mounting device prevents transmission of certain vibrations to the other device. It may for example prevent transmission of engine idling vibrations which the other device cannot conveniently be tuned to absorb. Other embodiments combine the stiffness-adjustment hydraulic system in one device with load-bearing springs which may be hydraulically damped resilient members.

Description

MOUNTING DEVICES

This disclosure has to do with mounting devices for adjusting the stiffness of a mounting connection, particularly although not exclusively in the context of engine mounting.

It is well known to include vibration and/or shock absorbing devices in an engine mounting connection. Some conventional devices absorb vibrations/shocks by the deformation of a load-bearing resilient elastomeric element bounding a hydraulic chamber which communicates with a secondary compensation chamber through an orifice to provide hydraulic damping of the deformation.

It may be difficult if not impossible to design a vibration/shock absorbing mounting device that can cope with all of the vibration/shock regimes envisaged in a given engineering context.

Proposals have therefore been made for adjusting the degree of hydraulic damping of the load spring's vibrations, in particular by enabling adjustment of the size, length or tortuosity of the damping orifice. For examples, see US-A-4768759, EP-A-248942, EP-A-278824, GB- A-2186052, GB-A-2135795 and GB-A-2191561. Another prior proposal, not hydraulically damped, uses alternative load- bearing springs with different characteristics in series, either of which can selectively be "short-circuited" using an adjustable rigid component: see US-A-5221078.

A general aim herein is to provide new mounting devices with a versatile stiffness adjustment capability. One preferred aim is to enable a very marked reduction in stiffness at selected times, so that a resilient vibration or shock-absorbing device is less subject to movements which it cannot adequately absorb. For example, a vehicle engine mount which adequately dampens driving vibrations may be unable to cope with vibrations of significantly different frequency or pattern e.g. while the engine is idling. With conventional devices these other vibrations are then transmitted to the frame/chassis and felt by the vehicle occupants.

Aspects of what we now put forward are set out in the claims.

One general measure now put forward is that a primary hydraulic chamber - provided between opposed rigid body portions of two relatively movable parts - has a deformable side wall which is non-resilient, or at least lacks load-bearing resilience, while being essentially non-stretching so that all relative movements of the body parts force a volume change in the primary chamber. The primary chamber is connected through an access opening to a secondary chamber, means such as a control valve being provided for adjusting the degree of opening. Conversely if the opening is substantially or completely shut, higher stiffness or complete rigidity can be achieved. By using a primary chamber wall without load-bearing resilience - contrasting with the hydraulic mounting devices conventionally used which bound the working chamber with a load-bearing spring - this proposal enables stiffness adjustment to be varied over a wide range, and determined substantially by selection of the properties of the access opening control and secondary chamber. For example, use of a secondary chamber whose wall also lacks load-bearing resilience enables a device adjustable to very low stiffness. This can be connected in series with another mounting device. Additionally or alternatively, a primary chamber or chambers can be connected to plural secondary chambers of varying response, and/or to tertiary or other chambers (not connected via an adjustable access opening) to provide qualitatively different effects e.g. hydraulically damped resilience. The passive deformation of the primary chamber(s) enables the selective driving of a wide variety of different responses.

Secondary hydraulic chamber(s) may also be formed between other opposed portions of the first and second body parts, to be compressed when the primary chamber expands and vice versa. For example a portion of the second body part may intervene between spaced portions of the first body part, with connected hydraulic chambers on either side of it. Chamber dimensions can be selected to match the resulting forced volume changes. Or, all or some of the volume compensation can be into chambers which are not rigidly volume-constrained in relation to the body part positions e.g having spring compliance or being freely expansible.

Appropriate orientation of opposed body portions and hydraulic chambers enables stiffness adjustability both axially and laterally relative to the mounting connection direction between the body parts' anchor points. As material for the inextensible chamber side walls, we find that a reinforced elastomer membrane, reinforced with e.g. reinforcement cords or filaments, shows suitable behaviour. The side wall desirably follows relative body part movements by bending or bend-movement compliance, e.g. as a bellows, substantially without bodily strains in the layer direction.

A complete seal of the access opening is not necessarily required for the closed condition, provided that adequate stiffness is presented for the vibrations to be passed. However for reliable transmission down to 1 Hz or 0.5 Hz e.g. for an engine mount, a complete seal is preferred.

It is preferred that substantially rigid regions of the opposed portions of the first and second body parts define opposed end surfaces of the hydraulic chamber(s), to force volume changes directly. Likewise, the end dimensions at the two opposite ends of the hydraulic chambers are preferably substantially the same so as to give a substantially linear relationship between positional displacement and volume displacement.

Stiffness contributed by the described hydraulic system between the first and second body parts in a higher-stiffness or rigid condition, for the or each movement direction, may be 1500 N/mm or more, preferably at least 2000 N/mm, even down at 1 Hz. Furthermore, preferably at least 5000 N/mm and more preferably at least 10000 N/mm at 400 Hz.

Conversely, the control means may be adjusted such that relatively free flow of hydraulic fluid through the access opening leads to a low-stiffness condition of the mounting device. For example, a stiffness in the direction of the or each relative body part movement of less than 100 N/mm and preferably less than 80 N/mm may be desired in this low-stiffness condition, in particular at frequencies at or above 10 Hz where conventional hydraulic damper channels are effectively 'choked' or rigid. To this end the deformable side wall of the or each primary (and optionally also secondary) hydraulic chamber connected between the body parts desirably has low and preferably negligible intrinsic stiffness, functioning as a non-resilient passive follower of the body part movements. The stiffness contribution of this hydraulic system can then be substantially determined by the freedom or otherwise of fluid flow through the access opening. These passive follower walls may therefore be formed as thin membranes e.g. 0.5 to 2 mm thick for reinforced elastomer or 3 to 7 mm thick for unreinforced elastomer. A spring or springs can be connected between the first and second body portions in parallel with the hydraulic system, to provide resilience and load-bearing capability in the device when the stiffness of the hydraulic system is reduced or eliminated. When the hydraulic system takes a rigid condition the spring is effectively "switched-out" .

Mechanical limit stops may be provided to act between the first and second body parts, to limit the extent of relative movement between them and prevent excessive deformation of the hydraulic chamber (s) , in particular to limit extension of non-resilient chamber side walls which are not adapted to resist tension.

The access opening can be formed through one of the body parts.

The flow control may comprise a valve member or obturator controllably movable to block or open the access opening. (Note that references here to an "opening" for access or for the valve member include the possibility of plural openings unless the context requires otherwise) . A resilient seal member may be provided for sealing the valve member against a body part against which it acts. For the preferred high stiffness at low frequencies a true seal is needed. A preferred form drives a sealing obturator down onto a sealing seat around the access opening. An alternative form of valve member is controllably rotatable about an axis e.g. the longitudinal connection axis of the device, and slides over the access opening. A drive for adjusting the or each control valve, e.g. a solenoid, may be incorporated in the device. The stiffness-adjusting system described may be connected in series with a vibration/shock absorbing load- bearing mounting device such as a hydraulically-damped mounting device. The stiffness adjuster can be operated in a high-stiffness or rigid condition in a regime for which the other mounting device is adapted, but adjusted to a low-stiffness condition in another regime in which the other device may undesirably transmit shocks/vibrations. Or, different functions may be combined in one device. The stiffness-adjusting system may have at least one hydraulic chamber, and particularly its primary non- resilient chamber, in common with a hydraulically-damped shock/vibration absorbing load-bearing mounting arrangement, load-bearing resilience of the device being provided by communication of this chamber with a volume compensation chamber having a wall which is a resilient load-bearing spring of the device. Communication is via a damping orifice, typically elongate. To combine this with an adjustable-stiffness or stiffness-release capability, the primary hydraulic chamber as required by the present proposals may be connectable selectively to a volume compensation chamber having a compliant or passive follower wall (for stiffness adjustment) and/or a volume compensation chamber having an active, load-bearing resilient wall, in addition to the hydraulically-damped connection. Indeed, the non-resilient primary chamber described herein may in a broad sense be regarded, simply as a passive working chamber whose communication(s) into one or more other mount-response determining chambers is/are selectively adjustable.

The hydraulic system is preferably pressurised and means such as an auxiliary pressure chamber may be provided for maintaining the system pressure e.g. over the device's lifetime.

Our proposals are now illustrated by way of example with reference to the accompanying drawings in which Fig. 1 is an axial cross-section of a first stiffness-adjustable mounting device;

Fig. 2 is a plan view of it;

Fig. 3 is a schematic plan showing the layout of hydraulic chambers; Fig. 4 is an axial cross-section of a second device;

Fig. 5 is a plan view of it;

Fig. 6 shows the second device connected in series with a hydraulically-damped mounting device;

Fig. 7 shows schematically how functions of stiffness-adjustment and hydraulic damping may be combined in one device; and

Fig. 8 is an axial cross-section of a practical embodiment of such a combination.

Referring firstly to Figs. 1 and 2, a stiffness- adjustable mounting device generally indicated at 1 is suitable for use in mounting a car engine. It has a rigid outer body portion 2, comprising a lower cup 21 with side walls 22 angled at 45° and a peripheral flange 23. A central rigid anchoring connector 4 is secured to the centre of the cup base 24, for connection of the device to one side of the engine mounting linkage.

The outer body portion 2 also has a cap 25 fixed across the top of the cup 21 by means of a peripheral crimp 26 engaging the cup flange 23. A rigid inner body portion 3 is housed between the outer cup 21 and cap 25, and is itself formed of two parts fixed together; an inner cup 31 nested coaxially with the outer cup 21 and a top plate 35 screwed to the top periphery of the inner cup and sealed against it with an O-ring 36. The top plate 35 has upwardly-projecting connector extensions 5 distributed around its circumference, for connection to the other side of the mounting linkage. In this example three are shown, with threaded unions 51 exposed upwardly by respective openings 26 through the outer top cap 25.

The side walls 32 of the inner cup 31 complement those of the outer cup 21, while the centre of the inner cup 31 forms an upwardly-projecting drive housing 33 housing a control solenoid 50. The inner top plate 35 has a circular central opening 37 into which the drive housing 33 projects, with an annular clearance 61 between them. Three circular openings 34 are provided through the side walls 32 of the inner cup and communicate with the annular space 61 above.

An axial hydraulic chamber 8 is defined at the top of the device by a reinforced rubber side wall 81, held in place by a lower annular securing bead 82 clamped around the inner top plate opening 37 by a clamping strip 83, and an upper annular securing bead 84 seating in an annular recess of the outer top cap 25. The reinforced deformable wall 81 is about 1mm thick, able to follow relative axial movements between the inner and outer body portions 2,3 with negligible resilience and low resistance by rolling/bending movement, but also negligibly stretchable under the hydraulic fluid pressure.

Lateral hydraulic chambers 9 are defined between the inner and outer cup sidewalls at the 120°-spaced locations of the three openings 34. Their deformable walls 91 are retained in position by inner and outer securing beads 92,94 secured by an inner clamping strip 93 and an annular outer cup recess.

Fig. 3 shows the arrangement of chambers. For stable operation, the hydraulic chamber volumes 8,9 defined within the deformable walls 81,91 are flat in shape, i.e. substantially shorter in their respective compression/expansion directions than in the transverse dimension. The upper and lower peripheries of each surrounding wall 81,91 are substantially the same diameter, to give a generally linear volume/displacement relation.

A hydraulic fluid of any suitable type (glycol is commonly used for automotive applications) fills the chambers 8,9 and the annular clearance 61 connecting them via the respective openings 34. The hydraulic system defined by the four chambers 8,9 is closed, and the transverse areas of the lateral chambers 9 less than that of the axial chamber 8 into which they communicate and selected so that, for relative movement in any direction between the inner and outer body portions 2,3, the total system volume is unchanged although that of individual chambers changes. The hydraulic fluid may be excess- pressurised or at atmospheric pressure. When reinforced rubber membranes are used, excess pressure is desirable to take up slight slack usually existing in the reinforcing cords.

Between the lateral chambers 9, lower limit stop engagements are provided by elastomeric pads 28 on the outer cup sidewalls 22 opposing outward stop surfaces 38 on the inner cup side walls 32, restricting to a predetermined degree the amount of downward and lateral movement possible for the inner body 3 relative to the outer 2. Likewise a top limit stop is formed by elastomeric pad segments 29 directed downwardly and inwardly around inside the outer top cap 25, at a predetermined spacing from the periphery of the inner body 3 to limit the upward and lateral movement of the inner body 3 relative to the outer 2.

A solid rubber biasing spring 7 is precompressed between the base 24 of the outer cup 21 and the base of the inner cup 31, so as to urge the inner body 3 against the upper limit stops in the free condition of the device, but countered by the nominal load on the device in situ so that the inner body 3 rides free between the limit stops as shown in Fig. 1. The spring also defines a predetermined minimum stiffness for the device as explained below. Fluid flow in the device is governed by a rotatable valve plate 10, having plate openings 101 which can be rotated into and out of register with the openings 34 of the inner cup 31. The rotation is controllably driven by the solenoid in the drive housing 33 through a drive shaft 102 fixed to a central hub portion 103 of the valve plate 101 which fits over the drive housing 33. The plate 101 conforms to the upper surface of the inner cup 31, fitting closely down the sides of the drive housing 33 to the cup side walls 32 and angling up along the side walls to cross the annular clearance 61 into an annular clearance slot 104 defined between edge regions of the inner body cup 31 and plate 35. It extends far enough to cover entirely each opening 34 through the inner cup 31. To strengthen the valve plate 101 against hydraulic forces across it, it has radially-extending stiffening ribs 105 on both sides. If desired, a sealing element (not shown) can be provided to seal between the edge of the plate 10 and the slot 104. A valve opening 101 is provided for each of the three lateral chambers 9. In the position shown in Fig. 1, each opening 101 allows a free fluid communication between the annular space 61 (and so the axial chamber 8) and the respective lateral chamber 9. With the plate 10 rotated 60° from that position, the openings 101 are entirely out of register with the. openings 34, which are then effectively closed by continuous portions of the plate 10 and fluid communication between the chambers 8,9 is blocked. In operation, for a rigid condition the valve plate 10 is turned to block fluid communication. Since the fluid is essentially incompressible, the chamber side walls 81,91 essentially inextensible and the chamber end walls (defined by the housing portions) essentially rigid, no relative movement can occur in any direction between the body components 2,3. In this condition, the stiffness of the device is over 2000 N/mm down to quite low frequencies e.g. 1 Hz.

When it is desired to modify these properties e.g. to effectively uncouple a series-connected hydraulically- damped device because vibrations are occurring which it cannot absorb, the stiffness adjusting device 1 can be switched to the low-stiffness condition with the valve plate openings 101 permitting full communication between the hydraulic chambers 8,9. Under these conditions the stiffness offered by the hydraulic system to relative movements between the body portions 2,3 is negligible. The valve opening dimensions and flow properties through the annular clearance 61 are such that the resonant frequency of the fluid mass within the hydraulic system is greater than the main frequencies (e.g. between 10 and 20 Hz for idling engines) in the regime in which the low- stiffness condition is used. For example, -it is greater than about 60 Hz. Up to such frequencies, the frictional resistance to fluid flow should also be negligible. Since the stiffness of the hydraulic system is essentially eliminated in this condition, stiffness of relative movement between the body parts 2,3 is governed by the spring 7 connected between them in parallel with the hydraulic system. The strength of this spring can be selected according to the minimum stiffness wanted. The axial rate of the spring is desirably 50 to 80 N/mm. The radial rates may be lower, e.g. below 50 N/mm. In this embodiment the radial rate is about 30 N/mm.

Fig. 4 shows a second embodiment of hydraulic stiffness adjuster. This has in common with the first embodiment the use of a central axial primary hydraulic chamber 8 connected through three inclined planetary secondary hydraulic chambers 9, and that all hydraulic chambers have reinforced elastomer membrane side walls without load-bearing resilience.

One difference is in the anchor points. Cup body 2 with its central anchor point 4 is at the top. Its side walls extend down and round and capture not a complete cap as in the first embodiment, but rather an annular flange 27 which provides the opposed supports for the secondary hydraulic chamber 9. The annular flange 27 leaves a central space through which the inner body part 3 can project, allowing another central anchor union 5. The rubber spring 7 is formed in three portions extending radially between the inner body and outer cup walls between the secondary hydraulic chambers. Radial limbs of the inner body carry limit stop buffers 28,29 which limit the relative movement of the two body parts by abutment against the cup base 22 or annular flange 27.

The side wall 81 of the primary chamber 8 is formed here by a rolling diaphragm rather than a bellows diaphragm: its attachments to the respective body parts are slightly radially spaced enabling rolling movement into a clearance, although the general dimensions of the opposed portions are still generally the same. A rolling diaphragm gives a lower stiffness and less stiffness variation than a bending bellows.

The primary chamber 8 occupies the upper part of a central cup cavity defined by the inner body part 3. The lower part of this cavity provides a pressurisation chamber 111 which serves to maintain an excess fluid pressure in the system during its life. The hydraulic fluid is pressurised to pre-stress the diaphragms and improve their bi-directional performance. Over time the diaphragms may creep, causing fluid pressure to drop and the device's response to change. To avoid this, pressurisation chamber 111 is defined by a partition 132 closing off the lower part of the cup cavity and sealed from the hydraulic chamber 8 by a ring seal 133. A dynamic dividing partition 136 bounds the pressurisation chamber 111 from below, mounted through a rigid support annulus 134 and precompressed elastomeric annulus 137 so as to create excess pressure in the pressurisation chamber 111. This communicates with the main hydraulic chambers 8,9 by way of a highly constricted passage 135 between inner body components leading through an accumulator chamber 139 into the communication space 61,93 between the primary and secondary chambers.

Communication between the primary and secondary chambers is via three radial channels 61 through the inner body wall 131, opening at valve seat openings 34. These openings are at the base of a cylinder in which acts the sealing plunger 10 of a solenoid 50, housed in a solenoid housing 33 fast with the inner body 3. Fig 5 shows the arrangement of the three solenoid housings (only one has the communication with the pressurisation chamber 111) . The sealing plunger 10 acts down onto the valve seat 34, giving a more positive and complete seal than in the first embodiment and enabling higher stiffnesses to be achieved over a greater frequency range. With the solenoids closed, this embodiment achieves stiffness of greater than 10000 N/mm at 400 Hz.

The peripheral disposition of the springs 7 and solenoids also achieves a more axially compact construction and central fastening top and bottom.

The device shown is suitable for use in series with a hydraulically-damped resilient engine mount, e.g as shown in Fig. 6. The damped mount 300 has the conventional load-bearing rubber spring 308 bounding a hydraulic working chamber 310, communicating through a damping partition 309 having an elongate damping orifice (detail not shown) with a compensation chamber 311 having a freely deformable wall 312. This combination can be used to avoid transmission of idling vibrations through the mounting by switching to the low-stiffness condition when idling, using an automated vehicle control system to switch the solenoids, closing them in other conditions to rigidify the device.

The skilled reader will understand that many variations within the described concepts are possible, depending on the context of use and the properties desired.

One possible variant is to use non-reinforced rubber or other suitable impermeable material for the deformable chamber walls. These may be formed with fold lines to assist passive deformation to follow the chamber volume changes. Alternatively, the chamber walls may be given a degree of spring resistance and thereby function as a spring connected in parallel with the fluid system, as proposed above.

Fig. 7 illustrates schematically some more radical variants. Outer and inner body portions 2,3 are arranged interposed, with a large-area upper hydraulic chamber 8 communicating axially through a duct 161 of the inner body 3 with a smaller-area lower hydraulic chamber 9. For a given linear displacement, the volume change in the upper chamber 8 will be much larger than that in the lower chamber 9 and so no movement at all can occur (i.e. there is a rigid condition) unless volume change mismatch compensation is provided. This is done by volume compensation chamber 12 of the inner body, communicating into the main duct 161 via a valve-controlled access opening 121. The compensation chamber 12 has a low- stiffness compliant follower wall 122, of low resilience by comparison with the mounting loads, to accommodate volume changes. Opening the access 121 by some suitable control mechanism enables flow into or out of the volume compensation chamber 12, allowing relative axial movement between the body parts 2,3.

Another feature shown in this embodiment (although it can be used independently from the feature just described) is incorporation of a hydraulically-damped load-bearing vibration/shock-absorbing capacity into the stiffness- adjustment device. One body portion 3 carries a volume compensation chamber 15, in communication with the hydraulic system 8,9,161 through a convoluted damping duct 151. A deformable wall 152 of the volume compensation chamber 15 is provided by a load-bearing resilient spring so that the device functions as a hydraulically-damped mounting device for relative movements between the body portions 2,3. The duct 151 may be permanently open to the hydraulic system, so that there is no truly rigid condition but rather a high-stiffness damped condition. When the valve 121 of the compliance chamber 12 is opened, volume compensation occurs preferentially in that chamber because of its much lower resistance to volume alteration, and the load-bearing spring stiffness given by the chamber 15 is effectively switched out. A low-stiffness spring may also be provided in parallel, as before. One or more chambers may be inclined to the axial direction, e.g. as described previously, to give lateral as well as axial stiffness control. The hydraulically-damped load bearing capacity may further include any features known for such devices and compatible in this context . For example a flexible diaphragm may be positioned in a limited volume compliance chamber across an opening communicating between the hydraulic system (duct 161, in this case) and the volume compensation chamber 15, to decouple small amplitude vibrations typical of high frequency noise.

Fig. 8 shows a practical embodiment of such a multiplex device. Rigid lower body cup 3 has an intermediate partition wall 409. Below the wall a tertiary hydraulic chamber 15 is bounded by a load-bearing spring compliance 152. The tertiary chamber 15 communicates through the partition 409 via an annular damping channel 151 with circumferentially spaced openings 412,413 into the primary hydraulic chamber 8. An upper body cup 2 provides the opposite end of the working chamber 8 above the partition 409. An upward side wall extrusion of the lower cup 3 is inturned around the upper cup 2 and opposes resilient stop buffers 28 on that inner cup 2, to limit separation of the two cups 2,3. A corresponding buffer 29 is provided on the lower face of the upper body 2, within the primary chamber 8, to limit their mutual approach. In addition to the damped connection to the tertiary chamber 15, the primary chamber 8 is connected upwardly, through a central opening in the lower face of the upper cup 2, to a hydraulic cavity within that cup. This cavity has a dual nature, being sub-divided by an inner partition 2a nested inside the outer cup 2 and mounted to it through an annular rubber compliance 322. This annular compliance 322 defines a quaternary hydraulic chamber 321 between the inner and outer shells 2,2a of the upper cup, with which the primary chamber 8 always communicates. The inner partition shell 2a defines in its top part a secondary hydraulic chamber, bounded at the top by a resilient spring compliance 122 (which may be as strong as or substantially weaker than the bottom spring compliance 152) , separated from the primary chamber 8 by a set of circumferential openings 61 through the partition shell's lower wall. The lower wall of the partition shell also forms a drive housing 33, extending down through the central opening of the upper cup 2, through the primary chamber and down into an indentation of the lower partition 409 to give a compact construction. A drive solenoid 50 is housed in this and its drive shaft 102 carries a closure member 10 movable perpendicularly into or out of sealing relation with the openings 61. With the openings 61 closed, the resilient response of this device is determined by the hydraulically damped communication of the primary chamber 8 with the tertiary chamber 15 and its spring compliance 152, also by the quaternary chamber 321 and its spring compliance 322, and carrying up and down with it in series non-responsive, internally rigid secondary chamber 12.

By activating the solenoid 50 to open the openings 61, the secondary chamber's compliance is introduced in series; if its spring 122 is made substantially weaker than those of the other compliances then it can effectively uncouple them and give a low-stiffness overall response.

The passive response of the primary chamber wall 81 enables this selection of a wide variety of responses within one device using a single working chamber.

Claims

CLAIMS :

1. A mounting device for load-bearing connection between structures subject to relative vibration, having first and second rigid body parts (2,3) with respective anchor points for connection to the respective structures; at least one primary hydraulic chamber (8,9) defined by opposed portions (25, 35;24, 31) of the respective rigid body parts and a surrounding side wall (81,91) which extends between the opposed body portions, the side wall (81,91) being passively deformable without load-bearing resilience to follow movements of the opposed body portions towards and away from one another, and substantially non-stretching for a given relative position of the body parts (2,3) whereby such movements require a corresponding volume change of the primary hydraulic chamber,* at least one secondary hydraulic chamber (9, 8;12) connectable with the at least one primary hydraulic chamber (8,9) through one or more access openings (61,34), and having a deformable wall (91,81;122) to accommodate volume changes compensating wholly or partially for those of the primary hydraulic chamber(s) , and access control means (10) acting at the access opening(s) , adjustable to control liquid flow between the primary and secondary hydraulic chambers and thereby adjust the stiffness of the connection between the rigid body parts (2,3) .

2. A mounting device according to claim 1 in which the access control means (10) is adjustable to a closed condition closing off the secondary hydraulic chamber from the primary hydraulic chamber.

3. A mounting device according to claim 1 or claim 2 in which said primary and/or secondary hydraulic chamber (9) is or are oriented to undergo said required volume change for relative movements of the opposed body portions both along and transverse to a load axis defined between the anchor points.

4. A mounting device according to any one of the preceding claims further comprising a spring (7) connected between the first and second body parts (2,3) in parallel with the primary chamber to provide a resilient response in a low-stiffness condition of the device.

5. A mounting device according to claim 4 in which the parallel spring (7) is undamped.

6. A mounting device according to any one of the preceding claims in which the or each hydraulic chamber side wall (81,91) without load-bearing resilience is of reinforced elastomer membrane.

7. A mounting device according to any one of the preceding claims in which the end peripheries of the or each surrounding side wall (81,91) of the respective opposed body portions have substantially equal dimensions .

8. A mounting device according to any one of the preceding claims in which liquid occupying the hydraulic chambers (8,9) is pressurised.

9. A mounting device according to any one of the preceding claims in which the access control means (10) comprises an obturator drivable into or out of blocking relationship with said access opening (61,34) .

10. A mounting device according to any one of the preceding claims in which the dimensions of the access opening(s) (61,34) is/are such that in a low-stiffness condition of the device the stiffness contributed by the primary and secondary hydraulic chambers (9,8) is not more than 100 N/mm at above 10 Hz.

11. A mounting device according to any one of the preceding claims in which in a high-stiffness condition of the device the stiffness contributed by the primary and secondary hydraulic chambers (9,8) is at least 5000 N/mm at 400 Hz.

12. A mounting device according to any one of the preceding claims comprising a limit stop (28) on a said body part (2,3) positioned to engage the other body part to limit the separation of said opposed body portions in a low-stiffness condition of the device.

13. A mounting device according to any one of the preceding claims in which the deformable wall (91,81) of at least one said secondary hydraulic chamber (9,8) is deformable without load-bearing resilience and is substantially non-stretching for a given relative position of the body parts.

14. A mounting device according to any one of the preceding claims in which at least one said secondary hydraulic chamber (9,8) is defined between further opposed portions of the relative body parts (2,3) whereby said movements require a corresponding volume change of said at least one secondary hydraulic chamber.

15. A mounting device according to any one of the preceding claims in which the first body part (2) has outer portions spaced along the load axis defined between the anchor points and the second body part (3) as an intervening portion lying between the spaced outer portions of the first body part (2) to provide said opposed portions, with a said primary hydraulic chamber (8,9) being on one side of the intervening portion and a said secondary hydraulic chamber (9,8) being on the other side.

16. A mounting device according to any one of the preceding claims in which the deformable wall (122) of a said secondary hydraulic chamber (12) is a resilient wall.

17. A mounting device according to any one of claims 1 to 15 in which none of the primary and secondary hydraulic chambers (8,9) has a deformable wall of load-bearing resilience.

18. A mounting device according to any one of the preceding claims further comprising a tertiary hydraulic chamber (15) having a deformable wall (152) of load- bearing resilience, connected to the primary hydraulic chamber (8) through an elongate damping channel (151) to give a hydraulically-damped resilient mounting.

19. A mounting connection comprising a mounting device according to any one of claims 1 to 17 connected in series with a hydraulically-damped mounting device (300) .