GB2298021A

GB2298021A - Vibration isolator

Info

Publication number: GB2298021A
Application number: GB9603429A
Authority: GB
Inventors: Michael Harold Radford
Original assignee: Barry Controls Ltd
Current assignee: Barry Controls Ltd
Priority date: 1995-02-17
Filing date: 1996-02-19
Publication date: 1996-08-21
Anticipated expiration: 2016-02-19
Also published as: GB9603429D0; GB2298021B

Abstract

A vibration isolator comprises an outer casing 301, 302 in which a core assembly 303 is movably held captive, the core assembly extending outside the casing, and suspended within the casing by at least one elastomeric member 318, pressurised fluid being supplied to the inside of the casing (e.g. to air bag 319) via a valve arrangement so that when the core assembly reaches a predetermined point of deflection within the casing, it actuates the valve to allow the fluid to inflate or exhaust from the casing, tending to return the core assembly towards its equilibrium position. The isolator may be used, for example, to support an engine on a locomotive chassis or an instrument platform in an aircraft. As shown, the casing comprises the upper and lower faces 301, 302 of an instrument platform, and the core assembly is pivotally connected at 313 to the main body of the aircraft. In alternative embodiments, two air bags may be used, or four air bags to be inflated or exhausted in two diagonally arranged pairs.

Description

IMPROVED VIBRATION ISOLATOR This invention relates to a vibration isolator, in particular, but not exclusively, for use in supporting an engine on the chassis of a locomotive or for suspending an instrument platform in an aeroplane.

In applications such as those mentioned above, a vibration isolator needs to be able to accommodate the large forces generated upon engine start-up or take-off in addition to absorbing the higher frequency, lower amplitude vibrations associated with general operation.

Known vibration isolators include so-called "soft" systems, in which vibration is generally absorbed by an elastic element. In a system with a linear loaddeflection characteristic, the application of a larger than normal load will lead to a proportionally large deflection, which it may not be possible to accommodate. If, alternatively, a system with a nonlinear load-deflection characteristic is used, in which an elastomeric element may be incorporated as an isolator at deflection extremes, the natural frequency of the system rises for the duration of an excess load application causing an unacceptable increase in vibrational energy transmission.These disadvantages are emphasised when such a system is in a snubbed condition, in other words when the deflection is at its maximum and the elastomeric element is compressed (or extended) substantially to its full extent, thereby directly transmitting a substantial amount of vibration. This is particularly problematic for captive systems in which the extent of deflection is limited by containment within a housing. Systems which combine linear and non-linear load-deflection characteristics generally also need to be carefully adjusted so as to ensure that the rapid transition of the system from soft to hard is achieved at the correct deflection. Any progressive settling of the elastomeric element will require this adjustment to be made at regular intervals.

In addition, the large deflect ions associated with soft vibration isolation systems can lead to excessive wear in the system, thereby reducing the operational life of the isolator and the body, such as the engine or instrument platform, which is to be isolated. It is therefore desirable to provide a system that will not degrade energy transmission beyond that of a normally snubbed system and in addition which will reduce the duration of the high stiffness snubbed condition when high loads are applied for extended periods. It is also desirable to reduce the high deflections associated with conventional vibration isolation systems.

According to the present invention, there is provided a vibration isolator comprising an outer casing in which a core assembly is movably held captive, the core assembly communicating with the outside of the casing, wherein the core assembly is suspended within the outer casing by means of at least one elastomeric member, and wherein pressurised fluid is supplied to the inside of the casing by way of a valve arrangement such that when the core assembly reaches a predetermined point of deflection within the casing it activates the valve arrangement so as to allow the pressurised fluid to apply a force tending to return the core assembly towards its equilibrium position.

A body to be isolated or suspended, such as an engine or an instrument platform, is mounted on one or more of the isolators through attachment to the core assembly of each isolator. Alternatively, the core assembly may be attached to a chassis or the like, and the outer casing may be used as the isolation support.

The pressurised fluid and valve arrangement has the advantageous effect of reducing the time for which the core assembly is held snubbed against the outer casing upon the application of an excess load, thereby reducing the transmission of vibration in the snubbed condition which is a disadvantage in known soft vibration isolators. In a preferred embodiment, the present invention provides conventional isolation means for movement in planes perpendicular to the major axis of the core assembly, but supplements through the application of pressurised fluid the force available from conventional elastomeric isolation members for movements in directions generally parallel to the major axis of the core assembly.Thus upon application of excess loads for extended periods in the direction of the major axis, the valve arrangement is activated so as to admit pressurised fluid which applies a force tending to restore the core assembly to its equilibrium position for as long as the excess load is applied.

Upon removal of the load, the resulting movement along the major axis of the core assembly activates the valve arrangement so as to shut off the supply of pressurised fluid and to exhaust the pressurised fluid supporting the excess load, thereby restoring the equilibrium position to match the original load.

In certain embodiments of the invention, the space inside the casing of the vibration isolator in which the core assembly is disposed is internally divided into two non-communicating chambers by an elastomeric diaphragm, and the core assembly is attached to the diaphragm. In these embodiments, the valve arrangement is preferably adapted such that when the core assembly reaches a predetermined point of deflection from its rest condition, the valve arrangement is activated so as to admit pressurised fluid to the chamber into which, and/or to exhaust or evacuate fluid from the chamber away from which, the deflection is taking place. The pressure difference between the two chambers results in a restoring force being applied to the diaphragm so as to return the core assembly towards its rest position.Where a very fast response is required, it is particularly advantageous to adopt an arrangement in which prevailing pressures are modified on both sides of the diaphragm. This is because the pressure difference across the diaphragm, and hence the force applied to the diaphragm, can be made greater than that obtainable merely by applying pressurised fluid to one side of the diaphragm without exhausting or evacuating fluid from the other side.

Alternatively, it may be sufficient merely to exhaust or evacuate the chamber away from which the deflection is taking place. Once the excess load has been removed, the elastic properties of the diaphragm will tend to return the core assembly to the equilibrium position when fluid is readmitted to the chamber which has been exhausted. These embodiments are particularly useful as vibration isolators for instrument platforms in aeroplanes, where the sudden application of "g" forces can be a problem.

The valve arrangement is advantageously adapted to be insensitive to fluid contaminants. This may be achieved through the provision of filter means and/or through careful selection of the valve type. In addition, the valve arrangement preferably provides a rapid response to the action of the core assembly in order to reduce the time which the system spends in the snubbed condition upon application of an excess load.

During the application of an excess force (typically as a result of an increase of torque reaction in the case of a suspended engine, or of the application of increased "g" forces in the case of an airborne installation), the spring stiffness will generally increase while supporting the same mass. This does mean that the resonant frequency of the suspended body on the vibration isolation system will rise, but without generally being accompanied by the conventionally associated increase in relative deflection. This has the advantage of enabling a relatively compact installation to be achieved.

In preferred embodiments of the present invention, there is provided a predetermined range of movement of the core assembly in the outer casing over which the core assembly does not activate the valve arrangement.

For the normal running of a suspended engine, for example, a degree of movement is required so that the system is not continually trying to compensate for the normal vibratory motion. By providing a predetermined degree of free movement between the core assembly and the valve arrangement, the system can settle to a position where the majority of the rapid positioning is taking place without the valve arrangement being needlessly activated. Fine-tuning of this position is automatically achieved through a negative feedback mechanism in which small displacements of the core assembly beyond its range of relatively free movement are countered by the accompanying small actuations of the valve arrangement, thereby maintaining the appropriate equilibrium position in normal use.

Suitable pressurised fluids for use in the present invention include air and other compressible fluids.

In embodiments suitable for installation on a locomotive, compressed air is advantageously utilised, since a compressed air supply is already installed on many locomotives for the control of other functions.

When a vibration isolator according to the present invention is to be installed as part of an instrument support platform or the like on an aircraft, a number of additional considerations need to be taken into account. The "g" forces to which the vibration isolator must respond are significantly higher and are applied significantly quicker than the forces encountered in locomotive applications.

According to a second aspect of the present invention, there is provided a vibration isolator comprising a frame or housing in which a core assembly is movably held captive, the core assembly communicating with the outside of the frame or housing, wherein the core assembly is suspended within the frame or housing by way of two or more deformable means, at least one of which comprises an inflatable chamber, and wherein pressurised fluid is supplied to said at least one chamber by way of a valve arrangement in such a way that when the core assembly reaches a predetermined point of deflection within the frame or housing it activates the valve arrangement so as to allow the pressurised fluid to inflate or exhaust from said at least one chamber thereby tending to return the core assembly towards its equilibrium position.

The frame or housing generally forms the body of an instrument platform, and the part of the core assembly communicating with the outside of the frame or housing is affixed to the main body of the aircraft.

Preferably, additional resilient suspension means, such as a strut extending between the upper and lower faces of the instrument platform and cooperating with an elastomeric or other resilient component of the core assembly, is provided which serves to provide isolation from lateral vibrations and/or additional isolation from vibrations perpendicular to the plane of the instrument platform. The inflatable chambers may take the form of air bags or the like.

The valve arrangement may be actuated through the physical movement of the frame or housing, or alternatively through means which monitor the pressure in the at least one chamber.

Two chambers may be provided, in which case one is adapted to resist "g" forces acting in a downwards direction relative to the plane of the instrument platform, and the other to resist "g" forces acting in an upwards direction, thereby helping to avoid the snubbed condition in which the core assembly transmits vibration directly to the platform. In this embodiment, the core assembly is rigidly affixed to the main body of the aircraft, for example to a bulkhead, and the twisting forces of the reaction moments generated during aerial manoeuvres are absorbed by the fixing means. In an alternative embodiment, four inflatable chambers are provided and the core assembly is affixed to the main body of the aircraft by pivotal mounting means.The chambers are disposed with two being located between the core assembly and the lower face of the instrument platform along the pivotal plane of the core assembly mounting means, and the other two being located in corresponding positions between the core assembly and the upper face of the instrument platform. The chambers are connected to the valve arrangement in such a way that each diagonally opposing pair of chambers inflate or deflate together. In this way, as well as reacting the "g" force components acting perpendicularly to the plane of the instrument platform, the system can absorb the twisting forces of the reaction moments within the platform structure and not apply these to the main body of the aircraft through the mounting means.

In a further development of this embodiment in which the core assembly is pivotally mounted on the main body of the aircraft, one of the complementary pairs of chambers (i.e. a pair comprising two chambers one on top of the other) may be replaced by a resilient support member or members. Such a support member may be in the form of a substantially solid elastomer extending between the upper and lower faces of the instrument platform and through which the core assembly is passed, or alternatively in the form of a pair of sprung members respectively extending from the upper and lower faces of the instrument panel and which grip or are attached to the core assembly. The core assembly, when considered in the frame of reference defined by the instrument platform, can then effectively pivot about the point where it is held by the-resilient support member.By varying the reaction force applied by the pair of inflatable chambers proportionally to the applied "g" forces, the twisting forces of reaction moments may still be absorbed internally within the instrument platform structure while allowing the structure of the vibration isolator to be considerably simplified. Either the pair of chambers nearest to or the pair furthest from the pivotal mounting may be replaced with a resilient support member.

In the three embodiments described above, a number of different inflation/deflation regimes may be applied. For illustrative purposes, it is assumed in the following that the maximum loading to be reacted is "9g", which is typical for aerobatic military aircraft.

Under a first regime, constant pressure to support the maximum "9g" loading is applied in one chamber (in the two chamber embodiments) or pair of diagonallyopposed chambers (in the four chamber embodiments), balanced by an "8g" loading in the complementary chamber or chambers in order to support normal "lug" operation. Instant reaction to the application of high "g" forces is then possible by exhausting the balancing chamber or chambers to the appropriate pressure for inertia loads of less than "9g". This regime, while allowing a very rapid response, has the disadvantage that high pressures must be applied, and consequently that high loads are continuously borne by the instrument platform structure.

Under a second regime, constant pressure to support a predetermined inertia loading of, say, "5g" is applied to one chamber or pair of chambers, balanced by "4g" in the balancing chamber or chambers for "lg" operation. Rapid response is then possible for applied loads of less than "Sg" by exhausting the balancing chamber or chambers to the appropriate pressure. For loads greater than "5g", the system responds through rapid pressurisation of the first chamber or pair of chambers, accompanied if required by an appropriate exhausting of the balancing chamber or chambers in order to achieve the necessary pressure difference to react the applied load.This regime has the advantage that the loads applied to the instrument platform structure by the chambers are lower, but the response time when high, i.e. greater than "5g", forces are applied is slower, since inflation is generally not as fast as deflation of the chambers.

Under a third regime, the pressure difference between complementary chambers is kept no higher than necessary, i.e. proportional to and just able to react the applied load. The valve arrangement is actuated in response to the position of the instrument platform relative to the static structure of the body of the aircraft, and/or by monitoring the pressure difference between complementary chambers, and enables an active response to the variation of the applied load through the appropriate inflation and/or exhaust of complementary chambers. In order to achieve a fast response time, the supply lines from the valve arrangement preferably have a relatively large bore, and/or a reservoir of high-pressure fluid may be provided.

A particularly preferred embodiment of the present invention uses just one inflatable chamber between the core assembly and the lower face of the instrument platform in order to react against the potentially high downwards "g" forces, the upwards "g" forces being reacted by a conventional resilient element.

Advantageously, the core assembly is pivotally mounted on the main body of the aircraft, and is further suspended within the instrument platform by a resilient support member as described above. In this embodiment, only the vertical downwards "g" forces, which are generally of the greatest magnitude, are reacted by the inflatable chamber in order to avoid the snubbed condition, forces in all other directions being reacted by conventional resilient means. Because only one inflatable chamber needs to be accommodated within the instrument platform structure, its volume may be greater than in the aforementioned embodiments, which in turn provides a softer spring for a given load carrying capacity and provides improved vibration isolation.Furthermore, because only one inflatable chamber is employed, the valve arrangement may be considerably simplified, thereby providing even greater reliability and efficiency.

In general, four vibration isolators according to the present invention may be used to support a typical instrument shelf, although it is clear that this number may be varied for particular applications.

Valve arrangements suitable for use with the second embodiment of the invention may take a number of forms. The basic requirement is that movement of the instrument platform relative to the static frame of the main body of the aircraft beyond predetermined limits serves to actuate the valve arrangement so as to inflate or exhaust the appropriate chamber or chambers in order to react against the applied "g" forces. By doing so, the system serves to reduce the risk of the platform becoming snubbed against the main body of the aircraft under the influence of high "g" forces and thereby becoming subject to direct transmission of vibration.The valve arrangement may be actuated directly by movement of the instrument platform, or, in embodiments where the core assembly is pivotally mounted on the main body of the aircraft, the valve arrangement may be actuated by movement of the core assembly. Alternatively or in addition, the valve arrangement may be actuated by sensors which-monitor the fluid pressure inside the inflatable chambers.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: FIGURE 1 is a plan view of one embodiment of the first aspect of the present invention; FIGURE 2 is a side elevation of the vibration isolator of Figure 1 with a partial cut-away showing the interior mechanism; FIGURE 3 is a view on section A-A of Figure 2; FIGURE 4 is a section through an alternative embodiment of the first aspect of the present invention; FIGURE 5 is a section through the elastomeric bush shown in Figure 4; FIGURE 6 is a plan view of one embodiment of the second aspect of the present invention; FIGURE 7 is a view on section A-A of Figure 6; FIGURE 8 is a plan view of a second embodiment of the second aspect of the present invention;; FIGURE 9 is a view on section A-A of Figure 8; FIGURE 10 is a plan view of a third embodiment of the second aspect of the present invention; FIGURE 11 is a view on section A-A of Figure 10; FIGURE 12 is a plan view of a fourth embodiment of the second aspect of the present invention; FIGURE 13 is a view on section A-A of Figure 12; FIGURE 14 is a cross-sectional view of a first valve arrangement; FIGURE 15 is a cross-sectional view of a second valve arrangement; FIGURE 16 is a plan view of the valve arrangement of Figure 15; FIGURE 17 is a perspective view of the valve arrangement of Figures 15 and 16; and FIGURE 18 is a cross-sectional view of a third valve arrangement.

Figure 1 is a plan view showing an upper part 5 of the housing of a vibration isolator attached to the main body (not shown) of the isolator by bolts 15. A main elastomeric member 4 connects the housing of the isolator to a core assembly, which is connected to a nut 1. The nut 1 may be locked with a pin 100. A hole 101 is defined in the top of the core assembly in order to accommodate a support member of a body to be isolated.

As shown best in Figure 2, one embodiment of the vibration isolator is assembled as follows: An air sealing washer 2 is fitted to a core assembly 8 followed by an upper snubber diaphragm 6, a main elastomeric member 4 and a second sealing washer 2'. The main elastomeric member 4 is connected to the shaft of the core assembly 8 by way of a bonded bearing 3 which permits rotation of the core assembly 8. A thread locking compound is then applied to the core assembly thread and a nut 1 is assembled and torquetightened against the shoulder on the core assembly 8.

The nut 1 can be further secured by drilling down the thread and inserting a pin lock 100. A lid 5 of the outer casing is attached to the main elastomeric member 4. An operating spool mounting bush assembly 20 is made up of an elastomeric bush 201 applied to a core 202. The core assembly 8 is connected to the bush assembly 20 by way of an annular plate 203. A valve mounting block 9 is then affixed to a lower snubber diaphragm 7 using a jointing compound, and outer casing components 5 and 11 and the upper and lower snubber diaphragms 6 and 7 are temporarily clamped together.

An exhaust valve seat 31 is assembled into the main air chamber casing component 11 using PTFE jointing tape and then adjusted to an assembly gauge in order to control the spacing between valve assembly seat surfaces 43, 44. After adjustment, the exhaust valve seat 31 is locked in position using tab 32 and screw 33 before the main air chamber 11 is removed, leaving the other casing components temporarily clamped. A valve assembly is attached to the valve mounting block 9 using bolts 13, and clamped to a valve operating spool 21 using a bolt 29 and a self-locking nut 30. Spacers 25, 251 and 28 are mounted on the spool 21 so as to hold inlet and outlet valve actuating washers 23, 231 held by the spacers 25, 251 and 28 at predetermined locations on the spool 21.Annular inlet and outlet valve bodies 22, 26 are located around the flanges of actuating washers 23, 231 respectively and are connected to the spool 21 by way of elastomeric diaphragms 27 which serve to prevent unwanted leakage of fluid through the valve assembly while still allowing a degree of relatively free axial and radial movement of the spool 21 in relation to the valve bodies 22, 26. A further diaphragm 24 connects, by way of the spacers 25 and 251 and clamping washers 14, the spool 21 to the valve mounting block 9 between the inlet and outlet valve sub-assemblies. The diaphragm 24 serves as a flexible inlet pressure retainer which permits a degree of relatively free axial and radial movement of the central spool 21 in relation to the main body of the vibration isolator.

An air inlet fitting 12 is fitted to the main air chamber 11 using PTFE sealing tape before assembly of the vibration isolator is completed by reattaching the main air chamber 11 to the remaining casing components 5, 6 and 7 using bolts 15 and ensuring that 'O'-ring 10 is correctly positioned.

Once assembled, the vibration isolator defines a main support chamber 42, an inlet chamber 41 and an outlet chamber 45. The main body of the core assembly 8 is held captive in the support chamber 42, and the support chamber 42 communicates with the outlet chamber 45 by way of at least one passage 46. A degree of vibrational damping for the core assembly 8 may accordingly be provided if the at least one passage 46 is narrow enough to offer significant resistance to fluid flow between the chambers 42 and 45.

In normal use, an initially applied load on the top of the core assembly 8 will cause the elastomeric element 4 to deflect axially downwards until an elastomer 81 on the base of the flange 82 of the core assembly 8 rests on the lower snubber diaphragm 7. The spool 21 of the valve assembly will be forced downwards and the top washer 23 will contact the rim of the inlet valve body 22 and cause an elastomeric integral seal 40 to be pulled out of contact with the base of the lower snubber diaphragm 7. This causes the inlet chamber 41 to become connected to the support chamber 42.

Application of fluid pressure to the inlet will cause the pressure in the support chamber 42 to increase until the combination of the upward force from the main elastomeric element 4 (due to its deformation from a neutral position) and the pressure acting on the underside of the elastomeric element 4 equals the applied load. The core assembly 8 will then rise due to the continued admittance of fluid and consequent increase in pressure until the flow of pressurised fluid is cut off by the elastomeric seal 40 recontacting the underside of the lower snubber diaphragm 7. Initial subsequent vibratory deflections of the core assembly 8 and the valve operating spool 21 downwards beyond this initial equilibrium position will momentarily actuate the inlet valve body 22 and admit more pressurised fluid.This raises the core assembly 8 slightly and thereby moves the system towards a stable equilibrium in which the core assembly 8 and the valve operating spool 21 are vibrating within the boundaries of relatively free movement provided by the valve assembly without actuating the inlet valve body 22.

Application of additional load to that already supported will deflect the core assembly 8 and the valve operating spool 21 downwards, thereby actuating the inlet valve body 22 and admitting further pressurised fluid which raises the core 8 and the spool 21 until equilibrium is once again achieved.

Upon reduction of the applied load, the equilibrium is lost and the now excess pressure in the support chamber 42 moves the core assembly 8 and the valve operating spool 21 upwards. This actuates the outlet valve body 26 at the base of the valve assembly and lifts it clear of the exhaust valve seat, thereby allowing fluid to escape from the support chamber 42 by way of the outlet chamber 45 until the core 8 and the spool 21 regain their equilibrium position at which point the outlet valve body 26 returns to the closed state. When the vibration isolator is not loaded, the tendency of the main elastomeric support 4 to try to regain its original shape will force the core assembly flange 82 of the core assembly 8 against the underside of the upper snubber diaphragm 6. In this configuration, the inlet valve body 22 is held in a closed position.This prevents ingress of pressurised fluid into the support chamber 42 until sufficient load is applied to cause the core assembly 8 to move the valve operating spool 21 thereby actuating the inlet valve body 22. This prevents wastage of pressurised fluid when the vibration isolator is not in operation.

Figure 3 is a plan view onto section A-A of Figure 2 and shows the relative positions of the valve mounting block 9, the valve operating spool 21, the inlet valve body 22 and the inlet valve actuating washer 23.

An alternative embodiment of the present invention is shown in Figure 4. This embodiment provides additional vibration damping through an annular elastomeric member 400 which connects the core assembly 8 to the inside edge of the support chamber 42 in such a way as to prevent leakage of pressurised fluid.

Further damping of radial vibration is provided by an elastomeric bush 401 which is fitted around the shaft of the core assembly 8. The outside of the bush 401 includes slots 500 as shown in Figure 5 so as to allow the bush 401 to be inserted relatively tightly into the top of the isolator casing component 5. The outside of the bush 401 is also provided with a friction material 402. Relatively small axial vibrations of the core assembly 8 are absorbed by flexing of the elastomeric member 400 and the bush 401. If the axial movements are above a predetermined magnitude, the friction material 402 on the bush 401 will slip against the inside surface of the casing component 5, thereby allowing the core assembly 8 to move sufficiently so as to activate the valve assembly and allow the ingress or egress of pressurised fluid, which will apply a force to the underside of the core assembly 8 and the elastomeric member 400. The vibration isolator may be provided with a dust cap 403. In contrast to the vibration isolator shown in Figure 2, the core assembly 8 of the isolator shown in Figure 4 is not free to rotate, since the flange 82 is secured to the elastomeric element 400.In order to allow the nut 1 to rotate upon the application of an excessive amount of torque, the core assembly 8 is provided with a rotatable axle member 404 to which the nut 1 is secured by way of a thread and a locking pin 405. A flange 406 is provided at the end of the axle member 404 remote from the nut 1 in order to prevent relative axial displacement between the axle member 404 and the core assembly 8. An 'O'-ring seal 407 may be provided to prevent leakage of fluid between the axle member 404 and the core assembly 8. If the axle member 404 is hollow, a sealing plug 408 may be used to prevent leakage of fluid though the centre of the axle member 8.

The valve arrangement need not be limited to the annular flat seat system described above. Suitable alternatives include accurately machined spool valves with no seals; spool valves with 'O'-ring seals; squashed tube valves (comprising a flexible tube squashed flat by a roller by direct mechanical action); elastomer flap valves (comprising a flap held shut and allowed to open by direct mechanical means such as a plate or a roller); sleeve valves (comprising a closefit directly mechanically coupled cylindrical sleeve fitted over a central core, with fluid being supplied through ports); flat slider valves (similar in principle to sleeve valves, only flat, and comprising a slider which progressively exposes ports admitting or exhausting fluid) and servo-controlled valves (comprising a flat seat valve pneumatically or hydraulically opened by exhausting balance fluid pressure from behind an operating diaphragm, and shut by restoration of pressure supplied by way of an auxiliary valve). Other types of valve which are relatively insensitive to fluid contamination may also be used.

Figures 6 and 7 show a vibration isolator according to the second aspect of the present invention, which is particularly suited for use as an instrument platform support in aeronautical applications. The instrument platform 300 comprises an upper face 301 and a lower face 302, and is mounted on the main body of the aircraft in which it is installed by way of a core assembly 303 which is rigidly fixed at one end to the main body of the aircraft and resiliently fixed at the other end to a mounting structure 304. The mounting structure 304 comprises a strut 305 extending between the upper 301 and lower 302 faces of the instrument platform and cooperating with an elastomeric or other resilient component 306 of the core assembly 303.The mounting structure 304 absorbs much of the vibration, both vertical and lateral, generated in the main body of the aircraft, and serves to hold the core assembly 303 captive within the instrument platform 300. In the embodiment of Figures 6 and 7, two air bags 307 and 308 are provided, one air bag 307 between the core assembly 303 and the upper face 301 of the instrument platform 300, and the other air bag 308 between the core assembly 303 and the lower face 302 of the platform. The air bags 307, 308 are bonded to the core assembly by way of elastomer mouldings 309 and are provided with a low friction sliding interface 310 on the surfaces which contact the faces 301, 302 of the instrument platform. Elastomeric bump stops 311 are provided on the parts of the core assembly which may, in extreme situations, temporarily contact the platform structure directly.A supply of pressurised air is taken to the air bags 307, 308 by way of a valve arrangement 312, which will be discussed in more detail hereinbelow. The valve arrangement is actuated by movement of the instrument platform in such a way that when the platform moves downwards under the influence of an applied "g" force, the pressure difference between the two air bags 307, 308 is adjusted so as to resist the applied force and thereby to keep the platform from becoming snubbed against the core assembly. This may be done by exhausting the upper air bag 307 and/or by further inflating the lower air bag 308. When the applied "g" force is removed, the pressure difference will tend to raise the platform 300, which will then activate the valve arrangement 312 so as to bring the pressure difference between the air bags 307, 308 back to an equilibrium condition.

Similarly, if an applied "g" force acts in an upward direction, the valve arrangement 312 serves to adjust the pressure difference between the air bags 307, 308 so as to react the applied force in a downward direction.

Figures 8 and 9 show a second embodiment of the second aspect of the present invention, in which the core assembly 303 is pivotally mounted 313 on the main body of the aircraft, and in which four air bags 314, 315, 316, 317 are provided, two on either side of the resilient mounting structure 304. In this embodiment, diagonally opposite air bags 314, 317 and 315, 316 are respectively paired, i.e. are inflated/exhausted together via common air paths through the valve arrangement 312. Because the core assembly 303 is pivotally mounted 313 on the main body of the aircraft, the core assembly will pivot relative to the instrument platform 300 about the resilient mounting structure 304.Through the pneumatic pairing of diagonally opposite air bags, this pivoting due to applied "g" forces may be reacted internally within the platform structure rather than being transferred to the interface between the core assembly 303 and the main body of the aircraft.

Figures 10 and 11 show an alternative embodiment in which the core assembly is pivotally mounted 313 on the main body of the aircraft, and in which only one pair of air bags 307, 308 is required. In this embodiment, the core assembly 303 is held captive by a resilient support member 318, which may comprise a substantially solid elastomer extending between the upper and lower faces 301, 302 of the instrument platform 300 and through which the core assembly 303 is passed, or alternatively in the form of a pair of sprung members respectively extending from the upper and lower faces 301, 302 of the instrument platform 300 and which grip or are attached to the core assembly 303. The resilient support member 318 forms the fulcrum about which the core assembly 303 pivots relative to the instrument platform 300 upon application of "g" forces. The valve arrangement inflates/exhausts the air bags 307, 308 in response to relative movement of the core assembly 303 to the instrument platform 300 so as to react against the applied "g" forces so as to avoid snubbing. By varying the reaction force applied by the air bags 307, 308 proportionally to the applied "g" forces, the reaction moments may be absorbed internally within the platform structure.

Figures 12 and 13 show a modification of the embodiment of Figures 10 and 11 in which only one air bag 319 is used, extending between the core assembly 303 and the lower face 302 of the instrument platform 300. The air bag 319 is only able to react downwards "g" forces, upwards "g" forces being reacted by a conventional elastomeric member 320. However, because only one air bag 319 is present, the space available between the core assembly 303 and the lower face 302 of the instrument platform 300 is maximised, and the volume of the air bag 319 can therefore be made larger, and the spring correspondingly softer, for a given load bearing capacity. Advantageously, the chamber 319 is divided into two parts by a flange 336 provided with a small hole 337. Upon rapid compression, air flow through the hole 337 provides an additional degree of damping.The ability of the system to react upwards "g" forces without becoming snubbed is reduced, but since it is usually the downwards "g" forces which have the greatest magnitude, the consequent simplicity of this embodiment outweighs the potential disadvantages.

The valve arrangement 312, instead of being located on the main body of the aircraft, may be mounted on the instrument platform 300 and be actuated by movement of the core assembly 303 relative to the platform 300. In a preferred embodiment, the valves are incorporated within the chamber 319, which leads to a particularly compact installation. For greatest sensitivity, the valves are located in a region of the chamber distal from the fulcrum formed by the core assembly 303 and the resilient support member 318; this is because movement of the core assembly 303 relative to the instrument platform 300 is greatest in this region.

Figure 14 shows a valve arrangement suitable for use with the four air bag embodiment of the present invention operating under the third inflation/deflation regime described above. Complementary pairs of air bags AA and BB pneumatically linked as shown. The valve arrangement comprises an upper spool valve 321 and a lower spool valve 322, which are actuated by direct contact of the instrument platform 300. If the platform 300 moves down, lower spool valve 322 is opened, allowing compressed air to inflate air bag pair AA and allowing air to exhaust from air bag pair BB.

This generates a reaction moment which tends to restore the platform 300 to the equilibrium position, thereby closing the valve 322 and keeping the platform from the snubbed condition. When the applied downwards "g" force is removed, the reaction moment will tend to raise the platform 300, thereby opening upper spool valve 321. When this happens, air bag pair AA is allowed to exhaust, and air bag pair BB is inflated, thereby bringing the platform 300 back to the equilibrium position and re-closing valve 321.

Instead of spool valves, poppet valves 324 may be used, as shown in Figure 15. Each poppet valve 324 is held closed by way of pressure applied to the inlet 325. When opened, the valves 324 allow the passage of pressurised air from the inlet 325 to the outlet 326.

The valve arrangement may be formed as a modular unit 327 which is connected to the instrument platform 300 by way of dampers or springs 328 in order further to isolate the platform 300 from vibration and additionally to reduce the chance of the system entering into "engine" mode, in which repeated reciprocal activation of the valve arrangement leads to a resonant condition.

Figure 17 shows a perspective view of such a valve arrangement as installed. The instrument platform 300 is suspended on a core assembly 303, which in turn is pivotally mounted 313 on the main body of the aircraft.

The valves are disposed in an upper unit 329 and a lower unit 330, both of which are mounted on the main body of the aircraft by way of dampers or springs 328.

A common supply line 331 provides pressurised air to the valve inlets, while supply lines 332 and 333 connect the valve outlets to the air bags (not shown) Instead of using the instrument platform 300 to actuate the valve arrangement, it is possible in embodiments where the core assembly 303 is pivotally mounted to use movement of the core assembly itself.

How this may be done using poppet valves is shown in Figure 18. By arranging the poppet valves as shown, with the exhaust valves 334 further from the pivotal mounting 304 than the inflation valves 335, there is achieved the advantage that the exhaust valves are opened just before the inflation valves, which can lead to a faster system response to an applied force, since it is generally quicker to exhaust an air bag than to inflate it.

Claims

CLAIMS:

1. A vibration isolator comprising an outer casing in which a core assembly is movably held captive, the core assembly communicating with the outside of the casing, wherein the core assembly is suspended within the outer casing by means of at least one elastomeric member, and wherein pressurised fluid is supplied to the inside of the casing by way of a valve arrangement such that when the core assembly reaches a predetermined point of deflection within the casing it activates the valve arrangement so as to allow the pressurised fluid to apply a force tending to return the core assembly towards its equilibrium position.

2. A vibration isolator as claimed in claim 1, wherein the valve arrangement is actuated only by deflection of the core assembly in directions parallel to a predetermined line.

3. A vibration isolator as claimed in claims 1 or 2, wherein the part of the casing in which the core assembly is disposed is internally divided into two non-communicating chambers by an elastomeric diaphragm to which the core assembly is attached.

4. A vibration isolator as claimed in claim 3, wherein the valve arrangement is such that, when the core assembly reaches a predetermined point of deflection from its rest position, the core assembly activates the valve arrangement to admit pressurised fluid to the chamber into which, and/or to exhaust pressurised fluid from the chamber away from which, the deflection is taking place.

5. A vibration isolator as claimed in any of the preceding claims, wherein the core assembly has a predetermined range of movement over which it does not actuate the valve arrangement.

6. A vibration isolator comprising a frame or housing in which a core assembly is movably held captive, the core assembly communicating with the outside of the frame or housing, wherein the core assembly is suspended within the frame or housing by way of two or more deformable means, at least one of which comprises an inflatable chamber, and wherein pressurised fluid is supplied to said at least one chamber by way of a valve arrangement in such a way that when the core assembly reaches a predetermined point of deflection within the frame or housing it activates the valve arrangement so as to allow the pressurised fluid to inflate or exhaust from said at least one chamber thereby tending to return the core assembly towards its equilibrium position.

7. A vibration isolator as claimed in claim 6, wherein one of the deformable means comprises a resilient suspension member which joins the core assembly to the housing or frame in such a way that the core assembly is resiliently movably held captive.

8. A vibration isolator as claimed in claims 6 or 7, wherein two chambers are provided, and wherein the valve arrangement is such that, when the core assembly reaches the predetermined point of deflection from its rest position, the valve arrangement is activated to admit pressurised fluid to the chamber into which, and/or to exhaust pressurised fluid from the chamber away from which, the deflection is taking place.

9. A vibration isolator as claimed in claims 6 or 7, wherein, when the isolator is installed, the core assembly is pivotally mounted on a surface exterior to the frame or housing.

10. A vibration isolator as claimed in claim 9, wherein four chambers are provided, two being located between the core assembly and an upper part of the frame or housing along the pivotal plane of the core assembly mounting means, and two being located in corresponding positions between the core assembly and a lower part of the frame or housing1 the chambers being connected to the valve arrangement in such a way that, in use, diagonally opposite pairs of chambers inflate and/or exhaust together.

11. A vibration isolator as claimed in claim 9, wherein two chambers are provided, one between the core assembly and an upper part of the frame or housing, and the other in a corresponding position between the core assembly and a lower part of the frame or housing.

12. A vibration isolator as claimed in claim 9, wherein only one chamber is provided, the chamber being located between the core assembly and a lower part of the frame or housing.

13. A vibration isolator as claimed in claim 9, wherein only one chamber is provided, the chamber being located between the core assembly and an upper part of the frame or housing.

14. A vibration isolator as claimed in any of claims 9 to 13, wherein the valve arrangement is activated by movement of the core assembly relative to the surface on which it is pivotally mounted.

15. A vibration isolator as claimed in any of claims 9 to 13, wherein the valve arrangement is located within the at least one chamber.

16. A vibration isolator substantially as hereinbefore described with reference to or as shown in the accompanying drawings.