Vibration damping arrangements embodying the invention, and to be described in more detail below by way of example only, are in the form of engine mounts for mounting engines in motor vehicles.

However, they may be used in other applications.

According to the invention, there is provided a vibration damping arrangement for damping relative vibrations between two rigid members, comprising a first rigid element for connection to one of the rigid members, a second rigid element for connection to the other rigid member, stiff resilient support material interconnecting the two rigid elements for resiliently flexing in response to the relative vibrations of the two rigid members, means defining a hollow chamber containing hydraulic fluid, the hollow chamber having a flexible wall arranged to flex in response to the relative vibrations whereby to cause corresponding pressure variations within the hollow chamber, valve means operable to at least partially release the pressure in the chamber, and electromagnetically operative control means for selectively rendering the valve means operative.

According to the invention, there is further provided a hydroelastic engine mount for damping relative vibrations between the engine of a vehicle and the body of the vehicle, comprising a first rigid fixing element for connection to one of the engine and the vehicle body, a second rigid fixing element for attachment to the other of the engine and the body, resilient support material extending between the two rigid fixing elements for resiliently flexing in response to vibrations between the two fixing elements, a first flexible-walled chamber defined in part by the resilient support material, a second flexible-walled chamber, a conduit interconnecting the two chambers whereby hydraulic fluid can be passed through the conduit between the two chambers in response to resilient flexing of the resilient support material such that movement of the fluid through the conduit damps the vibrations, valve means comprising a movable valve member mounted to be subjected on one side to the pressure in the first chamber and to be subjected on the other side to the pressure in the second chamber, and electromagnetic control means acting on the valve member and operative to switch the valve means between a first setting in which the valve member is substantially limited in movement and a second setting in which the valve member is substantially free to move.

Vibration damping arrangements embodying the invention, and in the form of engine mounts for motor vehicles, will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a section through one of the engine mounts ; and Figure 2 is an enlarged view of part of Figure 1.

As shown in Figure 1, the engine mount comprises a support 10 made of stiff resilient material such as rubber which is mounted between the engine of the vehicle and the vehicle body or chassis. The support 10 is generally frusto-conical in shape.

Its upper end is fast with a metal stud 12 by means of which an attachment is made to the engine of the vehicle. The base of the rubber support 10 is bonded to a metal armature 14 which in turn is rigid with a metal cover 15 carrying a stud 16 for attachment to the vehicle body. In this way, therefore, the engine of the vehicle is connected to the vehicle body by means of the stiff rubber support 10, which provides a resilient connection and helps to isolate engine vibrations from the vehicle body. In addition, however, the support assembly provides hydraulic damping as will now be explained.

The hollow interior of the support 10 provides a chamber ("working chamber") 22 of flattened conical shape and containing a suitable hydraulic fluid such as water with anti-freeze. The base of the working chamber 22 is defined by an upper plate or grill 24 which is supported by a valve body 26 made of stiff material, such as plastics material. The valve body 26 is held in position by a flange 28 surrounding an aperture in the lower part of the armature 14. The flange 28 supports the rigid metal cover 15 and a flexible membrane 34. The metal cover 15 defines a hollow interior 35 which is open to the atmosphere at 36. A chamber ("compensation chamber") 37 is defined between the flexible membrane 34 and the underside of the valve body 26.

Two means of interconnection between the working and compensation chambers 22,37 are provided. The first of these interconnections is via a helical or part-circular conduit 38 which is connected at one end to the working chamber 22 via a hole (not visible in the Figures) through the plate 24. At its other end, the conduit 38 is connected to the compensation chamber 37 ; the latter connection is also not shown in the Figures.

The second interconnection means between the chambers 22 and 37 is provided by a valve assembly 44. The valve assembly incorporates a flexible disc valve 46. The disc valve 46 may be made of rubber and has a flexible central portion 48 which is integral with a circular more rigid flange portion 50. The disc valve 46 is supported in a valve chamber 51 defined between the upper grill 24 and a lower grill 52. The upper grill 24 has apertures 58 and the lower grill 52 has apertures 60.

As shown in more detail in Figure 2, the disc valve 46 is attached to a flanged pole plate 70 which extends over the end of a core piece 71 carrying an electrically energisable coil 72.

The core piece is fixedly mounted within the valve chamber 51, in a circular recess 73. In Figure 1, the core piece 71 and coil 72 are indicated diagrammatically at 74.

When the coil 72 is electrically energised (as shown in the left hand half of Figure 2), the pole plate 70 becomes magnetically attracted to the core piece 71 and is prevented from moving within the valve chamber 51. The central flexible part of the valve 46 will be permitted a limited amount of flexibility. When the coil 72 is de-energized (as shown in the right hand half of Figure 2), the pole plate 70 is no longer attracted to the core piece 71. An amount of free play P therefore exists between the pole plate 70 and the core piece 71. A significantly greater corresponding vibratory movement of the disc valve 46 in the chamber 51 is thus possible.

In use, vibrations of the engine relative to the body are partly damped by the resilience and stiffness of the rubber support 10.

In addition, however, the support 10 acts as a piston or pump on the fluid within the working chamber 22. If the vibrations are of relatively high amplitude and relatively low frequency, the liquid is pumped to and fro between the working chamber 24 and the compensation chamber 37 through the conduit 38. The pressure in the compensation chamber 37 is maintained constant and substantially equal to atmospheric pressure, because of the flexibility of the membrane 34 and the connection to the ambient atmosphere via the aperture 36. Therefore, the compensation chamber 37 produces substantially no resistance to the fluctuating liquid movement. Damping is achieved by oscillation of the liquid mass in the conduit 38 which acts as a dynamic damper. Maximum damping is obtained at the resonant frequency which depends on the characteristics of the conduit.

However, the operation of the engine mount is modified by the valve assembly 44.

Initially, the condition will be considered in which the coil 72 is electrically energised, so that the disc valve 46 has substantially limited freedom of movement or play in the valve chamber 51.

In this condition and when the vibrations are of relatively high amplitude, the disc valve 46 is unable to follow the vibrations and is held substantially in a fixed position against the grill 52. There is therefore substantially no transfer of fluid pressure between the working and compensation chambers 22,37 via the valve 44. Pressure transfer therefore can only take place through the conduit 38.

When the engine vibrations have relatively low amplitude, the central part of the disc valve 46 is able to vibrate to a small extent within the valve chamber 51 and thus provides a pressure interconnection between the chambers 22 and 37, even though substantially no fluid flow is taking place under these conditions through the conduit 38 because the frequency of vibrations is substantially different from the resonant frequency of the conduit. Therefore, the flexibility of the disc valve 46 short-circuits the conduit 38 and reduces the effective stiffness of the engine mount so that transmission of vibrations from the engine to the vehicle body is correspondingly reduced.

If, now, the coil 72 is de-energised, the disc valve 46 is given amount of play P as shown in Figure 2. It is therefore able to vibrate not only in response to low amplitude engine vibrations but also at higher amplitude vibrations. In this way, therefore, it can short-circuit the conduit 38 not only at low amplitude vibrations but also at higher amplitude vibrations.

Therefore, the dynamic stiffness of the engine mount can be adjusted to be similar to that of the static stiffness. The stiffness of the engine mount can in this way be reduced when the engine is at idling speed, thus reducing the transmission of acoustic vibration to the vehicle body. Since the disc valve 46 is not connected to the body 10, the disc valve 46 has low inertia and is thus very responsive to vibrations.

Energisation of the coil 72, and thus the stiffness of the engine mount, can be automatically controlled, in dependence on engine speed, by means of a suitable device which is responsive to engine speed.