GB2266640A - Sealing means for flextensional sonar transducer assembly - Google Patents

Description

2266640 SONAR TRANSDUCER ASSEMBLY This invention relates to flextensional

sonar transducer assemblies, such as are used as high power acoustic energy sources in underwater sonar systems. Generally, such a transducer comprises a cylindrical hollow shell element of generally circular or elliptical cross-section which has fitted within it a driving mechanism, usually piezo-electric in nature.

In the more common elliptical transducer case, the driving mechanism is arranged to vibrate along the major axis of the elliptical element. This causes considerable changes in dimension about the minor axis and generates the required acoustic energy.

Because the transducer is intended to be immersed in a fluid, the hollow interior of the shell element containing the driving mechanism has to be sealed. This is usually done by fastening a pair of end plates to the ends of the hollow shell element using flexible seals. In addition, for optimum energy generation the shell is stressed or pre-loaded along its longitudinal axis and this is usually done by fastening the end plates in position by tie-rods which apply the required pre-load or stress.

The main problem associated with sonar transducers of this type is that of preventing leakage of fluid into the shell member from outside. The transducer is commonly used underwater and possibly at depths where the external water pressure becomes very large. This may cause deformation of the shell, particularly if this has a generally elliptical cross-section in which case the minor axis width of the shell will reduce under increasing pressure. At the same time, operation of the transducer results in continuous dimensional changes about the major and minor axes of the shell. For these reasons the problem of providing an effective long term seal is particularly difficult to solve. Various solutions have been tried, varying from the simple 0-ring seal shown in European patent application No. 215657 to the bonded seal discussed in British patent No.

2209645. In this latter case the seal is bonded to the shell and has a highly flexible area arranged to be in contact with the end plate of the transducer. None of the existing forms of seal has been entirely satisfactory in preventing the ingress of water over a long period of time.

It is an object of the present invention to provide a sonar transducer assembly having improved means for sealing the assembly against the ingress of a fluid under pressure.

According to the present invention there is provided a sonar transducer assembly arranged to be immersed in a fluid, which assembly includes a hollow cylindrical shell element, a pair of end plates arranged to be secured one to each end of said element to close off the volume within the element, and sealing means located between each end plate and the element, the sealing means comprising a moulding of flexible material impervious to said fluid and shaped to cooperate with the end plate and the end of the shell element to provide a first cylindrical portion fitting closely around the external surface of an end part of said element and having a second portion integral with the first portion and extending inwardly towards a longitudinal axis of the element to form a flange the inwardlydirected dimension of which is at least equal to the thickness of the element, the seal being maintained in a sealing relationship with the surface of the element by the pressure of the surrounding fluid and with the end of the element by the forces securing the end plate to the element In this specification and the claims attached hereto the term "cylindrical" is used to define a hollow body having a cross-section perpendicular to a longitudinal axis thereof which is either circular or elliptical.

According to a particular embodiment of the invention the seal may be moulded onto and bonded to the end plate.

The invention will now be described with reference to the accompanied-drawings, in which:

Figure 1 is a schematic view of a complete sonar transducer assembly; Figure 2 is a similar view of the hollow element of the transducer; Figure 3 is a sectional view of part of the end plate, element and seal according to a first embodiment of the invention; and Figure 4 is a sectional view of part of the end plate, element and seal according to a second embodiment of the invention.

Referring now to Figure 1, the main component of the transducer is the hollow shell element 10. This may commonly be of elliptical cross-section, though it may be of circular cross-section, if preferred. The shell element may be made from metal, or glass or carbon-reinforced plastics. The open ends of the element are closed by two end plates 11, one fastened across each end of the element 10. Between each end plate and the element is a flexible seal 12. The transducer assembly is held together by a number of tie bolts passing between the two end plates. One such tie bolt is shown at 13 by way of example.

In practice a number of such bolts, say eight, are provided, distributed around the circumference of the end plates.

Figure 2 shows the form of the hollow shell element 10.

This is, in the particular embodiment being described, of elliptical cross-section and made from a metal such as aluminium.

The interior of the element contains the transducer's piezo electric drive mechanism, though the present invention is not concerned with the details of this. The electrical connections to the drive mechanism are not shown.

Figure 3 shows one form of seal 12 in position between an end plate 11 and the element 10. The form of seal shown comprises a first part 14 which is formed to be a close fit around the outside of an end part of the element 10. A second part of the seal, formed integral with the first, is an inwardly directed flarige 15. The flange is formed so that it extends inwards for a distance at least equal to the thickness of the element 10. In the embodiment shown in Figure 3 the end plate 11 is formed with a centre portion 16 of greater thickness than that of the outer portion. To accommodate this the flange 15 of the seal is not formed at the etd of the cylindrical portion 14 but is spaced slightly from that end.

The transducer assembly is assembled by positioning the seals 12 between the end plates 11 and the element 10 and tightening the tie-bolts 13 to provide the necessary end load to the element 10 as described above. This action forms the seal between the end of the element and each end plate. As fluid pressure is applied to the outside of the assembly the portion 14 of the seal is forced into a close sealing contact with the external surface of the element. As the fluid pressure increases, so does the sealing effect.

Whilst the above arrangement of a separate seal 12 positioned correctly with respect to element 10 and end plate 11 is acceptable in many circumstances, several steps may be taken to improve the efficiency of the seal. One commonly-used procedure is to form the seal by a compression-bonding process on to the end plate. This bonds the material of the seal to that of the end plate and forms a virtually perfect fluid-tight seal. This also simplifies the assembly process. In addition it is possible to coat the inside of portion 14 of the seal 12 with a suitable sealing agent before the element 10 is inserted into the seal. This sealing agent is then cured to improve that portion of the seal. It is also possible to add fillets of sealant around the join of the element and the seal, as shown at 17 and/or around the join of the seal and the end plate 11 as shown at 18-.

Figure 4 shows a second embodiment in which the end plate 11 is of uniform thickness, the inwardly-directed portion of the seal 12 being then formed at one end of the other portion 14. If the seal is compression bonded to the end plate as described above then the two-thickness end plate of Figure 3 presents no particular advantages, since this serves mainly to locate the seal correctly during assembly.

A number of materials may be used for the seal 12. One commonly used material is a low-permeability, high carbon-black content neoprene rubber. Since, in use, the transverse dimensions of the shell element are continually changing then the primary requirement is for a inaterial which retains its flexibility under widely-varying temperature andpressure conditions.

In the embodiments described above the tie-bolts 13 have been shown located outside the shell element 10. 1 f the dimensions of the transducer are sufficient the tie-bolts may pass inside the element.

Claims (7)

Claims:

1. A sonar transducer assembly arranged to be immersed in a fluid, which assembly includes a hollow cylindrical shell element, a pair of end plates arranged to be secured one to each end of said element to close off the volume within the element, and sealing means located between each end plate and the element, the sealing means comprising a moulding of flexible material impervious to said fluid and shaped to cooperate with the end plate and the end of the shell element to provide a first cylindrical portion fitting closely around an end part of said element and having a second portion integral with the first portion and extending inwardly towards a longitudinal axis of the element to form a flange the inwardlydirected dimension of which is at least equal to the thickness of the element, the seal being maintained in a sealing relationship with the surface of the element by the pressure of the surrounding fluid and with the end of the element by the forces securing the end plate to the element.

2. An assembly as claimed in Claim 1 in which the seal is secured to the end plate by a compression bonding technique.

3. An assembly as claimed in either of Claims 1 or 2 in which the seal is secured to the element by a sealing agent.

4. An assembly as claimed in any one of Claims 1 to 3 in which each end plate has a centre portion of greater thickness than the outer portion.

5. An assembly as claimed in any one of the preceding claims in which a fillet of a sealing agent is applied to the joint between the end of the seal and the element.

6. An assembly as claimed in any one of Claims 1 to 5 in which the forces securing the end plate to the element are provided by a plurality of tie-bolts joining the two end plates.

7. A sonar transducer assembly substantially as herein described with reference to the accompanying drawings.