US3657581A

US3657581A - Power transducers

Info

Publication number: US3657581A
Application number: US26950A
Authority: US
Inventors: Leo Hoogenboom
Original assignee: Mechanical Technology Inc
Current assignee: Mechanical Technology Inc
Priority date: 1970-04-09
Filing date: 1970-04-09
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18

Abstract

A transducer arrangement is provided which achieves hard and uniform coupling between the transducer element and the transducer body member by mounting the transducer element in shrink-fit relationship with the body member. Conveniently, to assure uniform pressure on the transducer element, the element is shrink-fitted into an opening provided in the body member.

Description

Hoogenboom 51 Apr. 18, 1972 54-] POWER TRANSDUCERS 3,187,207 6/1965 [72] Inventor: Leo I-loogenboom, Ballston Lake, N.Y.

[73] Assignee: Mechanical Technology Incorporated, 3,390,559 8 Latham, N.Y. 3,339,090 8/1967 Jaffe et al ..310/8.7 X

[22] Filed: Apr. 9, 1970 FOREIGN PATENTS OR APPLICATIONS [21] Appl. No.: 26,950 118,239 2/1947 Sweden ..3I0/8.7

52 U.S. Cl ..fiwsizgsgsj ga (1)656 gj -ffig 'jg [5 I 1 Int. Cl. ..I I02k 7/00 Anomey joseph v. Claeys and Charles w- Heller [58] Field oISearch ..3l0/8.7,9.6,8.8,8.2,8.3, 57 ABSTRACT 517; 308/9, 5, 122' 29/447 5 A transducer arrangement is provided which achieves hard and uniform coupling between the transducer element and the 6 I R f d transducer body member by mounting the transducer element [5 l e erences l e in shrink-fit relationship with the body member. Conveniently, UNITED STATES PATENTS to assure uniform prdessure on the transducer element, the element is shrink-fitte into an opening provided in the body 3,104,334 9/1963 Bradley et a1 ..310/8.4 member 3,151,258 9/1964 Sonderegger.... .....3l0/8.7 3,471,205 10/1969 Farron et al. ..308/9 12 Claims, 7 Drawing Figures ,4 :2 5 4/ .50 j/ 4 42 43 4 r a I I 7 43L, l J44,

POWER TRANSDUCERS This invention relates generally to power transducers and more particularly to new and improved transducer devices and a method of making such devices. The invention has a wide range of applications, only some of which, wherein the invention is especially adapted and useful, are described in detail herein. Moreover, in the particular embodiments of the invention described in detail herein the transducer elements are of the electrostrictive (piezoelectric) type and are used as mechanical drivers although they may also be used as force sensors. It is to be understood that the transducer elements may be constructed of magnetostrictive materials as well. The invention described herein was made in the performance of work under NASA Contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

In recent years ultrasonic energy systems have found wide use in many commercial and industrial applications such as in cleaning, soldering and in bearings. Briefly, in such systems a transducer element of the magnetostrictive or electrostrictive (piezoelectric) type is driven from a-suitable source of oscillatory energy to produce the desired vibrations in the transducer element. As is well known, magnetostrictive transducer elements have the property of changing physical dimensions when subjected to an applied magnetic field while electrostrictive transducer elements have the property of changing physical dimensions when subjected to an applied electric field. Conversely when such elements are subjected to an applied force they have the property of modifying the applied field.

Thus, for example, when transducer elements are used as mechanical drivers they convert electrical energy to mechanical energy. The electrical energy may be supplied, for example, in the form of an oscillating electrical current. The resulting mechanical output is in the form of repetitive expansions and contractions of the transducer. The frequency of the oscillatory response (forced vibration) of the transducer corresponds to the frequency of the driving electrical output.

When the transducer elements are used as force sensors they convert mechanical force impulses to corresponding electrical impulses which can be measured by conventional means and a measure of the mechanical force imposed on the transducer is thus obtained. Calibration of such a device to obtain the mechanical force to electrical signal correspondence may be accomplished in any suitable manner known in the art.

The power generated by power transducers is transmitted to the working area by their supporting structure, the design of which will usually be determined by the transducer application. Efficient transmission of the power from the transducer element to the supporting body member, or other supporting structure, requires that hard and uniform coupling be provided between their interfaces. This is especially true when ultrasonic energy is involved. In addition, many very desirable power transducer elements are made of crystalline ceramic material, which materials have high compressive strength but low tensile strength. Accordingly, at higher power levels the transducer element must be preloaded and adequately supported to prevent failure due to internal tensile stresses. Many attempts have been made in the prior art to achieve the required preloading and hard and uniform coupling but none have been entirely satisfactory. For example, attempts have been made to preload the elements by clamping arrangements employing external bolts or other fastening means. Such an arrangement has the disadvantage that considerable power dissipation takes place in the bolted joints with severe local heating. The efficiency of power transfer is thereby much reduced and the structural failure rate is high.

It is a primary object of the present invention to provide a new and improved transducer arrangement which overcomes one or more of the foregoing prior art problems and in addition offers a number of distinct advantages in operation, ease of manufacture, and reliability.

It is another object of the invention to provide a transducer arrangement exhibiting superior transducer element support and giving high transmission efficiency between transducer element and supporting structure.

Another object of the invention is to provide a transducer arrangement exhibiting high reliability and long operating life.

Still another object of this invention is to provide a transducer arrangement wherein the energy density of transmission is high thereby making possible the use of smaller ceramic crystals and operation at lower voltages.

Yet another object of the invention is a transducer arrangement which readily lends itself to the use of mass production techniques with consequent cost savings.

A still further object of the invention is to provide a new and improved transducer arrangement having high dimensional stability permitting the device to be used as a mechanical reference, for example, in bearing metrology.

Briefly stated, in accordance with one aspect of the invention, there is provided a novel arrangement which achieves both preloading and supporting of the transducer element or elements in the supporting structure, or body, as well as hard and uniform coupling thereto. The new and improved transducer device comprises the combination of a body member and a transducer element in shrink-fit relationship therewith. The transducer element may be constructed by a magnetostrictive material or an electrostrictive material. Thus, in accordance with an embodiment of the invention a transducer element assembly may be provided which includes one or more transducer elements and a transducer element support means associated therewith. The transducer element assembly is mounted and supported in shrink-fit relationship with the supporting body member. Conveniently, this may beaccomplished by machining corresponding internal and external dimensions of the supporting body member, transducer elements, and mating transducer elements support means to tolerances of mechanical interference fit; mating surfaces being suitably shaped to ensure a hard coupling. If desired the transducer element may be shrink-fitted to the body member without utilizing any separate support means. The use of such a support means however allows for a very convenient means of applying an electric field across the element.

The novel features believed characteristic of this invention are set forth with particularity in the appended claims. Theinvention itself, together with its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is the schematic plan view of a transducer device constructed in accordance with one embodiment of the inventron;

FIG. 2 is the schematic cross-section of the arrangement shown in FIG. 1 taken along the line 2-2 and showing, in addition, attachment of suitable electrical connections;

FIG. 3 is a section view showing another embodiment of the invention;

FIG. 4 is a section view showing yet another embodiment of the invention;

FIG. 4a is a perspective view of the transducer element support member disposed between the rectangular transducer elements in FIGS. 3 and 4;

FIG. 5 is a perspective drawing partly in section of the frame of a conical spool bearing incorporating the present invention and FIG. 6 is the schematic cross-section of the bearing frame in FIG. 5 taken along the line 66.

Referring now more particularly to the drawings, there is shown in FIGS. 1 and 2 a transducer arrangement in accordance with one embodiment of the invention. As shown, the transducer device comprises a transducer element 11, which may be of a suitable magnetostrictive or electrostrictive material. As illustrated, element 11 is of hollow cylindrical configuration. To achieve maximum amplitude vibrations when subjected to an applied field in the radial direction, the material of transducer element 11 is polarized in the radial direction. A lengthwise slit 22 is provided in transducer element 11 to prevent the introduction of hoop stresses in the element during assembly or operation. Hoop stresses do not contribute to the generation of power by the transducer element but rather tend to cause damage to it. Transducer element 11 is shrink-fitted into a supporting body 12 of any desired shape. Element 11 has a transducer element support means, shown as a concentric pin 13 shrink-fitted into it, as shown. For example, transducer element 11 and pin 13 form a transducer element assembly which is disposed in shrink-fit relationship with the supporting body 12. The supporting body 12 and the internal pin 13 may be conveniently used as terminals as well as supports for the transducer device 10.

As illustrated in FIG. 2, pin 13 may be provided with suitable flexible members which make it possible to attach electrical leads at locations where they are not exposed to the adverse effects of high frequency vibrations. These members can also be used as points of attachment of external supports or suspensions for the transducer device. To this end, annular diaphragms l4 and 15 are provided which terminate in ring sections 16 and 17, respectively. Diaphragms l4 and 15 are formed integral with the pin 13 and the supporting body 12, respectively. Such members are operative to assure that vibrations of the pin 13 or body 12 are not transmitted to the face portions 18 and 19 of the ring sections 16 and 17, respectively. Power leads 20 and 21 may be attached to the face portions 18 and -19, as shown in FIG. 2. Oscillatory current may thus be conveniently supplied from a suitable power source (not shown) to the transducer element 11 through leads 20 and 21,

annular diaphragms

14 and 15, the internal pin 13, and the supporting body 12. When oscillatory current is so supplied, the transducer element 11 is set into a radial or thickness mode of oscillation. The hard and uniform coupling provided by the novel mounting arrangement of this invention assures that the mechanical energy so produced is transmitted to the desired working area of the transducer device.

Any suitable electrostrictive or magnetostrictive material may be used for the transducer elements 11. Some examples of materials known to exhibit highly magnetostrictive characteristics are permanickel, nickel and permendur. Especially desirable electrostrictive materials are the piezoelectric ceramic materials such as lead titanite and lead zirconite. One especially suitable electrostrictive material of this type is a ceramic material manufactured and sold under the designation PZT4 by the Clevite Corporation. Transducer elements 11 of such material can be readily obtained in finish machined form. For some hollow cylindrical transducer elements it is desirable, to assure uniform internal stresses during operation and allow for operation at low voltages, that the thickness of the transducer elements be made smaller than the radius thereof. For example, in one particular transducer arrangement the transducer thickness was made less than about oneeighth inch. Operation at low power input has the added advantage that operating temperatures are lower and any thermally caused frequency drift is much reduced.

The material of the supporting body 12 is not especially critical, although appropriate physical properties of the transducer element and supporting body should be properly matched. For example, the materials for element 11, pin 13 and body 12 should be selected so that their moduli of elasticity are approximately the same. If a transducer element of PZT4 ceramic material is used, a suitable material for supporting body 12 and pin 13 would be aluminum or titanium.

The geometric configuration of the supporting body 12 will usually be determined by the type and shape of transducer device desired, and the purpose for which the device is intended. The openings into which the transducer elements 11 are to be fitted can be machined accurately by conventional means, for example, by boring, broaching or any other suitable technique. The material of the internal pin 13 may be the same as that of the supporting body 12. The desired outside finish of pin 13 can be obtained in any suitable manner such as, for example, by grinding. The machining tolerances of all mating surface dimensions are such as to provide for a mechanical interference fit. Control of the degree of shrink-fit is important as this determines the power density which can be transmitted from the transducer element to the supporting body. Moreover, uneven or excessive loading of the element 11 may damage or depolarize it.

A preferred method of assembly in accordance with another aspect of this invention can best be explained by reference to FIGS. 1 and 2. Selecting transducer element 11 of Clevite PZT4 electrostrictive ceramic material and the supporting body 12 and internal pin 13 of aluminum, the desired compressive preload is achieved with an interference fit of 0.0006 to 0.003 inches per linear inch of corresponding component dimension. This is conveniently provided by heating the body 12 to a temperature in the range of about 250 to 280 C. The ceramic transducer element 11 is slit axially to remove and prevent hoop stresses from being developed and the pin 13 is inserted in the central opening thereof. The hollow cylindrical transducer element 11 with the pin 13 therein is cooled to about 20 C. and disposed in the opening in the heated body 12. When body 12 is returned to room temperature the transducer element assembly is supported and mounted in body member 12 and the transducer element 11 is in shrink-fit relationship with such body member and subjected to a preselected compressive loading. In a transducer device constructed as just described, the preloading of the transducer element 11 may be of the order of 2,000 to 10,000 psi. The critical maximum temperature to which the transducer element may be exposed is the Curie point of the material at which temperature the element depolarizes. The Curie point of Clevite PZT4 ceramic material, for example, is about 325 C. and the highest compressive load to which it should be sub jected is about 10,000 psi.

One criterion by which suitable design and machining tolerances of components and the correct assembly procedure can be assessed is the internal power loss under normal operating conditions which can be tolerated. This can be expressed conveniently as Q, the ratio of (energy stored in the transducer element at zero velocity/energy dissipated per cycle). The larger Q, the better the design and the higher the conversion efficiency of the device. Thus, for example, a prior art device using separate flanged flexures, large ceramic elements and clamping bolts was considered excellent with Q equal to about 300. On the other hand, the Q ofa device in accordance with FIG. 5 ofthis invention is about 2,500.

Other embodiments of the invention are illustrated in FIGS. 3 and 4. The basic concept involved in the arrangements illustrated in FIGS. 3 and 4 is the same as that already described. That is, the arrangement comprises a transducer element shrink-fitted to the body member. In the particular embodiments illustrated in FIGS. 3 and 4 the arrangement comprises a transducer element assembly including a transducer element and a transducer element support means. In these embodiments, however, the transducer element assembly comprises a pair of rectangular transducer element members with a sheet material member disposed therebetween. This assembly is then suitably shrink-fitted in a suitable cylindrical opening provided in the supporting body member. Since the transducer element assembly is of a rectangular configuration, a suitable insert means is provided to achieve a cylindrical surface which is convenient and effective in obtaining the required shrink-fit relationship. The insert also assures a pressure uniformity which otherwise may be difficult to obtain.

In the embodiments shown in FIGS. 3 and 4, therefore,

transducer elements

31 and 32 are shaped in the form of short rectangular parallelpipeds. The transducer element support means is in the form of a metallic sheet member 33 located between

elements

31 and 32. Thus, sheet member 33 provides an internal support, an external suspension point if needed, and serves also as one of the electrodes. An insert means is provided to achieve the desired shrink-fit relationship. As shown in FIG. 3, the arrangement includes an insert means 34 of U-shaped cross section. In the arrangement of FIG. 4, on the other hand, the insert means includes two cylindrical segments 35 and 36. The assemblies of transducer elements, sheet support members, and insert means are shrink-fitted into the body 37 of the device inthe manner previously described in detail in connection with the embodiment of FIG. 1. Although not shown in FIGS. 3 and 4, electrical connections may also be provided in the manner already described.

FIG. 5 illustrates the housing of a gaseous squeeze-film bearing incorporating the present invention. As is known in the art, in gaseous squeeze-film bearings one of the confronting bearing surfaces is made to undergo transverse oscillatory motion. Because of the viscous action and the non-linear nature of the squeeze motion, the pressure in the gas film is higher than ambient and a net load carrying capacity of the bearing is developed.

As shown in FIGS. 5 and 6, transducer elements 41 are of hollow cylindrical configuration. If such transducer elements are made of electrostrictive ceramic type material, the crystal structure of the elements is so oriented that their normal mode of oscillation is in the radial direction. Asshown, elements 41 are symmetrically located within appropriate openings in a supporting body 42. The supporting body 42, which may be of aluminum for example, terminates at each end in suitably

shaped end pieces

43 and 44. The

end pieces

43 and 44 of this bearing are shown as being of conical shape, adapted to cooperate with confronting bearing surfaces of

members

43 and 44 although they may be of any other suitable shape consistent with the bearing geometry desired and the particular bearing application. For example,

end pieces

43 and 44 would have a spherical shape if a spherical bearing geometry were used. The end pieces are connected with the central portion of body 42 through the weakened

sections

45 and 46, shown provided by the grooves 47 and 48.

The supporting body 42 is provided with six radial holes in its center plane, as shown in FIG. 6, into which the tubular ceramic transducer elements 41 are disposed. The tubular ceramic transducer elements 41 are slit axially to eliminate any hoop stresses during assembly and operation. Each transducer element 41 has a central opening 49 into which a suitable solid metal post 50, which may also be of aluminum, is shrink-fitted.

With the foregoing described arrangement the solid center posts 50 serve both as a convenient mounting means for the bearing assembly as well as one of the electrodes therefor. The outer end of the posts 50 may terminate in the integral annular diaphragm 51. An electrical connection can be conveniently made to the ring portion 52. The external suspension for the bearing may also be made to annular diaphragm 51.

In operation, the radial mode of oscillation of the transducer elements 41 imparts an axial mode of vibration to the central portion of the supporting body 42, which vibration is then transmitted to the

end pieces

43 and 44 through the weakened

sections

45 and 46. The weakened

sections

45 and 46 function as acoustic horns whereby the axial excursions of the body 42 are magnified at the

end pieces

43 and 44.

Another important feature of the present invention is that the dimensional stability of the transducer elements 41 does not affect the bearing gaps. Also, by making the thickness of the transducer elements 41 much smaller than the radius thereof, there is no appreciable effect on their. shrink-fit due to dimensional changes during operation at larger power densities.

The number of transducer elements employed and the location of such elements in the supporting body is determined by the method of loading and the performance characteristics of the transducer device sought. In the squeeze-film bearing embodiment, uniform motion is required. Consequently the transducer element should be symmetrically disposed about the periphery of the supporting body. Also, by such a disposition both feedback and vibration mode selectivity are achieved. With only one transducer element, mode control would be much more difficult.

In FIG. 6, the supporting body 42 is illustrated as having six holes, only three of which are shown to be provided with transducer element assemblies comprising tubular transducer elements 41 and center posts 50. In a particular bearing-constructed in accordance with this embodiment of the invention, satisfactory operation was obtained with transducer elements mounted in all six holes. Similarly, satisfactory but different operation was obtained with only three transducer elements arranged in the symmetric array illustrated in FIG. 6, that is, disposed apart. In both cases, resonant modes were found to exist at 11 Khz and 18 Khz. With the'embodiment employing only three transducer elements 41, however, one additional resonant mode was found to be present at about 27 Khz. It was found also that this 27 Khz. mode was very powerful and had a maximum axial amplitude at the cone tip of about 600 microinches for a thickness excursion of the transducer element 41 of less than 3 microinches.

While there has been described what are considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by United States Letters Patent is:

1. In combination:

a. a body member having an opening therein;

b. a transducer element assembly disposed within said opening in said body member and including a hollow cylindrical transducer element constructed of a material selected from the group consisting of electrostrictive and magnetostrictive materials and a cylindrical transducer element support member disposed in the interior transducer element and said transducer element support member being selected to exhibit similar moduli of elasticity and each being dimensioned to effect a broad area interference fit with each other in the range of about 0.0006 to 0.003 inches per linear inch and each of said body member and said transducer element support member including an integral annular diaphragm portion adapted to receive an electrical connection.

2. The combination recited in claim 1 wherein said transducer element is constructed of an electrostrictive material.

3. In combination:

a. a transducer element assembly including a pair of rectangular transducer elements constructed of a material selected from the group consisting of electrostrictive and magnetostrictive materials and a rectangular metallic support member thinner and no larger than said transducer elements disposed between broad surfaces of each of said transducer elements;

b. a metallic body member having at least one opening therein, said body member, transducer element and transducer element support member being selected to exhibit similar moduli of elasticity and each of said body member and said transducer element support members including an integral annular diaphragm portion adapted to receive electrical connections; and

. means for mounting and supporting said transducer element and said transducer element support member in stress producing relationship within an opening in said body member so that said transducer elements are subjected to a desired compressive loading.

4. The combination recited in claim 3 wherein said transducer element, said transducer element support means and the opening in said body member are dimensioned to effect a broad area interference fit with each other in the range of about 0.0006 to 0.003 inches per linear inch.

5. The combination recited in claim 4 wherein said transducer element is constructed of an electrostrictive material.

6. The combination recited in claim 4 wherein said transducer element is subjected to a compressive loading in the order of 2,000 to 10,000 psi.

7. In combination:

a. a body member defining a bearing housing having a radial opening in the periphery thereof and' a bearing surface adapted for cooperation with a relatively moveable confronting bearing surface;

b. a transducer assembly including a hollow cylindrical transducer element constructed of an electrostrictive material and a cylindrical transducer element support member disposed in the interior center opening of said hollow cylindrical transducer element, said body member, said transducer element and said transducer element support member being selected to exhibit similar moduli of elasticity and said transducer element assembly being disposed within the radial opening of said bearing housing so that said body member, said transducer element and said transducer element support means are in stress producing relationship with each other operative to subject said transducer element to a desired compressive loading;

c. means for supplying an oscillatory electric current to said transducer element to cause vibrations thereof which vibrations are transmitted to the bearing surface of said bearing housing. 8. The combination recited in claim 7 wherein the bearing housing defined by said body member includes a hollow cylindrical central portion terminating at each end in a portion 8 defining a bearing surface,

each of said end portions being connected with the central portion through a yieldable portion which is operative to magnify the vibrations transmitted to the body member from said transducer element.

9. The combination recited in claim 7 wherein said body member defines a bearing housing having a plurality of radial openings in the periphery thereof selected ones of which have transducer element assemblies disposed therein.

10. The combination recited in claim 9 wherein transducer element assemblies are disposed in radial openings spaced apart.

11. The combination recited in claim 7 wherein said body member and said transducer element support means each include integral annular diaphragm means which terminate in ring sections adapted to receive electrical connections for supplying said oscillatory electric current to said transducer element.

12. The combination recited in claim 11 wherein said body member defines a bearing housing having a plurality of radial openings in the periphery thereof and transducer element assemblies are disposed in openings spaced 120 apart.

3,657,581 Dated April 18, 1972 Leo Hoogenboom Patent No.

Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as' shown below: 1

' Column 6, line 31; after "interior" insert center opening. of said hollow cylindricaltransducer elemenmeaid body member, said Signed and sealed this 12th day of December 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Atteeting Officer I I Commissioner of Patents FORM PC4050 USCOMM-DC 60376-P69 U.$. GOVERNMENT PRINTlNG OFFICE I [99 0-368-31,

Claims

1. In combination: a. a body member having an opening therein; b. a transducer element assembly disposed within said opening in said body member and including a hollow cylindrical transducer element constructed of a material selected from the group consisting of electrostrictive and magnetostrictive materials and a cylindrical transducer element support member disposed in the interior transducer element and said transducer element support member being selected to exhibit similar moduli of elasticity and each being dimensioned to effect a broad area interference fit with each other in the range of about 0.0006 to 0.003 inches per linear inch and each of said body member and said transducer element support member including an integral annular diaphragm portion adapted to receive an electrical connection.

3. In combination: a. a transducer element assembly including a pair of rectangular transducer elements constructed of a material selected from the group consisting of electrostrictive and magnetostrictive materials and a rectangular metallic support member thinner and no larger than said transducer elements disposed between broad surfaces of each of said transducer elements; b. a metallic body member having at least one opening therein, said body member, transducer element and transducer element support member being selected to exhibit similar moduli of elasticity and each of said body member and said transducer element support members including an integral annular diaphragm portion adapted to receive electrical connections; and c. means for mounting and supporting said transducer element and said transducer element support member in stress producing relationship within an opening in said body member so that said transducer elements are subjected to a desired compressive loading.

6. The combination recited in claim 4 wherein said transducer element is subjected to a compressive loading in the order of 2, 000 to 10,000 psi.

7. In combination: a. a body member defining a bearing housing having a radial opening in the periphery thereof and a bearing surface adapted for cooperation with a relatively moveable confronting bearing surface; b. a transducer assembly including a hollow cylindrical transducer element constructed of an electrostrictive material and a cylindrical transducer element support member disposed in the interior center opening of said hollow cylindrical transducer element, said body member, said transducer element and said transducer element support member being selected to exhibit similar moduli of elasticity and said transducer element assembly being disposed within the radial opening of said bearing housing so that said body member, said transducer element and said transducer element support means are in stress producing relationship with each other operative to subject said transducer element to a desired compressive loading; c. means for supplying an oscillatory electric current to said transducer element to cause vibrations thereof which vibrations are transmitted to the bearing surface of said bearing housing.

8. The combination recited in claim 7 wherein the bearing housing defined by said body member includes a hollow cylindrical central portion terminating at each end in a portion defining a bearing surface, each of said end portions being connected with the central portion through a Yieldable portion which is operative to magnify the vibrations transmitted to the body member from said transducer element.

10. The combination recited in claim 9 wherein transducer element assemblies are disposed in radial openings spaced 120* apart.

12. The combination recited in claim 11 wherein said body member defines a bearing housing having a plurality of radial openings in the periphery thereof and transducer element assemblies are disposed in openings spaced 120* apart.