US3648983A

US3648983A - Apparatus for fluidizing materials

Info

Publication number: US3648983A
Application number: US11680A
Authority: US
Inventors: William B Campbell
Original assignee: I A ASSOCIATES Inc
Current assignee: I A ASSOCIATES Inc; Ia Associates Inc
Priority date: 1970-02-16
Filing date: 1970-02-16
Publication date: 1972-03-14
Anticipated expiration: 1989-03-14

Abstract

Apparatus for vibrating viscous media, slips or fluids to render them more fluid. The apparatus has a vibrator and one or more vibration amplitude control modules each comprising a pair of resilient pressurized air bags and a yoke connected at one end to the two air bags for attachment to a body to be vibrated.

Description

United States Patent Mar. 14, 1972 Campbell [54] APPARATUS FOR FLUIDIZING 3,166,22 1/1965 Schrader ..259/DIG. 42 MATERIALS 3,198,502 8/1965 Thompson ..259/91 X 3,498,384 3/1970 Ogura ..259/1 R X [72] Inventor: William B. Campbell, Columbus, Ohio [73] Assignee: l. A. Associates, Inc., Columbus, Ohio FOREIGN PATENTS R APPLICATIONS Filed: Feb. 1970 650,629 2/1951 Great Britain ..259/D1G. 42 [211 App]. No.: 11,680 Primary Examiner-Walter A. Scheel Assistant Examiner-Alan l. Cantor Attorney-Schiller & Pandiscio [52] US. Cl. ..259/1 R [51] Int. Cl. .3011 11/00 [57 ABSTRACT [58] Field of Search ..259/D1G. 41, DIG. 42, DIG. 43,

259/1316 44 l R 91 Apparatus for vibrating viscous media, sl|ps or fluids to render them more fluid. The apparatus has a vibrator and one or [56] References Cited more vibration amplitude control modules each comprising a pair of resilient pressurized air bags and a yoke connected at UNlTED STATES PATENTS one end to the two air bags for attachment to a body to be vibrated. 3,112,823 12/1963 Musschoot ..259/1 R X 3,153,834 1964 Boyer et a1 ..259/D1G. 42 Claims, 7 Drawing Figures 56 ,J Li V W r H- f t 1m ml 1 rTn rm .1. w\,\\\ as 2 g i I [I I W 52 I f I IL I I I 11 11 IJII 1: ![11] I LII 7 A 42 50 4 M I s 4 '-30 PATENTEDMAR 14 1912 SHEET 3 [IF 3 W/LL/AM B. CAMPBELL ATTORNEYS.

APPARATUS FOR FLUIDIZING MATERIALS This invention relates to flow of viscous media, slips and fluids and more particularly to novel apparatus and method for controllably varying the fluidity versus temperature behavior of viscous fluids.

The primary object of this invention is to provide a novel apparatus and method for controllably increasing the flow rates of viscous materials without direct application of heat.

A further object is to provide new and improved apparatus for facilitating movement and use of highly viscous materials such as ceramic mixtures, tar, food products, etc.

Another object is to provide a novel apparatus which capitalizes on the phenomenon of thixotropy to control the flow rates of viscous and plastic materials.

In its classic sense, a thixotropic material may be represented as a pasty gel which has the remarkable property of liquifying when subjected to internal mechanical stress (e.g., by shaking or vibrating or agitating) and of setting again to the gel form when the mechanical stress is terminated. By way of example, the phenomenon of thixotropy has been observed in colloidal'systems involving alumina, vanadium pentoxide, zirconium dioxide, stannic oxide, ferric oxide, certain gelatin sols, and even suspensions of fine clays such as bentonites. The viscosity of a thixotropic material will decrease as the amount of shear of the material increases. Thus, if a mixer blade is rotated in a thixotropic material, the faster the blade rotates the thinner the material becomes. If the mixer blade is stopped, the material will return to its original viscous or gel state. It is typical in looking atrate of shear versus shearing stress data for thixotropic materials to find that at low shear rates the structural breakdown of the materials has a greater influence on the shear stress than the shear rate. In other words, at low rates of shear, the shearing stress is affected more by the structure of the thixotropic material than by the rate of shear. Thus, in plotting rate of shear versus shearing stress for a typical thixotropic material, there is usually found an optimum point where the rate of shear produces the lowest apparent shearing stress or the lowest shearing stress gives a maximum rate of shear. This is the point of maximum fluidity.

As acknowledged above, it is known that a thixotropic material can be made fluid by subjecting it to vibration. The same is also true of other highly viscous or semisolid materials that are not generally considered to be thixotropic and that are not dialatent.

The present invention stems'from the realization that in terms of vibration input, the rate of shear is related to the frequency of vibration and the shearing stress can be related directly to the amplitude of vibration. I have discovered that for different materials there is an optimum amplitude and frequency of vibration at which maximum fluidity is realized at a given temperature level.

Accordingly, the specific object of the present invention is to provide novel apparatus for applying vibrations of controllable amplitude and frequency to viscous and semisolid materials, including thixotropic materials, to achieve improved flow characteristics, thereby facilitating movement and use of such materials. One use of the present invention is in the art of forming ceramic bodies. Typically, casting slurries must contain 30-40 percent water in order to be sufficiently fluid to conform readily to a mold. With the present invention, it is possible to make pasty casting mixes containing as little as 4-8 percent water to flow easily. Another application is in getting viscous material into and out of containers, e.g., in unloading tar or other highly viscous chemicals from railroad tank cars at temperatures substantially below that necessary for flow using normal pumping means, or in filling containers with highly viscous food products that are sensitive to heat.

The foregoing objects are achieved by novel apparatus which is made up of one or more vibration amplitude control modules, each comprising a pair of air bags contained between fixed supports, an operating member mounted between and supported by the two air bags and adapted to be attached to a body to be vibrated, and means for pressurizing both air bags to selected pressure levels. The apparatus also includes vibrator means for subjecting the module or the body to which the operating member of the module is attached to vibrations of variable frequency. The air pressure in the two bags provides control over the amplitude of vibration of the body to which the module is attached. By adjusting the frequency and amplitude of vibration, it is possible to modify the viscosity of a viscous material contained in or supported by the body to which the module is attached.

Other objects and features and many of the attendant advantages of the invention are set forth or rendered obvious from the following detailed description which is to be considered together with the accompanying drawing wherein:

FIG. 1 is a sectional view of a vibration amplitude control module constructed according to the present invention;

FIG. 2 is a plan view of the module shown in FIG. 1;

FIG. 3 is a side elevation of one form of apparatus using the module of FIGS. 1 and 2;

FIG. 4 is a plan view of the apparatus of FIG. 3;

FIG. 5 is a plan view showing the air supply system for the apparatus of FIG. 3;

FIG. 6 is a vertical section taken along line 6-6 of FIG. 5; and

FIG. 7 is a side elevation showing the'module-of FIGS. 1 and 2 with a vibrator unit attached mounted on a drum filled with viscous fluid.

Referring now to FIGS. 1 and 2, there is shown a vibration amplitude control module 2 which comprises'a rectangular housing 4 made up of four

side walls

6, 8, l0 and 12 and

opposite end plates

14 and 16. The bottom plate 14 fully closes off the lower end of the housing and preferably is secured in place by welding. The upper end plate 16 is secured by means of bolts and nuts to a pair of corner brackets 18 and 20 that are welded to the side walls of the housing. E'nd plate l6 is shaped so as to leave two opposite comer openings 22 and 24 in the upper end of the housing. Mounted within the housing are two axially aligned

air bags

26 and 28 which are conventional commercially available units and are illustrative of the type known in the art as single convolution airmounts which are commonly used for mounting machinery, instrument packages, etc., on air for vibration and shock installation. By way of example, airmounts as shown in FIG. 1 as well as other types that may be used in place of those shown in FIG. 1 are illustrated and described in the Airide Airmount Design Manual of Firestone Industrial Products Co. of Noble'sville, Ind. Essentially, the

air bags

26 and 28 are each made up of a circular ring 30 that is made of a suitable natural or synthetic elastomer such as neoprene and is molded so as to have a convoluted cross section with edge beads 32. The edge beads are secured to end attachments 34 that function to hermetically seal off the interior of the ring to form an air pressure chamber. The end attachments 34 may be secured to the beads by appropriate clamping means or by crimping as shown at 38. Preferably, the ring 30 is reinforced with nylon or rayon tire cord embedded therein.

The lower air bag is secured to end plate 14 by means of bolts 40 that screw into tapped holes formed in its bottom end attachment 34. The upper air bag is secured in the same manner to end plate 16. Disposed between the two air bags is an arm 42 that is made up of two

metal bars

44 and 46. Bar 44 is secured to the upper end attachment 34of air bag-26 by means of bolts 48 that are screwed into tapped holes in the end attachment. Bar 46 is secured in the same way to the lower end attachment of air bag 28. The two

bars

46 and 48 are secured together by bolts 50 as shown so'as to form the arm 42. The arm 42 is mounted so as to extend diagonally across the housing and affixed to its opposite ends are two

additional arms

52 and 54 that project out of the housing via the comer openings 22 and 24 respectively. Bolted to and connecting the outer ends of arms 52 and S4 is a cross-arm in the form of a flat bar 56. It' is to be noted that the

assembly comprising arrns

42, 52, 54 and 56 is hereinafter referred to as yoke 58 since its function is to connect the module to a surface or body which is to be vibrated. In addition to the foregoing elements, the module includes an air fitting 60 (see FIG. 6) mounted in one of the end attachments 34 of the two air bags whereby the bags may be connected to a source of pressurized air (not shown) for pressurizing to a selected pressure.

Various forms of vibrating apparatus for modifying the viscosity of materials may be constructed using one or more of the vibration amplitude control modules just described. Thus, it is possible to produce a vibrating table as shown in FIGS. 3-6 which is useful, for example, in liquidying thixotropic materials.

Referring now to FIGS. 4-6, there is shown a vibrating table comprising four of the modules 2 of FIGS. 1 and 2 mounted within a rectangular enclosure 64. The latter is welded or bolted onto the housings of the four modules which are disposed in the comers of the housing. Struts 66, in the form of box tubing or channel stock, are secured between adjacent modules so as to form a unitary frame structure which is supported at its four corners by legs 68 that rest on a floor or ground. Preferably, the frame structure is resiliently mounted to the four legs by airmounts 70 which may but need not be similar to

air bags

26 and 28. As seen in FIG. I, it is to be understood that the airmounts 70 have bottom and top end attachments like

air bags

26 and 28, with the bottom end attachments bolted to plates 72 on the upper end of legs 68, and the top end attachments bolted to the bottom end plate 14 of a module 2. Preferably, the airmounts 70 have a fixed air pressure, but it also is feasible for airmounts 70 to be fitted with air valves similar to conventional tire valves whereby they may be pressurized to whatever pressure level is desired. In this connection, it is to be noted that the function of airmounts 70 may be to merely reduce the transmissibility of vibrations to the legs 68 and the floor or platform on which the table is resting or they may be adjusted to contribute to the resonant tuning of the entire apparatus.

As seen in FIGS. 3 and 4, the illustrated apparatus also includes a table top 76 that is bolted to the cross-arms 56 of the yokes 58 of the four modules 2. The yokes 58 extend above the enclosure 64 far enough to assure that the table top will not engage the enclosure when it is being vibrated.

Located below the table top and preferably disposed symmetrically between the four modules 2 is a variable frequency vibrator unit 78 that is supported on a platform in the form of a rigid metal plate 80. Plate 80 is attached to the housings of the four modules in a suitable way, e.g., by welding or by means of brackets bolted to the modules. As used herein, the term variable frequency vibrator unit means a vibrator whose operating frequency is controllably variable. The vibrator may take any convenient form. Thus, it may be a direct or reaction type mechanical motor driven vibrator an air driven vibrator, a hydraulic vibrator, or an electromagnetic vibrator of the type similar in principle to a dynamic speaker. Since such vibrators are well known in the art, and since variable types of various frequency vibrators are usable in the present invention, it is not believed to be necessary to describe or illustrate vibrator unit 78 in detail. However, by way of example, the vibrator unit may be of the type available commercially from Stress Relief Engineering Co. of Costa Mesa, Calif. under the designation Formula 62, Model C, Slave Unit. Examples of other vibrators that may be used are set forth in chapter 25 of Shock and Vibration Handbook, Vol. II, Harris and Crede, McGraw-Hill Book Co., Inc., New York, 1961.

It is to be understood that the vibrator units operating controls (depicted as including an operating knob 81 for adjusting operating frequency) may be mounted in a wall of enclosure 64 in the space between two of the modules 2, preferably in a discrete control box 82 disposed as shown in FIGS. 3 and 4 or they may be mounted in a remote and free standing control box connected to the vibrator by power and air lines.

The system for pressurizing the

air bags

26 and 28 of the four modules is illustrated in FIGS. 5 and 6. The system comprises two

closed conduit loops

84 and 86. Loop 84 includes four tees 88, with each tee 88 connected by a conduit line 90 to air fitting 60 mounted in the end attachment of the lower air bag 26 of one of the modules 2. Loop 86 has four tees 92 each connected by a nipple 94 to the air fitting 60 of the upper air bag 28 of one of the modules 2. The

loops

84 and 86 have

additional tees

96 and 98 that are connected by

lines

100 and 102 to

conventional pressure regulators

104 and 106 which in turn are connected by lines 106 and 108 to a suitable source of pressurized air (not shown).

Regulators

104 and 106 are provided with control handles 1 l0 and 1 12 that are used to adjust the air pressures supplied by the two regulators to the upper and lower air bags of the four modules 2.

The vibrating table just described may be used for various tasks, as for example, to fluidize a ceramic molding composition. Assume that a plaster mold having a mold cavity of complex shape is mounted on the table top and that it is desired to fill said cavity with a ceramic molding composition comprising by weight 98 parts alumina, two parts clay and an electrolyte, and 2-8 parts water. Such a composition is quite dry and hence will not flow at room temperature. The composition is transferred to the mold using a spatula. Then with the

air bags

26 and 28 pressurized to a predetermined level, the vibrator unit is started and its operating frequency adjusted to a predetermined level. At the optimum combination of air bag pressure and vibrator frequency determined by prior tests, the ceramic mix will be fluidized and will flow so as to fully fill all parts of the mold cavity and void entrapped air.

With respect to vibration amplitude control, it is to be noted that the air bags function as variable springs. If both bags in a module are pressurized to 20 p.s.i., for example, a certain vibration amplitude will result. Changing the pressure in each bag to some other level, e.g., 60 p.s.i., will change the vibration amplitude and in addition, the force factor will also be different. Because the frequency also can be varied, it is possible to tune the table according to the material to be fluidized. For this reason, the apparatus shown in FIGS. 3-6 has many other uses. Thus, for example, it has been used to fluidize tar at a temperature 25 C. below its normal flow point temperature. It also has been used to fluidize chunks of shale.

It is to be noted that the apparatus of FIGS. 3-6 also may be modified for certain applications. Thus, for example, the table top 76 need not be horizontal and flat, but may be tilted and shaped like a trough so as to function as a vibrating chute, offering the advantage of fluidizing as well as directing the movement of materials that are normally very viscous or semisolid.

The table of FIGS. 3-6 also may be used as a controllable source of vibration for welding purposes. Thus, by clamping parts to be welded to the table and vibrating the table at a very low amplitude and a relatively high frequency, while welding is occuring, heated metal can be made to flow to the extent required to distribute stresses and avoid warpage.

A further advantage of the invention herein described is that the air pressure in each pair of

air bags

26 and 28 need not be identical but instead one bag may have a higher or lower pressure than the other. By unbalancing the pressures in the two bags of a module, it is possible to selectively control the acceleration of the table top when moving in one direction or the other under the influence of the vibrator unit, with the result that the vibration waveform of the table top will have different positiveand negative-going excursions. Similarly, the modules in a table need not all be at the same pressures; it is sometimes advantageous to have a symmetrical unbalance to get higher amplitudes. Thus, in the table of FIGS. 3-6, two of the modules at one side of the table may be set at one air pressure level and the other two set at another air pressure level, with the result that one side of the table top will vibrate at a different amplitude than the opposite side.

It is to be noted that the direction of vibration of the yokes 58 of the modules 2 or the table top is not critical, and for most purposes it is satisfactory to have the vibrator unit vibrate them vertically. However, for certain applications it may be desirable to have the table top vibrate horizontally. Thus, when it is desired to fill a mold that is relatively wide and long but shallow, it is preferred to vibrate the mold parallel to its length or width to assure that it will be completely filled and voided of air.

A further modification is the use of a fixed frequency vibrator unit (such as a Syntron-type vibrator) in addition to the variable frequency unit 78. This offers the advantage of providing a complex vibration waveform as well as increasing the vibration amplitude. It also is to be noted that more than one variable frequency vibrator unit may be used in the apparatus of FIGS. 3-6, and although not necessary, it is preferred that they be disposed symmetrically with respect to the table top.

FIG. 7 shows application of another embodiment of the invention which comprises a single vibration amplitude control module identical to the one shown in FIGS. 1 and 2. In this embodiment, the single module 2A has a variable frequency vibrator 78A secured to the bottom end plate 14 (not shown) of its housing 14. Although not shown, it is to be understood that vibrator 78A is electrically powered and is connected to a suitable power supply and that the two air bags of module 2A are connected to regulated air supplies. The module 2A is mounted on a drum 116, this being accomplished by attachment of its yoke 58 to a suitable bracket affixed to the drum. The drum is fitted with a spigot 118 whereby its contents may be discharged.

Assuming that the drum is filled with a highly viscous or semisolid material such as tar, liquification is achieved by pressurizing the air bags of the module 2A to predetermined pressure levels and operating the vibrator 78A at a predetermined frequency. The air bags work against each other as the applied vibration force causes the yoke to vibrate relative to the modules housing (or the housing may be considered to vibrate relative to the yoke) and the air pressures in the two bags determine the amplitude and force of the relative vibratory movement of the yoke and housing. Since the yoke is attached to the drum 116, it transmits vibrations to the drum at the operating frequency of vibrator 73A and with an amplitude and force limited by the two air bags. The vibrations applied to the drum produce internal stresses in the drums contents, whereupon the contents are rendered less viscous. The apparatus is operated with the optimum combination of bag air pressure and vibration frequency required to achieve maximum fluidization as determined by prior tests.

It is to be realized, of course, that fluidization of the contents of drum 116 may be achieved by disposing the drum on the table of FIGS. 3-6 and operating the table with the optimum combination of air pressures and vibrating frequencies required to achieve maximum fluidization.

Still other modifications of the invention are possible. Thus, for example, the

air bags

26 and 28 may have toroidal configurations or they may be replaced by two or three convolution airmounts or by other similar air cushion members. Furthermore, the table of FIGS. 3-6 may include as few as one module with the vibrator attached either to the table top or the module (in the latter case, the struts 66 would be extended and joined to each other to form a rectangular frame and the leg airmounts 70 would be attached directly to the struts). The table also could be made circular and provided with more than four of the modules 2. Still other modifications will be obvious to persons skilled in the art.

It is to be noted that as used herein the terms air bags, air cushions and airmounts are synonymous and denote flexible hollow air spring units pressurized or adapted to be pressurized with air or some other suitable compressible pressurizing fluid.

lclaim:

1. In vibrating apparatus having a member for mechanically applying vibrations to a body to be vibrated, a vibration amplitude control module comprising first and second spaced fixed supports, first and second mutually aligned pressurized air bags disposed between and connected to said air supports, an arm extending between and connected to said air bags, and means rigidly connecting said member and said arm for movement as a unit so that the amplitude of said vibrations is controlled by the fluid pressure of said air bags and the natural resonance of said body determined by its massand the vibration frequency.

2. The combination of claim 1 further including means for vibrating said member.

3. The combination of claim 1 further including means for vibrating said member at a variable frequency.

4. The combination of claim 1 further including means for independently varying the pressure in each of said air bags.

5. The combination of claim 1 wherein'said bags are pressurized to the same pressure level. i

6. The combination of claim 1 wherein said module comprises a housing with said supports and said air bags mounted therein, and further wherein said connecting-means-is an elongate arm that is connected to said member exterior of said housing.

7. Vibrating apparatus in the form of a table comprising a plurality of vibration amplitude control modules each comprising a pair of mutually aligned pressurized air bags constrained between fixed supports, alike plurality of arms each extending between and connected to one of. said pairs of air bags, a table top, table support means connected between said table top and said arms so that the amplitude or vibrations of said table top is controlled by said pairs of air bags, and vibrator means for vibrating said table top.

8. Apparatus according to claim 7 wherein each pair of bags comprises an upper bag and a lower bag, and further including first means for pressurizing the upper bags and second means for pressurizing the lower bags.

9. Apparatus according to claim 7 wherein said table'has four supporting legs and comprises four of said'modules and further wherein each of said modules is vertically aligned with one of said legs.

10. Apparatus according to claim 7 wherein said table comprises a frame made up of said modules and struts extending between said modules, and further wherein said vibrator means is attached to said frame.

11. Apparatus according to claim 10 comprising four symmetrically positioned modules, and further wherein said vibrator means is mounted substantially centrally ofsaid frame among said modules.

12. Apparatus according to claim 7 wherein each module comprises a housing, and further including'means connecting said housings so as to form a rigid frame, and further including legs supporting said frame.

13. Apparatus according to claim 7 further including resilient means between said frame and each of said legs.

14. Apparatus according to claim 7. wherein said table top is polygonal and each of said modules is located below a corner of said table top.

15. A vibrating table having a table top for mechanically applying vibrations to a body to be vibrated supportedby said table top, a vibration amplitude control module comprising first and second spaced fixed supports, first and second mutually aligned pressurized air bags disposedbetween and connected to said supports, an arm extending between and connected to said air bags, said arm also being connected'to said table top so that the amplitude of said vibrations is controlled by the fluid pressure of said air bags.

16. A vibrating table according to claim 15 further including a vibrator means for vibrating said table top at a variable frequency.

17. A vibrating table according to'claim 16 further including means for varying the pressure in said air bags.

18. A vibrating table according to claim 16 wherein said vibrator means is connected to said fixed supports.

19. A vibrating apparatus having a member for mechanically applying vibrations to a body to be vibrated, and means for vibrating said member, a vibration control module comprising first andsecond spaced plates fixed with-respect to each other, firstand second mutually aligned fluid-pressurized bags disposed between and connected to said supports, and a aligned fluid-pressurized bags disposed between and connected to said supports, a rigid U-shaped yoke having a first section extending between and secured to said bags and a second section affixed to said body, and means for vibrating said body, whereby the amplitude of vibration of said body in controlled at least in part by the fluid pressure of said bags.

Claims

1. In vibrating apparatus having a member for mechanically applying vibrations to a body to be vibrated, a vibration amplitude control module comprising first and second spaced fixed supports, first and second mutually aligned pressurized air bags disposed between and connected to said air supports, an arm extending between and connected to said air bags, and means rigidly connecting said member and said arm for movement as a unit so that the amplitude of said vibrations is controlled by the fluid pressure of said air bags and the natural resonance of said body determined by its mass and the vibration frequency.

5. The combination of claim 1 wherein said bags are pressurized to the same pressure level.

6. The combination of claim 1 wherein said module comprises a housing with said supports and said air bags mounted therein, and further wherein said connecting means is an elongate arm that is connected to said member exterior of said housing.

7. Vibrating apparatus in the form of a table comprising a plurality of vibration amplitude control modules each comprising a pair of mutually aligned pressurized air bags constrained between fixed supports, a like plurality of arms each extending between and connected to one of said pairs of air bags, a table top, table support means connected between said table top and said arms so that the amplitude or vibrations of said table top is controlled by said pairs of air bags, and vibrator means for vibrating said tAble top.

9. Apparatus according to claim 7 wherein said table has four supporting legs and comprises four of said modules and further wherein each of said modules is vertically aligned with one of said legs.

11. Apparatus according to claim 10 comprising four symmetrically positioned modules, and further wherein said vibrator means is mounted substantially centrally of said frame among said modules.

12. Apparatus according to claim 7 wherein each module comprises a housing, and further including means connecting said housings so as to form a rigid frame, and further including legs supporting said frame.

14. Apparatus according to claim 7 wherein said table top is polygonal and each of said modules is located below a corner of said table top.

15. A vibrating table having a table top for mechanically applying vibrations to a body to be vibrated supported by said table top, a vibration amplitude control module comprising first and second spaced fixed supports, first and second mutually aligned pressurized air bags disposed between and connected to said supports, an arm extending between and connected to said air bags, said arm also being connected to said table top so that the amplitude of said vibrations is controlled by the fluid pressure of said air bags.

17. A vibrating table according to claim 16 further including means for varying the pressure in said air bags.

19. A vibrating apparatus having a member for mechanically applying vibrations to a body to be vibrated, and means for vibrating said member, a vibration control module comprising first and second spaced plates fixed with respect to each other, first and second mutually aligned fluid-pressurized bags disposed between and connected to said supports, and a rigid yoke having a first section extending between and affixed to said air bags and a second section affixed to said member so that the amplitude of said vibrations is controlled by the spring constants of said bags.

20. In combination with a body to be vibrated, apparatus comprising a module having first and second spaced supports fixed with respect to each other, first and second mutually aligned fluid-pressurized bags disposed between and connected to said supports, a rigid U-shaped yoke having a first section extending between and secured to said bags and a second section affixed to said body, and means for vibrating said body, whereby the amplitude of vibration of said body is controlled at least in part by the fluid pressure of said bags.