GB1577516A - Oscillatory mechanisms - Google Patents

Description

(54) OSCILLATORY MECHANISMS (11) I, NORMAN ADRIAN DE BRUYNE, a citizen of the United States of America of 370G Brunswick Pike, Princeton, New Jersey, OS 540, United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the follow- ing statement:- The present invention relates to mechanisms of the type wherein a member, or a system of members, is arranged to perform oscillatory movements in operation of the mechanism, and it is an aim of the invention to provide mechanisms of this type wherein the transmission of power to that member or system, whereby its oscillation is caused or maintained, is by means of fluid pressure.

According to the invention there is provided a mechanism comprising at least one expansible chamber having an inlet connectible to a source of pressurised fluid and an outlet through which pressurised fluid supplied to the or each chamber can escape, and at least one oscillatory member associated with the or each expansible chamber, at least a part of which is arranged to move in an oscillatory path, and wherein the motion of the or each oscillatory member causes said outlet to be opened and closed at the respective ends of its cyclical path of travel, the consequent expansion of the respective chamber upon closure of the outlet therefrom delivering an impulse to the or each oscillatory member to open the outlet to thereby collapse the chamber until the oscillatory momentum of the member effects reclosure of the outlet to thereby produce a further said impulse thereto, whereby the oscillation thereof is caused or maintained.

In practice it has been found to be of advantage to include one or more flowregulating restrictions in one or more lines by means of which pressurised fluid is supplied to the or each expansible chamber, the restriction being as close upstream to the chamber as possible. The use of such restrictions can lead to an advantageous reduction in the overall fluid consumption and an increase in the rate of fall of pressure within the chambers after expansion, without decreasing the rate of rise of pressure within the chambers when their outlets are closed to such an extent as to undesirably limit the frequency of oscilla tion of the member or system which opens and closes the outlets.

It is generally preferred that the oscillatory member or system is one which is naturally oscillatory independently of the expansible chamber so that once it has been set oscillating the expansible chamber need only deliver relatively low power impulses thereto sufficient to maintain oscillation at the natural fre quency of the member/system. Nevertheless, it is equally within the scope of the invention that the oscillatory motion of the member or system be caused solely by the action of expansible chambers.

Again, it will normally be the case that the or each expansible chamber is mounted in a fixed position so disposed in relation to a portion of the oscillatory member or system that such portion closes the respective cham ber outlet in the course of the motion of the memlber/system and receive the consequentimpulse from the chamber. Nevertheless, it is equally within the scope of the invention for one or more chambers to themselves form part of an oscillatory system so that the respective chamber outlets are closed in the course of the motion of the system by contact with an adjacent stationary element or elements against which element the chamber reacts to deliver an impulse to the system.

All such variations as are discussed above will be exemplified subsequently.

The fluid pressure by which mechanisms according to rhe invention are operated may be either pneumatic or hydraulic but having regard to the fact that operation requires the intermittent escape of fluid from the expansible chamber pneumatic pressure will gener ally be preferred, the working fluid generally being air.

The invention will be described by way of examples with reference to the accompanying drawings, in which: Figures 1 to 5 respectively illustrate, purely schematically, some simple examples of mech anisms which employ the principles of the present invention; Figures 6 and 7 are respectively side and end elevation views of a practical laboratory shaker according to the invention; Figure 8 illustrates in section a detail of the shaker of Figures 6 and 7; Figure 9 illustrates, partly in section, a detail of a modified form of the shaker of Figures 6 and 7; Figure 10 shows in elevation another laboratory shaker; and Figure 11 shows in elevation a drive unit for a magnetic stirrer.

In each of Figures 1 to 5 reference numeral 1 indicates a respective chamber having an inlet connected (via a flow-regulating restriction), to a source of pressurised fluid, (not shown), and an outlet at 1A which, when open, pressurised fluid supplied to the chamber can escape. The chamber may comprise a flexible diaphragm or bellows so as to be expansible under the action of the pressurised fluid when the outlet 1A is closed, the chamber being so restrained that such expansion takes place in the sense of arrow 2.

Referring to Figure 1, the chamber 1 is in this case disposed adjacent to a spring leaf 3, rigidly anchored at one end 3A. The leaf can oscillate naturally, as indicated by arrow 4, flexing to the left and right, (in the sense of the Figure), of the median position in which it is illustrated, in the course of each excursion to the right the leaf coming into contact with and closing the outlet 1A of the chamber 1. Such closure of the outlet causes the chamber to expand in the sense of arrow 2, thus delivering a leftwardly directed impulse to the leaf whereby its oscillation is maintained. In Figure 2 the chamber 1 is disposed adjacent to a plate 5 secured to one end of a torsionally resilient bar 6 rigidly anchored at its other end 6A, the system 5/6 oscillating naturally in the sense of arrow 7.As before, the chamber 1 is so disposed that in the course of its oscillation plate 5 comes into contact with and closes the outlet 1A, the consequent expansion ob the chamber delivering an impulse to the plate in the sense of arrow 2 whereby the oscillation of the system is maintained.

In Figure 3 the chamber 1 is disposed adjacent to a member 8 resiliently borne so as to reciprocate naturally, as indicated by arrow 9, to the left and right, (in the sense of the Figure), of the median position in which it is illustrated. Again, in the course of each excursion to the right the member 8 comes into contact with and closes the outlet 1A of the chamber 1, the consequent expansion of the chamber delivering a leftwardly directed impulse to the member whereby its reciprocation is maintained.

In any of the embodiments described above with relation to Figures 1 to 3 a second ex pansible chamber 10 similar to chamber 1 may be added, the second chamber being arranged to deliver an impulse to the appropriate oscillating system in the direction of arrow 11 and out of phase with the first by one half of a cycle.

Figure 4 illustrates a mechanism in which a pair of expansible chambers 1 are employed to oscillate a member in a manner which is not a natural motion of the member 12 in the absence of the chambers, in contrast to the other mechanisms so far described with rela tion to Figures 1 to 3. In this case, the mem ber is exemplified as being constrained to run on a straight track 13 along which it shuttles in the sense of arrow 14 between the two chambers 1, each time a chamber is en countered the member 12 being arrested and closing the respective outlet 1A, the conse quent expansion of the chamber delivering an impulse to the member to return the member to the other chamber.

In Figure 5 a pair of chambers 1 are mounted at the ends of respective legs of a fork 15 pivotted at 116 so as to perform an oscillatory rocking motion in the sense of arrow 17. In the course of this motion the outlets lA of the chambers alternately come into contact with, and are closed by, a stationary element 18, the consequent expansion of the chambers reacting against element 18 90 as to deliver respective impulses to the fork in the opposite senses of direction to the arrows 2, whereby the motion of the fork is maintained. In this case, it will be noted, the chambers 1 themselves form a part of the oscillating system, in contrast to the mech anism so far described with relation to Figures 1 to 4.It will be appreciated however, that in any of the mechanisms of Figures I to 4 the chambers 1 and 10 oould, instead of being located in a fixed position so as to be cyclically contacted by the members 3, 5, 8 or 12, be mounted on those members so as to cyclically contact fixed elements in the nature of the mechanism of Figure 5.

It will be appreciated that it is the intention of Figures 1 to 5 merely to indicate some simple examples of the types of motion with which mechanisms according to the invention may operate, and they are not in themselves intended to represent complete practical embodiments of such mechanisms. In practice, mechanisms according to the invention may comprise oscillatory systems having a member which moves in the fashion of one of the elements 3, 5, 8, 12 or 15 (or with a more complicated motion), and through which the oscillation of the system is caused or maintained so that, for example, member 8 in Figure 3 might correspond to a portion of a linear vibratory conveyor and member 5 in Figure 2 might correspond to a portion of a spiral vibratory oonveyor.It is also proposed for mechanisms according to the invention to be employed for driving other mechanisms, wherein the oscillatory motion caused or maintained by the expansible chambers is merely an intermediate stage from which a further motion is derived. Thus, for example, the oscillatory motion of embodiments such as those shown in Figures 2 and 5 may be converted into a unidirectional stepwise rotary motion by a suitable "free-wheel" device or other escapement to drive e.g. a so-called magnetic stirrer.

Turning now to Figures 6 and 7, these illustrate a practical embodimem of a mechanism according to the invention in the form of a laboratory shaker. This comprises a right baseplate 22 to which are anchored the bottom ends of four upstanding spring steel strips 23.

The strips are arranged in two parallel pairs, the members of each pair being connected at their upper ends by respective rigid rods 24.

Rigidly depending from the mid-point of each rod 24 is a respective rigid plate 25, and borne between the rods 24 is a tray 26 adapted to support laboratory flasks, test tubes or the like for agitation. The elements 23, 24, 25 and 26 together with the contents of the tray, define the oscillatory system of the mechanism and it will be appreciated that the natural oscillatory motion of this system is with the strips 23 flexing leftwardly and rightwardly in the sense of Figure 6, the rods 24, plates 25 and tray 26 substantially reciprocating (at the contemplated amplitude of flexure of the strips 23).

Also secured to the baseplate 22 by means of brackets 17 are four expansible chamber assemblies 28, one disposed to each side of each plate 25 as shown in Figure 6. Details of the construction of each assembly 28 are discolsed in Figure 8. Referring to that Figure, reference numeral 29 denotes a diaphragm element in the form of an elastomeric moulding of e.g. neoprene or silicone rubber, which element comprises a circular base 30 from which extends a coaxial annular skirt 31 terminating an inwardly directed coaxial annular lip 32 parallel to the base. The base is formed with an annular kink 33 to facilitate the expansion of the chamber described below, and also has on the side opposite to skirt 21 a centrally disposed boss 34. Extending through the base and boss is a bore 35.

Sealedly received within the annular recess defined between the base, skirt and lip of element 29 ds a rigid disc 36 which is integral with or attached to a rigid tube 37. The disc 36 is itself recessed as shown in the Figure and there is accordingly defined between the disc and the base of element 29 a chamber 38' which communicates with the bore 35 of elements 29 and the bore 39' of tube 37. The tube is externally screwthreaded at 40 and a nut 41 is screwed onto the tube to secure the assembly to bracket 27, the lip 32 of element 29 being clamped between disc 36 and a rigid backing plate 42 having a flange 42A which prevents any tendency of the element 29 to be distorted radially outwardly as the nut 41 is tightened.

In operation, the free ends of the tubes 37 of each assembly 28 are connected by flexible tubing 38 (Figure 6) to a common source of pressurised air, such as a small air pump or compressed air cylinder, (not shown), regulated to deliver at a gauge pressure of only units of p.s.i., the respective lengths of tubing connected to each tube 37 incorporating flowregulating restrictions 39 immediately upstream of the tubes 37. So long as the plates 25 remain out of contact with the diaphragm elements 29 it will be appreciated that the air supplied to each chamber 38' will simply vent to atmosphere via the outlets defined by the respective bores 35, although the loss of air will not be great at the low working pressure contemplated.

From a consideration of Figure 6 however, it will also be appreciated that once the system 23/24/25/26 has been set oscillating each plate 25 will alternately come into contact with the bosses of the diaphragm elements 29 disposed to either side of the plates, the plates effectively closing the respective chamber outlets 35 by such contact. Closure of an outlet 35 causes the respective chamber 38 to pressurise so that the portion of the base of the respective diaphragm element 29 from kink 33 inwards is thrust away from the disc 36 thereby imparting a small impulse to the respective plate 25. In the embodiment of Figure 6 to 8 such an impulse is applied to each plate 25 at the end of each leftward and rightward excursion (in the sense of Figure 6), these impulses being sufficient to maintain the systems 23/24/25/26 in a constant state of oscillation at its natural frequency.

It is believed that a laboratory shaker of the form described above has considerable advantages over conventional laboratory shakers, which latter usually comprise a table to which is attached an electric motor driving an eccentric weight, or a tray which is reciprocated by an electric motor via a crank or similar mechanism. The pneumatically driven shaker described above does not require the provision of an electric motor, (and if such a motor is used to drive an air pump for the shaker this can be substantially remote from the shaker), and can accordingly be safely used in the presence of flammable vapours or combustible dust where the use of conventional shakers would involve the risk of explosion.When operating in an atmosphere which requires complete protection from the presence of oxygen the shaker can be operated from a pressurised source of inert gas, such as nitrogen, instead of air. The shaker is quiet in operation, consumes relatively little power, and once in operation can very readily be stopped and then restarted simply by arresting the motion of the tray by hand and then displacing the tray by hand, without having to interrupt the air supply to the expansible chambers.Changes in the frequency of oscillation can be effected by changing the inertia of the oscillatory system (e.g. by adding or removing weights from the tray), or by changing the flexural characteristics of the strips 23, (e.g. by effectively shortening or lengthening them by applying clamps at some point between their ends), and changes in the amplitude of oscillation can be effected e.g. by changing the distance by which the opposed pairs of expansible chamber assemblies 28 are spaced apart, togather perhaps with a change in the applied air pressure. In order to achieve agitation of the tray's contents at a temperature other than ambient, a temperature-controlled water bath may be added to the apparatus as indicated at 43 in Figure 7.It will be observed that whilst tray 26 is disposed within the bath the remaining elements 23/24/25 of the oscillating system together with the chamber assemblies 28, are disposed without the bath.

In a modified form of the shaker just described, the plates 25 are dispensed with and the expansible chambers are arranged to act directly upon the strips 23. In fact I have found it sufficient for one only of the strips 23 to be acted upon in this way, and such an arrangement may take the form illustrated in Figure 9.

Referring to Figure 9, the single strip 23 has four expansible chamber assemblies 28A to 28D associated therewith, the chamber assemblies being located by a support element 43 and arranged in two opposed pairs 28A, 28 > B and 28C, 28D. In operation, each opposed pair of chambers acts upon the strip 23 to maintain the oscillatory system in motion in a manner similar to that in which the chamber pairs act upon the plates 25 in the embodiment of Figure 6 and 7, in the present case chamber 28A acting in parallel with chamber 2 & and chamber 28B acting in parallel with chamber 28D. In such an arrangement it is advantageous to synchronise the action of the parallel chambers by putting them into a mastemand-slave relationship.

Tlitis, whereas the diaphragm elements of assemblies 281A and 28B are provided with outlets 35 as before, the diaphragm elements of assemblies 28C and 28D have solid bosses 34 with no such outsets. Assembly 28C is connected to the pressure source (not shown) in parallel with assembly 28A through flexible tubing 38A and 38iC and a common restriction 39A whilst assembly 28D is connected to the pressure source in parallel with assembly 28B through flexible tubing 38B and 38D and a common restriction 39B, the pressure within both chambers 28A and 28C being determined by the opening and closing of the outlet of 2d81A and the pressure within both chambers 28B and 28D being determined by the opening and closing of the outlet of 28B.

In the embodiment of Figure 9 the amplitude of oscillation of the illustrated strip 23, and therby of the entire oscillatory system, can be changed by changing the elevation of the pair of chamber assemblies 28A and 28B.

To this end those assemblies are mounted upon a carriage 44 vertically slidable upon support element 43, the tubes 37 of the chamber assemblies passing through respective vertical slots 431A. The carriage can be locked to the support element by any suitable means, such as a set screw, (not shown), in any position within the range of travel permitted by slots 43A.

Referring to Figure 10, the illustrated shaker comprises an outer casing 101 within which it suspended a frame 102 of rectangular planform. The frame 102 is suspended by four spring steel strips 103 in a similar arrangement to that of the strips 23 in Figures 6 and 7, the strips 103 being rigidly anchored at thetir upper ends and conferring upon frame 102 the ability to perform an oscillatory motion relative to casing 101, at the contemplated amplitude of flexure of the strips 0103 the motion of frame 102 being substantially reciprocatory. Supported by frame 4102, upon a similar series of four spring steel strips 104, is a second rectangular frame 105.

As illustrated, the planes of the strips 104 are perpendicular to the planes of the strips 103 so that frame 105 can perform an oscillatory motion relative to frame 102, (similar to the motion of the tray 26 relative to the baseplate 22 in Figures 6 and 7), which is perpendicular to the motion of the frame 102 relative to casing 101). Thus, when the entire system 102, 103, 104, 105 is set oscillating frame 105 is adapted to perform a motion relative to casing 101 which is the vector sum of the two above descri;bed perpendicular motions, and this motion is regulated by a crank mechanism 106 linking casing 101 and frame 105 so that the frame performs a circular orbital motion in a hirizontal plane.The frame 105 supports a table upon which laboratory flasks or the like can be secured for agitation.

Secured to the casing 101 adjacent to the side 102A of the lower frame 102 are a parallel pair of expansible chamber assemblies 107 similar to the assemblies 28 illustrated in Figure 8, two further such assemblies (not shown), being provided adjacent to the side of the frame 102 opposite to side 102A. In operation, the expansible chamber assemblies are connected via flow-regulating restrictions to a source of pressurised air and, once the system 102, 103, 104, 105 has been set oscillating, they maintain the entire system in oscillation at its natural frequency by the delivery of impulses to the substantially reciprocatory frame 102 in an entirely analogous manner to that in which the corresponding assemblies act in the laboratory shakers of Figures 6 to 9.However, it is possible to force the system to oscillate at higher frequencies than the natural frequency by increasing the air pressure supplied to the expansible chamhers. The action of the chambers in each parallel pair is preferably synchronised by putting them into a master-and-slave relationship as in the embodiment of Figure 9.

The shaker described herein has the same advantages of explosion -proofness, low power consumption, quietness and ease of operation as are enjoyed by the shakers of Figures 6 to 9. It has the additional advantage, however, that it agitates with an orbital rather than a reciprocatory motion and is thereby adapted to swirl liquids, this being particularly desirable in microbiological and tissue culture work.

If required, additional, inclined strips 103A may be provided for bracing the strips 103 against shear displacements during the oscillation of the shaker.

Turning now to Figure 11, the illustrated magnetic stirrer drive unit comprises a hori zontaily aligned permanent magnet 1111 fast on a shaft 1112 which is borne for rotation about a vertical axis by a ball bearing 113. In use, the magnet 111 is employed to rotate a second magnet (not shown) within a vessel of liquid to be stirred, as is conventional.The shaft 1.12 is arranged to be rotated by a crank 114. The crank 114 is in turn arranged to be driven by the oscillation of a horizontal rod 115, the crank pin ll4A being borne by a low-fiiction plastics moulding 116 (e.g. of polypropylene), fast on one end of the rod *1115. Rigid with the rod 115 is a plate 117 having a plane face 1 17A perpendicular to the axis of the rod.

The drive unit further comprises an expansible chamber assembly 118. This assembly is of a similar form to the assemblies 28 illustrated in Figure 8, but its diaphragm element has an extended, bored boss 119 which is lined by a brass tube 120 communicating with the interior of the expansible chamber. The end of rod 115 remote from its connection to the moulding 116 lies loosely within tube 120, that is to say it does not occupy the full internal cross-section of the tube but leaves a space through which the interior of the expansible chamber can communicate with atmosphere.

In operation, the expansible chamber assembly is connected via a flow-regulating resriction to a source or pressurised air, as before.

The disposition of parts is such that as magnet 1,11 rotates rod 115 performs a generally reciprocatory motion, (subject to the eccentricity of crank 114), with the face 117A of plate 117 cyclically opening and closing the passage provided from the interior of the expansible chamber to atmosphere. Accordingly, in the manner described hereinbefore, once the rotation of magnet 111 1 has been initiated the expansible chamber cyclically delivers impulses to the plate 117 to maintain the magnet 111 in rotation. Since in this case the overall mechanism is not naturally oscillatory, the frequency of oscillation, (or in other words the rotational speed of magnet 111), can be varied quite widely by varying the strength of the impulses delivered by the expansible chamber, viz by varying the applied air pressure.

In order to ensure that the drive unit is self starting, i.e. to ensure that the expansible chamber is sealed and will thereby deliver an initial impulse when the air supply is first connected, a stationary magnet 121 may be employed to influence magnet 1 111 to move the mechanism into the appropriate initial position, this influence of course being overcome when the air supply is connected.

As in the case of the shaker described with reference to Figure 10, the described drive unit is explosion-proof, consumes little power, is easy to operate and quiet in operation. It also has the advantage over conventional elec tric-motor drive units of generating no perceptible heat below the rotating magnet, heat generation being a considerable problem with electric-motor drive units, particularly in microbiological and tissue culture work. I have found a drive unit of the type described to be effective in typical "light duty" stirring at a rotational speed of, say, 100 R.P.M. (revolutions per minute) with an overall air consumption of about 3.5 litres/minute at a supply gauge pressure of 3 to 4 p.s.i. (pounds per square inch).This corresponds to a power consumption of about 2.2 watts and compare very favourably with a typical conventional drive unit employing a 25 watt shaded pole motor, the greatest proportion of the power consumed by which is represented by the generation of useless and undesirable heat.

Although the expansible chamber/crank mechanism of Figure H has been described in terms of its use in driving a magnetic stirrer it will be appreciated that such a mechanism will be of use in many other situations calling for a low-power rotary drive.

Three such expansible chamber assemblies 1;18 having connecting rods 115 acting on a common crank pin and spaced apart around the pin at 1200 intervals can be used for the generation of a three-phase 'fluctuating air supply suitable for driving the three-phase "A.C.A." stirrers described in U.K. Patent 1,485,741. In such an arrangement pressurised air would be supplied simultaneously to each chamber via individual restrictions, the fluctuating pressure within each chamber caused by the cyclic opening and closing of its outlet being tapped off to an individual stirring motor diaphragm assembly in the three-phase stirrer.

WHAT I CLAIM IS: - 1. A mechanism comprising at least one

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. Figures 6 to 9. However, it is possible to force the system to oscillate at higher frequencies than the natural frequency by increasing the air pressure supplied to the expansible chamhers. The action of the chambers in each parallel pair is preferably synchronised by putting them into a master-and-slave relationship as in the embodiment of Figure 9. The shaker described herein has the same advantages of explosion -proofness, low power consumption, quietness and ease of operation as are enjoyed by the shakers of Figures 6 to 9. It has the additional advantage, however, that it agitates with an orbital rather than a reciprocatory motion and is thereby adapted to swirl liquids, this being particularly desirable in microbiological and tissue culture work. If required, additional, inclined strips 103A may be provided for bracing the strips 103 against shear displacements during the oscillation of the shaker. Turning now to Figure 11, the illustrated magnetic stirrer drive unit comprises a hori zontaily aligned permanent magnet 1111 fast on a shaft 1112 which is borne for rotation about a vertical axis by a ball bearing 113. In use, the magnet 111 is employed to rotate a second magnet (not shown) within a vessel of liquid to be stirred, as is conventional.The shaft 1.12 is arranged to be rotated by a crank 114. The crank 114 is in turn arranged to be driven by the oscillation of a horizontal rod 115, the crank pin ll4A being borne by a low-fiiction plastics moulding 116 (e.g. of polypropylene), fast on one end of the rod *1115. Rigid with the rod 115 is a plate 117 having a plane face 1 17A perpendicular to the axis of the rod. The drive unit further comprises an expansible chamber assembly 118. This assembly is of a similar form to the assemblies 28 illustrated in Figure 8, but its diaphragm element has an extended, bored boss 119 which is lined by a brass tube 120 communicating with the interior of the expansible chamber. The end of rod 115 remote from its connection to the moulding 116 lies loosely within tube 120, that is to say it does not occupy the full internal cross-section of the tube but leaves a space through which the interior of the expansible chamber can communicate with atmosphere. In operation, the expansible chamber assembly is connected via a flow-regulating resriction to a source or pressurised air, as before. The disposition of parts is such that as magnet 1,11 rotates rod 115 performs a generally reciprocatory motion, (subject to the eccentricity of crank 114), with the face 117A of plate 117 cyclically opening and closing the passage provided from the interior of the expansible chamber to atmosphere. Accordingly, in the manner described hereinbefore, once the rotation of magnet 111 1 has been initiated the expansible chamber cyclically delivers impulses to the plate 117 to maintain the magnet 111 in rotation. Since in this case the overall mechanism is not naturally oscillatory, the frequency of oscillation, (or in other words the rotational speed of magnet 111), can be varied quite widely by varying the strength of the impulses delivered by the expansible chamber, viz by varying the applied air pressure. In order to ensure that the drive unit is self starting, i.e. to ensure that the expansible chamber is sealed and will thereby deliver an initial impulse when the air supply is first connected, a stationary magnet 121 may be employed to influence magnet 1 111 to move the mechanism into the appropriate initial position, this influence of course being overcome when the air supply is connected. As in the case of the shaker described with reference to Figure 10, the described drive unit is explosion-proof, consumes little power, is easy to operate and quiet in operation. It also has the advantage over conventional elec tric-motor drive units of generating no perceptible heat below the rotating magnet, heat generation being a considerable problem with electric-motor drive units, particularly in microbiological and tissue culture work. I have found a drive unit of the type described to be effective in typical "light duty" stirring at a rotational speed of, say, 100 R.P.M. (revolutions per minute) with an overall air consumption of about 3.5 litres/minute at a supply gauge pressure of 3 to 4 p.s.i. (pounds per square inch).This corresponds to a power consumption of about 2.2 watts and compare very favourably with a typical conventional drive unit employing a 25 watt shaded pole motor, the greatest proportion of the power consumed by which is represented by the generation of useless and undesirable heat. Although the expansible chamber/crank mechanism of Figure H has been described in terms of its use in driving a magnetic stirrer it will be appreciated that such a mechanism will be of use in many other situations calling for a low-power rotary drive. Three such expansible chamber assemblies 1;18 having connecting rods 115 acting on a common crank pin and spaced apart around the pin at 1200 intervals can be used for the generation of a three-phase 'fluctuating air supply suitable for driving the three-phase "A.C.A." stirrers described in U.K. Patent 1,485,741. In such an arrangement pressurised air would be supplied simultaneously to each chamber via individual restrictions, the fluctuating pressure within each chamber caused by the cyclic opening and closing of its outlet being tapped off to an individual stirring motor diaphragm assembly in the three-phase stirrer. WHAT I CLAIM IS: -

1. A mechanism comprising at least one

expansible chamber having an inlet connectible to a source of pressurised ,fluid and an outlet through which pressurised fluid supplied to the or each chamber can escape, and at least one oscillatory member associated with the or each expansible chamber, at least a part of which is arranged to move in an oscillatory path, and wherein the motion of the or each oscillatory member causes said outlet to be opened and closed at the respective ends of its cyclical path of travel, the consequent expansion of the respective chamber upon closure of the outlet therefrom delivering an impulse to the or each oscillatory member to open the outlet to thereby collapse the chamber until the oscillatory momentum of the member effects reclosure of the outlet to thereby produce a further said impulse thereto, whereby the oscillation thereof is caused or maintained.

2. A mechanism as claimed in claim 1 wherein one or more flow-regulating restrictions are included in one or more lines by means of which pressurised fluid is supplied to the or each expansible chamber, the restriction being close upstream to the chamber.

3. A mechanism as claimed in claim 1 or 2 wherein the oscillatory member or members is or are naturally oscillatory independently of the expansible chamber or chambers so that once oscillation is effected the expansible chamber or chambers need only deliver relatively low power impulses thereto sufficient to ,maintain oscillation at said natural frequency.

4. A mechanism as claimed in any preceding claim wherein the or each expansible chamber is mounted in a fixed position so disposed in relation to a portion of the or each oscillatory member that such portion closes the respective chamber outlet in the course of the motion thereof and receives the consequent impulse from the chamber.

5. A mechanism as claimed in claim 1, 2 or 3 wherein the or each expansible chamber forms part of an oscillatory-system so that the or each respective chamber outlet is closed in the course of the motion of the system by contact with an adjacent stationary element or elements against which element or elements the chamber or chambers react to deliver an impulse to the system.

6. A mechanism as claimed in any preceding claim wherein the or each chamber comprises a diaphragm sealed around its periphery to a rigid support, said outlet being an opening in the diaphragm, said inlet being an opening in the rigid support.

7. A laboratory shaker comprising a holder or support for one or more objects to be shaken and a mechanism as claimed in any preceding claim connected to said holder or support and operative to effect shaking movement of the holder or support.

8. A shaker as claimed in claim 7 wherein a plurality of spring strips are each anchored at one end thereof to a base, the strips being interconnected so as to oscillate in unison and the strips supporting said holder or support.

9. A shaker as claimed in claim 8 wherein the strips are upstanding from the base.

10. A shaker as claimed in claim 8 or 9 wherein the strips are interconnected by rods so as to oscillate in unison.

11. A shaker as claimed in claim 10 wherein one or more plates are connected to said rods so as to be moved thereby to form said member or members arranged to perform oscillatory movements in operation of the mechanism.

12. A laboratory shaker comprising a mechanism as claimed in any one of claims 1 to 6 and a naturally oscillatory system inoluding a first member which is arranged to perform an orbital motion and a second member which is arranged to perform a substantially reciprocatory motion, the first member being adapted to bear the articles to be agitated and the second member being effective to cyclically open and close one or more of said expansible chamber and thereby receive impulses whereby the oscillation of the system is maintained.

13. A laboratory shaker comprising mechanism as claimed in any one of claims 1 to 6, which mechanism includes a member arranged for generally reciprocatory motion, said member being effective to cyclically open and close one or more of said expansible chambers and thereby receive impulses whereby its oscillation is caused or maintained, and a crank mechanism arranged to be driven by said member for producing a rotary motion.

14. A laboratory shaker substantially as described with reference to and as illustrated in Figures 6 to 8 of the accompanying drawings.

15. A laboratory shaker substantially as described with reference to and as illustrated in Figures 6 to 8 modified as in Figure 9 of the accompanying drawings.

16. A laboratory shaker substantially as described with reference to and as illustrated in Figure 10 of the accompanying drawings.

17. A laboratory shaker substantially as described with reference to and as illustrated in Figure 11 of the accompanying drawings.