GB2410750A

GB2410750A - Automatic balancing device

Info

Publication number: GB2410750A
Application number: GB0402529A
Authority: GB
Inventors: Neil William Phillips
Original assignee: Dyson Technology Ltd; Dyson Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2004-02-05
Filing date: 2004-02-05
Publication date: 2005-08-10
Also published as: GB0402529D0; WO2005075726A1

Abstract

An automatic balancing device 50; 150; comprising a cavity 52, 152, containing a volume of fluid 62, 162, and a plurality of bodies 60, 160, in contact with the fluid 62, 162, wherein the density of each of the bodies 60, 160, is less than the density of the fluid 62, 162. Automatic balancing forces which counterbalance an out-of-balance mass 72 in a rotating body are created as a result of the displacement of the fluid 62, 162, by the buoyant bodies 60, 160, to create an uneven distribution of the fluid about its axis of rotation 22, 122. The balancing device may be used in a washing machine.

Description

24 1 0750 Automatic Balancine Device The invention relates to an automatic

balancing device for counterbalancing an out-of balance mass present in a rotating body Particularly, but not exclusively, the invention relates to an automatic balancing device for use in washing machines and for counterbalancing out-of-balance masses present in a washing machine during washing, rinsing and/or spinning cycles.

Automatic balancing devices for counterbalancing out-of-balance masses are known in many different applications However, the most complex out-ofbalance situations occur when both the position and magnitude of the outof-balance mass is unpredictable and the speed of rotation is variable, as in the case of a washing machine Further complications arise in the case of a washing machine because the magnitude and position of the outof-balance mass can vary with time during a single laundering cycle.

Many different automatic balancing devices have been proposed and used in washing machines and many of these are effective at counterbalancing outof-balance masses at speeds above the critical speed (ie. the speed of resonance of the system). Examples of this type of automatic balancing device are shown in GB1,035,033; GB1,092,188, WO 93/23687; WO 95/32372; US5,813,253; US5,862,553; DE1 912 481. All of the devices shown in these documents make use of the phenomenon by means of which, at speeds of rotation above the critical speed, freely rotating counterbalancing masses automatically take up positions in which the out- of-balance mass is counterbalanced. In each of the aforementioned arrangements, the freely rotating counterbalancing masses are constrained to move in a circular path, either by being mounted about an axle which is concentric with the axis of rotation of the rotating body, or by being located in a circular race.

These known arrangements all have various disadvantages. In arrangements wherein the counterbalancing masses are mounted centrally, the centre of mass of each counterbalancing mass is normally located radially closer to the axis of rotation than the centre of mass of the out-of-balance mass. In order to provide sufficient counterbalancing capability, the mass of the counterbalancing masses must be relatively large. This increases the overall weight of the machine in which the device is utilised and can add to the cost of manufacture as well. In arrangements in which the counterbalancing masses are arranged to run in a race, the race must be manufactured to very high tolerance levels which also increases manufacturing costs. Such arrangements are also commonly very noisy as a result of collisions between the counterbalancing masses in the race. Finally, due to the high contact pressures applied between the components of the known arrangements, high strength materials such as steels must be used, which adds to manufacturing costs and the overall mass of the machine.

Out-of-balance masses present in rotating bodies can also be balanced to some extent using fluid balancing rings in which a volume of fluid is constrained to move in an annular channel which is caused to rotate with the body Such systems have been used to balance the wheels of vehicles and in other applications. However, fluid balancing rings have a limited capacity to balance an out-of-balance load due to the fact that, at high speeds, centrifugal forces tending to disperse the fluid evenly around the channel overcome the forces urging the fluid into the position in which the out-of-balance mass is counterbalanced. Fluid balancing rings are generally capable of counterbalancing approximately 60% of an out-ofbalance load.

It is an object of the invention to provide an automatic balancing device in which some or all of the disadvantages of the prior art are reduced or eliminated. It is a further object of the invention to provide an automatic balancing device which is effective in counterbalancing an outof-balance mass without adding significantly to the overall mass of the product in which it is utilised. It is a further object of the invention to provide an automatic balancing device which is economical to produce in comparison to

prior art devices.

The invention provides an automatic balancing device comprising a cavity containing a volume of fluid and a plurality of bodies in contact with the fluid, wherein the density of each of the bodies is less than the density of the fluid.

An arrangement in which the density of the bodies is less than the density of the fluid in the cavity allows the mass of the fluid itself to be used to counterbalance the out-of- balance mass. This is in complete contrast to the known balancing devices which use the mass of the freely rotating bodies to counterbalance the out-of-balance mass. In the arrangement of the present invention, the lightweight bodies are used to displace a fixed volume of the fluid and so create a distribution of the fluid within the cavity which is unevenly spread about the axis of rotation of the out-of-balance mass The balancing forces created by the presence of an excess of fluid in one area of the cavity provide the counterbalance to the out-of-balance mass. This arrangement is also highly advantageous over known fluid balancing rings because the presence of the lightweight bodies reduces the influence of the centrifugal forces on the distribution of the fluid within the channel. This provides an automatic balancing device having a balancing capability far greater than that which can be achieved by known fluid balancing rings.

Another advantage of the arrangement of the present invention is that the contact pressures developed in the arrangement are lower than those developed in many of the prior art devices mentioned above. This means that the materials from which the components of the automatic balancing device of the present invention is made do not need to be of such high strength as has previously been the case. This can have a significant cost benefit. Also, the manufacturing methods used to form these components may be different and this too may lead to reductions in costs.

A further advantage of a device according to the present invention is that there is little noise generated by collisions between the low- density bodies.

Preferably, the fluid is a liquid such as water. Water is preferred because it is environmentally friendly, readily available and safe in the event of a spillage It is also convenient to be able to introduce the fluid to the system in situ, giving the advantage of lower weight during transportation. If the invention is utilised in a washing machine, the water could be introduced to the automatic balancing device during the first wash cycle performed by the machine after purchase However, it is envisaged that other liquids, for example oils, could be used, or even relatively dense gases. It is desirable that the density of each body is considerably less than the density of the fluid, being preferably less than 50% of the density of the fluid, more preferably less than 30% and most preferably less than 10% of the density of the fluid The greater the difference between the respective densities, the greater the size of the out-of-balance mass which lO the automatic balancing device is able to balance. The density of the fluid can be increased by the addition of suitable additives: for example, salt can be added to water in order to increase its density.

In a preferred arrangement, the cavity comprises an annular channel in which the bodies are freely movable. In such an arrangement, it is preferable that the bodies are spherical, ovoid or cylindrical in shape so that they contact the annular channel at a point or along a line. This is advantageous because friction between the channel and the bodies is minimised and the bodies tend to roll within the channel instead of sliding.

Further reductions in friction can be achieved by adding suitable lubricants to the fluid.

It is also preferred that a gap of at least 3mm, preferably 5-6mm, be provided between the wall of the channel and the body. Using such an arrangement enables the bodies to move easily to positions within the channel which achieve good balancing. This increases the efficiency of the device.

In a preferred embodiment, each of the bodies has the same size and shape as the remaining bodies. However, it is also envisaged that one or more of the bodies could have a larger volume and/or a higher density than the remaining bodies Such an arrangement would allow for fine adjustment of the device during use.

It is envisaged that an automatic balancing device of the type described above could be used singly or in combination with other such balancing devices. Preferably, when a s plurality of balancing devices are used to balance a single rotating body, the balancing devices are spaced along the axis about which the body rotates Alternatively or additionally, the balancing devices can be provided at different radial distances from the axis about which the body rotates.

The invention also provides a washing machine incorporating one or more balancing devices as described above.

Embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Figure I is a schematic front view of a washing machine incorporating an automatic balancing device according to the present invention; Figure 2 is a front view of a first embodiment of the automatic balancing device forming part of the washing machine of Figure 1; Figures 3a, 3b and 3c show three different possible shapes of the bodies forming part of the balancing device according to the invention; Figure 4 is a front view of the automatic balancing device of Figure 2 shown in combination with the tub of the washing machine of Figure l and in a first position; Figure 5 is a view similar to Figure 4 but showing the device in a second position; Figure 6 is a view similar to Figure 5 of a second embodiment of the automatic balancing device of the invention; Figure 7 illustrates a third embodiment of the automatic balancing device according to the invention; Figure 8 illustrates a fourth embodiment of the automatic balancing device according to the invention; Figure 9a and 9b are, respectively, front and side views illustrating a system in which three devices according to the invention are used in combination; Figure 10 illustrates a fifth embodiment of the automatic balancing device according to the invention; Figure 11 illustrates a sixth embodiment of the automatic balancing device according to the invention; and Figure 12 illustrates a seventh embodiment of the automatic balancing device according to the invention.

Figure 1 shows a washing machine 10 incorporating an automatic balancing device according to the invention. In conventional manner, the washing machine 10 has an outer casing 12 inside which is mounted a watertight tub 14. The tub is mounted inside the housing 12 via springs and dampers 16 in a known manner so that excursions of the tub brought about by the operation of the washing machine 10 can be accommodated within the housing 12 A perforated drum 18 is rotatably mounted within the tub 14 and a motor 20 is provided for rotating the drum within the tub about an axis 22. A door 24 is provided so as to allow access to the interior of the drum 18 for the purpose of introducing laundry items thereto. The arrangement of and sealing between these components is all well known and so will not be described any further here Also well known is the operation of a such washing machine to carry out a washing process on laundry items placed within the drum l 8.

The washing machine l O has an automatic balancing device 50 secured to the external surface of the drum 18. The automatic balancing device 50 is located inside the tub 14 and rotates with the drum 18 about the axis 22 when the drum 18 is driven by the motor 20. The automatic balancing device 50 is shown in greater detail in Figure 2 The automatic balancing device 50 comprises a cavity consisting of an annular channel 52 having an outer wall 54 and an inner wall 56. Each of the outer wall 54 and the inner wall 56 is centred about the axis 22 about which the drum 18 rotates (see arrow 58) The inner wall 56 can be formed by a portion of the outer wall of the drum 18, although it is preferred that the channel is formed separate from the drum 18 and secured thereto during manufacture of the washing machine 10. The channel 52 is closed by side walls (not shown), giving the channel 52 a rectangular cross-section. (It will be appreciated from the description below that the shape of the cross- section of the channel is not important and that a circular, elliptical or other cross-sectional shape can be provided) A plurality of bodies 60 are located within the channel 52. In the example shown, eight bodies 60 are provided, although this number is not critical. Each body 60 has a circular cross-section. A fluid 62 surrounds the bodies 60 and fills the remainder of the channel 52 so that the bodies 60 are completely submerged in the fluid 62 The bodies 60 have a density which is low compared to the density of the fluid 62. In the example shown, the fluid 62 is water and the bodies 60 are machined from a polyeurythane foam so that the density of the bodies 60 is approximately 40% of the water 62. The bodies 60 are thus buoyant. Different materials can be used to form the bodies so that their density is less than 30%, preferably less than 10%, of the density ofthe water 62.

The diameter of each of the bodies 60 is smaller than the depth d of the channel so that, when the bodies 60 are in contact with the inner wall 56, a gap is present between the outer extremity of each body 60 and the outer wall 54 of the channel 52. In the embodiment shown, the size of the gap is approximately 5-6mm, although a gap of as little as 3mm is acceptable. A gap smaller than approximately 3mm has been found to restrict the flow of the fluid about the bodies 60 which in turn restricts the free movement of the bodies 60 within the channel 52. A gap greater than approximately 6mm has been observed to have little benefit over a gap of 5-6mm.

Figures 3a, 3b and 3c illustrate three possible shapes of the buoyant bodies 60 suitable for use in a device as described above Figure 3a shows a simple spherical body 60a which, in a channel having either a rectangular or a circular cross-section, would contact the channel 52 at only a single point The diameter of the spherical body 60a would be at least 3mm, and preferably 5-6mm, less than the depth d of the channel 52. Figure 3b shows a body 60b having a cylindrical shape, the body 60b being arranged in the channel 52 so that movement of the body 60b within the channel 52 occurs by rolling.

Again, the diameter of the body 60b would be at least 3mm, and preferably 5-6mm, less than the depth of the channel 52 Figure 3c shows a body 60c similar to that shown in Figure 3b but having a cylindrical aperture 64 running along the longitudinal axis thereof. This allows the water 62 to penetrate the aperture 64 and thus reduce the amount of water displaced by the body 60c in comparison to that displaced by the body 60b An envisaged advantage of such an arrangement will be discussed below.

It will be apparent to a skilled reader that variations in the shapes illustrated are perfectly possible. For example, the cylindrically-shaped body 60b shown in Figure 3b may be adapted to have hemispherical ends and an ovoid shape would also be possible.

Equally, the planar end of the body 60b may have a plurality of small, hemisperical nodules located thereon. This reduces the area of contact between the body 60b and the wall of the channel 52 which thus reduces friction. It is also possible to provide bodies having a wedge-shaped cross-section if desired, although it is envisaged that the best results will be obtained when the bodies 60 contact the walls 54, 56 of the channel 52 at a point or along a line of contact.

In the event of the buoyant bodies 60 having a cylindrical shape as shown in Figure 3b, and the annular channel 52 having a rectangular crosssection, the axial dimension of the buoyant bodies 60b is chosen so that a small gap is present between each end of the respective body 60b and the side wall of the channel 52. The size of this gap is chosen so that a minimum amount of friction is generated between the body 60b and the channel 52 but also so that the alignment of the cylindrical body 60b is maintained (ie the body 60b is not permitted to twist in the channel 52) The bodies 60a, 60b, 60c can be manufactured by machining from a foam material such as polyeurythane expanded foam or epoxy cellular foam A thin coating of a waterproof or lubricating substance can be applied to the foam body. Alternatively, any or all of the bodies 60a, 60b, 60c can be manufactured from a thin-walled plastics material or a metal and left hollow or filled with a low density gas or foam. It is preferred that the bodies 60a, 60b, 60c will be formed from a relatively stiff material and will have a smooth outer surface so that the coefficient of friction between the bodies 60a, 60b, 60c and the water 62 and the bodies 60a, Gob, 60c and the inner wall 56 of the channel 52 is as low as possible. This improves the maneouvrability of the bodies 60a, 60b, 60c within the channel 52 The density of each of the bodies 60 is made as low as possible in comparison to that of the water 62. The lower the relative density of the bodies 60, the greater the counterbalancing force which can be generated and thus the larger the out-of-balance mass which can be balanced. The bodies 60 have a density of less than 50% of the water 62. It is preferred that the bodies will have a density of less than 30% of the water. A density of less than 10% is more preferred.

Figure 4 illustrates the automatic balancing device 50 of Figure 2 when the drum to which it is attached is either stationary or rotating at low speeds of the sort used to carry out a washing cycle on laundry items 70 located within the drum 18. As the drum 18 rotates about the axis 22 in the direction of arrow 58, the laundry articles 70 are tumbled in a wash liquid in a known manner. The speed of rotation of the drum 18 is insufficient to cause the laundry articles 70 to adhere to the outer wall of the drum 18.

Although the channel 52 is caused to rotate with the drum 18, the buoyant bodies 60 remain collected in the uppermost portion of the channel 52 as shown in Figure 4 and little significant movement of the bodies 60 occurs. As the speed of rotation of the drum 18 increases, one or more of the bodies 60 may be dragged all or part-way around the channel but this will have no balancing effect on the drum.

Figure 5 illustrates the operation of the automatic balancing device of the present invention when the speed of the drum 18 has passed the critical speed and the automatic balancing effect is operational As before, the drum 18 rotates about the axis 22 in the direction of the arrow 58 The out-of-balance load created by the presence of the laundry articles is represented by the mass 72. As the speed of the rotation of the drum 18 increases, the water 62 is rotated with the channel 50 and a pressure gradient is developed across the channel 52. Hence the buoyant bodies 60 move towards, and ultimately contact, the inner wall 56 of the channel 52 whilst remaining submerged in the water 62. When the speed of rotation of the drum 18 is above the critical speed, forces caused by the excursion of the drum and acting on the water 62 in the channel 52 cause the buoyant bodies 60 to move towards positions which allow the out-of-balance 1 S mass 72 to be counterbalanced.

Because the bodies 60 have a density which is considerably lower than that of the water 62, the bodies 60 themselves do not have a counterbalancing effect on the out-of- balance mass 72. Rather, the bodies 60 displace a given volume of water 62 corresponding to the volume of the respective body 60 from the area of the channel 52 in which the body 60 is located. This leads to an uneven distribution of water 62 in the channel 52. It is this uneven distribution of the water 62 which creates counterbalancing forces which then counterbalance the mass 72 in the drum 18 Looking at Figure 5, the bodies 60 displace water from the area of the channel 52 near to the out- of-balance mass 72 which in turn creates an excess of water on the opposite side of the channel 52. This excess of water counterbalances the mass 72. The dynamics ofthe system mean that the bodies 60 will move along the channel 52 until an optimum counterbalancing position of the excess water is reached A second embodiment of the invention is illustrated in Figure 6. In this embodiment, the volume of water 62 present in the channel 52 is insufficient to fill completely the volume of the channel not occupied by the bodies 60 An air pocket 66 is thus present in the channel 52 alongside the bodies 60 and the volume of water 62. When the drum 18 is at rest or rotating only at low speeds, the air pocket 66 is located at the uppermost area of the channel 52. However, when the drum 18 rotates at higher speeds, centrifugal forces will cause the water 62 to be forced towards the outer wall 54 of the channel 52 and so the air pocket 66 distributes itself about the inner wall 56 as illustrated in Figure 6 The bodies 60 are thus only partially submerged in the water 62.

A third embodiment of the invention is illustrated in Figure 7. In this embodiment, the automatic balancing device 150 comprises a cavity consisting of a disc-shaped chamber 152 centred about an axis 122 about which the drum to which the device 150 is attached rotates. Two buoyant bodies 160 are freely-rota/ably mounted about an axle 121 which is also centred on the axis 122. The bodies 160 are spaced along the axle 121 so that each body 160 rotates in a separate plane. The interior of the chamber 152 not occupied by the bodies 160 is filled with a liquid 162, in this case, glycerol.

When the device 150 is caused to rotate with the drum of the washing machine at speeds above the critical speed, the bodies 160 displace corresponding volumes of glycerol 162 to cause an uneven distribution of glycerol within the chamber 152 The imbalance of the glycerol about the axis 122 causes forces to be created which will counterbalance the out-ofbalance load present in the drum due to the load of the laundry articles. The forces acting on the glycerol 162 then cause the bodies 160 to rotate about the axis 122 so that they approach the position in which the glycerol provides the maximum counterbalancing effect It will be appreciated that the shape of the bodies 160 shown in Figure 7 is not intended to be limiting and that other shapes are possible Furthermore, whilst only two bodies 160 are shown in Figure 7, it would be possible to provide more than two similar bodies spaced along the axle 121 if desired The devices 50, 150 shown in the drawings are not shown to scale and relative dimensions are not intended to be correct Experimentation has shown that, in relation to devices of the sort illustrated in Figures 2 to 6, cylindrical bodies having a diameter of 35mm located in an annular channel having a depth of 42mm allows a good degree of manouevrability of the bodies and so achieves an acceptable degree of reliability in successfully counterbalancing an out-of-balance mass These measurements mean that the diameter of each body 60 is approximately 83% of the radial depth of the channel 52, although it is envisaged that the diameter of each body can be between 75% and 90% of the radial depth of the channel 52 without any serious detriment to the performance of the device being perceived.

It is to be noted that, in each of the embodiments shown in Figures 2 to 6, a total of eight bodies 60 have been provided in the channel 52 Also, in the embodiments shown in Figures 2 to 6, each of the bodies 60 has been identical to the remaining bodies 60.

Figure 8 illustrates an embodiment in which the number of bodies has been increased and the volume of one of the bodies has been increased in comparison to the remaining bodies In other respects, Figure 8 is similar to Figure 5 in that it shows an automatic ] 5 balancing device 250 according to the invention, comprising an annular channel 252 in which are located a plurality of freely-movable buoyant bodies 260 of the type described above. The channel 252 is filled with water 262 or another liquid of relatively high density compared to the bodies 260.

Thirteen buoyant bodies 260 are provided in the channel 252. In addition, a single additional buoyant body 261 is provided This additional buoyant body 261 has a larger diameter than the bodies 260, although it is still dimensioned so that a gap of at least 3mm is provided between the body 261 and the outer wall 254 of the channel 252 when the additional body 261 is pressed against the inner wall 256.

The role of the additional buoyant body 261 is to provide a degree of fine tuning to the device Because the buoyant bodies 260 are light, the friction forces acting to oppose their movement towards their optimum positions may are relatively large. This means that the buoyant bodies 260 may not reach their optimum positions. The movement of the additional buoyant body 261 is less restricted by friction forces and thus is more likely to reach its optimum position.

In an alternative arrangement having an effect similar to that of the arrangement shown in Figure 8, the buoyant bodies 260 have a cylindrical shape as shown in Figure 3b The additional body 261 is, however, replaced by a buoyant body which has the same outer diameter as the cylindrical bodies 260 but has an aperture running along its axis as shown in Figure 3c. Water 262 fills the aperture in the additional body 261. This has the effect of displacing less of the dense fluid 262 than is displaced by each of the cylindrical bodies 260 and so the effective overall density of the additional body 261 appears to be greater than that of the cylindrical bodies 260. The forces which tend to move the buoyant bodies 260, 261 into a position in which balancing is achieved will thus be more likely to achieve the optimum position for the additional body 261 than for the cylindrical bodies 260.

Varying the number of bodies in the channel has an effect on the capability of the automatic balancing device to counterbalance out-ofbalance masses. Ideally, the number of bodies 660 is selected so that, when the bodies 660 are located directly adjacent one another in the channel 652, they extend circumferentially around substantially half of the channel 652, as shown in Figure 12. To be more precise, the balancing capability of the device 650 is maximised when the centres of the bodies 661 in the end positions lie diametrically opposite one another. The speed with which the bodies 660 take up their optimum positions can, however, be increased by reducing the number of bodies 660 provided in the channel 652 to less than the number shown in Figure 12 If the bodies 660 are arranged so as to occupy approximately 120 of the circumference of the channel 652, the balancing capability will be lessened but the time taken to achieve maximum balancing is reduced Hence, a compromise needs to be struck between balancing capability and speed of achieving balancing.

Devices of the type described above can be used to great effect singly or in combination with similar devices. For example, a plurality of annular channels of the sort described in connection with Figures 2 to 6 and 8 can be provided in an arrangement shown in Figures 9a and 9b As can be seen, three separate channels 352a, 352b, 352c are provided. Each channel 352a, 352b, 352c has different radial dimensions so as to lie inside one another in a single plane. Each channel 352a, 352b, 352c is filled with a relatively dense liquid 362 (different liquids can be provided in different channels if required) and each channel 352a, 352b, 352c has a plurality of buoyant bodies 360 provided therein Each channel 352a, 352b, 352c may contain bodies having a density or volume which is different from that of the bodies in the other channels if desired It is also possible toenhance the balancing capability of a system which is made up of several automatic balancing devices of the type described above by providing a plurality of such devices which are spaced along the axis of rotation of the drum of the washing machine or other rotating body For example, referring to the washing machine illustrated in Figure 1, a single device 50 of the type illustrated in Figures 2 to 4 may be provided at the front of the drum 18 whilst a similar device 50 may be provided at the rear of the drum 18 Alternatively, a device of the type shown in Figures 9a and 9b could be provided at either end of the drum 18. Still further, a device of the sort described above may be utilised at one end of a rotating drum whilst a more traditional device may be employed at the other end. The operating characteristics of the two systems can be tuned to optimise their combined performance if desired (ea. each balancing system can be tuned to have different resonant speeds) It is believed that the buoyant bodies forming part of an automatic balancing device according to the invention do not contribute greatly to the excursion of the rotating body at speeds below the critical speed. However, it is envisaged that, if the buoyant bodies could be distributed approximately evenly around the annular channel shown in Figures 2, 4, 5, 6, 8 and 9 at speeds below and passing through the critical speed, then this would be an advantage. Figure 10 shows an arrangement in which such a distribution is possible. As can be seen from Figure 1O, the annular channel 452 has recesses 453 formed in the outer surface 454 thereof The number of recesses 453 corresponds to the number of bodies 460 present in the channel 452 and each recess 453 communicates with the channel 452 via an opening through which a respective body 460 may pass.

Each recess 453 also has an undercut portion 455 at each end thereof shaped so as to receive a respective body 460. The recesses are shaped and dimensioned so that only one buoyant body may enter (and remain in) any one recess at any one time.

In use, when the drum of the washing machine (or other rotating body) is rotating at low speeds, the buoyant bodies 460 will generally gather in the upper part of the channel 452 However, the buoyancy of each body 460 will cause it to rise into a recess 453 through the respective opening and be carried with the channel 452 in a circular path about the axis 422. Each body 460 will rise into a recess 453 when an empty recess 453 is presented at the top of the channel 452. The undercut portions 455 then deter the body 460 from leaving the recess 453 as the body 460 is carried around the circular path. The body 460 is able to exit the recess 453 as it passes the lowermost part of the path but, as long as the speed of the channel 452 is sufficient, the body 460 will remain held in the recess 453. Even distribution of the buoyant bodies 460 about the axis 422 can thus be achieved.

Uniform distribution of the bodies in the channel can also be achieved in other ways For example, shallow recesses formed in the outer wall of the channel and shaped to receive the bodies can have permanent magnets or electromagnets associated with them.

The magnets are arranged radially outwardly of each recess and the bodies are arranged to consist of or incorporate a magnetic material This can be achieved, for example, by providing a foam body with a thin steel sleeve on the outer surface thereof. Such an arrangement can also improve the rolling resistance of the body in the channel. The strength of attraction between the bodies and the recesses formed in the outer wall can be selected so that, below a defined speed of rotation of the channel, the bodies will be held in the recesses and carried around the circular path. Above the defined speed of rotation, the forces urging the buoyant bodies towards the centre of rotation will overcome the magnetic attraction forces and the buoyant bodies will move out of the recesses so as to be able to move freely and allow the desired automatic balancing to take place Of course, in a case wherein electromagnets are used to retain the bodies in an evenly-distributed arrangement about the channel, the bodies can be released simply

by switching offthe electromagnetic field

In a further variation of the above-described invention, the low-density bodies can be provided in combination with high-density bodies in the same cavity. Such an arrangement is illustrated in Figure 11. In Figure 11, the channel 552 contains both buoyant bodies 560 and non-buoyant bodies 560' The channel 552 has a depth which is sufficient to allow the buoyant bodies 560 and the non-buoyant bodies 560' to pass one another within the channel 552 without colliding and without exerting significant drag forces on one another. In use, when the rotating body exceeds critical speed, the non-buoyant bodies 560' are flung to the outer wall 554 of the channel 552 and the buoyant bodies 560 are pressed to the inner wall 556 of the channel 552. Each set of bodies 560, 560' is able to contribute to the automatic balancing of the rotating body without impacting on the benefit afforded by the other set of bodies 560', 560.

The invention is not intended to be limited to the specific embodiments described above. Alternative arrangements will be apparent to a skilled reader. For example, liquids other than water and glycerol may be used in the cavities: indeed, heavy gases may also be used. The number, size and density of buoyant bodies may be varied, as may the shape thereof.

The invention has application in many and various fields, the preferred application being that of a washing machine, as has been mentioned above However, it is envisaged that the invention has application in fields as diverse as vehicle wheels, propellers, turbines, motors, angle grinders and centrifuges of the type used, for example, in medicine, industry and nuclear environments

Claims

CLAI1\IS 1. An automatic balancing device comprising a cavity containing a

volume of fluid and a plurality of bodies in contact with the fluid, wherein the density of each of the bodies is less than the density of the fluid.
2. An automatic balancing device as claimed in claim 1, wherein the fluid is a liquid.
3 An automatic balancing device as claimed in claim 2, wherein the liquid is water.
4. An automatic balancing device as claimed in any one of the preceding claims, wherein the density of each body is less than 50% of the density of the fluid.
An automatic balancing device as claimed in claim 4, wherein the density of each body is less than 30% of the density of the fluid.
6. An automatic balancing device as claimed in claim 5, wherein the density of each body is less than 10% ofthe density ofthe fluid
7. An automatic balancing device as claimed in any one of the preceding claims, wherein the bodies are manufactured from a foamed material.
8. An automatic balancing device as claimed in any one of the preceding claims, wherein the bodies are hollow.
9. An automatic balancing device as claimed in any one of the preceding claims, wherein each of the bodies is at least partially submerged in the fluid.
10. An automatic balancing device as claimed in claim 9, wherein each of the bodies is fully submerged in the fluid
11 An automatic balancing device as claimed in any one of the preceding claims, wherein the cavity is completely filled by the bodies and the fluid.
12. An automatic balancing device as claimed in any one of the preceding claims, wherein the cavity comprises an annular channel
13. An automatic balancing device as claimed in claim 12, wherein the bodies are shaped so as to contact the annular channel at a point or along a line of contact.
14 An automatic balancing device as claimed in claim 13, wherein the bodies are spherical, cylindrical or ovoid in shape.
15. An automatic balancing device as claimed in claim 14, wherein the diameter of each body is between 75% and 90% of the radial depth of the annular channel.
16. An automatic balancing device as claimed in claim 14 or 15, wherein the radial depth of the annular channel is at least 3mm greater than the diameter of each body
17. An automatic balancing device as claimed in claim 16, wherein the radial depth of the annular channel is substantially 5mm greater than the diameter of each body.
18 An automatic balancing device as claimed in any one of claims 12 to 17, wherein the bodies occupy at least 120 of the circumference of the annular channel.
19. An automatic balancing device as claimed in claim]8, wherein the bodies occupy substantially half of the circumference of the annular channel
20. An automatic balancing device as claimed in any one of claims 12 to 19, wherein the annular channel incorporates means for distributing the bodies around the channel
21 An automatic balancing device as claimed in claim 20, wherein the means for distributing the bodies around the channel comprise a plurality of recesses formed in the outer wall of the channel.
22. An automatic balancing device as claimed in claim 20 or 21, wherein the means for distributing the bodies around the channel comprise a plurality of magnets spaced around the outer wall of the channel.
23. An automatic balancing device as claimed in any one of the preceding claims, wherein each of the bodies has the same size and shape as the remaining bodies.
24 An automatic balancing device as claimed in any one of claims 1 to 22, wherein at least one of the bodies has a density which is closer to that of the fluid than the remaining bodies.
25. An automatic balancing device as claimed in any one of claims 1 to 22, wherein at least one of the bodies has a volume which is larger than that of the remaining bodies.
26. An automatic balancing device as claimed in any one of claims 1 to 11, wherein the cavity comprises a cylindrical chamber and the bodies are rotatably mounted therein about a central axis
27 An automatic balancing device as claimed in claim 26, wherein the bodies are axially spaced along the axis so as to rotate in separate planes
28. An automatic balancing device as claimed in claim 26 or 27, wherein at least two bodies are provided.
29. An automatic balancing device substantially as hereinbefore described with reference to any one of the embodiments shown in the accompanying drawings.
30 Apparatus comprising a body rotatable about an axis and including at least one automatic balancing device as claimed in any one of the preceding claims.
31. Apparatus as claimed in claim 30, comprising a plurality of automatic balancing devices as claimed in any one of claims I to 29.
32. Apparatus as claimed in claim 29, wherein the said automatic balancing devices are spaced along the said axis.
33 Apparatus as claimed in claim 31 or 32, wherein the said automatic balancing devices are provided at different radial distances from the said axis.
34. Apparatus as claimed in any one of claims 30 to 33, wherein the apparatus is a washing machine