Automatic balancing device

Provided herein is an automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body. The automatic balancing device includes a chamber with an outer wall and an axis, first and second counterbalancing masses constrained to move freely in a circular path about the axis within the chamber, and a viscous fluid provided in the chamber so as to provide viscous coupling between the outer wall of the chamber and each of the counterbalancing masses. The counterbalancing masses may be arranged so that the first counterbalancing mass leads the second counterbalancing mass when the device is in use. A constraint may be provided with the device so as to prevent the first counterbalancing mass from leading the second counterbalancing mass by more than substantially 180°.

FIELD OF THE INVENTION

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 the washing machine during washing and spinning cycles.

BACKGROUND OF THE INVENTION

Automatic balancing devices for counterbalancing out-of-balance masses are known in many different applications. However, the most complex out-of-balance situations occur when both the position and size of the out-of-balance mass is unpredictable and the speed of rotation is variable, as in the case of a washing machine. Many different automatic balancing devices have been proposed and used in washing machines and many of these are effective at counterbalancing out-of-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; U.S. Pat. Nos. 5,813,253; 5,862,553; 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 rotatable counterbalancing masses automatically take up positions in which the out-of-balance mass is counterbalanced. However, it is recognised in some of the aforementioned documents that, at speeds below the critical speed, freely rotating counterbalancing masses act so as to exacerbate the excursion of the rotating body due to the presence of the out-of-balance mass. In these cases, the counterbalancing masses are proposed to be locked in a fixed position with respect to the chamber in which they are located when the body is rotating at a speed below the critical speed. In the case of U.S. Pat. No. 5,813,253, a roller locates in a recess in order to prevent the balancing masses from moving along the annular path in which they run. The roller is released from the recess when the body exceeds the critical speed. In GB1,092,188, the counterbalancing masses are pivotably mounted about an axle with locking members provided to lock the masses in a fixed position with respect to the chamber in which they are housed when the speed of rotation is below critical. The locking means release when the speed is above critical. It is also envisaged in this prior art document that the counterbalancing members could be locked together, so that they have a zero net out-of-balance effect until the critical speed has been exceeded.

The known mechanisms by means of which counterbalancing masses can be locked in a position in which they have a zero net out-of-balance effect are generally difficult and expensive to manufacture. They are susceptible to damage in view of the movement of the counterbalancing masses, which can sometimes be quite violent. However, in arrangements which do not lock the counterbalancing masses, acceleration of the drum of the washing machine from a below-critical speed to an above-critical speed can cause extreme excursion, especially at the critical speed.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body in which, during acceleration through the critical speed, the amount of excursion of the rotating body is minimised. It is a further object of the present invention to provide an automatic balancing device which is economic to manufacture and less susceptible to damage than known arrangements. It is a further object of the present invention to provide a method of operating a washing machine in which rotation of the drum can be accelerated through the critical speed with the minimum of excursion.

The invention provides an automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body, the automatic balancing device comprising a chamber having an outer wall and an axis, first and second counterbalancing masses constrained to move freely in a circular path about the axis and within the chamber, and a viscous fluid provided in the chamber so as to provide viscous coupling between the outer wall of the chamber and each of the counterbalancing masses when the device is in use, characterised in that the counterbalancing masses are adapted and/or arranged such that, in use, the first counterbalancing mass leads the second counterbalancing mass, and constraining means are provided so as to prevent the first counterbalancing mass from leading the second counterbalancing mass by more than substantially 180° when the device is in use.

Preferably, the first counterbalancing mass has a moment of inertia which is lower than that of the second counterbalancing mass. More preferably, the viscous coupling between the first counterbalancing mass and the outer wall of the chamber is higher than the viscous coupling between the second counterbalancing mass and the outer wall of the chamber.

The arrangement on this invention has been found to be effective in reducing the amount of excursion of the rotating body in comparison to an arrangement utilising no counterbalancing means, at least when used in a washing machine. Indeed, the claimed arrangement is capable of reducing the maximum excursion of the rotating drum of a washing machine to a level significantly below that currently achieved by commercially available washing machines which utilise a known balancing device. The reduction in excursion is particularly important as the rotating body passes through the critical speed as the excursion is largest at this speed. Reducing the excursion, particularly at the critical speed, allows washing machines and other similar devices to be manufactured with larger drums because less provision for excursion needs to be made. Also, less ballast needs to be provided.

The arrangement of this invention is also very simple to manufacture and reliable in operation. In place of the known locking arrangements described in the prior art discussed above, the constraining means can take a very simple form which results in higher reliability and lower failure rates than more complex arrangements. The manufacturing cost of the claimed arrangement is also lower than the cost of the known prior art arrangements.

The arrangement of this invention is believed to operate in the following manner. When the rotating body is rotated at a speed below critical but sufficient to cause the counterbalancing masses to rotate within the chamber, the counterbalancing masses spread apart because the first counterbalancing mass leads the second counterbalancing mass. As the speed of the device increases, the spread of the masses will increase until, when the spread is at or near 180°, they are prevented from spreading apart any further by the constraining means. In this position, i.e., when the masses are spread apart by approximately 180°, they contribute little or nothing to the out-of-balance mass present in the rotating body and the excursion of the rotating body is not exacerbated. However, at the same time, the excursion of the rotating body causes the masses to be drawn towards one another. Because the masses are free to move with respect to the chamber (within the constraints applied by the constraining means), they move towards one another thereby effecting a partial balancing of the out-of-balance mass in the rotating body. This movement of the masses affects the phase and size of the excursion of the rotating body which then causes further movement of the masses in response. Hence the masses are in constant movement with respect to the chamber, continually moving into a position in which the out-of-balance mass is partially balanced. This results in a reduction of the excursion of the rotating body.

It is also believed that, as the speed of rotation of the rotating body approaches resonance (the critical speed), the counterbalancing masses tend towards a position in which the out-of-balance mass is fully balanced. Thus the amount by which the excursion of the rotating body is reduced increases as the rotating body approaches resonance.

The invention also provides a method of operating a washing machine having a drum and incorporating the automatic balancing device described above, the method comprising the steps of:(a) rotating the drum and the automatic balancing device at a speed below the critical speed of the washing machine whilst allowing each of the counterbalancing masses to rotate freely about the axis;(b) causing the first counterbalancing mass to lead the second and any further counterbalancing mass;(c) preventing the first counterbalancing mass from leading the second and any further counterbalancing mass by more than substantially 180°; and(d) increasing the speed of rotation of the drum to a speed above the critical speed of the washing machine.Preferably, the method comprises the further steps of:(e) detecting variations in the amplitude of excursion of the drum due to the presence of an out-of-balance load therein and the counterbalancing masses;(f) detecting a minimum amplitude of excursion; and(g) commencing the increase in the speed of rotation of the drum to a speed above the critical speed of the washing machine at a time when the amplitude of excursion is at or close to a minimum.

Preferably, the speed of rotation of the drum is increased from a speed below the critical speed of the washing machine to a speed above the critical speed of the washing machine at a rate of between 5 rpm/s and 50 rpm/s, more preferably between 5 rpm/s and 15 rpm/s.

It is believed that the rate of acceleration of the drum can have an effect on the amount by which the excursion of the rotating body is reduced. Slower rates of acceleration have been shown to improve the effect. It is believed that this is due to the fact that slower rates of acceleration allow the counterbalancing masses time to adopt new, advantageous positions which will minimize the excursion experienced as the rotating body passes through resonance.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a typical environment in which an automatic balancing device is useful and desirable.FIG. 1shows a washing machine10having an outer casing12and a tub14mounted inside the outer casing12by way of a system of springs and dampers15. A perforated drum16is mounted inside the tub14so as to be rotatable about an axis18. In this embodiment, the axis18extends horizontally although this is not essential. A hinged door20is located in the front face of the outer casing12in such a manner that, when the door20is in a closed position (as illustrated), the tub14is sealed in a watertight manner. The door20is openable to allow articles of laundry to be placed inside the drum16prior to the commencement of a washing cycle to be carried out by the washing machine10. Flexible seals22are also provided between the drum16and the door20so that moderate movements of the drum16with respect to the outer casing12can be tolerated.

The drum16is mounted in a rotatable manner by way of a shaft24which is supported in cantilever fashion in the washing machine10and driven by a motor26. The shaft24passes through the tub14and into the interior thereof so as to support the drum16. The drum16is fixedly connected to the shaft24so as to rotate therewith about the axis18. It will be understood that the shaft24passes through the wall of the tub14in such a manner as to cause no rotation of the tub14. Such mounting arrangements are well known in the art. The washing machine10also includes a soap tray28for the introduction of detergent, one or more water inlet pipes30leading to the tub14via the soap tray28, and a water drain32communicating with the lower-portion of the tub14.

All of the features thus far described in relation to the washing machine10are known per se and do not form essential parts of the present invention. Common variants of any or all of these features may therefore be included in a washing machine capable of incorporating or utilising an automatic balancing device according to the invention if desired.

The present invention relates to an automatic balancing device suitable for use in a washing machine of the general type described above. In the embodiment shown, the automatic balancing device50is located on the distal end of the shaft24, inside the drum16and adjacent the rear wall16athereof. However, an automatic balancing device50of the type described below can also be located on the outside of the drum16, still adjacent the rear wall16athereof, but on the side facing the tub14. It is also possible to provide the automatic balancing device at other positions along the shaft24; for example, between the tub14and the outer casing12, although such an arrangement is not the preferred position.

The automatic balancing device50is illustrated in a rest position inFIG. 2. The automatic balancing device50has a cup-shaped cover52which, together with a circular plate53lying next to the rear wall16aof the drum16, forms an outer wall54defining a chamber56. The cover52is fixedly attached to the circular plate53so that the chamber56is liquid-tight for reasons which will be explained below. The means of connection between the cover52and the circular plate53are not material to the invention. The circular plate53can be attached to the rear wall16aof the drum16if desired. The cover52is fixedly attached to the distal end24aof the shaft24and is rotatable therewith so that, when the shaft24is rotated by the motor26about the axis18, the cover52is rotated as well. The shaft24passes through the chamber56from the circular plate53to the cover52.

Two counterbalancing masses80a,80bare rotatably mounted on the shaft24. The masses80a,80bare mounted on the shaft24via bearings58so that the masses80a,80bare able to rotate freely about the shaft. Means (not shown) are provided on the shaft24for preventing the masses80a,80bfrom moving axially along the shaft24, their positions being maintained with respect to one another and with respect to the cover52. The chamber56contains a pool of oil60(or other viscous liquid) which is sufficiently deep to ensure that the counterbalancing masses80a,80bare partially submerged as shown inFIG. 2. Indeed, it is preferred that there is sufficient oil60in the chamber56to ensure that, when the automatic balancing device50is rotated at a speed sufficient to distribute the oil60about the periphery of the chamber56, the counterbalancing masses80a,80bare still partially submerged in, or at least in contact with, the oil60.

The counterbalancing masses80a,80bare identical to one another. The configuration of one of the masses80ais shown in detail inFIGS. 3aand3b. Essentially, the mass80acomprises a disc portion82awhich is relatively slim in depth and circular in plan view. A mass portion84ais provided on the disc portion82aand is rigidly fixed thereto. Indeed, the mass portion84acan be formed integrally with the disc portion82aif desired. In the embodiment shown, the mass portion84aoverlies an area of the disc portion82aamounting to approximately one quarter, but the extent of overlie is not important. What is important is that the shape of the mass portion allows the centre of gravity of the mass80ato be located a significant distance from the centre of the disc portion82a. A central aperture86ais located in the geometric centre of the disc portion82aand passes through both the disc portion82aand the mass portion84a. The aperture86ais dimensioned so as to receive the bearings58by means of which the mass80ais mounted on the shaft24. The function of the mass portion84ais to provide the counterbalancing mass80awith an eccentric centre of gravity and sufficient mass to allow the mass80ato function as a counterbalance to an out-of-balance load present in the drum16of the washing machine10during its operation.

The mass portion84ais delimited by an edge or lip88a. The edge or lip88acomprises a surface which extends generally perpendicular to the circular faces of the disc portion82a. The edge or lip88aincludes two abutment surfaces90a,92awhose function will be described below.

The mass80acarries a pin94awhich protrudes beyond the surface of the disc portion82abut does not protrude beyond the surface of the mass portion84a. The pin94aextends beyond the surface of the disc portion82a, and perpendicular thereto, by an amount which is greater than the smallest distance a between the counterbalancing masses80a,80bas shown inFIG. 2. However, the distance protruded by the pin94ais not as great as the larger distance A between the two masses80a,80b, as will be explained below.

The counterbalancing mass80bis identical to the counterbalancing mass80adescribed above. Parts of the counterbalancing mass80bwill now be referred to using the reference numeral assigned to the corresponding part of mass80a, but with the letter “a” replaced by the letter “b”.

The arrangement of the counterbalancing masses80a,80bin the automatic balancing device50is shown inFIG. 2. The first counterbalancing mass80ais positioned to the left as shown and the second counterbalancing mass80bis positioned to the right. As has been mentioned, the distance a between the masses80a,80bin the area where the mass portions84a,84boverlap is less than the distance by which the pin94bprotrudes beyond the disc portion82bof the mass80b. However, the distance A between the disc portions82a,82bis greater than the extent of protrusion of the pin94bfrom the disc portion82b. Thus the distal end of the pin92bis constrained to travel between the abutment surfaces90a,92apassing across the disc portion82aand not across the mass portion84a. At each end of the possible travel path of the pin94brelative to the mass80a, the pin94bwill contact one of the two abutment surfaces90a,92aof the other mass80a.

The rest position of the two masses80a,80bis shown inFIG. 4a. The fact that the masses80a,80bare identical means that, in the absence of the pin94b, the masses80a,80bwould lie alongside one another in alignment. However, because the pin94bmay not lie alongside the mass portion84aof the mass80a, it abuts against the abutment surface90aof the mass80a. Hence the masses80a,80blie slightly out of alignment in the rest position as shown inFIG. 4a. It will be appreciated that, in the arrangement shown inFIG. 2, the pin92aof the mass80ais in fact redundant.

In operation, the automatic balancing device50operates as follows. Rotation of the drum16is effected by rotation of the shaft24. The automatic balancing device50rotates with the shaft24and the drum16so that the whole of the outer wall54of the chamber56rotates at a relatively high speed. By this we mean that the speed of rotation of the drum16, and thus the chamber56, is sufficient to create centrifugal forces which will overcome gravitational forces and so maintain the load contained within the drum pressed against the wall thereof but is below the critical speed of the washing machine. Hence the counterbalancing masses80a,80bare not in a position to perform an automatic counterbalancing function. However, the viscous coupling provided by the oil60between the outer wall54of the chamber56and the counterbalancing masses80a,80bwill cause the counterbalancing masses80a,80bto rotate about the shaft24. Due to the dynamics of the system, the masses80a,80bwill rotate about the shaft24at a rotational speed which is lower than that of the shaft24and the drum16. Because of the shaping of the masses80a,80b, and particularly the fact that a significant proportion of the surface of the mass80bfacing the circular plate53is spaced further from the circular plate53than the whole of the surface of the mass80afacing the cover52, the viscous coupling between the outer wall54and the mass80awill be higher than the viscous coupling between the outer wall54and the mass80b. Hence, the mass80awill rotate at a speed closer to that of the drum16and the chamber56than the mass80b. The result of this is that the mass80awill become the leading mass and that the mass80bwill trail behind it. Moreover, the mass80awill rotate about the shaft24at a higher angular velocity than the mass80b. Furthermore, the abutment of the pin94bagainst the abutment surface90awill eliminate any possibility of the mass80afrom lagging behind the mass80b. In the embodiment shown, the location of the pin94band the abutment surface90awill in fact ensure that the mass80aalways leads the mass80b.

As the drum16and chamber56rotate about the shaft24in the direction shown by the arrow B inFIG. 4b, the trailing mass80bwill trail further behind the leading mass80a. The masses80a,80bthus spread apart with the mass80aleading and the mass80btrailing behind it by an increasing amount. However, as the angle by which the mass80btrails the mass80aapproaches 180° (seeFIG. 4b), the pin94babuts against the abutment surface90athus preventing the mass80bfrom lagging behind the mass80aby more than 180°. In the embodiment shown, the mass80bis constrained to lag the mass80bby less than 180°. Even so, the combined effect of the counterbalancing masses80a,80bon the rotating body in this position is small.

It has been found that, by allowing the counterbalancing masses80a,80bto rotate freely within the constraints described above, the maximum amount of excursion of the drum16can be kept to a minimum as the drum16is accelerated from the speed described above, through the critical speed to a spin speed high enough to extract water from the load carried in the drum16. Spin speeds are commonly 1400 to 1600 rpm at present. It is advantageous if the rate of acceleration of the drum16is kept to a moderate rate: a rate of increase of speed of between 5 and 50 rpm per second is generally regarded as favorable, with a rate of increase of 5 to 15 rpm/s being more favorable still. Hence, in operation, the speed of the drum16is increased at a rate of between 5 and 50 rpm/s (preferably between 5 and 15 rpm/s) from the relatively high speed mentioned above to a suitable spin speed (typically 1400 to 1600 rpm) without constraining or otherwise locking the counterbalancing masses80a,80bto one another or to the outer wall54of the chamber56. In this way, the excursion of the drum16at the critical speed is reduced in comparison to other systems. This allows the dimensions of the drum16to be maximized for any given size of washing machine and/or the risk of damage occurring when large loads are spun at high speeds to be minimized. Above the critical speed, the masses80a,80bautomatically adopt positions which will counterbalance any out-of-balance present in the drum16, as is well known.

Optionally, a sensing device27(shown in dotted lines inFIG. 1) may be connected to the motor26. The sensing device27detects the current drawn by the motor26and/or the running speed of the motor26. As the masses80a,80badopt different relative positions, and thus perform a greater or lesser counterbalancing effect over time, either a minimum current drawn by the motor26or a maximum speed of rotation of the motor26will be indicative of a position of greatest counterbalancing effect. At these positions, the excursion of the drum16will be at a minimum for a given speed. It is believed to be beneficial to detect when these periods of minimum excursion occur and to initiate the acceleration of the drum16to the required spin speed at a point in time when the excursion is at or close to a minimum. Experimental results have shown that initiating the acceleration at such a time is advantageous in many cases.

In a method of operating the washing machine ofFIG. 1, when it is desired to rotate the drum at speeds high enough to extract wash liquor or rinse water by spinning, the drum is first rotated at a speed which is sufficient to stick the washload to the walls of the drum but below the critical speed. The masses80a,80bare allowed to rotate freely about the shaft24, although the engagement of the pin94bagainst abutment surfaces90a,92aprevents the mass80bfrom leading the mass80aat all or from lagging the mass80aby more than 180°. The difference in the viscous coupling between the outer wall54of the chamber56and each of the masses80a,80bcauses the masses initially to spread apart. Thereafter, the masses80a,80bwill redistribute themselves under the influence of various forces and will, periodically, take up positions which cause maxima and minima of excursion of the drum16and the shaft24. If the motor26has coupled to it a sensing device27as described above, the speed of and/or current drawn by the motor26is monitored and a minimum value of either characteristic is sensed. The drum16is then accelerated to the desired spin speed. This will inevitably require the speed of rotation of the drum to pass through the critical speed, at which the excursion of the drum16is greatest. The rate of acceleration is between 5 and 15 rpm/s but could be as high as 50 rpm/s. By allowing the masses80a,80bfreedom to rotate about the shaft24during the acceleration step, the maximum excursion is kept as low as possible. Also, by initiating the acceleration of the drum16from the lower speed at a point when the excursion is at or close to a minimum (as sensed by the sensor27), the excursion experienced by the drum16at the critical speed is minimized. Above the critical speed, the masses80a,80bposition themselves, as is well known, so as to counterbalance the out-of-balance load within the drum16and so the excursion of the drum16at speeds above critical is greatly reduced and, in some cases, eliminated.

Keeping the maximum excursion of the drum16to a minimum is beneficial because a lower provision for excursion then needs be built into the machine and thus the size of the drum16can be increased in comparison to other machines. Also, the risk of damage occurring due to excessive excursion of the drum16is reduced.

A second embodiment of the invention is illustrated inFIGS. 5,6and7.FIG. 5is a cutaway front view of an automatic balancing device150having an outer wall154delimiting a cylindrical chamber156. A shaft124, which is rotatable about an axis118, passes through the chamber156. Two counterbalancing masses180a,180bare mounted on the shaft124via bearings158so as to be freely rotatable about the shaft124. The counterbalancing masses180a,180bare shaped so as to increase in breadth with distance from the shaft124. As with the masses80a,80billustrated inFIGS. 2,3and4, the effect of this is to space the centre of gravity of each counterbalancing mass180a,180baway from the axis118.

Located between the counterbalancing masses180a,180bis a collar190which is shaped and dimensioned to be supported on and freely rotatable about the shaft124.

Projecting outwardly from a first annular face192aof the collar190are two diametrically opposed pins194a. A single pin194bprojects outwardly from a second annular face192bof the collar190, the pin194bbeing aligned with one of the pins194a. The pins194a,194bproject sufficiently far from the respective annular surfaces192a,192bto engage with the counterbalancing masses180a,180bas they rotate about the shaft124.

The dimensions of the collar190and the positioning of the pins194a,194bthereon are such that the pins194aabut against the edges of the counterbalancing mass180awith little or no play as shown inFIG. 5. Thus the collar is held in a substantially fixed position with respect to the counterbalancing mass180a. However, since the counterbalancing mass180bis acted upon by only the single pin194b, it is able to move relative to the counterbalancing mass180abetween a first position in which the masses180a,180bare aligned and a second position in which the masses180a,180bare diametrically opposed. The positioning of the pin194bis such that the mass180ais not allowed to lag behind the mass180b. As in the previous embodiment, the viscous coupling between the outer wall154and the mass180ais higher than that between the outer wall154and the mass180b. This can be achieved in any suitable way.

One way to vary the viscous coupling between the outer wall154and the respective masses180a,180bis illustrated inFIG. 8a. Here, the mass180ais arranged so as to be closer to the outer wall154than the mass180b. An alternative way of ensuring that the viscous coupling between the outer wall154and the mass180ais higher than that between the outer wall154and the mass180bis to provide the surface of the mass180awith an uneven or textured finish, as illustrated inFIG. 8b. This non-planar finish need only be applied to the part of the surface of the mass180afacing the outer wall, if desired, although more of the surface can be finished in this way. Further alternative means for ensuring that the viscous coupling between the outer wall154and the mass180ais higher than that between the outer wall154and the mass180bwill be apparent to a skilled reader.

In the embodiments described above, only two counterbalancing masses have been provided. It is possible to provide more than two masses and an arrangement showing the use of three masses is shown inFIG. 9. Each mass280a,280b,280cis mounted in a chamber256on a shaft224so as to be freely rotatable thereabout. Collars290a,290bare mounted between each pair of masses280a,280band280b,280cas shown. The collars290a,290bare similar to the collar190shown inFIG. 6and described above and operate in the same way. However, it is to be noted that the side of each collar290a,290bfrom which two diametrically opposed pins project is arranged so as to face the central mass280b. This ensures that the other masses280a,280care prevented from leading the central mass280bor from lagging the central mass280bby more than 180°. In order to ensure that the viscous coupling between the outer wall254of the chamber256and the central mass280bis higher than the viscous coupling between the outer wall254of the chamber256and the outer masses280a,280c, fins281are provided on the outer edge of the mass280b. The distal edges of the fins281lie very close to the outer wall254of the chamber256and provide high viscous coupling for the central mass280b.

Each of the embodiments described above makes use of a plurality of counterbalancing masses which are identical to one another. By this we mean that the masses are identical in shape and are made from material of the same density so that the centres of gravity of each mass lie at the same point. Different arrangements for ensuring that the first (leading) mass has a different viscous coupling with the wall of the chamber from that of the or each other mass have been described above. However, it is possible to arrange for the first mass always to lead the or each other mass in different ways. One such way is to provide the first mass with a lower moment of inertia than the or each other mass. This can be achieved by ensuring that the centre of gravity of the first counterbalancing mass lies closer to the axis of the chamber than the centre of gravity of the or each other mass. One way to achieve this is to arrange for the mass of the first counterbalancing mass to be lower than that of the or each other counterbalancing mass. This is possible by manufacturing the first counterbalancing mass (or a part thereof) from a material having a lower density than that of the other counterbalancing mass or masses; by providing the mass portion382aof the first counterbalancing mass380awith one or more hollow cavities383a(seeFIG. 10a); or by providing the first counterbalancing mass480awith a number of mass-reducing through-holes481a(seeFIG. 10b). Alternatively, the mass portion582aof the first counterbalancing mass580acan be shaped and configured so that, with respect to the or each other counterbalancing mass, the centre of gravity lies closer to the axis518than that of any other counterbalancing mass used in the same system (seeFIG. 10cin which the shape of the mass portion582bof another counterbalancing mass is shown in dotted outline). In this arrangement, it is envisaged that the mass of each counterbalancing mass is the same as that of the other counterbalancing masses.

Thus it can be seen that the first counterbalancing mass can be arranged to lead the or each remaining counterbalancing mass by providing it with either a higher viscous coupling with the chamber wall or a lower moment of inertia than that of the or each remaining counterbalancing mass. Other means for arranging for the first counterbalancing mass to lead the or each remaining counterbalancing mass will be apparent to a skilled reader.

The skilled reader will also appreciate that it is possible to rely solely on the means for arranging for the first counterbalancing mass to lead the or each remaining counterbalancing mass and to dispense with any further constraining means which will physically prevent the first counterbalancing mass from lagging the or any other counterbalancing mass. Referring to the embodiment shown inFIGS. 1 to 4, the abutment of the pin92aagainst the abutment surface90aprevents the first counterbalancing mass80afrom lagging the second counterbalancing mass80b. However, it is possible to rely on the higher viscous coupling between the first counterbalancing mass80aand the chamber wall54to ensure that the first counterbalancing mass80aleads the second counterbalancing mass80band so the abutment surface90acan be dispensed with. A similar change could be made to the embodiments illustrated inFIGS. 5 to 10.

The scope of the invention is not limited to the embodiments described above. It will be appreciated that the shape of the counterbalancing masses can be varied almost indefinitely as long as the essential counterbalancing function is achieved. Indeed, the counterbalancing masses provided in a single automatic balancing device as described above may be different in shape from one another and do not have to be identical. Different numbers of masses may be provided and alternative means for constraining the relative position of the masses may be provided. A further alternative arrangement contemplated within the scope of the invention is the use of the rear wall of the drum to form part of the outer wall of the chamber of the automatic balancing device. Other variations will be apparent to a skilled reader.