Automatic Balancing Device
The invention relates to an automatic balancing device for counterbalancing an out-of-
balance mass present in a body which is rotatable about an axis. Particularly, but not
exclusively, the invention relates to an automatic balancing device which is suitable for
use in a washing machine for counterbalancing out-of-balance masses in washing
machines during washing and spinning cycles.
Automatic balancing devices for counterbalancing out-of-balance masses in rotating
bodies are known. Many work on the well-known principle that, at speeds above the
critical speed of the system in which the body is rotating, freely-rotatable
counterbalancing masses will automatically take up positions in which the out-of-
balance mass is counterbalanced. It has also been recognised that, if these
counterbalancing masses are left unconstrained at speeds below the critical speed, they
exacerbate the excursion of the rotating body which is highly undesirable. In order to
remove this problem, devices have been proposed in which, at speeds below critical, the
counterbalancing masses are locked in a balanced position about the axis so that, instead
of having a detrimental effect on the system, they have no effect at all. Examples of
such systems are shown in US 5,813,253 and GB 1,092,188.
GB 2,388,849 discloses an improved automatic balancing system suitable for use in a
washing machine in which constraining means are permanently provided on the two
counterbalancing masses so as to limit the separation of the masses at speeds both above
and below critical. A certain amount of counterbalancing at below critical speeds can
be achieved with this system. This system has merit but suffers from the disadvantage
that the amount of counterbalancing achievable below the critical speed varies with time
and so the point at which the speed of rotation is increased to and through the critical
speed needs to be carefully controlled in order to achieve the best results, The fact that
the same constraints are applied to the counterbalancing masses at speeds both above
and below critical can also inhibit the effect of the masses in some cases.
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An object of the invention is to provide an automatic balancing system in which the
counterbalancing masses are able to provide at least partial counterbalancing at sub-
critical speeds but are also free to provide a full counterbalancing effect at speeds above
the critical speed. It is a further object of the invention to provide an automatic
balancing system by means of which the maximum excursion of the rotating body is
minimised reliably and simply.
The invention provides an automatic balancing device for counterbalancing an out-of-
balance mass present in a body which is rotatable about an axis of a dynamic system
having a critical speed, the automatic balancing device comprising a plurality of
counterbalancing masses, each of which is movable in a circular path about the axis so
as to generate a balancing force, the balancing forces combining, in use, to produce a
resultant balancing force which is variable between a minimum value and a maximum
value, characterised in that the automatic balancing device is configured so that, at a
first speed of rotation of the body which is below the critical speed, the movement of at
least one of the counterbalancing masses is restrained so that a substantially constant,
non-zero resultant balancing force is produced, the said resultant balancing force being
freely movable about the axis, and, at a second speed of rotation of the body which is
above the critical speed, the counterbalancing masses are free to adopt a position in
which the out-of-balance mass is counterbalanced.
The production of a non-zero resultant balancing force, as a result of the restraint of at
least one of the counterbalancing masses, allows an out-of-balance mass in the body to
be partially counterbalanced at below-critical speeds. Ensuring that the resultant
balancing force is substantially constant eliminates or reduces the amount of variation in
the counterbalancing capability over time. This means that, when the speed of rotation
of the body needs to be increased to and through the critical speed, there is no need to
exercise the level of control which would otherwise need to be exercised in order to
keep the maximum excursion to a minimum. The benefits of keeping the maximum
excursion to a minimum are well understood.
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Preferably, the second speed of rotation is any speed above a predetermined speed
which is above the critical speed of the said system. This reduces the potential for
unwanted oscillations which may occur if the counterbalancing masses are free to move
at all speeds above the critical speed.
It is preferred that the minimum value of the resultant balancing force is zero to allow
complete balancing to take place when there is no out-of-balance mass in the body.
It is preferred that, at the first speed of rotation, the resultant balancing force is less than
half, more preferably between 5% and 35%, and still more preferably between 15% and
20% of the maximum value of the resultant. It has been found that these values reliably
provide an adequate amount of counterbalancing for a range of out-of-balance values in
the practical application of a washing machine.
Preferably, the automatic balancing device further comprises restraining means, the
restraining means being operative at the first speed of rotation and inoperative at the
second speed of rotation. Such an arrangement allows different modes of operation to
be used for below-critical and above-critical speeds, thus ensuring that the benefits of
each mode of operation can be enjoyed without compromising the operation of the
device in either mode.
In a preferred embodiment, two counterbalancing masses are pivotably mounted about
the axis. When the restraining means are operative, the angle between the balancing
forces generated by the counterbalancing masses is between 140° and 175°, preferably
between 155° and 165°. Again, it has been found that these values provide an adequate
amount of counterbalancing for a range of out-of-balance values in a practical
application, particularly in the context of a washing machine.
In an alternative embodiment, at least three counterbalancing masses are provided and,
when the restraining means are operative, all but one of the counterbalancing masses are
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prevented from moving with respect to one another so that no resultant balancing force
is produced, the remaining counterbalancing mass being freely pivotable about the axis.
This arrangement has the advantage of being relatively simple to construct.
In a further alternative embodiment, which is primarily suitable for use with a vertical
axis arrangement, the counterbalancing masses are supported on a support surface
having a central portion, an annular race arranged axially outwardly of the central
portion, and an upwardly inclined portion extending between the central portion and the
annular race, the restraining means comprising a cylindrical lip arranged between the
central portion and the upwardly inclined portion. The counterbalancing masses are
formed as spherical balls which are dimensioned so as to form a continuous circle
immediately inwardly of the cylindrical lip and at least one of the spherical balls has a
reduced mass in comparison to the mass of the remaining balls. Preferably, the number
of balls is at least two and is not a factor of the total number of balls. This type of
arrangement has the advantage that, apart from the balls, no moving parts are required
and that, when the balls are arranged inside the lip, the presence of the reduced-mass
balls will ensure that a fixed resultant balancing force is produced.
The invention also provides a mechanism for counterbalancing an out-of-balance mass
present in a body which is rotatable about an axis, comprising a first automatic
balancing device as previously described and a second automatic balancing device as
previously described, the first and second automatic balancing devices being arranged
coaxially but spaced apart from one another along the said axis.
The invention further provides a method of counterbalancing an out-of-balance mass
present in a body which is rotatable about an axis, the body being provided with a
balancing device having a plurality of counterbalancing masses, each of which is
moveable in a circular path about the axis, the method comprising the steps of:
(a) rotating the body at a speed which is below the critical speed of the
system of which the body forms a part so that each counterbalancing mass generates a
balancing force;
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(b) restraining the movement of at least some of the counterbalancing
masses in such a manner that a substantially constant, non-zero resultant balancing force
is produced, the said resultant balancing force being freely moveable about the axis;
(c) increasing the speed of rotation of the body to a speed above the critical
speed of the system of which the body forms a part; and
(d) removing the restraint from the counterbalancing masses.
The benefits of the method according to the invention are similar to those of the
apparatus according to the invention.
Preferably, the step of restraining the movement of at least some of the
counterbalancing masses includes connecting all of the counterbalancing masses to one
another to prevent relative movement therebetween whilst still allowing rotation of the
connected counterbalancing masses about the axis. More preferably, the resultant
balancing force produced thereby is between 5% and 35%, advantageously between
15% and 20% of the maximum possible resultant balancing force. As before, these
values provide an adequate amount of counterbalancing for a range of out-of-balance
values.
Further advantageous and preferred features are set out in the subsidiary claims.
Embodiments of the invention will now be described with reference to the
accompanying drawings in which:
Figure 1 is a schematic sectional side view of a washing machine incorporating an
automatic balancing device according to a first embodiment of the invention;
Figure 2 is a schematic side sectional view, on an enlarged scale, through the automatic
balancing device forming part of the washing machine of Figure 1;
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Figure 3 is a front view of the essential parts of the automatic balancing device of
Figure 2 showing the counterbalancing masses latched together;
Figure 4 is a front view of a latch forming part of the automatic balancing device of
Figure 2, the latch being shown on a greatly enlarged scale;
Figure 5 is a front view similar to Figure 3 showing the counterbalancing masses
unlatched and in an intermediate position;
Figure 6 is a front view similar to Figure 3 showing, on a reduced scale, the
counterbalancing masses unlatched and in a position in which the resultant balancing
force is at a minimum value;
Figure 7 is a front view similar to Figure 3 showing, on a similarly reduced scale, the
counterbalancing masses unlatched and in a position in which the resultant balancing
force is at a maximum value;
Figure 8 is a front view of an automatic balancing device according to a second
embodiment of the invention showing two counterbalancing masses held in a restrained
position;
Figures 9a and 9b are three-quarter views of a catch forming part of the device of Figure
8, the catch being shown in the restraining and unrestraining positions respectively and
on an enlarged scale;
Figures 10a and 10b are sectional side views of the device of Figure 8 with the catches
shown in restraining and unrestraining positions respectively;
Figure 11 is a front view of an automatic balancing device according to a third
embodiment of the invention showing two counterbalancing masses held in a restrained
position;
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Figure 12 is a front view of an automatic balancing device according to a fourth
embodiment of the invention showing all but one of the counterbalancing masses held
in a balanced position;
Figures 13a and 13b are, respectively, plan and side views of a fifth embodiment of an
automatic balancing device according to the invention and showing the position of the
counterbalancing masses at the second speed of rotation;
Figures 14a and 14b are, respectively, plan and side views of the automatic balancing
device of Figures 13a and 13b and showing the position of the counterbalancing masses
at the first speed of rotation;
Figures 15a and 15b are, respectively, plan and isometric views of a sixth embodiment
of an automatic balancing device according to the invention and showing the position of
the counterbalancing masses at the first speed of rotation; and
Figure 15c is an enlarged view of the catch shown in Figures 15a and 15b.
Figure 1 illustrates a typical environment in which an automatic balancing device is
useful and desirable. Figure 1 shows a washing machine 10 having an outer casing 12
and a tub 14 mounted inside the outer casing 12 by way of a system of springs and
dampers 15. A perforated drum 16 is mounted inside the tub 14 so as to be rotatable
about an axis 18. In this embodiment, the axis 18 extends horizontally although this is
not essential and the axis 18 could be inclined to the horizontal. Indeed, the entire
arrangement could be rotated through 90° so that the axis is arranged vertically or
substantially vertically. A hinged door 20 is located in the front face of the outer casing
12 in such a manner that, when the door 20 is in a closed position (as illustrated), the tub
14 is sealed in a watertight manner. The door 20 is openable to allow articles of laundry
to be placed inside the drum 16 prior to the commencement of a washing cycle to be
carried out by the washing machine 10. Flexible seals 22 are also provided between the
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drum 16 and the door 20 so that moderate movements of the drum 16 with respect to the
outer casing 12 can be tolerated.
The drum 16 is mounted in a rotatable manner by way of a shaft 24 which is supported
on the tub 14 and driven by a motor 26. The shaft 24 passes through the tub 14 and into
the interior thereof so as to support the drum 16. The drum 16 is fixedly connected to
the shaft 24 so as to rotate therewith about the axis 18. It will be understood that the
shaft 24 passes through the wall of the tub 14 in such a manner as to cause no rotation
of the tub 14. Such mounting arrangements are well known in the art. The washing
machine 10 also includes a soap tray 28 for the introduction of detergent, one or more
water inlet pipes 30 leading to the tub 14 via the soap tray 28, and a water drain 32
communicating with the lower portion of the tub 14.
All of the features thus far described in relation to the washing machine 10 are 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 washing machine 10 shown in Figure 1 incorporates an automatic balancing device
50 according to the invention. The automatic balancing device 50 is located on the rear
wall 16a of the drum 16, remote from the door 20, and is arranged to rotate with the
drum 16. The automatic balancing device 50 is shown more clearly in Figure 2. It
consists of a wall 52 which delimits a cylindrical chamber 54. Part of the wall 52 can
be formed by the rear wall 16a of the drum 16. An axle 56 extends across the chamber
54, the axle 56 lying coincident with the axis 18 about which the drum 16 rotates.
Supported on the axle 56 are two counterbalancing masses 60, 70. The
counterbalancing masses 60, 70 are axially spaced along the axle 56 and are mounted
thereon by way of bearings (not shown) so as to be freely rotatable about the axis 18
and within the chamber 54.
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A viscous fluid 58 (eg. oil) is provided in the chamber 54. The amount of oil 58 is
selected to ensure that, when the wall 52 of the chamber 54 is rotated with the drum 16,
there is sufficient viscous coupling provided between the wall 52 and the
counterbalancing masses 60, 70 to cause the counterbalancing masses 60, 70 to rotate
about the axle 56. This technique is well known.
The counterbalancing masses 60, 70 are shown in front view in Figure 3. Both
counterbalancing masses 60, 70 are generally the same shape, although this is not
essential. Each counterbalancing mass 60, 70 is shaped so that its centre of mass 62, 72
is spaced away from the axis 18. It will be understood that, as the counterbalancing
masses 60, 70 rotate about the axis 18, a balancing force FB passing through the
respective centre of mass 62, 72 will be generated. Each counterbalancing mass 60, 70
has a relatively small inner portion 64, 74 through which the axle 56 passes and which
has a radially outer edge 65, 75 which lies relatively close to the axle 56. Each
counterbalancing mass 60, 70 also has a relatively large outer portion 66, 76 having a
radially outer edge 67, 77 which lies close to the wall 52 of the chamber 54. Each
counterbalancing mass 60, 70 also has an enlarged portion 68, 78 on one side of the
inner portion 64, 74 for reasons which will be explained below.
Shown in Figures 3 and 4 are the means by which the counterbalancing masses 60, 70
are restrained at speeds below the critical speed of the system in which they are used, ie.
the tub 14 as it is mounted in the washing machine 10. The restraining means comprise
a moveable latch 80 which is mounted on one of the counterbalancing masses 60. The
latch 80 is positioned on the enlarged portion 68 of the counterbalancing mass 60 and
on the side face thereof adjacent the other counterbalancing mass 70 so that the latch 80
lies in the same plane as the other counterbalancing mass 70. The latch 80 is rotatably
mounted about an axis 82 and has a head portion 84 which is urged in an anticlockwise
direction, as indicated by arrow A in Figure 4, by a torsion spring 86. One end 86a of
the spring 86 is seated in a recess in the latch and the other end 86b is seated in the side
face of the counterbalancing mass 60. The other counterbalancing mass 70 includes a
recess 88 which is formed in the inner portion 74 adjacent the enlarged portion 78. The
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recess 88 is shaped so as to receive the head portion 84 of the latch 80. The enlarged
portion 78 extends radially outwardly beyond the radially outer edge 75 of the inner
portion 74 for reasons which will be explained below.
The shape and mass of the latch 80 and the characteristics of the spring 86 are selected
so that, at a predetermined speed of rotation of the counterbalancing masses 60, 70, the
head portion 84 of the latch 80 will move radially outwards against the bias of the
spring 86 about the axis 82. The predetermined speed of rotation at which this will
happen is selected to be above the critical speed of the system.
The operation of the automatic balancing device 50 will now be described in the context
of a washing machine. When the drum 16 of the washing machine 10 is rotating at
speeds below the critical speed of the system, so in normal washing or rinsing mode, the
wall 52 of the chamber 54 will rotate at relatively slow speeds about the axis 18. If the
counterbalancing masses 60, 70 are not already latched together, the counterbalancing
masses 60, 70 will oscillate gently with respect to one another until the head portion 84
of the latch 80 becomes aligned with the recess 88. The head portion 84 will then drop
into the recess 88 under the influence of the spring 86. The counterbalancing masses
60, 70 then become latched together so that they cannot move with respect to one
another although the latched masses 60, 70 can still rotate together about the axis 18.
When the counterbalancing masses 60, 70 are latched together, as shown, in Figure 3,
their respective centres of mass 62, 72 are held at a fixed distance from one another so
that the balancing forces FB generated by the rotation of the counterbalancing masses
60, 70 about the axis 18 act in directions which are at a fixed angle a to one another. In
this embodiment, the angle a is substantially 160° but this angle can be varied between
as little as 140° and as much as 175°. What is important is that the balancing forces FB
generated by the rotation of the counterbalancing masses 60, 70 combine to produce a
resultant balancing force FR which is non-zero in magnitude. The resultant balancing
force FR has a constant magnitude which is smaller than the magnitude of either of the
balancing forces FB. However, although the counterbalancing masses 60, 70 are latched
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together, they are still able to rotate about the axis 18. Hence the resultant balancing
force FR is also able to rotate about the axis 18.
The resultant balancing force FR has been found to be effective in partially
counterbalancing the out-of-balance mass present in the drum 16 at speeds below the
critical speed of the washing machine system. Whilst full counterbalancing is not
possible in many cases, primarily because the out-of-balance mass is too great to be
counterbalanced by the comparatively small resultant balancing force FR, it is still
possible to achieve partial counterbalancing which reduces the maximum excursion of
the tub 14 as the speed of rotation of the drum 16 increases. Indeed, as the speed of
rotation of the drum 14 approaches the critical speed, the effect of the resultant
balancing force FR increases and so the benefit to be had also increases.
The benefit of this partial counterbalancing is that, if the maximum excursion of the tub
14 is kept to a minimum, the space provided between the tub 14 and the casing 12 (in
which the excursion of the tub 14 is accommodated) can be reduced. This means that,
for a given size of casing, a larger tub 14 and drum 16 can be provided. This results in
higher peripheral speeds being achievable during spinning cycles and washing machines
being able to handle larger out-of-balance loads.
When the counterbalancing masses 60, 70 are latched together as shown in Figure 3, the
rotational speed of the drum 16 can be increased through the critical speed of the
system. The maximum excursion of the tub 14 is kept to a minimum by retaining the
counterbalancing masses 60, 70 in the latched configuration. When the drum 16 has
accelerated through the critical speed to an above-critical speed, the counterbalancing
masses 60, 70 must be released so that full counterbalancing of the out-of-balance mass
in the drum 16 can be achieved. As has been explained above, the shape and mass of
the latch 80, and the characteristics of the spring 86, have been chosen so that, at a
speed above the critical speed of the system, the head portion 84 will move radially
outwardly against the bias of the spring 86 under centrifugal forces. The head portion
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84 thus becomes disengaged from the recess 88 and the counterbalancing masses 60, 70
are thus free to rotate with respect to one another.
In the configuration shown in Figure 5, the head portion 84 of the latch 80 is completely
disengaged from the recess 88. The counterbalancing masses 60, 70 are free to take up
positions in which the out-of-balance mass in the drum 16 is completely
counterbalanced, in the same way as has been achieved in many prior art devices. The
position of the enlarged portion 68 of the counterbalancing mass 60 (on which the latch
80 is mounted) is such that the inner portion 74 of the counterbalancing mass 70 does
not come into contact with any part of the latch 80. However, the shape of the
remainder of the counterbalancing mass 70 does provide limits to the relative movement
between the counterbalancing masses 60, 70 and the extremes of movement are shown
in Figures 6 and 7.
In Figure 6, the counterbalancing masses 60, 70 are positioned diametrically opposite
one another. The balancing forces FB act in opposite directions so that no resultant
balancing force is produced. The minimum resultant balancing force is therefore zero in
this embodiment. In this position, the latch 80 abuts against the enlarged portion 78 of
the counterbalancing mass 70. In Figure 7, the latch 80 abuts against the edge of the
outer portion 76 and the counterbalancing masses 60, 70 lie substantially side by side.
The balancing forces FB generated by the rotation of the counterbalancing masses 60, 70
are substantially aligned and thus the resultant balancing force is at its maximum
possible value of 2 x FB.
At these extremes of rotational movement, the resultant balancing force FR is at its
minimum and maximum respectively. The concept behind the invention resides in that,
at sub-critical speeds, the counterbalancing masses 60, 70 are held fixed with respect to
one another so that the resultant balancing force FR is not zero (as has been the case with
all the known prior art) but is not allowed to vary substantially in magnitude. The
resultant balancing force FR is allowed to rotate about the axis 18 so that partial
counterbalancing of the out-of-balance mass present in the drum 16 can be achieved.
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Ideally, the resultant balancing force FR is held at a fixed value which is between the
minimum value achievable by the freely-rotatable counterbalancing masses 60, 70 (as
shown in Figure 6) and the maximum achievable value (as shown in Figure 7). Ideally,
the resultant balancing force FR is held at between 5% and 35% of the maximum
achievable value and tests have shown that holding the resultant balancing force FR at
between 15% and 20% is particularly advantageous in the context of a washing
machine. In the embodiment shown in detail in Figures 2 to 7, the angle a can be
selected according to the application in which the device 50 is to be used. It is believed
that the angle a should be selected so that the magnitude of the resultant balancing force
FR should be approximately one third of the largest expected out-of-balance mass
present in the rotating body. Angles of between 140° and 175° are expected to give
good results in most applications, In the application of a washing machine, angles of
between 155° and 165° appear to be favourable and 160° has been found to be
particularly effective.
Whilst the drum 16 is rotating at speeds above the critical speed (ie. during the spinning
cycles), the latch 80 remains in the position shown in Figures 5 to 7. Counterbalancing
of the out-of-balance mass in the drum 16 is achieved as normal. When the rotational
speed of the drum 16 drops below the predetermined speed at which the latch 80
disengages from the recess 88, the head portion 84 moves inwardly under the action of
the spring 86 until it touches the radially outer edge 75 of the inner portion 74 of the
counterbalancing mass 70. If the counterbalancing masses 60, 70 are rotating with
respect to one another, the head portion 84 will slide over the radially outer edge 75 of
the inner portion 74 of the counterbalancing mass 70 until the head portion 84 becomes
aligned with the recess 88. The head portion 84 then drops into the recess 88
whereupon the counterbalancing masses 60,70 become re-latched in the position shown
in Figure 3. The counterbalancing masses 60, 70 will then remain latched together in
this position until the rotational speed of the drum 16 exceeds the speed at which the
latch 80 has been designed to become released from the recess 88. However, it is not
important that the counterbalancing masses 60, 70 are latched together during the
washing and rinsing cycles: it is only essential that the counterbalancing masses 60, 70
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are latched together as the speed of rotation of the drum 16 increases towards the critical
speed of the system so that the maximum excursion is minimized as the drum 16
accelerates through the critical speed.
A second embodiment of the invention is shown in Figures 8 to 10b. In this second
embodiment, the automatic balancing device 150 again comprises a wall 152 which
defines a cylindrical chamber 154. A viscous fluid (not shown) is provided in the
chamber 154 to provide viscous coupling between the wall 152 and the
counterbalancing masses 160, 170, 190. These counterbalancing masses 160, 170 are
again supported next to one another on an axle 156 so as to be freely rotatable about the
axis 118, which is again concentric with the drum of the washing machine in which the
device 150 is used.
The counterbalancing masses 160, 170 are generally semicircular in front view, as can
be seen from Figure 8. Their centres of mass 162,172 are located at a distance from the
axis 118 as before. As each counterbalancing mass 160,170 rotates about the axis 118,
a balancing force FB1 is generated, the balancing force FB1 acting in a direction which
passes through the respective centre of mass 162,172.
A third counterbalancing mass 190 is also provided in the chamber 154. This third
counterbalancing mass 190 is also freely rotatably mounted about the axle 156. The
third counterbalancing mass 190 is smaller and less massive than the counterbalancing
masses 160, 170, but it also generates a balancing force Fb1 as it rotates about the axis
118. A maximum resultant balancing force will be produced when the balancing forces
FB1, Fb1 generated by each counterbalancing mass 160, 170 190 are aligned. The
counterbalancing masses 160, 170, 190 are also able to adopt positions relative to one
another such that there is no resultant balancing force.
When all three counterbalancing masses 160, 170, 190 are unrestrained and the device
150 is rotating at speeds above the critical speed of the system, they will assume
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positions about the axis 118 which will counterbalance any out-of-balance mass present
in the drum of the washing machine, in a known manner.
However, at speeds below the critical speed, it is necessary for at least one of the
counterbalancing masses 160, 170, 190 to be restrained so that a non-zero resultant
balancing force, which is able to rotate about the axis 118, is produced. This is
achieved by the provision of catches 180 on the counterbalancing masses 160, 170
which, at sub-critical speeds, prevent relative rotation therebetween so that no resultant
balancing force is produced by the two larger counterbalancing masses 160, 170. In the
embodiment shown, one catch 180 is provided on each of the counterbalancing masses
160, 170 as shown in Figure 8. The catch 180 itself is shown in more detail in Figures
9a and 9b and its operation is illustrated in Figures 10a and 10b.
Each catch 180 is located on an edge face 164, 174 of the respective counterbalancing
mass 160, 170 close to the radially outermost edge 166, 176 thereof. The catch 180 is
pivotably mounted on the counterbalancing mass 160, 170 by a pin 182 which is
eccentrically positioned in the catch 180. The catch 180 is dimensioned so that the
breadth b of the catch 180 is not greater than the axial depth d of the counterbalancing
mass 160, 170. It is also dimensioned and positioned so that, when the catch 180 lies
along the edge face 164, 174 of the respective counterbalancing mass 160, 170, the
distal end 184 of the catch 180 does not protrude beyond the outermost edge 166,176 of
the counterbalancing mass 160,170.
Each catch 180 is biased under the action of a spring (not shown) similar to that
illustrated in Figures 3 and 4. The direction of bias is illustrated in Figure 9a by arrow
B. At speeds of rotation below the critical speed of the system, the action of the spring
urges the catch 180 in the direction illustrated so that the catch 180 projects beyond the
front or rear surface of the respective counterbalancing mass 160, 170. However, the
shape and mass of the catch 180 and the characteristics of the spring are selected so that,
at a predetermined speed of rotation, which is not less than the critical speed of the
system, the centrifugal forces acting on the catch 180 will cause it to move against the
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action of the spring about the pin 182 in a direction illustrated by arrow C in Figure 9b.
This will bring the catch 180 into a position in which it is aligned with the edge face
164,174 of the counterbalancing mass 160,170 and does not project beyond the surface
thereof. At no time does either catch 180 interfere with the free rotational movement of
the third counterbalancing mass 190.
The catches 180 operate in the following manner. At speeds of rotation below the
critical speed of the system, the catches 180 will be urged, under the action of the
spring, towards the position shown in Figure 9a. If the counterbalancing masses 160,
170 are in an overlapping position, the distal end 184 of each catch 180 will rest on and
slide over the facing surface of the opposite counterbalancing mass 160, 170. As soon
as the counterbalancing masses 160, 170 come into the position shown in Figure 8, the
catches 180 will move into the positions shown in Figure 10a so that relative rotation
between the counterbalancing masses 160, 170 is prevented. In this position, the
balancing forces FB1 generated by the rotation of the counterbalancing masses 160, 170
will be equal and opposite and thus there will be no resultant balancing force produced
by the two counterbalancing masses 160,170.
However, the third counterbalancing mass 190 remains unrestrained and able to rotate
about the axis 118. The total resultant balancing force produced when the catches 180
are in operation is thus equal to the balancing force Fb1 described above and is freely
rotatable about the axis 118. By selecting the shape and mass of the third
counterbalancing mass 190, this balancing force can be selected to be less than either of
the balancing forces FB1 generated by the counterbalancing masses 160, 170. Ideally, it
is selected to have a magnitude of less than one half, preferably approximately one
third, of the maximum expected out-of-balance mass in the drum of the washing
machine in which the device 150 is to be used. This ensures that the out-of-balance
mass will be at least partially counterbalanced at speeds below the critical speed of the
system. This is highly advantageous in that the maximum excursion of the drum is kept
to a minimum as the drum approaches the critical speed of the system.
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Once the drum has passed through the critical speed of the system, the counterbalancing
masses 160, 170 must be released to allow them to counterbalance the out-of-balance
mass in the drum. This is achieved, as has been described, by selecting the shape and
mass of the catches 180 and the characteristics of the spring to allow the catches 180 to
rotate about the pins 182 at a predetermined speed which is above the critical speed. At
that speed, the catches 180 move to the positions shown in Figure 10b so that neither
counterbalancing mass 160, 170 is restrained any longer. The three counterbalancing
masses 160, 170, 190 are thus able to adopt positions which achieve the desired
counterbalancing effect at high speeds.
As with the previous embodiment, it is not essential that the catches 180 are operative at
all lower speeds of rotation. However, as the speed of the device 150 drops below that
at which the catches 180 move to the position shown in Figure 10b, it is likely that the
counterbalancing masses 160,170 will at some stage adopt the position shown in Figure
8. At that time, the catches 180 will move back into the positions shown in Figure 10a
under the action of the springs and the counterbalancing masses 160, 170 will again
become restrained.
The third embodiment, which is illustrated in Figure 11, is a variation on the second
embodiment described above and includes many of the same features. The automatic
balancing device 150a has a chamber 154a in which two counterbalancing masses 160a
and 170a are mounted about an axis 118a. The arrangement is the same as mat shown
in Figure 8, except that no third counterbalancing mass is provided in the arrangement
of Figure 11. Furthermore, the second counterbalancing mass 170a is formed so as to
have three large holes 171 therethrough. This means that the mass of the second
counterbalancing mass 170a is significantly less than that of the first counterbalancing
mass 160a.
The automatic balancing device 150a operates in a manner which is very similar to that
in which the device 50 shown in Figures 1 to 7 operates. At speeds below the critical
speed, the latches 180a restrain the movement of the counterbalancing masses 160a,
18

170a relative to one another. At these speeds, because the masses of the
counterbalancing masses 160a, 170a are different, a resultant balancing force will be
produced even though the counterbalancing masses 160a, 170a are latched in a
diametrically opposed position. The magnitude of this resultant balancing force will
remain constant because the counterbalancing masses 160a, 170a cannot move relative
to one another, but it is free to rotate about the axis 118a because the counterbalancing
masses 160a, 170a can also rotate together about the axis 118a. However, the size and
position of the holes 171 can be selected so that the criteria mentioned above are
fulfilled; ie. the resultant balancing force when the counterbalancing masses 160a, 170a
are latched together is between 5% and 35%, preferably between 15% and 20%, of the
maximum achieveable resultant balancing force.
When the device 150a achieves a speed above the critical speed of the system in which
it is used, and the catches 180a move to their inoperative position as described above in
relation to the second embodiment, the counterbalancing masses 160a, 170a are free to
adopt positions in which the out-of-balance mass in the rotating body of the system is
counterbalanced. Unlike the first and second embodiments described above, the
different masses of the counterbalancing masses 160a, 170a mean that, in the event that
there is no out-of-balance mass present in the rotating body, some resultant balancing
force will always remain. In the application of a washing machine, it is extremely
unlikely that there will be no out-of-balance mass present in the drum and so an
embodiment of this sort has application in washing machines.
A fourth embodiment of the invention is illustrated in Figure 12. In this embodiment,
the automatic balancing device 250 comprises two separate, annular ballraces 260, 270
which are arranged to be concentric with the axis 218 about which the drum, or other
rotating body in which the out-of-balance mass to be counterbalanced is located, rotates.
The first ballrace 260 is of the type which is known in the art. It comprises an annular
race 262 in which a plurality of identical balancing balls 264 are located. A viscous
fluid such as oil (not shown) provides viscous coupling between the wall of the race 262
and the balls 264. The balls 264 are dimensioned so that, when they lie adjacent one
19

another, they occupy less than half of the race 262 so as to maximize their balancing
effect. A mechanism (not shown), which is operative at speeds below the critical speed
of the system in which the device 250 is used, is provided for fixing the balls 264 at
equispaced positions around the race 262. When the balls 264 are held in those
positions, they are balanced about the axis 218 and no resultant balancing force is
produced. An example of a suitable mechanism for retaining the balls 264 in the
predetermined positions (as shown in Figure 12) is shown and described in US 5 813
253. Other suitable mechanisms will be apparent to a skilled reader.
The second ballrace 270 has a very simple construction. It consists of a simple annular
race 272 in which a single ball 274 is located. No mechanism is provided for fixing the
ball 274 in any given position. Viscous coupling is again provided by a viscous fluid
such as oil.
In operation, and when the device 250 is rotating at speeds above the critical speed of
the system, the mechanism by means of which the balls 264 are held in their fixed
positions about the axis 218 is inoperative. The balls 264, as well as the ball 274, are
free to adopt positions within their respective races 262, 272 in which the out-of-balance
mass present in the drum or other rotating body is counterbalanced in a known manner.
However, when the device 250 drops to a speed at which the mechanism becomes
operative, the balls 264 in the outer race 262 will become fixed in their predetermined,
balanced positions. In these positions, no resultant balancing force is produced by the
balls 264.
Because the ball 274 is not restricted in any way, it remains free to move about the axis
218. The balancing force FB2, which is the balancing force generated solely by the ball
274, is now the only balancing force which has any effect and so is equal to the resultant
balancing force of the device 250. This resultant balancing force can be selected to be
equal to as much as half of the maximum resultant balancing force produced when the
balls 264 are all located adjacent one another by appropriate selection of the size and
mass of the ball 274.
20

Because there is only one ball 274 present in the ballrace 270, there must be a resultant
balancing force of constant magnitude produced when the device 250 is rotated. If
more than one ball were present in the ballrace 270, it would be possible for those balls
to adopt a balanced arrangement which would result in no resultant being produced, or
for the resultant balancing force to be variable. The concept behind the invention is to
provide a constant resultant balancing force which is moveable about the axis 218
which is achieved by the arrangement shown in Figure 12.
At speeds below the speed at which the restraining mechanism becomes operative, the
resultant balancing force FB2 is used to partially counterbalance the out-of-balance mass
present in the rotating body in which the device 250 is used. As the speed of the device
250 then increases towards the critical speed of the system, the maximum excursion of
the body is kept to a minimum by virtue of the partial counterbalancing. When the
rotating body has accelerated to a speed above the critical speed of the system, the
mechanism is released to allow the balls 264 to contribute to the counterbalancing effect
and so provide effective counterbalancing of a wide range of out-of-balance masses.
The previously described embodiments are all primarily suitable for use with bodies
which rotate about a horizontal (or substantially horizontal) axis, although they could
also be used in machines having a substantially vertical axis. The fifth embodiment,
which is illustrated in Figures 13a, 13b, 14a and 14b, is however well suited for use
with a body which rotates about a vertical (or substantially vertical) axis. In the
embodiment, the device 350 consists of a support surface 360 which is mounted
concentrically with the axis 318 about which the body in which the out-of-balance mass
to be counterbalanced is present. The support surface 360 comprises a circular central
portion 362 surrounded by a cylindrical lip 364. An inclined portion 366 extends
upwardly and outwardly from the upper edge of the lip 364 to a cylindrical wall 368 and
an overhanging lip 370. The uppermost part of the inclined portion, the cylindrical wall
368 and the overhanging lip 370 combine to form an annular race 372.
21

A plurality of balancing balls 374 are provided on the upper surface of the support
surface 360. In the embodiment shown, sixteen balls 374 are provided. All of the balls
374 have the same diameter. The diameter of the balls 374 is chosen so that, when the
balls 374 are arranged at the outermost extremity of the central portion 362, ie. abutting
against the lip 362, then the balls 374 fit around the circumference of the central portion
without play, as shown in Figure 14a. The balls 374 are also dimensioned so that they
will fit into the annular race 372 in a manner which allows them to roll therein. The
height of the lip 364 is chosen so as to be slightly less than the radius of the balls 374
for reasons which will be explained below.
Three of the balls 374 are manufactured from a material which is significantly lighter
than the material from which the other balls 374 are manufactured. The number of balls
which are so manufactured can be varied but only within certain limits. It is acceptable
for only one of the balls 374 to be lightweight but, if more than one of the balls is a
lightweight ball, the number of lightweight balls must not be a factor of the total number
of balls. The reasons for this will become clear as the operation of the device 350 is
explained.
When the device 350 is rotating at low speeds, the balls drop downwards under the
influence of gravity and fall into the central portion 362, as shown in Figures 14a and
14b. As has been explained, the balls 374 fit snugly around the outer part of the central
portion 362 and so are prevented from moving with respect to one another as the device
350 rotates. If all the balls 374 were of the same mass, no resultant balancing force
would be produced because the individual balancing forces would all be equidistantly
spaced about the axis. However, because three of the balls 374 are substantially lighter
than the other, a resultant balancing force is produced. Its magnitude will depend upon
the position of the lightweight balls, which is not controlled. It will be greatest when
the three lightweight balls lie next to one another and least when they are as close to
being equidistantly spaced as the geometry of the arrangement will allow.
22

If the number of lightweight balls is greater than one and a factor of the total number of
balls 374, there is a possibility that the lightweight balls will position themselves so as
to be equispaced about the axis 318. This would produce no resultant balancing force
and so is not permitted (unless the mass of each lightweight ball were different: from the
other lightweight balls).
In this configuration, and at speeds below the critical speed, the resultant balancing
force is used to partially counterbalance the out-of-balance mass in the rotating body.
As the speed of rotation increases and approaches the critical speed, the
counterbalancing effect of the device 350 increases. The maximum excursion of the
rotating body is thus minimized at the most crucial point.
As the body passes through the critical speed, the centrifugal forces acting on the balls
374 increases to such an extent that the balls 374 ride over the lip 364 and onto the
inclined portion 366. This is only possible if the height of the lip 364 is less than the
radius of the balls 374 although the height of the lip 364 must be sufficient to maintain
the balls 374 in the central portion 362 at speeds below the critical speed. The balls 374
then travel upwardly across the inclined portion 366 to the annular race 372 in which
there are no restraints on any of the balls 374. At these high speeds, the balls are free to
adopt positions in which the out-of-balance mass in the rotating body is
counterbalanced.
It will be appreciated that, as the speed of the rotating body slows to below-critical
speeds, the balls 374 descend across the inclined portion 366 and fall back into the
central portion 362. The positions in which the lightweight balls appear when the balls
return to the central portion 362 may not be the same as the positions in which they
appeared the previous time the balls 374 were located in the central portion but that
does not matter. As long as the balls 374 are not equispaced about the axis 318, a
constant resultant balancing force will still be produced.
23

A sixth embodiment of the invention is shown in Figures 15a to 15c. In this
embodiment, the automatic balancing device 450 again comprises a wall 452 which
defines a cylindrical chamber 454. A viscous fluid (not shown) is provided in the
chamber 454 to provide viscous coupling between the wall 452 and the
counterbalancing masses 460, 470. The counterbalancing masses 460, 470 are
supported next to one another on an axle 456 so as to be freely rotatable about the axis
458, which is concentric with the drum of the washing machine or other dynamic
system in which the device 450 is used.
At speeds below the critical speed, the counterbalancing masses 460, 470 are restrained
so that a non-zero resultant balancing force FR, which is freely movable about the axis
458, is produced. This is achieved by the provision of a catch 474 on the
counterbalancing mass 470 which, at speeds below the critical speed, is received by a
notch 464 on the other counterbalancing mass 460. The catch 474 is shown located in
the notch 464 in Figures 15a to 15c.
The catch 474 is positioned close to an outer circumferential edge 476 of the
counterbalancing mass 470. This allows the catch 474 to be at least partially submerged
in the viscous fluid at all speeds of rotation. This reduces noise and wear on the catch
474 and the counterbalancing masses 460,470. The catch 474 is pivotably mounted on a
pin 474a which extends from an edge face 478 of the counterbalancing mass 470 in a
substantially circumferential direction. Attached to the pin 474a is a spring 474b. The
spring 474b applies a biasing force to the catch 474 which urges the catch 474 towards
the axis 458.
The catch 474 operates in the following manner. At speeds of rotation below the critical
speed of the system, the catch 474 will be urged towards the axis 458, as described.
When the counterbalancing mass 460 is moving in an anti-clockwise direction relative
to the counterbalancing mass 470 (see the arrow 480 shown in Figure 15a), the
counterbalancing masses 460,470 will become oriented such that a ramp portion 466 of
counterbalancing the counterbalancing mass 460 is adjacent to the catch 474. The catch
24

474 will be displaced by the ramp portion 466 in a direction away from the axis 458. As
the counterbalancing masses 460, 470 continue to move relative to one another, the
catch 474 will contact an abutment surface 468 and become trapped in the notch 464. In
this position, relative rotation between the counterbalancing masses 460, 470 will be
prevented and the balancing forces FB3 generated by the rotation of the counterbalancing
masses 460,470 will combine to give a fixed resultant balancing force FR.
As discussed above, the catch 474 is able to engage with the notch 464 if the
counterbalancing mass 460 is moving in an anti-clockwise direction relative to the
counterbalancing mass 470. However, the catch 474 is also able to engage with the
notch 464 when the counterbalancing mass 460 is moving in a clockwise direction
relative to the counterbalancing mass 470, provided that the relative speed of rotation
between the counterbalancing masses 460, 470 is low. At higher speeds, the catch 474
will not engage with the notch 464 and the counterbalancing masses 460, 470 will
continue to move relative to one another until the relative speed is lower.
The unlocking of the counterbalancing masses 460, 470 is achieved in the following
way. The shape and mass of the catch 474 and the characteristics of the spring 474b are
selected such that, at or above a pre-determined speed which is greater than the critical
speed, the centrifugal forces acting on the catch 474 are sufficient to overcome the
biasing force of the spring 474b. This allows the catch 474 to pivot about the pin 474a
and move radially outwards to a position where it is not located in from the notch 464.
The counterbalancing masses 460, 470 are then free to assume positions about the axis
458 which will counterbalance any out-of-balance mass present in the drum of the
washing machine (or other dynamic system) in a manner similar to the previous
embodiments.
The invention is not limited to the precise details of the embodiment described above, as
will be apparent to and appreciated by the skilled reader. Variations and modifications
are intended to be encompassed by the scope of the claims. For example, in the
embodiments illustrated, the restraining means (the latch 80 of the first embodiment, the
25

catches 180, 180a of the second and third embodiments, the non-illustrated restraining
means of the fourth embodiment, the cylindrical lip 364 of the fifth embodiment and the
catch 474 of the sixth embodiment) are designed to hold the relevant counterbalancing
masses in fixed positions relative to one another. However, it is to be understood that
some play can be allowed between the restraining means and the counterbalancing
masses whilst still maintaining a beneficial effect. In the first embodiment, the recess
88 can be made larger in the circumferential direction than the depth of the head portion
84. This will allow some relative movement between the counterbalancing masses 60,
70 whilst the restraining means (latch 80) is operative. This movement can be as much
as several degrees. Similarly, in the second and third embodiments, a certain amount of
play can be allowed between the catches 180,180a and the edge faces 164, 174 of the
relevant counterbalancing masses 160, 170; 160a, 170a and, in the fifth embodiment,
play can be allowed between the balls 364 when they are positioned at the outermost
part of the central portion 362 and against the cylindrical lip 364. In each of these
cases, whilst the magnitude and position of the resultant balancing force produced
whilst the restraining means are operative may vary somewhat, the variation is
insufficient to detract from the benefit achieved by the invention.
Other variations which are intended to fall within the scope of the invention include the
provision of additional counterbalancing masses and counterbalancing masses of
different shapes in the first and second embodiments, alternative latching mechanisms
in the first, second and third embodiments, additional ballraces in the fourth
embodiment, ballraces spaced axially instead of radially in the fourth embodiment, and
different numbers of balls and variations in size in the fifth embodiment.
Two or more of the devices described above can be combined to produce a mechanism
in which a first of the devices is positioned on one side of the rotatable body and a
second of the devices is positioned on the other side of the rotatable body. The devices
are then spaced along the axis about which the body rotates. The devices are coaxial.
The devices are preferably identical but this is not essential. This is advantageous in
that balancing of a wide range of out-of-balance masses present in the rotating body can
26

be counterbalanced effectively, both above and below the critical speeds, without
requiring either automatic balancing device to be particularly large in dimensions or
mass.
27

We Claim:
1. An automatic balancing device for counterbalancing an out-of-balance mass
present in a body which is rotatable about an axis of a dynamic system having a critical
speed, the automatic balancing device comprising a plurality of counterbalancing
masses, each of which is movable in a circular path about the axis so as to generate a
balancing force, the balancing forces combining, in use, to produce a resultant balancing
force which is variable between a minimum value and a maximum value, characterised
in that the automatic balancing device is configured so that, at a first speed of rotation of
the body which is below the critical speed, the movement of at least one of the
counterbalancing masses is restrained so that a substantially constant, non-zero resultant
balancing force is produced, the said resultant balancing force being freely movable
about the axis, and, at a second speed of rotation of the body which is above the critical
speed, the counterbalancing masses are free to adopt a position in which the out-of-
balance mass is counterbalanced.
2. An automatic balancing device as claimed in claim 1, wherein the second speed
of rotation is any speed above a predetermined speed which is higher than the critical
speed.
3. An automatic balancing device as claimed in any one of claims 1, 2 or 3,
wherein the minimum value of the resultant balancing force is zero.
4. An automatic balancing device as claimed in any one of the preceding claims,
wherein, at the first speed of rotation, the resultant balancing force is less than half of
the maximum value of the resultant.
5. An automatic balancing device as claimed in claim 4, wherein, at the first speed
of rotation, the resultant balancing force lies in the range 5% to 35% of the maximum
value of the resultant.
28

6. An automatic balancing device as claimed in claim 5, wherein, at the first speed
of rotation, the resultant balancing force lies in the range 15% to 20% of the maximum
value of the resultant.
7. An automatic balancing device as claimed in any one of the preceding claims,
further comprising restraining means, the restraining means being operative at the first
speed of rotation and inoperative at the second speed of rotation.
8. An automatic balancing device as claimed in claim 7, wherein the restraining
means are movable between an operative position and an inoperative position.
9. An automatic balancing device as claimed in claim 7 or 8, wherein the
restraining means comprise interengaging means which, when operative, limit the
movement of at least one counterbalancing mass relative to at least one other
counterbalancing mass.
10. An automatic balancing device as claimed in claim 9, wherein the
counterbalancing masses are pivotably mounted about the axis and the interengaging
means, when operative, prevent relative movement between at least two
counterbalancing masses whilst permitting pivotal movement about the axis.
11. An automatic balancing device as claimed in claim 10, wherein two
counterbalancing masses are provided and, when the interengaging means are operative,
the angle between the balancing forces generated thereby is between 140° and 175°.
12. An automatic balancing device as claimed in claim 11, wherein the angle
between the said balancing forces is between 155° and 165°.
13. An automatic balancing device as claimed in claim 10, wherein at least three
counterbalancing masses are provided and, when the interengaging means are operative,
all but one of the counterbalancing masses are prevented from moving with respect to
29

one another so that no resultant balancing force is produced, the remaining
counterbalancing mass being freely pivotable about the axis.
14. An automatic balancing device as claimed in claim 13, wherein the remaining
counterbalancing mass generates a balancing force which is smaller than the balancing
force generated by any of the other counterbalancing masses.
15. An automatic balancing device as claimed in any one of claims 10 to 14,
wherein the interengaging means comprise at least one latch or catch which is mounted
on a first of the counterbalancing masses and which interengages with a second of the
counterbalancing masses.
16. An automatic balancing device as claimed in claim 15, wherein the latch or
catch is configured so as to release the second counterbalancing mass at the second
speed of rotation of the body about the axis.
17. An automatic balancing device as claimed in claim 15 or 16, wherein the latch is
located on an outer circumferential edge of the first counterbalancing mass.
18. An automatic balancing device as claimed in claim 9, wherein the
counterbalancing masses comprise a plurality of bodies, a first of the bodies being
located in a first annular race and the remaining bodies being located in a second
annular race, the interengaging means, when operative, acting so as to fix the bodies
located in the second annular race in positions so that no resultant is produced, the first
body being freely movable within the first annular race.
19. An automatic balancing device as claimed in claim 18, wherein the bodies are
spherical balls.
20. An automatic balancing device as claimed in claim 19, wherein the balls in the
second annular race are all the same size and mass.
30

21. An automatic balancing device as claimed in claim 20, wherein, when the
interengaging means are operative, the balls in second annular race are equidistantly
spaced about the axis.
22. An automatic balancing device as claimed in claim 7, wherein the
counterbalancing masses are supported on a support surface having a central portion, an
annular race arranged axially outwardly of the central portion, and an upwardly inclined
portion extending between the central portion and the annular race, the restraining
means comprising a cylindrical lip arranged between the central portion and the
upwardly inclined portion.
23. An automatic balancing device as claimed in claim 22, wherein the
counterbalancing masses comprise a plurality of spherical balls.
24. An automatic balancing device as claimed in claim 23, wherein at least one of
the spherical balls has a reduced mass which is significantly less than that of the
remaining balls.
25. An automatic balancing device as claimed in claim 24, wherein at least two of
the spherical balls have a reduced mass which is significantly less than that of the
remaining balls.
26. An automatic balancing device as claimed in claim 25, wherein the number of
balls having a reduced mass is not a factor of the total number of balls.
27. An automatic balancing device as claimed in any one of claims 23 to 26,
wherein the height of the cylindrical lip is less than the radius of the smallest of the
spherical balls.
31

28. An automatic balancing device as claimed in any one of claims 23 to 27,
wherein all of the spherical balls have the same diameter.
29. An automatic balancing device as claimed in any one of claims 22 to 28,
wherein the counterbalancing masses are dimensioned so that, when arranged
immediately inwardly of the cylindrical lip, the counterbalancing masses form a
continuous circle about the axis with substantially no play.
30. An automatic balancing device substantially as hereinbefore described with
reference to any one of the embodiments shown in the accompanying drawings.
31. A mechanism for counterbalancing an out-of-balance mass present in a body
which is rotatable about an axis, comprising a first automatic balancing device as
claimed in any one of the preceding claims and a second automatic balancing device as
claimed in any one of the preceding claims, the first and second automatic balancing
devices being arranged coaxially but spaced apart from one another along the said axis.
32. A mechanism as claimed in claim 31, wherein the first and second automatic
balancing devices are substantially identical to one another.
33. A mechanism as claimed in claim 31 or 32, wherein the first and second
automatic balancing devices are arranged on either side of the body.
34. A method of counterbalancing an out-of-balance mass present in a body which is
rotatable about an axis, the body being provided with a balancing device having a
plurality of counterbalancing masses, each of which is moveable in a circular path about
the axis, the method comprising the steps of:
(a) rotating the body at a speed which is below the critical speed of the
system of which the body forms a part so that each counterbalancing mass generates a
balancing force;
32

(b) restraining the movement of at least some of the counterbalancing
masses in such a manner that a substantially constant, non-zero resultant balancing force
is produced, the said resultant balancing force being freely moveable about the axis;
(c) increasing the speed of rotation of the body to a speed above the critical
speed of the system of which the body forms a part; and
(d) removing the restraint from the counterbalancing masses.

35. A method as claimed in claim 34, wherein the step of restraining the movement
of at least some of the counterbalancing masses includes connecting all of the
counterbalancing masses to one another to prevent relative movement therebetween
whilst still allowing rotation of the connected counterbalancing masses about the axis.
36. A method as claimed in claim 35, wherein the counterbalancing masses are
connected in a position which produces a resultant balancing force of between 5% and
35% of the maximum possible resultant balancing force.
37. A method as claimed in claim 36, wherein the counterbalancing masses are
connected in a position which produces a resultant balancing force of between 15% and
20% of the maximum possible resultant balancing force.
38. A method as claimed in any one of claims 34 to 37, wherein" the balancing
device is as claimed in any one of claims 1 to 30.
39. A method of counterbalancing an out-of-balance mass present in a body which is
rotatable about an axis, the method being substantially as hereinbefore described with
reference to any one of the embodiments shown in the accompanying drawings.

The invention provides an automatic balancing device (50; 150; 150a; 250; 350) for
counterbalancing an out-of-balance mass present in a body (12) which is rotatable about
an axis (18; 118; 118a; 218; 318), the automatic balancing device (50; 150; 150a, 250;
350) comprising a plurality of counterbalancing masses (60, 70; 160, 170, 190; 160a,
170a; 264, 274; 374), each of which is movable in a circular path about the axis (18,118,
118a, 218, 318) so as to generate a balancing force (FB; FB1, Fb1; FB2). In use, the
balancing forces (FB; FB1, Fb1; FB2) combine to produce a resultant balancing force
(FR) which is variable between a minimum value and a maximum value. The automatic
balancing device (50; 150; 150a; 250; 350) is configured so that, at a first speed of
rotation of the body (12) about the axis (18, 118, 118a, 218, 318), the movement of at
least one of the counterbalancing masses (60, 70; 160, 170, 160a, 170a; 264, 274; 374), is
restrained so that a substantially constant, non-zero resultant balancing force (FR) is
produced, the said resultant balancing force (FR) being freely movable about the axis (18,
118, 118a, 218, 318). At a second speed of rotation of the body (12) about the axis (18,
118, 118a, 218, 318), the counterbalancing masses (60, 70; 160, 170, 190; 160a, 170a;
264, 274; 374) are free to adopt position in which the out-of-balance mass is
counterbalanced. The device (50; 150; 150a; 250; 350) allows at least partial
counterbalancing of the out-of-balance mass at speeds below the critical speed of the
system in which it is used and this reduces the maximum excursion of the body (12)
through the critical speeds.