AUTOMATIC BALANCING DEVICE

The invention relates to an automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body. 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 magnitude of the out-of-balance mass is unpredictable and the speed of rotation is variable. All of the devices shown in these documents make use of the phenomenon by means of which, at speeds of rotation above the critical speed, freely rotating counterbalancing masses automatically take up positions in which the out-of-balance mass is counterbalanced. In each of the aforementioned arrangements, the freely rotating counterbalancing masses are constrained to move in a circular path, either by being mounted about an axle which is concentric with the axis of rotation of the rotating body, or by being located in a circular race. These known arrangements all have various disadvantages. In arrangements wherein the counterbalancing masses are mounted centrally, the centre of mass of each counterbalancing mass is normally located radially closer to the axis of rotation than the centre of mass of the out-of-balance mass. In order to provide sufficient counterbalancing capability, the mass of the counterbalancing masses must be relatively large. This increases the overall weight of the machine in which the device is utilised and can add to the cost of manufacture as well. In arrangements in which the counterbalancing masses are arranged to run in a race, the race must be manufactured to very high tolerance levels which also increases manufacturing costs. Such arrangements are also commonly very noisy as a result of collisions between the counterbalancing masses in the race. Finally, due to the high contact pressures applied between the components of the known arrangements, high strength materials such as steels must be used, which adds to manufacturing costs and the overall mass of the machine. Out-of-balance masses present in rotating bodies can also be balanced to some extent using fluid balancing rings in which a volume of fluid is constrained to move in an annular channel which is caused to rotate with the body. Such systems have been used to balance the wheels of vehicles and in other applications. However, fluid balancing rings have a limited capacity to balance an out-of-balance load due to the fact that, at high speeds, centrifugal forces tending to disperse the fluid evenly around the channel overcome the forces urging the fluid into the position in which the out-of-balance mass is counterbalanced. Fluid balancing rings are generally capable of counterbalancing approximately 60% of an out-of-balance load. It is an object of the invention to provide an automatic balancing device in which some or all of the disadvantages of the prior art are reduced or eliminated. It is a further object of the invention to provide an automatic balancing device which is effective in counterbalancing an out-of-balance mass without adding significantly to the overall mass of the product in which it is utilised. It is a further object of the invention to provide an automatic balancing device which is economical to produce in comparison to prior art devices. The invention provides an automatic balancing device comprising a cavity containing a volume of fluid and a plurality of freely-movable bodies in contact with the fluid, wherein the density of each of the freely-movable bodies is less than the density of the fluid. An arrangement in which the density of the freely-movable bodies is less than the density of the fluid in the cavity allows the mass of the fluid itself to be used to counterbalance the out-of-balance mass. This is in complete contrast to the known balancing devices which use the mass of the freely rotating bodies to counterbalance the out-of-balance mass. In the arrangement of the present invention, the lightweight bodies are used to displace a fixed volume of the fluid and so create a distribution of the fluid within the cavity which is unevenly spread about the axis of rotation of the out-of-balance mass. The balancing forces created by the presence of an excess of fluid in one area of the cavity provide the counterbalance to the out-of-balance mass. This arrangement is also highly advantageous over known fluid balancing rings because the presence of the lightweight bodies reduces the influence of the centrifugal forces on the distribution of the fluid within the channel. This provides an automatic balancing device having a balancing capability far greater than that which can be achieved by known fluid balancing rings. Another advantage of the arrangement of the present invention is that the contact pressures developed in the arrangement are lower than those developed in many of the prior art devices mentioned above. This means that the materials from which the components of the automatic balancing device of the present invention is made do not need to be of such high strength as has previously been the case. This can have a significant cost benefit. Also, the manufacturing methods used to form these components may be different and this too may lead to reductions in costs. A further advantage of a device according to the present invention is that there is little noise generated by collisions between the low-density bodies