Bearing assembly

A bearing assembly particularly suitable for the rotor arrangement of an electrical motor or generator includes a bearing located in a housing. The bearing has an inner race, an outer race, and a plurality of ball bearings between the inner and outer races. A portion of the housing is made up of a deformable flexible wall which permits the housing to conform to the shape of the outer race member, rather than forcing the outer race to conform to the shape of the housing, as was experienced hitherto.

FIELD OF THE INVENTION

This invention relates to a bearing assembly for, for example, the rotor of an electrical machine such as an electrical motor or generator.

BACKGROUND OF THE INVENTION

Electrical motors are widely used for many different applications and are commonly used in domestic appliances. For example, in a vacuum cleaner a motor is used to drive a fan that causes dirty air to be sucked through a dirty air inlet. The dirty air passes through some form of separation device such as a cyclonic or bag separator that separates dirt and dust from the airflow, and finally the air is exhausted from an air outlet.

Switched reluctance machines have become increasingly popular in recent years. In a switched reluctance motor, a stator has sets of poles that are sequentially energised to rotate a rotor into line with the energised pair of poles, under the influence of the magnetic fields associated with each set of poles. By rapidly switching between different pairs of poles, it is possible to cause the rotor to rotate at a very high speed.

A bearing assembly is employed to rotatably support the shaft of the rotor with respect to the stator. A typical bearing assembly comprises a bearing, which comprises a plurality of ball bearings held between an inner race and an outer race, and a housing for supporting the bearing against the shaft of the rotor.

A problem which may be encountered with conventional bearing assemblies is that the pressure of the housing against the outer race of the bearing may cause the bearing to deform.

SUMMARY OF THE INVENTION

The invention provides a bearing assembly comprising a bearing having an inner race and an outer race, the assembly further comprising a housing for the bearing, wherein a portion of the housing comprises a deformable wall.

The invention further provides a housing for a bearing having inner and outer races, wherein a portion of the housing comprises a deformable wall.

The provision of a deformable wall permits the housing to conform to the shape of the outer race member, rather than forcing the outer race to conform to the shape of the housing, as was experienced hitherto.

The deformable wall has a thickness of approximately 0.5 mm, but could have a thickness of between 0.1 mm to 2 mm.

Preferably, the deformable wall is an integral part of the housing. Thus, the housing and wall may be manufactured from a single process, such as die-casting, cold-forging or moulding.

Advantageously, the housing also contains means for supplying lubrication to the bearings, to ensure smooth running of the rotor assembly throughout the lifetime of the bearings.

Advantageously, the housing is made of a thermally conductive material. In conventional bearing assemblies, the bearings can get hot in use and, at very high rotational speeds, may even overheat. The provision of a thermally conductive housing for the bearing permits heat generated by the bearing to be dissipated. Thus, the bearing can be run at higher speeds than hitherto.

The invention is suitable for supporting rotor assemblies comprising a rotor on a shaft. Preferably, the bearing assemblies are located at both ends of the shaft. Resilient means may be provided on the housings to permit the rotor assembly to rotate about its own centre of mass, with little excursion. In conventional rotor assemblies having hard mounted bearing assemblies, the rotor tends to rotate about its geometric centre, which may cause radial stress on the bearings, thereby reducing their lifetime.

The resilient means may take the form of at least one o-ring attached to each housing. Preferably, a pair of o-rings is provided on each housing to equally support the bearing, with one ring attached to each end of each housing.

The invention is applicable to switched reluctance machines, and is particularly useful in such machines that operate at high speeds of, say, 100,000 revolutions per minute.

While the following embodiments describe the invention as applied to motors which are used to drive a fan in a vacuum cleaner, it will be appreciated that the invention can be applied to both motors and generators, for any type of application, and is not limited to vacuum cleaners or the field of domestic appliances.

Like reference numerals refer to like parts throughout the specification.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 to 3show a rotor assembly indicated generally by the reference numeral1. The rotor assembly1comprises a rotor shaft2having a rotor member3. The rotor member3comprises an axially laminated stack of steel plates, arranged to form a pair of poles3a,3b. The shaft2also carries a coaxial impeller4having a plurality of blades5arranged to direct fluid flow from the shaft to the periphery of the impeller in tangential directions. The shaft also carries a pair of balance rings and a position indicator in the form of an optical encoder disc6, to enable the rotational position of the rotor member3to be determined in use.

Bearing assemblies7,8are provided on the shaft2. Each bearing assembly7,8comprises a bearing9,10supported on the shaft2by a housing11,12. The bearings9,10are arranged to press-fit into their respective housings11,12and also to press-fit onto the rotor shaft2. Each bearing9,10comprises an inner race9a,9b, an outer race10,10band a plurality of ball bearings (not shown) held between the races. The bearings9,10permit the rotor3to be rotatably supported in a stator13, such as is shown inFIG. 4.

In accordance with the invention, each bearing housing11,12includes a flexible deformable wall14,15. Each wall14,15comprises a thin metallic portion of the housing11,12and is arranged to be radially deformable. Thus, in use, the wall14,15of the housing11,12tends to conform to the contours of the outer race9b,10bof the respective bearing9,10, with which it is held in close contact.

In previous arrangements with press-fit housings having rigid walls, the housing tended to push against the outer race of the bearing and deform it. It has been proposed to alleviate this problem by manufacturing the housings with greater accuracy, but this has involved more costly manufacturing techniques. Typically, housings are formed by means of cold forging or die casting techniques, neither of which are able to provide the manufacturing tolerances required for accurate push-fit housings. The invention permits the housing to be manufactured using these known techniques. Cold forging is an industrial process in which metal workpieces are squeezed between suitably contoured dies, typically at room temperature, to form a final desired shape. Die-casting is accomplished by forcing molten metal under high pressure into metal dies.

The housings also contain respective reservoirs16,17of fluid, such as grease, which are arranged to provide lubrication to the bearings9,10in use. Typically, the ball bearings are coated with grease that, over time, gets pushed out of the races. The reservoirs16,17of grease supply the ball bearings with lubrication throughout their lifetime.

The bearing housings11,12are supported against the stator13by resilient means18,19. In this embodiment, the resilient means18,19is provided in the form of o-rings18a,18b,19aand19b. Each of the housings11,12carries a pair of o-rings18a,18band19a,19b. The o-rings of each pair are located at positions on the housings that correspond with the end portions of the bearings inside the respective housing. This soft mounting of the rotor shaft against a stator assembly permits the rotor assembly1to find its own centre of rotation in use. Thus, the rotor assembly1rotates about its own centre of mass, with little excursion. The provision of soft-mounted bearing assemblies also provides a reduction in transmitted loads and vibration.

The bearing assemblies7,8are located at the extreme end portions of the rotor shaft2. This feature aids the balancing of the shaft2during assembly.

The stator13comprises a stack of steel laminations arranged to have four inwardly projecting salient poles. Two of the poles13aand13b, diametrically opposite each other, are shown inFIG. 4. Each pole supports a winding20a,20b, which together form a first phase. The other diametrically opposite poles (not shown) similarly accommodate respective windings, which represent a second phase. Each winding comprises a large number of turns (e.g. 50+ turns) of an insulated electrical conductor around the respective stator pole.

The stator13and windings20are encapsulated by plastics material21by means of an injection-moulding process, by which plastic granules are melted, then injected into a mould cavity under pressure to create the required shape. During this process, the aperture22for the rotor assembly and an end cap23for receiving one of the bearing housings11are formed simultaneously.

An optical encoder disc6, or chopper, is disposed on the rotor shaft2. The disc6is associated with an optical sensor arranged to detect the rotation position of the disc and, hence the rotor member3. Signals from the optical sensor are transmitted to a controller (not shown). The encoder disc6has a diameter smaller than that of the rotor member3, which facilitates manufacture of the rotor assembly. During manufacture, the components of the rotor assembly are assembled on the shaft, and the entire rotor assembly is simply slotted into the aperture22provided for the rotor member3, with the housing11abutting the end cap23. Previously, the individual components of the rotor assembly were balanced separately before being incorporated into the motor or generator, produced a less than ideal balance condition of the completed rotor assembly. However, the rotor assembly of the present invention may be completed before final assembly of the motor, so that the complete rotor assembly may be balanced in one operation.

The controller is electrically connected to the drive circuit, to which the windings on each of the stator pole portions are connected. Torque is produced by switching current on in each phase winding in a sequence, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position.

The impeller4rotates with the rotor shaft2and thus draws air into the motor. The bearing assembly8forms a nose cone located at the end of the shaft, upstream of the impeller4. Hence, air being drawn in by the impeller4will firstly flow over the bearing assembly8.

Heat generated by the bearing10is dissipated by the bearing housing12, which is made of thermally conductive material. The airflow over the bearing assembly8serves to cool the bearing housing12.

There is also provided an inlet24for a second airflow for the bearing assembly7at the other end of the shaft. Heat generated by the bearing9is dissipated by the thermally-conductive housing11, which is cooled by the flow of air from the inlet24.

FIG. 5shows one example of a vacuum cleaner30in which the motor may be used. The motor-driven impeller4draws dirty air into the cleaner30via a nozzle31and a hose and wand assembly32. The dirty air enters a separator33, which serves to separate dirt and dust from the dirty air. The separator33can be a cyclonic separator, as shown here, or some other separator, such as a dust bag. Cleaned air leaves the separator33before entering the motor housing located within the main body34of the cleaner. A pre-motor filter is typically placed in the airflow path before the impeller to filter any fine dust particles that were not separated by separator33.

In use, the motor rotates the impeller4at a very high speed (of around 100,000 rpm). The pumping action of the impeller4draws air through the cleaner. The air then flows over the bearing housings and is redirected by the impeller blades5through diffusion outlets25into the scroll26.

A post-motor filter may be placed in the airflow path after the scroll. However, the provision of a brushless motor reduces the requirement for such a filter. The cleaned air is then exhausted from the cleaner to the atmosphere via a suitable outlet.

Variations to the described embodiments will be apparent to a skilled person and are intended to fall within the scope of the invention. For example, the deformable wall need not be an integral part of the housing. The wall could be manufactured from a different material to that of the rest of the housing.

The deformable wall may comprise a portion, or a plurality of portions, of the wall surrounding the bearing.

The provision of a deformable flexible wall provides reduced noise and vibration to rotor arrangements owing to reduced compressive forces on the bearings. The housings also assist in the dissipation of heat generated by the bearings in use.

The invention has been described with reference to bearings in which a plurality of ball bearings provides rotational motion. However, the invention is also applicable to other types of bearings, such as those employing rollers.

The bearing assembly of the invention is equally applicable to the rotor arrangements of motors and generators, not necessarily of the switched reluctance type, and may be employed in appliances other than domestic vacuum cleaners, such as lawn mowers, air conditioners, hand dryers and water pumps.