GB2201841A - An electric motor thrust bearing basing arrangement - Google Patents

Abstract

The motor, for example a permanent magnet direct current electric motor, has a moving coil armature, i.e. an ironless armature (10). Permanent magnet means are provided to bias one end of a motor shaft (12) into contact with a thrust face (17) to ensure that brush gear (7) follows a consistent track along a commutator (11). In a preferred embodiment a permanent magnet (16) defines the thrust face and the shaft is of high magnetic permeance. Other arrangements are disclosed including repulsion systems biasing non-magnetic thrust bearing together. <IMAGE>

Description

AN ELECTRIC MOTOR This invention relates to an electric motor and particularly but not exclusively to a fractional horsepower pmdc electric motor.

In electric motors equipped with a commutator and brush gear for conducting electricity to the armature of the motor, it is desirable that the brush gear follows a consistent track along the commutator. To achieve this the shaft of the motor should not move in an axial direction during operation. It is therefore common in motors in which the armature includes a steel lamination stack to offset the centre line of the stack from the centre line of the stator magnet field and to rely upon the magnetic forces between the stator and the armature to draw one end of the motor shaft against a thrust face.

However, this solution cannot be used with moving coil armatures because of the absence of iron.

According to the present invention there is provided an electric motor comprising a motor frame, a moving coil armature, including a commutator and a motor shaft, supported for rotation by the motor frame, brush gear supported by the motor frame, a thrust face provided by or mounted with respect to the motor frame, and permanent magnet means for biassing one end of the motor shaft into contact with the thrust face.

Preferably, the permanent magnet means comprises at least one permanent magnet having its magnetic field axially oriented.

In one embodiment of the invention, the motor shaft or a part provided thereon adjacent to the thrust face is of high magnetic permeance and a permanent magnet defines, or is provided adjacent to, the thrust face.

In another embodiment of the invention, a permanent magnet is provided on the shaft adjacent to the thrust face and a part of high magnetic permeance defines, or is provided adjacent to, the thrust face.

In yet another embodiment of the invention, a first permanent magnet is provided at one end of the motor frame and a second permanent magnet is provided on the motor shaft adjacent to the first permanent magnet, the first and second permanent magnets being appropriately magnetized so that the forces of attraction or repulsion therebetween bias one end of the motor shaft into contact with the thrust face.

Advantageously, said one end of the motor shaft is part spherical.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates one embodiment of an electric motor according to the present invention, and Figures 2-4 are fragmentary views illustrating further embodiments of an electric motor according to the present invention.

Referring to Figure 1, the motor shown therein is a fractional horsepower permanent magnet direct current motor having a moving coil armature winding 10 and a commutator 11 mounted fast on a shaft 12 having a part spherical end. The shaft 12 is journalled for rotation in journal bearings 13 supported, respectively, by a can-like motor casing 15 which forms part of the motor frame, and by a stationary central iron core 14 secured to the base of the casing 15. Moving coil armatures, such as described above, are commonly known in the art as ironless armatures, because of their lack of iron in the armature. The motor conventionally also includes two segmental magnets 8 supported by the casing 15 and brush gear 7 supported by a plastics end cap 9, which forms a further part of the motor frame and which closes the open end of the casing 15..

The shaft 12 is of high magnetic permeance and a permanent magnet 16 is mounted in a seat in the plastics end cap 9. The magnet 16 defines a thrust face 17 and has two poles which are aligned with the axis of the motor. The magnet 16 draws the part spherical end of the shaft 12 into contact with its thrust face 17 so as to maintain the shaft 12 and hence the commutator 11 in a fixed axial position relative to the motor frame during operation of the motor.

In the embodiment shown in Figure 2, the thrust face 17 is independent of the magnet which in this case is an annular magnet 18 supported by the end cap 9 and surrounding the part which defines the thrust face 17. This part could as shown be a separate component supported by the end cap 15 or it could be an integral part of the end cap. Again the magnet 18 has two poles aligned with the axis of the motor.

In the embodiment shown in Figure 3, the shaft 12 is of low magnetic permeance and in order that the shaft 12 is drawn towards the thrust face 17 defined by the magnet 16, a washer 19 of high magnetic permeance is provided as an interference fit on the end of the shaft 12 adjacent to the magnet 16. Alternatively, the washer 19 could be in the form of a permanent magnet and the magnet 16 could be replaced by a disc of high magnetic permeance.

In the embodiment shown in Figure 4, the shaft 12 is again of low magnetic permeance, but in this case an annular permanent magnet 20 is provided as an interference fit on the shaft 12. The magnets 16 and 20 are arranged with unlike poles adjacent to one another so that the magnet 20 and hence the shaft 12 is attracted to the magnet 16. The two magnets could, of course, be arranged at the opposite end of the shaft with like poles adjacent so that the repulsion forces between the magnets urge the shaft 12 into contact with a non-magnetic thrust face.

The above embodiments are given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention defined by the appended claims.

Claims (7)

1. An electric motor comprising a motor frame, a moving coil armature, including a commutator and a motor shaft, supported for rotation by the motor frame, brush gear supported by the motor frame, a thrust face provided by or mounted with respect to the motor frame, and permanent magnet means for biassing one end of the motor shaft into contact with the thrust face.

2. An electric motor as claimed in claim 1, wherein the motor shaft or a part provided thereon adjacent to the thrust face is of high magnetic permeance and wherein a permanent magnet defines, or is provided adjacent to, the thrust face.

3. An electric motor as claimed in claim 1, wherein a permanent magnet is provided on the shaft adjacent to the thrust face and wherein a part of high magnetic permeance defines, or is provided adjacent to, the thrust face.

4. An electric motor as claimed in claim 1, wherein a first permanent magnet is provided at one end of the motor frame and a second permanent magnet is provided on the motor shaft adjacent to the first permanent magnet, the first and second permanent magnets being appropriately magnetized so that the forces of attraction or repulsion therebetween bias one end of the motor shaft into contact with the thrust face.

5. An electric motor as claimed in anyone of the preceding claims, wherein said one end of the motor shaft is part spherical.

6. An electric motor substantially as hereinbefore described with reference to the accompanying drawings.

7. An electric motor as claimed in anyone of the preceding claims, the motor being a fractional horsepower p.m.d.c. motor.