GB2344697A - Resilient thrust bearing assembly - Google Patents

Abstract

A thrust bearing assembly 30 for an electric motor is mounted in a bearing retainer recess 44 of an end cap 14 behind a sleeve bushing 28. The thrust bearing assembly 30 has a thrust plate 50 supported by a backing sheet 52 which is resiliently deformable to provide the thrust plate with a spring loaded action to bias the thrust plate against the end of the shaft of the motor. Hardenable material 46 may be injected behind the assembly to form a preload.

Description

Electric Motor This invention relates to an electric motor and has particular application to small size permanent magnet electric motors as used, for example, in accessories in vehicles. It is in this context that the invention will be described. The invention also relates to a thrust bearing assembly for use in such a motor.

In a vehicle, accessories and the small electric motors they contain are subject to vibration. This vibration can cause an audible clicking noise to be emitted from the motors. In the past, because vehicle interiors have had a fairly high background noise level, the clicking sound has been tolerated. In recent years, great advances have been made in reducing the background noise level in vehicles, especially in luxury class vehicles. So much so that the clicking noise is now a major cause of annoyance to vehicle occupants. Attempts to quieten the motors by concealing them in sound absorbing chambers or material have not addressed the root cause and have increased the size of the motors in applications where space is critical.

It has been discovered that the clicking noise is caused by the vibrations causing the rotor of the motor to move or slide axially through the bearings, striking the thrust plate or thrust bearing with sufficient force to produce the clicking noise. The sliding or axial movement is permitted by the sleeve bearings and the end play or gap between the shaft and the thrust bearing. In a mass produced motor, end play is needed to ensure that the shaft is free to rotate and does not bind on the thrust plate. Although the gap can be reduced to a very small value, it cannot be eliminated altogether due to manufacturing tolerances and price constraints on mass produced motors.

Accordingly, the present invention seeks to provide an electric motor and a thrust plate assembly therefor which eliminates or at least mitigates the generation of the clicking noise.

Accordingly, the present invention in one aspect thereof, provides an electric motor comprising a stator, sleeve bearings associated with the stator, a rotor having a shaft journalled in the sleeve bearings for rotation about an axis, a thrust bearing supported with respect to the stator and arranged to limit axial movement of the rotor in one direction, wherein the thrust bearing has a rubbing surface for contacting a stop on the shaft and a resilient backing member for urging the rubbing surface into substantially continuous contact with the stop.

According to a second aspect, the present invention provides a thrust bearing assembly for use in an electric motor comprising a bearing member having a rubbing surface for engaging an abutment and a resilient member arranged to urge the rubbing surface into contact with the abutment.

Preferably, the resilient member is fixed to the bearing member as this makes handling easier.

Brief Description of the Drawings Preferred embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure lisa partial sectional view of a small electric motor incorporating a thrust plate assembly in accordance with a first embodiment of the present invention; Figure 2 is an enlarged sectional view of an end cap of the motor of Figure I illustrating the thrust plate assembly; Figure 3 is a sectional view of a part of the end cap of Figure 2 illustrating a thrust plate assembly according to a second embodiment; and Figure 4 is a view of a thrust plate assembly for mounting on a shaft of the motor in accordance with a third embodiment.

Detailed Description of the Preferred Embodiments A miniature permanent magnet d. c. electric motor 10 is shown in Figure 1 in partial section to illustrate its construction. The motor 10 has a casing 12 in the form of a hollow cylindrical can having a closed end and an open end. The open end is closed by an end cap 14 which will be described in more detail hereinafter. The casing 10 has a ring magnet 16 forming a permanent magnet stator. Located within the stator is a rotor 20 comprising a rotor core 22 mounted on a shaft 24 adjacent a commutator 26.

Rotor windings wound around the core and terminated on the commutator as is commonly known, have been omitted for ease of drawing.

The shaft 24 is journalled for rotation in sleeve bushings 28 mounted in the closed end of the casing and in the end cap, respectively, and extends through a hole in the closed end of the casing 12 to engage a load (this is known as the drive end). To limit axial movement of the shaft through the bushing, the non-drive end 25 of the shaft 24 bears against a thrust bearing assembly 30 located behind the bushing 28 of the end cap 14.

This thrust bearing assembly 30 will be described in greater detail later.

The drive end of the shaft has a spacer sleeve 31 located between the rotor 22 core and the bushing 28 of the casing 12. Located between the spacer 31 and the bushing 28 is a fibre washer 32 which functions as a thrust bearing as it provides a low friction sliding surface between the sleeve and a thrust face formed as an abutment on the bushing. Because of its function and its appearance, it is commonly referred to as a thrust washer.

The end cap 14 is shown in more detail in Figure 2. The end cap is of a two part construction comprising a first part 34 forming a brush/terminal support portion of insulating material such as nylon and a second part 36 forming a bearing support portion of metal such as sheet steel.

The first part 34 supports the motor terminals 38 which in turn support the fingerleaf brushes 40 (only one shown) used to electrically connect the associated motor terminals to the commutator 26. The brushes 40 are connected to the motor terminals 38 by upset riveting shown generally at 42.

The second part 36 is a generally flat part with a stamped recess 44 forming a bearing housing or retainer for the bushing 28. Between the bushing 28 and the bottom of the recess 44 is the thrust bearing assembly 30 and a spacer ring 48. The bushing, being a press fit in the recess, retains the thrust bearing assembly 30 and the spacer ring 48.

The first and second parts 34,36 are joined together initially by posts (not shown) formed on the first part and passing through holes in the second part before being deformed to prevent their withdrawal and finally, by being clamped together by the motor casing during assembly.

The thrust bearing assembly 30 comprises a thrust bearing or plate 50 of hard low friction material such as nylon with a backing sheet 52 of elastomer or rubber such as a silicone rubber sheet or neoprene.

The thrust plate 50 contacts directly the end 25 of the motor shaft 24 and the end play is set such that the shaft length between the non-drive end 25 of the shaft and the contact face of the thrust washer 32 is slightly greater than the distance between the thrust face of the drive end bushing 28 and the contact face of the thrust plate 50 with the backing sheet 52 in an uncompressed state so that in use, the backing sheet, due to its compressibility and resilient nature, will enable the thrust plate 50 to remain in contact with the end 25 of the shaft while allowing slight end play or axial movement of the shaft 24 without applying excessive axial force to the shaft which would otherwise significantly increase the friction of the contact between the shaft 24 and the thrust bearings 32,50. In this manner, the shaft 24 is prevented from bouncing on the thrust plate 50 eliminating the audible knocking or clicking noise.

In a second embodiment, as shown in Figure 3, the second part 36 of the end cap has a hole in the bottom of the bearing retainer recess 44. In order to set the initial shaft end play and to set a preload on the thrust plate after the motor has been assembled, a hardenable resin 46 is pumped or injected into the bearing recess 44 through the hole to force the thrust plate 50 into contact with the shaft and to compress the backing sheet 52. The value of the preload is dependent on the injecting pressure of the resin.

Once hardened, the resin 46 provides axial support for the backing sheet/thrust bearing assembly 52,30.

In addition or alternatively, the thrust washer 32 could have or be supported by an elastomeric backing sheet to spring load the thrust washer in a manner similar to the thrust bearing assembly described in the first embodiment.

Thus, as shown in Figure 4, the thrust washer 32 having a central hole 33 for the motor shaft has a backing member 54 of elastomeric material, e. g., neoprene, having a similar annular shape. The backing member 54 is located between the thrust 32 and the spacer 31.

While three embodiments of the invention have been described, other variations will be obvious to those skilled in the art. All such variations are considered part of the invention as defined by the following claims. By working the invention, the audible clicking noise generated by many motors for automotive accessories can be eliminated or at least reduced to an acceptable level.

Claims (10)

1. Claims 1. An electric motor comprising: a stator, sleeve bearings associated with the stator, a rotor having a shaft journalled in the sleeve bearings for rotation about an axis, a thrust bearing supported with respect to the stator and arranged to limit axial movement of the rotor in one direction, wherein the thrust bearing has a rubbing surface for contacting a stop on the shaft and a resilient backing member for urging the rubbing surface into substantially continuous contact with the stop.
2. 2. A motor according to claim 1, wherein the thrust bearing is substantially flat having two planar surfaces, one planar surface being the rubbing surface and the other planar surface being in contact with the resilient backing sheet.
3. 3. A motor according to claim 1 or claim 2, wherein the backing material is of rubber sheet material.
4. 4. A motor according to any one of the preceding claims, wherein the stop is the end of the shaft.
5. 5. A motor according to any one of the preceding claims, wherein the axial gap is set after assembly of the motor by injecting a hardenable material behind the thrust washer and resilient backing member to provide a preload force on the thrust washer by partially resiliently deforming the backing member in the axial direction.
6. 6. A thrust bearing assembly for an electric motor comprising a bearing member having a rubbing surface for engaging an abutment of a motor and a resilient member arranged to urge the rubbing surface into contact with the abutment.
7. 7. A thrust bearing assembly according to claim 6, wherein the resilient member is fixed to the bearing member.
8. 8. A thrust bearing assembly according to claim 6 or claim 7, wherein the resilient member is of rubber sheet material.
9. 9. An electric motor substantially as hereinbefore described with reference to the accompanying drawings.
10. 10. A thrust bearing assembly substantially as hereinbefore described with reference to the accompanying drawings.