A sheet metal bearing for an electric motor

Abstract: A self-aligning bearing for a fractional-horsepower PMDC electric motor is formed from sheet metal coated with polytetrafluoroethylene to facilitate intermittent, short-term operation. A tubular first part (1) provides a journal-bearing surface (4). A second part (5) extends radially from a first end (2) of the first part (1) to provide radial support for the first part (1). A third part (8) extends axially from a radially outer second end (7) of the second part (5) so as to surround the first part (1) and is provided with a part-spherical outer mounting surface (10) for engagement with a bearing support (9).

Description: Field of the invention The invention relates to bearings for fractional-horsepower PMDC electric motors having shafts of, typically, 2 to 4 mm diameter. These bearings can be produced from sheet-metal and then coated on their rotary load bearing surfaces with friction-reducing material. Bearings such as these are particularly, but not exclusively, suitable for use as self-aligning bearings. Background Art A bearing for a fractional-horsepower PMDC electric motor conventionally comprises a first part having axially spaced first and second ends and providing a journal-bearing surface, a second part having radially inner and outer first and second ends which is connected to the first part and provides radial support for the first part, and a third part for engagement with a bearing support. Thus, for a self-aligning bearing having a cylindrical journal-bearing surface and a part-spherical outer surface engageable with the bearing support, the first, second and third parts constitute radially inner, intermediate and outer parts of a ring member and, to provide lubrication, the ring member is normally formed from sintered material which is sufficiently porous to hold, typically, up to 20% of its volume of lubricating oil. Bearings of this construction function quite satisfactorily, but the cost of manufacture is significant, particularly where the outer surfaces of the bearings have to be part-spherical for self-aligning purposes. Moreover, in some applications, for example: car door locking motors, where the motor is only required to operate intermittently for a small number of revolutions the major problem is reducing static friction, or "stiction". There is insufficient time to develop a friction-reducing oil film and so the cost of providing a lubrication bearing is wasted. In fact, the presence of oil which does little to reduce friction can cause damage by flowing onto the contact surfaces of the commutator because, as a result of the limited operation of the motor, there is insufficient heat to burn this oil to thereby prevent the formation of an insulating coating. The generic DE-A-3 332 323 discloses a self-aligning bearing made from sheet metal. The bearing has a hollow, cylindrical portion forming the journal bearing surface, and a dish-shaped portion formed at one end of the cylindrical portion and extending axially towards the other end. The dish-shaped portion has a convex part-spherical outer surface for bearing against a complementary surface of the bearing housing. A multi-fingered spring fixed to the bearing housing bears against the inner surface of the dish-shaped portion, either directly or via an intermediate ring, to retain the bearing within the bearing housing. A low friction coating is applied to the journal bearing surface. While such bearings may function satisfactorily, they do require the use of a retaining spring and possibly, an intermediate ring adding to material cost and requiring special attention during assembly. Disclosure of the Invention The purpose of the present invention is to provide a bearing for a fractional-horsepower PMDC electric motor which can be more economically manufactured than conventional bearings such as sintered bearings. This is achieved by providing a bearing of sheet metal in which the outer, part-spherical mounting surface has a maximum diameter, as measured perpendicular to the axis of the bearing, in a plane spaced from the second part, which is greater than the diameter of the second part and greater than the diameter of the second end of the third part; and the second end of the third part is axially adjacent the second end of the first part. Thus, according to the invention, there is provided a sheet metal, self-aligning bearing, for a fractional-horsepower PMDC electric motor, comprising a tubular first part having axially spaced first and second ends and providing a journal-bearing surface coated with non-liquid friction-reducing material, a second part, for providing radial support for the first part, having a first end connected to the first end of the first part and a radially outer second end; and a third part, for engagement with a bearing support, having a first end connected to the second end of the second part, an axially spaced second end which is radially spaced from the second end of the first part, and an outer, part-spherical mounting surface engageable with the bearing support; wherein the outer, part-spherical mounting surface has a maximum diameter, as measured perpendicular to the axis of the bearing, in a plane spaced from the second part, which is greater than the diameter of the second part and greater than the diameter of the second end of the third part; and the second end of the third part is axially adjacent the second end of the first part. By this means, it is possible to form a bearing from sheet metal by a simple pressing and/or swaging or spinning process. The non-liquid friction-reducing material may comprise a fluorocarbon polymer such as polytetrafluoroethylene (PTFE) which may be applied to the sheet metal, in known manner, before the sheet metal is subjected to the forming process to produce bearings. Moreover, where the bearings are formed from a continuous strip of sheet metal, the entire outer surface of the strip need not be coated, as in an immersion coating process, but may be coated, for example: by spraying, on only that much of one side of the strip as is necessary to ensure that the journal-bearing surface of the tubular first part of each bearing is coated with friction-reducing material. Although this will normally involve unnecessarily coating parts of the surface of the bearing other than the required journal-bearing surface, reduction in the amount of friction-reducing material can result in substantial cost saving. Unfortunately, this economising measure is offset by the fact that although precoating may facilitate the metal-working forming process, the forming process inevitably damages the coating. It has therefore been found better to form the bearings from uncoated sheet metal and then to coat the entire surface of each bearing in an immersion coating process. Immersion coating processes, for this purpose, may be used to provide very satisfactory fluorocarbon polymer coats. However, in view of factors such as the difficulties encountered in the provision and maintenance of uniform coating thickness on the journal-bearing surface of the tubular first part of each bearing, better results are obtainable by use of the known process of electroless codeposition of nickel and fluorocarbon polymer particles to form a composite coating in which fluorocarbon polymer particles are dispersed within a layer of nickel. Best results, so far, have been obtained in a codeposition process such as this in which the fluorocarbon polymer particles are of PTFE. In a preferred embodiment, in order to form the part-spherical outer mounting surface, the second part may be formed with at least three circumferentially extending segments which are separated by cuts which extend from an edge of the third part which is remote from the second part. The cuts may extend beyond the hereinbefore mentioned plane in which the part-spherical mounting surface has its maximum diameter and, in order to facilitate formation of the part-spherical surface, the blank from which the bearing is pressed may be relieved by removing sheet metal on opposite sides of each cut, in a preliminary stamping process. A bearing embodying the present invention is hereinafter described, by way of example only, with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 is a cross-sectional elevation of an assembly incorporating a bearing according to the present invention; Figure 2 is an end elevation of the bearing shown in Figure 1; and Figure 3 is an end elevation of a blank which has been partly formed in the process of producing a bearing as shown in Figure 2. Description of the Preferred Embodiment As shown in Figure 1, a sheet metal bearing has a first part 1, providing a journal-bearing surface 4 which is coated with non-liquid friction-reducing material such as polytetrafluoroethylene, or some other solid lubricant material, and this first part 1 has first and second ends 2 and 3. The bearing also has a second part 5 extending radially from the first end 2 of the first part 1. The second part 5 has radially inner and outer first and second ends 6 and 7 respectively connected to the first end 2 of the first part 1 of the bearing and to a third part 8 of the bearing extending axially around the first part 1. The third part 8 of the bearing has a part-spherical outer mounting surface 10 which is seated in a part-spherical seat 19 formed in a bearing support 9. The friction-reducing coating on the journal-bearing surface 4 reduces static friction between the journal-bearing surface 4 and a shaft (not shown) mounted in the bearing. If the shaft is not exactly aligned with the axis of the bearing, the bearing is able to align itself with the shaft by virtue of its part-spherical outer mounting surface 10 engaging the part-spherical seat 19 formed in the bearing support 9. As shown in Figures 2 and 3, formation of the part- spherical mounting surface 10 is facilitated by providing cuts 15 which extend radially inwards from the edge 11 of the blank from which the bearing is formed. The cuts 15 divide the third part 8 of the bearing into three circumferentially extending segments 12, 13 and 14. As shown in Figure 3, the blank is notched to provide relief between the circumferentially extending segments 12, 13 and 14 and thereby facilitate formation of the part-spherical outer mounting surface 10. The coating of friction-reducing material may be applied to all surfaces rather than to just the journal-bearing surface 4. It may be applied by any convenient manner including immersion coating processes or spraying. Alternatively, an electroless codeposit technique may be used to provide PTFE particles dispersed with a layer of nickel. Clearly, whenever all surfaces of a bearing according to the invention are coated with a non-liquid friction-reducing material, precautions must be taken to ensure that the reduction of friction on any particular surface does not detract from performance of the bearing. Thus, in the embodiment shown, the pressure between the engaging surfaces 10 and 19 must be adjusted to provide the required friction. If this is impracticable, the coating of friction-reducing material should not be applied to this particular surface of the bearing. The sheet metal forming bearings according to the invention may be of any bearing metal, such as brass, but in the embodiment shown, bronze is preferred to brass because of its greater resilience.

Claims: A sheet metal, self-aligning bearing, for a fractional-horsepower PMDC electric motor, comprising a tubular first part (1) having axially spaced first and second ends (2, 3) and providing a journal-bearing surface (4) coated with non-liquid friction-reducing material, a second part (5), for providing radial support for the first part (1), having a first end (6) connected to the first end of the first part (1) and a radially outer second end (7); and a third part (8), for engagement with a bearing support (9), having a first end connected to the second end (7) of the second part (5), an axially spaced second end which is radially spaced from the second end (3) of the first part (1), and an outer, part-spherical mounting surface (10) engageable with the bearing support (9); characterized in that: the outer, part-spherical mounting surface (10) has a maximum diameter, as measured perpendicular to the axis of the bearing, in a plane spaced from the second part (5), which is greater than the diameter of the second part (5) and greater than the diameter of the second end of the third part (8); and the second end of the third part (8) is axially adjacent the second end (3) of the first part (1). A bearing, according to Claim 1, in which all surfaces are coated with non-liquid friction-reducing material. A bearing, according to Claim 2, in which the non-liquid friction-reducing material is an electroless codeposit comprising fluorocarbon polymer particles dispersed within a layer of nickel. A bearing, according to any preceding claim, in which the second end of the third part (8) of the bearing is formed with at least three circumferentially extending segments (12, 13, 14) separated by cuts (15), each cut extending from the second end of the third part (8) beyond the plane spaced from the second part (5).

Inventors: STROBL GEORGE, STROBL, GEORGE
Applicants: JOHNSON ELECTRIC SA, JOHNSON ELECTRIC S.A.
Application year: 1992
Main classification: F16C23
IPC codes: F16C33, F16C17, F16C23, B32B15, H02K5