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) is connected to and diverges radially from the second end (3) of the first part (1) for engagement with an abutment surface (17) of 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. DE 3332323 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. CH-A-220818 discloses a plain bearing in accordance with the preamble of claim 1 having a tubular first part, disposed within a circular section aperture in a bearing support, and radially outwardly extending parts at opposite ends of the first part. The first part is waisted so as to define a space between the first part and the bearing support aperture except at the ends thereof and so that the first part only makes contact with a rotatable shaft passing therethrough at a position mid-way between the ends of the bearing when the bearing is in an unloaded condition. If the bearing is loaded, the first part deforms to increase the area of contact between the shaft and the bearing. Due to its deformability, the bearing also has some self-aligning property. Disclosure of the Invention The purpose of the present invention is to provide a self-aligning bearing assembly for a fractional-horsepower PMDC electric motor which can be more economically manufactured than conventional bearings such as sintered bearings. According to the invention there is provided a self-aligning bearing assembly for a fractional horsepower PMDC electric motor comprising a bearing support having a circular section aperture a radially extending support surface formed at one end of the aperture and an abutment surface formed on the other end of the aperture and a sheet metal bearing having a tubular first part, disposed within the aperture, with axially spaced first and second ends and a journal bearing surface, a second part having a first end connected to the first end of the first part and a radially outer second end, and a third part connected to the second end of the first part and diverging radially outward from the second end of the first part, wherein the journal bearing surface is coated with a non-liquid friction-reducing material; the aperture tapers radially inward from the support surface to the abutment surface and wherein the third part of the bearing engages the abutment surface and thereby holds the second part of the bearing in frictional contact with the support surface of the bearing support so as to provide radial support for the first 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 the bearings, 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. Advantageously, the resilience of the bearing support material, the resilience of the bearing itself, and the outer diameter of the third part of the bearing is such that the third part can pass through the aperture and snap into position with the first part of the bearing disposed within the aperture and supported at its second end by the bearing support material around the aperture, with the second part in engagement with the support surface of the bearing support, and with the third part in engagement with the abutment surface so as to hold the second part of the bearing in engagement with the support surface. In a bearing assembly such as this, the first end of the first part of the bearing is given radial support by the second part of the bearing making frictional contact with the support surface of the bearing support and the second end of the first part is given radial support by the bearing support material engaging the first part of the bearing. However, if the shaft supported by the bearing is not aligned with the axis of the bearing, the bearing is able to tilt into a self-aligning position. In this case, the second part of the bearing slides over the support surface of the bearing support. The movement of the first end of the first part of the bearing is accommodated within the tapering aperture formed in the bearing support. A bearing embodying the present invention is hereinafter described, by way of example only, with reference to the accompanying drawing. Brief Description of the Drawings Figure 1 is a sectional side elevation of an assembly incorporating a bearing according to the present invention. Description of the Preferred Embodiment As shown in the single figure of the drawings, 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 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. 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. The first end 6 is connected to the first end 2 of the first part 1 of the bearing. A third part 8 of the bearing, adjacent the first part 1, extends radially outwardly by diverging radially from the second end 3 of the first part 1. As shown, the bearing support 9 is provided with a support surface 16, extending perpendicular to the axis of the bearing, an abutment surface 17 facing away from the support surface 16, and an aperture 18 which tapers, in two stages, from the support surface 16 to the abutment surface 17. The bearing support 9 is made from resiliently deformable material such as moulded plastic. The bearing is dimensioned so that, as a result of the resilience of the material of the bearing support 9 or of the bearing itself, the third part 8 of the bearing can be pressed through the aperture 18 to snap into position with the third part 8 engaging the abutment surface 17 so as to hold the second part 5 of the bearing in frictional engagement with the support surface 16 and so that the bearing support material surrounding the aperture 18 provides radial support for the second end 3 of the first part 1 of the bearing. Radial support for the first end 2 of the first part 1 of the bearing is provided by frictional engagement of the second part 5 with the support surface 16. If the shaft is not precisely aligned with the axis of the bearing, the bearing is able to tilt as a result of frictional movement between the second part 5 of the bearing over the support surface 16 and movement of the first part 1 of the bearing within the tapered aperture 18. In practice, the surface of the aperture 18 need only diverge from the second end 3 of the first part 1 of the bearing at an angle of 2 to 3 degrees. For the sake of clarity, the angle of divergence shown has been greatly exaggerated. 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 an immersion coating process or spraying. Alternatively, an electroless codeposit technique may be used to provide PTFE particles dispersed within 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, as the performance of this bearing depends on frictional engagement between the inner surface of the second part 5 and the surface 16 of the bearing support 9, the pressure between the engaging surfaces 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 suitable metal, such as brass or bronze.

Claims: A self-aligning bearing assembly for a fractional horsepower PMDC electric motor comprising a bearing support (9) having a circular section aperture (18) a radially extending support surface (16) formed at one end of the aperture (18) and an abutment surface (17) formed on the other end of the aperture (18) and a sheet metal bearing having a tubular first part (1), disposed within the aperture (18), with axially spaced first and second ends (2,3) and a journal bearing surface (4), a second part (5) having a first end (6) connected to the first end (2) of the first part (1) and a radially outer second end (7), and a third part (8) connected to the second end (3) of the first part (1) and diverging radially outward from the second end (3) of the first part (1), characterised in that the journal bearing surface (4) is coated with a non-liquid friction-reducing material ; the aperture (18) tapers radially inward from the support surface (16) to the abutment surface (17); and in that the third part (8) of the bearing engages the abutment surface (17) and thereby holds the second part (5) of the bearing in frictional contact with the support surface (16) of the bearing support (9) so as to provide radial support for the first end of the first part. A bearing assembly according to claim 1 wherein the resilience of the bearing support, the resilience of the bearing and the outer diameter of the third part (8) of the bearing are such that the third part (8) can pass through the aperture (18) and snap into position. A bearing assembly according to claim 1 or claim 2 wherein the non-liquid friction-reducing material is an electroless codeposit comprising fluorocarbon polymer particles dispersed within a layer of nickel.

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