Abstract:
In a bearing assembly or a gearing with a least two bearing elements or gearing elements that are adapted for movement relative to each other, at least one bearing element or gearing element comprises a polyimide coating on a carrier material.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a bearing or drive assembly with at least two bearing elements or drive elements that are adapted for movement relative to each other. 
   As a known bearing assembly of the above-described type, a ball bearing comprises a plurality of steel balls that are positioned between an inner ring and an outer ring, and the balls are kept in spaced relationship by a cage. A lubricant reduces friction and wear in the ball bearing and simultaneously protects the bearing elements against corrosion. For special applications, which are expected to function as much as possible without a lubricant, the ball bearing may also include very expensive bearing elements of a ceramic material, of steel coated with carbide in a vacuum, or a solid material of plastic. 
   BRIEF SUMMARY OF THE INVENTION 
   One aspect of the present invention is the provision of improvements to a bearing or drive assembly of the type described above. In accordance with one aspect of the present invention, a bearing or drive assembly includes at least two bearing elements or drive elements that are adapted for movement relative to each other, with at least one of the bearing elements or drive elements including a polyimide coating on a carrier material. 
   The fact that at least one bearing or drive element comprises a polyimide coating on a carrier material permits reducing the lubricant needs and thus the operating costs to a great extent in comparison with lubricated bearing elements of steel. Preferably, a supply of lubricant is no longer required during operation, and production costs are considerably reduced in comparison with ceramic bearing elements, carbide-coated bearing elements, and bearing elements of a solid material of plastic. Since bearing elements of solid plastic material exhibit a very high absorption of water, which may lead to a brittleness of the material, the bearing assembly of the present invention has the additional advantage that the bearing and drive elements with a polyimide coating absorb negligibly little water, i.e., they are hydrolytically stable. Furthermore, the polyimide coating of the present invention facilitates a longer service life of the bearing or drive assembly. 
   It is possible to utilize the bearing or drive assembly of the present invention in a large variety of technical fields, such as motive power engineering or automobile manufacturing, and in all dimensional ranges, i.e., both in the case of very small dimensions, such as for fine and finest mechanics, and in the case of very large dimensions, such as for windmills. In the case of antifriction bearings, the rings and/or rolling bodies may be coated. The bearing or drive assembly of the invention may also be in the form of a gearing, for example, a toothed gearing, a plain bearing, a linear guideway, for example, with rails adapted for relative movement toward one another, or as a chain drive. 
   A very thin formation of the polyimide coating is simpler and more cost-favorable to produce, and increases the advantages of hydrolytic stability and longer service life. As protection against corrosion of the carrier material that normally consists for the most part of iron, a passivation coating is applied to the carrier material below the polyimide coating, i.e., chronologically before applying the latter. To the extent that high pressure loads are to be expected in the bearing assembly, a pressure-resistant coating may be additionally applied to the passivated carrier material below and/or inside the polyimide coating. 
   Preferably, the polyimide coating is prepared from a dissolved prepolymer (prepolymer solution), which is directly obtained from the raw materials or by reversing the polymerization. In a next step, it is preferred to immerse the bearing or drive element into this prepolyimide, so that a thin polyimide film forms. While it is not necessary to add special lubricants in this process, such an addition is possible as an option. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention is described in greater detail with reference to two embodiments illustrated in the drawings, in which: 
       FIG. 1  is a top view of a first embodiment; 
       FIG. 2  is a partial cutout view of the first embodiment; and 
       FIG. 3  is a partial cutout view of a second embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
   In the first embodiment, a bearing assembly  1  is constructed as a ball bearing. Between an outer ring  3  with a larger diameter and an inner ring  5  with a smaller diameter, a plurality of balls  7  are arranged, which are kept in spaced relationship by a ball cage  9 . In known manner, the balls  7  abut an inside wall of the outer ring  3  and an outside wall of the inner ring  5 . The inside wall of the outer ring  3 , the outside wall of the inner ring  5 , and preferably also the balls  7  are coated by the following method. 
   To begin with, a metallic carrier material  11 , preferably machinery steel, receives a very thin passivation coating  13 , preferably between 0.05 μm and 5 μm, to which a polyimide coating  15  is applied. In comparison with the thickness or with other dimensions of the metallic carrier material  11 , the polyimide coating is very thin, preferably less than 30 μm. It is temperature-resistant up to at least 260°–300° C., and absorbs negligibly little water, i.e., it is hydrolytically stable. Its small thickness also prevents peeling under loads. The passivation coating  13  has an even smaller thickness. 
   For a very simple and cost-favorable production of the aforesaid hybrid coatings, the metal carrier material  11  is initially passivated. Used as raw materials of the polyimide are, for example, a dianhydride, for example, 1,2,4,5 benzenetetracarboxylic 1,4:4,5 dianhydride, and a diamine, for example, 1,5 diamino-5-methylpentane. From these raw materials, a prepolymer of the polyimide is prepared in a solution. The passivated bearing element  3 ,  5 , or  7  that is to be coated is then immersed into this solution, so that a thin film forms. For the polymerization and for removing the solvent, the bearing element  3 ,  5  or  7  undergoes a subsequent tempering, whereby the final polyimide coating  15  is formed. Before using the bearing assembly  1 , a minimal initial lubrication may occur by depositing one drop of a lubricant on the bearing assembly. During its operation, the bearing assembly  1  will need no additional lubricant, i.e., it is permanently lubricated. In a modified variant, the drop of lubricant or an additive having a comparable effect is already added to the prepolymer solution, which will then likewise improve the frictional properties, i.e., the coefficient of sliding friction or friction. 
   A second embodiment is largely similar to the first embodiment, but is intended for special cases of application. Between the passivation coating  13  and the polyimide coating  15 , a pressure-resistant coating  17  is additionally provided, such as, for example, a silicate. Likewise in this second embodiment, the total thickness of the polyimide coating  15 , the pressure-resistant coating  17 , and the passivation coating  13  is a great deal smaller than the dimensions of the carrier material  11 , i.e., the bearing element  3 , 5 , or  7  is again very thinly coated. 
   Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.