Patent Application
20060026838
This invention relates to roller bearings with a roller race made from flat sheet metal. More particularly this invention relates to a roller bearing of the type including a roller race made from high carbon flat sheet metal.
One method of making a roller bearing is to form a race with end flanges from flat sheet metal. The sheet metal is subjected to a forming process to form the flat stock into a shell 12 which is closed or flanged at one end 14 and open at the opposite end 16. Examples of the forming process include deep drawing, stamping or cold forming.
The flat sheet metal used to form the shells has typically been made of low carbon steel, for example, a steel having a carbon content of less than 0.35%. To provide a sufficiently hard raceway, the shell is subjected to a process to add carbon to the surface, for example, carburizing, carbonitriding, or the like. All of these processes in which carbon is added to the surface are collectively referred to hereinafter as carburizing. One example of a carburizing process includes exposing the shell, either with or without the internal components installed, to a high carbon atmosphere at high temperatures, for example, a temperature greater than 850° C., for an extended period of time such that carbon from the atmosphere diffuses into the shell. The process produces a high-carbon, hardened surface layer, while the core of the shell, between the hardened surfaces, remains a lower carbon, softer area. The process can be expensive and time consuming, particularly when a deeper hardened surface layer is desired.
U.S. Pat. No. 6,682,227 discloses a bearing assembly manufactured from a cold-rolled strip wherein the carbon content is between 0.30-0.55%. After the shell is drawn, the bearing is still subjected to a carburizing treatment wherein carbon is added to the surface of the bearing. The 6,682,227 patent describes a process in which the bearing is maintained in a high carbon atmosphere at a temperature of approximately 850° C. for 25 minutes and then quenched. The bearing assembly is then subjected to a tempering process wherein the assembly is heated to approximately 170-200° C. for a certain time and then cooled. While the duration of carburizing may be reduced, the process is still required, thereby still requiring the provision of a high carbon atmosphere at high temperature for some time.
Another potential drawback for bearing shells formed from lower carbon flat sheet steel is the surface quality of the bearing raceway. Low carbon steels often experience microtearing during the forming process, thereby resulting in a surface with irregularities. In some applications, such irregularities are acceptable. However, in more precise applications, the shell must be subjected to a finishing process, for example, grinding, in order to smooth the bearing surface. Again, this process is expensive and time consuming.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
The present invention provides a roller bearing assembly and a method of forming a rolling bearing assembly. The bearing assembly includes a shell formed from a cold-rolled strip having a carbon content greater than 0.65%. The formed shell has a circumferential raceway and a plurality of rolling elements are positioned in the shell. In one aspect of the invention, the circumferential raceway has an as hardened surface hardness of at least 58 HRC without carburizing of the formed shell. In another aspect of the invention, the cold-rolled strip has a carbon content greater than 0.65% and an ASTM grain size no. of 8 or finer. The circumferential raceway has a surface roughness Ra less than or equal to 18 microinchs without the application of any finishing process to the formed shell. In another aspect of the invention, the cold-rolled strip has a carbon content greater than 0.65% and a plain strain forming limit value (FL0) greater than or equal to 0.25.
The present invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Certain terminology, for example, “top”, “bottom”, “right”, “left”, “front”, “frontward”, “forward”, “back”, “rear” and “rearward”, is used in the following description for relative descriptive clarity only and is not intended to be limiting.
Referring to
Conventionally, shells formed through forming flat sheet stock, for example, through deep drawing, stamping or cold forming, have generally been limited to low carbon or medium carbon steels with a carbon content less than or equal to 0.55%.
The present inventors have found that the bearing shell 12 of the present invention may be formed from flat sheet stock that is a high carbon steel having a carbon content of 0.65% or greater. The material preferably has a chemical composition of: % carbon 0.65-0.90; % manganese 0.75-0.90; % chrome 0.15-0.25; % silicon 0.15-0.30; % phosphorus 0.025 max; % sulfur 0.025 max; and % aluminum 0.020-0.050. The preferred material also has an ASTM grain size no. of 8 or finer. For example, the inventors have produced the present bearing assembly 10 utilizing SAE-1074 carbon steel strip stock having an ASTM grain size no. of 8 or finer. While the inventors have found SAE-1074 carbon steel strip stock to be preferable, other high carbon steels may be utilized. It is preferred that such high carbon steels have an ASTM grain size no. of 8 or finer and a plain strain forming limit value (FL0) greater than or equal to 0.25. As indicated in
Roller bearing assemblies 10 manufactured using the preferred material provided a shell 12 having very good concentricity and roundness. The shell 12 also has a heat treated bearing hardness, for example, greater than 58 HRC, without any post-forming carburizing process due to the high carbon content. The formed shell 12 is typically tempered to increase the surface toughness, but generally does not require a carburizing treatment to harden the bearing surface. The formed shell 12 also has a substantially uniform carbon content through the cross-section of the shell 12. Furthermore, due to the fine grain structure, the formed shell 12 has a smooth surface without any additional finishing. For example, the formed shell 12 will preferably have an average surface roughness Ra of less than 18 microinchs and preferably less than 8 microinchs without any grinding or other supplemental finishing of the formed shell 12.
