Rolling member

Details Rolling member Rolling member

2000-05-24

2001-05-08

Yee, Deborah (Department: 1742)

Metal treatment

Stock

Ferrous

C148S906000, C384S492000, C384S912000

Reexamination Certificate

active

06228184

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to rolling bearings used in precision instruments such as HDD (hard disk drive) or VTR, food machines, aircraft, machine tools and semiconductor-related instruments, linear guide bearings and rolling apparatuses such as ball screw apparatuses, and more particularly to rolling apparatuses excellent in characteristics such as resistance to fretting, acoustic performance, corrosion resistance and workability obtained by improving material compositions.
BACKGROUND OF THE INVENTION
In general, materials which have been used for rolling bearings include SUJ2 for bearing steels, SUS 440C and 13Cr martensitic stainless steels for stainless steels, and steel products corresponding to SCR 420 and SCM 420 for case hardening steels. The rolling bearings are used while being subjected to repeated shear stress under high contact pressure. In order to ensure rolling fatigue strength resisting the shear stress, therefore, hardening and tempering are performed to bearing steels, hardening, subzero treatment and tempering to stainless steels, and hardening and tempering to case hardening steels after carburizing or carbonitriding to realize a hardness of 58 to 64 in HRC.
However, these rolling bearings are used under a great variety of circumstances, and the use of ball bearing steels causes the possibility of early rusting by salt damages in regions adjacent to sea, invasion of water or sea water, or exposure or use under corrosive circumstances such as wetted conditions and the like.
Then, in the rolling bearings used under corrosive circumstances or particularly requiring the prevention of rusting, high carbon Cr martensitic SUS 440C has hitherto been used as stainless ball bearing steels excellent in corrosion resistance and having a hardness of 58 or more in HRC which is necessary for the bearings.
Further, the rolling bearings used in various spindles, various spindle motors and swing arms of HDD devices are required to have excellent rotational and acoustic performances and resistance to fretting, and to be small in torque fluctuations. However, the conventional stainless steels contain a number of coarse eutectic carbides having a size of more than 10 &mgr;m, so that it is difficult to obtain desired working accuracy. For example, the acoustic performance thereof is liable to be inferior to those of the rolling bearings made of ball bearing steels.
Then, in the spindle motors particularly used for driving magnetic discs for rotation and requiring sufficient rotational and acoustic performances, the rolling bearings made of ball bearing steels are used in many cases for this reason. In contrast, the rolling bearings for swing arms used for driving the swing arms performing access positioning to effective areas of the magnetic discs are used under swinging conditions. It is therefore difficult to form oil films between the rolling elements and races, so that torque fluctuations and torque spikes are developed by fretting wear to cause harm to the read function of the HDD devices in some cases.
Accordingly, high carbon Cr stainless steels good in resistance to fretting are used in many cases in the rolling bearings for swing arms, and in recent years, 13Cr martensitic stainless steels improved in rotational and acoustic performances are often used as stainless ball bearing steels for the rolling bearings used in the swing arms of the HDD devices and the like.
On the other hand, in the case of linear guide bearing apparatuses comprising guide rails and sliders, or ball screw apparatuses comprising screw shafts and nuts, JIS-SUS 440C and 13Cr martensitic stainless steels (carbon contents 0.6% to 0.7%) and further precipitation hardening type stainless steels such as JIS-SUS 630 are used as corrosion-resistant stainless materials used therein.
Case of Rolling Bearings
In the high carbon Cr stainless steels as described above, when the content of C and Cr is increased, for example, when C is contained in an amount of more than 0.6% by weight, a number of coarse eutectic carbides having a size of more than 10 &mgr;m are formed coupled with a large amount of Cr. These not only reduce the fatigue strength, toughness and resistance to corrosion of the rolling members, but also deteriorate workability such as the malleability and machinability.
Further, the presence of the coarse eutectic carbides sometimes adversely affects the acoustic performance of the rolling bearings. The acoustic performance indicates the degree of noise developed by signals generated in operation of the rolling bearings, and often causes a serious problem in relatively small-sized stainless steel rolling bearings used in precision instruments such as HDD devices which are apt to be easily damaged by vibration.
That is to say, the vibration developed in the bearings largely depends on the configurational accuracy of outer races, inner races or shaft elements, and rolling elements thereof. Accordingly, when materials containing the coarse eutectic carbides are used for the bearings, the coarse carbides inhibit the achievement of desired accuracy in finishing the bearings, and further, the difference in wear between grounds and the eutectic carbides arises also in rotation operation to cause a reduction in accuracy of roughness and the like. Furthermore, these eutectic carbide particles interfere with one another at their contact surfaces, resulting in increased noise.
As described above, the coarse eutectic carbides not only deteriorate the acoustic performance of the bearings, but also become sources of stress concentration to decrease the fatigue strength, and further to cause deterioration in toughness and resistance to corrosion. Accordingly, the high carbon Cr martensitic stainless steels such as SUS 440C not only have no sufficient resistance to corrosion and no mechanical strength, but also is extremely poor in acoustic performance, and is further high in cost. It has been therefore impossible to suitably use it for the rolling bearings used under corrosive circumstances or the rolling bearings used in various spindles, various spindle motors and the swing arms for the HDD devices.
Further, these rolling bearings are fixed with adhesives in many cases, and adhesion of rust preventive oil raises various problems. For example, the adhesive strength is decreased, or the rust preventive oil chemically reacts with the adhesives to contribute development of rust and further to generate out gas, which adheres to disc surfaces to reduce reliability of the HDD devices. Accordingly, the rolling bearings are completely degreased in many cases. It is therefore considered that stainless steels are better for the rolling bearings. However, stainless steels are high in cost compared with bearing steels such as SUJ2. Moreover, it contains a number of coarse eutectic carbides, although it is good in resistance to corrosion and resistance to fretting compared with SUJ2. It is therefore difficult to obtain target working accuracy. Further, these rolling bearings tends to be inferior to ones made of SUJ2 in acoustic performance, so that it is difficult to use them for the rolling bearings for spindle motors requiring high rotational and acoustic performances. Furthermore, with respect to the rolling bearings for various spindles, further improvements in resistance to fretting during conveyance and operation of the spindles have been desired.
JP-B-5-2734 (the term “JP-B” as used herein means an “examined Japanese patent publication”) discloses martensitic stainless steel rolling bearings significantly improved in acoustic performance and fatigue strength in which the content of C and Cr is decreased to inhibit formation of eutectic carbides. However, they are inferior to SUJ2 in the size of carbides, and are not only poor in acoustic performance, but also take cost into no consideration. Further, this publication discloses no resistance to fretting which is a required characteristic for the rolling bearings for HDD swing arms at all, and further describes no workability at all.
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