Bearings – Rotary bearing – Antifriction bearing
Reexamination Certificate
2003-02-04
2004-10-12
Footland, Lenard A. (Department: 3682)
Bearings
Rotary bearing
Antifriction bearing
C384S573000, C384S625000
Reexamination Certificate
active
06802651
ABSTRACT:
TECHNICAL FIELD
This invention relates to a cage for use in a rolling bearing.
BACKGROUND ART
Heretofore, as a cage for use in a rolling bearing under going large load, a machined cage made of high strength brass excellent in the mechanical strength has been used. While the cage is excellent in the slidability and the wear resistance due to self-lubricity, since the material cost is high, the fabrications cost is high, and the yield is low, it is used only for special applications.
On the contrary, a pressed cage formed by fabricating a cold rolled steel sheet typically represented by SPCC or a hot rolled steel sheet typically represented by SPDH by pressing into a predetermined shape is advantageous in view of the cost compared with the machined cage made of high strength brass, but it is poor in the slidability and the wear resistance. Therefore, it has been conducted to improve the slidability and the wear resistance of the pressed cage by applying nitriding thereby forming a hard nitride layer on the surface of the cage.
Generally, when nitriding is applied to an iron and steel member comprising a low carbon steel such as SPCC or SPDH (iron and steel material with carbon content of 0.25% by weight or less), nitrogen and iron are reacted on the surface of the iron and steel member, by which nitrogen atoms diffuse into the iron and steel member and a layer comprising a nitrogen compound (nitride layer) is formed on the surface. The nitride layer forms a phase at higher nitrogen concentration toward the surface of the member. That is, the phase of the nitride layer changes from the vicinity at the boundary with the base material at the innermost site toward the outermost surface in the order of &ggr;′ phase (Fe
4
N), &egr; phase (Fe
2-3
N), and &xgr; phase(Fe
2
N).
In the nitriding conducted in existent pressed cage (“Tufftride method” and gas softening nitriding), since nitriding is applied at a high temperature of 550-600° C., the &egr; phase and the &xgr; phase at the surface tend to form a porous structure. When the distribution for the hardness in the surface layer portion of the nitride layer obtained by the gas softening nitriding method by a nano-indentation method accurately at an interval of several micro orders, while the hardness is Hv 600 or more at the depth of about 10 &mgr;m but it is sometimes less than Hv 500 at the outermost surface as shown at (b) in the graph of FIG.
3
. In the measurement for the hardness by an existent micro-vickers tester, the distribution for the hardness on the surface layer of the nitride layer can not be measured at an interval of several &mgr;m.
Further, in the existent method describe above, the nitride layer is formed at a large thickness by making the processing time longer upon nitriding or increasing the nitrogen potential in the treating atmosphere while considering the decrease in the thickness of the nitride layer by wear during use of the cage. In accordance therewith, the nitrogen concentration in the nitride layer becomes higher and most of the nitride layer sometimes forms the &xgr; phase. The &xgr; phase is poor in the toughness and tends to suffer from sharing fracture easily by sliding friction with rolling elements and tends to drop easily from the surface of the cage. As a result, it is difficult to obtain the effect of improving the slidability and the wear resistance.
On the other hand, Japanese Published Unexamined Patent Publication Hei 10-2336 describes that fluoridation of replacing the oxide on the surface of the cage with a metal fluoride film before nitriding forms a uniform and dense nitride layer comprising nitrides with an average grain size of 1 &mgr;m or less on the surface of the cage. However, since the wear resistance of the cage greatly depends on the type and the hardness of the nitride layer, the slidability and the wear resistance are sometimes insufficient by merely making the nitride grain size finer on the outermost surface like the cage described in the above noted publication.
Further, Japanese Published Unexamined Patent Publication 2001-90734 proposes formation of a dense nitride layer of 3-20 &mgr;m thickness and a nitride layer of a porous structure of 2-25 &mgr;m thickness on the surface of a pressed cage by nitriding. According to this proposal, both the surface hardness and the lubricant retaining performance of the pressed cage are made favorable and improvement is expected for the wear resistance. However, even by the proposal, no sufficient wear resistance can sometimes be obtained in a severe lubrication circumstance in which a lubricant film is less formed.
Further, it is also necessary to prevent degradation of the mechanical strength of the cage by the nitriding.
The present invention has been accomplished taking notice on the subject in the prior art as described above, and it is an object thereof to provide a cage more excellent in the slidability and the wear resistance than usual in a cage in which a nitride layer is formed on the surface by using an iron and steel material with carbon content of 0.25% by weight or less, fabricating the same into a predetermined shape and applying nitriding, and to increase the mechanical strength of the cage.
DISCLOSURE OF THE INVENTION
For solving the subject described above, the present invention provides a cage in which a nitride layer is formed on a surface by using an iron and steel material with a carbon content of 0.25% by weight or less and fabricating the same into a predetermined shape and then conducting nitriding, having a nitride layer with a hardness of 650 or more in vickers hardness (Hv) at a thickness of 3 or more and 15 &mgr;m or less.
By the way, even when it is intended to measure the distribution of the hardness in the direction of the depth of a thin layer such as a nitride layer in the measurement for the hardness by an existent micro-vickers tester at an interval of several micron meters, no accurate measurement is possible since reading error is large or indentation diameter increases due to excessive load, but accurate measurement is possible by the nano-indentation method. Accordingly, the hardness of the nitride layer in the cage of the present invention should be measured by a nano-indentation method. Further, when it is measured by the nano-indentation method, measurement is conducted in a state of where the indentation diameter is at least 5 to 50% for the thickness of the nitride layer.
The cage of the present invention is obtained by applying the nitriding to the cage formed of an iron and steel material such that formation of the &xgr; phase is suppressed and a nitride layer comprising &ggr;′ phase, &egr; phase or(&ggr;′+&egr;)phase is formed. As the method, it can be mentioned that the reaction rate of nitriding is retarded and the temperature for nitriding is preferably 400 to 550° C. and, more preferably, 400 to 540° C.
Further, when nitriding is applied at relatively high temperature after forming the cage by pressing from a steel sheet formed of a low carbon steel, since work strains during pressing are relieved during nitriding, the cage tends to form deformation. However, the heat treatment deformation caused to the cage can be suppressed by applying the nitriding in the temperature range described above that is a temperature range in which recrystallization dose not occur in the low carbon steel.
When the temperature for the nitriding is 560° C. or higher or the treatment time is longer, since the thickness of the nitride layer exceeds 15 &mgr;m in which the &xgr; phase is formed to the outermost surface of the nitride layer making it porous and deteriorating the hardness and the toughness of the nitride layer, no good slidability and wear resistance can be obtained. On the contrary, when the thickness of the nitride layer is less than 3 &mgr;m, the thickness of the nitride layer tends to become not uniform possibly forming a portion where the nitride layer is not formed, good slidability and wear resistance can not be obtained reliably.
Good slidability and wear resista
Miyamoto Yuji
Ueda Koji
Footland Lenard A.
NSK Ltd.
LandOfFree
Retainer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Retainer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Retainer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3313860