Abstract:
A fine nitride layer comprising a compound layer and a diffusion hardened layer of hardness being Hv900 or higher is formed, through a low temperature nitriding method at temperature in the range of 300 to 400° C., in a guiding part. The guiding part guides the circumferential surfaces of retainers  7   a  to  7   c  and is formed at any one part of inner ring raceways  3   a  to  3   c  and outer ring raceways  4   a  to  4   c  as well as the inner circumferential surfaces of the outer rings  5   a  to  5   c , to thereby realize such rolling bearings at low cost, having excellent abrasion resistance, and good corrosion resistance, if needed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is to improve abrasion resistance of rolling bearings to be incorporated in rotation supporters of jet engines for aircraft or gear boxes and to be served under circumstance at high temperatures, thereby to heighten endurance of rolling bearings. 
     In rotation supporters of jet engines for aircraft or gear boxes, rolling bearings  1   a  to  1   c  as shown in FIGS. 1A to  1 C are incorporated as bearings for aircraft. Three kinds of rolling bearings  1   a  to  1   c  are shown in FIGS. 1A to  1 C. The rolling bearing  1   a  of FIG. 1A is a ball bearing of single row deep groove type with four point-contact points in which two-piece housing inner ring  2   a  is provided. The rolling bearing  1   b  of FIG. 1B is an angular ball bearing having an inner ring  2   b  of counter bored type. The bearing of FIG. 1C is a single row cylindrical roller bearing. 
     In each of these cases, the rolling bearings  1   a  to  1   c  have inner rings  2   a  to  2   c , outer rings  5   a  to  5   c , a plurality rolling elements  6   a  to  6   c  rotatably disposed therebetween, and retainers  7   a  to  7   c , respectively. The inner rings  2   a  to  2   c  are made of iron based alloy and respectively have inner ring raceways  3   a  to  3   c  in outer circumferential surfaces thereof. The outer rings  5   a  to  5   c  are made of iron-alloy and respectively have outer ring raceways  4   a  to  4   c  in inner circumferential surfaces thereof. The plurality rolling elements  6   a  to  6   c  are rotatably disposed between the inner ring raceways  3   a  to  3   c  and the outer ring raceways  4   a  to  4   c , respectively. The retainers  7   a  to  7   c  rotatably hold the rolling elements  6   a  to  6   c , respectively. The outer circumferential surface of each of the retainers  7   a  to  7   c  is prevented from displacement in an radial direction thereof by guiding the outer circumferential surface of respective retainer  7   a  to  7   c  by means of one part of the inner circumferential surface of the outer ring  5   a  to  5   c  which (one part) is disposed outside of the outer ring raceway  4   a  to  4   c.    
     For using the rolling bearings  1   a  to  1   c  of the structures as shown in FIGS. 1A to  1 C in applications particularly requiring large corrosion resistance in bearings for aircraft, there are many cases manufacturing the inner rings  2   a  to  2   c  and the outer rings  5   a  to  5   c  with SUS440C (the iron-alloy containing C 1.05 wt %, P 0.040 wt % or lower, S 0.030 wt % or lower, Mn 1.00 wt %, Si 1.00 wt %, Cr 17.00 wt %, Mo 0.75 wt %, the balance being Fe and unavoidable impurities). 
     Using SUS440C under circumstances at high temperatures as jet engine for aircraft or gear boxes, it did not always have enough corrosion resistance and so was disadvantageous in view of securing endurance. That is, since the inner rings  2   a  to  2   c  and outer rings  5   a  to  5   c  constituting the rolling bearings  1   a  to  1   c  to be applied to usage as above mentioned are tempered at high temperatures in the course of the production, the hardness in the surfaces including the inner ring raceways  3   a  to  3   c  and the outer ring raceways  4   a  to  4   c  is around Hv650 at the most. This value is poor in the corrosion resistance comparing with the hardness Hv being about 700 of heat resistant bearing steels such as M50 (the iron-alloy containing C: 0.80 wt %, P: 0.030 wt % or lower, S: 0.030 wt % or lower, Mn: 0.30 wt %, Si: 0.25 wt %, Cr: 4.00 wt %, V: 1.00 wt %, Mo: 4.25 wt %, the balance being Fe and unavoidable impurities). On the other hand, the heat resistant bearing steels as M50 are difficult to secure a sufficient endurance in term of corrosion resistance. 
     In view of such circumstances, there are cases of using heat resistant and corrosion resistant steels such as Pyrowear 675 (trade name=iron-alloy containing C: 0.07 wt %, P: 0.005 wt % or lower, S: 0.003 wt % or lower, Mn: 0.65 wt %, Si: 0.40 wt %, Cr: 13.00 wt %, V: 0.60 wt %, Mo: 1.80 wt %, Ni: 2.60 wt %, Co: 5.40 wt %, the balance being Fe and unavoidable impurities). However, if fabricating the inner rings  2   a  to  2   c  and the outer rings  5   a  to  5   c  of the rolling bearings  1   a  to  1   c  with such heat resistant and corrosion resistant steels, being accompanied with carburizing treatments for hardening surfaces, Cr carbides are precipitated in the surfaces of the inner rings  2   a  to  2   c  and the outer rings  5   a  to  5   c , so that the corrosion resistance in the surface is lowered and it is still difficult to secure the enough endurance. 
     Further, in case of using the rolling bearings  1   a  to  1   c  as shown in FIGS. 1A to  1 C for supporting such as turbines of jet engines for aircraft, parts guiding the outer circumferential surfaces of the retainers  7   a  to  7   c  often causes abrasion or damage in parts of the inner circumferential surfaces of the outer rings  5   a  to  5   c . For avoiding such accidents, the conventional technique carries out a treatment for forming a hard film such as TiN in the guiding part of the outer circumferential surfaces. But since this hard film treatment is very expensive, and not only it invites cost-up of the rolling bearings  1   a  to  1   c , but also the hard film often comes off, it does not always bring about an effect of improving a secure abrasion resistibility. 
     Besides, with respect to the rolling bearings  1   a  to  1   c  to be used to jet engines for aircraft or gear boxes, in a case of a so-called hybrid bearing where the inner rings  2   a  to  2   c  as well as the outer rings  5   a  to  5   c  are made of bearing steels, but the rolling elements are made of ceramic in order to heighten a seizure resistance, the improvement of the abrasion resistibility in the inner rings  2   a  to  2   c  and the outer rings  5   a  to  5   c  become a furthermore problem. That is, in the hybrid bearing, since the contacting condition between the inner rings  2   a  to  2   c  and the rolling elements  6   a  to  6   c  and/or the contacting condition between the outer rings  5   a  to  5   c  and the rolling elements  6   a  to  6   c  takes place between materials of heterogeneous kinds, the seizure resistance of the rolling elements  6   a  to  6   c  can be improved in comparison with the bearing having the rolling elements made of bearing steels (as well as the inner and outer rings). However, the hardness of the bearing steel comprising the inner rings  2   a  to  2   c  and the outer rings  5   a  to  5   c  is far lower than that of the ceramic composing the rolling elements  6   a  to  6   c , so that when foreign matters as dust or dirt go into the bearings  1   a  to  1   c , the inner raceways  3   a  to  3   c  and the outer raceways  4   a  to  4   c  are considerably worn. 
     SUMMARY OF THE INVENTION 
     In view of these situations, the invention has been made to realize such rolling bearings at low cost, having excellent abrasion resistance, and good corrosion resistance, if needed. 
     In addition, the invention has also been made to realize a method of making such rolling bearings. 
     The above-mentioned object can be achieved by a rolling bearing according to a first aspect of the invention comprising an inner ring, an outer ring, a plurality of rolling elements rotatably disposed therebetween, and a retainer. The inner ring is made of iron-alloy and has an inner ring raceway in an outer circumferential surface thereof. The outer ring is made of iron-alloy and has an outer ring raceway in an inner circumferential surface thereof. The plurality of rolling elements are rotatably disposed between the inner ring raceway and the outer ring raceway. The retainer has the inner circumferential surface and the outer circumferential surface and rotatably holds each of the rolling elements. Either of the inner and outer circumferential surfaces of the retainer is guided by a part of either of the outer circumferential surface of the inner ring and the inner circumferential surface of the outer ring for preventing the displacement in the radial direction of the retainer. 
     In the rolling bearing of the invention, a fine nitride layer comprising a compound layer and a diffusion hardened layer and has hardness of Hv900 or higher is formed in a part which guides the circumferential surface of a retainer by means of at least one part of any surface of the inner ring raceway surface and the outer ring raceway surface as well as the outer circumferential surface of the inner ring and the inner circumferential surface of the outer ring. 
     The above-mentioned object can also be achieved by a method of making the above-mentioned rolling bearing, according to a second aspect of the present invention, comprising the steps of: 
     performing a low-temperature nitriding treatment at 300 to 400° C. so as to form nitride layer in surfaces of the inner ring and the outer ring of the rolling bearing. 
     As the low-temperature nitriding treatment, an NV nitriding treatment being “NV-300” of Daido Hokusan K.K. may be desirably used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B, and  1 C show partially cross sectional views illustrating three examples of the conventional rolling bearings; 
     FIGS. 2A,  2 B, and  2 C show partially cross sectional views illustrating three examples of the rolling bearings according to the present invention; 
     FIG. 3 shows a front view of the testing apparatus used for confirming effects brought about by the invention; 
     FIG. 4 shows a graph showing the experimented results as to the M50-made test pieces; 
     FIG. 5 shows a graph showing the experimented results as to the M50NiL-made test pieces; and 
     FIG. 6 shows a diagram showing distributions of the residual stress in the M50NiL-made test pieces. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, first to three embodiments of the rolling bearing according to the present invention will be described. 
     In rotation supporters of jet engines for aircraft or gear boxes, rolling bearings  11   a  to  11   c  as shown in FIGS. 2A to  2 C are incorporated as bearings for aircraft. Three kinds of rolling bearings  11   a  to  11   c  according to first to third embodiments according to the present invention are shown in FIGS. 2A to  2 C. The rolling bearing  11   a  of FIG. 2A according to the first embodiment is a ball bearing of single row deep groove type with four point-contact points in which two-piece housing inner ring  12   a  is provided. The rolling bearing  11   b  of FIG. 2B according to the second embodiment is an angular ball bearing having an inner ring  12   b  of counter bored type. The bearing of FIG. 2C according to the third embodiment is a single row cylindrical roller bearing. 
     In each of these cases, the rolling bearings  11   a  to  11   c  have inner rings  12   a  to  12   c , outer rings  15   a  to  15   c , a plurality rolling elements  16   a  to  16   c  rotatably disposed therebetween, and retainers  17   a  to  17   c , respectively. The inner rings  12   a  to  12   c  are made of iron based alloy and respectively have inner ring raceways  13   a  to  13   c  in outer circumferential surfaces thereof. The outer rings  15   a  to  15   c  are made of iron-alloy and respectively have outer ring raceways  14   a  to  14   c  in inner circumferential surfaces thereof. The plurality rolling elements  16   a  to  16   c  are made of ceramic and are rotatably disposed between the inner ring raceways  13   a  to  13   c  and the outer ring raceways  14   a  to  14   c , respectively. The retainers  17   a  to  17   c  rotatably hold the rolling elements  16   a  to  16   c , respectively. The outer circumferential surface of each of the retainers  17   a  to  17   c  is prevented from displacement in an radial direction thereof by guiding the outer circumferential surface of respective retainer  17   a  to  17   c  by means of one part of the inner circumferential surface of the outer ring  15   a  to  15   c  which (one part) is disposed outside of the outer ring raceway  14   a  to  14   c.    
     However, it is also possible to guide the inner circumferential surface of the retainer with the outer circumferential surface of the inner ring so as to prevent the displacement of the retainer in the radial direction. 
     In each of the rolling bearings  11   a  to  11   c  of the invention, a fine nitride layer  20  is formed in a part which guides the outer circumferential surface of each of the retainer  17   a  to  17   c . The fine nitride layer  20  is provided on at least one part of the inner circumferential surface of each of the outer rings  15   a  to  15   c.    
     However, note that when the retainers  17   a  to  17   c  are guided by the outer circumferential surface of the inner rings  15   a  to  15   c  respectively, it is possible to provide such fine nitride layer  20  on at least part of outer circumferential surface of each of the inner rings  15   a  to  15   c . Further, the fine nitride layer  20  may be provided on both of the inner and outer circumferential surfaces of the inner and outer rings. 
     The fine nitride layer  20  comprises a compound layer  20 A as a surface layer and a diffusion hardened layer  20 B disposed below the compound layer  20 A. The fine nitride layer  20  has hardness of Hv900 or higher. 
     The fine nitride layer  20  is formed by a low-temperature nitriding treatment performed at 300 to 400° C. As the low-temperature nitriding treatment, an NV nitriding treatment being “NV-300” of Daido Hokusan K.K. may be desirably used. 
     In case the iron based alloy such as stainless steels is carried out with the low temperature nitriding at 300 to 400° C., not the carburizing treatment, a secondary hardening does not take place, and the nitrogen goes into the metallic structure, so that the hardness in the surfaces of the inner rings  12   a  to  12   c  and the outer rings  15   a  to  15   c  fabricated with the above iron-alloy is heightened. Thus, the abrasion resistance in the inner rings  12   a  to  12   c  and the outer rings  15   a  to  15   c  is increased in that the hardness in the surface is heightened. Besides, the surfaces of the inner rings  12   a  to  12   c  and the outer rings  15   a  to  15   c  are not effected with the secondary hardening, and since CrN is not precipitated in the surfaces, the corrosion resistance does not go down. Accordingly, if a material having an excellent corrosion resistance is used as the iron-alloy, the corrosion resistance can be enough secured in the inner rings  12   a  to  12   c  and the outer rings  15   a  to  15   c.    
     On the other hand, depending on an ordinary gas nitriding treatment which has conventionally been used to hardening of surfaces of the iron-alloys, since the treating temperature is high as 480 to 600° C., not only the surfaces of the inner rings  12   a  to  12   c  and the outer rings  15   a  to  15   c  are effected with the secondary hardening, but also CrN is precipitated in the surfaces. Those undesirably worsen the corrosion resistance in the surface of the iron-alloy. 
     If performing the low temperature nitriding treatment on the above M50 or M50NiL (iron based alloys containing C 0.13 wt %, P 0.030 wt % or lower, S 0.030 wt % or lower, Mn 0.30 wt %, Si 0.25 wt %, Cr 4.00 wt %, V 1.20 wt %, Mo 4.25 wt %, Ni 3.50 wt %, the balance being Fe and unavoidable impurities), a residual compresive stress is increased and both of fracture toughness and rolling fatigue life are heightened. In particular, as the surface hardening is increased, the rolling fatigue life when foreigners are mixed is remarkably improved. 
     EXAMPLES 
     Reference will be made to experiments for confirming effects brought about by the invention. The experiments were practiced using a testing apparatus (Polymet testing apparatus) as shown in FIG.  3 . This testing apparatus presses loading disks  9 ,  9  against a pillar-like testing piece  8  rotated by a motor (not shown) by a loading apparatus  10 , said loading disks  9 , 9  being located at two positions in opposite sides in the diameter direction of the testing piece  8 . The rolling contacts between the outer circumferential surface of the test piece  8  and the outer circumferential surfaces of the loading disks  9 , 9  are dropped with a lubricant oil (compound gas turbine oil) of a fixed amount from an oil supply apparatus to provide lubrication on the rolling contacts. 
     The diameter of the test piece  8  was 9.5 mm, the length was 76 mm, the rotation speed was 12500 min −1 , and the contacting pressure at the rolling contacts was 4.8 Gpa. The parameter Λ value) of oil pressure was 0.9. The test pieces  8  of M50 and M50NiL were many prepared for testing under these condition, and with respect to each kind of the test pieces, those practiced with the low temperature nitriding treatment and those not practiced therewith were produced. The tests were continued until peeling appeared on the surfaces per each kind of the test pieces, and total rotation numbers until peeling (the number of repeating stress) were recorded. The results are shown in FIGS. 4 and 5. 
     In FIGS. 4 and 5 showing the experimented results, FIG. 4 shows the results as to the M50-made test pieces, and FIG. 5 shows the results as to the M50NiL-made test pieces. In each of them, ◯ designates the results of those not practiced with the low temperature nitriding treatment, while  designates the results of those practiced therewith. As seen from FIGS. 4 and 5, in the case of the test pieces of the low temperature nitriding treatment, the peeling resistibility is largely improved. 
     FIG. 6 shows the results of measuring the residual stress in the surfaces of the test pieces. Also in FIG. 6, ◯ designates the results of those not practiced with the low temperature nitriding treatment, while  designates the results of those practiced therewith. From FIG. 6, it is assumed that in the case of the test pieces of the low temperature nitriding treatment, the residual compressive stress is largely improved, and this fact contributes to the large improvement of the peeling resistibility. 
     The experiments were carried out with the test pieces made of M50 and M50NiL, and in the case of the test pieces made of the stainless steel, similar differences arise in those practiced with the low temperature nitriding treatment and those not practiced therewith. Accordingly, if the low temperature niriding treatment is performed on the inner rings and the outer rings made of the stainless steels, the excellent abrasion resistibility can be provided in addition to the excellent corrosion resistibility. 
     While there has been described in connection with the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the invention. 
     Being composed and worked as mentioned above, the rolling bearing of the invention can contribute to improvements of the endurance in the rotation supporters of jet engines for aircraft or gear boxes.