Rolling element bearing having improved wear characteristics, and rolling element for such bearing

Abstract: A rolling element bearing comprises an outer ring and an inner ring which enclose a bearing space containing rolling elements, which rings consist of steel. At least one of the rolling elements comprises at least partially a zirconium material which is harder or stiffer than the material of the other rolling elements. Any indentations (3), in particular the projecting parts (5) thereof, which are formed by solid contamination particles upon compression between the raceway (2) in question and an rolling element of about the same hardness, are now compressed as soon as a rolling element with a substantially higher hardness overrolls them. Thereby, said smooth character of the raceways is restored. Only small holes (4) are left, which do not have a reduced or no negative influence on the running properties of the bearing. In these small holes minute quantities of lubricating oil (9) are collected, which help the lubrication of the rolling element in case of oil starvation.

Description: The invention is related to a rolling element bearing according to the preamble of claim 1. Normally, the rolling elements of such bearing are of steel as well, in particular of steel with about the same hardness properties as the steel of the rings. US-A-3 764 188 discloses such a rolling element bearing. From EP-A-89155, a bearing is known in which all rolling elements consist of a steel shell and a core of a zirconium material. This known bearing therefore does not have the capacity of smoothing any indentations in its raceways, having regard to the fact that all contacting surfaces consist of steel and therefore have the same hardness. During rotation of a bearing, the rolling elements roll over the raceways of the rings. Any contaminations, for instance small wear particles, which are on the raceways or the rolling elements become compressed between these elements. Hard wear particles, for instance small steel particles which have come off of one of the components of the bearing or have entered from the outside, resist such compression and lead to the formation of minute indentations in both the raceways and the rolling elements. It has been found that in particular the indentations in the raceways have a negative influence on the useful bearing life. Also. they lead to increased noise during running due to unsmooth running and increased play, caused by wear of the raceway surfaces. These indentations take a craterlike shape, whereby a small central hole is surrounded by a ridge of material that has been pushed upwardly and projects above the raceway. Conversely, the rolling elements are indented as well. Due to the cyclic character of the load exerted by the rolling elements, crack formation and fatigue may occur in the rings at the location of high stress concentration usually located just under the ridge of a dent. Although it has already been proposed to protect the bearings against such detrimental contamination particles by providing seals and filters, nevertheless the ingress thereof cannot always be totally prevented and therefore the formation of indentations cannot excluded. The object of the invention is therefore to alleviate the consequences of the occurrence of such indentations. This object is achieved in that at least one of the rolling elements at least has an outer surface shell of a zirconium material which is harder than the material of the other rolling elements. Any indentations, in particular the projecting parts thereof, which are formed by solid contamination particles upon compression between the raceway in question and an rolling element of about the same hardness, are now compressed as soon as a rolling element with a substantially higher hardness overrolls them. Thereby, said smooth character of the raceways is restored. Only small holes are left, which do not have a reduced or no negative influence on the running properties of the bearing. In these small holes minute quantities of lubricating oil are collected, which help the lubrication of the rolling element in case of oil starvation. Also, the stress concentration and thus the risk of crack formation and wear is basically reduced. Furthermore, the bearing will run more quietly. Normally, the other rolling elements consist of steel with about the same mechanical properties as the steel used in the rings. Despite its hardness, the zirconium material has a modulus of elasticity and thermal expansion which are about the same as those of steel. Also, their Poisson's ratio are about the same. Furthermore, their specific weights do not differ much. The advantage is that the application of such rolling elements of zirconium material amongst the other steel elements do not lead to dynamic imbalances nor to differences in expansion under varying temperature conditions. Thereby, under all circumstances a smooth operation of the bearing is obtained. The added benefits of this solution is the low cost of raw materials for the making of Zirconia ceramics and the possible ease of processability and workability of Zirconia. For instance, grinding can be used as possible finishing process with feed rate of 200-1200 um/min, (for steel typical fine grinding feed rate is about 1500 um/min). Initial cost saving on the Silicon Nitride counterpart is estimated of a factor five. A second embodiment of this invention is the use of Zirconia in the making of hybrid rolling elements produced by direct sintering or press-fitting or chemical or mechanical bonding of Zirconia ceramic on steel. This technology is made possible by the great similarity in the elastic constants of Zirconia ceramic and steel, thermal expansion and Young's modulus is practically equal to the one of steel. Furthermore risk of unbalance mass is also avoided, in case of concentricity errors of the two part, because similar density between the steel and Zirconia materials. Rolling element bearings wherein al rolling elements consist of ceramic material are already known. Having regard to the fact that in these bearings all rolling elements exert the same compressing action on the raceways, the development of such indentations with high ridges is prevented. However, due to the higher hardness and stiffness properties of these rolling elements, the stresses induced in the rings are also higher thus leading to a reduced life or capacity for such types of bearings. In the bearings according to the invention, most of the rolling elements are made in steel; thus the deformation is shared equally between the contacting bodies leading to lower stress and preservation of the dynamic capacity of the bearing. The invention will further be explained with reference to an embodiment shown in the figures. Figure 1 shows a partly side view of an outer ring, having indentations, of a bearing with normal rolling elements. Figure 2 shows a partly view of the outer ring, having small holes, in a bearing comprising at least one stiffer or harder rolling element. Figure 3 shows a ball shaped rolling element. The outer ring 1 of a rolling element bearing, the other parts of which have not been shown, has a raceway 2 over which the rolling elements may roll. Any solid contamination particles which get caught on the surface of the rolling elements and the raceway 2, will lead to the crater-like indentations, comprising a small hole surrounded by a ridge or projecting part 5. In turn, these projecting parts 5 will lead to stress concentration and to indentations in the rolling elements themselves. The presence of such indentations in both the rings and the rolling elements of the bearing lead to early fatigue and crack formation. Also higher vibrations and a noisy running of the bearing are obtained. According to the invention, at least one of the rolling elements is of substantially harder or stiffer nature, e.g. is made of a zirconium material. Such rolling element, when overrolling the indentations 3, will compress at least the protruding parts 5 thereof, as shown in figure 2. In this figure 2, an outer ring 6 is shown, of which the projecting parts have been compressed down, giving rise to lands 7 which correspond to the normal, non-damaged surface 8 of the ring 6. The holes which are formed by the indentations are left over. The risk of fatigue and therefore crack formation and wear in ring of a bearing having at least 1 stiffer or harder rolling element is considerably lower than in the normal bearings. The presence of the small holes 4 leads to a better lubricating of the raceway 8 and the rolling elements. In the holes 4 minute amounts of oil 9 are collected, which give better lubricating characteristics. The rolling element shown in figure 3 consists of an outer shell 10 of a zirconium material such as partial stabilised zirconia ceramic with tetragonal polycristals. Its core 11 consists of a suitable steel. of course, the rolling element according to the invention may also full consist of such zirconium material. In summary the following advantageous embodiments are made possible by the inventions. Improved wear/contamination resistant bearing with hybrid rolling elements. Bearing for difficult lubrication, thermal and environmental conditions (grease lubrication at high speeds, high temperature, corrosive environments, hydrogen corrosion, vacuum etc). Bearing for superior life expectancy for normal operating conditions (better resistance against surface initiated fatigue). Also the following problems are solved: Unbalance mass in hybrid rolling bearings. Uneven bearing internal clearance due to temperature changes in hybrid rolling elements bearings. Use of combined hybrid sintering technology (steel and ceramic coupled) in bearing components. Use of press fitting or chemical or chemical bonding technology (between ceramic and steel) for making bearing parts.

Claims: Rolling element bearing, comprising an outer ring (1) and an inner ring which enclose a bearing space containing rolling elements, which rings (1) consist of steel and whereby at least one of said rolling elements comprises a material which is different from the material of the other rolling elements characterised in that at least one of the said rolling elements has at least an outer surface shell (10) of a zirconium material which is harder or stiffer than the material of the other rolling elements. Bearing according to claim 1, wherein at least two rolling elements consisting at least partially of a harder material are provided which are regularly spaced in circumferential direction. Bearing according to claim 1 or 2, wherein each other rolling element consists of steel. Bearing according to claim 3, wherein each rolling element consisting at least partially of a harder material has an outer shell (10) comprising zirconium material. Bearing according to claim 4, wherein zirconium shell (10) encloses a steel core (11). Bearing according to claim 3, wherein each rolling element of a harder material fully consists of zirconium material. Bearing according claim 5, 6 or 7, wherein the zirconium material is a zirconia ceramic ZrO 2 . Bearing according to claim 7. wherein the zirconium material is a partial stabilised zirconia ceramic. Bearing according to claim 8, wherein the zirconia ceramic comprises an alloy of 95% ZrO 2 . 5% Y 2 O 3 and 0.01% Al 2 O 3 .

Inventors: DE VRIES GERARDUS, GABELLI ANTONIO, VEENHUIZEN PIETER ABRAHAM, DE VRIES, GERARDUS, GABELLI, ANTONIO, VEENHUIZEN, PIETER, ABRAHAM
Applicants: SKF ENG & RES CENTRE BV, SKF ENGINEERING & RESEARCH CENTRE B.V.
Application year: 1997
Main classification: F16C33
IPC codes: F16C33