Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
2001-10-04
2002-11-26
King, Roy (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
Reexamination Certificate
active
06485581
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rolling bearing holding a main spindle of a machine tool.
2. Description of the Related Art
A main spindle of a machine tool is required to rotate with high precision and small increase in temperature for maintaining high working accuracy. High precision and a small increase in temperature during rotation are also required for a bearing supporting the main spindle. To meet these requirements, a small amount of grease and a slight amount of oil (oil mist or air oil) are employed in bearings for lubrication.
A small temperature rise in bearings for use in the main spindle of the machine tool must be maintained to prevent a degradation in working accuracy caused by the thermal deformation of the main spindle. Thus such bearings are used in the demanding condition of high-speed rotation with a small amount of lubricant oil. Under such a harsh condition, roughness and wear on the raceway caused by a slight shortage of oil film, damage such as peeling and smearing, and a shortened lifetime caused by seizing may be problems. A shortage of oil film thickness can be caused by, for example, an entry into the bearing of cutting oil or chippings of the workpiece, an increase in the working load, and heat generation in the motor. Roughness and wear in the raceway may cause noise during use and degradation in the rotation accuracy of the main spindle. As the rotation speed of the main spindle is expected to further rise in the future, it is very important to prevent the damage described above.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a bearing that has an excellent durability and an excellent surface damage resistance when used in the main spindle of a machine tool.
In the bearing for use in a main spindle of a machine tool according to a first aspect of the present invention, at least its raceway is made of steel containing by mass, C (carbon): no less than 0.6% and not more than 1.3%; Si (silicon): no less than 0.3% and not more than 3.0%; Mn (manganese): no less than 0.2% and not more than 1.5%; P: at or less than 0.03%; S (sulfur): at or less than 0.03%; Cr (chrome): no less than 0.3% and not more than 5.0%; Ni (nickel): no less than 0.1% and not more than 3.0%; Al (aluminum): at or less than 0.050%; Ti (titanium): at or less than 0.003%; O (oxygen): at or less than 0.0015%; N (nitrogen): at or less than 0.015%; and the rest is composed of Fe (iron) and unavoidable impurities. The raceway is tempered after either quenching or carbonitriding and its surface hardness presents at least HRC (Hardness of Rockwell C-scale) 58 after tempering.
The steel of the above composition, if it is quenched and tempered, has an excellent rolling fatigue resistance even with no carbonitriding. Thus it is possible to omit the carbonitriding process and thereby reduce the manufacturing cost thereof. Although it is preferable to omit carbonitriding process in terms of a reduction in the manufacturing cost, an excellent rolling fatigue resistance can be attained by applying carbonitriding instead of quenching.
Besides, the steel of the above composition is cheaper than precipitation hardening bearing steel such as M50.
A correlation is recognized between the surface hardness of bearing components made of the steel of the above composition and rolling fatigue life: higher surface hardness is likely to provide longer rolling fatigue life. Thus the rolling fatigue life is extended in the invention by making the surface hardness HRC 58 or higher. If the surface hardness is less than HRC 58, the rolling fatigue life tends to become significantly shorter, and fluctuations in useful life increase.
The improvements disclosed in this invention provide an inexpensive and highly rolling fatigue-resistant bearing for use in the main spindle of a machine tool. The bearing for use in the main spindle of a machine tool may be an angular contact ball bearing or a cylindrical roller bearing.
The following is an explanation of the preferable range of each component contained in the steel according to the present invention. The term “%” as used herein means percentage by mass, unless indicated otherwise.
C: 0.6 to 1.3%
Carbon is a component essential for ensuring a strength high enough for roller bearings. In this invention, the percentage of carbon content is at least 0.6% in order to attain a predetermined hardness after heat treatment. Carbides play an important role in extending rolling fatigue life; however, it has been found that large particles of carbide are generated and then shorten the rolling fatigue life if the percentage of carbon content exceeds 1.3%. The upper limit of carbon content is, therefore, determined as 1.3%.
Si: 0.3 to 3.0%
It is preferable to add silicon because Si prevents softening at high temperatures and improves the heat resistance of bearings. The lower limit of the percentage of silicon content is determined as 0.3% because such effects do not appear if Si content is less than 0.3%. The heat-resistance of bearings is increased as Si content increases; however, if the Si content exceeds 3.0%, the effect of silicon addition reaches a maximum and workability at high temperatures and machinability decreases. Therefore, the upper limit of the silicon content is determined as 3.0%.
Mn: 0.2 to 1.5%
Manganese is an element used for deoxidation of steel and the improvement of quenching properties. Since at least 0.2% of Mn addition is required to attain such effects, the lower limit of the Mn content is determined as 0.2%. On the other hand, if more than 1.5% of Mn is contained in steel, its machinability decreases significantly. Thus the upper limit of Mn content is determined as 1.5%.
P: 0.03% or less
Phosphorus segregates in an austenite grain boundary and thereby decreases the toughness and rolling fatigue life of steel. Therefore, its content is limited to 0.03%.
S: 0.03% or less
Sulfur harms the hot working ability of steel, and decreases the toughness and rolling fatigue life of steel, forming non-metallic inclusions. Its upper limit is, therefore, determined as 0.03%. It is preferable to make the S content as low as possible since sulfur exerts such negative effects on steel. However, since sulfur has the effect of improving machinability, sulfur may be included at up to 0.05%.
Cr: 0.3 to 5.0%
Chrome is an element which plays an important part in the present invention. This element is added to steel to improve its quenching properties, increase hardness by forming carbides and extend useful life. Since steel has to contain Cr at a level of at least 0.3% to provide a predetermined amount of carbide, the lower limit of Cr content is determined at 0.3%. On the other hand, if its content exceeds 5.0%, large carbides are generated and then rolling fatigue life is shortened. The upper limit of Cr content is, therefore, limited to 5.0%.
Ni: 0.1 to 3.0%
Nickel is also an important element in this invention, preventing the change in texture during rolling fatigue at high temperatures and the decrease in hardness at high temperatures, thereby extending the rolling fatigue life of the bearing. In addition, the addition of Ni leads to higher toughness and longer life under the existence of foreign substances as well as an improvement in the corrosion-resistance. Since steel has to contain Ni at a level of at least 0.1% to attain these effects, the lower limit of Ni content is determined as 0.1%. However, if the Ni content exceeds 3.0%, a large amount of austenite remains in the steel after quenching and the predetermined hardness cannot be attained. Besides, the cost of steel rises with the addition of Ni. Thus the upper limit of Ni content is determined as 3.0%.
Al: 0.050% or less
Aluminum is used as a deoxidizer during steel manufacturing. Since Al forms oxide inclusions with a high hardness and shortens the rolling fatigue life, Al content should be reduced as much as possible. Also if Al content exceeds 0.050%, the rolling fatigue life of the bearing is significantly
Mizutani Mamoru
Tako Hiroshi
Arent Fox Kintner & Plotkin & Kahn, PLLC
King Roy
McGuthry-Banks Tima
NTN Corporation
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