In the past, traditional methods for fixing anti-friction bearing races into openings or onto shafts have been rather limited, the most preferred and widely used being an interference fit in which the mating portion of a bearing race is sized slightly smaller or larger than the respective shaft or opening it is designed to fit. Of course, the degree of "interference" between the bearing race and its shaft or opening and the materials thereof dictates the method used to get the race into place. A small degree of interference facilitates a press fit wherein the race may be evenly pressed into place by a machine press, while a greater degree of interference usually necessitates heating material around an opening or cooling a shaft to take advantage of thermal expansion or shrinkage to enlarge or reduce an opening or shaft into/onto which a race is to be fitted. Also, the bearing may be cooled or heated to some extent to increase the effect. One particular example of this is in the automotive engine refurbishing industry wherein when valve seats of cylinder heads deteriorate or become damaged, it is most preferable, if the cylinder head is fabricated from aluminum, to replace them by placing the head in a suitable oven and heating the head. The coefficient of thermal expansion is greater for the aluminum alloy head than the hardened steel alloy of the valve seats, and upon reaching a selected temperature in excess of engine operating temperature, the valve seats may be removed with little or no applied force. Naturally, the degree of interference between the valve seats and their openings in the head is selected to tightly hold the valve seats in the head at engine operating temperature (180.degree. to 220.degree. F.) but yet become loose in the oven (approximately 500.degree. F.). After removing the damaged seats, one quickly puts the new seats in after reheating the head, with the seats being at room temperature, or the seats may be cooled in a freezer (approximately 0.degree. F.) to further shrink their diameters and facilitate convenient installation.
The interference fit technique and methods for assembly work well enough in automotive and other industries, but for specialized application such as high-speed cryogenic pumps found in rocket motors, problems arise. Particularly, in an upper main roller bearing assembly of the Space Shuttle Main Engine (SSME) Alternate Turbopump Design (ATD) of the fuel turbopumps, difficulty was experienced with the inner race of the bearing occasionally cracking after a short period of time. The race, constructed of 440C steel hardened to an Rc (Rockwell C) of 56 to 62, possesses a coefficient of thermal expansion of 4.05.times.10.sup.-6 in/in of averaged over a temperature range of +70.degree. to -400.degree. F., while the shaft, constructed of Inconel.TM.718 or IN100, possesses a coefficient of thermal expansion of 4.8.times.10.sup.-6, likewise averaged. 440C is used for the race because it can be hardened to the desired hardness, has excellent general corrosion and rust resistance, which is important because only small amounts of dry film transfer lubricant are used in the bearing, and it must tolerate long storage times unprotected by lubricant film. Further, 440C is not subject to hydrogen embrittlement, important because of the hydrogen-rich environment around the bearing when in use. While it is believed there are other materials that may perform adequately, 440C appears to be the optimum design choice for the above-stated reasons.
Bearing in mind that these components are subjected, when in operation, to extremely cold temperatures of approximately -400.degree. F., followed by a return to ambient temperature, calculations show that, given a shaft size of 2.8739".+-.0.0001", the difference in contraction between the shaft and the opening in the race due to the differences in coefficients of thermal expansion will cause a reduction in diameter of the shaft 0.0010" more than the reduction of the opening in the race, loosening the interference fit. Further, centrifugal growth of the race at operating speed (approximately 37,341rpm) causes the opening in the race to expand an additional 0.0028" more than the shaft, further loosening the fit. From this, it is seen that in order to have a tight fit of the inner race on the shaft at cryogenic operating temperatures, the opening in the race must be undersized by approximately 0.004" at room temperature, with 0.005" being considered the minimum acceptable and providing 0.001" interference at cryogenic operating temperature and operating speed.
At least this degree of interference (0.001") is required to prevent occasional slippage between the race and shaft, causing the race to "creep" around the shaft, resulting in galling and other damage thereto. Worse yet, at room temperature, tensile stresses in the race, on the order of 48,100 PSI at 0.005" interference, promotes stress corrosion cracking over the storage times typical for rocket motors and their components, which could result in a cracked race. This problem was manifested during assembly of the race to the shaft wherein, when the shaft was cooled in liquid nitrogen to reduce its diameter, and the race was heated to expand its diameter so that the race could be easily positioned on the shaft, the race would occasionally crack as room temperature equilibration occurred between assembled shaft and race.
Responsive to this problem, the manufacturing of the race developed to a point where meticulous and expensive fabrication techniques to largely eliminate stress concentration points at a microscopic level were required in order to produce a sufficiently durable race at the aforestated stress levels. With the bearing ground and polished to perfection, the fit was sized at what was believed to be a best-obtainable balance between overstressing the race at room temperature on one hand and not having a tight enough fit at cryogenic operating temperature on the other. These efforts, however, are obviously expensive and time-consuming and demonstrate the need for a robust bearing race for the described purpose that is not so difficult to manufacture and assemble.
Attempts by others to overcome problems relating to anti-friction bearing races becoming loose owing to different thermal expansions are not directed to similar problems encountered in a cryogenic environment. Pertinently, U.S. Pat. No. 4,283,096, issued Aug. 11, 1981, to Picard et al., discloses an anti-friction bearing having an outer race restrained from thermal and centrifugal growth by a ring disposed thereabout and having a modulus of elasticity higher than, and a coefficient of expansion lower than, the outer race. However, the application of Picard et al. is directed toward turbine engines and does not address the problem of an inner race becoming loose on a pump shaft due to different degrees of shrinkage from extreme cold. Further, in the restrained outer race of Picard et al., the ring which restrains the race is not a part of the bearing itself but an additional component.
It is, therefore, an object of the present invention to provide an inner race for an antifriction bearing that will not loosen at cryogenic temperatures and is not under such high tensile stress at ambient temperature that will promote stress corrosion cracking.