US 3283376 A
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Description (OCR text may contain errors)
Nov. 8, 1966 J. HOCKIN 3,283,376
METHOD OF INVESTMENT CASTING 0F BALL BEARINGS Filed Feb. 23, 1962 United States Patent 3,283,376 METHOD OF INVESTMENT CASTING 0F BALL BEARINGS John Hockin, 4305 S. Maple Ave., Brookfield, Ill. Filed Feb. 23, 1962, Ser. No. 175,178 3 Claims. (Cl. 22200) This invention relates to a new and improved method of casting ball bearings utilizing the essential features of investment casting as modified to produce the maximum benefits of investment casting techniques and the maximum physical properties attainable by conventional sand casting.
While this invention relates generally to the casting of any type of ball bearing, its most effective economic benefits are realized in the casting of ball bearings from wearresistant alloys such as the cobalt and similar base alloys. Such ball bearings are cast from such alloys for use in bearings operating at high temperatures and for use in particularly corrosive environments. An example of the use of such bearings is in nuclear submarines where relatively high temperatures and highly corrosive influences are involved and in which the operation of the bearings is critical to the effective operation of the unit.
At the present time, such ball bearings cast from wearresistant alloys are made by the old method of sand casting. The sand casting technique necessitates the production of a new mold for each bearing. Because two halves are necessary to complete the mold, the resultant bearing is usually imperfect in having a circumferential projection and because the sand casting necessitates a relatively large gate area in order to produce a sound casting, expensive subsequent machining operations are necessary to reduce the casting to a relatively round element within the tolerances permitted and to eliminate the parting lines caused by the mold halves.
Therefore, it is an object of this invention to provide a new method of casting ball bearings which utilizes modifications of the investment casting technique to eliminate the imperfections caused from the use of mold halves and to reduce substantially the imperfections resulting from the use of a large gate area, to significantly reduce the extent of subsequent machining operations to a minimum, while providing ball bearings having a higher static fracture load than normally obtainable by sand casting, and a greater resistance to plastic deformation under load than would normally be expected from investment castmg.
In adapting the techniques of investment casting to suit the requirements of casting ball bearings, it is recognized that investment casting has always involved the pouring of molten metal into a heated investment mold. In fact, prior to this invention, it was thought that investment casting techniques were limited to the use of the heated mold, it being impossible to otherwise achieve a satisfactory castlng in most cases.
However, in casting ball bearings of wear-resistant alloys, it was found that the pouring of molten metal into a heated mold resulted in a less rapid cooling of the casting metal, the balls produced not being as hard or as resistant to plastic deformation as those produced by sand casting, which utilizes the principle of poring a molten metal into a cold sand mold.
Therefore, it is a further object of this invention to provide an improved method of investment casting which permits and teaches the pouring of molten metal into a relatively cold investment mold in order to achieve casting hardness, without jeopardizing the superior metallurgical structure of the investment casting.
Particularly when dealing with wear-resistant alloys such as the cobalt base alloys, the expense of the alloy is a critical economic factor demanding the elimination of ice waste, and, because such alloys melt at suchhig-h tempera tures, also reduction of the amount of said alloys which must be melted. This waste is often expressed in terms of ratio of runner to mold cavity.
It is a still further object of this invention to teach and provide a method of casting wear-resistant alloy ball bearings which minimizes the amount of gate and runner which is allowed to solidify during casting.
Further objects of the invention are to teach and provide a method of casting wear-resistant ball bearings in a manner which permits the bearing to be automatically separated or easily broken from the solidified gate portion and which produces a substantially spherical bearing element with a sufficiently smooth surface of a dimensional tolerance which eliminates grinding of the gate area.
Further objects and advantages of this invention will i be evident from an examination of the particular description and from the accompanying drawings, in WhlCll- FIG. 1 is a cross-sectional view of the pattern to be used to form the investment mold;
FIG. 2 is a view in cross-section of a plurality of such patterns sprued together before the formation of the investment mold;
FIG. 3 is a cross-section of the investment mold formed by the pattern shown in FIG. 2; and
FIG. 4 is an enlarged cross-sectional view of a ball bearing formed pursuant -to the method of this invention.
The basic techniques of investment casting are set forth in my copending application Serial No. 110,417, filed May 16, 1961 and now Patent No. 3,156,023. As used in this application, the term investment casting is intended to include the use of monolithic shells.
In the basic process, a pattern is usually formed by injecting a heat-disposable wax or plastic into a die to make a pattern. In the case of the production of a ball bearing, and particularly a ball bearing made of a wear-resistant alloy such as a cobalt base alloy, the pattern is formed as shown in FIG. 1 of a substantially spherical portion 2 to which is connected an outwardly projecting stud 4 which is substantially conical, the apex of the stud being secured to the spherical portion 2 and the base portion projecting outwardly therefrom so that the minimum diameter of the stud occurs at the intersection with the spherical portion 2 and the maximum diameter of the stud occurs outwardly therefrom.
These patterns can be made so that they can be easily attached to or formed integrally with a sprue such as shown at 6 in FIG. 2 to form a cluster. As shown in FIG. 2, the sprue 6 comprises an elongated cylindrical rod which terminates at one end in a frusto-conical mouth portion 8 which forms the pouring cup for the molten metal when the investment mold is formed. The individual ball and gate patterns 1 may be secured to the central sprue 6 by heating the ends of the studs or in other manners of attachment known and commonly used in the industry.
When the ball and gate portions 1 are secured to the sprue 6 to form a cluster, an investment mold is made as shown in FIG. 3 according to the conventional manner, or in accordance with the invention shown in my copending application Serial No. 110,417.
In FIG. 3, it will be particularly noted that the gate portions 10 are what are referred to as pinpoint gates, the diameter of the opening between the spherical portion and the remaining portion being substantially less than /3 of the diameter of the spherical portion. This ratio is far less than is conventionally thought necessary as a minimum for the production of radiographically sound castings.
After the investment mold is thus formed, the pattern material is then removed, usually by the method of plac- 3 ing the mold in a high temperature oven which is fired at temperatures between 1300 and 2100 F. to remove all pattern material and carbonaceous matter. The mold may remain in this high temperature oven for some time, depending upon the size of the mold, type of investment, and complexity of pattern.
At this point, the present method and the conventional methods used in investment casting depart radically. For in conventional investment casting, after the pattern has been removed and the investment mold is still at a high temperature, the molten metal is poured into the molds to form the casting. In the casting of a wearand corrosion resistant ball bearing, hardness of the casting is desirable and pouring the molten metal into a hot mold reduces significantly the resistance of the casting to plastic deformation and hardness.
Therefore, the next step is to cool the investment mold to a temperature substantially lower than that used to remove the pattern material. This may be achieved by permitting the investment mold to cool gradually to room temperature, or some type of refrigeration where more rapid cooling may be desired.
The molten base or casting metal to be used is heated to above its melting point in any type of high temperature furnace and poured into a ladle. poured into the investment mold through the pouring cup 8 and through the runner 6 until the runner is completely filled. The molten metal is allowed to remain'in the runner for from 2 to 30 seconds, depending upon the size of the ball and the time required for it to solidify in the mold cavity, and then the mold is lifted and the molten metal remaining in the runner 6 may be, in the preferable mode of the method, poured back into the ladle. This, of course, obviates wasting metal which would ordinarily solidify in the runner and can further reduce the amount of metal which need be used for successive operations.
Also, as a unique result of using this method, the metal in the mold cavity solidifies before that in the runner, and Whether the molten metal in the runner is returned to the ladle or not, the solidification of the metal in the spherical mold cavity automatically serves it from the metal in the gate portion because of the shrinkage. Otherwise, because of the pinpoint gate, the gate can be easily and simply broken from the spherical portion 2 to yield a substantially spherical ball bearing which is sufficiently exact in dimension to eliminate or significantly reduce subsequent machining.
Ball bearings produced according to the method previously described have been found to exhibit a high degree of radiographical soundness.
The theories which explain this phenomenon have not been verified but are here postulated for Whatever aid they may be to others working in this art.
One of the fundamental problems of investment casting is the fact that the molten metal tends to harden around the periphery of the mold before it hardens in the center. metal in the central portion of the casting which, if the gate is not sufiiciently large, results in a depression in the gate area. This depression or imperfection obviously would be objectionable in a spherical ball bearing. However, in the instant method, it is possible that the relatively large volume of molten metal flowing through the pinpoint gate heats up the gate to a temperature substantially higher than the mold area surrounding the spherical ball. This permits the liquid in the gate and the runner to feed the ball as the cooling and solidification occurs, and obviates shrinkage on the surface of the ball. This, then, could explain the obtaining of a sound casting. It is also possible that this small gate solidifies before the ball, and any shrinkage occurs internally within the ball,
Usually, this results in a shrinkage of the The metal is then which is not critical to the elfectiveness of the ball in operation. This latter conjecture has not been demonstrated detectably by radiographic or other inspection methods. In any event, the use of the extremely small pinpoint gate and the relatively cold investmentmold departs radically from what has been known of effective investment casting techniques in the past.
I have conducted extensive tests relating to the castings produced by the aforedescribed process. I have found that such castings embody both the advantages of investment casting and those of sand casting, while minimizing or eliminating the objectionable features of each. More specifically, the aforedescribed process results in eliminating the parting lines caused by sand molding, thus reducing the amount'of machining required to produce ball bearings within the dimensional tolerances required, while producing ball bearings having a higher static fracture load than normally obtainable by sand casting, and a greater resistance to plastic deformation under load than would normally be expected from investment casting;
1. In a method of casting high temperature wear'and corrosion resistant alloy ball bearings, the steps of:
providing an investment mold having a pin point gate. leading to a spherical cavity, the gate narrowing 1n the direction of the cavity until the gate at the cavity is, substantially less than one third of the diameter of the spherical cavity, but of sufficient diameter to permit the flow of molten metal therethrough,
heating a high temperature wear. and corrosion resistant allow to a temperature above the melting point of the alloy, pouring the molten wear and corrosion-resistant alloy into the investment mold, where the alloy flows through the pin point gate into the spherical cavity,
the temperature of the mold atthe time of beginning the pouring being substantially lower than 1300 degrees F., the minimum temperature required to burn out the .pattern material, used to form the .mold, but not lower than room temperature,
whereby the high temperature of the molten alloy passing through the pin point gate heats the gate to a tempera ture so much higher than that to which the mold surrounding the cavity is raised that the molten alloy in the mold cavity solidifies before the molten alloy within the gate.
2. The method recited in claim 6 wherein the high,
temperature wear and corrosion-resistant allow is a cobalt base alloy.
3.The method of casting high temperature wear and corrosion resistant alloy ball bearings recited in claim 1 wherein the investment mold provided has a plurality of spherical cavities and pin point gates connected to a central sprue to form a cluster.
References Cited by the Examiner UNITED STATES PATENTS 1,104,037 7/1914 Custer 22-200 1,156,093 10/1915 Pack- 22-200 2,375,486 5/ 1945' Morin 22200 OTHER REFERENCES Precision Investment Casting by Cady, Reinhold Publishing Co., New York, 1948, pp. 8, 9, 16, 17 and, 19.
Investment Castings by Wood et al., Reinhold Publishing Co., New York 1952, pp. 115, 153, and 154.
WILLIAM J. STEPHENSON, Primary Examiner.