US3746073A - Method of casting hollow metal balls - Google Patents

Abstract

A cast steel ball having a centrally located spherical void and an uninterrupted outer surface is provided for use in mills used to pulverize material. The ball is produced by a centrifugal casting method in which the axis of rotation of the ball is constantly changed during solidification of the molten steel. A mold arrangement made of a chill material and having no risers is provided in order that the outer surface of the ball is uninterrupted and of substantially the same grannular structure throughout.

Description

United States Patent 1191 Schmuck et a1.

[ METHOD OF CASTING HOLLOW METAL BALLS [75] lnventors: James E. Schmuck, Tempe, Ariz.;

Walter C. Troy, National City, Calif.

[73] Assignee: AMSTED Industries, llncorporated,

Chicago, 111.

22 Filed: Dec. 1, 1971 211 App]. No.: 203,593

[52] U.S. Cl. 164/116 [51] Int. Cl B22d'l3/06 [58] Field of Search 164/116,115,114; 425/429, 435

[56] References Cited UNITED STATES PATENTS 2,325,019 7/1943 Rubissow 164/114 X 3,104,423 9/1963 Kemper 425/429 Y 3,528,133 9/1970 Morse 425/435 1 July 17, 1973 2,222,266 11/1940 Rubissow 164/115 3,614,976 10/1971 Bolling et al 164/114 X FOREIGN PATENTS OR APPLICATIONS 1,020,076 2/1966 Great Britain 164/1 16 Primary ExaminerRobert D. Baldwin Attorney-Walter L. Schlegal, Jr. et a1.

[57] ABSTRACT A cast steel ball having a centrally located spherical void and an uninterrupted outer surface is provided for use in mills used to pulverize material. The ball is produced by a centrifugal casting method in which the axis of rotation of the ball is constantly changed during solidification of the molten steel. A mold arrangement made ofa chill material and having no risers is provided in order that the outer surface of the ball is uninterrupted and of substantially the same grannular structure throughout.

3 Claims, 6 Drawing Figures Patented July 17, 1973 3,746,073

3 Sheets-Sheet 1 fizz/91:51.5

Patented July 17, 1973 3,746,073

3 Sheets-Sheet 2 m g 68 hm.

METHOD OF CASTING HOLLOW METAL BALLS This patent relates to a cast metal ball having a void located at its approximate geometric center and to a method and apparatus used in making the ball.

Metal balls have been used in mills for pulverizing coal used in firing boilers. As contact is made between the ball and the material to be crushed a given amount of the balls outer surface is worn away. A hard abrasion resistant outer skin is therefore advantageous to prolong the useful life of the ball. Furthermore, it is desirable to have balls with a relatively smooth outer surface so that during rotation of the mill the material to be crushed is located between the outer surface of the ball and a surface of the mill. Voids in the outer surface of the ball allow material to locate therein requiring added rotation to pulverize the material and possibly the passage of particles that are larger than desired.

These balls have traditionally been made by forging. Limitations of the forging process, however, prevent the use of high alloy materials that have desirable abrasion resistant qualities. It has therefore been necessary to accept high wear rates for forged balls.

A cast ball has recently come into use. The balls are cast out of high alloy materials in sand molds which create a good grain structure. It is difficult, however, to feed a cast ball in a mold. Risers must be relatively large thereby creating, in the area of feed, a surface having a lower abrasion resistant quality than an adjacent surface contacting the mold. During use of such balls certain sections of the outer surface will wear faster than other sections. Such differential wear is highly undesirable.

It is further desirable for the balls to have a centrally located void. This void has been accomplished to a degree by the use of a core suspended by a cross-shaped support in the mold. These supports, however, leave either holes in the outer surface of the ball or high wear surface areas. Furthermore, after the casting solidifies the center of the balls contain the material of the core which is generally sand. During use of the balls, the outer skin is worn away thereby opening the sand filled cavity to its surroundings and allowing the sand to be mixed with the coal or the substance being pulverized. Such mixing is highly undesirable. Movement of the core prior to or during casting also creates problems. A ball having an off centered void has un unfavorable dynamic balance which to a degree affects the opera- 7 tion of the mill.

BRIEF DESCRIPTION OF THE INVENTION The above problems and others have been solved by applicants by providing an alloy iron ball having a centrally located spherical void and an uninterrupted chillsolidified body. In producing these balls, molten metal is poured into a spherical cavity located in a stationary graphite mold. After a given period of time the mold is slowly rotated on a horizontal axis that corresponds with the geometric center of the ball. As the metal inside the cavity solidifies a shrinkage void develops inside the ball. Since the ball is being rotated, the molten metal within the ball continuously washes over the inside of the solidifying skull causing the skull to increase in thickness without the formation of discontinuities thereby locating the void toward the center of the ball. When the skull is thick enough to allow safe handling of the ball with melt inside, the ball is removed from the mold and centrifugally spun by a method which causes random location of the spin axis. By constantly changing the spin axis, the shrinkage void becomes centrally located within the ball.

To accomplish the random location of the spin axis the partially solidified ball may be placed inside of a cylindrical drum which is rotated on a horizontal axis. The drum is spun at an angular velocity sufficient to centrifugally cast the melt in the ball. To change the axis of rotation of the ball, the drum is either oscillated or rotated on a vertical axis. The oscillational or rotational movement is resisted by the angular momentum of the spinning ball causing a certain amount of skidding between the ball and the surface of the drum and a corresponding changing of the axis of rotation of the ball. Such skidding may be aided by the presence of graphite particles. The solidified ball is removed from the drum and heat treated in an appropriate manner.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. '1 is a schematic sectional view of a mold arrangement used in casting metal balls taken on the centerline of the casting cavity.

FIG. 2 is a side elevational view of a machine used to cast metal balls and illustrates the mold arrangement of FIG. 1 in an operational position.

FIG. 3 is a top plan view of the machine illustrated in FIG. 2.

FIG. 4 is a schematic side elevational view, partially in section, of an apparatus used for spinning a metal ball during solidification.

FIG. 5 is a sectional view of a cast metal ball taken on a line which corresponds to line A--A in FIG. 2 and embodying features of the invention.

FIG. 6 is a view of another cast metal ball sectioned similar to the one illustrated in FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS A mold arrangement 10, schematically illustrated in FIG. 1, comprises mating upper and lower blocks 12 and 14 made of graphite or another chill material defining a cavity 16. A passage 18 is located in upper block 12 for venting air during casting. Cured thermosetting sand 19 may be used to prevent molten metal from entering passage 18 and permit the passage of air.

Cavity 16 is filled with molten metal from the bottom through a passage 20, which may be lined with clay, cured thermosetting sand or the like. The lining material is chosen with the idea of controlling the cut-off time, i.e., the time it takes the metal to freeze within passage 20 during casting. A runner 22 made of cured thermosetting sand has a vertical passage 23 in comm unication with a horizontal passage 24 which communicates with passage 20. It is desired that the horizontal passage 24 extends beyond passage 20 for several inches to achieve favorable casting conditions. The passage 23 extends above the upper portion of cavity 16 in order that molten metal introduced into passage 23 will completely fill cavity 16 as a result of the force of gravity. Runner 22 may be detachably connected to mold arrangement 10 in any one of a number of known ways (not shown).

A machine 26 (FIG. 2 and 3) may be used to rotate the mold arrangement 10 about an axis A-A that passes through the geometric center of cavity 16. A frame 28 has sections 30 and 32 respectively around blocks 12 and 14. Spaced L shaped members 34 are secured to each section 30, 32 and contact an outer surface of each block 12,14. The sections 30 and 32 along with the L shaped members 34 are drawn toward one another by bolts 36 and nuts 38 to tightly engage the mold blocks 12 and 14.

A pair of spaced circular shafts 40 and 42 in axial alignment are connected to section 32. Each shaft 40, 42 is mounted on a pair of rollers 44 secured to machine 26 proximate mold arrangement 10 in a manner to permit easy rotation of the shafts about axis AA. Circular retaining plates 46 are secured to each shaft 40,42 on opposite sides of each pair of rollers 44 to prevent axial movement of the shafts. A crossbar 48 is connected to the end of shaft 40 for rotating the mold arrangement 10 byhand.

An electric variable speed motor 50 on machine 26 is connected to a gear reducer 52 in a known manner. A standard roller chain 54 interconnects sprocket wheels 56 and 57 which are respectively connected to a shaft on gear reducer 52 and shaft 42 in a manner to transmit rotational movement therebetween. Shaft 42, and accordingly mold arrangement 10, may be rotated by motor 50 throughout a range of angular velocities.

ln operation, cavity 16 is sprayed with a known mold wash. The two mold blocks 12 and 14 are then mated and clamped together by frame 28. Bar 48 is then turned by hand to position mold arrangement 10 so that pouring passage 20 is vertical and located at the bottom of cavity 16. Runner 22 is then secured to mold arrangement 10. Molten metal is poured into passage 23 of the stationary mold arrangement 10. The cavity 16 should be filled in about 20 seconds.

At the completion of the pour it is desirable to seal passage 20 as soon as possible. A known slide gate (not shown), may be used for this purpose or chill rods made of steel may be inserted into passage 23 to speed up the freezing of molten metal therein. If the chill rods are used, after a period of time, mold 10 may be slowly rotated by hand to dump excessive metal out of runner 22. With a 12% inch diameter ball, mold 10 may be hand rotated about 30 seconds after cavity 16 is filled.

After the ingate 22 is removed, mold 10 may be slowly rotated at an angular velocity of from to 20 rpm by motor 50. This rotation serves to prevent internal separation of the solidifying metal by maintaining the solidifying skin at a temperature at which the molten metal will bond or weld. The ball should never be left at one location long enough to allow the skin to chill to a temperature which is too low to permit bonding or welding of the molten metal. Mold arrangement should be rotated for a sufficient period of time to allow the molten metal to form a skin that is thick enough to allow safe handling of the cast ball with molten metal inside.

After approximately 3 minutes of hand rotation of mold 10 the solidified skull ofa 12% inch diameter ball is generally strong enough to permit removal of the partially liquid ball so that it may be centrifugally spun. The transfer of the ball to the centrifugal casting machine must be completed in less than 45 seconds because of the possibility of developing an internal separation caused by the ball being located in one position for a period of time. An apparatus for performing the centrifugal casting of the ball on a randomized spin axis is illustrated in FIG. 4.

As previously noted, the ball is moved from mold arrangement 10 of machine 26 to a drum 58 as quickly as possible. The ball should not be permitted to remain in a static position during transfer. The drum 58 is rotated on an axis B-B which passes through a shaft 60 mounted in known pillow blocks 62. A known motor pulley arrangement 64 is used to rotate shaft 60 and drum 58 at an angular velocity suitable for the ball being cast, e.g., 150 rpm for a 12% inch diameter ball. Higher angular velocities are needed for smaller diameter balls and lower ones for larger balls.

As previously noted, during this stage of solidification of the molten metal within the ball, the axis of rotation of the ball should be constantly changed. This change may be accomplished in drum 58 by mounting drum 58 and the motor-pulley arrangement 64 on a turntable 66 that may be moved a number of degrees upon an axis that is angular relative to axis B--B. Where possible, 360 rotation of turntable 66 is preferred and may be achieved by mounting the turntable on rollers 68 and rotating the turntable by hand or known power means. Multi-axial spinning of the ball is predicated upon the fact that skidding between the ball and drum 58 occurs when turntable 66 rotates or oscil lates. The skidding may be aided by adding a solid lubricant, such as graphite, to drum 58. When the lubricant is added and spin speeds are within the specific range, the ball will spin at right angles to the spin axis B--B of drum 58. Since the spin axis B-B is con stantly changing due to movement of turntable 66 the spin axis of the ball will also be constantly Changing.

The ball should be spun in drum 58 until the outer surface of the ball has reached a temperature which indicates the completion of the solidification process. This temperature may be determined by an optical pyrometer or other means. The temperature, of course, will vary depending upon the given metal used. The solidified ball may thereafter be removed from drum 58 I and heat treated.

It is necessary to maintain certain variables during casting within given limits. The temperature of the molten metal during casting is critical. If the metal is too cold pouring wrinkles and laps will appear on the outer surface of the ball. If the molten metal as poured is too hot, longer time intervals are required in order to accomplish the desired solidification. in brief the pouring temperature must be selected in accord with casting characteristics of the particular material, as is well known in the art.

A cross-section of a typical cast ball 76 is shown in FIG. 5. The periphery or outer surface 78 of ball 76 is preferably smooth, tough and abrasion resistant. The shrinkage void 80 is substantially spherical but may have a slightly rough or erose inner surface 81. The skin 82 of the ball between the inner and outer surfaces 81 and 78 is preferably uniform in hardness, from outside to inside but may be slightly less abrasion resistant toward void 80.

It should be appreciated that the presence at void 80 is an essential affirmative element in configurational design of ball 76. It is not a necessary evil related to shrinkage but is rather a definite benefit. Elastic reactions are different in a solid ball as compared with a hollow ball both in service and during the occurence of transient stresses in manufacture. For these reasons the void requires engineering attention, particularly with respect to its centralization. Ill-centered voids will both disturb the residual state of stress and impair the dynamic balance of ball 76.

Balls 83 having a slightly elongated center void 84, please note FIG. 6, may be manufactured with the arrangement illustrated in FIGS. 1, 2 and 3. Even though a ball 83 is not as acceptable as a ball 76, they may be used successfully in many applications. A ball 83 with a slightly elongated or oval shaped but centrally located void 84 may be produced in cavity 16 by rotating mold after the passage 20 has frozen at an angular velocity of from 10-200 rpm with best results occuring in the 100 to 150 rpm range. Angular velocities above 150 rpm tend to elongate the shrinkage void 84 along the axis A-A of rotation while angular velocities lower than 100 rpm tend to form voids having extremely jagged internal surfaces and possibly may form a plurality of shrinkage voids.

Rotational movement of mold arrangement 10 during the casting ofa ball 83 must exist for a sufficient period of time in order to permit solidification of all the molten metal' within the ball. Such time period, of course, will depend upon molten metal temperature at pouring, the diameter of the cavity and the temperature of blocks 12 and 14. Upon solidification of the molten metal, balls similar to the one illustrated in FIG. 6 may be heat treated.

Regardless which method is used to produce the ball, the molten metal therein must be constantly moved. Prolonged static periods will only result in improper welding of the molten material to a cold inner skin. It has been determined that such improper welding will result in internal separation cracks which have undesirable effects on the radial heat flow within the ball. Portions of the ball lying outboard from a separation crack will heat more rapidly than other portions of the ball because of their partial isolation by a heat flow interface. The different rates of heating leads to extremely high axial compression stresses on the portion of the ball outboard from a separation crack with the usual consequence of severing the ball on a diametrical plane normal to the separation crack. It is, therefore, desirable to have a homogeneous skin with a uniform grain structure at any given radial distance from the centroid.

What is claimed is:

l. A method of casting spherical metal articles with a centrally located spherical void comprising the steps of: filling a spherical cavity of a mold with molten metal; then permitting a portion of the molten metal to solidify at the surface of the cavity forming a selfvsustaining homogeneous skull; then quickly removing the partially solidified article from the cavity and placing the partially solidified article in a drum disposed on a horizontal axis; then rotating the drum about the horizontal axis while simultaneously rotating the drum about a vertical axis disposed outside of the drum so that the article is spinning on a constantly changing axis; and continuing such rotation until the article has solidified.

2. The method of claim 1 wherein the mold is rotated during the step of partially solidifying the molten metal at the surface of the cavity. j

3. The method of claim 1, and including the step of lubricating the internal surface of the drum prior to placing the partially solidified article therein.

Claims (3)

1. A method of casting spherical metal articles with a centrally located spherical void comprising the steps of: filling a spherical cavity of a mold with molten metal; then permitting a portion of the molten metal to solidify at the surface of the cavity forming a self-sustaining homogeneous skull; then quickly removing the partially solidified article from the cavity and placing the partially solidified article in a drum disposed on a horizontal axis; then rotating the drum about the horizontal axis while simultaneously rotating the drum about a vertical axis disposed outside of the drum so that the article is spinning on a constantly changing axis; and continuing such rotation until the article has solidified.

2. The method of claim 1 wherein the mold is rotated during the step of partially solidifying the molten metal at the surface of the cavity.

3. The method of claim 1, and including the step of lubricating the internal surface of the drum prior to placing the partially solidified article therein.

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