US2513814A - Piston - Google Patents

Description

July 4, 1950 G. L. MOORE 2,513,814

PIs'roN Filed Jan. 21, 1947 2 Sheets-Sheet l llllllllll George L. M00 e July 4, 1950 G. L. Moons PISTON Filed Jan. 21. 1947 ffy 2 Sheets-Sheet 2 INVENTR.

Patented July 4, 1950 PISTON George L. Moore, Hudson, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Application January 21, 1947, Serial No. 723,344

2 Claims.

This invention relates to pistons for internal combustion engines or for like purposes and is principally concerned with problems attendant upon the use of a piston material, such as magnesium or aluminum alloy, which is relatively expansible in the sense that it has a higher coeiiicient of expansion than the material of the cylinder in which the piston is designed to operate.

The object of this invention is to provide a piston, particularly a piston having a skirt of oval cross section, which may be fitted to a satisfactory clearance in the cylinder when cold, which, when heated in operation, will not expand sufiiciently to produce excessive -scufiing or friction, but which has a skirt portion forming an effective operational seal with the cylinder wall and serving to center the piston during its operation in the cylinder. Further objects of the invention include the provision of pistons which will satisfactorily perform such functions without the necessity of including in the piston structure control members or struts made of material different in coeiiicient of thermal expansion than the material of the piston body or of slotting or otherwise breaking the continuity of the lower portion of the piston skirt to control the expansion thereof or the flow of heat therein.

The piston of this invention has a skirt which is, in large part, of the conventional oval shape such as is usually produced by a cam grinding operation. The major axis of the oval shape defined by the outer surfaces of the skirt is substantially at right angles to the axis of the wrist pin bosses and may be of constant length or of progressively decreasing or increasing length from top to bottom of the skirt if it is desired, in accordance with well known principles, to provide a tapering skirt. In accordance with this invention, however, the minor axis of the oval shape increases abruptly in length at a point below the wrist pin bosses of the piston so that, in effect, the surface of the skirt portion of the piston denes two oval shapes which, having a common major axis, blend in the thrust faces of the skirt and gradually separate in the area under the wrist pin bosses. Thus under each wrist pin boss the lower portion of the skirt forms a raised area which blends into the shape of the upper skirt in the region of the thrust faces.

The invention, its purposes and objects may be more readily described and explained with reference to specific embodiments thereof such as shown in the accompanying drawings in which:

- conventionally located in the skirt 2.

Fig. 1 is an elevation view of a piston constructed in accordance with this invention;

Fig. 2 is an elevational view taken at right angles to the view in Fig. 1;

Fig. 3 is a diagram on which is projected the vvarious mentioned axes of the piston shape, the

divergences in` dimensions being greatly exaggerated for purposes of illustration.

Fig. 4 is an elevational view of another type of piston constructed in accordance with this invention.

Fig. 5 is an elevational view taken at right angles to the View in Fig. 4.

The piston shown in Figs.1 and 2 of the drawing is, except as indicated, of conventional shape and includes a head portion I to which is appended a skirt 2. Transverse slots 3 separate the thrust faces 4 of the skirt from the head portion l. Further slots, an example of which is the downwardly extending slot 5 indicated in dotted line in Fig. 2, may also be provided. Wrist pin bosses, openings of which are shown at 6, are

Conventional grooves 1 are provided in the head portion I for the reception of piston rings. The piston is formed, as by cam grinding, with a skirt portion the surfaces of which form an oval shape. The major axis of this oval lies, as indicated in Fig. 3, at right angles to the axis of the wrist pin bosses and extends between the thrust faces 4. In the piston of this invention the major axis is of the same length throughout the height of the skirt unless, as above mentioned, the skirt is tapered, in which case the major axis gradually increases in length from top to bottom or bottom to top, as the case may be, of the piston skirt. Throughout the greater portion of the height of the piston skirt the shorter or minor axis, which is disposed at right angles to the major axis, as indicated in Fig. 3, likewise is of constant length, or of constantly increasing length if a skirt taper is provided, but, in accordance with this invention, the minor axis` abruptly increases inlength at a point below the wrist pin bosses so that the lower portion of the skirt defines an oval of greater minor axis than the upper portion of the skirt. Preferably this lower portion of the skirt is not great in height and lies well below the wrist pin bosses.

Thus the skirt of the piston shown in Figs. 1 and 2 is fashioned to define a lower oval and an upper oval shape which form a juncture at a point below the wrist pin bosses and which differ only in the increased length of the minor axis of the lower shape thus resulting in the formation of a. step, ledge or shoulder 8 (see Figs. 1 and`2) below the wrist pin boss openings, which ledge gradually disappears in the direction of the thrust faces 4 as the two oval shapes blend together to conform to the same major axis. To achieve the objects of this invention the divergence in length between the minor axis of the lower ovate shape and the upper ovate shape need not be large, a divergence in the order of .004 to .006 inch usually being suflicient in passenger car sizes of pistons.

In operation the lower skirt portions below the shoulders or offsets 8 assume, under operational temperatures, a shape closely approximating that.

of the cylinder in which the piston operates, thus forming an effective seal between the bottom portion of the `skirt and the cylinder wall, over a surface which while great enough in area to effect this purpose and to prevent sidewise motion or tilting of the piston in the cylinder, is yet suiciently small to avoid excessive friction with the cylinder wall.

A piston having a skirt of the shape described may be formed in any suitable manner. A convenient method is to grind a conventionally cast piston skirt to form the two ovate shapes.

It is also possible, if desired, to so cast the piston as to roughly form offset portions below the wrist pin boss opening which may later be finally shaped and dimensioned in the grinding operation. Such a piston is illustrated in Figs. 4 and 5. In this piston, which comprises a head portion I to which is appended a skirt II, the thrust faces I2 of the skirt are separated from the head portion I0 by transverse slots I3. A slot I4 extends downwardly in one of the thrust faces. Wrist pin bosses, the openings of which are shown at I5, are located in the skirt I I. Conventional grooves I6 are provided in the head portion I0 for reception of piston rings. The

vbottom edge of skirt I I is arcuate in shape under each wrist pin opening I so that the length of the skirt reaches a maximum through the thrust face portion and a minimum through the boss carrying portion. A relief portion I8, formed in the piston at the.t1me of casting, extends around each wrist pin opening and downwardly to within a short distance from the edge of the skirt when it terminates, thereby forming a step, ledge or shoulder I9 below the wrist pin openings I5 which ledge blends into the thrust faces The piston is cam ground so that the skirt I2. portion is oval in shape, the major axis of the oval lying at right angles to the axis of the wrist pin bosses and extending between thrust faces I2. This major axis is of the same length throughout the height of the skirt unless the skirt be tapered, in which case the major axis gradually increases in length from top to bottom or bottom to top, as the case may be, of the piston skirt. The minor axis, which is at right angles to the major axis, abruptly increases in length at the shoulder or ledge I9 below the wrist pin openings. so that the lower portion of the skirt defines an oval of greater minor axis than the ovate portion of the skirt above the shoulder I9. In operation the piston performs in the manner described with reference to the piston illustrated in Figs. 1 and 2.

Among the advantages to be derived from the use of the principles of this invention is the fact that the described objects and results may be obtained without resort to inserts, control members or struts made of material different from the alloy which forms the body of the pi.- ton, thus allowing the use of a ,completely homogeneous piston. Also the lower portion of the skirt need not, as is the case in some other pistons designed to achieve similar objects, be slotted or otherwise broken in continuity in order to provide means by which excess circumferential expansion may be absorbed.

This application is a continuation in part of my copending application Serial No. 522,999, filed February 19, 1944, now abandoned.

I claim: y

1. An internal combustion engine piston formed of a relatively expansible alloy and comprising a head, a skirt depending from said head and wrist pin bosses disposed in said skirt; the surfaces of the skirt defining two ovate shapes having a common major axis at right angles to` the axis of the wrist pin bosses, one of said ovate shapes having a longer minor axis than the other ovate-shape and being defined by the lower portion of the skirt and joining the ovate shape of lesser minor axis at a point below said wrist pin bosses, the change from said lesser to said longer minor axis being abrupt at said point and the respective lengths of said lesser and said longer minor axes being such as to prevent operational contact between the cylinder wall and-` said' piston skirt over that portion of said ovate shape of lesser minor axis which includes and* is adjacent to the termini of said minor axes.`

2. An internal formed of a relatively expansible alloy and comprising a head, a skirt depending from said head and wrist pin bosses disposed in said skirt; the surfaces of the skirt defining two ovate shapes having-'a common major axis at right angles to the axis of the wrist pin bosses, one of said ovate shapes having a longer minor axis than the other ovate shape and being defined by the lower portion of the skirt and joining the ovate shape of lesser minor axis at a point below said wrist pin bosses, the change from the longer minor axis of the lower ovate shape to the lesser minor axis of the upper ovate shape being abrupt, whereby the lower ovate shape forms a stepped portion under said wrist pin bosses, which portion merges gradually into the thrust faces of the piston, the respective lengths of said lesser and said longer minor axes being such as to prevent operational contact between the cylinder wall and said piston skirt over that portion of said ovate shape of lesser minor axis which includes and is adjacent to the termini of said minor axes.

GEORGE L. MOORE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name v Date 1,623,328 Austin Apr. 5, 1927 2,044,074 Jardine June 16, 1936 2,046,789 Norton July '7, 1936 2,262,132 Berry Nov. 11, 1941 combustion engine pistonv l

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