US1638898A - Piston - Google Patents

Description

Aug. 16,

' G. D. WELTY PISTON Original Filed Aug. 2, 1924 4 INVEN OR I W amvu A;%RNEY Patented Aug. 16, 1927. p

UNITED STATES f 1,638,898 PATENT OFFICE.

GEORGE DONALD WELTY, F CLEVELAND, OHIO, ASSIGNOR TO ALUMINUM COMPANY OF AMERICA, 01 PITTSBURGH, PENNSYLVANIA, A CORPORATION OF PENNSYL- VAN IA.

PISTON.

Original application filed August 2, 1924, Serial No. 729,683} Divided and this application filed Juno 28,1925. Serial No. 39,676.

It also relates to a method of making an annular metallic article adapted to expand or contract radially a predetermined and controllable amount when split lengthwise. It further relates to a tubular article characterized by possessing increased resistance to contraction or collapse when split lengthwise for a part or all of its length.

My invention. is based on the discovery that internal strains of compression or tens sion may be put into a tubular, metallic artie cle by cooling the outside and inside at different rates and that these strains may be predetermined and controlled so that when the wall of the article is split in a longitudinal direction throughout all or part of its length, it will tend to expand or to con-' tract'according to the nature and the magnitude of the strains which have been set up. The preferred method of practicing my invention consists briefly of suitably quench- '"ing or cooling the exterior or interior of a suitably heated annular or tubular article at an unequal rate to set up internal strains of tension or compression of the desired magnitude, then splitting the article in a lengthwise direction, the determination of the surface to be cooled the more rapidly de- 40 pending on whether expansion or contraction of the article, aftersplitting, 'is desired.

In the drawings accompanying this specification and forming a part thereof, in which I have illustrated two different structures embodying my invention.

Figure 1 is a perspective view of a tubular article upon which my invention may be practiced.

Figure 2 is a cross sectional view taken on line 22 of'Fig. 1 showing a tube subjected to my improved process before and after being split lengthwise.

F igure'3 is a View corresponding to Fig.

example, my method of treatment may 2 but showing, before and after splitting, a tube subjected to the variation of my process to cause contraction after splitting.

Figure 4 is a cross sectional view of a conventional form of piston provided with circumferentially and longitudinally extending intersecting slits in the skirt, and

1 Figure 5 is a cross sectional view taken substantially on line 55 of Fig. 4 showing beforeand after splitting the piston of-Fig. 4 which had been subjected to my method to produce expansion or splitting.

Annular metallic articles when suddenly cooled or quenched from fairly'high temperatures have internal strains set up on them which strains depend on the nature and extent of the chilling. If the rate of chilling is unequal on the'inner and outer surfaces of the article, the strains tend to distort the article when it is split lengthwise, the distortion occuring in a direction dependent upon the place of application of the greater chilling action and to an amount dependent upon the extent of the chilling actionl For example, an annular or tubular article may be quenched in a manner to cause the article, when split lengthwise, to contract or' to expand, or to remain unchanged; and by suitably regulating the extent of the quenching of the art cle, the amount of this contraction or expansion can be predetermined and repeated with a high degree of uniformity.

My invention is broadly applicable to metallic articles which are, in general, an-- nular, tubular 'or continuous hollow shells and it is not restricted to such articles when composed of any particular metal or alloy. It may be practiced on cast articles'or those which have been previously worked. For be practiced on drawn or extruded. tubes of steel, aluminum alloys or other metals, as

well as on pistons for: internal combustion engines, composed of cast iron or alu minum base alloys.- It will, of course, be understood that articles of various shapes, sizes and compositions will require different quenching temperatures and that the determination of such temperatures will depend upon the composition and nature of the article in question, and the amount of distortion desired in the treated article. In general the strains are proportional to the severity of the chilling consequently substantially the same amount of distortion may be obtained by quenching from different temperatures provided the quenching agent or the temperature thereof is suitably varied to cause substantially the same rate of cooling. For example, an article may be cooled from a high temperature by a relatively slow cooling medium and a similar article may be given substantially the same properties by cooling from a lower temperature with a more rapid cooling agent. The specific examples given hereinafter will serve, however, to guide those skilled in the art in practicing my invention on articles of various sizes, shapes, natures and compositions.

In the drawingsFigs. l, 2 and 3 are various views of a drawn steel tube 1. This tube, when heated to about 1500 F. and then cooled by quenching the inner surface, will possess the property of expanding from the dotted line position to the full line position of Fig. 2 When it is split lengthwise as by split 2. After being'split, the tube 1 may still retain the property of further expansion when it is heated sufliciently or when it is struck or jarred so as to relieve the internal strains. If the outer surface instead of the inner surface is quenched from a temperature of about 1500", the tube will then possess the property of contracting upon being .slit lengthwise, as shown in Fig. 3 where the dotted lines indicate the position of the tube before splitting and the full lines the position after splitting. It will be understood that these indicated changes are greatly exaggerated in the figures, the actual change being only a few thousandths of'an inch. With a three inch tube having walls 1/16" thick the expansion or contraction may amount to between about .001 and .003 when quenched from 1500 F. in water at a temperature of about 50 F.

In general, the more severe the quenching the greater will be the tendency of the article to expand or contract, that is, distort and when the quenching of a number of similar articles is of substantially the same severity, the articles will possess the tendency of distortion to sul'lstantially the same extent.

My invention is of particular utility when applied to an internal combustion engine piston of the so-called split skirt type, wherein one or more. of the bearing faces of the skirt are provided with intersecting circumferential and longitudinal slits. In pistons of this type treated in accordance with this invention there. is a tendency toward expansion of the free endsol the sections in the skirt along the lengthwise slit, as contrasted with a corresponding tendency to contraction, or to contraction of one section and expansion of another section in permanent mold cost pistons not so treated.

Equal expansion of these split sections is desired because it tends to maintain undiminished the original bearing area of the skirt and to make possible close fitting of the piston to its cylinder and maintenance of substantially the original clearance during the major part of or all the life of a piston.

In applying my invention to tubular articles having a closed end, as illustrated by an internal combustion engine piston, distortion may be produced in the same manner as in the tube shown in Figs. 1 to 3. inclusive, except that in the piston the actual distortion may be confined to a portion only of the skirt, whereas in Figs. 1, 2 and 3 it has been illustrated as being more or less uniformly distributed throughout the entire circumferential length of the article.

Figs. 4 and 5 show a conventional form of piston 3 having a circumferentially extending slit 4; and a longitudinally extendingslit 5 interesecting therewith, these slits being formed in what is known as one of the two bearing faces 6 of the piston. The dotted and full lines in Fig. 5 indicate respectivcly,.tl1e positions before and after slitting of a piston skirt subjected to my method to cause expansion after splitting,

the amount of change being greatly exaggerated. The piston 3, when cast in a permanent mold of an aluminum base alloy containing say between about 4% and about 16% of copper or copper and other heat responsive hardening metals, such as silicon and magnesium, with or without nickel, manganese, chromium, and with or without other metals, may be subjected to my improved process in one or more different ways.

It may be suitably quenched or may be cooled more rapidly on the inner than on t the outer surface, as it comes from the mold in a highly heated condition, and then split; or it may be cooled in the open air to room temperature after being removed from the mold then reheated at a high temperature for the desired length of time, quenched more rapidly on the inner than on the outer surface and then split; but preferably it should be cooled in the open air to room temperature after leaving the mold, then heated to a high temperature for the desired length of time, quenched more rapidly on the inner than on the outer surface, then aged at a low temperature as compared with the high temperature just mentioned, in order to improve such properties as hardness and elastic limit, then machined and split just before putting into the motor. The unequal cool ing or quenching step of my method may be accomplished by bringing water, oil or other cooling fluid into the interior thereof, after which the skirt may be split substantially, as shown in Figs. f and 5, with the result that the portions of the skirt adjacent to the slits 4' and '5 will tend to assume subiztantially the full line positions shown in When a diametrical expansion of from about .001 or .005" is desired in the skirt of a piston about 3 inches in diameter and composed of an aluminum base alloy of the foregoing composition, I may quench the interior of the piston from temperatures of 700 F. to 950 F. but to produce the same result at the lower temperature the temperature of the quenching medium should be correspondingly. lower so as to produce the same severity of chilling.

The preferred temperature is 950 F. with such compositions because at such temperature the article possesses the maximum potentiality for hardening.

The temperature of the quenching medium such as water, to be used with a piston of the foregoing compositionand at 950 F. is about F. when an expansion of from .001" to about .005 is desired. I

Although water has been mentioned hereinabove as a suitable quenching medium, it will be understood by those skilled in the art that oil may be similarly employed, but when the same severity of chill is desired the temperature of the oil should be considerably lower than that of the water. For example, when the cast iron tube mentioned hereinabove is to be quenched from a temperature' of about 1500 in oil instead of water, thedoil should be'maintained. at somewhere about 30 F. in order to obtain the same severity of chill as when the water is employed at a temperature of about 50 F.

Pistons of such composition when quenched in accordance with my invention not only expand when split but possess the property of tending to expand still more when struck,

jarred or subjected to the ordinary usage of pistons in internal combustion engines. They are thus capable of resisting any tendency of the skirt portions along the slits to spring inwardly when the skirt is split or to be pressed inwardly by the cylinder walls.

When the inner and outer surfaces of an article heated in accordance with my invention are quickly cooled or quenched at substantially the same rate, the internal strains are either absent or tend to balance each other and the article tends neither to expand nor contract when split lengthwise thereafter.

This application-is a division of my copending application Serial No. 729,683%, filed August 2nd, 1924.

I believe the strains set up by my treatment to be strains of compression and tension; and that in general when one surface of an annular article is quenched by my method that surface has compression strains set up in it while the opposlte surface has tension strains set up in 1t. This belief and theory may notv be the real explanation,

however, and consequently I do not wish to be bound thereby. The scope of my invention is set forth in what is claimed.

What is claimed is: a

1. A piston casting for an internal combustion engine characterized by possessing internal unrelieved strains in the skirt thereof which are capable of causing outward movement of portions of the skirt when the skirt is split.

2. A- piston casting characterized by possessing unrelieved internal compression strains near the inner surface thereof which are capable of causing. outward movement of1 portions of the. skirt when the skirt is sp it.

3. A piston casting composed of aluminum alloy and characterized by possessing unrelieved internal compression strains near the inner surface thereof which are capable of causing outward movement of portions of the skirt when the skirt is split.

In testimony whereof I hereunto afiix my signature this 3d day 0f'June,.1925.

GEORGE DONALD WELTY.

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