US2238086A - Piston - Google Patents

Description

April 15, 1941 P. L. BOWSER PISTON Filed y 6, 1937 2 Sheets-Sheet 1 INVENTOR. Percy L. Bowser ATTORNEYS April 15, 1941.

P. L. BOWSER 2,238,086

PISTON Filed July 6, 19:57 2 Sheets-Sheet 2 Fiy. 8

Fly. 6

INVENTOR. Percy L. Bowser ATTORNEYS.

atcnted Apr. 15, 1941 UNITED STATES PATENT- OFFICE PISTON poration of Delaware Application July 6, 1937, Serial No. 151,983

4 Claims. (Cl. 309-13) This invention pertains to trunk pistons such as are used in internal combustion engines. More particularly the invention applies to pistons constructed of lightmetals such as aluminum and its alloys.

A princpal object of this invention is to provide a piston structure such that the piston may be given a uniform bearing against the cylinder walls at all operating temperatures.

Another object is to provide a light metal piston in which the skirt or bearing portion has incorporated therein a less expansible metal such as iron or steel, so proportioned that the rate of expansion of the skirt approaches that of the cylinder in which the piston works.

Another object is to provide a skirt structure whereby the flow of heat therefrom to the cylinder wall may be controlled.

Another object is to provide a novel structure for supporting the skirt portion upon the piston head.

Further objects will appear from the following description taken in connection with the accompanying drawings in which:

Figure l is a side view of a'piston embodying this invention.

Figure 2 is a section on line 2-4 of Figure 1.

Figure 3 is a view similar to Figure 1 illustrating another embodiment of this invention.

Figure 4 is a section on line 4-4 of Figure 3.

Figure 5 is a section on line 5 5 of Figure 4.

Figure 6 is a side view of the control member.

Figure 7 is an end view of Figure 6 and Figure 8 is a diagram illustrating, for the purpose of explanation, a probable action of the piston during expansion.

In accordance with an embodiment of this invention the piston comprises a head and a skirt, the latter having cylinder-bearing walls. all preferably constructed of light metal such as cast aluminum or its alloys. A combined supporting and control member constructed of a less expansible metal such as iron or steel is provided in the form of a hoop extending circumferentially of the skirt and rigidly secured thereto and to the head by casting the aluminum around the control member. The skirt is separated from the head at its upper portion so as not to be aiiected directly by the expansion thereof and the control member is so positioned as to extend circumferentially across gaps provided in the skirt structure so that the circumferential expansion of the control member is combined with that of the skirt in order to limit the total expansion of the latter.

' which the wrist-pin may extend. Surrounding Referring now to the drawings I designates the piston head which has cast integrally herewith and so as to extend downwardly therefrom a pair of wrist-pin bearings 2. These bearings are bored as indicated at 3 to receive the wristpin. The skirt portion is provided with cylinderbearing walls 4 on a diameter transverse to that oi the wrist-pin axis. These walls I are separated from the. bearings 2 by gaps 5 in the casting. The lower end-of the skirt is formed into a circumferentially continuous rim 6.

In the embodiment of Figures 1 and 2 the wrist-pin bearings 2 are joined integrally with the rim 6 whereby the cylinder-bearing wall portions 4 are connected by a continuous body of cast metal to the head I. In the embodiment of Figure 3 the bearings 2 are separated from the rim 6 by horizontal gaps I.

When the piston is cast there is first placed in the mold and suitably positioned therein a support or control member 8. This member is constructed of a less expansible metal such as relatively thin flexible steel and is preferably formed as shown in Figures 6 and 7. At the ends of one diameter thereof openings 9 are provided through these openings may be grouped smaller perforations Ill through which the cast metal may flow to form a strong interlock. At the ends of the transverse diameter one or more openings 1 I may be formed or in certain cases such openings may be omitted. At points intermediate the open--' ings 9 and II additional groups of perforations I2 have a purpose which will be explained presently. This member is placed in the mold so as to extend circumferentially of the piston to be cast and with the axis of the perforations 9 coinciding with the wrist-pin axis. The metal is then poured into the mold to cast the piston around this member in the form illustrated in Figures 1 to '5 inclusive.

When the piston has been cast in this manner the member 8 will be embedded in the cast metal of the wrist-pin bearings 2 with the openings 9 surrounding the bore 3. The cast metal flows through the perforations l0 and thereby forms a series of interlocking members which assist in holding the member 8 rigid with respect to the wrist-pin bearings and the head. In order .to secure a still better interlock the openings 9 may be formed with teeth as shown in Figure 6. At the side edges of the cylinder-bearing portions 4 the cast metal is formed into longitudinally extending lugs l3 on the inside of the pis-- ton. The perforations I! are so positioned as come within these lugs and the casting metal flows through said perforations'to form a securely interlocking connection between the piston skirt and the member t at these points. The lugs is are formed so as to surround completely the member 8 so that the latter is entirely embedded in this portion of the skirt. At the openings H the mold core is so formed that it does not permit the cast metal to flow laterally over the edges of the perforation but the cast metal enters this perforation and its interior surface is formed flush with the inner surface of. the member 8. By this arrangement the control member is interlocked with the skirt against movement therealong but is free of the skirt as far as radial movement is-concerned since the metal within the perforations H shrinks away from the sides of the perforations and remains comparatively loose therein.

It will be seen therefore that the skirt is formed with two diametrically opposite cylinderbearing walls 4. These are separated from the head I by horizontal slots H. In both embodiments, Figures 1 and 3, the'member 8 extends circumferentially around the interior of the piston being embedded in the bearings 2 and securely interlocked therewith and being similarly embedded and interlocked with the cylinderbearing walls 4 at the side edges thereof. The

member 8 thus forms a hoop encircling the piston and provides a strong connectionbetween the head and the skirt. In the embodiment of Figure 1 the metal of the bearings extends to the rim 6 to facilitate the heat flow, but that is in many cases not necessary. The skirt may be provided with lugs 20 which may be machined to balance the piston; these lugs are really continuatlons of the bearing bosses.

The member 8 also provides a control member whereby the expansion of the skirt is controlled. It will be noted that the complete circle of the skirt is formed partly of cast metal at the cylinder-bearing walls 4 and partly of less expansible metal provided by the member 8. The parts of the member 8. which extend across the gaps extend in circumferential relation to the rest of the skirt and therefore their expansion combines directly with that of the cast metal to provide the total expansion of the skirt. There are four such sections totaling approximately half the circumferential extent of the walls 4. These portions of the control member are preferably, so proportioned with reference to the members 4 as to provide in conjunction therewith a total rate of expansion for the skirt approaching that of the cylinder in which the piston works.

Referring to Figure 8 which represents only one-quarter of the complete piston, the region surrounding the ends of the wrist-pin bearings 2 is preferably recessed as indicated at l5 .(Flgures 1 and 3) and the ends of the bearings 2 are also machined to a diameter sufficiently smaller than the cylinder bore to insure ample clearance at the highest working temperature because these hearings expand with the head and at a considerably higher rate than the cylinder. In order to accommodate sufllcient metal in the bearings 2 on the outside of the member 8 to provide for making a good casting and also to allow for removing suflicient metal to obtain the above clearance, the member 8 may be formed to slightly oval shape with its smaller diameter along the wrist-pin axis. The bearing faces of the walls 4 are also formed with a slightly elliptical eccentricity, also with he minor axis of the aasaoee ellipse along the wrist-pin axis. This relation is illustrated, to a greatly exaggerated scale in Figure 8. The bearing walls t are formed to such a diameter transverse to the wrist-pin axis as to have a normal operating clearance in the cylinder when cold. When the piston heats up, since its heating is produced by the combustion gases acting on the head i, the head will be much hotter than the skirt, particularly the bearing walls Q. Accordingly the head expands to a greater extent than the skirt and the wrist-pin bearings 2 move outwardlyalong the wrist-pin axis to a greater extent than the bearing walls 4 move outwardly along the bearing axis. During such expansion therefore those sections of the member 8 which extend between the bearings 2 and the walls 43 are carried outwardly by the outward movement of the bearings 2 and, as they are positioned in circumferential relation to the piston skirt they tend to draw the side edges of the walls 4, and particularly the upper corners of those edges, outwardly so as to tend to straighten out the walls 4 or to reduce their eccentricity. This causes the walls 4 to bear on the cylinder walls over an increasing area as the piston expands until at the highest working temperature this bearing area extends substantially throughout the face of the wall 4. At the same time the. tension exerted along the wrist-pin axisby the outward movement of the bearings tends to contract the skirt along the transverse axis, and this contraction tends to counterbalance the thermal expansion along said transverse axis. This growth of the piston under rise of temperature is illustrated in dotted lines in Figure 8.

It will be seen therefore that this invention provides a piston in which the excessive rate of expansion of aluminum is compensated for by introducing into the circumference of the skirt one or more sections of less expansible metal, and these sections are so proportioned and so positioned that their expansion may combine directly with that of the aluminum to control and limit the total expansion of the skirt., This control furthermore is exercised in such a manner that the extent to which the skirt bears upon the cylinder wall increases with the rise of temperature. It will also be noted that by the greater growth of the piston head along the wrist-pin axis the outward movement on bearings 2 tends to stretch the skirt along that axis. This tendency in turn tends to contract the skirt along the transverse axis and thus compensates for expansion of the skirt in the latter direction.

It will also be noted that the structure ofthe member 8 as a closed ring or hoop is such as to give it great strength as a support for the skirt.

Being a unitary element, the fact that it is rigidly interlocked with the bearings 2"'and also with the skirt at the lugs It enables it to withstand side stresses tending to roclcythe skirt relatively to the head. As these stresses are applied to the member 8 in an edgewise direction, said member is able to resist such stresses with great strength. The additional interlock afforded by that portion of the casting metal which enters the openings I l adds to the strength of the structure in this respectsince it supports the member 8 against edgewise movement. At the same time it does not interfere with the free expansion of the bearing walls 4 since those walls may move away from said member in a radial direction. However, this last interlock is not essential in all cases and may be omitted from certain types of pistons.

It will be noted that those portions of the control member 8 which extend fromone of the wall sections 4 to the other pass through, and are embedded in the outer portions of the wrist-pin bosses. These portions of the member 8 provide control elements which are bowed between the wall sections 4, so that the intermediate portions of said bowed elements are locked to the bosses near the outer circumference of the skirt. This spaces the points of connection to the bosses as far apart on the piston diameter as practicable and obtains the advantage of the increased expansion of the greater extent of cast metal between these points so that a correspondingly greater amount of control is obtained. This arrangement also places the free portion of the member 8 between the bosses and the walls 4 in a circumferential position, so that their controlling action on said walls may be exerted by a substantially tangential force. By the use of special metals, such as Invar steel, practically any degree of control may be obtained.

While the action of this piston has been described with reference to certain theories, believed to be correct, it will be understood that these theories may be in error, but the structure has been found to operate successfully, and that the use of such theories is intended to clarify the a unitary whole it is believed that certain individual features or subcombinations thereof may be useful without reference to other features or the complete combination, and it is understood that the employment of such separate features or subeombinations is contemplated bythis invention when within the scope of the appended claims.

It is further obvious that various changes in details of construction may be made within the scope of the appended claims without departing from the spirit of this invention, and it is understood therefore that the invention is not limited to the specific details shown or described.

Having thus described the invention, what is claimed is:

1. A trunk piston, comprising, a head, a skirt having a cylinder-bearing portion separated from said head and formed with a slightly elliptical eccentricity, wrist-pin bearings rigidly connected with said head, and a radially-flexible hoopshaped control member extending circumferentially of said skirt within said cylinder-bearing portion but rigidly secured thereto at the edges thereof and interlocked therewith longitudinally at the middle portion thereof but free thereof r for radial movement relative thereto during expansion, and being embedded in the outer portions of said bearings, said control member being of less expansibility than said head and skirt.

2. A trunk piston, comprising, a; casting of light metal providing a head, wrist-pin bearings depending therefrom, a skirt providing a flexible cylinder-bearing wall separated from said head and along its side edges from said pin bearings, positioned on an axis transverse to the wrist pin axis and having its bearing face formed with a slightly elliptical eccentricity; and a radially flexible control member of a lower rate of expansion than the casting metal, embedded in the outer portions of said bearings and extending along the circumferential curve of the skirt to connect said bearings with the lateral edges of said wall, said member being embedded in said edges but lying wholly radially within said wall beyond said edges.

3. A trunk piston, comprising, a casting providing a head, wrist-pin bearings depending from said head, oppositely positioned bearing walls, said walls being separated from said head and along their side edges from said pin bearings and having their bearing faces formed with a slightly elliptical eccentricity; and an arcuate controlling member of a lower rate of expansion than said head and walls and radially flexible, said member being positioned with its are extending along the piston circumference continuously from one of said bearings to the other to connect the outer portions of said bearings with the lateraledge portions of said walls and having parts beyond said edge portions which are free of and radially within said walls.

4. A trunk piston, comprising, a casting providing a head, wrist-pin bearings depending from said head, a skirt having oppositely positioned bearing walls separated from said head and along their side edges from said pin bearings, said walls having their bearing faces formed with a slightly elliptical eccentricity; and a control member of flexible sheet metal having a lower rate of thermal expansion than the casting metal, said member being of substantially circular hoop form and of suiilcient vertical width to extend above and below said pin bearings, positioned with its are extending along the piston circumference to connect said bearing walls to said pin bearings, and embedded in the casting at the outer portions of said pin bearings and at the side edges of said bearing walls.

- PERCY L. BOWSER.

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