US2315403A - Composite piston - Google Patents

Description

H. H. DILLON- COMPOSITE PISTON March 30, 1943.

Filed Sept. 24, 1940 INVENTOR.

u N. W

H 7 m%fi. H

Patented Mar. 30 943 2,315,403 COMPOSITE PISTON- Harold H. Dillon, Cleveland, O'hio,assignor, by

mesne assignments, to Aluminum Gompanyci America,

Pennsylvania Pittsburgh, Pa.,'-a' corporation of Application September 24, 1940, Serial No. 358,117

aoiajims.v (chaos-14) This invention relates broadly to pistons for internal combustion engines and more specifically to pistons comprising separately skirt portions.

The most commonly used pistons for internal formed head and g t combustion engines have been integral one piece castings ofaluminum alloy in orderto obtain, at a practical cost, the strength," ruggedness and durability essential for operation, combined with the advantages of the light weight and high'heat conductivity of aluminum alloy. The problems that have arisen in connection with such pistons I flow mainly from'the relatively high coeflicient of thermal expansion of the aluminum alloy as compared with the cast iro'n'f'rom which the env gine cylinders are commonly made.

The principal object of the present invention is to combine in a practical manner a piston head portion, which may be a casting of aluminum or other light metal alloy, and a skirt portion which may be formed from sheet'steel or other ferrous metal in such a manner thatthe assembled pis'- tori will be able'at'all times to properly transmit andwithstand the explosion, inertia, and thrust forces to which it is subjected; a close clearance will be maintained at all temperatures between the skirt or guiding portion of the piston and skirt at any operating temperature; and'by providingthe upper end of the skirt and the head with angularly disposed abutting surfaces arranged so as to remain in firm sliding contact without stressing 'or distorting the skirt at all operating temperatures and acting to prevent relative sliding movement of the head and skirt portions along the wrist pin axis or relative tilting of the parts about this axis. In the preferred embodiment of the invention the abutting surfaces on the skirt and head are conical and arranged at an angle proportional to the ratio between the radius of a point on the conical surfaces andgthe vertical distance ofthe same from V a horizontal plane through the wrist pin axis.

Referring to the drawing wherein'the preferred v embodiment of the invention is'illus'trated:

Fig. 1 is an elevational view of the piston embodied in the present invention with parts thereof shown in section for the sake of clarity;

Fig. 2'is a vertical sectional view ofthe piston showing the angle of the inclined surface between the cylinder wall; and distortion or stressing of I the skirt from the different thermal expansion and contraction of the material of the head portion will be prevented. Other objects are to eonnect the two portionsof the pistontogether m a simple, practical and economical .manner; to

- utilize the wrist pin by which the piston is connected to the usual connecting rod for holding the two parts of the piston together without subjecting the skirt to distorting forces resulting:

irom'the relative movement, during expansion and contraction, of the wrist pin bosses carried by the-head; and to center the skirt portion with respect to the head portion and hold the two parts against relative movement along the wrist pin axis and against relative tilting about the wrist pin axis by engagement-of the upper end of the skirt with the head, without imposing stresses on the skirt portion or permitting clearance to develop betwen the skirtancl head during the difierential expansion and contraction of the parts. v

, These and other objects whichwill appear hereinafter are attained by providing the head bosses receiving a wrist pin, with the skirt bosses spaced sumciently from the head bosses to prevent the head bosses from exerting pressure on the and. skirt portions. of the piston with aligned the head and skirt portion with a plane through the'center of .the wrist pin bosses, the section be ng taken on a line 2-2 of Fig. 1;

Fig. 3 is a transverse sectional view of the piston showing the wrist pin bosses in: the skirt.

spaced from the bosses formed in the depending webs or pillars of the head portion, the section "being taken on a line 3 3 of Fig. 1; and,

Fig. 4 is a diagrammatic view: of the piston showing the method of determining the angle of the inclined surfaces.

Referring to Fig. 2. the piston disclosed therein comprises a separately formed upperorheadpor- The head tionlll and a lower or skirt portion l l. portion has cast integral'therewith webs l5 and depending pillars l2 which have wrist p'in bosses l3 formed in the loweriends thereof.

In the formation of the piston it is desirable that the headand integral pillars H be formed of a light, strong and durable metal which possesses a relatively high heat conductivity. Thus it is preferable that the head and pillar portions .of

the piston be cast ofsome high expanding metal such'as aluminum or an aluminum alloy. A plural iy f grooves I! are provided in the peripheral surface of the piston-headfor the reception of V the conventional piston rings, not shown. Formed on the lower circumferential edge of the head subjacent-the lowermost ring grove I there is an inclined or tapered surface l6 which is adapted to engagea complementalsurface pro,- vided on the upper edge of the skirt H,

The skirt portion l! which is formed by press-1 ing, spinning or welding a low expanding metal such as steel, encompasses the depending pillars l2- and is disposed in abutting relation with the head ll. Formed in opposed sides of the skirt ortion and in axial alignment with the bosses I! in the pillars I! are wrist pin bosses I! which have steel bushings I! inserted therein and brazed thereto. Pressed within the steel bushings II are bronze bushings I! which are adapted to receive the wrist pin 22. The wrist pin also extends through the bosses II to hold the head and skirt together, in addition to providing for a pivotal connection of the piston to the connecting rod of the engine in the usual manner. The outer edges of the bosses I! are separated from the inner edges of the bosses H by spaces 20. The upper circumferential edge of the skirt has an inturned. inclined surface 2| formed thereon upon which the tapered surface I! of the head is disposed to provide for a sliding engagement between the head and skirt portions.

In the manufacture of the piston the head is preferably cast in a permanent mold from aluminum or other light metal alloy and the wrist pin bosses II in the pillars I! are cored out in the usual manner. The inclined surface I on the skirt and the mating surface II on the head are ilnish machined separately at the desired angle before finishing the openings in the bosses.

II or I! and prior to the finishing of the exterior surface of the skirt. The parts arethen assembled with the mating inclined surfaces I. and 2! pressed together into firm yet slidable contact in substantially the relation desired in the completed piston, and while the head and skirt portions are held in their desired concentric relation in a suitable jig or fixture the holes in both sets of wrist pin bosses II and H are finish bored simultaneously. The piston is then assembled with a snug wrist pin in place after which the exterior surface of the skirt is ground to the desired diameter and finish.

By finish boring the bosses in both portions of the piston at the same time, in this manner, the desired contact between the inclined surfaces is obtained with theaxes of the two sets of bosses in exact alignment, providing a true surface contact and a free sliding movement between the head and skirt at all temperatures in the completed piston. The inclined surfaces on the head and skirt are pressed together, preferably with just sufficient force to hold the head against rocking or sliding laterally with respect to the skirt, which is guided in the cylinder, and with insumcient force to distort the skirt or hinder the sliding of the surfaces during expansion and contraction.

The proper inclination of the surfaces I and II with respect to the wrist pin axis is determined by the ratioof the vertical height of the pillars II, from the wrist pin axis to a point on the inclined surfaces, divided by the radius of the piston at that point. During heating, the pillars expand vertically, causing the wrist pin and the skirt to move downwardly with respect to the head. At the same time the head expands radially, tending to move the inclined surface it of the head radially outward. The skirt, being composed of a material of a lower coefilcient of expansion, expands both vertically and radially to a lesser extent than the head. With the inclined surfaces disposed at the proper angle, the two expansions act to slide the inclined surface of the head over the inclined surface cf the skirt without increasing or decreasing the contact pressure between these surfaces. Theoretically, assuming a uniform heating of all parts, the correct inclination is reached when the tangent of the angle of the inclined surfaces from the horizontal is equal to the ratio of the height from a horizontal plane through the wrist pin axis to any point on the inclined surface and the radius of the piston to said point on the inclined surface.

I This relationship is illustrated in Fig. 4 of the drawing. In the particular embodiment illustrated, the angle a of the inclined surfaces with the wrist pin axis is approximately 40 while the angle 1: of the inclined surfaces with the vertical axis of the piston is approximately 50. Thus with the inclined surfaces disposed at such an angle the height of the pillars, designated as d, from the wrist pin axis to a point 0 on said surfaces is the tangent of angle a multiplied by the radius, designated 1, of the piston to the point c.

This relationship remains true and determines the proper angleof inclination for any size or relative proportionof the piston. However, in certain designs the operating temperature of the webs may vary slightly from the operating temperature of the head, requiring a corresponding adjustment of the angle of inclination.

The space 2| between the wrist pin bosses l3 and the bosses II in the skirt permits the bosses I! to be carried outwardly along the wrist pin by the diametrical expansion of the head without imposing any stress upon the skirt. The pillars I! are made relatively rigid with the head in order to properly withstand and transmit the explosion and inertia forces between the head disc and the wrist pin. Thus the bosses ii are separated during expansion in accordance with the diametric'al expansion of the head disc, while the skirt bosses i1 move apart in accordance I with the lesser'expansion of the skirt. Such relativemovement of the bosses I! along the wrist pin axis is permitted because the bosses II are freelyrotatable and slidable longitudinally on the wrist pin. The bosses it are held equally spaced or centered with respect to the skirt bosses IT at all temperatures by the engagement of the inclined surfaces I6 and 2| which, as previously explained, remain in contact at all temperatures not only to hold the head and skirt against relative tilting movement but also to hold the skirt centered with respect to the head. The wrist pin may be held from sliding out of the piston into engagement with the cylinder wall by being secured'to the connecting rod or may be of the full floating type with suitable means (not shown) to prevent the pin escaping from the bosses ll.

By this construction a strongand durable piston is provided in which the inertia and explosion forces transmitted between'the head and the wrist pin are carried by the pillars l2 integral with the head portion and bearing directly on the wrist pin through the bosses If. The lateral thrust forces are transmitted from the wrist pin directly to the skirt bearing in the cylinder through the bosses l1 bearing upon the wrist pin. The skirt is preferably made of sheet steel or other ferrous metal so that its coeflicient of thermal expansion is substantially the same as that of the cylinder. Thus the skirt can be ground to a substantially circular sectional contour with but a small clearance from the cylinder wall and this close clearance will be maintained skirt portions are secured together solely by the wrist pin while the sliding joint between the inclined surfaces I6 and 2| as previously explained maintains the parts in correct concentric relation without transmitting any'of the large forces to which the piston is subjected.

Although the foregoing description is necessarily ofa detailed character, in order that the o I invention may be completely set forth, it is to be understood that the specific terminology is not intended to be restrictive or confining, and that variousrearrangements of parts and modifications of detail may be resorted to without departing from the scope or spirit of the invenand holding said skirt and head together, said head and skirt being formed with inclined complemental engaging surfaces disposed above said wrist pin bosses, said surfaces being inclined at an angle the tangent of which is proportional to the ratio of the height of a point on the inclined surface above the wrist pin axis to the distance from said point to the axis of the piston, y 2. A piston comprising a head having depending pillars carrying wrist pin bosses, a separate skirt having aligned wrist pin bosses formed and holding said skirt and head together, said head and skirt being formed with inclined compiemental engaging surfaces disposed above said wrist pin bosses, said surfaces being inclined at an angle the tangent of which is theratio 01' the distance of a point on the inclined surface to a horizontal plane through the wrist pin axis, to the radius of thepiston to that point, the outer edges of the said bosses carried by the head being spaced inwardly along the wrist pin from the inner edges of the bosses formed in said skir 10 when the piston is cold. v

3. A piston comprising a head having depend ing pillars carrying wrist-"pin bosses formed of a single piece of aluminum alloy, and a separate skirt formed of ferrous metal having aligned wrist pin bosses, the upper portion of said skirt having an inclined bearing surface engaging a;

complemental bearing surface formed on the head, and a wrist pin passing through the'wrist outer edges of the bosses carried by the head be- 1 'ing spaced inwardly along'the wrist pin from pin bosses carriedflby said head and the bosses formed in said skirt and constituting the sole means holding the head and skirt together, the

the inner edges of the bosses formed in the skirt when the piston is cold, andsaid inclined bearing surface being arranged at such an angle that the difierential radial expansion of said head and said skirt is compensatedby the differential axial therein, a wrist pin passing through said bosses expansion of said skirt and said pillars, so that said bearing surfaces maintain said skirt centered with respect'to the head at all temperatures.

HAROID H. DILLON

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