US1951646A - Piston manufacture - Google Patents

Description

March 20, 1934. c. R. BUTLER PISTON MANUFACTURE Original Filed Sept. 27. 1922 2 Sheets-Sheet l IN V EN TOR. C/ffi/PL 5 f? 5072 5R,

ATT RJ'VEY.

March 1934- c. R. BUTLER I 5 ,646

PI STON MANUFACTURE Original Filed Sept. 27. 1922 2 Sheets-Sheet 2 I N V EN TOR. CHAR; E5 6. 51/72 51?,

WJTTZZNEY.

Patented Mar. 20, 1934 UNITED STATES PATENT OFFICE No. 590,800. Divided and this August 1, 1930, Serial No. 472,464

application 4 Claims. (01. 22-200) This application is a division of my prior application Serial No. 590,800, filed September 27,

, 1922, and relates to pistons of aluminum, aluminum alloy, or light-weight metal, and particular- 5 ly to methods of producing such pistons.

The use of light-weight metals, such as aluminum and aluminum alloys, has many advantages in making pistons of internal combustion engines, because of the light weight and because of the bearing-metal qualities of these metals. However, there has been a disadvantage because the coeflicient of expansion of aluminum is about double that of iron, so that as the engine gets hot the clearance of the piston is decreased, with the result that the clearance has generally either had to be too great when the engine was cold or was so small when the engine got hot that the piston would seize.

In my prior United States Letters Patent No. 1,532,121, granted April '7, 1925, I disclosed a piston which overcomes these-difliculties. My present invention is an improvement and development of that set forth in such prior patent. The piston of this application, like that of my afore- 5 said prior patent, is a composite piston embodying a head and skirt of light-weight metal and parts such as bands of ferrous metal which have a lower coeflicient of thermal expansion and which control the diameter of the piston and limit its rate of thermal expansion to an extent such that it will not bind or seize in its associated cylinder.

It is the object of this invention to produce such a piston which will be free from any internal stresses, particularly those which might deform the piston and adversely affect the expansioncontrol exercised by the ferrous-metal parts of the piston. Further, it is my object to make such a piston by an effective, simple, and inexpensive method, which will enable it to be sold at a cost competitive with the old piston.

In carrying out my invention, fundamentally, I provide in the skirt' of the piston one or more longitudinal slots, which are wavy so that they will not score the cylinder. I preferably extend these longitudinal wavy slots from a transverse slot in a plane between the piston head and'the wrist-pin bosses to the lower end of the pston skirt, or at any rate to a point below'the wristpin bosses. I embed in the light metal of the piston skirt one or more rings which cross the longitudinal wavy slots, and'which are of a metal, such as-steel, having a smaller co-eflicient of expansion than has the light metal of the piston skirt; and I particularly provide one of these rings at the upper end of the piston skirt between the piston head and the transverse plane 0! the wrist-pin bosses. I provide a reinforcing rib of the piston metal in the longitudinal plane 01', the wrist-pin bosses, to give greater effect to the expansion of the piston metal in that planeso that by the compulsion of the embedded rings the variation 'in the piston diameter at'the slipper portions, especially between the piston head and the transverse plane of the wrist-pin bosses, will be even less than that of iron, although it is a. positive quantity. I form the longitudinal slots in the casting 'of the piston, by yielding slotforming cores, whereby not only the waviness of the longitudinal slots is easily obtainable, but the metal when it freezes is free to expand circumferentially and in consequence there are set up in the piston skirt no stresses such as those which when the slots are formed by sawing tend to deform the piston. I use these slot-forming cores as supports for the reinforcing rings in the casting of a. piston. In making the piston I use a metal mold, and use a core which is carried (preferably by a breach-block joint) from a metal plate of this metal mold, so that all the external surfaces of the piston are quickly cooled by contact with metal, while the interior parts are still hot so that metal may still flow toward the cooled surfaces to minimize shrinkage and blow holes.

The accompanying drawings illustrate my invention: Fig. 1 is a perspective view of a. complete piston embodying my invention, with a sector cut out to showthe internal construction; Fig. 2 is a perspective view of the two steel rings and of the slot-forming cores which hold such rings in proper relative position in casting; Fig. 3 is a perspective view, from above, of the mold cover; Fig. 4 is a transverse section through the piston, in the plane of the transverse slots; Fig. 5 is a vertical section through the piston-forming mold and core, showing the piston cast, the section being taken substantially on the line 55 of Fig. 6, but showingthe core in elevation; Fig. 6 is another vertioal section through the mold and core, taken on the line 6-6 of Fig. 5, and showing the core in section so that the riser appears; Fig. 7 is a plan of the mold and core of Fig. 5, before the piston is poured; Fig. 8 is a vertical central section through the piston, in the plane of the wrist-pin bosses, in the position in which the piston is cast in the mold shown; and Fig. 9 is a. horizontal section on the line 9-9 of Fig. 5.

The piston shown has the usual head 10 with its piston-ring grooves 11, and a skirt which is divided into segments, 12 and 13, shown as two ,in number.

have only two segments, div.'.ded by longitudinal slots 1'7 in a longitudinal plane substantially perpendicular to the longitudinal plane of the wristpin bosses 16, each of which thus lies at about the middle of its section 12 or 13. An advantage of having only two sect ons of the skirt is that the whole skirt may receive heat from the piston head by conduction, and transmit such heat to the cylinder wall.

The longitudinal slots 17 are preferably wavy slots, as is clear from Fig. 1, so that although these slots are in general longitudinal they will not score the cylinder wall. The slots 1'7 are preferably formed when the p ston is cast, instead of being cut or sawed subsequently, so that they are in the piston when the piston cools, and there are no stresses set up which would tend to distort the piston skirt if such a slot were sawed or cut afterward. Each slot 1'7preferably runs from the lower end of the piston skirt past the transverse plane of the wrist-pin bosses 16 to about the plane of the lowermost piston-ring groove 11; where each longitud'nal slot opens into a transverse slot 18, which is formed in the bottom of the lowermost piston-ring groove 11 and extends from the slot 1'7 in both directions toward but not to the longitudinal plane of the wrist-pin bosses. This leaves the piston skirt joined to the piston head directly over the wristpin bosses, for the transmission of thrust from said head to such bosses; and also for the transmission of heat from the head by conduction to the skirt around the ends of the transverse slots 18, which heat spreads over the whole piston skirt by conduction (if there are only two slots 1'7) and is transmitted therefrom to the cylinder walls, for the better cooling of the piston.

To supplement the transmission of thrust from the piston head to the wrist-pin bosses, I preferably provide an internal rib 19 in the plane of the wrist-pin bosses, which pin projects radially inward from the skirt and from the sides of the piston head above the bosses and preferably downward from the inner surfaces of the piston-head top, as is clear from Figs. 1, 5, 6, and 8. This longitudinal-thrust-rib reinforcement 19 is not essential in all aspects of my invention, but I prefer its use both for the transmission of this thrust and also for controlling the expansion of the piston as it is heated, as will be explained later.

The two sections 12 and 13, though separated by the slots 17 so far as the main metal of the piston is concerned, are interconnected by embedded rings 20 of another metal. These rings 20 are made of a metal with a low thermal coefflcient of expansion, such as iron or steel, preferably steel for strength, while the remainder of the piston is made of the desired light metal, such as aluminum or aluminum alloy, which has a much higher thermal coefficient of expansion, usually about double that of steel. The rings 20 are continuous rings, and are tight on the piston body at all times. The obtaining of this tightness is materially assisted by the fact that the slots 1'7 are formed during the casting oper ation. While the rings 20 may take various forms, as explained in my prior patent, for simplicity I have shown only a plain ring of uniform cross-section, as is illustrated in Fig. 2, without here explaining the interlocking features which are explained in my aforesaid co-pending application. These rings 20 extend circumferentially around the whole piston, and are completely embedded in thickened parts of the wall of the piston skirt, though exposed where they cross the slots 17. There is at least one such ring in' a transverse plane between the piston head and the transverse plane of the wrist-pin bosses; and preferably there is another such ring near the bottom of the piston skirt. This is illustrated in Fig. 1. However, the number of these rings is not essential, and I have shown two rings because that is the number I prefer, as I have obtained excellent results with that number.

In operation, the reinforcing rings 20 control the expansion of the piston skirt. They are made of metal which is stronger than the body of such skirt, and they compel such skirt body to expand in definite directions and with definite restrictions. Because the coefficient of expansion of steel or iron is only about one-half that of aluminum or ordiary aluminum alloys, such rings hold the body of the piston skirt when it is heated in operation from expanding diametrically as it would ordinarily do in conformity with its coefficient of expansion. Instead, the rings 20 act as tracks, and compel the expansion of the skirt to be mainly circumferential, the skirt segments sliding to some extent on the rings 20 and closing or opening the slots 1'7 to a greater or less extent as the piston gets hotter or colder. However, the expansion is not wholly circumferential, as because the piston head is connected to the piston skirt over the wrist-pin bosses the expansion of the piston head diametrically forces the wrist-pin bosses apart to some extent, in greater amount than the coefficient of expansion of the steel rings 20 permits; and therefore by thus spreading slightly the steel rings 20 over the wrist-pin bosses the expansion of such ring 20 in the plane of the slots 1'7 is prevented from being as great as it otherwise would be. This therefore draws in relatively, but not absolutely,

the portions of the skirt near the longitudinal slots 1'7, which are the slipper portions of the skirt, so that the diameter at such slipper portions varies less even than is required by the coefiicient of expansion of steel, although such piston skirt itself is made of aluminum or aluminum alloy. This permits the piston to be made with a very small clearance on the slipper-diameter, especially between the wrist-pin bosses and the piston head, and slightly increases this clearance as the engine gets hot, instead of decreasing it as with the old aluminum-alloy piston; but still permits the piston on the slipperdiameter to have a positive coefficient of expansion.

To take care of the unequal expansion on the different diameters, and because such small clearances are not necessary over the wrist-pin bosses, the piston skirt is preferably relieved on the diameter of the wrist-pin bosses, as indicated by the stippled portion 21 in Fig. 1.

In making the piston above described, I prefer to use a mold having a base 25, a cylindrical side wall 26 rising from this base and provided with an outwardly extending flange 2'7 at the top, and a core-supporting cover or closing plate 28 removably set in a tapered recess in the outer face a lowance for shrinkage and machining. The

upper face of the base 25 is shaped to give the desired contour to the top surface of the, piston head, whether such top surface is fiat as shown or is of some other shape. The cover or closing plate 28 carries a depending core 29, most conveniently a sand core, for giving the desired shape to the interior of the piston, the core being provided with suitable grooves and depressions for forming the wrist-pin bosses 16, the rib 19, and the thickened wall-portions which are to contain the reinforcing rings 20.

As shown, the piston is poured upside down, through a riser hole 30 in the center of the core. which riser hole communicates with the core-depressions which are to form the wrist-pin bosses 16, as is clear from Fig. 6. The rings 20 are held in proper position during pouring by longitudinal compressible core-bars 31 which are to form the slots 17; which core-bars 31 project inward into longitudinal slots 32 formed at the desired locations on the core 29. The outer edge portions of the bars are crimped as shown in Fig. 2, to form wavy lines which will produce the wavy slots 17. Each core-bar 31 is made of a folded strip of sheet metal between the two walls of which is a layer of yieldable material, such as asbestos 33, so that by the yielding of the core bars the piston skirt may be kept free from internal strains while cooling. The core bars 31, which are themselves held by being mounted in slots in the core 29, in turn support the reinforcing rings 20, and have holes 34 through them for the passage of such rings, as is clear from Fig. 2. Slits 35 connect these holes 34 to the edges of the core bars, so that the end portions of the core bars beyond such slits may be bent (as one end is indicated in Fig. 2) to openthe holes 34 for the insertion of the rings 20, after which the bent end-pieces of thecore bars are bent back into the plane of the body of the core bars. The slits 35 for the ring 20 which is near the bottom ofthe piston skirt, or near the top of the mold if the piston is castupside down, extend to the inner edges of the core bars 31, so that when the bent end-portions are bent back in place they may project into the slots 32 and thus be held in proper position. The slits 35 for the ring 20 which is near the piston head, or near the bottom of the mold in casting the piston upside down, extend to the outer edgesoftlie' core bars 31, which are the folded edges, and when returned to place may be held in proper position by pins 36 inserted in the fold to close the slits, as is clear from Fig.2. In order to assist in positioning the rings 20 concentrically with the piston, I preferably provide several outwardly projecting buttons 3'7 on the core 19 in the plane of each rin to bear against the inner surface of such ring.

The core 29, as already stated, is carried from 1 the cover or closing plate '28. I prefer to do this by a breach-block lock. The cover plate has a central hole 38 through it, longer in one direction than in the other, and with inwardly projecting fingers 39 on the smaller diameter of the hole. The upper surfaces of these arc-shaped fingers 39 are inclined, like parts of a screwthread. The core 29 has a central extension 40 which passes through the hole 38, and which has laterally projecting fingers 41 at two opposite points. The distance from tip to tip of these fingers 41 is sufiiciently'small to permit the extension 40' to be inserted through the hole 38 when such fingers are on the longer diameter of such 1 hole; but when'the core after such insertion is turned on its axis, the fingers 41, the under faces may be handled as a unit; and the rings 20 and core bars 31 may be handled as part of this unit. This complete unit may be put in place in the mold 26, and will be positioned by the fit of the cover plate 28 in the tapered recess in the surface of the flange 27. The cover plate 28 is preferably provided with upstanding fingers 43 for facilitating the handling of the cover plate.

When the rings 20 and core bars 31 are assembled in place on the core 29, and the core 29 is mounted to hang from the cover plate 28, and the cover plate with these parts is put in place on the rest of the mold 25--26, the piston may be poured.

The melted aluminum or aluminum alloy, or other light metal, is poured through the riser hole 30 (in the model shown), to a level near the top of the extension 40; which is higher than any part of the finished piston is to be. This metal runs out through the depressions for the wristpin bosses 16 into the space for the piston skirt and piston head, and completely fills such space. The metal which is to form the outer surfaces of the piston is in contact with the metal of the mold; the upper surface of the piston resting upon the base 25, the cylinder side walls of the piston bearing against the inner surfaces of the side walls 26 of the mold, and the lower edge of the skirt bearing against the under surface of the cover plate 28. Thus all these surfaces of the piston cool quickly because of their contact with the metal of the mold. As the poured metal cools its contracts, but the still molten metal in the interior of the piston and the riser hole causes extra metal to flow into the piston body to make up for such contraction until actual solidification takes place; so that the surfaces of the piston will not contract. awayfrom the mold or rings or leave air holes in the piston surface. Any gases which are set free in the pouring may escape through the space between the extension 40 and the walls vof the opening 38; and the under face of the cover plate is provided with radial grooves 44 for facilitating this escape of gases to prevent the formation of blow holes in the casting. As the cylinder contracts, the two sections 12 and 13 of the piston skirt may slide on the rings 20, so that they will not have internal stresses set up in them; and this sliding is facilitated by the compressibility of the core bars 31.

When the poured piston is cold, the fingers 41 of the sand core are broken off and the cover plate 28 removed, after which itv is easy to remove the poured piston from the mold and to clean out from it the sand of the core; Then it is easy to remove the core bars 31, after first cutting off or bending back the end portions thereof. This leaves the rings 20 embedded in the core, but exposed in the wavy slots 1'7 which are formed in the casting of the piston. The piston is then machined, part of which is the cutting of the piston ring grooves 11 and of the transverse slots 18, and is then ready for use.

The step of forming the longitudinal slots 17 in the piston during the casting operation is'an important one and has a marked effect on the a nates internal stresses which might otherwise interfere with the expansion-control exercised by the bands 20. If the piston skirt, as formed during the casting operation, were circumferentially continuous-4. e., uninterrupted by the longitudinal slots 17-11; is obvious that that portion of the piston metal lying immediately within a band 20 would, by reason of the greater rate at which it contracts when the piston cools, tend to shrink radially away from the inner surface of the band, and thus tend to create between the inner surface of such band and the piston-metal immediately adjacent to it a gap or void. The presence of such a gap or void would permit the metal of the piston-skirt to expand at its normal rate until it engaged the inner surface of the band 20, and this uncontrolled expansion might be suificient to cause the gripping or seizing action which it is the object of this invention to avoid.

Tests which I have made indicate that a gap or void such as is above suggested may actually exist in a piston cast with a circumferentially continuous skirt. By comparing the rates of thermal expansion of two pistons, identical to each other except that the longitudinal slots were formed in one piston during the casting operation while in the other such slots were out in the skirt after the piston was cast, I find that the skirt of the former, when subjected to gradually increasing temperature, expands substantially at a rate corresponding to the coefllcient of thermal expansion of the bands 20, while the other piston, when similarly heated, expands first at a rate corresponding to the coefficient of expansion of the piston-metal and subsequently at a lesser rate corresponding to the coefficient of expansion of the bands 20.

I claim as my invention:

1. The method of making a piston having a head and a depending skirt, which consists in placing in the annular space between the skirtforming wall of a mold cavity and an associated core an assembly comprising circumferentially spaced longitudinally extending core-bars and a core-surrounding ring of different metal from said piston, subsequently pouring the molten piston-metal into the mold-cavity, and, after the piston has solidified, withdrawing it from the mold and removing said core bars to leave circumferentially, spaced longitudinal slots in the piston skirt.

plane perpendicular to that of said wrist-pin bosses and a reinforcing ring of different metal from the piston skirt in position to be embedded in the skirt and crossing slots formed therein by the core-bars, subsequently ppuring molten pieton-metal into the mold cavity, and, after the piston has solidified, withdrawing it from the mold and removing said core bars to leave circumferentially spaced longitudinal slots in the piston skirt.

3. The method of making a metal piston having a head and an annular skirt and a continuous ring embedded in the skirt, said ring being of material having a lower coefilcient of expansion than the metal of the piston, which process comprises supporting the ring concentrically in the annular skirt-forming portion of a piston mold, also placing in the skirt-forming portion of the mold a core-bar which extends longitudinally on both sides of the ring and radially across the skirt-forming portion of the mold, pouring molten metal into the mold to form the piston, withdrawing the piston from the mold after it has solidified, and removing the core-bar from the piston skirt to leave a longitudinal slot therein.

4. The method of making a metal piston having a head and an annular skirt and a continuous ring embedded in the skirt, said ring being of material having a lower coefllcient of expansion than the metal of the piston, which process comprises supporting the ring concentrically in the annular skirt-forming portion of a piston mold, also placing in the skirt-forming portion of the mold two circumferentially spaced core-bars which extend longitudinally on both sides of the ring and radially across the skirt-forming portion of the mold, pouring molten metal into the mold to form the piston, withdrawing the piston from the mold after it has solidified, and removing the core-bars from the piston skirt to leave longitudinal slots therein.

CHARLES R. BUTLER.

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