USRE20256E

USRE20256E - Piston ring

Info

Publication number: USRE20256E
Authority: US
Publication date: 1937-01-26

Description

. 26, 1937. w. WUER'IFEL PISTON RING originalii l ed Sept. 9, 1930 2 Sheets-Shae}. 2

INVENTOR WILLIAM WUERFEL ATTORN EY I Reissues! Jan. 26, 1937 UNITED .STATES PATENT OFFICE I l'ls'roN RING William Wueriel. Philadelphia. rs., a-ignor to", Wllhenlng llanniactnring compl m We! phigla aeorporationoillelaware 4 Original No. 1313392, dated July '1. 1031, serial No. 480,786, September 9, 1930. Application ior reissue July a, 1931. Serial No. 549,530

1': cum. sea-4s):

My invention relates to certain new and useiul piston ring construction adapted particularly ior controlling theoil within the cylinders 01 internal combustion engines and the like, and it relates ,5 more particularly to a piston ring construction wherein a plurality oi split, annular rin s are operativsly disposed within piston, and are so related-to each other and to the ring groove, as to aiiord eiiective means for preventing excess oil irom passing the piston into the combustion chamber; 'at the same time permitting a propenand sumcient lubrication oi the cylinder wall and minimizing the frictional resistance and wear due to the rings, and which will also'tend to eil'ect a pressure proof seal betweu pistonandcylinder. t v

This application is a continuation, in part, oi my earlier flled copending applications, Serial Number 425,638 and Serial Number 456,350.

It is an object oi my invention to provide an eilective oil control ring which will be efllcient in the conservation oi the oili within the crank case oi the engine, which will nevertheless permit sumcient lubrication, and which at the; same 2 time will aiiotd a minimum irictional resistance to the reciprocation oi the piston and a minimum wear and heating eiiect. With the above and other objects in view, whi

will appear more iully irom the iollowing de- 3o tailed description, my invention consists oi a 40 ring and-at least one'oi which is positioned on the lowermost part oi the corresponding ring, and recesses or notches in. and extending,trans-' versely across, one or moreoithe abutting c'ontiguous side walls or iaces oi said plurality'oi s rims, and means operativelv interposed between the innercylindrical wall or bottom" oi ring groove. and theinner peripheral suriaces of each oi said rings, ior exerting auniiorm outward 11-,

dialpre'asure upon each-oisaid rihgsi-andbne 0 01 presenting a plane side-seatingsurface in tive; juxtaposition ,tothe respective sideewall oi thering-receiving groove having an eiiective area substantially diilerent irom the eiiective area oi the side-seating iuriaoe presented 55 bytheotherrlng in iustapoeitionto the other aring groove oi'the.

each oi, Figures 1 and 4 to 'l inclusive,

side-wall oi the: ring-receiving groove, and the, lower and upper, outer peripheral edges of the successive cylinder-contact or cylinder-sealing suriac'es presenting substantially diflerent angles for producing substantially diiierent "oil-scrap- 5 ing'" eil'ects during the two opposed strokes oi reciprocatory movements'oi the piston. My invention iurther consists 0! other ieatures 'oi construction which will appear more fully irom theiollowing detailed description oi my invention and the accompanying drawings.

For the purpose oi illustrating my invention. I have shown in the accompanying drawings iorms thereoi which are at present preierred by me. 'sincethe same have been iound in practice 5 to give satisiactoryand reliable results, although it is to be understood thatthe various instrumentalitiesoi which my invention consists can u be variously arranged andorganiaed and that my inventionis not limited to theprecise ar- 20 rangement and organization oi the instrumentalities as herein shown and described.

Figure 1 represents a side elevational view. partly in section (on line 1-4 oi Figure 2), with the fragmentary portion oi the cylinder and-piston.

Figure 2 represents a section taken general on lined-I oi Figure l. Figure 3 represents an elevation oi the radial expander spring. a

Figures-4 to '7 inclusive re nt side-elem 1 ticnal views, similar to Figure 1, illustrating modified embodimmts oi my invention.

In carrying out my invention, I may utilize any conventional piston ll adapted ior operation '35 within any conventionalcylinder Ii and any suitable number oi ring-receiving groovbs: said piston ring it and ylinder ii being illustrated inthe drawings in a iragmentary manner. novel piston ring construction includes an 40 upper and a lower split annular designated by numerals i2 and it. W vely. the aggregate overail width oi which nds generally with the width oi thefrl g groove ilz-there being however a llilht lateral '01 5 axial clearance betweenitlie rings and the side wallsoi theringgrooveindicatedat II. The successive individual and separate annular rings are preferably, though not necessarily. oi equal. overall width and are chamiered or otherwise set oil or rabbeted' upper and outer circumierential edges, a's-will be seenin Th'eupperand lower chamiera u e u re? sw a. c and'D- oiuieu per chamas I fer II andthelower rabbetinggroove ll, (Figures 1 and 4) form corresponding upper and lower annular oil-collecting chambers. These successive and spaced non-communicating annular oilcollecting chambers, formed by the'upper and lower chamfers ll and II, as in the case ofthe forms of my invention shown in.l"igures 5, B and '7, or the upper chamfer l6 and lower rabbeting groove It in the modiflcations of my intions is and of the rings l2 and II, to about one-half of the width of the corresponding It will thus be seen that below each of the circumferential annular oil chambers is a comparatively. narrow annular cylindrical contact surface I! and 20, respectively, adapted for contactwith the cylinder wall ll.

It will be observed that the lower circumfer-- ential corners or edges 2i and 2-2 respectively, of

each of the ring members I2 and II, are formed of two surfaces at a right angle to eachother.

i. e.; the lower side wall of the corresponding ring member and the cylindrical contact portions l9 and 20 respectively. The upper outer circumv in advance of the lower right angular edge 22;

ferential corners or edges 28' and 24 respectively. are, on the other hand, both formed of two surfaces meeting at an obtuse angle to each other as in the modifications illustrated in Figures 5, 6 and 7;or at lea'st the upper edges 23 are formed at an obtuse angle. while the lower edge 24 is formed at a right angle; as in the, modifications showninFiguresland i i r The eil'ect of the pair of successive lower right angular edges 2| and 22', and the pairof successive upper edges, oneor both of which are at an obtuse angle, is to provide a greater oil scraping action on-the down-stroke of the piston ,and a lesser oil scraping action on the up-stroke of the piston;--with a resultant tendency to cause a general removal .of oil from the cylinder wall in the direction of the crank case and with the further resultant tendency; on the downstroke, to gather a greater part of the affected oil to gather a lesser part of the aflected oil -in ad chamber l1 or l8,'and to gather a still lesser I or II and I8, asthe casemaybe.

vance. of the lower edge 2-1, into the annular part of the affected oil into the upper annular chamber It. If the particular ring groove is not the uppermost groove of the piston, then a certain amount of oil is scraped into the upper; annular chamber It by the'ring aboveit in the this construction and relationship of constituent elements, is to effect an eflicient lubrication of 7 cylinder walls by reason of the release of'some of the entrapped oil in the chambers ll and II In one or both of the abutting or con side walls or faces of the'rlngs l2 and It, a series (any suitable number) of relatively shallow recesses 2i of'generally uniformdepth, are provided. extending generally transversely orradially across the. inner side or face of the ring: .with theirouterendstermlnatingintheeormerrabbeting or chamfer of the corresponding ring or in the cylindrical cylinder-contact or cylindersealingsurfaceofthering andwiththeirinner ends terminating on the inner circumference thering. Throughthese'reoeles 2laswellas possibly through the slight clearance indicated at some inner peripheries of the rings J2 and "and -"bottom 21, respectively. of the ring groove l4.

ll,whichclearancemayshiftfromthebottomof the groove to the topof the groove or divide therebe'tween (or which may also be between'the two rings) a portion of the oil is transmitted in aninwardradialdirectionintotheringgrowe. it behind the rings i2 and II, from whence the excessoilisdrainedbackintothecrankcase through the drain hole 2| in the wall of the piston. It is this portion of the .oil which otherwise enters the combustion chamber and. results in what 'isgenerally referred to as "oil-pimping.

The other portion of the oil adhering to the cylinder wall is scraped down and into the crankcase by the'low'ermost edge 22 of the-ring ll.

vThe recesses 25 are preferablyofuniform depth and made with outwardly divergent side walls or edges as indicated in Figure 2. ,The outwardly divergent side walls or edges of-the recesses 25 are convexly curved and preferably have a common center at-a point outside of the radially outermost boundary of the ring.

Intermediate the inner circumferential walls,

of the rings l2 and i8, and the inner wall or bot tom 21 of the ring groove is, aradially acting gat ribbon expander spring 28 is provided. of the general type disclosed in my Patent Number 1,707,035, preferably having a sinuous'corrugated formation giving rise to opposite and staggered contact portions adapted for contacting with the e The expander spring 2. is preferably narrowed at the inner contact points 20 thereof, and slotted longitudinally by means of' the slots 20 at the outer contact portions ll thereof. By reason of the'slottlng of the outer contact portions 8|, two independently flexible contact points 32 and II are provided for contacting engagement with the two separate rings l2 and I2 respectively. This independent flexibility and resultant equalimtion is further augmented by the narrowing at the inner points of contact 29. This narrowing also reduces the impedance to the passage of the. oil. Due to the limited independent flexibility of the upper and lower contact'points 32 and 33. the rings i2 and II are urged outwardly by the expander spring 28 -pvith an equalised or compensated outward radial pressure. Thus any diiference in over-all radial width of the rings l2 and I; or any variation inthe diameter of the cylinder in an axial direction, will not cause a firmer contact between the cylinder wall and one of the rings than between the cylinder wall and the other of the two rings. bees-use any such slight diiferencelis compensated for or equalised by the of the upper and lower edges o'f'the two rings may be maintained, free of any variation which might otherwise be occasioned by diiferenee' in.

contact pressure between the rings and wall fiat. ribbon, radial expander spring 2' e xpands the rings l2 andv Itindependently of any otherexpandingmeanassforinstancatension in the rlnfl. or said radial expander spring augmentstheoutwardtensionsetintothebody of the rings by any suitable means. as for in stance theheat-shaping or heat-tensionin'g disjclosed ifithe PedrichlsKolb Patent No 1,284,439. Thus the splitannular rings l2and I! may be setto' give an outward tension,'by heat-shaping thenuor otherwise, and a suitably tensioned flat.

ribbon. radial expander spring 2| is operatively interposed between theinner peripheries of the "ls throughout the entire periphery thereof. and so ring members It and-it and the inner wall of the ring-receivi s groove, to center such rings with respect to the piston, so that the various oil controlling functions of the rings and their constituent elements. are made more uniform jthat the collapse of the rings is prevented. The eflective width of the flat. ribbon, expander spring is' made less than the'width of thering-receiving groove,soastopermitfllefreepassageofoil fromtheradialoilpassagewaysofthering,'

through the oil chamber formed between the inner peripheries of ring members and the inner wall of the ring-receivinggroove. into the oil draining apertures 20.

The clearance between the aggregate over-all width the two rings it and lland the width of the groove II is preiflerably made about fivethousandths cfaninch (more or less). This clearance. when used in conjunction-with the motor oils of a viscosity within the viscosity range commonly employed for internal combustion engines, will produce a suiiicient radial passageway I between the outer and inner circumference of the ringsandwillatthesametimemaintainaiilm or layer of oil, by reason of the viscosity. cohesion and adhesion of'the oiL-and possibly also by an eifect similar to, if not in fact of, the capillary action due to the surface tension or the oil in the comparatively narrow clearance. Because of the greater side-contact surface or side-seating be- .tween the lower side face of the lower ring II a and the adjacent side-wall of the ring-receiving edge I! of the lower ring it toconvey a portion 'of-the oil from the cylinder wall into the rear groove. than between the upper side-face or sidesesting surface of the upper ring if and theadiacentside-wall of the ring-receiving groove, the clearance will tend to form more between the lower ring and the adjacent lower side wall of the ring groove, as indicated particularly at II in Figure l. This'coacts with the lowerscraping of the ring groove it to the drain holes ll, "lhe pil aifected bythe edges Ii and I4 is conveyed totherearoftheoiigroovebytherecessesfl. The

clearance II is greater than that to "at" the width of the ring groove:-the normal "flt" clearance being about one-thousandth or one and one-half thousandth. while the clearance forming a part of my present invention is approximately five-thousandths of an inch (more or less) sumcient to maintainthe layer of oil. which will not only .aiford a draining means but whichwiii also ail'ord a cushion for the rings.

Theresult of thisnovelconstructicmaspcinted out hereinabove, is a highly emcient oil controlling or oil conserving means; which will at the same time maintain a suihcient and proper lubrication of the cylinder wall and reduceto a minimum the frictional resistance to the action of the rings.

I am aware that the-invention maybe embodied in other specific iorms without considered in all respects as illustrativeand not restrictive. rsferencebeing had to theapp'ended claim'sratherthantotheforegcingdescriptionto indicate thelcopeof theinvention.

Ham thus'described my invention. what I herebyclaim as newanddes'lreto sec'ureby Letters Patent, is:

1.1hecombinationwithacflindenofaysstnn adapted to an annular and wear incidental departing from the spirit or essential attributes thereof. and I therefore desire .the present embodiment to be are integral with each other, and or an axially divided, multi-piece, split, annular metallic piston-ring having anouter annular oil-collector groove in'the outer cylindrical surface thereof, a plurality of peripherally distributed generally radialoilp extending fromsaid outer annular oil-lltor groove through the pistonring, into an oil chamber formed between the inner peripheral surface of the piston-ring and the inner wall of the ring-receiving groove; the

.dimensicns of said radial passageways in a direction generally concentrically with the periphery of the ring being greater than their dimensions in a direction generally parallel to the axis of the ring. and lateral or axial clearance between piston ring and ring-receiving groove in excess of the normal clearance required to fit the pistonring into said groove, suiiicient to permit the formation and maintenance of a layer of oil in said lateral clearance between piston-ring and ring-receiving 'groove;e ach of said ring members having an outer side-seating surface for "side-seating engagement with the respective sidewalls of the ring-receiving groove, and the alientive area of one of said two side-seating surfaces being substantially less than the eii'ectlve area of the other of said side-seating surfaces.

2. The combination with a cylinder and able-- ton adapted to'trav'el therein, the latter having an annular ring-receiving groove therein, the de-- lining walls of which are integral with each other, of an axially divided two-piece. split, annular cast-iron piston-ring having an outer annular oil-collector groove in the outer cylindrical surface thereof, divided between the two annular ring members comprising said piston-ring, a series of peripherally spaced and generally radial oil passageways extending from said outer annular oil-collector groove through the piston-ring, into an oil chamber formed between the inner peripheral surface of the piston-ring and the bottom of the'ring-receiving groove, the dimensions of said radial passageways in a direction concentric of the ring, and lateral or axial clearance between piston-ring and ring-receiving groove in excess -of the normal clearance required to lit the pistonring into thegroove, suillcienttopermit the formation and maintenance of a layer of oil. in said lateral clearance between piston-ring and ring- 1 receiving groove;--esch of said ring members having an outer side-seating surface for sideseating engagement with the respective side-walls of the ring-receiving groove; and the eifective area of one of said two side-seating surfaces benular piston-ring. having'a plurality of general I! radial oil passageways peripherally distributed with the periphery of the ringbeing greater than their dimensions in a direction parallel to the axis a m fm m u w .v .m u, 1 47 H. a m m W... mZ m a. I... wim ur mmpmm mmwmwmm W. WWWM WW WWWW WW WWWMWMWWW MWWWWWWWWWWW WWWWW WWMWWWWWWWWWWWWWWWW WWWWWWW W WWW mm a, W m MW m ..W W; W Wm WMWWWWWWWQWMW WW W W. W, WW WWWW W a W m W W WW W W W MW..W WW W WW W W m W WW WWW. W W W M W WW MW W W W W WWWWW WW WW M MW WWM ZWW M WW ram W WZ W. WW.WWW W W m WW W WW W WW. W W rm WWWWWWMWW WW WWW W M W WWW W. WWW WW W W WWWW W WWW W WW WWWW WWWWW W W W WW WW WWWWWW WWWWW WV WWWQMWWW .WWWWMWNWWWW .WWWWWWWM ..WWMWWWM WWWW .MMWWWWWMWWWWWWWMMWWWMM w WWW. W WWWWWWW WWWWWWWW WWWW W WWWWWWWW WW WWWWW W W W WWWWWW W WWW WWWWW WW W WW WW WWW WWW WWW WWWW W WWWWWW W WW W WW W ..MWWWWW MWWWM WWWWWMMMWWM Fm WWWWMWWWWWMMWMWWWMW WW WWMW WWW W W: w W :MWWFMM WW W WW WWMW WW Z W w mm a m W m M.. u mm v WWWW WWWWW W WWWWWW W W W WWWW WW WWW W WW WW WW W r W ..W., W w W W W W. W WW m. n W m W 4 m Mm mMm mm mmwmm m W m m m m WWW m mmm/ m MM um a W a a v, m m n mmm mm m mmmm nm w mmnm WW W WW W M WW M W WWWW W WW W W WW WWWWWWWW WW WW WWWWWWWW WWWM WWWWW WWWWWWWWW WWWWWWWW WWWMWWW WWWWWWWW ,W WW

into said groove, sufiicient to permit the formation and maintenance of a layer of oil in said lateral clearance between piston-ring and ringreceiving groove, and a single expander spring outer side-seating surface for side-seating engagement with the respective side-walls of the ring-receiving groove, and the elective area of theupper of said two side-seating surfaces beingsubstantially less than the elective area of the other of said side-seating surfaces.

8. Thecombination with a cylinder and a p ton adapted to travel therein, the latter having an annular ring-receiving groove therein, the

' defining walls of which are integral with each other, of an axially divided two-piece, split, an-

nular cast-iron piston-ring having an outer annular oil-collector groove in the outer cylindrical surface thereof, divided between the two an nular ring members comprising said piston-ring,

a series of peripherally spaced and generally ratheir outer ends opening in the outer peripheral dial oil passagewaysv extending from said outer annular oil-collector groove through, the pistonring, into an oil chamber formed between the inner peripheral surface of the piston-ring and the bottom of the ring-receiving groove, the dimensions of said radial passageways in a direction concentric with the periphery of, the ring being greater than their dimensions in a direction parallel to the axis of the ring, lateral or axial clearance between piston-ring and rin'g-receivlng groove in excess of the normal clearance required to fit the piston-ringinto the groove, suiiicient to permit the formation and maintenance of a layer of-oil, in said lateral clearance between piston-ring and ring-receiving groove, and T. single expander spring intermediate the inner peripheral wall of the ring-receiving groove "nularplston and each of said ring members for exerting a 'circumferentially distributed outward radial pressure upon each of said ring members;each of said ring members having an outer side-seating surface for side-seating engagement with the re,- spective side-walls of the ring-receivinggroove, and the elective area of one of said two sideseating surfaces being substantially less than the elective area of the other of said side-seating combination with a cylinder. of a piston face tho cylin radialpassagewaysina centricwiththepcrlpheryoftheringbeing & some normslclearance required to iit the piston-ring ring and ring-receiving groove, sumciently in excess of the normal clearance required to fit the piston ring into said ring-receiving groove, to permit the formation and maintenance of a layer of oil in said lateral clearance between pistonring and ring-receiving groove; and a single expander spring intermediate the inner peripheral wall of the ring-receiving groove and each of said ring members for exerting a circumferentially distributed outward radial pressure upon each of said ring-members;each of said ring members having-an outer side-seating surface for sideseating engagement with the respective sidewalls of the ring-receiving groove, and the elective area of the upper of said two side-seating surfaces being substantially less than the elective area of the other of said side-seating surfaces. v

' 10. The combination with a cylinder, of a piston adapted to travel therein, the latter having an annular ring-receivinggroove therein. the

defining walls of which are integral with each other, and of an axially divided'two-piece, split annular metallic piston ring, having a series of peripherally spaced and generally radial oil passageways extending through the ring with cylinder-contact surface of the piston ring and serving to scrape oil from the cylinder wall and to gather the said 011 and to pass the same through the ring into an oil chamber form'ed bej tween the inner peripheral surface of the pistonring and the bottom of the ring-receiving groove, the dimensions of saidradial passageways in a direction concentric with the periphery of the ring being greaterthan their dimensions in a direction parallel to the axis of the ring, lateral or axial clearancebetween piston ring and ring receiving groove, suiiiciently in excess of the normal clearance required to fitthepiston ring into the groove, to permit the formation and maintenance of a layer of oil in said lateral clearance between piston-ring and ring-receiving groove, and oil drainage means extending through the piston wall to drain the excess oil gathered by the piston-ring, and a single expander spring intermediate'the inner peripheral I wall of the ring-receiving groove and each of said ring members for exerting a circumferentialiy distributed outward radial pressure upon each of said ring members;eachof said ring members having an outer side-seating surface for sideseating engagement with the respective side-walls of the ring-receiving groove, and the elective area of one of said two side-seating surfaces being substantially less than the elective-area of the other of said side-seating surfaces.

11. The combination with a cylinder, of a piston adapted to travel therein, having an annular ring-receiving groove, the defining walls of which are integral with each other, and of an axially divided multi-piece, split, annular metallic pistonering, each ring member comprising said multi-piece annular cylindrical cylinder-contact surface portion, continuous and uninterrupted annularly, except for the very slight interruption at the split, the axial width of said continuous and annulariy uninterrupted cylinder-contact surfaceportions being less than the over-all axial widths of said ring mem-- bers, respectively. s series of peripherally spaced and generally radialoilpassagewa'yl extending through the-pistonring, having their outer ends int u-m'ediatessidcentinumisandannularly 1mleast in parts, so as to permit the generally free passage of. oil from said radial oil passageways or oil-collector groove in the outer cylindrical' surface thereof, a plurality of peripherally distributed and generally radial oil passageways extending from said outer oil-collector groove through the piston-ring, to an oil chamber formed between the inner peripheral surface of the plston-ring and the inner wall of the ring-receiving groove and said passageways being bounded by the ring members; the dimensions of said generally radial oil passageways in a direction generally concentric with the periphery of the pistonrlng being greater than their dimensions in a direction generally parallelto the axis of the pistonring; each of said ring members having an outer receiving groove; an axially divided, multi-piece,

side-seating surface for side-seating engagement with the respective side-walls of the ring-receiving groove, and the effective area of the sideseating surface of the ring member nearest to the head of the piston being substantially less than the effective area of the side-seating surface of the other ring member.

16. In combination with a cylinder and a piston adapted to travel therein "and having an annular ring-receiving groove, the defining walls of which are integral with each other and having oil-drainage means extending through the piston-wall, adapted to drain oil from the ringsplit, annular metallic piston-ring having an outer oil-collector groove in the outer cylindrical surface thereof, apluraiity of peripherally distributed and generally radial oil passageways extending from said outer oil-collector groove through the piston-ring, to an oil' chamber formed between the inner peripheral surface of the piston-ring and the inner wall of the ringreoeiving groove and said passageways being bounded by the ring members; the dimensions of said generally radial o'il passageways in a direction generally concentric with the peripheryof the piston-ring being greater than their dimensions in adirectlon generally parallel to the axis of the piston-ring; and the depth of said radial oil passageways-at their outer ends, in a direction parallel to the axis of the piston-ring, being generally the same astheir corresponding depth at their inner ends, and a ribbon-expander spring operatively interposed between-the inner peripheral surfaces of-each of said ring members and the inner wall of the ring-receiving groove and within the said oil chamber formed between said surfaces, to exert a circumferentially distributed outward radial pressure upon said ring members;

said ribbon expander spring having an effective cross-section less than the width of the ring-receiving groove at least in parts, so as to permit the generally free passage of oil from said radial oil passageways of said piston-ring through said 011 chamber formed between the inner peripheral surface of the piston-ring and the inner wall of the ring-receiving groove and into said oil drainage means extending through the wall of the piston. V a w 17. In combination with a cylinder and a piston adapted to travel therein and having an annular ring-receiving groove, the defining walls of a which are integral with each other andhaving oil-drainage means extending through thepistonwall, adapted to drain oil from the ring-receiving groove; an axially divided, multi-piece, split, annular metallic piston-ring, the component members of which are capable of independent expansion and contraction. said piston-ring having a plurality of circumferentially distributed oil passageways extending from the outer periphery of-said piston ring to the inner periphery thereof, to an oil chamber formedv between the inner peripheral surface of the piston-ring and the inner wall of the ring-receiving groove; the dimensions of said oil passageways in a direction generally concentric with the periphery of the piston-ring being greater than their dimensions in a direction generally parallel to the axis of the piston-ring; said oil passageways having side walls extending generally at a right angle to the axis of the piston ring and having outwardly divergent end walls, and a ribbon expander spring operatively interposed between the inner peripheral surfaces of each of said ring members and the inner wall of the ring-receiving groove and within said oll chamber formed between said surfaces, to exert a circumferentially distributed outward radial pressure upon each of said'independently flexible ring members; said ribbon expander having an eflective cross-section less than a the width of the ring-receiving groove at least in age means extending through the wall of the piston.