CA1063786A - Piston ring forming method - Google Patents

Abstract

ABSTRACT
A method and apparatus for forming piston rings are disclosed, A split piston ring made by conventional processes is locally heated along its inside surface for a short distance either side of the split. This causes a short length of the ring on either side of the split to curl into a slight inward protrusion due to internal yielding of the restrained heated area and subsequent tensioning during cooling.
The inwardly curved configuration prevents catching of the ring's ends on valve ports of ported cylinder engines.

Description

This invention relates to a piston ring forming rnethod and apparatus, and more particularly to a method and apparatus for induclng a slight additional in~ard curva~ure, oten called negative point protrusion, on either side of the split of a piston ring.
The split piston rings formed by conventional processes are generally somewhat out of round in order that the ring exert an even pressure around the cylinder wall-when it is compr~ssed and installed therein. Sometimes the ends of such a ring tend to spring outwardly to a greater extent that the rest o the rin~ due ~o the original oblong shaping of the ring. This tendency can cause problems in internal combustion engines having ported valves within the cylinder walls. The ends of a ring may spring sLightly outwardly as they pass over a valve port opening, then snag upon the upper ox lower edge of the valve port as the piston continues to travel. Generally, the effect of the snagging is not to ca~se serious problems with the ring itse~f but rather to drop meta~
- filings within the cylinder, which can cause serious difEiculties - and result in early wear. It is therefore desirab~e to produce a ;~
slight additional inward curvature on the piston ring at each end - - adjacent the split. This negative point protrusion o~ the ring brings the ring's ends slightly inward within the cylinder 50 that they cannot catch on the edges of the valve ports.
- Previously, face lapping of a ring, wherein the ring is compressed into a cylinder similar to a iring cylinder and recip- -~rocated therein with a fine grinding compound, was sufficient to prevent serious problems of ring end snagging in service as described above The stresses within the ring would be more or ~;
; less equalized during lapping by removal o material from areas whicb tend to exert ~reater outward pressure, including t~e r~ng ends. ~lowever, certain newer engines have placed piston rings under more demanding conditions, o~ten requiring actual recession of the ring ends to avoid snagging on ported valves. Particularly in turbocharged engines, which involve much higher pressures within the firing cylinders, pressurized gases entering the space bet~een the piston and the ring exert greater outward pressures on the ring causing it to push outwardly on the cylinder wall with greater force. As a result, the ring ends are forced outwardly enough to allow them to catch on the ported valves o~ the cylinder wall.
Special grinding operations such as "cam" profile grind-ing have been used to remove material from the outer faces of ring ends~ resulting in a recession o~ the ring face at its ends~ Also, negat;ve point protrusion of piston ring ends has been done by ;~
cold bending. However, these ~.ethods can result in irregularities in surface finish, uneven bending, and some loss of ring strPngth. -~
The methods are also generally more costly than that of the present disclosure.
Heat treatments have been used ln a variety of ways in the manufacture of piston rings. See, for example, U. S. Patent `
~os. 2,487,587; 2,280,552 and 3,377,682. In Patent No. 2~487,587 ring edges are hardened through induction heating. Patenk No.

2,280,552, also relating to ring hardening by heat treating, recog nizes that a split ring expands as a result of localized heat treatment. In Patent No. 3,377,682 a coil of rings is radially stretched, then heat treated to i'set" the stretch. In Patent ~o.
2,081,257, packing rings axe deormed into a smalLer radius ~nder `; pressure, then heated to keep them in the deformed configuration.
The present învention provides a piston ring shapin~
method and apparatus capable o~ quickly and efficiently deforming . ' , - i . . .
!, ' :' ~ . - ' ' ' .;

~ 7 ~ ~
the piston ring end~ into a slig~ in~ard curl, or negative point protrusion, in the area immediat~ly adjacent the split.
According to the method o:E the invention, a piston ring which has been completed according to conventional processes, including face lapping, is subjected to rapid heating along its inside surface for a short distance on either side of the split.
The rapid neating permeates only a portion of the depth of the ring, causing the inner side of the ring to tend to expand. The heated surface, however, cannot expand because of its restxaint by ;
-10 the remainder of the thickness of the ring and by a fixture which may be used to confine the ring. The inner side of the ring in the heated area therefore goes into compression and yields~ i.e., the mo~ecules slip and the affected are~ is no longer under com~
pression. As the heated portion of the ring co~ls, it i~ brought into tension. The tension causes the short heated length of the ring on either side of the split to curl slightly inwardly to a deformed position with the ring ends on the order of a few thou~
sandths of an inch from their original position. Fallowing defor-mation, the physical properties of ~he ring such as grain orienta-tion and hardness are not noticeably diferent. -In a preferred orm of the invention, a piston ring isheated by means of electric current. The ring is posîtioned to ~ -be engaged at both tips on either side of the split, at the inside surface of the ring, by a bridging contact. A pair of electrodes of narrow contact area are positioned one on either side of the split an equal distance therefrom, ~lso on the inside of the ring.
When electrical energy is applied, current flows from one electrode through a portion o the ring, through the bridging contact and through the other ring portion to the electxode of opposite charge.
Electxode arrangement can altexnatively include two electrodes of ~ 3 -.: , . - . : .,, . . ~,

3'786 similar charge on either side of the split, with a center "ground"
electrode of opposite charge contacting both ends of the ring at the split. In either case, electrical energy is applied for a short period, from a fraction of a second to several seconds, depending upon the magnitude of the current, the ring size and the amount of negative protrusion desired. Such use of direct contact electrical resistance heating obtains the desired ring profile without any material removing operations, is versatile in that the magnitude and geometry of the resulting negati~e point protrusion can be easily varied, and leaves a ring surface which is free of irregularities in surface finish. In addition, the method and apparatus are economically implemented and easily automated.
The invention also encompasses a piston rin~ formed according to the above-described method. The physical character- -istics of such a piston ring are superior to those of rings shaped according to prior art negative point protrusion and grinding methods in that the radius of curvature o~ the negatively protruded portions of the ring is substantially uniform, the finish of the ring is protected from damage caused by mechanical gripping and `~
bending equipment, and the internal structure of the ring in the newly shaped area is left essentially unchanged, assuring continu-ing strength and wear resisting qualities.
Fig. 1 is a schematic plan view showing the apparatus and indicating the method of the present invention;
Fig. 2 is an enlarged sectional view taken along the line 2-2 of Fig. 1, Fig. 3 is an enlarged view showing a portion of the appar-` atus of Fig. 1, with some parts removed for clarity, and illustrating the method of the invention;
Fig. 4 is a schematic, somewhat exaggerated representation ' ;'~ , .. . ..
;

~' .: ' ' . , 7 ~

showing the piston ring of Fig. 3 following a shaping operation performed according to the invention; an~
Fig. S is a sectional view taken along the line 5-5 of Fig. 3.
In the drawings, Fig. 1 shows a piston rin~ shaping apparatus accordin~ to the invention and generally indicat~d by the reference nu~er 1~. The apparatus includes a base plate 117 a slidable piston ring retaining fixture 1~ which is opera~ed by engaging means 16, and ring heating means 18 affixed to the base ll. The fixture engaging means 16 preferably comprises a connect-ing rod 17 linked to a pneumatic or hydraulic cylinder (not shown~
or any other reciprocal mechanical driving means. The slidable ~ :
fixture 14 may be guided on the base plate 11 by means o~ slotted holes 19 and pins 21 extending upward from the base plate ll and ..
closely fitted within the holes 19, or by other suitable guiding means7 A piston ring 22 with its gap 23 oriented to the right in :
Fig. 1 is placed in the retaining fixture 14 when the fixture 14 is in an unengaged loading position to the right in ~i~. 1, shown by dashed lines. The ring 22 is inserted into a circular opening -24 of the fixture l~, and is in a somewhat compressed condition ; - since the opening 24 is smaller than the uncompressed diameter of .. the ring 22. The engaging means 16 then draws the fixture 14 to . . .
the left, to the position shown in solid lines in Fig. 2~ wherein -~ the interior face of the piston ring 22 is engaged by the heating ., means 18. The fixture 14 is preferably non-metallic, at least in ~
the areas contacting the ring 22. The piston rino 22 is one which ::
has been completed according to conventional processes~ includin~
i -face lapping, so that the entire face of the ring is smooth prior ; to the shaping operation described herein.

The heating means 18 preferably utilizes electrical ~37B6 resistance heating, through electrodes 26 and 27 of ~pposite charge. The electrodes, sho~ engaged by the piston ring 22 in Fig. 1, contact the ring with a bias force o preferably about ten pounds each. The bias force is provided by the engaging means 16.
The heating means 18 and its engagement with the ring 22 are shown in greater detail in Figs. 3 and 5. A spring-loaded bridging contact 28 with a prefexably tapered shape engages the interior surfaces o the ring at both sides of the spLit, making contact at the corners defined by the ring ends. The purpose of the bridging contact 28 is to provide electrical con~inuity between the two ring portions being heated, so that current can flow from one of the electrodes 26 and 27 across the interior surf~ce o the ring 22 to the other. The electrodes 26 and 27 each fonm a narrow line of contact down the interior face of the ring 22, as shown in Figs. 3 and 5. Each of the electrodes 26 and 27 is supported by a contact block 29 or 30 of electrical~y conducti~e material. The contact blocks 29 and 30, which are shaped in accordance with the curvature of the ri~g 22 but spaced therefrom as shown in Fig. 3, act as heat sinks or heat convected and radiated from the heated ring 22. The blocks 29 and 30 are affixed to respective electrically conductive mounting blocks 31 and 32, separated from one another by an insulator 33. Power is supplied - to the electric ring heater 18 by means of a pair of electrical co~ductors of opposite charge (not shown) connected to the contact , ~ blocks 31 and 32 and to a power sou~ce ~not shown), and the blocks ;- 31 and 32 are accordingly insulated from the base plate 11 by ., .
insulator sheets 34.
For cooling of the heating means 18, including the mount-ing blocks 29 and 30, coolant passages are provided through the blocks 31 and 32. Wat~r or other coolant flows -into the blocks .

.. . . ` ................ .. ...... .. .. .
.. . . '; . . .... . ...

~ 7 ~6 throllgh inlet condui~s 35 (see Figs 1 and 2~ and exits through outlet conduits 36.
As indicated in Fig. 2~ the bridging CQntaCt 28 is con-nected to a plunger 37 which is slidable within a plunger sleeve 38 of electrically insulative material, seated ~ithin a cavity formed between the two blocks 31 and 32. Inside the plunger sleeve 38, a co~pression spring 3g provides a bias force on the bridging contact 28 against the ends of the piston ring 22. The bias force preferably exceeds the ten pound force exerted by the contacts 26 and 27 against the interior ring surface.
As an alternative electrode arrangement, the central bridging contact 28 can be appropriately connected as an electrode ~ -of one charge, serving as a "ground" electrode, and the outer electrodes 26 and 27 can carry the opposite charge. In this way, -current flow through the ring 22 can be regulated individually on each side of the gap 23 for individual shape control, i~ de~ire~.
When electrical energy is applied through the contacts 26 and 27 to the piston ring ~2, the current flows between the con-: tacts along the interior surface 22a of the ring 22, only slightly penetrating the depth of the ring. This causes the area alo~g the ~ .current flow path to heat up immediately, tending to expand. The remaining thickness of the ring~ however, prevents expansion and the affected ar-ea 22a goes into compression. Although the restrain-ing effect of the unheated thickness of the ring is suf~icient to .i prevent expansion, the confinement of the ring provides additional !
restraint. Since the ring is unable to expand, the internal : .
structure of the ring yields and the affected area is no longer under compression. As the ring cools, the affected area 22b tends to contract and goes into tension. As indicated in Fig. ~, when the contacts a~e removed and the shape-altered ring ~' is removed ~ 37~6 from the confining fixt-lre 14, the tension along the inner surface 22b of the ring draws the ring ends ln~ardly, defining a generally constant radius R2 of curvature smaller than the radius Rl o the remainder of the ring 22'. The altered ring shape is shown some-~hat exaggerated in Fig. 4.
The amount of negative point protrusion obtained on a piston ring is determined by a number of variables: the diametex of the ring; the distance between the electrodes, i.e., the length - of the affected ring end portion 22a; the thickness o~ the ring, the heating time; and the temperature of the affected area during heating. The temperature reached during heating is controlled by the magnitude of the electric current applied as well as the time duration of application.
In a particular application o the method of the invention, piston rings of 9~1/16 inch diameter and .300 inch thickness rom face to interior surface were used. The electrodes were spaced - about 1 1/4 inch from the gap Electrical power settings were regu-lated to plovide negative point protxusion of .0015 inch to .0035 inch~ with power applied from about one-half second to two seconds.
The temperatures of the heated portions of the ring we~e approxi~
mately 400F.
During other trials of negative point protrusiQn usi~g direct contact resistance heating, it was found that temperatures ; - ranged from about ~00 to l500F, with the temperature preferably maintained between 500 and 1000F. For a piston ring of about 150 inch thickness~ the heating depth should be about .~30 inch. The heating depth should always be less than half the ring thickness, and is preferably in the range of about t~enty per cent of the ; thickness. This is controlled by the tempera~ure as well as the time duration of heati~g. The ~ime duration of heating may be from .~ .
~ - 8 -.

about one-llalf second to ten seconds, bu~ i~ preferably between about one-halE second and two seconcls, The distance between each electrical co~tact and the adjacent ring er,d should be about five to twenty per cent of the diameter oE the ring. Electric power required ~or heating is approximately 3 ~o S kilowat~s at a fre quency of about 450 kilohertz. Generally the desired amount o~
negative point protrusion is abou~ .001 to ,002 inch.
Flame heating and electrical induction heating were also used to heat piston rings for negative point protrusion. Both of these heating methods were successful, providing the desi~ed change in ring shape, but the results were not as consistent as those obtained with direct contact resistance heating.
., , .

. . .
.,. ~.
,, '~` .

.' .

,'-` ' ' - ~
, - , ,.
.~ -'' , -''' ' ' :
'''; '' - - ' ' ' : - ' ' ' .: .

.. . . . .

.
: _ g _ .-..... ,.. ,,. - . , .
: ~ , . . ~ . . .

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for bending inward the ends of a formed piston ring such that a portion of the length of the ring on either side of the ring gap defines a smaller radius of curvature than the remainder of the ring, comprising:
applying heat to the inside surface of the ring along said portions of the length of the ring to a depth less than half the thickness of the ring, to a temperature sufficient and for a period of time sufficient to cause the internal structure of the heated portion of the ring to yield under thermal compression; radially compressing the ring prior to or during heating, restraining the ring against radial expansion during heating and cooling; and allowing the heated portion of the ring to cool.

2. The method of claim 1 wherein said heat applying step comprises passing an electrical current along the inside surface of the ring.

3. The method of claim 1 which further includes the step of confining the piston ring into a fixed position prior to heating.

4. A piston ring having a ring gap and having a portion of the length of the ring on either side of the ring gap defining a smaller radius of curvature than the remainder of the ring, the piston ring being made by the method of claim 1.

5. A method for effecting negative point protrusion of a piston ring, comprising heating a portion of the inside surface of the ring on either side of the ring gap by passing electrical current through a portion of the depth of the ring adjacent the inside surface, through a portion of the length of the ring on either side of the gap for a predetermined time period, radially compressing the ring prior to or during heating, restraining the ring against radial expansion during heating and cooling, and cooling the heated ring.

6. The method of claim 5 which further includes the step of confining the piston ring to a fixed position prior to the application of the electric current.

7. A piston ring having a ring gap and having negative point protrusion at the ring gap thereof, the piston ring being made by the method of claim 5.