US3613164A - Piston construction for high pressure apparatus - Google Patents

Abstract

THE INVENTION PERTAINS TO A PISTON STRUCTURE FOR HIGH PRESSURE WORK IN WHICH A SMALLER END OF THE PISTON IS SUBJECTED TO PRESSURE AND WHEREIN THE PISTON IS DIVIDED INTO A FIRST SMALLER TAPERED END PART WHICH IS INTRODUCED INTO A CHAMBER SMALL END FOREMOST TO EXERT HIGH PRESSURE ON A SAMPLE IN THE CHAMBER, AND INTO A SECOND LARGER PART SEPARATE FROM THE SMALLER END PART AND WHICH LARGER PART DELIVERS THE THRUST FROM A PRESS TO THE LARGER END OF THE SMALLER END PART SO THAT IF PISTON BREAKAGE OCCURS IT WILL BE CONFINED TO THE SMALLER END PART.

Description

O 19, 1 7 B. J. NEMETH ETAL 3,613,164

PISTON CONSTRUCTION FOR HIGH PRESSURE APPARATUS Filed Aug. 7, 1968 m 38 Tl IIIIIH 36 FIG-2 IN VUN'R m5 8. J. NEMETH' B. c. BOECKELER MAWQM United States Patent 01' fice 3,613,164 Patented Oct. 19, 1971 3,613,164 PISTON CONSTRUCTION FOR HIGH PRESSURE APPARATUS Bela J. Nemeth and Benjamin Clark Boeckeler, Greensburg, Pa., assignors to Kennametal Inc., Latrobe, Pa. Filed Aug. 7, 1968, Ser. No. 750,809 Int; Cl. B30b 11/32 US. Cl. 1816.5 Claims ABSTRACT OF THE DISCLOSURE The invention pertains to a piston structure for high pressure work in which a smaller end of the piston is subjected to pressure and wherein the piston is divided into a first smaller tapered end part which is introduced into a chamber small end foremost to exert high pressure on a sample in the chamber, and into a second larger part separate from the smaller end part and which larger part delivers the thrust from a press to the larger end of the smaller end part so that if piston breakage occurs it will be confined to the smaller end part.

The present invention relates to high pressure apparatus of a generally known type in which two tapered pistons are introduced into a cavity small end foremost from opposite ends of the cavity to compact a Work sample therein and to develop extremely high pressures on the work sample.

The particular high pressure apparatus to which the piston of the present invention pertains is usually known as the belt or girdle design. As mentioned, such devices, in general, are -well known in the high pressure field. Typical devices of this nature are shown, for example, in US. Pats. 2,941,247 and 2,941,248.

Heretofore, however, it has been not uncommon for the pistons of such devices to fracture due to the high stresses developed therein in exerting the extremely high pressures at which samples are sometimes worked in the high pressure apparatus. Because of the high pressures to which the pistons are subjected, they are necessarily made of a hard high compressive strength material such as cemented carbides of which cemented tungsten carbide is an example. The pressures encountered exceed the yield strength of the best steels and, therefore, materials such as cemented carbides are resorted to in order to make the apparatus workable.

While cemented carbides, in particular, tungsten carbide, are adequate for sustaining the pressures imposed thereon with respect to resisting deformation, it is nevertheless the case that the pistons will sometimes fracture. With devices of the known type, the piston is a rather large member having a small tapered end part at one end that is introduced into the cavity or chamber of the high pressure apparatus and a substantially larger cylindrical opposite end part at the other end integral with the small end part and which is subjected to pressure as by the platen or bed of an hydraulic press or other pressure exerting mechanism. The larger cylindrical part is usually slightly tapered and is held in a heavy steel ring by being press fitted or shrink fitted therein by known procedures. With a piston constructed in the manner referred to above, breakage of any part of the piston involves discarding of the entire piston. Such pistons are naturally quite expensive and are difficult to make and breakage of a piston is, therefore, a serious matter.

Furthermore, when the pistons are made as large members including both a small end and a large end, a rather large body of the cemented carbide must be worked and the possibility of pits and flaws and voids within the body of the carbide is greater than if a smaller body of carbide is worked. Since the carbide in the piston is being worked in a range somewhere near the limits of the strength thereof, when the piston is in service it follows that any flaws whatsoever in the body of the carbide have a strong tendency to induce breakage of the piston and thereby materially shorten its working life and increase the cost of operating the high pressure apparatus. Furthermore, since the larger ends of the pistons are mounted in heavy steel support rings, the replacement of a piston involves considerable time and labor. This is especially true because the mounting of the piston in the steel ring must be done with great precision.

According to the present invention, the foregoing problems that have existed in connection with high pressure apparatus for a long time are solved by the relatively simple expedient of making the small tapered end of the piston which enters the chamber of the high pressure apparatus separate from the larger end thereof which is subjected to pressure by the pressure developing mechanism, such as a hydraulic press. For the sake of clarity and convenience, in the following description and claims, the smaller end of the piston structure is referred to as a piston tip while the larger end is referred to as a pressure block.

With the piston so divided, breakage is substantially confined to the smaller separate end, or piston tip portion of the piston structure so that, in the event of breakage, less loss is involved and less loss of time occurs.

Unexpectedly, it has been found that misalignment of the piston tip with the working chamber, and the development of regions of extreme stress in the piston tip at the interface of the piston tip and pressure block, represents no serious problem and can easily be completely compensated by the provision of a shim or washer of suitable hardness between the piston tip and the pressure block.

The, forming of the small end of the piston structure as a separate member insures a higher quality for this member because a smaller volume of cemented carbides is being worked. Furthermore, the smaller end of the piston, when it is made as a separate member, can be compounded so as to have the very best qualities for its intended purpose.

Further, the pressure block can also be compounded to have the most desirable qualities. For example, while the piston tip must have hardness and high compressive strength, it must not be so brittle that it will easily fracture. The pressure block, on the other hand, works at less stress than the smaller part of the piston tip and can be made more brittle and thus more resistant to deformation.

The exact nature of the present invention will become more apparent upon reference to the following detailed specification, taken together with the accompanying drawings in which:

FIG. 1 is a somewhat diagrammatic sectional view showing a typical high pressure apparatus and including therein piston structures according to the present invention;

FIG. 2 is an elevational view of a piston and pressure block therefor according to the present invention drawn at enlarged scale; and

FIG. 3 is a fragmentary sectional view drawn at still greater scale showing some of the pertinent dimensional characteristics of the piston and the pressure block therefor.

Referring to the drawings somewhat more in detail, the apparatus shown therein comprises a belt or girdle 10 press fitted or shrink fitted into an outer supporting steel ring 12. Belt 10 forms a central chamber into which the upper piston 14 and lower piston 16 extend small end foremost. Within the chamber is a sample 18 which is to be subjected to high pressure. The pressure considered in connection with apparatus of this nature may rise to nearly 100,000 atmospheres, although pressures below 50,000 atmospheres are more common. The sample is usually contained within an electrically conductive tube 20 and an electrically conductive disk 22 is positioned at each end of tube 20 and in electrical contact therewith. Each disk 22 is engaged by a ring of electrically conductive material, indicated at 24, by means of which the disk is maintained in electrical contact with its respective pistons 14, 16.

A sleeve-like gasket 26 of electrical insulating materia surrounds tube 20, and disks 28 of electrical insulating material are disposed inside rings 24.

Between belt and sleeve 26 are gasket members 30 which seal the high pressure chamber. Gaskets 30 are advantageously made of two parts, an outer part 30a adjacent the ring 10 and extending into surrounding relationship with sleeve 26, and an inner part 30b which is engaged by the tip of the pertaining piston and which also surrounds the pertaining ring 24. A conical metal element 32 is disposed between the parts 30a and 30b of gasket 30.

The sleeve 26 and disks 28 and the gaskets 30 are made of material having the properties of thermal insulation and which are electrically non-conductive and which are somewhat deformable at high pressures without, however, losing shear strength. The inner ends of the gaskets are at the high internal pressure of the cell while the outer ends are at atmospheric pressure so that maintenance of shear strength is of the utmost importance to prevent the gaskets from being extruded from between the piston tips and the belt. A number of materials are suitable for this purpose and among such materials are pyrophyllite, catlinite, talc, and other stones and ceramic materials. With respect at least to the gaskets 30', it is important that they confine the extremely high pressures developed within the working chamber of the apparatus while, at the same time, they permit advancement of the piston tips 14, 16, into the chamber. These materials must have the quality of flowing under pressure without, however, being flowable to the degree that pressure will be lost within the working chamber.

Each piston tip, as will be seen in FIG. 1, tapers outwardly from its working end to a larger base region, and the base region is engaged by a pressure block 34. Each pressure block 34 is substantially larger in diameter than its pertaining piston tip and is also preferably formed of a cemented carbide. Each block 34 is preferably tapered on its periphery and is press fitted or shrink fitted into an annular supporting steel ring 36. The upper pressure block may be engaged by a platen 38 of a press while the lower pressure block rests on bed member 400 of the press. In this manner, the necessary pressure can be exerted on the pressure blocks to drive the piston tips into the working chamber of the high pressure apparatus.

Turning now to FIGS. 2 and 3, it will be noted therein that between the base end of each piston tip and the pressure block pertaining thereto is a metal disk 42 of a relatively hard material, hard steel, for example, but which is nevertheless somewhat softer than the carbide material of the pressure block and the piston tip. The provision of disk 42 permits irregularities in the base end surface of the piston tip and in the surface of the pressure block adjacent thereto to be compensated, and also permits slight misalignments of the piston tip with the pressure block to be compensated and, furthermore, protects the base member from fragments of the piston tip in the event of failure thereof by absorbing or deflecting broken piston tip parts to prevent them from being driven into or impacting against the base member.

The piston tip shown in FIGS. 2 and 3, which is piston tip 16 of FIG. 1; will be seen to be a substantially frustoconical member having a smaller work end 44 which is introduced into the chamber of the high pressure apparatus, and a larger working end 46 to which pressure is ap- '4 1 plied by the pertaining pressure block 34. The peripheral wall of the piston is in the form of a smooth straight cone and the ends are also straight, being in the form of flat planes. At the junctures of the end faces with the sidewall, radii are formed so as to eliminate regions of extremely high stress in the piston.

As will be seen in FIG. 3 the curvature where the base surface 46 of the piston joins the side wall thereof is indicated R1 and this radius may be up to about 10% of the maximum diameter of the piston. The same, or a smaller, radius can advantageously be used where working end face 44 joins the sidewall of the piston.

FIG. 3 will also show that the end face 46 of the piston tip may have a slightly tapering peripheral portion 48 and that, likewise, pressure block 34 also has a tapering peripheral portion 50. The radius R2 of the central planar portion of base 46 of the piston tip is smaller than radius R3 of the central planar portion of the end of block 34 adjacent to the piston so that the piston tip end will always be fully supported on a flat surface even if there is some slight lateral shifting of the piston tip relative to the pressure block.

The angle A pertaining to the sidewall of the piston tip can vary from about 10 to 45 degrees and it has been found that an angle of 20 degrees produces highly satisfactory results.

The angle B pertaining to the tapered peripheral portion of the larger end of the piston tip may also vary considerably, say, from 0 to about 15 degrees with satisfactory results.

The angle C pertaining to the tapered peripheral portion of pressure block 34 can vary from 0 to 50 degrees and is not critical. Angles on the order of 45 degrees have proved quite satisfactory for this purpose. The 45 degree taper at the working end of the pressure block permits easy grinding of the flat end of the block to restore it to a planar condition after the piston tip has been used long enough to detract from the flatness of the working end thereof.

The taper of the piston tip is provided so that the stress per unit area at the working end 44 of the piston tip, and which approaches the maximum permissible stress of the material in the piston tip is substantially reduced at the base end 46 thereof. The reduction in unit working stress is such that there is little chance that the piston tip will fracture at its larger end; such fractures ordinarily commencing near the smaller working end of the piston. Further, the rounded corner at the larger end of the piston tip prevents the extreme outer edge of the larger end of the piston tip from being stressed by the pressure block to the point of fracturing the piston tip. The size of pressure block 34 is selected so that the unit working pressure at the side thereof to which pressure is applied by the member 38 or 40 is below the yield strength of common steel castings.

It will also be noted that the arrangement is such that an electrical current can be passed through the sample in the working chamber in order to develop high temperatures therein if so desired, although it will be understood that the invention is equally useful in both cold and hot processes.

From the foregoing it will be appreciated that the separate piston tips, according to the present invention, are easler to manufacture than the integral piston tips and pressure blocks according to the prior art.

The smaller piston tip made in accordance with the present invention will weigh only about 12% as much as the pressure block and only about 11% as much as if the piston tip and pressure block were made in a single piece. Inasmuch as the piston tip is in the form of a smaller piece, the quality thereof is better because it will contain proportionately fewer surface and internal flaws. which could lead to premature failure.

Furthermore, it is less expensive and less time consuming to grind the small piston tip than to grind an entire piston tip and pressure block combination.

Also, since failure substantially always occurs in the piston tip, it is this particular item which has to be replaced. The pressure blocks are ordinarily fitted within a heavy retaining steel ring and it will be obvious that it is a much simpler, and a much cheaper, matter to replace a broken piston tip than it is to replace a larger unit consisting of a piston tip integral with a pressure block and the whole fixed by press fitting or shrinking into a heavy steel ring.

It will furthermore be evident that the design of the present invention does not in any way interfere with the passage of current through the apparatus to develop high temperatures within the sample in the cavity of the high pressure apparatus if it is desired to do so.

A further benefit of making the piston tip separate from the pressure block is that one and the same pressure block can accommodate piston tips of different size and shape thereby substantially reducing tool inventory. Also, it has been found that piston tip life can be enhanced by periodic reheating thereof which seems to relieve stresses set up by the deformation which the piston tip undergoes in use. Such reheating is necessary only for the piston tip and can be done quite easily when the piston tip is separate from the pressure block, as taught by the present invention.

Modification of the present invention falling within the scope of the appended claims will occur to those skilled in the art.

We claim:

1. In a high pressure apparatus; a ring of hard high strength material forming a high pressure chamber circular in cross section and having outwardly tapering openings at the opposite ends, axially symmetrical tapering frusto conical piston tips of hard high strength cemented metal carbide material, each said piston tip having flat parallel end faces and rounded corners at the juncture of the side wall thereof with said end faces and receivable small end foremost in a respective said opening, tapering frusto conical gasket means having inside surfaces engaging said piston tips and outside surfaces engaging the confining walls of the said openings and deformable under pressure, axially symmetrical pressure blocks of a hard high strength cemented metal carbide material separate from said piston tips and coaxial therewith and engaging the large ends of said piston tips for exerting thrust thereon, each said pressure block being substantially larger in cross sectional area than its pertaining piston tip so that the unit pressure developed in the block when pressed against the piston tip will be substantially lower than the unit pressure developed in the large end of the pertaining piston tip, each said block having a flat circular central region larger in diameter than the large end of the pertaining piston tip on the side facing the pertaining piston tip, each said piston tip being free of lateral support between the respective said gasket means and the pressure block pertaining thereto, and a relatively thin metal disk larger in diameter than the large end of the piston tip interposed between the large end of each piston tip and the said central region of the pertaining pressure block, said metal disk being of such hardness as to prevent extrusion thereof laterally when the respective pressure block is pressed against the respective piston tip but softer than the material of said pressure block and piston tip so as to be deformable to distribute stresses uniformly over the interengaging surfaces of said pressure block and piston tip.

2. An apparatus according to claim 1 in which said metal carbide is tungsten carbide.

3. An apparatus according to claim 1 in which each said pressure block also includes an annular region tapering outwardly and upwardly from the periphery of said fiat central region to the outer periphery of the block.

4. An apparatus according to claim 1 in which each piston tip, at the end thereof adjacent its pertaining pressure block, has an annular peripheral region which tapers away from the pressure block toward the adjacent rounded corner of the piston tip thereby providing the piston tip with a circular central planar area opposed to the said fiat central region of the pertaining pressure block.

5. An apparatus according to claim 4 in which the said annular peripheral region of the piston tip at the end thereof adjacent the pressure block which tapers from the central planar area of the piston to the adjacent rounded corner is disposed at an angle of up to about 15 degrees from the plane of the said central planar area.

References Cited UNITED STATES PATENTS J. HOWARD FLINT,

US. Cl. X.R.

1816 R, high pressure dig.

JR., Primary Examiner

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