US3049385A - Piston rubbers - Google Patents

Description

Aug. 14, 1962 R. E. SMITH 3,049,335

PISTON RUBBERS Original Filed Feb. 28, 1958 2 Sheets-Sheet l M L. .l lh.

Aug. 14, 1962 R. E. SMITH 3,049,385

PISTON RUBBERS Original Filed Feb. 28, 1958 2 Sheets-Sheet 2 Rev/ ab f. Jaw/h ATTORNE J United States -ate 7 Claims. (Cl. 277-212) This invention relates to high pressure fluid pumps for moving abrasive fluid such as drilling fluid, and more particularly 'to piston rubbers for use in such pumps.

This application is a continuation of application Serial No. 718,350, fil ed February 28, 1958, and now abandoned.

The invention is particularly applicable to the field of high pressure pumps for drilling fluids and the like, as it provides a piston rubber With a backup portion which will expand into contact with a worn liner without taking a permanent set.

Drilling fluid is highly abrasive and therefore mud pumps conventionally employ pistons having a rubber cup-shaped member for slidably engaging the cylinder wall. It has been recognized for some time that it is necessary to back up the piston rubber to prevent its extruding under pressure and being chewed ofl between the pump liner and the metal parts of the piston.

Among other reinforcing materials, various fabrics such as cotton, Dacron and nylon have been tried. These fabrics were all woven from multi-fibered threads of material.

Due to the twisted multi-strand threads used in said fabrics, they all have a tendency to wick. It had been found that even where a thread is carefully bonded in frictioning material such as rubber or synthetic rubber, there remains in the twist of the threads small voids. Thus, when the end of the thread is exposed due to abrasion of the rubber with the pump liner, liquid will travel due to wicking or capillary action along the fiber.

- When liquid is forced or drawn into the fabric, it tends to destroy some, and in some instances all, of the physical properties of the fabric since the fluid is often composed of the various chemicals which go into the drilling mud as well as oil or water. Liquid also helps to fray the end of twisted strands of the fibers.

The void spaces in the fibers also allow a minute amount of movement of the strands in each fiber which in turn, due to friction and movement, tends to break down the bond between the various plies of fabric in the rubber. This breakdown is similar in nature to the breakdown between plies in automobile tires.-

Many of the fabrics previously used have been unable to recover from strain. They tend to take a permanent set in expanded position against the liner. This results in excess friction on the suction stroke and short lift of the piston rubber. This shortcoming of many fabrics is pronounced in pumps for drilling fluids as liner and rubber wear is pronounced, and the piston rubber and its backup must expand to a greater diameter as wear progresses to bridge the gap between the piston and liner. Of course, where materials in the backup member take a permanent set, an additional strain is induced on the bond between the individual plies of fabric, and between the backup as a Whole and the face stock of the piston rubber where the two are molded integral.

It has been found that a vastly superior piston rubber backup can be formed by reinforcing the backup with a monofilament fiber. Not only is a monofilament fiber much stronger in resisting a bending force than a group of twisted fibers having the same overall diameter, it is also not subject to a Wicking action. The monofilament has a single periphery surface which may be securely bonded to the frictioning material and there are no voids within the monofilament strand which will permit capillary action of liquids.

It has also been discovered that the monofilament fiber, when properly selected, will have good elongation and recovery characteristics. Thus by properly selecting the fiber, as much as recovery is possible and there will be little or no permanent set taken in the rubber back-up, at least due to the nature of the backup material employed.

It has been specifically discovered that synthetic monofilament fibers such as nylon, rayon, Teflon (polytetrafluorethylene) and Dacron (polyethylene glycol terephthalate) fabric will provide a superior reinforcing material for piston rubber backups. In particular, monofilament nylon and in turn monofilament nylon fabric has the ability to recover from strain more completely than many other fabrics subjected to an equivalent amount of strain. This ability of monofilament nylon fabric, coupled with its property of high ultimate elongation, has proved to be advantageous over other multi-strand twisted fiber fabric backed piston rubbers. The reason being that oil field pumps very often are used to pump abrasive fluid and even when they are not, wear on the liner wall as well as the piston rubber takes place. With this being the case, the piston rubber backup must expand to a greater diameter as wear progresses to bridge the gap between the piston and liner and thus back up the piston rubber face as it is desired to do. Monofilament nylon fabric reinforced piston rubber backups, on the pressure stroke, expand and bridge this gap and on the return or suction stroke contract to approximately the original size. Most other fabrics woven from multi-strand twisted fibers will expand on the pressure stroke of the pump, but due to their inability to recover from the strain imposed, take a permanent set with the backup at a larger diameter than when new. This causes drag on the liner during the suction stroke as well as the pressure stroke. There results added wear on the piston rubber backup as well as the liner. Also, more pump horsepower is needed to over come the added friction.

It is an object of this invention to provide a piston rubber reinforced with a fibrous material which is nonwicking.

Another object is to provide a piston rubber having re a permanent set and without weakening the adhesion:

between the frictioning material and reinforcing fibers.

Another object is to provide a piston rubber in which reinforcing fibers are used which are abrasive-resistant.

Another object is to provide a piston rubber with a reinforcing material in which the individual threads of the weave are stronger'than those heretofore used against bending action.

Other objects, features and advantages of this invention will be apparent from the drawings, the specification and claims.

In the drawings wherein there are shown illustrative embodiments of this invention, and wherein like reference numerals indicate like parts:

FIGURE 1 is a schematic elevation of a pump of the type employed to circulate drilling fluid with parts broken away to illustrate the working end of the pump;

FIGURE 2 is a view partly in section and partly in elevation of the Working piston of the pump of- FIG- URE 1 when the pump is stationary and thepiston rubbers are not subjected to a pumping force;

FIGURE 3 is a view similar to FIGURE 2 illustrating the condition of the pump rubbers when the piston is moving in the direction of the arrow on the connecting rod;

FIGURE 4 is a view similar to FIGURE 3 illustrating the piston moving in the opposite direction and the similar but reverse action of the two piston rubbers;

FIGURE 5 is a view on an enlarged scale of a fragment of a piston rubber and backup with a section of the backup cut away to illustrate the construction of the piston rubber backup;

FIGURE 6 is a view on an enlarged scale of a modified form of piston rubber and backup with sections cut away to illustrate the construction of the backup when it is formed separately from the piston rubber and when an additional backup of conventional form is employed with the monofilament backup of this invention; and

FIGURE 7 is a view on an enlarged scale of a monofilament fiber.

Referring now to the drawings, the pump 10 and pump piston 11 are generally conventional in form with the exception of the particular fibers used in forming the backup reinforcing material. The reinforcing material is bonded in the piston rubber backup in the conventional manner.

The backup may be constructed as shown in FIGURE 5 with a plurality of layers 12 through 17 bonded together with frictioning material such as rubber or synthetic rubber in the conventional manner. It is preferred that the backup be integral with the piston rubber face 18 as shown in FIGURE 5. However, it may be formed as a separate annulus 19 designed to reside behind the piston rubber face 21 as shown in FIGURE 6.

The monofilament fiber now available is relatively expensive. If desired, a reinforcing annulus 22 constructed in the conventional manner with reinforcing cotton duck may be interposed between the piston rubber face 21 and the monofilament backup annulus 19. In this instance the monofilament fibers take the majority of punishment at the point of greatest wear and yet the cheaper reinforcing annulus adds rigidity to the piston rubber. Also natural fiber or other relatively inexpensive fiber could be interwoven with the monofilament fiber. As a third alternative, layers of monofilament and other fabrics could be intermixed.

It will be noted that the monofilament fabric is shown in FIGURE 5 in a straight over and under weave. This is preferred, but other weaves which will give the desired stiffness while permitting elongation of the individual fibers may be used.

It is pointed out that the weave employed is preferably rather loose as suggested in FIGURE 5 so that the frictioning material may form a complete bond with each strand of fiber instead of with the faces of closely woven fabric, as has been the past practice.

Any monofilament fiber which can be bonded with frictioning material will provide a non-wicking backup material. When it is to be used in a pump of the type illustrated, the monofilament fiber should be able to withstand approximately 320 F. temperature (molding temperature for natural or synthetic rubber). This temperature is normally encountered in molding the piston rubbers. Of course, it might be possible to reduce this temperature somewhat by lengthening the molding time, but it is desirable to follow conventional practice.

If the backup is formed separately from the piston rubber, different frictioning material might be used which would not require high molding temperature.

The material should be able to withstand both the temperature induced by friction and the temperature of the fluid to be pumped for prolonged periods.

The monofilament fiber should be inert to the fluids to be pumped. However, it is pointed out that only the ends of the fiber will beexposed to fluid, and this would retard decomposition of the fibers. Thus in some instances the pump might move fluids which would attack the fibers, but the life of the pump rubbers would be lessened.

In conventional cotton duck reinforced piston rubbers the reinforcing action decreases with wear and piston life. Not only is the gap to be bridged larger, but the duck has been weakened due to fraying and wicking action. Monofilament fibers are not subject to these two disadvantages and will retain their strength to give a superior reinforcing action throughout the life of the piston rubber.

The fibers should be sufiiciently stiff to provide the necessary backup without being so brittle that they would tend to break off. In other words, in the intended service they should deform or bend with reciprocation of the piston.

The monofilament fabric should be characterized by low liquid absorption. The presence of liquid will cause swelling and this in turn will cause the piston rubber to fold over the retainer plates 24. Also liquid will tend to break down the bond between the filament and the rubber.

The monofilament fiber should have as great a resistance to abrasion as possible to increase its useful life. Lowering the abrasive-resistant characteristics of the fiber may result in a lower quality product.

In addition to the characteristics outlined above, it is preferred that the monofilament fiber have superior elongation and recovery characteristics. The elongation of the fibers should be such that the backup n'ng can expand from a size less than the diameter of a new liner to a size which will accommodate normal wear of both the liner and piston. It is preferred that the elongation be at least equal to cotton. Thus, with wear of the liner and backup member, the fibers permit expansion of the backup member to accommodate progressive wear of the liner and backup member.

In order to avoid excess friction on the suction stroke with resultant wear and increased horsepower, the recovery characteristics of the filaments should be adequate to contract with the frictioning material to a diameter less than the diameter of the liner on the suction stroke.

Of course the monofilament fiber must be such as to give a good bond with rubber and synthetic rubber.

Referring again to the weave of the fibers, it will be noted from FIGURE 5 that the straight weave provides a maximum number of fiber ends at the outer periphery of the backup. Therefore, a minimum number of runs such as fiber 25 will be present at the periphery of the backup. The minimum number of such runs is desirable, as there is a tendency for the entire fiber to be pulled free when it runs along the surface of the backup.

It is pointed out that the monofilament fiber takes up much less space than a twisted fiber of comparable strength and, therefore, the monofilament fiber makes possible more fibers per inch.

Of the known synthetics which may be produced in monofilament form, nylon is preferred. It has excellent elongation and recovery properties and very low liquid retention and swell characteristics. It is of course stiff enough to provide the necessary strength for backup and is sufficiently abrasive-resistant to give long life. Extensive tests with backups made from nylon have proved it to be vastly superior to the twisted fiber generally in use in reinforcing material today.

Dacron will provide a superior monofilament fiber for use in this invention, but nylon is preferred as the ultimate elongation of Dacron is not as high as nylon.

The known characteristics of Teflon indicate that it will probably work. However the stiffness, toughness and recovery characteristics of continuous strand monofilament Teflon are not known. These unknown characteristics would, of course, affect the usefulness of the fiber.

Saran has all of the characteristics of a good fiber except that it loses strength at about 200 F. As molding temperatures are above 300 F., this fiber could not be I used under present-day processes of molding. This fiber is satisfactory if utilized in a backup which does not require molding temperature above 200 F. One way in which this might be accomplished is to form the backup separate from the piston rubber face and utilize a frictioning material which does not require above approximately 200 F. molding temperature.

Fiberglass cannot be used as it is too brittle, and tests indicate that the fibers break ofi in use.

Rayon could be used, but its liquid retention and swell characteristics are considerably higher than nylon. They are not such, however, that they would be prohibitive unless too many fibers were used. A further disadvantage is that the material swells and loses strength in the presence of strong alkalis.

Of course, there may be other fibers which have the desired characteristics and other fibers may be hereafter developed which will have the desired characteristics.

From the above discussion of fibers, it is believed apparent that the selection of the particular fiber used will depend upon the service conditions and upon the quality of product desired. Several presently known synthetics are usable, but their characteristics are such that different quality products will be obtained. Nylon is preferred for all service conditions.

Referring now to FIGURES 2, 3 and 4, the action of the piston rubber backup is illustrated. In the form shown, the piston rubber 26 includes an integral backup portion 26a. The piston rubber is mounted on the piston in the conventional manner with opposed rubbers on opposite sides of the piston 11. As will be noted from FIG- URE 2, the recovery characteristics of the frictioning rubber and backup fibers are such that the diameter of the backup is less than the inner diameter of liner 27 when there is no pressure in the cylinder differential across the piston.

In FIGURE 3 the piston is shown moving in the direction of the arrow on the connecting rod 28. It will be noted that the suction piston rubber is contracted, and the backup is not in engagement with the liner. On the compression or power stroke side of the piston the fluid being compressed is acting on the pressure face of the rubber and also flows to the inner annulus of the backup portion 26a to exert a radial force and urge the backup into firm engagement with the liner 27. This is indicated by the radially outward movement of the bore 29 of the backup portion of the piston rubber. The space between the piston and bore is exaggerated for clarity of the drawings.

When the direction of the piston is reversed as shown in FIGURE 4, the relationship of the two piston rubbers is reversed to that shown in FIGURE 3. Now the piston rubber on the connecting rod side of the piston is expanded so that its backup is in firm engagement with the liner, and the other piston rubber is contracted to provide minimum friction on the suction stroke.

From the above discussion it is believed apparent that high elongation and recovery characteristics are preferred when this invention is employed in the type of pump illustrated. However, it will be understood that where it is not necessary to accommodate for Wear of the backup material or the member which it slidably engages, a monofilament fiber might be employed having poor elongation and recovery characteristics. The monofilament fiber should be superior to conventional backup materials in such service in providing a nonwicking fiber which, as explained above, will increase the life of the backup.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made Within the scope of the appended claims without departing from the spirit of the invention.

What is claimed is:

1. A high pressure pump comprising a cylinder, a

piston, and a piston rubber mounted on said piston, said piston rubber including an annular body portion having an outer peripheral surface which is subjected to Wear when the piston rubber is in use, said body portion formed of rubber and at one end having a plurality of monofilament fibers arranged in layers substantially throughout the thickness of the body and a rubber frictioning material bonding all of staid fibers together, each monofilament layer extending transverse to the axis of the body portion with the extremities of a majority of the fibers so disposed with respect to the outer peripheral surface of the body portion that said fiber extremities are subject to wear in use, said monofilament fibers being of a synthetic material having the property of withstanding molding temperatures, resisting abrasion, being stiff and non-brittle, the property of low liquid absorption, and also the property of good elongation to expand with the frictioning material into engagement with the cylinder on the compression stroke, and the property of recovering with the frictioning material on the suction stroke to a diameter less than the diameter of the cylinder.

2. A piston rubber adapted to reciprocate within a cylinder to pump fluid comprising, an annular body portion having an outer peripheral surface which is subjected to wear when the piston rubber is in use, said body portion formed of rubber and at one end having a plurality of monofilament fibers arranged in layers substantially throughout the thickness of the body and a rubber frictioning material bonding all of said fibers together, each monofilament layer extending transverse to the axis of the body portion with the extremities of a majority of the fibers so disposed with respect to the outer peripheral surface of the body portion that said fiber extremities are subject to wear in use, said monofilament fibers being of a synthetic material having the property of withstanding molding temperatures, resisting abrasion, being stiff and non-brittle, the property of low liquid-absorbing capacity and also the property of good elongation to expand with the frictioning material into engagement with the cylinder on the compression stroke and the property of recovering with the frictioning material on the suction stroke to a diameter less than the diameter of the cylinder.

-3. A piston rubber as defined in claim 2, wherein each layer is formed of a fabric produced by weaving said monofilament fibers.

4. A piston rubber as set forth in claim 2, wherein the material of the monofilament fibers is selected from the synthetic material group of nylon, rayon, polytetrafluorethylene and polyethylene and glycolterephthalate.

5. A piston rubber for an elastic piston which is adapted to reciprocate within a cylinder to pump fluid, said piston rubber including, an annular body portion of substantially circular shape in cross-section with its outer peripheral surface being subjected to wear when the member is in use, said body portion being formed of a plurality of monofilament fibers arranged in layers substantially throughout the thickness of the member and a frictioning material bonding all of said fibers together, each monofilament layer also extending transverse -to the axis of the body portion with the extremities of a majority of the fibers so disposed with respect to the outer peripheral surface of the body portion that said fiber extremities are subject to wear in use, said monofilament fibers being constructed of nylon.

6. A piston element which is reciprocable within a cylinder to pump fluid comprising, an elastic annular body having sealing lips at one side and having monofilament fibers embedded in an area at the end opposite said sealing lips, the monofilament fibers being arranged in adjacent layers in planes normal to the axis of the body and extending substantial-1y throughout the entire cross-sectional area of the body with the extremities of a majority of the fibers in close proximity to the outer peripheral surface of the body whereby wear of said surface will result in wear of said fiber extremities, said monofila-ment fibers being of a synthetic material having the property of being stifi and non-brittle, the property of low liquidabsorbing capacity and also the property of good elongation and recovery characteristics.

7. A piston element as defined in claim 6 wherein the References Cited in the file of this patent material of the monofilament fibers is selected from the 10 2,307,511

UNITED STATES PATENTS Sharp et a1 Feb. 13, 1940 Barnes et al Feb. 26, 1952 Taylor May 4, 1954 Stillwagon May 25, 1954 Fleming Sept. 24, 1957

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