FIELD OF THE INVENTION

The present invention pertains to fluid seals of the radial type. More specifically, it pertains to the formation of an elastomeric radial lip seal having in combination therewith a bonded wear insert fabricated from polytetrafluoroethylene with a hydrodynamic surface and, preferably, a dust lip.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (i.e., PTFE) is, in many respects, an ideal dynamic sealing material. It has a low coefficient of friction so that it will slide easily generating very little heat, especially when running dry, and it is wear-resistant so that it is not easily abraded. Moreover, PTFE is non-porous and not permeable to liquids in addition to being very stable in the sense that it is not chemically or thermally reactive, thus being attacked by very few solvents, oils, etc. Apart from the above advantages, it remains resilient over an extremely wide temperature range.

On the negative side, however, PTFE has two properties that make it a very difficult material from which to make fluid seals which can utilize the aforenoted good properties. In the first place, PTFE cannot be molded into odd shapes as can common

elastomers and plastics, such as the thermoplastics and the ther osetting plastics. In practice, the PTFE must be compacted from the powdered form in which it is produced and then sintered, much as in the manner powder metal parts are fabricated. Of further significance, is the fact that PTFE has no liquid phase, as do most plastics, and it sublimates directly from the solid state to the gaseous stage at a temperature in excess of 550 F.

Secondly, PTFE is not easily elastic and it cannot be made flexible as the nitrile rubbers, the polyacrylate rubbers, the silicone rubbers or others which are the usual materials from which seals are made. The so-called rubbers, being easily elastic, will conform to and establish intimate contact with any surface against which they are pressed. In so doing, they can establish a positive seal to prevent the passage of fluid at such an interface. PTFE, on the other hand, is stiffer and tends to bridge over high spots in the surface against which it is pressed. This leaves minute channels through which fluids can migrate under static conditions when there is no relative movement between seal and sealing surface. Such a condition dictates that if a successful seal is to be made from a material that will not establish intimate contact, it must include some mechanism that will return migrating or leaking fluid back to the oil side of the seal. This is most easily done by making the seal hydrodynamic, such as, providing hydrodynamic flutes on the sealing lip of the seal. Of significance, as noted above, is the fact that PTFE has no appreciable liquid phase. Accordingly, it will not flow into odd-shaped molds as will most elastomers. Instead, it must be machined and/or coined into useful shapes, such as sealing lips with hydrodynamic flutes as taught in United States of America Patents Nos. 3,857,156, 3,929,341, 3,939,551, and 3,985,487, which are assigned to the assignee of the present application. In these patents, the PTFE sealing lips with hydrodynamic flutes were formed by turning in their entirety or turned and coined in a die before assembly into a seal. The

turning and coining operations, of course, are relatively complicated and expensive. Another known prior art approach is comprised of a combination of elastomeric radial lip seal and a PTFE wear surface at the sealing lip, such as shown in Federal Republic of Germany Offenlegungsschrift No. 24 35 675. However, the latter described prior art does not suggest formation of a PTFE wear insert having a hydrodynamic wear surface.

As can be appreciated from the foregoing description, the prior art has been unable to satisfactorily and economically produce a fluidic seal which includes a polytetrafluoroethylene wear insert that is fabricated in an economical and easy fashion, such that the wear insert has a hydrodynamic surface formed thereon in a simple molding process.

Also, such prior art seals do not have a dust protection lip thereon. Such seals lead to rapid deterioration of the shaft.

SUMMARY OF THE INVENTION

Briefly stated, the present invention overcomes the several drawbacks associated with heretofore known seals and processes which are adapted for use in the formation of such fluid seals employing solid, deformable and non-moldable seal lips or, alternatively, seal lip wear insert materials. The present invention is also believed to be an improvement over known seals and processes adapted for use in the formation of such fluid seals having a fluted hydrodynamic surface that can be used for sealing purposes.

Broadly, in accordance with the spirit and scope of the present invention, there is a novel and improved method of forming an elastomeric radial lip seal with a bonded, solid, deformable and non-moldable wear insert material having a hydrodynamic surface positioned at the sealing lip through molding under heat and pressure. Such method embodies the steps of positioning a seal case member coated with a suitable bonding agent on a lower preheated mold core forming a portion of a mold

cavity with said core having assembled thereto a negatively imprinted surface having a hydrodynamic fluted configuration at a preselected location, and positioning a generally annular deformable and non-moldable solid material treated with a suitable bonding- agent on the lower core of the mold situated so as to be in close proximity to the preselected location having the fluted imprinted surface. This method contemplates positioning a generally annular, uncured elastomeric material adjacent the solid, deformable and non-moldable annulus, which has a volume greater than the volume of a closed mold cavity, and entrapping the elastomer, and deformable and non-moldable annuli and the seal case member by heating and compressing under pressure which is sufficient in magnitude to force portions of the deformable annulus onto the negatively imprinted fluted surface of one of the mold members by hydrostatic pressure generated as the elastomer is being cured, such that a positive imprint is formed on the deformable and non-moldable annulus which is generally complementary to the negatively imprinted surface on the seal mold. Such method envisions maintaining pressure and heat for a predetermined period of time to insure curing of the elastomeric annulus and the bonding agents to form a unitary radial lip seal having the deformable and non-moldable annulus bonded to the molded elastomeric annulus material with the latter being bonded to the seal case member; and then removing the unitary molded radial lip seal from the mold cavity. In a preferred embodiment, polytetrafluoroethylene forms the insert material.

Additionally, the present invention can include a hydrodynamic, elastomeric radial lip seal with a wear insert bonded to the elastomeric seal body at the sealing lip wherein the seal body is angularly formed and dimensioned such that it has an auxiliary dirt excluding lip spaced axially apart from the wear insert and adapted to engage a shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 shows in cross-section an open compression mold with individual elements of the radial lip seal of the present invention in position prior to the molding operation; FIGURE 2 shows in cross-section the mold of FIGURE 1 in the closed position during the molding operation;

FIGURE 3 shows a partial cross-section of the seal of the present invention as it is removed from the mold and indicating the trim line; FIGURE 4 shows a partial cross-section of the seal of the present invention installed on a shaft; and

FIGURE 5 is a cross-section of another embodiment of the present invention showing an extension of the elastomeric body of the seal forming an auxiliary dust excluding lip.

DETAILED DESCRIPTION

FIGURE 1 of the drawings shows in cross-section the relative positions of the seal components of the present invention as they are placed in a compression mold prior to the molding operation. It will be understood that, but for the exceptions to be mentioned, the compression molding that is performed is accomplished in a standard manner by conventional components. Accordingly, the heat, pressure and time for such process vary in accordance with known practice in this field and do not form a part of the invention. The lower mold element or core 10 is held in position on the lower platten of a press (not shown) by a centrally located bolt 12. Likewise, the upper mold element or ring 14 is held in position on the upper platten of a press (not shown) by a centrally located bolt 16.

The mold core 10 has an outer circumferential shoulder 18, the function of which is to position the seal case 20 which may be made of an appropriate metallic substance and coated in a conventional manner with any suitable bonding agent. The mold

core 10 of the present embodiment includes a profile 22 that may substantially form most of the internal surface of the seal, and a circumferential external ridge 24 positioned at the axially outboard end of the fluted surface 23. The PTFE (polytetra- fluoroeth lene) wear insert 26 is snapped over the ridge 24.

Such insert 26 is pretreated with any suitable bonding material as is the case 20 in a known manner, such as brushing, rolling dipping or spraying. The mold is preheated to the cure temperature of the elastomer being used, preferably, prior to positioning of the seal metal case 20. Also, the ridge 24 enhances proper positioning of the wear insert. Such ridge is not found on conventional compression molding components. Moreover, it is within the spirit and scope of the invention that other solid, deformable and non-moldable materials are envisioned for use besides polytetrafluoroethylene. It will be appreciated that such material must be able to deform under the hydrostatic pressures developed for curing the elastomer. The term "snapped" is used to describe the positioning function of the ridge 24 which has a larger outside diameter than the inside diameter of the wear Insert 26. The insert 26 must be stretched to get past the ridge 24 so that it may be positioned in close proximity with a hydrodynamic flute configured surface 23. The small circumferential external ridge 24 is made on the core 10 of the mold and is positioned at the axially outboard end of the fluted surface 23. The wear insert 26 in its initial form is a simple flat washer. During the positioning, the stretching of the inside diameter of the wear insert 26, without substantially altering the outside diameter, causes the originally flat washer to warp and assume the generally conical shape as shown in FIGURE 1 in close proximity with the hydrodynamic configured surface 23 on the mold core 10. Additional stretching of the wear insert 26 makes it tend to lie down and cover the area of the mold core 10 which has the profile fluted surface 23.

The hydrodynamic patterned surface 23 is shown in FIGURE 1 to be a unidirectional helical groove. For purposes of

illustration, this patterned surface is used throughout the description. It being understood that the present invention is not limited to this specific pattern since any hydrodynamic pattern that can be imprinted on the mold core 10 can be embossed on the wear insert 26, be it left-handed or right-handed, unidirectional, or any of the various patterns of bi-directional flutes.

With continued reference to FIGURE 1, the uncured ring of elastomer or prep 27 is positioned around the lower taper 28a and rests in the vicinity of the wear insert 26. The volume of the elastomeric prep 27 is intended to be greater than the volume of the cavity formed within the mold when the upper ring 14 is closed on the lower core 10. As is well-known in the art, this disparity in volumes creates extremely high hydrostatic pressure within the mold as the elastomeric prep 27 is heated within the closing mold and becomes semi-liquid. The semi-liquid elastomeric material flows throughout the mold cavity filling it in its entirety (as shown in Fig. 2) whenever in the closed mold position illustrated in FIGURE 2. The excess semi-liquid elastomeric material of the prep 27 is trapped between the male taper 28a of the core 10 and the female taper 28b of the ring 14. These tapers 28a and 28b are very accurately machined for a perfect fit with each other. The angles of the tapers are precise and equal and generally within a range of 0 to 4 degrees as measured relative to the base of the respective mold element 10 or 14 so that as the mold is closed, the relative axial movement of the cores is much greater than the radial closure of the surface 28b as it approaches surface 28a. The result of this differential in closing rates produces extremely high pressures within the mold as the passage between surface 28a and surface 28b closes down to essentially zero. It has been discovered that, in fact, this pressure is great enough to permanently deform or coin the PTFE wear insert 26 into intimate engagement with the hydrodynamic patterned surface 23 of the mold core 10. The hydraulic pressure is maintained throughout the cure cycle of the elastomer.

The profile 30 of the outer ring 14 forms the external contour of the sealing element. The bottom or closure surface 32 is designed to clamp the seal metal case member 20 against the mold cavity 10.slightly before the tapered surfaces 28a and 28b come together. Tn so doing, the exit of the excess semi-fluid elastomer is restricted in the seal metal area before the final high pressure is attained. The final closure of the tapered surfaces 28a and 28b requires that the seal metal 20, which is not hardened, be indented or coined slightly by the closure surface 32 of the mold ring 14.

FIGURE 2 shows the mold ring 14 completely closed down on the mold core 10 and squeezing the seal metal 20 at position 40. At this stage, the elastomer 27 (seen in FIGURE 1) has become semi-liquid and completely fills the mold cavity to take the new shape as shown at 42. Internal pressure has built-up within the mold and the PTFE wear insert 26 has been embossed with the hydrodynamic fluted surface 23 of the mold core 10 as at 44. The so-called "hat" 46 of the seal is non-functional in that it is not part of the final seal configuration as shown in FIGURES 3 to 5. In terms of molding the elastomeric material forming the hat, it simply fills an area between mold parts that do not touch so as to ensure complete mold closure between the squeeze position 40 and the taper 28 which is the interface between male taper 28a and female taper 28b shown in FIGURE 1. The ring cavity 48 is a space that is provided in the mold to contain the excess of the elastomeric prep 27, as seen in FIGURE 2, that will not fit in the closed mold. The excess is known as flash and is indicated by reference numeral 50. It is important to have some flash so as to insure that the elastomeric prep 27 is large enough to fill the mold cavity and generate the pressure necessary to emboss the PTFE wear insert 26.

The mold which is shown in FIGURE 2 is kept closed for a period of time which maintains heat and pressure on the elastomer 27. The heat and pressure cause cross-linkages to develop chemically within the elastomer and the bonding agents which are

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applied to the metal case 20 and the PTFE wear insert 26. This cross-linking cures the elastomer and bonding agents and insures that all components adhere to each other as a unit during removal from the mold. FIGURE -3 shows in cross-section, a section in detail of the seal as it would appear after removal from the mold. The elastomeric body of the seal 42 is bonded to the seal metal case 20, as at 60, and to the embossed PTFE wear insert having the fluted surface 44 as at 62. Dotted line 66-66 indicates where the excess rubber of the "hat" is cut away or trimmed from the seal body 42.

A spring groove 64 is optional and depends on the eventual application in which the seal is to be used. A garter spring 70, if applied to the finished seal in the position of the spring groove 64, will increase the radial pressure of the sealing lip on the shaft 72, as shown in FIGURE 4. Increased radial pressure will increase friction and wear which may be detrimental to an application. On the other hand, it will tend to make the hydrodynamic pattern 44 of the wear sleeve 26 lie down on the shaft 72 and increase the hydrodynamic efficiency of the seal.

FIGURE 5 shows a cross-sectional view of another embodiment of a seal similar to the one shown in FIGURE 4 in which the elastomeric body 42 of the seal has been extended axially inwardly and angularly away from the primary seal at 44 to form an auxiliary dust excluding lip 74. In some applications where the operational environment is particularly dirty, the dust excluding lip 74 can greatly extend the useful life of the seal. More accurately, it should be stated as greatly extending the life of the shaft which is the "other half" of the seal. Without such an auxiliary lip 74, dirt has a tendency to get worked into the area between the PTFE wear insert 26 and the rotating shaft 72. The mechanics of the wear process that ensues is that the PTFE wear insert 26 , being softer than the metallic shaft 72 and dirt particles, is indented by such dirt particles. Accordingly,

the dirt particles become embedded and entrapped by the PTFE wear insert 26. Entrapped in this manner, the dirt now becomes the surface of the sealing lip. Most dirt contains particles of sand which consists to a great extent of quartz crystals. Quartz is, of course, harder than steel so now the wear insert 26 has become a piece of sand paper that rapidly erodes away the steel shaft 72. Even the eroded particles from the shaft become embedded in the PTFE wear insert 26 to further accelerate the wear on the shaft. All of this occurs during operation because the dirt is stationary on the PTFE wear insert 26 and its sharp corners slide around the rotating shaft. The unprotected PTFE wear insert 26 without lip 74 does not wear, but the exposed shaft wears to a point where the seal can no longer function on the destroyed shaft surface. The dust excluding lip means 74 is, therefore, a very important part of the seal, especially in conjunction with a PTFE wear insert 26 at the sealing lip since it overcomes the noted drawbacks resulting from a seal absent a dust lip.

It should be apparent from the foregoing that the invention is capable of exploitation in the manufacture of seals. While the invention has been described in connection with the preferred embodiment, it is not intended to limit the invention to the particular form set forth above, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.