Polytetrafluoroethylene shaft sealing element and manufacturing method thereof

Abstract

A polytetrafluoroethylene (PTFE) rotating shaft seal assembly includes an annular seal housing mountable within an inner bore of a housing and a sheet made of PTFE. An outer region of the sheet is secured by the sealing housing and an inner region of the sheet extends to a central opening of the sheet. The sheet forms an embossed fluid pumping indentation on a first side thereof and an embossed flexible indentation on an opposite second side thereof. The sheet also includes an embossed static sealing tape portion surrounding the central opening and having no fluid pumping indentation. A method of manufacturing such a sheet and seal assembly is also provided.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This international (PCT) patent application claims priority to and the benefit of U.S. patent application No. 18 / 178,084, filed on March 3, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates generally to dynamic polytetrafluoroethylene (PTFE) shaft seals and, more particularly, to the structure and method of making PTFE shaft seals. Background Art

[0004] Dynamic shaft seals are commonly used to seal openings in a housing through which a rotating shaft extends. The seal acts between the housing and the shaft to prevent lubricant from escaping the housing and typically also keeps dust and debris out of the housing.

[0005] One type of dynamic seal utilizes a wafer made of PTFE material that is initially disc-shaped and is clamped around its periphery by a rigid sealing sleeve. The inner peripheral edge of the PTFE wafer is free, and during the process of installing the seal on the shaft, the inner peripheral edge of the wafer is stretched and the inner working surface of the wafer is conformed to the shaft. The working surface of the PTFE wafer is sometimes formed into a groove pattern through a cutting or embossing process, and the grooves operate in a dynamic mode to pump fluid from the inner working surface back to the housing. Among them, cutting is a known technique that forms grooves of the desired size, shape and pattern on the wafer by removing material from selected areas of the wafer. Embossing is also a known process in which the portion of the wafer to be embossed is subjected to a high-intensity localized compressive load, resulting in permanent deformation in the affected area and the formation of permanent indentations. Through the displacement of the material during the embossing process, these permanent indentations form fluid dynamic groove characteristics and corresponding raised or displaced areas. While molded PTFE sheets perform well in dynamic mode, they can struggle to prevent fluid leakage in static mode, where the shaft is not rotating, due to the lack of dynamic pumping action. This issue is particularly pronounced in designs where the molded structure extends to the inner periphery of the working surface. Furthermore, improving the static sealing capability of a shaft seal can negatively impact dynamic sealing performance, as the pumped fluid must maintain a clear return path.

[0006] One aspect of the present invention addresses this issue by achieving an appropriate balance in design and functionality between the dynamic and static performance of a shaft seal, enabling the seal to operate effectively in both dynamic and static modes, thereby preventing fluid leakage when the shaft is rotating (dynamic mode) and also preventing fluid leakage when the shaft is stationary (static mode). Summary of the Invention

[0007] A polytetrafluoroethylene (PTFE) rotary shaft seal assembly includes an annular seal housing mountable within an inner bore of a housing, and a thin sheet made of PTFE, with an outer region of the sheet secured by the seal housing and an inner region extending to a central opening of the sheet. The sheet has embossed fluid pumping indentations formed on a first side and embossed flexibility indentations formed on an opposing second side. The sheet further includes an embossed static seal band portion surrounding the central opening and lacking the fluid pumping indentations.

[0008] This PTFE shaft seal provides improved dynamic sealing performance by embossing pumping indentations, while the embossed static sealing ring provides improved static sealing performance when the shaft passes through the shaft opening of the housing. More specifically, the static sealing band is designed to fit tightly or tightly against the shaft when the shaft is stationary to prevent fluid (e.g., oil) from leaking from the interior of the housing; however, in dynamic mode, it works in conjunction with the embossed pumping indentations so that any fluid escaping in dynamic mode is pumped back into the housing by the hydrodynamic effect of the indentations and through the static sealing band in the return direction.

[0009] A method for manufacturing a PTFE shaft seal assembly is also provided, comprising preparing an annular sheet of PTFE material having a uniform thickness, the sheet having an outer region, an inner region, and opposing first and second sides. The method further comprises pressing and permanently deforming the inner region of the sheet between opposing embossing dies to extrude a plurality of embossed dynamic fluid indentations on the first side of the sheet, and further embossing a static sealing band of reduced thickness, the static sealing band being free of fluid indentations and surrounding a central opening of the sheet. An advantage of this method is that it can form a PTFE sheet having both embossed dynamic fluid pumping characteristics and embossed static sealing characteristics. The embossing of the static sealing band results in a thickness that is less than the thickness of the original pre-embossed sheet, thereby providing sufficient sealing performance to prevent oil leakage under static conditions, while allowing oil to pass under the static sealing band and return to the oil side of the seal under dynamic conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Other objects, advantages and features of the present invention will be further clarified in the following detailed description and accompanying drawings, in which: Figure 1 is a front perspective view of a PTFE sheet preform; Figure 2 It is along Figure 1 An enlarged cross-sectional view taken along line 2-2; Figure 3 is with Figure 2 A similar view, but showing an embossed PTFE sheet; Figure 4 This is a partial cross-sectional view of the PTFE seal assembly in use; Figure 5 is a schematic diagram of the PTFE imprinting process; and Figures 6A-6F is a schematic diagram showing the various stages of the thin film imprinting process. DETAILED DESCRIPTION

[0011] Figure 1 and Figure 2 An embodiment of a PTFE sheet preform 10, also referred to as a gasket, flat gasket, or disc, is shown. It can be planar and cut from a sheet of polytetrafluoroethylene (PTFE). The figure shows the preform in its preformed state, before any structural features are embossed onto its surface. Sheet 10 can have a generally cylindrical structure, including a cylindrical outer surface 12 (having a predetermined diameter) and a cylindrical inner surface 14 (having a relatively smaller predetermined diameter). Outer and inner surfaces 12, 14 can be concentrically arranged. Cylindrical inner surface 14 defines a central hole or opening 16 extending through the sheet. The size of opening 16 may depend on the size of the shaft to be extended through opening 16, as will be described further below. Sheet 10 has a first side (or front side) 18 and an opposing second side (or back side) 20. In the pre-formed stage of the sheet 10 shown in the figure, the first side 18 and the second side 20 are planar and parallel and define a predetermined unembossed thickness t1 of the sheet 10, which is uniformly distributed from the outer surface 12 to the inner surface 14 in the initial state. Figure 2 As shown. Thickness t1 may also be referred to as original thickness or unembossed thickness. The sheet 10 includes a radially outer region (or portion) 22 and a radially inner region (or portion) 24, both portions 22, 24 having the same original thickness t1 at the initial stage of the preform sheet.

[0012] Figure 3 An embodiment of the sheet 10 after embossing is shown. The outer region 24 retains its original, preformed state (e.g., Figure 1 and Figure 2 ), i.e., it has not been embossed and retains its original thickness t1. The inner region 22 has been embossed so that permanent embossed features are formed on its surfaces 12, 14. These features are formed due to the plastic deformation of the PTFE material in this region 22 during the embossing process. These embossed features are not formed by cutting or removing material, but are achieved by permanent displacement (plastic deformation) of the material during the embossing process. The embossed features include fluid pumping channels, grooves or indentations 26 pressed into the first side surface 18. Each groove 26 has an inclined sidewall that extends from the bottom (or valley) 28 of the groove to the surface 18. The areas between adjacent grooves 26 form peaks 30, which engage with the shaft 32 during operation, as shown Figure 4 By comparison Figure 2 and Figure 3It can be observed that the peaks 30 protrude below (i.e., are offset relative to) the first side surface 18 of the outer region 22. This is due to the fact that the embossing of the inner region 24 permanently displaced a certain amount of the inner region material downwardly below the original plane of the first side surface 18. It can also be seen that the valleys 28 of the grooves 26 lie within or substantially lie within the same plane as the first side surface 18 of the unembossed outer region 22.

[0013] Figure 3 Also shown are flexure channels, grooves, or indentations 34 permanently impressed by embossing into the rear side 20 of the inner region 24. These features are arranged generally opposite the fluid pumping grooves 26. Each flexure groove 34 has a sloped sidewall extending from the bottom of a valley 36 of the groove 34 to the surface 20. The areas between adjacent grooves 34 form peaks 38. It can be observed that the flexure grooves 34 on the rear side 20 are aligned with the peaks 30 on the front side 18. The permanent material displacement produced during the embossing of the flexure grooves 34 is reflected in the offset peaks 30 formed on the front side 18, which protrude below the first side plane 18 of the outer region 22. In other words, the PTFE material permanently displaced during the embossing of the recessed flexure grooves 34 is at least partially accommodated by the displacement of material on the opposite side during the formation of the offset peaks 30. However, the dimensions of the embossing mold used in the embossing process are preferably designed so that the peaks 30 themselves are embossed and permanently compressed on at least a portion of the surface, and excess PTFE material that would otherwise fill the peaks 30 is displaced radially inward toward the inner diameter 14 of the sheet 10 during the embossing process.

[0014] Figure 3 The illustrated embodiment further illustrates the formation of an embossed static sealing band 40 adjacent to and surrounding the aperture 16. The static sealing band 40 is circumferentially continuous and lacks the fluid channels 26 on its inner side 18. The inner surface of the static sealing band is smooth and continuous, without grooves, channels, or other embossed features, to prevent the passage of fluids (e.g., oil) under static operating conditions (as described below). The embossed maximum thickness t2 of the embossed static sealing band 40 is less than the original thickness t1 of the preform sheet 10. This reduction in thickness is due to the compression and permanent deformation of the radially innermost region of the sheet 10, which results in that region being thicker than the original thickness t1 of the preform sheet 10. Figure 1 and 2The illustrated embodiment of the sheet preform 10 shows an initial, pre-embossed sheet state that is permanently thinned and permanently elongated. The front side 42 of the sealing band 40 is displaced (or offset) downward relative to the first side 18 of the unembossed outer region 22 and is preferably located in the same plane as the peak 30. This permanent downward offset or displacement of the front side 42 of the sealing band 40 material is a result of the embossing process. The opposing rear side 44 of the sealing band 40 is permanently located below the rear side 20 of the unembossed outer region 22, also as a result of the embossing process. Thus, after embossing, the sealing band 40 exhibits an offset, thinned, and elongated configuration relative to the unembossed, preformed state of the sheet 10. The embossed surface of the sealing band 40 includes the front side 42, the rear side 44, and the inner diameter surface 46, which now forms the aperture 16 but has a correspondingly reduced diameter due to the elongation of the sealing band 40 during the embossing process. It can be further seen that the front surface 42 of the embossed static sealing band 40 is in the same plane as the valleys 28 and 36 of the grooves 26 and 34 respectively, while the rear surface 44 thereof is located above the valleys 28 and 36 of the grooves 26 and 34 respectively.

[0015] The peak-to-peak embossed thickness t3 of the embossed inner region 24 is greater than the original thickness t1. The valley-to-valley embossed thickness t4 of the embossed inner region 24 is less than the original thickness t1 and less than the embossed thickness t2 of the static sealing band 40. The center plane of the material separating valley 28 from valley 36, having a thickness t4, is displaced relative to the center plane of the unembossed outer region 22.

[0016] The size and shape of the fluid pumping grooves 26 are preferably consistent across all grooves 26. The radially inner sidewall of a groove 26 preferably has a greater inclination angle than its opposing radially outer sidewall. The grooves may have a sawtooth shape, with the radially outer sidewall being steeper than the radially inner sidewall. The peaks 30 of the fluid pumping grooves 26 may have an inclined surface that rises toward the outer region 22, resulting in the peaks 30 also exhibiting a sawtooth structure. The flex grooves 34 may be deeper and wider than the fluid pumping grooves 26 and have different sizes and shapes. The valleys 36 and peaks 38 of the flex grooves 34 are both arc-shaped. The flex grooves 34 and their corresponding valleys 36 and peaks 38 may be uniform or may be variably designed to control the desired flexibility of the sheet 10 during operation. The depth of the fluid pumping grooves 26 is less than that of the flex grooves. The valley-to-valley spacing of the pumping grooves 26 is the same as the valley-to-valley spacing of the flex grooves 34. The embossed static sealing band 40 is separated from the nearest adjacent valley 36 of the flex groove by the intervening valleys 28 of the fluid groove 26 and the intervening peaks 38 of the flex groove 34 .

[0017] Figure 4A schematic diagram illustrates an embossed sheet 10 forming part of a PTFE rotary shaft seal assembly 50. The assembly 50 comprises a rigid, annular seal housing 52, which may be fabricated in a known manner from an annular outer housing portion 54 and an annular inner housing portion 56. The outer region 22 of the sheet is positioned between opposing annular radial flanges of the housing portions 54, 56. The housing 52 is then deformed to clamp and secure the sheet 10 between the housing portions 54, 56, while the inner region 24 of the PTFE sheet extends from the radially inner side of the housing 52 to the free annular opening 16 thereof. One way to achieve the installation of the sheet 10 is to deform the axially inner lip of the outer housing portion 54 onto the inner housing portion 56 while simultaneously compressing the portions 54, 56 axially. An elastomeric gasket 58 may be positioned between the sheet 10 and the outer seal portion 54. The seal assembly 50 is installed within a housing 60 to be sealed, with the housing 52 pressed into an associated housing groove surrounding the shaft opening 62. The interior of the housing 60 is filled with a fluid (e.g., lubricating oil), while the external environment is typically air. Figure 4 The "OIL" side and the "AIR" side of the seal assembly 50 when in use are marked. The shaft 64 extends through the shaft opening 62, and its outer diameter running surface or sealing surface is relatively large in size relative to the stamped hole 16 of the sheet 10. When the shaft 64 passes through the seal assembly 50, it causes the hole 16 in the sheet 10 to elastically stretch so as to fit tightly against the running surface of the shaft 64. The running surface may be the shaft itself or a sleeve, which may be part of the shaft 64 or part of the seal assembly 50. The elastic deformation of the sheet 10 causes the inner region 24 to form an annular structure, so that the front side 42 of the stamped static sealing band 40, as well as part or all of the stamped fluid grooves 26 and their associated peaks 30, are all inverted on the shaft 64; at the same time, the inner region is deflected and extends axially along the shaft 64 toward the oil side of the seal assembly 50.

[0018] During operation, the installed seal assembly 50 can operate in either static or dynamic conditions (or modes). As the shaft 64 rotates, any oil that might have passed through the static sealing band 40 from the housing 60 is drawn into the grooves 26. The resulting motion of the rotating shaft 64 relative to the static grooves 26 creates a dynamic pumping action, effectively driving the fluid back to the oil side, preventing it from leaking out of the housing 60 through the shaft opening 62. It's common for the housing 60 to be under positive pressure during operation, which exacerbates the tendency for oil to be forced through the static sealing band 40. This is then driven back through the fluid pumping grooves 26 in dynamic mode. When the shaft 64 is stationary, the grooves 26 do not generate a dynamic pumping action. The sealing of the shaft opening 62 relies entirely on the static sealing band 40. Because this sealing band is die-cast and relatively thin, it facilitates the passage of oil in dynamic mode thanks to the grooves 26. These grooves capture the oil and dynamically return it back into the housing 60, past the static sealing band 40. Typically, static sealing strips are thick-walled to enhance static sealing performance, but this requires grooves or some form of surface channeling to facilitate dynamic oil return in dynamic mode. The present invention employs a different approach, employing a stamped static sealing strip 40 without any grooves or oil channeling. By stamping the static sealing strip 40 without extending the groove 26, the static sealing strip 40 is circumferentially continuous and forms a complete 360-degree static seal around the shaft 64 when stationary, preventing oil leakage from the housing 60.

[0019] Figure 5 and Figures 6A-6F The process steps of embossing the above-described features into the radially inner portion 24 of the PTFE sheet preform 10 are illustrated, while the radially outer portion 24 of the sheet preform is not embossed. Figure 5 It is schematically shown that an unembossed sheet preform 10 is introduced between upper and lower embossing dies 66, 68, the dies are driven to force closed and emboss the embossing into the sheet 10, and the embossed sheet is subsequently removed. Figure 6A The embodiment structure of the upper embossing mold 66 and the lower embossing mold 68 arranged relative to each other is further shown in detail. It is understood that one or both of the molds 66, 68 are controlled by an adapted molding machine, which embosses the sheet 10 by forcing the molds toward each other and separates the molds when the sheet is loaded and unloaded. The molds 66, 68 have opposing embossing surfaces, which are shaped to form embossed features on the above-mentioned sheet 10. It can be seen that the molds 66, 68 have a series of ridges 70, 72 and cavities 74, 76 respectively. The embossing process begins by placing the unembossed preformed sheet 10 between the molds 66, 68, as shown. Figure 6A As shown, the sheet 10 is placed on the ridge 72 of the lower die 68 with its inner diameter surface 14 spaced from the upstanding inner wall 78 of the lower die 68 .

[0020] Figure 6B The initial movement of the dies 66, 68 toward each other is shown, causing the ridges 70, 72 to bite into the opposing sides 18, 20 of the inner portion 24 and begin to compress the inner portion 24. The localized reduction in cross-section of the sheet 10 at the ridges causes the inner portion that will eventually form the embossed static sealing band 40 to be elongated (i.e., reduces the aperture 16 of the sheet 10), which is now facing the inner wall 78 of the lower die 68. At this point, the sheet 10 is constrained by its outer surface 12 and inner surface 14 against the inner wall 78 and outer wall 80 of the lower die 68, respectively.

[0021] Figure 6C Further closing of the dies 66, 68 is shown, from which it can be seen that the compression of the ridges 70, 72 begins to force the inner portion 24 of the sheet 10 to be displaced into the cavities 74, 76 of the dies 66, 68. It can also be seen that the outer side 54 of the static sealing band-forming portion 40 is fully engaged by the corresponding wall of the upper die 66, thereby forcing the opposite inner side 56 to be displaced into the opposite innermost cavity 82 of the lower die 68.

[0022] Figure 6D and 6E The further progressive closing of the embossing dies 66, 68 is shown, from which it can be seen that the PTFE material continues to displace further into the cavities 74, 76, and 82, with the first cavities 74, 76 being completely filled, the endmost cavity 82 and the opposing wall 84 also being filled, and beginning to compress the static sealing band-forming portion 40 of the sheet 10, reducing its thickness to less than the original sheet thickness t1. At this stage, the end face 46 is also restrained and embossed. The displaced material / extruded material from these blocked and filled areas continues to enter the available space in the cavity 74 of the upper die 66.

[0023] at last, Figure 6F The embossing dies 66, 68 are shown in a fully closed position, with the interior region 24 fully engaged and compressed by the die surfaces, its inner surface 46 permanently deformed (embossed), and its two opposing surfaces 18, 20 also permanently deformed (embossed). Only the very tips of the peaks 38 on the rear side 20 are not embossed (i.e., these tips are not fully engaged and compressed by the surfaces of the dies 66, 68 to produce the permanent deformation because there is excess space in the corresponding upper die cavity 75, allowing the tips to remain unembossed). Of course, the entire interior region 24 (including the tips of the peaks 38) can be fully embossed.

[0024] Obviously, in light of the above teachings, many modifications and variations of the present invention are possible. Therefore, it should be understood that the present invention can be practiced in ways different from those specifically described while still falling within the scope of the present invention.

Claims

1. A polytetrafluoroethylene (PTFE) rotary shaft seal assembly comprising: an annular sealing housing mountable in the inner bore of the housing; a sheet made of PTFE having an outer region secured by the sealing housing and an inner region extending to a central opening of the sheet; The sheet includes an embossed fluid pumping indentation formed on a first side thereof; and wherein the sheet includes an embossed static sealing band portion surrounding the central opening and containing no fluid pumping indentations.

2. The assembly according to claim 1, wherein The embossed static sealing tape portion has a defined maximum embossed thickness t2, and the outer region of the sheet inside the housing is not embossed, and its unembossed thickness t1 is greater than the embossed thickness t2 of the static sealing tape portion.

3. The assembly according to claim 2, wherein The sealing assembly includes a flexure indentation imprinted on an opposing second side of the sheet, and wherein the sheet has an unmounted state in which the interior region freely extends inwardly of the housing, and an mounted state in which a rotating shaft having a functional outer running surface passes through the sealing assembly, causing the opening of the sheet to stretch and causing at least a portion of the fluid pumping indentation and the static sealing band to fall over and engage the running surface of the shaft.

4. The assembly of claim 3, wherein the fluid pumping indentation and the flexure indentation each comprise a plurality of peaks separated by corresponding valleys, and wherein: The embossed thickness t3 of the inner region, measured between the bottoms of the valleys of the fluid pumping indentation and the flexure indentation, is less than the unembossed thickness t1 of the outer region and less than the embossed thickness t2 of the static sealing band.

5. The assembly according to claim 4, wherein The embossed thickness t4 of the inner region, measured between the peaks of the fluid pumping indentation and the flexure indentation, is greater than the unembossed thickness t1 of the outer region and greater than the embossed thickness t2 of the static sealing band.

6. The assembly of claim 5, wherein a peak of the fluid pumping indentation is located below the first side surface of the unembossed outer region.

7. The assembly of claim 4, wherein the depth of the valleys of all embossed fluid channels is the same.

8. The assembly according to claim 4, wherein The valleys of the embossed flexure channels are offset from the valleys of the embossed fluid channels.

9. The assembly of claim 4, wherein at least a portion of the peaks of the flexure indentations are in the same plane as the second side surface of the outer region.

10. The assembly of claim 4, wherein: Between the embossed static sealing band and the nearest adjacent valley of the deflection indentation, there are intervening valleys of the deflection indentation and intervening peaks of the deflection indentation.

11. The assembly according to claim 4, wherein The valleys of the embossed fluid indentations are generally coplanar with the first side surface of the unembossed outer region, while the valleys of the embossed flexure indentations are located below the second side surface of the unembossed outer region.

12. A method for manufacturing a PTFE shaft sealing sheet, comprising: preparing an annular sheet preform of PTFE material having a uniform thickness and having an outer region, an inner region, and opposing first and second sides; The sheet preform is embossed, including pressing and permanently deforming the interior region between opposing embossing dies to emboss a plurality of embossed dynamic fluid indentations on the first side, and further embossing a reduced thickness static sealing band portion that does not contain fluid indentations and surrounds the central opening of the sheet, thereby forming an embossed sheet.

13. The method according to claim 12, wherein An outer region of the embossed sheet is unembossed after the steps of pressing and deforming the inner region, and wherein the unembossed outer region has a thickness greater than the thickness of the reduced thickness static sealing tape portion of the inner region.

14. The method of claim 12, wherein the step of embossing the sheet preform comprises embossing a flexure indentation on the second side of the interior region of the sheet.

15. The method according to claim 12, wherein: The laminar preform has a central opening of a predetermined initial diameter, and wherein the step of embossing the laminar preform comprises reducing the diameter of the opening during embossing of the static sealing tape portion.

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