Radial seal and method of making

A method of manufacturing a radial shaft seal assembly. The method includes the step of stretching an inner diameter of a ring-shaped polytetrafluoroethylene seal radially outward from an axis with a mandrel to a first stretched condition on a mold core element. The method also includes the step of defining a substantially enclosed mold cavity around the polytetrafluoroethylene seal stretched over the mold core element with at least one mold element. The method also includes the step of locating a rigid casing within the mold cavity spaced from the stretched polytetrafluoroethylene seal. The method also includes the step of introducing a liquefied rubber elastomer into the mold cavity. The method also includes the step of molding the liquefied rubber elastomer under heat and pressure such that the rigid casing and polytetrafluoroethylene seal in a stretched state are bonded to a solid rubber elastomer member. The method also includes the step of removing the collectively bonded polytetrafluoroethylene seal and rigid casing and rubber elastomer member from the mold core element without stretching the inner diameter of the polytetrafluoroethylene seal beyond the first stretched condition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to radial seals. More particularly, the invention relates to a method for making an improved fluoropolymer radial seal, such as a radial shaft seal, that is bonded directly to an elastomeric casing layer.

2. Description of the Prior Art

Radial shaft seals that are designed for use in sealing the main rotating shaft of vehicle air conditioner compressors, superchargers, power steering pumps, and engine crankshafts may utilize multiple sealing elements designed such that a first sealing element facing the fluid or gas to be sealed is an elastomer, such as a natural or synthetic rubber. The elastomer generally has sufficient flexibility and resilience to provide a seal against the shaft. A second stiffer, lower friction, and more chemically resistant sealing element is generally positioned behind and in tandem with the elastomeric seal such that an axial gap is provided between the sealing edge of the stiffer wear-resistant seal and the back sealing edge of the more resilient elastomeric sealing element. The second sealing element is generally made from a fluoropolymer, such as polytetrafluoroethylene (PTFE), or a filled PTFE material which incorporates one or more known filler materials to control the mechanical, tribological or other properties of the PTFE.

Generally in the art, the elements of such seal structures have been typically assembled together and then are clamped together in a unit using a crimping process. In such a process, a rubber element and the PTFE component are crimped between two rigid casings to form a seal. The PTFE component is also typically crimped between the rubber element and one of the rigid casings. It is known in the art to utilize a flat PTFE washer or preformed conical-shaped structure that is bonded or clamped to form the overall seal.

Other radial shaft seal designs have also been proposed which do not utilize crimping or clamping of the elastomer and PTFE component into a rigid casing, but rather utilize a metal casing to which the PTFE sealing element is attached by molding an elastomeric member to both the PTFE sealing element and the metal casing. In such designs, the PTFE element may be used only as a bearing member to support and control the load of the elastomeric sealing element, such that the sealing function is entirely performed by the elastomeric sealing element. An example of such a seal configuration is shown in U.S. Pat. No. 4,274,641 to Cather. In this configuration the PTFE bearing member and the elastomeric sealing lip are bonded in tandem and are both in contact with the shaft surface. Similarly, in U.S. Pat. No. 6,428,013 to Johnston et al. several seal designs are disclosed where both the PTFE sealing element and elastomeric element are in contact with the shaft surface on which sealing is to be affected.

Still other seal designs have also been proposed which do not incorporate an elastomeric sealing element and which rely entirely on a PTFE sealing element to provide the fluid seal. One such radial shaft seal design is described in U.S. Pat. No. 4,650,196 to Bucher et al. In Bucher et al., the PTFE element is bonded over a portion of its length to an elastomeric casing which is in turn bonded to a rigid casing. Similarly, in Johnston et al. several seal designs which incorporate a PTFE sealing element as the primary sealing element are disclosed.

One limitation of the related art radial shaft designs, such as those described above, is that the PTFE sealing element does not seal along its entire length. For example, in the designs of Johnston et al. the PTFE sealing element is not in contact with the shaft along its entire length. This is also the case for the PTFE member of Bucher et al. leading to a sub-optimal use of the available PTFE sealing material. Furthermore, these radial seal designs also provide limited control of the sealing pressure applied either by the PTFE sealing element itself to the shaft or other sealing surface, or else by the combination of the elastomeric casing and the PTFE sealing element to the shaft or other sealing surface because of the limited contact area of the PTFE. In addition to the limitations noted above, related art radial shaft seal designs also have known limitations with respect to installation of the seals onto the shaft or other member to be sealed. Many of the known designs where the PTFE lip is the primary sealing lip have the free end of the radial sealing lip facing the fluid side, usually the oil side, of the sealed region. These configurations are known to be difficult to install onto circular shafts and the like, necessitating the use of special fixtures and installation tools, and special assembly precautions or methods to assemble such seals on shafts so as to avoid nicking or otherwise damaging the surface of the PTFE material, and thus destroying the functionality of the seals. Fluoropolymer sealing materials, such as PTFE, are known to be very susceptible to nicking or other surface damage to the sealing surface which can compromise their ability to seal effectively. Reverse lay down configurations of the PTFE sealing element, where the free end of the sealing element faces away from the oil side of the installation, have been proposed, such as in Johnston et al., in order to enhance the ability to install such seals and lessen the susceptibility to nicking, inverse folding, or creasing during installation. However, such seal configurations are still believed to be subject to other limitations, such as those described above.

SUMMARY OF THE INVENTION

In summary, the invention is a method of manufacturing a radial shaft seal assembly. The method includes the step of stretching an inner diameter of a ring-shaped polytetrafluoroethylene seal radially outward from an axis with a mandrel to a first stretched condition on a mold core element. The method also includes the step of defining a substantially enclosed mold cavity around the polytetrafluoroethylene seal stretched over the mold core element with at least one mold element. The method also includes the step of locating a rigid casing within the mold cavity spaced from the stretched polytetrafluoroethylene seal. The method also includes the step of introducing a liquefied rubber elastomer into the mold cavity. The method also includes the step of molding the liquefied rubber elastomer under heat and pressure such that the rigid casing and polytetrafluoroethylene seal in a stretched state are bonded to a solid rubber elastomer member. The method also includes the step of removing the collectively bonded polytetrafluoroethylene seal and rigid casing and rubber elastomer member from the mold core element without stretching the inner diameter of the polytetrafluoroethylene seal beyond the first stretched condition.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT

A plurality of different embodiments of the invention are shown in the Figures of the application. Similar features are shown in the various embodiments of the invention. Similar features have been numbered with a common reference numeral and have been differentiated by an alphabetic designation. Also, to enhance consistency, features in any particular drawing share the same alphabetic designation even if the feature is shown in less than all embodiments. Similar features are structured similarly, operate similarly, and/or have the same function unless otherwise indicated by the drawings or this specification. Furthermore, particular features of one embodiment can replace corresponding features in another embodiment unless otherwise indicated by the drawings or this specification.

FIG. 1shows a portion of an apparatus300for making or manufacturing a radial shaft seal. The apparatus300can stretch a preform72of a radial seal and includes a pusher78and a mandrel64. The mandrel64extends a longitudinal axis67and includes a frustum portion65ending in maximum radial edge73. The mandrel64is integrally formed with a mold core element98that is cylindrical.

FIG. 1also shows a ring-shaped polytetrafluoroethylene seal in the condition of the preform72. The preform or washer72has an inside diameter or surface62and an outside diameter or surface74. The flat fluoropolymer washer72is placed onto an upper end76of the mandrel64. The diameter of the upper end76of the mandrel64is smaller than the inner diameter62of fluoropolymer washer72.

The expandable pusher or installation tool78is in the form of a plunger having an expandable lower section, such as a plurality of separate expandable fingers80. The expandable fingers80each have a contact surface82which is used to engage the flat washer72as the installation tool is moved into contact with the flat washer72and mandrel64in the direction shown by arrow84. As the fingers80of the expandable pusher or installation tool78move along a tapered outer surface86of the frustum portion65of the mandrel64, the contact surfaces82of the expandable fingers80engage an upper surface88of fluoropolymer washer72causing it to slide down the outer surface86of mandrel64. The mandrel64may have one or more tapers and is constructed such that the diameter of an outer surface87of the element98is greater than the inner diameter62of fluoropolymer washer72. The taper of the surface87can be a linear profile, a convolute profile, an involute profile, or any other suitable tapered profile for pre-stressing the preform72.

The size of the preform72together with the size of the mandrel64and element98determine the amount of stretching that is performed on the fluoropolymer seal preform72and the amount of pre-stress imparted to pre-stressed fluoropolymer seal32(shown inFIG. 2). The washer72and mandrel64may be selected such that the diameter of the outer surface87of mandrel64is greater than the outer diameter of fluoropolymer seal preform72. Thus, washer72may be pre-stressed on both the inner diameter62and outer diameter74.

Referring now toFIGS. 1 and 2, the washer72slides down the outer surface86of the mandrel64by lowering the plunger78with respect to the mandrel64. As a result, the washer72is plastically stretched and becomes the seal32. The seal32will be in a pre-stressed condition at a first end90at least. The inner diameter62of the pre-form72corresponds to the end90of the pre-stressed seal32. The level of stress along the length of the pre-stressed fluoropolymer seal32can vary because the portion of the washer72adjacent to the inner diameter62of the fluoropolymer washer72is stretched to a greater extent than portion adjacent to the outer diameter74. The outer diameter74may also be expanded as it glides downward along the mandrel64, thereby pre-stressing a second end92of fluoropolymer seal32.

U.S. patent application Ser. No. 11/224,362 provides additional details of an exemplary embodiment of the present invention with respect to positioning the preform or washer72on the element98. The '362 application is incorporated by reference in its entirety as teachings for an exemplary embodiment of the present invention. Other methods for positioning the a seal on mandrel may be practiced in connection with the present invention.

FIG. 2shows the seal32disposed on the element98and a mold cavity70defined around the pre-stressed seal32. In the exemplary embodiment of the invention, a plurality of mold elements100,102,104,108cooperate with the element98to define the mold cavity70. In alternative embodiments of the invention, the mold cavity70may be defined by less than five mold elements or by more than five mold elements.

Referring now toFIG. 3, the mold cavity70includes a first portion112that extends sleeve-like a first distance in a direction represented by arrow114, the direction being parallel to the axis67(shown inFIGS. 1 and 2). The first portion112extends from a first end116proximate to the edge73and a second end118spaced from the first end116. The mold cavity70also includes a second portion120spaced radially outward of the first portion112with respect to the axis67(shown inFIGS. 1 and 2). The second portion120extends sleeve-like a second distance in a direction represented by arrow122, the direction being parallel to the axis67. The second portion120extends between first and second ends124,126that are spaced from one another. In operation, a casing30can be disposed in the second portion and thus bifurcate the second portion120into two sub-sleeve portions136,138that are radially spaced from one another with respect to the axis67and in communication with one another at the first end124.

The mold cavity70also includes a third portion128that extends radially with respect to the axis67between both of the second ends118,126to place the first and second portions112,120in fluid communication with one another, the second portion120connected through the sub-sleeve136in the exemplary embodiment of the invention when a casing30is in the mold cavity70. The third portion128extends transverse to the axis67. The cross-section ofFIG. 3shows a chaplet134; the chaplet134is not present in all cross-sections of the mold cavity.

As a result of the configurations of the first, second and third portions112,120and128, the cross-section of the mold cavity can be U-shaped, J-shaped, W-shaped, V-shaped, H-shaped, or any other shape that generally turns on itself as it extends between the first end116of the first portion112and the first end124of the second portion120.

Referring now toFIGS. 2 and 3, an injection port130is disposed proximate the first end124of the second portion120in the exemplary embodiment of the invention. As a result, the injection port130is a substantially maximum distance away from the edge73and the end90that is under the maximum pre-stress. In a molding operation, the casing30can be disposed in the mold cavity in spaced relation to the seal32. Liquefied rubber elastomer can be directed through the injection port130and fill the mold cavity70. The liquefied rubber elastomer can flow around the chaplet134. The liquefied rubber elastomer can be molded under heat and pressure such that the rigid casing30and polytetrafluoroethylene seal32in a stretched state are bonded to a solid rubber elastomer member.

A comparison betweenFIG. 2and the figures of the '362 application reveals that the seal32is not disposed in an annular notch defined in the mandrel64. A surface132is defined by the mold element108and is disposed proximate to the end90to support the seal32against movement during the molding operation. A radial gap of 0.0005-0.0025 can be defined between the end90and the surface132. A larger gap may not be desirable; a larger gap may allow a sufficient quantity of PTFE material and/or liquefied rubber elastomer to penetrate between the surface132and the end90and form a burr. The burr would be present on the lead sealing edge of the radial seal assembly and would therefore require trimming.

Upon completion of a molding operation, the collectively bonded polytetrafluoroethylene seal32and rigid casing30and rubber elastomer member (a complete radial seal assembly) can be removed from the element98without stretching the inner diameter62(or end90) of the polytetrafluoroethylene seal32beyond the first stretched condition. The mold element108and the element98can be moved away from one another and the radial seal assembly can be moved in the direction represented by arrow136.

FIG. 4shows a mold structure for practicing a second, alternative embodiment of the invention. A polytetrafluoroethylene seal32aand a rigid casing30aare disposed in a mold cavity70ahaving an annular configuration centered on an axis67a. The mold cavity70ais defined by a plurality of mold elements98a,100a,102a,104a, and108a. An injection port130ais a substantially maximum distance away from an end90aof the seal32. In a molding operation, liquefied rubber elastomer can be directed through the injection port130and fill the mold cavity70. The liquefied rubber elastomer can be molded under heat and pressure such that the rigid casing30aand polytetrafluoroethylene seal32ain a stretched state are bonded to a solid rubber elastomer member.

In the second exemplary embodiment of the invention, a first end90aof the polytetrafluoroethylene seal32ais stretched radially outward from the axis67ato a first stretched diameter. A second end92aof the polytetrafluoroethylene seal32ais stretched radially outward from the axis67ato a second stretched diameter. The second stretched diameter is greater than the first stretched diameter in the second exemplary embodiment of the invention; the two diameters were the same in the first exemplary embodiment of the invention. Making the stretched diameter of the second end greater than the stretched diameter of the first end does not necessarily imply that the second end is stretched more than the first end. As set forth above, the ends (90,90a,92,92a) shown in the cross-sectional views correspond to the inner and outer diameters of the ring-like seal and the extent of stretching for each end is the stretching of those diameters. In the first embodiment, the outer diameter is less stretched than the inner diameter because the diameters are substantially equal when the seal32is in the stretched condition. In the second embodiment, the inner and outer diameters are more similarly stretched because the outer diameter (represented by the end92a) is greater than the inner diameter. The stretched diameter of the outer end (92or92a) can be selected in view of the expected operating environment. For example, if it were desirable for the outer diameter of the ring-like seal32to be biased to contract radially inwardly at a relative greater rate, the second embodiment of the invention may be the preferred. On the other hand, if it were desirable for the outer diameter of the ring-like seal32to be biased to contract radially inwardly at a relatively weaker rate, the first embodiment of the invention may be the preferred.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims. In addition, the reference numerals in the claims are merely for convenience and are not to be read in any way as limiting.