Abstract:
A mechanical seal surrounding a rotary shaft to prevent a fluid (i.e., a liquid or gas) from flowing across the seal, the seal including a rotatable seal ring that includes a rotatable seal face, where the rotatable seal ring is rotatable around a rotational axis of the rotary shaft, and a stationary seal ring spatially fixed relative to the rotatable seal ring and having a stationary seal face to engage the rotatable seal face to form the mechanical seal, where the stationary seal ring includes about 1% to about 30%, by weight, graphite, and about 70% to about 98%, by weight, polytetrafluoroethylene. Also, a graphite containing teflon sealing ring used to form a mechanical seal around a rotary shaft, where the teflon sealing ring includes a first seal face capable of engaging a second seal face on a second sealing ring surrounding the rotary shaft to form the mechanical seal. The teflon sealing ring may include graphite, polytetrafluoroethylene, and optionally, non-graphite carbon.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to mechanical seals for rotating shaft machinery. Specifically, the invention includes the use of graphite containing polytetrafluoroethylene sealing elements to form seals around a rotatable shaft.  
         [0002]     Mechanical seals may be used to prevent fluids (e.g., hydrocarbon gases, water) from escaping a confined space by flowing around rotating shaft machinery extending into that space from outside. These seals have application in a wide variety of devices and processes where such machinery is used, including dishwashers, washing machines, mixers, pumps, and many other kinds of machinery.  
         [0003]     In early applications, it was common to make mechanical seals from cloth or rope wedged between the rotating shaft and the fluid containment wall. Later seal designs replaced cloth and rope with elastomeric materials (e.g., O-rings) clamped or stretched around the rotating shaft. While these rotating seals are simple to implement, they have relatively short lifetimes due to wear at the sealing interface, heat damage from friction, and damage from corrosive fluids absorbing into the sealing materials. In addition, when these materials slip around the rotating shaft, they may wear away protective films (e.g., a protective oxide film) formed on the surface of the shaft, allowing corrosive fluids to corrode away the underlying shaft material and further weaken the seal.  
         [0004]     Later seal designs included mechanical seals that had a seal interface formed between two seal rings, one of which may rotate with the rotating shaft while the other remains fixed. The rings, which may be coaxial to the rotational axis of the shaft, have opposite facing sealing faces that form a fluid seal by making contact with each other. The materials used to make the seal rings may be chosen to minimize the effects of friction and wear at the contacting sealing faces.  
         [0005]     A conventional mechanical seal ring design includes forming the stationary seal ring out of a hard material (e.g., silicon carbide (SiC)) and the rotating seal ring out of graphite containing carbon. The planar grain structure of graphite makes it a natural lubricant for the sealing interface, and its thermal conductivity provides for the fast dissipation of the frictional heat generated as the rotational seal ring rotates against the stationary ring. Unfortunately, production of the graphite containing carbon rings is time consuming and expensive.  
         [0006]     Typically, the process starts by mixing graphite and carbon particulates with an organic binder material (i.e., glue) and baking the mixture in an oven. Because the goal of the baking is to decompose the binder into carbon without forming CO and/or CO 2 , the mixture needs to bake very slowly (e.g., about 30 to 60 days depending on the materials) at temperatures of 500° C. or more. Furthermore, the oven has to be purged of ambient air to prevent the oxygen in the air from reacting with carbon in the baking sealing ring. Thus, there remains a need for materials that can be used in the sealing rings of mechanical seals that do not require the time and energy intensive production steps of graphite containing carbon.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     Embodiments of the invention include a mechanical seal surrounding a rotary shaft to prevent a fluid (i.e., a liquid or gas) from flowing across the seal. The seal may include a rotatable seal ring that includes a rotatable seal face, where the rotatable seal ring is rotatable around a rotational axis of the rotary shaft. The seal may also include a stationary seal ring spatially fixed relative to the rotatable seal ring and having a stationary seal face to engage the rotatable seal face to form the mechanical seal. The stationary seal ring may be made from about 1% to about 30% (by weight) graphite, and about 70% to about 98% (by weight) polytetrafluoroethylene. The seal ring may also optionally include about 1% to about 30% (by weight) non-graphite carbon.  
         [0008]     Embodiments of the invention also include a mechanical seal surrounding a rotary shaft to prevent a fluid from flowing across the seal. The seal may include a stationary seal ring that includes a stationary seal face, where the stationary seal ring is positioned around the rotary shaft. The seal may also include a rotatable seal ring that is rotatable relative to the stationary seal ring and has a rotatable seal face to engage the stationary seal face to form the mechanical seal. The rotatable seal ring may be made from about 1% to about 30% (by weight) graphite, and about 70% to about 98% (by weight) polytetrafluoroethylene. The seal ring may also optionally include about 1% to about 30% (by weight) non-graphite carbon.  
         [0009]     Embodiments of the invention may also include a graphite containing teflon sealing ring used to form a mechanical seal around a rotary shaft. The teflon sealing ring may include a first seal face capable of engaging a second seal face on a second sealing ring surrounding the rotary shaft to form the mechanical seal. The teflon sealing ring may be made from about 1% to about 30% (by weight) graphite, and about 70% to about 98% (by weight) polytetrafluoroethylene. The seal ring may also optionally include about 1% to about 30% (by weight) non-graphite carbon.  
         [0010]     Additional features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods particularly pointed out in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a simplified sectional view of a mechanical seal assembly according to embodiments of the invention;  
         [0012]      FIG. 2  shows a simplified sectional view of another mechanical seal assembly according to embodiments of the invention; and  
         [0013]      FIG. 3  shows a flowchart of the process steps for making a sealing ring according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     The present invention includes mechanical seals that include sealing rings made from graphite loaded tetrafluoroethylene (PTFE). As noted above, the graphite provides a natural lubricant for the sealing interface, and its thermal conductivity provides for the fast dissipation of the frictional heat. However, the graphite loaded PTFE rings of the present invention do not require complex, time consuming and energy intensive production processes, and therefore may be made at a significantly reduced cost. Moreover, the higher oxidation and corrosion resistance of PTFE make graphite loaded PTFE sealing rings suitable for mechanical seals exposed to a wide variety of chemical environments.  
         [0000]     Exemplary Mechanical Seal Assembly  
         [0015]     Referring now to  FIG. 1 , a sectional view of a mechanical seal assembly  100  according to embodiments of the invention is shown. The assembly  100  may be used to form a seal around rotatable shaft  102  through the engagement of opposing sealing faces on rotatable seal ring  110  and stationary seal ring  112 . The seal rings  110  and  112  may be part of a seal flange  104  that separates a sealed fluid side from an opposite side (e.g., a non-sealed side, a shaft motor side, etc.).  
         [0016]     In some embodiments, the rotatable seal ring  110  and shaft  102  may be separated by a gap that keeps the two components from making direct physical contact. This gap may be maintained by o-ring  106  that helps keep shaft  102  aligned with seal flange  104 . O-ring  106  may physically contact both the shaft  102  and gland gasket  114  in seal flange  104 , and may help prevent fluid from leaking across the surface of shaft  102 . The o-ring  106  may rotate with shaft  102  and make rotational contact with gasket  114 . Another o-ring  107  may be placed around rotatable seal ring  110  to help maintain the ring  110  in position around shaft  102 . O-ring  107  may rotate with rotatable seal ring  110  and may make rotatable contact with gasket  114 .  
         [0017]     Engagement of the opposing sealing faces on rotatable seal ring  110  and stationary seal ring  112  may be assisted by spring  108  urging the rotatable seal ring  110  towards the stationary seal ring  112 . While rotatable seal ring  110  may be urged towards stationary seal ring  112 , the stationary seal ring  112  may be prevented from moving away from the rotatable seal ring  110  by boot  116 .  
         [0018]     Either the rotatable seal ring  110  or stationary seal ring  112  may include graphite loaded PTFE. The graphite loaded PTFE may also optionally include carbon (e.g., about 1% to about 30%, by weigh, non-graphite carbon). When one of the seal rings  110  or  112  includes graphite loaded PTFE, the other ring may include harder materials, such as alumina (Al 2 O 3 ), silicon carbide (SiC), diamond (C), steel, stellites (i.e., alloys of cobalt with varying amounts of chromium, nickel, iron, tungsten, and/or silicon) and/or tungsten carbide (WC, W 2 C), among other materials.  
         [0019]     In some embodiments, seal rings  110  and  112  engage each other with physical contact at the sealing faces on each ring. The contact may cause the graphite in the graphite loaded PTFE to be deposited in a layer at the interface of the sealing faces that helps form the mechanical seal between the sealing rings  110  and  112 . In additional embodiments, there may not be physical contact between the sealing rings  110  and  112  when they engage one another to form the mechanical seal. In these non-contacting sealing embodiments, one of the sealing rings may have groves, trench patterns, etc. etched into the sealing face that helps create the mechanical seal through fluid dynamics effects occurring in the gap between the sealing rings  110  and  112 .  
         [0020]      FIG. 2  shows a sectional view of another mechanical seal assembly  200  according to embodiments of the invention. Assembly  200  may be used to form a seal around rotatable shaft  202  through the engagement of opposing sealing faces on rotatable seal ring  210  and stationary seal ring  212 . A groove may be formed in an edge of the stationary seal ring  212  to holding o-ring  214  in place around the circumference of the seal ring  212 . O-ring  214  may sealingly engage a portion of the shaft housing (not shown) to prevent fluid, particulates, etc., from passing over the seal ring  212  and escaping out the end of shaft  202 . The material used in o-ring  214  may depend on materials and operating environment for assembly  200 . O-ring materials may include, for example, natural and synthetic rubbers.  
         [0021]     The rotatable seal ring  210  may be part of a rotatable seal assembly  206  that also includes external housing  207  and shaft elastomer  209  for rotatably coupling the ring assembly  206  to the shaft  202 . The rotatable seal assembly  206  may be urged into engagement with the stationary seal ring  212  with the aid of spring  205 . Spring  205  may be set to a steady pressure (e.g., at least 4 psi) at the interface of the sealing rings that helps form the mechanical seal. Seal flange  204  may act as a stop for an end of spring  205  that is opposite the end engaging rotatable seal assembly  206 .  
         [0022]     Similar to the embodiments described in  FIG. 1 , either the rotatable seal ring  210  or the stationary seal ring  212  may be made from graphite loaded PTFE. The seal ring may also include carbon fibers. When one seal ring is made from graphite loaded PTFE, the other may be made from harder materials, such as alumina, silicon carbide, diamond, steel, stellites, tungsten carbide, etc. The mechanical seal between seal rings  210  and  212  may be formed through a contacting or non-contacting seal interface.  
         [0023]     The mechanical seals shown in  FIGS. 1 and 2  may be used in a variety of applications, including re-circulation/boiler feed pumps, sump pumps, sewage pumps, wastewater pumps, irrigation pumps, food and beverage pumps, drainage pumps, chemical pumps, well pumps, compressors, pool pumps, spa pumps, jetted tub pumps, grinder pumps, and hydraulic pumps, among other applications. As noted above, the mechanical seals may also be used in a number of appliances, including dishwashers, washing machines, etc.  
         [0000]     Exemplary Method of Forming PTFE Seal Ring  
         [0024]      FIG. 3  shows a flowchart that includes steps for forming a graphite loaded PTFE seal ring according to embodiments of the invention. The process includes mixing together precursor materials  302  that may include powders of synthetic graphite (e.g., about 1% to about 30%, by weigh) with a PTFE resin (e.g., about 70% to about 98%, by weight). In some embodiments, non-graphite carbon (e.g., about 1% to about 30%, by weight) may also be added to the mixture. The non-graphite carbon may include any form of carbon other than graphite, such as coal particulates, soots, and carbon black, as well as covalent network solid forms of carbon such as diamond and diamond-like carbon. The non-graphite carbon may include substantially rounded carbon particulates and/or carbon fibers.  
         [0025]     The base resin of PTFE may be mixed with a predetermined weigh percentage of graphite and carbon to produce a mixture with about 5% to about 30% (by wt.) graphite in PTFE. Examples of mixtures include about 5% (by wt.) graphite in PTFE, about 10% (by wt.) graphite in PTFE, about 15% (by wt.) graphite in PTFE, and about 27% (by wt.) graphite with about 3% (by wt.) non-graphite carbon in PTFE, among other mixtures.  
         [0026]     The mixture may be formed into a seal ring  304  by pouring the mixture into a mold having the shape of the seal ring. For example, the mixture may be poured into a compression mold where the sealing ring is formed under a predetermined amount of pressure. The uncured sealing ring may then be placed into a furnace a cured  306  using a multi-stage heating process to form the graphite loaded PTFE sealing ring. For example, heating process may start out at room temperature and then be ramped up to about 550° F. over the course of 4 hours. The ring is then held at 550° F. for about 1 hour before the temperature is increased further to about 700° F., over the course of a few hours (e.g., 3 hours) and held at the higher temperature for a few more hours (e.g., 3 hours). Then the ring may be cooled from 700° F. to 500° F. over a period of 4.5 hours, and held at 500° F. for another hour. Finally, the ring may be cooled from 500° F. to about 80° F. over the course of about 4 hours, before being removed from the furnace.  
       EXAMPLES  
       [0027]     The wear characteristics of mechanical seals using graphite loaded PTFE seal rings according to embodiments of the invention were tested for 50 hours and 250 hours of continuous operation. The mechanical seals were formed by a sealing face of a rotary sealing ring made from 5%, by wt., graphite in polytetrafluoroethylene that engaged an opposite sealing face on a stationary seal ring made from 99.5% alumina (AD995).  
         [0028]     The rotary sealing ring was placed on a 0.5 inch diameter shaft that was set to rotate at 1750 rpm while engaging the stationary sealing ring. The sealing faces on the seal rings were pressed together at contact pressures ranging between 4 to 5.5 psi to ensure that the sealing faces maintain a mechanical seal throughout the test. All tests were performed in an ambient environment at a temperature of about 19° C. After a preset number of hours of continuous operation (i.e., 50 hours or 250 hours) the rotating shaft was stopped and the length of rotary seal ring was measured to determine the extent that material had been worn off the ring. Table 1 lists the wear measurement results for three 5% graphite PTFE rotary seal rings after 50 hours of continuous operation, and a fourth 5% graphite PTFE rotary seal ring after 250 hours of operation.  
                                     TABLE 1                           Mechanical Seal Wear Test Results            Test Conditions   Test 1   Test 2   Test 3   Test 4               Test Duration   50 hours   50 hours   50 hours   250 hours       Shaft Speed   1750 rpm   1750 rpm   1750 rpm   1750 rpm       Contact Pressure   5.238 psi   5.267 psi   5.121 psi   4.191 psi       Environment Temp   19° C.   19° C.   19° C.   19° C.       Rotary Seal Ring   5% Gr PTFE   5% Gr PTFE   5% Gr PTFE   5% Gr PTFE       Rotary Seal Ring Thickness   0.2510 inches   0.2490 inches   0.2495 inches   0.2490 inches       Stationary Seal Ring   AD995   AD995   AD995   AD995       Test Results       Rotary Seal Ring Wear   0.0020 inches   0.0005 inches   0.0015 inches   0.0015 inches       Stationary Seal Ring Wear   0.0000 inches   0.0000 inches   0.0000 inches   0.0000 inches       Temperature Generation   3° C.   3° C.   6° C.   3° C.       Coefficient of Friction   0.04   0.04   0.04   0.03                  
 
         [0029]     The test results in Table 1 show that the 5% graphite PTFE rotary seal ring used in the mechanical seals consistently showed little wear after 50 hours of continuous operation, only losing between 0.5 and 2 mils (i.e., thousands of an inch) of material from a quarter inch thick seal ring. The longevity of the low wear properties was confirmed in the 250 hour test (i.e., Test #4) which measured a loss of only 1.5 mils of material from the quarter inch, 5% graphite PTFE, rotary seal ring.  
         [0030]     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.  
         [0031]     Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, or groups.