Patent Publication Number: US-9841141-B2

Title: Lubrication method for housing having an exclusion seal

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure is directed to a lubrication method for a shaft sealed by an exclusion seal, and, more specifically, toward a method of controlling the volume and/or the pressure of a lubricant in a housing to maintain a lubricating film between the shaft and the exclusion seal. 
     BACKGROUND 
     Automatic lubrication systems are known that include mechanical dosing and pumping components. These components may be configured to provide a fixed dose of lubricant (grease or oil or other suitable lubricant, referred to hereinafter generally as “grease”) at predetermined intervals. The intervals and doses are conventionally set by manually adjusting each of the individual dosing devices. A lubrication system that provides a greater degree of control and more automated control over lubricant dosing is disclosed in U.S. Patent Application Publication No. 2013/0015019 A1 for an “Advanced Lubrication System,” which disclosure is hereby incorporated by reference. 
     An example of a mechanical system that requires lubrication is a bearing assembly encased in a bearing housing. These systems include a housing containing the bearing, and the housing has a free volume, that is, a volume potentially finable with lubricant when lubricant is pumped into the housing through a lubrication inlet. A shaft extends through an opening in the housing, and a seal is provide between the shaft and the housing, either on the shaft or on the housing. Seals may either be fixed to a housing and have a radially inwardly extending lip configured to ride on or engage a rotatable shaft or may be fixed to a shaft and have a radially outwardly extending lip configured to ride on or engage the opening in the housing. The following discussion will generally discuss seals mounted on a housing but applies equally to shaft-mounted seals. 
     Bearing housings also generally include one or more pressure relief valves (purge valves), which may comprise, for example, spring-biased check valves. When the pressure in the housing reaches a certain level, 5 psi, for example, the relief valve opens and allows lubricant to escape through the relief valve until the pressure drops below the certain level. However, bearing pressure relief valves are notoriously unreliable and prone to frequent failure. When these valves fail, lubricant may leak uncontrollably from the housing and produce an under-lubrication condition for the bearing which could rapidly lead to bearing damage or failure if not quickly addressed. 
     Conventional lubrication systems are generally configured to provide lubricant based on the needs of the bearing or component being lubricated. When a determination is made that the bearing requires additional lubricant, a pump may be actuated to pump additional lubricant into the bearing housing. The housing is generally completely full of lubricant and pressurized to some extent 2-5 psi, for example. The pressure in the housing while the bearing is operating will generally be lower than the pressure at which the pressure relief valve opens. Adding a sufficient lubricant to the bearing housing can increase the pressure to a level greater than 5 psi and cause lubricant to be expelled through the pressure relief valve. Adding lubricant intermittently or periodically to the bearing housing without raising the pressure above 5 psi helps ensure that adequate lubrication is provided for the bearing at all times. 
     Bearing housings are used in many different environments. In the mining field and various industrial fields, bearing housings are exposed to dust, dirt and other contaminants which will damage a bearing if they are able to enter the bearing housing. Exclusion seals are particularly useful for providing a seal between a shaft and a bearing in such environments. Exclusion seals are configured to keep dirt and other contaminants out of a bearing housing. When exclusion seals are new, they accomplish this exclusion of dirt by maintaining a tight seal between the shaft and the housing. The interface between the shaft and seal is so tight that dirt and other contaminants impinging on the shaft/seal interface are unable to fit between the shaft and seal. When exclusion seals wear, they provide a less secure seal and are less able to physically block contaminants from entering the bearing housing. However, beneficially, worn exclusion seals allow a small amount of grease to leak from the bearing housing, and this outward leakage of grease helps push contaminants that accumulate or impinge against the shaft/seal interface away from the interior of the bearing housing. 
     To maximize an exclusion seal&#39;s ability to exclude contaminants, it would generally be desirable to maintain as tight a seal as possible between the seal and the shaft. However, the friction produced at the shaft/seal interface and thus the frictional drag on the shaft is related to the pressure with which the seal engages the shaft. High pressure also causes the seal to wear faster that a seal that engages the shaft with a light pressure. Therefore, a balance must be struck between keeping the seal as tight as possible to keep contaminants out of the bearing housing and using a loose seal to minimize the energy usage required to drive a shaft in physical engagement with the seal. 
     It would be desirable to provide a bearing system that avoids the problems of conventional, unreliable, pressure relief valves and that also ensures a high degree of contaminant exclusion by an exclusion seal, even as the seal wears. 
     SUMMARY 
     These and other problems are addressed by the present disclosure, a first aspect of which comprises a method that includes providing a bearing housing configured to house a bearing, the bearing housing enclosing a free volume for receiving a lubricant and including a shaft opening and providing a shaft in the shaft opening. The method also includes filling the free volume with the lubricant and providing an exclusion seal between the shaft opening and the shaft. The exclusion seal is repeatably and non-destructively shiftable from a sealing state in which the exclusion seal substantially prevents the lubricant from leaving the free volume and a release state in which the exclusion seal allows the lubricant to escape between the exclusion seal and the shaft. The exclusion seal is shiftable from the sealing state to the release state by increasing a pressure in the bearing housing to above a first level and is shiftable from the release state to the sealing state by decreasing the pressure to below the first level. The method also includes continuously maintaining a film of the lubricant between the exclusion seal and the shaft by repeatedly causing the pressure of the lubricant in the free volume to rise above and fall below the first level. 
     Another aspect of the disclosure comprises a method that includes providing a bearing housing configured to house a bearing, the bearing housing enclosing a free volume for receiving a lubricant and including a shaft opening, and the bearing housing not including a purge valve. The method includes providing a shaft in the shaft opening, filling the free volume with the lubricant and providing an exclusion seal between the shaft opening and the shaft. The exclusion seal is repeatably and non-destructively shiftable from a sealing state in which the exclusion seal substantially prevents the lubricant from leaving the free volume and a release state in which the exclusion seal allows the lubricant to escape between the exclusion seal and the shaft. The exclusion seal is shiftable from the sealing state to the release state by increasing a pressure in the bearing housing to a level greater than or equal to 10 psi and is shiftable from the release state to the sealing state by decreasing the pressure to below the level. The method also includes providing a pump for pumping the lubricant into the bearing housing, the pump being capable of causing the pressure in the bearing housing to exceed the level and continuously maintaining a film of the lubricant between the exclusion seal and the shaft by repeatedly operating the pump to cause the pressure of the lubricant to alternately rise above and fall below the level. 
     A further aspect of the disclosure comprises a method that includes providing a bearing housing configured to house a bearing, the bearing housing enclosing a free volume for receiving a lubricant and including a shaft opening, providing a pump for pumping the lubricant into the bearing housing, providing a shaft in the shaft opening and providing an exclusion seal between the shaft opening and the shaft. The exclusion seal is configured to allow the lubricant to escape past the exclusion seal when the pressure is above a first level. The method also includes providing a controller operably connected to the pump for controlling the pump, providing a pressure sensor configured to measure a pressure in the bearing housing and to send an output signal to the controller indicative of the measured pressure, adding the lubricant to the bearing housing while measuring the pressure, determining a maximum pressure obtainable by adding the lubricant to the bearing housing, and if the maximum pressure is not greater than a predetermined pressure providing a signal indicative of a worn seal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects and features of the present disclosure will be better understood after a reading of the following detailed description together with the attached drawings, wherein: 
         FIG. 1  is a schematic sectional side elevational view of a bearing housing including a bearing that can be lubricated with an embodiment of the present disclosure. 
         FIG. 2  is a schematic diagram of a first embodiment of a lubrication system according to the present disclosure including the bearing housing of  FIG. 1 . 
         FIG. 3  is a schematic diagram of a second embodiment of a lubrication system according to the present disclosure including multiple bearing housings of  FIG. 1 . 
         FIG. 4  is a schematic diagram of a third embodiment of a lubrication system according to the present disclosure including multiple bearing housings of  FIG. 1 . 
         FIG. 5  is a sectional side elevational view of a first seal suitable for use in the present disclosure. 
         FIG. 6  is a sectional side elevational view of a second seal suitable for use in the present disclosure. 
         FIG. 7  is a sectional side elevational view of a third seal suitable for use in the present disclosure. 
         FIG. 8  is a flow chart illustrating a first method according to the present disclosure. 
         FIG. 9  is a flow chart illustrating a second method according to the present disclosure. 
         FIG. 10  is a flow chart illustrating a third method according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein the showings are for the purpose of illustrating presently preferred embodiments of the disclosure only and not for the purpose of limiting same,  FIG. 1  illustrates a housing  10  containing a bearing assembly  12  that includes an inner ring  14 , an outer ring  16  and a plurality of rolling elements  18  between the inner ring  14  and the outer ring  16 . The bearing assembly  12  supports a shaft  20  for rotation relative to the housing  10 , and an inner seal  22  and an outer seal  24  seal a free volume  26  inside the housing, which free volume  26  is completely or substantially filled with a lubricant, such as grease. The free volume  26  includes the spaces inside the bearing assembly  12  where the grease makes contact with the rolling elements  18 , the region immediately adjacent to the shaft  20  where the grease is churned when the shaft  20  rotates, and the regions axially away from the bearing assembly  12  and radially away from the shaft  20  where the grease does not actively perform a lubricating function but where it is available to move into contact with the bearing assembly  12  or the shaft  20  as the shaft  20  and roller elements  18  move in the housing  10  and mix the lubricant. 
     The inner seal  22  and the outer seal  24  are preferably exclusion seals, that is, seals specifically configured to keep debris and contaminants out of the free volume  26  and away from the bearing assembly  12 . Such seals are configured to allow grease to escape from inside a bearing housing when the pressure in the bearing housing exceeds a predetermined level. Such a pressure increase may be caused, for example, by adding additional grease through the lubrication inlet  28  (illustrated in  FIGS. 2-4 ). However, conventional bearing housings include pressure relief valves (purge valves) that allow grease to escape from the free volume  26  before the pressure reaches the predetermined level. In conventional housings sealed with exclusion seals, therefore, the exclusion seals generally do not allow grease to escape unless they are very worn and near the end of their service life, in which case they may allow some leakage of grease. In the present disclosure, on the other hand, the bearing housing  10  does not include a pressure relief valve, and the exclusion seals provide the only exit path for grease to leave the bearing housing  10 . The amount of grease and the rate at which grease can be forced out of the bearing housing  10  is controllable by controlling the pressure in the bearing housing  10 . 
       FIG. 2  schematically illustrates the bearing housing  10  of  FIG. 1  in a lubrication system according to an embodiment of the present disclosure. The lubrication system includes a controller  30  having at least one signal input  32  and at least one signal output  34 , a lubrication pump  36  and the bearing housing  10  described above in connection with  FIG. 1 . The lubrication pump  36  includes a signal input  38  connected to the at least one signal output  34  of the controller  30  by a lubrication signal output line  40  and a grease outlet  42  connected to the lubrication inlet  28  on the bearing housing  10  by a lubrication line  44 . 
     A sensor  46  is mounted on the bearing housing  10  and configured to sense one or more conditions associated with the bearing housing  10  and/or the grease in the free volume  26  of the bearing housing  10 , which conditions may include a temperature of the grease or a pressure in the bearing housing  10 . To this end, the sensor  46  includes one or more probes (not illustrated)) that extend into the grease in the free volume  26  in the bearing housing  10 . The sensor  46  produces, periodically or intermittently, one or more output signals on the sensor output line  48  which sensor output line  48  is connected to the signal input  32  of the controller  30 , and the one or more output signals are indicative of the temperature of the grease, the pressure in the bearing housing  10  or both. The signals may be sent at any desired interval, every  15  seconds, for example, and may comprise digital or analog signals or any combination of signals from which the controller  30  can extract information regarding the conditions sensed by the sensor  46 . 
     A wired configuration of the controller  30 , lubrication pump  36  and sensor  46  is illustrated  FIG. 2 . However, one or more of the controller  30 , the lubrication pump  36  and the sensor  46  may communicate wirelessly with one another without exceeding the scope of this disclosure. To this end, the controller  30  is shown as having an antenna  31 , and the lubrication pump  36  and the sensor  46  can be provided with suitable transmitters and/or receivers and configured to communicate wirelessly with the controller  30  via any of various known wireless communication protocols. It should be understood that any wired connection discussed herein could be replaced with a wireless connection within the scope of the present disclosure. 
     The controller  30  may comprise a programmable general purpose computer running software configured to receive signal inputs from the sensor  46  and to produce output signals for controlling the lubrication pump  36 . The output signals may comprise a single instruction to cause the lubrication pump  36  to cycle and output a fixed volume or dose of grease, or, alternately, the signal produced by the controller  30  may include information for setting the volume of grease to be dispensed in a given dose by the lubrication pump  36  if the lubrication pump  36  is configured to receive and act on such information. As discussed in greater detail below, the controller  30  includes a first memory  60  for storing information about the operating state and lubrication requirements of the bearing assembly  12  and a second memory  62  (which may comprise a location in the first memory  60 ) for storing rules or algorithms for producing suitable output signals based on the input signals received from the sensor  46 . 
       FIGS. 3 and 4  illustrate two further embodiments of lubrication systems in which the same reference numerals are used to identify elements common to the first embodiment. These embodiments are generally similar to the embodiment of  FIG. 2  except that the controller  30  controls the lubrication of multiple bearings housed in multiple bearing housings  10 ′.  FIG. 3  includes an SLV injector  50  which divides and distributes a quantity of grease provided by the lubrication pump  36  to multiple bearing housings  10 ′—four bearing housings  10 ′ in this example. In order to work effectively with the SLV injector  50 , each of the lubrication inlets  28  of the multiple bearing housings  10 ′ includes a valve (not illustrated). The controller  30  may control the lubrication pump  36  as discussed above, and, alternately or in addition, may communicate with the SLV injector  50  via injector communication line  52 . When an SLV injector  50  is used that can be adjusted to provide different lubrication amounts to different ones of the bearing housings  10 ′, this control information can be provided by the controller  30 . 
       FIG. 4  illustrates a lubrication system that includes a multi-point lubricator  54 . The multipoint lubricator  54  provides greater control over the amount of grease provided to various ones of the multiple bearing housings  10 ′. The bearing housings  10 ′ are identical to the bearing housings  10 ′ of  FIG. 3 , and each lubrication inlet  28  in this embodiment also includes a valve (not illustrated). The controller  30  may communicate solely with the lubrication pump  36  as in the first embodiment or may, alternately or in addition, communicate with the multipoint lubricator  54  via multipoint communication line  56  to adjust the dose volume and/or lubrication interval for each of the bearing housings  10 ′. 
     Conventional lubrication systems generally dispense grease based on the requirements of the mechanical device being lubricated, for example, a bearing assembly like bearing assembly  12 . Conventional bearings may include sensors for measuring vibrations indicative of an under-lubricated bearing condition, and lubricant may be added to the conventional bearing housing when this under-lubrication condition is sensed. Other systems may dispense lubricant on a periodic basis, which period is calculated to ensure that the bearing does not become under-lubricated. However, in conventional bearing housings, when too much lubricant is used, it will be expelled through a pressure relief valve. It is therefore generally desirable to use no more lubricant than the amount required to keep the bearing lubricated because additional lubricant pumped into the housing will be expelled through a pressure relief valve and wasted. 
     The present disclosure is directed to a method and apparatus for controlling the pressure in the bearing housing  10  to ensure that enough lubricant is expelled from between the inner seal  22  and the shaft  20  and from between the outer seal  24  and the shaft  20  to maintain a film of lubricant between the shaft  20  and the inner and outer seals  22 ,  24 . This may be accomplished by controlling the volume of lubricant added and/or by controlling the rate at which lubricant is added. That is, a sufficient volume of lubricant must be added to the bearing housing  10  to cause some of the lubricant to escape between the shaft  20  and the inner and outer seals  22 ,  24 , and the lubricant must be added sufficiently often that the shaft/seal interface does not dry out between lubricant releases. Because the bearing housing  10  does not have a purge or pressure relief valve, the amount of grease that escapes between the inner and outer seals  22 ,  24  and the shaft  20  is affected by the pressure in the bearing housing  10 . 
     Beneficially, the inner seal  22  and the outer seal  24  may be balanced, that is, constructed such that substantially the same amount of grease leaks from between each of the seals  22 ,  24  and the shaft  20  at a given pressure. This helps ensure that a film of lubricant is continuously maintained between both seals and the shaft  20  and that one of the inner and outer seals  22 ,  24  does not dry out before additional lubricant is released. Alternately, the seals can be configured to allow the release of lubricant preferentially to one side, e.g., the side that is most vulnerable to the ingress of contamination. Or, if one seal comprises a lip seal and the other seal comprises a labyrinth seal as discussed in greater detail below, the labyrinth seal (which is a non-contacting seal) can be configured so that substantially all grease purges from between the lip seal and the shaft. This may be done because the lip seal is the seal that primarily benefits from the movement of grease past the seal. Grease in the labyrinth seal may help prevent the ingress of contaminants through the labyrinth seal, but such a seal does not require grease to reduce friction because the two portions of the labyrinth seal are not in contact. 
     The pressure in the bearing housing may be controlled by the lubrication pump  36  and by the force with which the inner and outer seals  22 ,  24  engage the shaft  20 . It will be assumed that the lubrication pump  36  can be configured to provide grease at an arbitrarily high pressure, high enough to expel grease from any exclusion seals under consideration herein. Alternately, a plunger or other structure (not illustrated), could be controlled to periodically enter the free volume  26  to increase the pressure in the bearing housing  10  by decreasing the free volume  26 . Changing the pressure and/or volume of grease in the free volume  26  is one factor controlling how much grease escapes from the shaft/seal interface Another factor that affects how much grease escapes at a given pressure is the design of the inner and outer seals  22 ,  24 . 
     Multiple factors affect how tightly a seal engages a shaft. These factors include the material from which the seal is constructed and its flexibility, and well and the length, thickness, number and configuration of various lips or seal portions that make contact with the shaft and how close the seal is mounted to the shaft. Persons of ordinary skill in the art understand how to modify these variables to produce seals that engage shafts with a desired force. For purposes of this disclosure, the force with which a seal engages a shaft will be described in terms of the pressure of grease that can be maintained in the bearing housing by the seal rather than a direct indication of the force exerted by a seal against a shaft. Thus, instead of stating that a seal engages a shaft with “x pounds of force per linear inch,” for example, the seal would be described by the pressure at which lubricant will begin to leak from between the shaft and the seal. In conventional newly installed exclusion seals, this pressure must be lower than the pressure at which a pressure relief valve opens. In the present disclosure, for systems without pressure relief valves, the pressure will be set to a higher level than in conventional systems—to 10 psi or 20 psi, for example or even higher. 
     Another factor that must be considered when selecting an exclusion seal for an application is the amount of friction created by the exclusion seal sliding on a rotating shaft. Making the seal press hard on the shaft increases its ability to exclude foreign contaminants, but at the same time, disadvantageously, increases friction and thus the amount of energy required to rotate the shaft. However, by ensuring that a lubricating film of grease is always present between the seal and the shaft, the present disclosure allows a seal to maintain a greater contact pressure against the shaft without unduly increasing friction. 
     For example, an exclusion seal set to withstand 20 psi of pressure in a conventional bearing housing could make a shaft difficult to turn. It would also substantially prevent any grease from leaking from a conventional bearing housing because the conventional pressure relief valves would open at approximately 5 psi. However, in the present disclosure, grease is provided to the bearing housing  10  at a sufficient rate to ensure that a quantity of the grease sufficient to lubricate the shaft/seal interface leaks from between the shaft  20  and the inner and outer seals  22 ,  24 . Such seals therefore provide improved resistance against the ingress of foreign contaminants, and the repeated expulsion of grease from between the seals  22 ,  24  and the shaft  20  forces contaminants that accumulate near the shaft/seal interface away from the seal. Beneficially, a shaft on which such seals are mounted may require generally the same amount of energy to turn as a shaft using a conventional unlubricated exclusion seal while at the same time providing a greater ability to exclude foreign material from the bearing housing  10 . 
     In addition, the seal of the present disclosure will retain its sealing ability down to a pressure of approximately 2 psi, the same as conventional seals. Unlike conventional seals, the seals of the present disclosure will last longer than conventional seals, first, because they have further to wear—from being able to retain 20 psi down to being able to retain 2 psi, for example, unlike conventional seals that may when new be able to retain about 4 psi to and also require replacement when they can only retain about 2 psi, for example, and also because the presence of the lubricating film between the shaft  20  and the inner and outer seals  22 ,  24  reduces friction thus a rate of wear. 
     Various exclusion seal configurations can be used with the presently disclosed system. A first seal  70  is illustrated in  FIG. 5  inserted between a portion of a bearing housing  10  and a shaft  20 . The free volume  26  is located on the left side of the seal  70  in  FIG. 5 . The seal  70  comprises a circumferential metal case  72  having a radially outwardly facing channel  74  in which is mounted a seal body  76  and a seal lip  78  extending at an angle to the radial direction and terminating at a seal contact surface  80 . The seal lip  78  is connected to the seal body  76  at a seal hinge  82 , and the material from which the hinge  82  and the seal lip  78  are formed may be resilient so that the seal hinge  82  presses the contact surface  80  against the shaft  20  with a certain force when the contact surface  80  is pressed radially against the shaft  20  by the bearing housing  10 . To adjust, in this case, increase the contact force between the contact surface  80  and the shaft  20 , a metal spring  84  is provided. The metal spring  84  has a first end  86  mounted between the seal body  76  and the metal case  72 , a flexed center portion  88  generally following the curve of the seal hinge  82 , and a second end  90  overlying the seal lip  78  and pressing the seal lip  78 , and in particular the contact surface  80  at the end of the seal lip  78 , against the shaft  20 . 
     The thickness and composition of the seal hinge  82  and the seal lip  78  and the spring force provided by the metal spring  82  affect how tightly the contact surface  80  presses against the shaft  20  and thus the pressure in the free volume  26  that can be maintained by the seal  70 . As will be appreciated from this drawing figure, when the pressure in the free volume  26  is greater than the pressure that can be retained by the seal  70 , the contact surface  80  separates slightly from the shaft  20 , and grease from the free volume  26  escapes from the free volume  26  between the contact surface  80  and the shaft  20 . The seal  70  thus has two operating configurations: a sealed configuration that retains grease in the bearing housing  10  and a release configuration that allows grease to escape. The seal  70  can shift repeatedly and non-destructively between these configurations. Allowing a sufficient volume of grease to escape and to escape at a sufficient frequency ensures that a film of lubricant is continuously present between the contact surface  80  of the seal  70  and the shaft  20  to keep friction at the interface of the contact surface  80  and the shaft  20  at an acceptably low level. 
     A second seal  90  suitable for use with the presently disclosed system is illustrated in  FIG. 6 . The seal  90  includes a seal body  92  from which project a first or inner seal lip  94  that is connected to the seal body  92  by a first seal hinge  96  and which terminates in a first contact surface  98  on the shaft  20  and a second or outer seal lip  100  that is connected to the seal body  92  by a second seal hinge  102  and which terminates in a second contact surface  104  on the shaft  20 . The outer seal lip  100  is separated from the inner seal lip  94  by an interlip space  106 . A stiffening band  108  is mounted on a radially inner side of the seal body  92  between the inner seal lip  94  and the outer seal lip  100  to increase the stiffness of the seal  90 . 
     The seal  90  is pressed against the shaft  20  by the bearing housing  10 , and the pressure maintainable in the free volume  26  of the bearing housing is determined by the material and configuration of the first and second seal lips  94 ,  100 . This double lip construction provides benefits in the present system when used in highly contaminated environments because the first and second lips  94 ,  100  provide additional protection against the ingress of contaminants. Grease exiting the free volume  26  must pass between the first contact surface  98  and the shaft  20 , enter and/or circulate in the interlip space  106  and then pass between the second contact surface  104  and the shaft  20  before exiting the bearing housing  10 . Even if contaminants manage to penetrate the outer seal lip  100  of the seal  90 , they may thereafter become trapped in the interlip space  106  and eventually be expelled by additional grease passing through the interlip space  106  on its way out of the bearing housing  10 . Contaminants thus must travel past two seal lips and find their way out of the interlip space  106  while moving upstream against an exiting flow of grease in order to reach the interior of the bearing housing  10  and potentially cause damage. 
     While such double lip seals provide good protection, they may be unsuitable for some conventional applications. In a conventional system, one without a constant film of lubricant between the shaft  20  and the seal  90 , the friction produced by two lips might be too high and cause excessive drag on the shaft  20  and cause the seal to wear so quickly that frequent replacement would be required. However, by continuously maintaining a lubricating film between the first contact surface  98  and the shaft  20  and between the second contact surface  104  and the shaft  20 , the exclusion benefits provided by this double lip construction can be obtained without creating excessive drag. 
     A third seal  110  useable with some embodiments of the present disclosure is illustrated in  FIG. 7 . The seal  110  comprises a labyrinth seal with a shaft-mounted element  112  having an outer surface  114  and a housing-mounted element  116  having an outer surface  118 . The outer surface  114  of the shaft-mounted element  112  is complementary to the outer surface  118  of the housing-mounted element and faces the outer surface  118  of the housing-mounted element. The outer surfaces  114 ,  118  are not linear and include at least one change of direction along a direction of the shaft axis such that grease in the free volume  26  must follow a tortuous path  120  from the free volume  26  to the outside of the housing. The radial separation of the shaft-mounted element  112  and the housing-mounted element  116 , together with the number of bends and direction changes along the tortuous path  120  affect the pressure in the free volume  26  required to drive grease through the tortuous path and out of the bearing housing  10 . The seal  110  can thus be configured to prevent the release of grease at pressures below a first level and to allow the release of grease through the tortuous path  120  at pressures above the first level. When two labyrinth seals are used together, on inner and outer sides of a housing, for example, the seals can be configured to allow the release of grease preferentially through one of the two seals or configured so that substantially equal amounts of grease are released through both seals. 
     To lubricate a contact seal, such as the first seal  70  and the second seal  90  discussed above, the controller  30  may be configured to cause the lubrication pump  36  to add a given quantity of grease to the bearing housing  10  each hour. The amount of grease added may be based on various factors. For example, to ensure that the lubricating film is maintained, the controller  30  may be configured to cause the lubrication pump  36  to dispense a given volume of grease in smaller doses than normal. If a given bearing requires 5 cc of grease per hour to be adequately lubricated, the controller  30  may dispense at least one cc of grease every  12  minutes to ensure that the first and second seals  22 ,  24  are lubricated. It may be preferable to dispense a greater volume of grease per hour than is conventionally required when only the lubrication requirements of the bearing are considered. Therefore, in the foregoing example, the controller  30  may be configured to cause the lubrication pump to dispense 1.5 or 2.0 cc of grease every 12 minutes. In general, the interval between dispensing doses of grease will be shortened and enough grease will be added to the free volume  26  to ensure that some portion of the grease escapes from the bearing housing  10  with each dose. 
     In some applications, it may be possible to use the signal provided by the sensor  46  to control the dispensing of grease. That is, the pressure in the bearing housing  10  may be measured to obtain an average pressure and then the controller  30  may be actuated to dispense a sufficient quantity of grease into the bearing housing  10  to raise the pressure by a predetermined amount, 2 psi, for example. The average pressure will represent the pressure that the inner and outer seals  22 ,  24  are capable of retaining a give state of wear, and increasing the pressure ensures that this pressure is exceeded so that grease will leak from between the shaft  20  and the inner and outer seals  22 ,  24 , and lubricate the shaft/seal interface. However, in other applications, the movement of the bearing assembly  12  or other system being lubricated may cause pressure fluctuations that make it difficult or impossible to accurately measure pressure or to add grease based on instantaneously measured pressure levels in the bearing housing. In those cases, grease can be added a time basis, which time basis can be determined empirically, if necessary, by determining how long it takes for a given grease at a given shaft/seal interface at a give temperature to wear away. The controller  30  can thus be configured to cause grease to be dispensed sufficiently frequently to maintain the lubricating film. 
     The signal provided by the pressure sensor also allows the controller  30  to perform a seal test and determine when the inner and outer seals  22 ,  24  have worn to the point that they require replacement. A seal test is performed when the bearing assembly  12  in the bearing housing  10  is not operating. To perform a seal test, the controller  30  causes the lubrication pump  36  to pump lubricant into the bearing housing  10  as rapidly as practicable in order to raise the pressure in the bearing housing  10  while monitoring the pressure level in the bearing housing  10 . If the pressure cannot be raised to above a predetermined pressure, 2 psi for example, the controller  30  determines that the inner and outer seals  22 ,  24  are no longer providing adequate protection against the ingress of foreign matter, and the controller  30  generates an output signal that indicates that the inner and outer seals  22 ,  24  need to be replaced. 
     The seal test can produce a binary output—that is, the seal either passes or fails the seal test. Optionally, the controller  30  may provide an indication of the state of wear of the inner and outer seals  22 ,  24  by producing different output signals based on the maximum pressure that is obtainable during the pressure test. For example, assume that a particular seal or set of seals can maintain a pressure in the bearing housing of 20 psi when new and that such seals require replacement when they can no longer maintain a 2 psi pressure in a bearing housing. The controller  30  may be configured to output signal indicating that the seal is “nearly new” when the seal is capable of maintaining a pressure of greater than 10 psi, a signal indicating that the seal is “somewhat worn” when the maximum pressure obtainable is between 5 psi and 10 psi, and a signal indicating that the seal will soon require replacement when the maximum pressure obtainable is between 2 psi and 5 psi. Alternately or in addition, the controller  30  can provide an indication of the actual maximum pressure that was obtained during the test. In this manner, during a monthly maintenance of the system, for example, maintenance personnel can determine whether seal replacement is required and know when replacement imminent. 
     The controller  30  may also beneficially use the temperature signal output from the sensor  46  to confirm that lubricant has been added to the bearing housing  10 . Each dose of lubricant added to the bearing housing  10  will increase the pressure in the bearing housing  10  before grease escapes. Because temperature is proportional to pressure in a fixed volume system, the periodic pressure increases caused by the addition of lubricant can be detected by detecting transient temperature increases in the grease. This method may only be effective when the added dose of lubricant is sufficiently large to change the temperature by an unambiguously measurable amount. However, it is believed that the addition of, for example, 5 cc doses of lubricant may be sufficient to briefly raise the temperature of the lubricant in a manner that allows a lubrication event to be confirmed. The controller  30  may be configured to monitor these temperature increases and ensure that a temperature increase follows each addition of lubricant. If a temperature increase is not detected, the controller  30  may provide an indication of a leak or blockage in the lubrication line or another problem with the system that requires attention. 
     A method according to the present disclosure is illustrated in  FIG. 8 . This method includes a step  130  of providing a bearing housing configured to house a bearing, the bearing housing enclosing a free volume for receiving a lubricant and including a shaft opening, a step  132  of providing a shaft in the shaft opening, a step  134  of filling the free volume with the lubricant, a step  136  of providing an exclusion seal between the shaft opening and the shaft, the exclusion seal being repeatably and non-destructively shiftable from a sealing state in which the exclusion seal substantially prevents the lubricant from leaving the free volume and a release state in which the exclusion seal allows the lubricant to escape between the exclusion seal and the shaft, the exclusion seal being shiftable from the sealing state to the release state by increasing a pressure in the bearing housing to above a first level and being shiftable from the release state to the sealing state by decreasing the pressure to below the first level, and a step  138  of continuously maintaining a film of the lubricant between the exclusion seal and the shaft by repeatedly causing the pressure of the lubricant in the free volume to rise above and fall below the first level. 
     Another method according to the present disclosure is illustrated in  FIG. 9  and includes a step  140  of providing a bearing housing configured to house a bearing, the bearing housing enclosing a free volume for receiving a lubricant and including a shaft opening, the bearing housing not including a purge valve, a step  142  of providing a shaft in the shaft opening, a step  144  of filling the free volume with the lubricant, a step  146  of providing an exclusion seal between the shaft opening and the shaft, the exclusion seal being repeatably and non-destructively shiftable from a sealing state in which the exclusion seal substantially prevents the lubricant from leaving the free volume and a release state in which the exclusion seal allows the lubricant to escape between the exclusion seal and the shaft, the exclusion seal being shiftable from the sealing state to the release state by increasing a pressure in the bearing housing to a level greater than or equal to 10 psi and being shiftable from the release state to the sealing state by decreasing the pressure to below the level, a step  148  of providing a pump for pumping the lubricant into the bearing housing, the pump being capable of causing the pressure of the lubricant in the housing to exceed the level, and a step  150  of continuously maintaining a film of the lubricant between the exclusion seal and the shaft by repeatedly operating the pump to cause the pressure of the lubricant to alternately rise above and fall below the level. 
     Another method according to the present disclosure is illustrated in  FIG. 10  and comprises a step  152  of providing a bearing housing configured to house a bearing, the bearing housing enclosing a free volume for receiving a lubricant and including a shaft opening, the bearing housing not including a purge valve, a step  154  of providing a shaft in the shaft opening, a step  156  of filling the free volume with the lubricant, a step  158  of providing an exclusion seal between the shaft opening and the shaft, the exclusion seal being repeatably and non-destructively shiftable from a sealing state in which the exclusion seal substantially prevents the lubricant from leaving the free volume and a release state in which the exclusion seal allows the lubricant to escape between the exclusion seal and the shaft, the exclusion seal being shiftable from the sealing state to the release state by increasing a pressure in the bearing housing to a level greater than or equal to 10 psi and being shiftable from the release state to the sealing state by decreasing the pressure to below the level, a step  160  of providing a pump for pumping the lubricant into the bearing housing, the pump being capable of causing the pressure of the lubricant in the housing to exceed the level, and a step  162  of continuously maintaining a film of the lubricant between the exclusion seal and the shaft by repeatedly operating the pump to cause the pressure of the lubricant to alternately rise above and fall below the level. 
     The present invention has been discussed herein in terms of presently preferred embodiments. Additions and modifications to these embodiments will become apparent to persons of ordinary skill in the art upon a reading of the foregoing disclosure. It is intended that all such additions and modifications form a part of the present invention to the extent they fall within the scope of the several claims appended hereto.