Abstract:
An air or oil dryer module is provided to remove or reduce moisture from a lubricant using a pressurized air supply to inject dry air into a lubricant sump. As the “dry air” or air with a minimized moisture content, gathers moisture from the lubricant a relative humidity sensor detects an increase in air moisture. At a pre-determined relative humidity level, a fresh supply of air is added into the lubricant sump, purging the higher relative humidity level air from the sump. A method of reducing moisture content of a lubricant contained in an enclosed space is also disclosed and is performed by agitating the lubricant, providing a humidity sensor in fluid communication with the enclosed space, measuring the humidity of the enclosed space and comparing the humidity measurement with a pre-selected humidity level, introducing dry air into the enclosed space if the humidity level equals or exceeds the pre-selected humidity level, until the saturated humidity level is less than the pre-selected humidity level.

Description:
This application claims the benefit of and is a continuation-in-part of copending application Ser. No. 09/457,026 filed on Dec. 8, 1999, which is a continuation in part of application Ser. No. 09/207,373 filed Dec. 8, 1998 now abandoned, which is a continuation in part of application Ser. No. 09/820,124 filed Mar. 9, 1997, now U.S. Pat. No. 6,878,842 which claims benefit of provisional application No. 60/085,643 filed May 15, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     Lubrication is an important aspect of maintaining machinery in proper operating condition. Machine elements such as bearings, journals, shafts, and joints require proper lubrication between their moving surfaces to decrease friction, prevent contamination, reduce wear and dissipate heat. Improper lubrication is likely to lead to premature component wear and component or system failure. 
     When determining the optimal lubrication between moving machine elements, many factors should be considered. These factors include the mode of operation of the machine, the type of machine element to be lubricated, the environment of the machine, the operating speed of the machine, the lubricant&#39;s viscosity, the lubricant&#39;s temperature, the lubricant&#39;s ingredients, and the lubricant&#39;s condition. 
     Prior art lubricators, such as the TRICO OptoMatic oiler, supply a constant level of lubricant within a lubricant reservoir to a machine element. The lubricant level is predetermined for the particular application and cannot be changed during the operating time of the machine to which the constant level lubricator is attached. Although this type of lubricator provides reasonable performance in many steady-state operations, multiple variables can create unacceptable operating conditions and lead to premature wear, or even failure, of machine elements. The variables include “on” and “off” operating modes (machine cycling), oil viscosity, machine speed, lubricant temperature, lubricant condition, and lubricant vessel pressure. 
     Other devices, such as the TRICO Hydrolert indicate by LED signals the status of the equipment&#39;s lubrication such as lubricant condition within acceptable levels, lubricant condition at the upper limit of acceptable levels, and lubricant condition immediate action required. This device is effective because an operator is signaled only when the lubricant condition is at the upper limit of acceptable levels or if immediate action is required. This reduces maintenance costs and productivity is enhanced. 
     With specific regard to lubricant relative humidity, or saturated relative humidity, one prior form of lubricant qualitative control utilized an in-line water removal filter. Another form of lubricant qualitative control is to dilute the existing amount of lubricant with a fresh supply of lubricant, reducing the overall lubricant moisture content by introducing a portion of lubricant with a lower moisture content. Still another form of moisture removal is to expose the air surrounding the lubricant to a desiccant material that removes humidity from the air. 
     SUMMARY OF THE INVENTION 
     The system of the present invention relates generally to the field of lubrication and specifically to the field of devices which deliver a lubricant to a machine element, such as a bearing in a pump. 
     The system of the present invention provides continual on-line monitoring that eliminates the need for expensive repetitive lubrication checking and time-consuming laboratory oil sampling. Designed for installation on rotating equipment or other lubricant reservoirs, the system of the present invention provides real time information on lubricant variables such as lubricant water content, with the added flexibility of providing real time information and control of other variables such as temperature, viscosity, and particulate matter. 
     The present invention is directed at lowering lubricant water content to acceptable levels. It has been discovered that moisture removal from oil is an important objective to achieve optimum lubricant condition. Lubricants such as oil have an initial moisture content, and often the initial moisture content is unacceptable. Additionally, harsh machine operating conditions such as pump wash-downs and condensation occurring during changing plant temperature and humidity conditions can contribute to potentially harmful lubricant moisture levels. 
     If the lubricant is not agitated such as during an off operating mode, a continuous oxygenated layer forms a barrier on top of the lubricant. This oxygenated layer prevents moisture from both entering and leaving the lubricant. When the lubricant is agitated, such as during an “on” equipment operating mode, the oxygenated layer is broken. “Dry” air, or air with moisture significantly removed, assists in withdrawing the moisture from the oil by a continuous moisture equilibrium interaction between the agitated lubricant and the dry air above the lubricant. It has also been found that an increase in dry air temperature increases the rate of moisture removal from the lubricant. 
     The present volumetric lubricant dispenser controls water content in a lubricant to provide optimum operating conditions and extend the useful life of lubricants. An automatically controlled system is adjusted by means of a microprocessor that receives input from various system sensors and adjusts the lubricant humidity level accordingly through an air drying process, that, in turn lowers the lubricant moisture level. In the automatic mode, the entire system is constantly monitored. 
     The present volumetric lubricant dispenser is designed to control the water content of lubricant to a rotating element or machine part or other lubricant reservoir. The rotating element may include a bearing or a journal or the combination of the two. In a typical application, a bearing includes an inner and an outer race. Between the races a plurality of rolling elements are positioned, usually balls. If the outer race is attached to the bearing housing, the inner race and the rolling elements are rotated into and out of a lubricant reservoir. The reservoir is maintained within the bearing housing. 
     For the purpose of qualitative lubricant control, the present invention may be described as a system intended for controlling lubricants, both mineral (natural) and synthetic based, by measuring the condition characteristics of the lubricant, including water or moisture content. 
     Accordingly, an apparatus for qualitative lubricant condition control and monitoring may include but not be limited to the following illustrative description. The apparatus comprises at least one lubricant condition-sensing module. The module may be of any suitable type for sensing a desired lubricant characteristic; in this case water or moisture content. The lubricant sensing module is communicatively coupled to a qualitative control mechanism. For example, the qualitative control mechanism could be a microprocessor. The qualitative control mechanism is also communicatively coupled to at least one response mechanism, e.g., an air pump or compressor. The qualitative control mechanism is capable of actuating at least one response mechanism in response to a signal received from at least one lubricant condition-sensing module and interpreted by the qualitative control mechanism. 
     A method of reducing moisture content of a lubricant contained in an enclosed space is also disclosed and is performed by agitating the lubricant, providing a humidity sensor in fluid communication with the enclosed space, measuring the humidity of the enclosed space and comparing the humidity measurement with a pre-selected humidity level, introducing dry air into the enclosed space if the humidity level equals or exceeds the pre-selected humidity level, until the saturated humidity level is less than the pre-selected humidity level. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially cut away front elevational view of the workpiece to which the invention is directed. 
     FIG. 2 is a schematic view of an oil dryer module and transducer communicatively coupled with the workpiece. 
     FIG. 2A is a partially cut away view of portions of an oil dryer module supplying logic to the oil dryer module. 
     FIG. 2B is a schematic view of an oil dryer module. 
     FIG. 3 is a schematic view of an air dryer sensor system applied to a lubricant sump containing lubricant. 
     FIG. 4 is a schematic view of an oil dryer module and transducer communicatively coupled with the workpiece, the oil dryer module further comprising a controller and a communication output and input port. 
     FIG. 5 is a schematic view of an oil dryer module and transducer communicatively coupled with the workpiece, with a CE control module in-line. 
     FIG. 6 is a schematic view of an oil dryer module and transducer communicatively coupled with the workpiece, with an indicator module in-line, the indicator module communicatively coupled with an external computer. 
     FIG. 7 is a schematic view of an oil dryer module and transducer communicatively coupled with the workpiece, with a digital readout indicator module in-line, the digital readout indicator module communicatively coupled with an external computer. 
    
    
     DETAILED DESCRIPTION 
     Although the disclosure hereof is detailed and exact to enable those skilled in the art to practice the invention, the physical embodiments herein disclosed merely exemplify the invention which may be embodied in other specific structure. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims. 
     The present invention in one form comprises a qualitative lubricant condition sensing and control apparatus. The invention is intended to allow flexibility of components based on user preference. For instance, a user may have particular concern for one or more lubricant conditions, such as the lubricant&#39;s viscosity, the lubricant&#39;s temperature, the lubricant&#39;s ingredients, and the lubricant&#39;s condition (purity), in addition to the lubricant water saturation or saturated relative humidity (“SRH”). In this sense, the invention is a modular system of interrelated components that are usable together or tailored for specific applications. 
     For the purpose of the present invention, air drying, oil drying, and lubricant drying are used interchangeably to describe the nature of the desired process and result, because air drying in an enclosed space results in oil or lubricant drying for lubricants in the enclosed space. Also for the purpose of defining the present invention, an agitated lubricant is desired to encourage the lubricant/air moisture equilibrium interaction by preventing formation of the oxygenated barrier on top of the lubricant. Two types of humidity readings are relevant to the present invention. The first is “saturated relative humidity” used to describe the water content present in a lubricant. The second is “relative humidity” used to describe the water content present in air contained in an enclosed space containing air and the lubricant. 
     The lubricant is “agitated” during routine machine operating conditions, such as when the machine is running. Additionally, some machines such as centrifugal pumps may contain internal components such as a “slinger disc” or a “flinger ring” that contribute to or enhance lubricant agitation. 
     Also for the purpose of defining the invention, a void space in a sump is referred to as an enclosed volume. The enclosed volume is enclosed with respect to the workpiece to define a system that minimizes the introduction of air from the workpiece operating environment such as a pump farm. The enclosed volume, is however, coupled with external elements to which the invention is directed, i.e., an oil drying module and related components. 
     The workpiece to which the invention is directed is best shown in FIG. 1, although the invention can be applied in different work environs. In FIG. 1, the workpiece is shown as a centrifugal pump P with a motor M. The invention provides an optimal lubrication condition to machine element or bearings  20  by minimizing the relative humidity of the air surrounding the bearings  20 , also decreasing the percent saturation of water in lubricant  12 , such as oil. In FIG. 1, the bearings  20  are viewed from the side, with their front profile generally doughnut shaped. The lubricant  12  is contained within a fluid reservoir, or bearing housing  60 . Typically, two bearings  20  are located within the bearing housing  60 . Bearing  20  includes an inner race, an outer race, and a plurality of rolling ball elements positioned between inner race and outer race (not shown). In order for bearing  20  to perform its function of maintaining a low friction precision relationship between a rotating element or shaft  14  and bearing housing  60 , the bearing  20  must operate in an environment where it receives proper lubrication. 
     To achieve proper lubrication, bearing housing or reservoir  60  is filled with a predetermined amount of lubricant  12 . When the machine, of which bearing  20  is a component, is in a static state, the amount of lubricant  12  within the housing  60 , the pressure of the lubricant  12 , the distribution of the lubricant  12  within the housing  60 , and the temperature of the lubricant  12  are constant. 
     It has been discovered that moisture removal from oil is an important objective to achieve optimum lubricant condition. Lubricants such as oil have an initial moisture content. If the lubricant  12  is not agitated and is in a static state, such as during an off operating mode, a continuous oxygenated layer forms a barrier on top of the lubricant  12 . This oxygenated layer prevents moisture from both entering and leaving the lubricant  12 . 
     Once the machine is set in motion and journal  14  begins to rotate, the lubricant&#39;s characteristics begin to change. These characteristics can continue to change during the run cycle of the machine. When the lubricant  12  is agitated, such as during an on operating mode, the oxygenated layer is broken, facilitating and allowing the removal of moisture from the lubricant  12 , in addition to allowing the introduction of moisture to the lubricant  12  from air contained in the enclosed void space V. 
     This relationship between humidity removal in a static state and an agitated state is shown in the following Table 1: 
     
       
         
               
             
           
               
                   
               
             
             
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
     Table 1 demonstrates the enhancing effects of agitation on moisture removal from a lubricant. A seven (7) hour test was conducted with air relative humidity evels measured every fifteen (15) minutes on two sealed flasks containing identical volume and type of lubricant. The first flask remained static and undisturbed throughout the test. The second flask was slightly agitated throughout the test to simulate pump operating conditions. Both flasks contained an initial relative humidity in the air of approximately 43%. As Table 1 shows, the relative humidity in the air of the agitated flask gradually increased throughout the test, while the relative humidity in the air of the static flask remained virtually constant. The rise in the relative humidity in the air of the agitated flask demonstrates the release of saturated relative humidity from the lubricant into the air by the continuous moisture equilibrium interaction between the agitated lubricant and the dry air above the lubricant. The air above the lubricant acts as a sink for the saturated relative humidity from the lubricant. 
     Referring now to FIG. 2, an oil dryer module  150  is shown positioned to be communicatively coupled with the void space V on the workpiece. The oil dryer module  150  is capable of receiving electric signals from a transducer T. If the transducer T detects a humidity reading above a pre-determined level, the transducer T sends a signal to the oil dryer module  150  through electrical circuitry as described later. In response to a humidity reading above the pre-determined level, the oil dryer module  150  initiates air removal from the void space V, and also alternatively illuminates an LED  55  to signal elevated humidity levels. 
     Removed air enters the oil dryer module  150  from an air intake line  48  coupled with an air intake coupling  48 C. The oil dryer module  150  then compresses the air with an air compressor  25  as shown on FIG. 2B, removes the moisture from the air, and returns the dried air to the void space V, as described later. The extracted moisture leaves the oil dryer module  150  from the drain port  49 , where the water can be captured by a receptacle (not shown). The dried air is returned to the void space V through an air-out coupling  50 C and an air-out line  50 , communicatively coupled with the void space V. 
     Referring to FIG. 2, the oil dryer module  150  is equipped with an indicating mechanism  55 , such as an LED that can indicate visually whether an alarm condition, or an RH reading above a pre-selected level. In addition to the visual indication of an alarm condition through the LED indicator  55 , the oil dryer module  150  is equipped with a dry contact alarm output  70  and an electrical output  71  intended for coupling with a computer (not shown) for data storage and analysis. Additionally, the oil dryer module  150  is provided with power through the power inlet P. 
     In the embodiment shown in FIGS. 2 and 2A and  2 B, the oil dryer module  150  is internally controlled by a printed circuit board  70  for providing signals to actuate the solenoid valve  44  and the air compressor  25 . The transducer T provides an input to the printed circuit board  70 , which contains a controller or microprocessor  32 . The air compressor  25  is controlled by an output of the printed circuit board  70 . When the transducer T feeds input into the microprocessor  32  of the printed circuit board  70  indicating an air moisture condition at or above the pre-selected limit, the microprocessor  32  sends a signal to actuate the air compressor  25  and a solenoid valve  44 , which in combination purges the wet air and replaces the wet air with dry air. The microprocessor  32  simultaneously sends a signal to the LED  55 , illuminating the LED. The dry air is pumped and circulated through the void space V until the predetermined humidity and/or temperature levels are reached, which indicate that the lubricant has, again, reached the desired level of humidity or temperature. 
     Referring now to FIG. 2B, a small air compressor is housed within the oil dryer module  150  to force air through the air-in line  48  and thus the air-out line  50 . The air exchange is a closed-loop purge with the air-in line  48  and the air-out line  50  in fluid communication with the same void V. The air compressor  25  controls the flow of dry compressed air into the sump void space V by drawing wet air from the void space V and replacing the wet air with dry air. From the air compressor  25 , the wet air enters into a high pressure chamber  26 , where the wet air is condensed and separated into air and water components. At a predetermined level, the microprocessor  32  opens the solenoid valve, allowing the moisture to leave the oil dryer module  150  for capture in a receptacle (not shown). The amount of water captured in the receptacle is available for measurement and data trending to show efficacy or problem pumps. The dried air is returned to the void space V through the air-out line  50 . 
     Although the system depicted in FIG. 2B shows a closed-loop air drying system, with the air from the void space V dried and then returned to the void space V, an open-loop air drying system (not shown) would be preferable for larger scale oil drying projects. In an open-loop air drying system embodiment, the wet air from the void space V would be expelled from the void space V by the introduction of dry air from an external source, such as “shop air” or air from a plant air compressor. The microprocessor  32  would initiate air introduction from the shop air source until the humidity level in the void space V falls below the pre-determined level. In this embodiment, the wet air from the void space V is not withdrawn and subsequently condensed and separated, but is instead purged and replaced with the introduction of dry shop air. Another alternative to shop air is the addition of other gases, such as nitrogen, to reduce the relative humidity and thus the saturated relative humidity. 
     Referring now to FIGS. 2 and 3, as pictured in FIG. 2, the transducer T refers collectively to an RH sensor  50 , a temperature sensor  46 , and their respective electrical couplings with the oil dryer module  150 , although the temperature sensor  46  is not required. Comparing FIGS. 2 and 3, in FIG. 3, the RH sensor  42 , temperature sensor  46 , and a transducer electrical line  52 , used to communicate with the oil dryer module  150  are pictured as entering the void space V individually. However, as pictured in FIG. 2, it is preferable to bundle the elements together to minimize connections as designated by the transducer T. One convenient way to communicatively couple the transducer T with the void space V is through threaded connections. However, any known coupling mechanism capable of withstanding a hot and oily work environment could perform suitably. 
     Referring now to FIG. 3, the RH sensor  42  measures the humidity of the air in the equipment housing or sump void space V. Preferably, the RH sensor  42  is not submerged in the lubricant, but instead measures the humidity in the air, although the RH sensor  42  could be submerged in the lubricant and perform suitably. The RH sensor  42  collects the humidity information and converts it into electrical signals, which the microprocessor  32  may then utilize in association with a predetermined microprocessor program as fully described in co-pending application Ser. No. 09/457,026 incorporated herein by reference. The use of the temperature sensor  46  is preferable to most precisely correlate the reading from the RH sensor  42 , although un-correlated data could also perform suitably. 
     The RH sensor  42  measures the relative humidity of the air at predetermined intervals utilizing a timer (not shown). While it is presently believed best to measure humidity at predetermined intervals, humidity could be measured continuously in a stream of data, e.g., in an analog type system. 
     One RH sensor  42  that has been found to perform suitably is a Model EMD 2000 humidity sensor of the type sold by General Eastern, a Banthorpe PLC Company, located at 20 Commerce Way, Woburn, Mass. 01801-1057. 
     In its commercially preferred form the RH sensor  42 , through the microprocessor  32  is capable of turning on or off indicating mechanism  55 , e.g., a light or LED, when a predetermined safe level of humidity is present. The sensing mechanism  55  is also capable of switching the monitored equipment on or off at a predetermined control point or other set condition indicating acceptable humidity condition. 
     Alarm operating mode is indicated by a red light. This indicates the lubricant is at high moisture levels for optimum lubricant performance and protection. During this mode the oil humidity is greater than or equal to the pre-selected humidity reference. 
     Again comparing FIGS. 2 and 3, the air-in line  48  and air-out line  50  can be coupled with the sump at any location above the level of the lubricant  12  in the reservoir  60 . However, it is preferable to have the air-out line  50  positioned near the transducer T. This positioning provides a more accurate humidity reading to the transducer T because the withdrawal of wet air from the void space V would lead the wet air past the transducer T. This avoids the potential of the transducer T measuring air freshly returned from the oil dryer module  150 , which would provide a humidity reading dryer than the overall humidity level within the void space V. 
     Referring now to FIG. 4, an alternative embodiment of the oil dryer module  150  is shown. In this embodiment, the oil dryer module  150  is additionally equipped with a communications port  56 , which can comprise a commonly available communications port such as a BUS port. 
     Referring now to FIG. 5, a second alternative embodiment is shown. In this embodiment, the oil dryer module  150  and transducer T are communicatively coupled with a CE control module CM in-line. In this embodiment, the oil dryer module  150  is not internally controlled as described above, but is externally controlled by the CE control module CM. This embodiment adds flexibility to the oil drying system to allow monitoring and control of additional lubricant variables such as viscosity, power, and vibration. 
     In this embodiment, the transducer T is coupled with the CE control module CM. The CE control module CM is supplied with a network communications port CP for connection to a remote network (not shown). A dryer driver cable  59  communicatively couples the CE control module CM with the oil dryer module  150 . 
     Referring now to a third alternate embodiment as shown in FIG. 6, the oil dryer module  150  and transducer T are communicatively coupled with the workpiece, with an indicator module I in-line, the indicator module I communicatively coupled with a remote computer (not shown). In this embodiment, the oil dryer module  150  is controlled by the remote computer, with the control and transducer T signals carried through the indicator I, and to the oil dryer module  150  and remote computer as necessary. The communication between the indicator module I and the remote can be can be accomplished through numerous mechanisms, such as a common BUS port and cable arrangement as shown. An oil dryer cable  61  communicatively couples the indicator module I with the oil dryer module  150 . 
     This embodiment also adds flexibility to the oil drying system to allow monitoring and control of additional lubricant variables such as viscosity, power, and vibration. With the additional viscosity, power, and vibration, the user can then operate the remote computer to provide valuable lubricant information and correlation for prediction for multiple devices, such as in a pump farm. The remote computer can be used for data storage, trending, and oil dryer module  150  control. The user can analyze this information to detect trouble machinery or machinery in need of repair or replacement, and to predict maintenance. For instance, if a particular piece of machinery within a larger pump farm is requiring more frequent lubricant control, that piece of machinery, the data trending could indicate possible trouble machinery. 
     Additionally, the use of indicator module I can facilitate indication and control for a plurality of oil dryer modules  150  deployed on a plurality of workpieces. For instance, as shown in FIG. 6, the indicator module I has  8  ports for facilitating  8  oil dryer modules  150  deployed on  8  workpieces. Thus, the indicator module I acts as a central coupling hub between the oil dry modules  150  and the remote computer. 
     If a temperature sensor is deployed, the sensor monitors the temperature of the lubricant. A bearing operating in an poorly lubricated environment will dissipate excess heat generated by the bearing into the lubricant. If the lubricant begins to break down, it cannot properly perform its function of providing a reduced friction substrate between the bearing components and dissipating heat away from the bearing. If the lubricant temperature exceeds a known level, the microprocessor  32  signals drive motor  100  to add more lubricant to the bearing housing  60 . The additional lubricant  12  helps to rejuvenate the present lubricant and helps to dissipate excess heat. 
     If a vibration sensor is employed, the vibration sensor monitors the vibration of bearing housing  60 . Again, improper lubrication of bearing  20  may result in a vibration in excess of a preset maximum vibration level. When the maximum vibration level is exceeded, sensor  140  sends a signal to the microprocessor  32  whereby additional lubricant  12  is again added to the bearing housing  60 . The additional lubricant  12  dissipates the vibrational energy through a larger volume of lubricant  12 . 
     If a pressure sensor is employed, the pressure sensor constantly monitors the lubricant pressure within bearing housing  60 . If the pressure falls below a predetermined setting, improper lubrication of bearing  20  is likely to occur. In this situation, the microprocessor  32  would send a signal to the drive motor thereby adding more lubricant  10  to the bearing housing  60 , increasing the lubricant level, and increasing the lubricant pressure above the required minimum. 
     Additionally, any level sensor, such as an elastohydrodynamic (EHD) sensor may be added to the present invention to provide a signal to indicate if lubricant should be added or subtracted from the bearing housing  60 . 
     The relevant aspects of sensing and control are fully described in co-pending application Ser. No. 09/457,026 and incorporated herein by reference. One skilled in the art will readily appreciate that the air purging cycle of the present invention can be controlled through the alarm sequence of co-pending application Ser. No. 09/457,026. 
     Referring now to a fourth alternate embodiment as shown in FIG. 7, the oil dryer module  150  and transducer T are again communicatively coupled with the workpiece, with the indicator module I in-line, the indicator module I communicatively coupled with the remote computer (not shown), such as a personal computer. In this embodiment, the indicator module I is equipped with digital readout indicator module DR in-line, the digital readout indicator module DR communicatively coupled with the external computer. The digital readout DR can display a pre-selected lubricant condition parameter such as an analog RH readout. This value is also relayed to the remote computer. 
     The mechanisms described herein allow for the collection of data and the storage of that data in a memory system for future reference. Also, the stored data can be used to identify situations that are not desirable when data is indicating a trend toward undesirable conditions so those problems may be identified before they occur. The data may be made available for use by algorithms (computer programs) programmed into the microprocessor. The data may also be provided to a user via means of a read out structure so that the user may use a look-up table or other predetermined reference material and then make an appropriate condition monitoring decision regarding the lubricant. 
     The foregoing is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.