Abstract:
Energy loss in a dip lubrication system is reduced by reducing the immersion depth of the gear within a pool of oil. This can be accomplished by increasing the pressure within the dip lubrication system which effectively reduces the flow rate of the oil so that the oil remains separated from the oil pool for a longer period of time thereby reducing the oil level and the immersion depth of the gear within the oil pool. Alternately, this can be accomplished by substituting a higher density gas for air which has the same effect. In a third embodiment the immersed gear includes wind vanes that direct air against the oil pool creating a trough which effectively reduces the immersion depth of the gear within the oil pool.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority of application Ser. No. 61/683,261 filed Aug. 15, 2012, the disclosure of which is hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a lubrication system and method for reducing power loss related to dip lubrication systems. 
     BACKGROUND 
     In dip lubrication systems, also referred to as splash lubrication systems, components such as gears are rotated through an oil sump. The rotating components then splash the lubricant on adjacent parts, thereby lubricating them. Drive axles and transmissions typically have several gear sets that are splash lubricated from an oil sump or reservoir. As the gears turn in the oil, the gears and bearings are coated with the circulating oil. At high speeds, the gears are essentially pumping the oil, creating a force corresponding to energy or shear losses in the fluid. Although one does not want to unduly reduce the amount of lubricant in the system, the immersion depth of the component into the oil relates to power loss. The deeper the component is immersed in the oil, the greater the power loss. Accordingly, it is desirable to reduce power loss without decreasing the overall volume of the lubricant within the system. 
     There is a need for a lubrication system and method for reducing power loss, such as in dip lubrication systems, that addresses present challenges and characteristics such as those discussed above. 
     SUMMARY 
     The present invention is based on the discovery that power loss in a dip lubrication system can be reduced using the gas in the system. The present invention is further premised on the realization that the immersion depth of a component, such as a gear, into an oil sump in a splash lubrication system can be reduced by either reducing the rate at which the oil returns to the sump or by pushing the oil away from the gears in the sump. More particularly, by adjusting the internal pressure of the dip lubrication system to increase the density of the gas within the system, effectively reduces the return flow rate of the oil to the sump thereby reducing the immersion depth and energy loss. Alternately, the dip lubrication system can be filled with a gas denser than air which will have the same effect. In an alternate embodiment, air or other gas can be directed at the location of the sump where the gear is immersed, pushing some of the oil to either side to create a trough effect and thereby reduce immersion depths and thereby power loss. 
     According to an exemplary embodiment, a lubrication system for reducing dip lubrication power loss comprises a housing defining an internal volume and a component rotatably mounted within the housing. The housing is fluidly sealed for containing a gas of greater density than air at atmospheric pressure. The housing is also configured to hold a pool of a lubricant. The component is mounted within the housing such that a portion of the component is immersed into the pool of the lubricant. Furthermore, the component is configured to rotate through the pool for splashing the lubricant within the housing. 
     In one aspect, the housing contains the gas. The gas is compressed in order to have a pressure greater than atmospheric pressure. In another aspect, the gas has a specific gravity greater than air. 
     According to another exemplary embodiment, a method for reducing power loss for a dip lubrication system includes fluidly sealing a housing defining an internal volume with a rotatable component therein. The method includes pumping a gas into the internal volume such that the gas has a greater density than air at atmospheric pressure. In addition, the method includes containing the gas with greater density than air at atmospheric pressure within the internal volume. 
     In one aspect, the method further includes collecting a pool of a lubricant within the internal volume of the housing and immersing a portion of the component within a pool of the lubricant to an immersion depth. The method also includes rotating the component through the pool of the lubricant to splash a portion of the lubricant within the housing. In addition, the method includes reducing the immersion depth of the component while maintaining a generally fixed volume of lubricant within the internal volume and returning the portion of splashed lubricant back to the pool of the lubricant. 
     Various additional objectives, advantages, and features of the invention will be appreciated from a review of the following detailed description of the illustrative embodiments taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below serve to explain the invention. 
         FIG. 1  is an overhead cross sectional view partially broken away of an exemplary axle according to the present invention; 
         FIG. 2  is a rear view partially in cross section of the axle shown in  FIG. 1 ; 
         FIG. 3  is a perspective view of a dip lubrication test system. 
         FIG. 4A  is a chart of rundown results for distilled water. 
         FIG. 4B  is a chart of rundown results for oil #1. 
         FIG. 4C  is a chart of rundown results for oil #3. 
         FIG. 5A  is a chart of churning power loss consumption for distilled water. 
         FIG. 5B  is a chart of churning power loss consumption for oil #1 
         FIG. 5C  is a chart of churning power loss consumption for oil #4. 
         FIG. 6A  is a chart of churning power loss consumption at a constant speed interval at 1 bar. 
         FIG. 6B  is a chart of churning power loss consumption at a constant speed interval at 2 bar. 
         FIG. 6C  is a chart of churning power loss consumption at a constant speed interval at 0 bar. 
         FIG. 7  is a chart of churning power loss consumption for distilled water with an added surfactant. 
         FIG. 8  is a non-dimensional chart relating a power parameter to Reynolds number for various pressures. 
         FIG. 9  is a diagrammatic rear view depiction of a splash lubrication system according to the present invention. 
         FIG. 10  is a diagrammatic side cross sectional view of the splash lubrication system of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is an improvement to a splash lubrication system which can be used with virtually any splash lubrication or dip lubrication system. These can be, for example, the drive axle for a motor vehicle, a transmission, gear boxes for various applications such as helicopters, wind turbines, and the like. These splash lubrication systems have a component such as a gear that is immersed in a pool or sump of oil and rotates to splash the oil about the system to thereby lubricate other gears or moving parts. 
     One exemplary system is shown in  FIG. 1  which is the drive axle  10  for a motorized vehicle. This representation is diagrammatic and for illustration purposes only. Normally, such systems operate at ambient pressure filled with oil and air. But, according to the present invention, an internal volume  12  of an axle  10  is maintained at super-atmospheric pressures. As shown, the system has a sealed housing  14  with a gas inlet valve  16  that allows air or other gas to be pumped into the housing  14  to increase the internal pressure within the housing  14 . The housing  14  provides a gas tight containment system, also referred to herein as fluidly sealed. 
     The system further includes rotary seals  18 ,  20  and  22  at each axle  24 ,  26  and  28  permitting the axles  24 ,  26 ,  28  to rotate without allowing the gas to escape the housing  14  maintaining the internal pressure. 
     The internal pressure will be above atmospheric. Generally, it can be, for example, two bars, three bars or four bars, or higher. As long as the pressure of the gas does not cause a negative interaction of the gas with the lubricant, the pressure can be increased. Likewise, the internal pressure cannot exceed the effective sealing capacity of the respective seals  18 - 22 . 
     With reference to  FIG. 1 , the axle includes various gears with a planetary gear  30  which rotates into the pool  32  of oil. This splashes the oil around the interior housing  14  causing oil to contact the remaining gears  34 . As the planetary gear  30  rotates, only a certain portion  31  of the gear  30  is actually immersed in the pool  32  of oil, although with the axle the component that rotates in the oil is a gear, it can be virtually any rotating structure in place of the gear. 
     By way of example and observation,  FIG. 3  shows an embodiment of a lubrication system in the form of a dip lubrication test system  34 . The dip lubrication test system  34  is generally a simplified representation of the drive axle  10  for a motorized vehicle shown in  FIG. 1  and includes the gear  30  having the portion  31  immersed within the pool  32  of lubricant within an enclosure  36 . The enclosure  36  includes a cylindrical housing  38  having a pair of polycarbonate endplates  40  sealing the internal volume  12  therein with o-rings (not shown). The gear  30  is rotatably driven within the enclosure  36  by a drive mechanism  42  having an electric motor  44 , a drive belt  46 , a clutch  48 , and a spindle  50 . The electric motor  44  has selective, variable speed settings and is operatively coupled to the drive belt  46  for rotating the clutch  48 . The clutch  48  is removably connected to the spindle  50  so that the spindle  50  may be either rotatably driven by the electric motor  44  or freewheel in position. The spindle  50  extends through at least one of the endplates  40  and into the internal volume  12 . In this respect, the gear  30  is fixed to the spindle  50  such that the gear  30  may be rotatably driven or freewheel within the enclosure  36  for measuring the effects of power loss due to dip lubrication as the portion  31  of the gear  30  moves through the pool  32  of lubricant. According to an exemplary embodiment, the electric motor  44  is a 1 horsepower AC motor for rotatably driving the gear  30  to a maximum speed of 335 rad/s. Also, internal volume  12  of the enclosure  36  without the gear  30  is 1.77 dm 3  and the lubricant volume is 0.128 dm 3 . The dip lubrication test system  34  also includes thermocouples (not shown) for measuring initial lubricant temperature and the lubricant temperature between tests.  FIGS. 1-3  show a lubricant fill level along the gear  30  of approximately 25% of a radius of the gear  30 . Please note that use of a proper quantity of the lubricant, such as oil or water, is important in any dipped lubrication system. Improper quantity of lubricant may be detrimental to the system and may include such effects as increased gear bulk temperatures and increased gear failure risks. In these cases, the oil amount required for the lubrication may still be sufficient but the oil circulation and cooling is controlled by the lubricant properties. 
     According to an exemplary embodiment, the gear  30  and the oil within the dip lubrication test system  34  has the following respective qualities: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Gear Dimensions 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 m (module) 
                  4 mm 
               
               
                   
                 α (pressure angle) 
                 20° 
               
               
                   
                 z (number of teeth) 
                 38 mm 
               
               
                   
                 b (tooth face width) 
                 40 mm 
               
               
                   
                 R o  (gear outer radius) 
                 80 mm 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Lubricant Material Properties 
               
             
          
           
               
                   
                   
                   
                   
                 Surface 
               
               
                   
                 Kinematic 
                   
                 Density, 
                 tension, 
               
               
                   
                 viscosity, cSt 
                   
                 g/cm 3   
                 mN/m 
               
             
          
           
               
                   
                 40° C. 
                 100° C. 
                 15° C. 
                 20° C. 
               
               
                   
                   
               
             
          
           
               
                   
                 Oil #1 
                 14.8 
                 3.3 
                 0.855 
                 21.21 [8] 
               
               
                   
                 Oil #2 
                 30.4 
                 4.6 
                 1.011 
                 27.75 [8] 
               
               
                   
                 Oil #3 
                 196.4 
                 23.6 
                 0.842 
                   30 [8] 
               
               
                   
                 Oil #4 
                 157 
                 15.1 
                 0.884 
                 30.81 [8] 
               
               
                   
                 Dist. Water 
                 0.65 
                 0.29 
                 1.000 
                  72.8 [8] 
               
               
                   
                   
               
             
          
         
       
     
     The testing methodology of the dip lubrication system  34  is based on the inertia rundown technique. This method relies on the determination of the churning torque and power due to drag acting on the gear  30  by churning of the oil. After the gear  30  reaches a constant, desired speed, the clutch  48  disengages from the gear  30 , which, in turn, decelerates while rotating on the spindle  50  within the surrounding oil. The dip lubrication test system  34  also includes a data acquisition system (not shown) operatively connected to a speed sensor (not shown) for collecting speed data related to the gear  30  during use. 
     In order to rotate the gear  30  within the enclosure at various pressures, a pump  52  connects to the internal volume  12  within the enclosure  36  via the inlet valve  16 . With the aid of the pump  52 , the pressure within the enclosure  36  may be selectively varied to atmospheric pressure, negative pressure, or positive pressure. As described herein, negative pressure may also be referred to as a vacuum and positive pressure may be referred to as compressed gas. According to exemplary embodiments, the dip lubrication test system  34  was operated through inertia rundown at 0 bar, 1 bar, and 2 bar. Generally, the term “1 bar” may be used interchangeably with “atmospheric pressure.”  FIGS. 4A-4C  show the inertia rundown for various lubricants, such as distilled water, relatively low viscosity oil #1, and relatively high viscosity oil #3. 
     During operation of the dip lubrication test system  34 , the total torque acting on the gear  30  is a product of the moment of inertia of the gear (I g= 0.00282 kg·m 2 ) and the angular deceleration. Therefore, by capturing the gear  30  deceleration after the declutching from the electric motor  44 , the torque exerted by test oil is evaluated using Newton&#39;s second law of motion represented for rotation as: 
     
       
         
           
             T 
             = 
             
               
                 
                   I 
                   g 
                 
                 ⁢ 
                 a 
               
               = 
               
                 
                   I 
                   g 
                 
                 ⁢ 
                 
                   
                     ⅆ 
                     w 
                   
                   
                     ⅆ 
                     t 
                   
                 
               
             
           
         
       
     
     With negative pressure within the enclosure  36 , the only restraining torque during rundown is churning loss on the gear  30  and relatively small contribution of mechanical loss from the bearing housing (not shown) and oil seal related to the gear  30  rotatably mounted on the spindle  50 . With compressed gas, such as air within the enclosure  36 , both churning of liquid and air windage contribute to overall losses. To remove the effect of these losses from data collected by the data acquisition system, liquid churning power losses were computed by subtracting the power loss measured when the housing was filled to the prescribed level with liquid, from that obtained when the housing was empty of liquid. According to an exemplary embodiment of the dip lubrication testing system  34 , these losses were approximately 7.5 W at 310 rad/s and calculated based on the following equations for churning torque and churning power loss, respectively:
 
 T   ch   =T −( T   b   +T   z )
 
 P   ch   =wT   ch  
 
       FIGS. 5A-5C  show the power loss comparison for distilled water, oil #1, and oil #4, respectively. As described above, oil #4 is considerably more viscous than oil #1. As such, the losses for oil #4 were greater within the vacuum than when gas was present. However, there was little effect of air pressure for the less viscous fluids (Oil #1, water). 
       FIGS. 6A-6C  show variation of the churning power loss with the viscosity at constant speed intervals for the atmospheric pressure, compressed gas, and vacuum, respectively. For atmospheric pressure and compressed gas, there are approximately constant power losses with viscosity increase for oils. At higher speeds, power loss initially increases and then decreases before increasing again with the viscosity. It is believed that the effect of gravity causes the oil to stick on the surface of the enclosure  36  and gear  30  for a longer time before draining back to the sump as viscosity of the oil increases. For the vacuum, there is a rapid increase in the power loss as oil viscosity increases. For comparison, vacuum churning loss increases up to 4 times relative to atmospheric pressure for high speed. However, the power loss related to water tends to not be as affected by the air pressure variation given that compressed gas gives lower losses compared to the other two conditions. 
     The results for water show relatively higher power losses than might be expected from extrapolating the results for the oils in  FIGS. 6A-6C . However, water has a somewhat higher density (1 g/cm3) and a much higher surface tension than any of the oils. For this reason, surfactants may be used with water to reduce the surface tension and increase the contact of two materials. Detergent, such as dishwashing detergent, may be used to lessen the surface tension of water. The addition of surfactants has a stabilizing effect on the water bubbles. Water with surfactant shows reduction in the losses proving effect of surface tension as shown in  FIG. 7  for both atmospheric pressure and compressed gas. In this respect, atmospheric pressure power loss is higher than compressed gas, whereas churning power loss for compressed gas with surfactant is less than 1.4 times compared to atmospheric pressure with no surfactant. 
     Still greater insights into the effect of pressure within the enclosure  36  on power loss may be appreciated by performing a dimensional analysis on the above findings as related to the Reynolds number, Froude number, and Bond number. 
     The Reynolds number is defined from momentum as: 
     
       
         
           
             Re 
             = 
             
               
                 wR 
                 P 
                 2 
               
               
                 v 
                 0 
               
             
           
         
       
     
     The Froude number relates to the dominant effect for free-surface flows and is represented as: 
     
       
         
           
             Fr 
             = 
             
               
                 
                   w 
                   2 
                 
                 ⁢ 
                 
                   R 
                   p 
                 
               
               g 
             
           
         
       
     
     The Bond number is a ratio of body forces to surface tension forces and is represented as: 
     
       
         
           
             Bo 
             = 
             
               
                 ρ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   gR 
                   p 
                   2 
                 
               
               τ 
             
           
         
       
     
     Finally, a power parameter is calculated as pressure multiplied by immersed gear surface area and pitch velocity. The power parameter is calculated at each pressure within the enclosure  36  and compared over the Froude number and the Reynolds number. 
     By way of comparison, it becomes clear that at higher speeds and atmospheric pressures, the power loss is lower relative to vacuum and compressed air. However, as speed decreases, compressed air results in lower losses while vacuum is detrimental. With decreasing Reynolds number, the power loss remains somewhat constant. There is also a considerable reduction in the power loss for the compressed air. 
       FIG. 8  shows a 3D plot of the Reynolds number, Froude number and Power parameter for pressure within the enclosure  36  at 0 bar, 1 bar, and 2 bar. The variation of power loss with the Reynolds number is distinctly different for high and low Froude numbers. For low viscosity, air density variations do not affect the power parameter greatly, except for initial speeds where the vacuum causes higher power loss. As speed and viscosity increase, the power loss remains constant to a certain point and then varies significantly with the air pressure. Relative to atmospheric pressure, the power loss for the vacuum increases rapidly with speed, whereas the power loss for compressed air decreases for low to medium speeds. Specifically, the maximum churning power loss variation for the gear  30  within the vacuum is approximately 3.74 times higher than at atmospheric pressure. However, the maximum churning power loss variation for the gear  30  within compressed air is approximately 2.24 times lower than at atmospheric pressure.  FIG. 8  also includes a dividing line to indicate at which Froude number and Reynolds number pressure within the enclosure  36  does not significantly affect the power parameter. 
     The dominant feature is the large change in the power loss parameter with Froude number (ratio of inertial to gravitational force). This is consistent with an observation that the return of the fluid to the sump and the immersion of the gear  30  are entirely dependent upon gravity. For this reason, it is believed that the losses in dipped lubrication are, in large part, due to acceleration of the fluid, limited by gravitational replenishment of the oil into the pool within the enclosure  36 . 
     The variation of the power parameter with Reynolds number, as shown in  FIG. 8 , is an indication of the effect of viscous forces. However, the results showing that higher viscosity lubricants do not always give a higher power parameter at a particular Froude number suggest that viscous forces do not control the net behavior of the system. Accordingly, it is believed that viscosity lubricants are slower to return to the pool under the influence of gravity and, in turn, yield a lower immersion depth with reduced inertial losses. 
     Furthermore, introducing the surfactant into the water, as indicated in  FIG. 7 , tends to reduce the surface tension by about 25% and resulted in a reduction in the power loss of up to approximately 20%. Most of the oils had similar Bond numbers, but more aqueous fluids may have lower values due to higher surface tensions. For this reason, lower power losses are obtained with an intermediate viscosity lubricant and the lowest available surface tension. 
     In general, the pressure within the enclosure  36  affects the density of the gas therein and reduces power loss. It is believed that the denser air flowing radially from the rotating gear  30  disturbs the free surface of the lubricant. In turn, the gear  30  becomes more shallowly immersed and the flow rate of the accelerated lubricant is reduced, leading to lower power losses. The effect is evidently increased if the fluid is more viscous since the return flow of the more viscous fluid is already diminished. 
     In an embodiment of the present invention, the same effect can be achieved by altering the gas within the axle  10 . For example, normally the interior of an axle will include air and will be vented. The present invention provides the sealed housing  14  for the axle, utilizing seals  18 ,  20  and  22 . In this second embodiment, the air is replaced with a gas which is denser than air. The gas can be any dense gas which is inert with respect to the oil. Preferably, it should be a gas which is not flammable or combustible. An exemplary gas could be, for example, argon, as well as other gaseous compositions such as halogenated hydrocarbons. 
     Of course, a higher density gas can be incorporated within the present system and at super-atmospheric pressure to further increase the density of the gas thereby reducing the flow rate of the lubricant and in turn reducing the immersion depth. This operates in the same manner as discussed above. 
       FIGS. 9 and 10  show another embodiment of the present invention. As shown in  FIGS. 9 and 10 , the rotating component such as a gear  60  in a dip lubrication system  58 , has a plurality of wind vanes  62  and  64  on the sides of the gear  60 . The vanes  62 ,  64  extend only part of the way to the peripheral edge  66  of the gear  60  so that they do not extend into the pool of oil  68 , which would increase power loss. Rotation of the gear  60  causes a downward flow of air as indicated by arrows  70 , which cause a trough or depression  72  to form in the oil at the point of immersion thereby reducing immersion depth. 
     As shown, the vanes  62 ,  64  are on both sides of the gear  60 . However, the vanes  62 ,  64  can be on only one side of the gear  60  or can be positioned on the shaft (not shown) attached to the gear  60  or any location whereby rotation of the gear  60  will cause rotation of the vanes  62 ,  64 . But, preferably the vanes  62 ,  64  are attached to at least one side of the gear to thereby cause the trough to form in the oil immediately alongside the gear. 
     The plurality of radially extended vanes,  62 ,  64  is positioned around the gear  60  so that a downward air flow is maintained. This will also create an upward airflow, which in turn will maintain oil suspended above the gear, also reducing the oil level and the immersion depth. This embodiment is also more efficient if the gas within the system is either denser than air or the pressure is super-atmospheric. This permits all the embodiments of the present invention to be utilized at the same time. 
     While the present invention has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative dispensing assembly and method and illustrative examples shown and described. Accordingly, departures may be from such details without departing from the scope or spirit of the general inventive concept.