Patent Publication Number: US-2005126749-A1

Title: Heat pipe cooler for differential assembly

Description:
The present application is a Continuation-in-part of U.S. application Ser. No. 10/143,752 filed on May 14, 2002 and is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a heat pipe cooler for a differential assembly and particularly to an effective evaporative heat pipe assembly for transferring heat from the differential lubricant.  
      2. Description of Related Art  
      Typically axles are lubricated by a reservoir of oil in the sump which is circulated by the moving components. This is known as splash lubrication. The operating temperature of a splash lubricated axle assembly or other torque transmission device is generally dependent on the torque being transmitted, the ambient temperature, the speed of rotation and airflow over the device.  
      The operating temperature can be just above the ambient temperature to more than 200 degrees F. above ambient. Operating temperatures significantly above 250 degrees F. can begin to cause problems with the durability of the components in the axle as well as the lubricant itself. These temperatures are generally encountered at higher speeds and/or torques such as high speed highway driving or trailer towing. Therefore it is desirable to avoid these higher temperatures as much as possible. It is the intention of the present invention to minimize these high operating temperatures.  
      It is known to provide cooling conduits within an axle assembly in order to avoid high operating temperatures. These conduits are positioned about a majority of the differential assembly and contain a hydraulic fluid from another device which is capable of cooling the lubricant in the axle.  
      It is also known to have a differential assembly with a heat pipe having heat-absorbing portions extending through an oil sump as well as externally disposed heat-transmitting devices.  
      Additionally, it is know to have a heat pipe including a closed tube having a heat exchange medium comprised of a plurality of fins.  
      Thus, the aim underlying the present invention lies in providing an effective fluid lubricant cooling area that is readily utilizable, without significantly increasing the costs of production, the required space, and weight.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to a heat pipe cooler for an axle having a housing and removable cover. The heat pipe is secured to the cover and includes a heat-receiving zone, a heat-emitting zone and a working fluid, wherein the working fluid is evaporated in said heat-receiving zone and condensed in said heat-emitting zone to promote heat transfer between said lubricant and an external environment. The heat-receiving zone is at least partially immersed in the lubricant and contains heat-dissipating fins to promote heat transfer. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
      A better understanding of the present invention will be had when reference is made to the accompanying drawings, wherein identical parts are identified by identical reference numbers and wherein:  
       FIG. 1  is rear view of a differential cover plate with integral heat pipe assembly according to the present invention.  
       FIG. 2  is a sectional view taken along lines  2 - 2  of  FIG. 1 .  
       FIG. 3  is rear view of a differential cover plate with integral heat pipe assembly according to an alternate embodiment of the present invention.  
       FIG. 4  is a sectional view taken along lines  4 - 4  of  FIG. 3 .  
       FIG. 5  is a sectional view of a heat pipe secured to a differential cover according to an alternate embodiment of the present invention.  
       FIG. 6  is a sectional view of a heat pipe secured to a differential cover according to an alternate embodiment of the present invention.  
       FIG. 7  is a perspective view of a heat pipe secured to a differential cover according to an alternate embodiment of the present invention.  
       FIG. 8  is a front exposed view of the embodiment of  FIG. 7 .  
       FIG. 9  is an enlarged partial exposed view of the embodiment of  FIG. 8 .  
       FIG. 10  is a sectional view of a heat pipe secured to a differential cover according to an alternate embodiment of the present invention.  
       FIG. 11  is a perspective of an installed differential assembly employing the heat pipe of the present invention.  
       FIG. 12  is a front view of a heat pipe secured to a differential housing according to an embodiment of the present invention.  
       FIG. 13  is a side view of the embodiment depicted in  FIG. 12 . 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION  
      The present invention is directed to a heat pipe assembly for cooling lubricating fluid in a differential axle assembly. Preferably an evaporative heat pipe assembly is employed to increase the amount of heat transfer from the lubricant to the external environment. The assembly includes a hermetically sealed heat pipe containing a working fluid which is secured to a removable cover. The working fluid is selected to be vaporizable and condensable within the working temperatures of the differential assembly. The purpose of the heat pipe is to decrease the temperature of the lubricating fluid in the axle assembly. If the temperature of the lubricant gets to high, the lubricant will break down decreasing viscosity and its ability to effectively lubricate the gear assembly. The present invention effectively maintains the fluid lubricant at acceptable operating temperatures.  
      The heat pipe contains a working fluid such as water, or a sodium based fluid or other acceptable working fluids. In liquid form, the working fluid collects at the bottom of the heat pipe due to the force of gravity. The bottom of the heat pipe is submersed in the pool of axle lubricant and causes the working fluid to vaporize (latent heat of vaporization) thus reducing the temperature of the lubricating oil. The vapor then travels up to a condensing portion where the vapor condenses giving up heat to an external environment. The condensed liquid then returns to the evaporative portion where the cycle continues. The arrangement increases the amount of heat that can be transferred from the axle lubricant. The structure of the preferred embodiments of the present invention will no be explained.  
       FIGS. 1-2  depict differential cover plate  1  with integral heat pipe assembly  3 . A hollow loop-shaped pipe  4  forming a continuous closed loop containing the heat transfer fluid  5  is integrally formed with a bolstered portion  7  of the cover  1 . The heat pipe is simply placed within a molding during casting of the cover plate  1 . The integral casting facilitates good contact between the heat pipe  4  and bolstered portion  7  to promote heat transfer there between as well as firmly position the heat pipe. The thick bolstered portion  7  is formed with heat dissipating fins  9  on an external surface to enhance heat transfer. As can be seen a first elongated portion  11  of the heat pipe is disposed within the confines of the cover  1  and consequently within the axle housing. As can be seen in  FIG. 1 , the heat pipe is below the level of the lubricant  13 , which lies just below access bore  14 , and thus the first elongated portion of the heat pipe remains submersed in the axle lubricant during operation of the axle assembly. A second elongated portion  12  remains substantially embedded within the bolstered portion  9 . The working fluid vaporizes in the first elongated portion  11  as it extracts heat from the hot lubricant. The vapor migrates to the second elongated portion where it condenses and gives off heat through the bolstered portion  7  to the external environment. As can be readily seen the integral heat pipe and cover assembly provide a simple solution to increasing heat dissipation-through the axle assembly without requiring modification to the axle assembly or remaining housing. Such an arrangement allows for easy retrofit applications to existing differential assemblies. The arrangement also allows for the heat pipe to be easily assembled with the cover away from the vehicle to which the axle is installed.  
       FIGS. 3-4  represent an alternate embodiment of the present invention. As in the previous embodiment, heat pipe assembly  23  is integrally secured to the cover plate  21 . As in the previous embodiment the heat pipe is preferably cast with the cover plate forming an integral connection between the heat pipe and cover plate. According to this embodiment ( FIGS. 3-4 ), the heat pipe extends through a modified bolstered portion  27  of the cover plate  21 , such that the second elongated portion  31  is disposed external to the cover plate and axle housing. A plurality of heat dissipating fins  28  is secured to the second elongated portion of the heat pipe to promote heat transfer to the external environment. The remaining portions of the embodiment of  FIGS. 3-4  remains nearly identical to that of the previously described embodiments of  FIGS. 1-2 .  
      The heat pipe assembly  3 ,  23  may be filled with a sodium-based working fluid. However, the specific fluid and concentrations of soluble elements may be chosen to maintain a temperature to which the heat is to be transferred which is largely dependent on the desired operating temperature of the lubricating oil such as not to exceed 200 degrees. While this present embodiment does not include a capillary structure or wick, one may be incorporated to the extent it enhances the evaporative cycle. The working fluid must also be chosen to be compatible with the material of the heat pipe to avoid undesirable reactions. For example, the heat pipe  3 ,  23  and cover  1 , may be made of aluminum, copper or stainless steel or other suitable materials.  
       FIGS. 5-6  depict sectional views of additional embodiments of the present invention. Referring to  FIG. 5 , a rear cover  101  is secured to a differential housing containing differential gearing to allow differential rotation between a pair (one shown) of output shafts  205 . A pinion gear  206  drives a ring gear  207  which in turn rotates a differential case containing differential gears (schematically represented by reference numeral  208 ) as is commonly known in the art. The cover  101  is simply bolted to a rear potion of the housing as is conventional in the art. The cover plate  201  preferably contains an integrally formed reservoir  202  in fluid communication with pool of lubricant  213  in the housing  201 . The heat pipe assembly  203  contains a first evaporative end  211  disposed within the pool of lubricant  213  and includes a plurality of heat dissipating fins  211   a  also submersed in the lubricant. The heat pipe extends through the cover plate  1 , reservoir  202 , to the external environment to condensing portion  212  which also includes a plurality of heat dissipating fins  212   a . The heat pipe assembly  203  also preferably includes a wick member  216  disposed within said heat pipe to promote transfer of working fluid from said evaporative heat receiving zone  211  to the condensing heat emitting zone  212  through capillary action. The wick member may be made for example of a metal mesh.  
       FIG. 6  represents a slightly modified version of the embodiment depicted in  FIG. 5 . In this embodiment, the evaporative portion of the heat pipe  213  extends substantially vertical within the reservoir  202  as to substantially horizontal as the heat pipe  211  of the embodiment shown in  FIG. 5 .  
      In the embodiments of  FIGS. 6-9  two heat pipes  203   a ,  203   b , are employed each containing separate evaporative cycles as in the previous embodiments. However, a first set of heat dissipating fins  412   a  interconnects each of the heat pipes in the heat dissipating/condensing zone of the assembly. A second set of heat dissipating fins  411   a  interconnects each of the heat pipes  203   a ,  203   b  in the heat receiving/evaporating zone of the assembly as shown in  FIGS. 8-9 .  
      As previously indicated the specific materials may be selected based on the material specified for the cover plate and operating temperature of the lubricating fluid of the axle assembly. The heat pipe, or envelope may be composed of a durable material able to withstand environmental corrosion while being lightweight. Aluminum, copper and even some types of plastics may be employed. In the embodiments shown in  FIG. 1-4  the rear cover is preferably made of die cast aluminum and the heat pipe envelop made of aluminum with a sodium based working fluid contain therein. In the remaining embodiments metals such as aluminum, stainless steel or copper may be employed. Such metals provide sufficient strength to be able to contain high vapor pressures as higher working temperatures. The envelope may also be somewhat flexible, or bendable such as tygon tubing. The function of the wick is to transport the working fluid from the condenser to the evaporator via capillary action. The shape may be varied in form and can be made of various materials such as a metal screen. However, powdered metals, metal foams, sintered metals, felt type metals as well as some types of plastics or even glass my be employed so long as it is compatible with the working fluid and heat pipe envelope material. A wick may also be omitted forming a thermosyphon as in the embodiments of  FIGS. 1-4 . It is finally noted that certain combinations of working fluid and envelopes which may produce non-condensable gasses should be avoided. These gases may result from a chemical reaction between the envelope and working fluid. For example, an aluminum envelope and water will produce AIO and hydrogen gas. Excessive accumulation of non-condensable gasses will cause the heat pipe to fail. However, it is well within the knowledge of one of ordinarily skill in the art to avoid such unacceptable combinations as such are well documented in the art.  
      While the foregoing invention has been shown and described with reference to a preferred embodiment, it will be understood by those possessing skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. For example additional exemplary embodiments are shown in  FIGS. 1-13  which are also beleive to be within the spirit and scope of the present invention. For example, referring to  FIG. 10 , the heat pipe may extend within the housing beyond the cover plate. The heat pipe may also extend substantially horizontally as it emerges from the cover plate as shown in  FIG. 11 .