Abstract:
A ventilation device for transmissions with lubricant comprising water, the device being disposed above the transmission ( 1 ) and being connected thereto by way of a line ( 10 ) and comprising a pressure equalization opening ( 13 ) that leads to the surrounding environment, is to operate without loss of evaporating water to the environment. To this end, the device comprises at least one container ( 17 ) designed as a condenser for evaporated water rising through the line ( 10 ), the container being provided with a cooling element for dissipating heat ( 15 ), the condensate passing from said container ( 17 ) back to the transmission ( 1 ). In one variant, two containers containing a fluid are described which communicate with one another by way of a U-shaped channel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a ventilation device for transmissions with a lubricant containing water, wherein the ventilation device is connected to the transmission housing via a line and has a pressure equalization opening leading to the environment. 
         [0002]    In oil-lubricated transmissions there is a ventilation opening, which allows pressure equalization with the environment when the temperature in the transmission rises or falls due to operating conditions and which is usually equipped as a device with installations for retaining oil mist and for keeping away dirt from the environment. 
         [0003]    WO 2007/098523 A1 discloses a lubricant containing water, which in addition to a high proportion of water contains an agent lowering the freezing point, such as glycol, and further additives and/or suspended matters. Further details as well as the advantages to be obtained with such a lubricant can be gathered from the above-mentioned publication. 
         [0004]    DE 22 20 565 discloses a recirculating cooling system for oil-lubricated reduction transmissions. A fan recirculates air containing oil vapors from inside the transmission through a heat exchanger back into the transmission. The condensed oil is returned separately. There is no pressure equalization with the environment. 
         [0005]    Water has a very low vapor pressure in comparison to lubricating oil, so that it evaporates easily. This leads to the fact that not only the amount of lubricant in the transmission becomes less and less, which soon results in the destruction of the transmission, but also the composition of the lubricant changes because the proportion of water becomes smaller and smaller. 
         [0006]    Thus it is the object underlying the invention to remedy this disadvantage and to counteract the loss of water to the environment. 
       SUMMARY OF THE INVENTION 
       [0007]    According to the invention, this is achieved by the fact that the ventilation device comprises at least one container formed as a condenser to which evaporated water passes through the line, said container being provided for this purpose with means for dissipating heat, wherein the condensate passes from said container back to the transmission. 
         [0008]    When the transmission is heated up due to operating conditions, the air which is present in the lubricant-free space expands and there is increased evaporation, so that a mixture of air and water vapor flows through the line into the evaporating device. In the evaporating device the water vapor is condensed and the air is allowed to escape into the environment. Thus a separation of air and water vapor and a condensation of the water vapor occur simultaneously with the pressure equalization. 
         [0009]    Heat dissipation is required for the condensation, which heat dissipation is achieved in various ways. Either by a group of pipes mounted inside the at least one container and flown through by a suitable cooling agent or by cooling ribs arranged at the outside of the container, or a combination of both. If the transmission is part of a motor vehicle, the cooling ribs should be exposed to the air stream, if possible. This is easily possible due to the fact that the device does not need to be mounted at the transmission itself, but is connected thereto by a line. If the line continuously rises and its cross section is sufficiently dimensioned, it can at the same time serve as a return line for condensed water. In this way, only a single tube is required between the transmission and the device. 
         [0010]    However, the device can also be arranged at the same height as or lower than the transmission. In these cases the line, at least in the container, leads downwards and ends below the level of the condensate. In this way, the condensate can be drawn back in the case of a pressure drop in the transmission. 
         [0011]    In an advantageous further development of the invention, the device consists of two containers arranged essentially at the same geodetic height and containing a liquid, wherein the deepest points of the two containers communicate with one another via a U-shaped duct and wherein the line opens into one of the containers and the other container has the pressure equalizer opening at its top. The two containers are interconnected in the manner of a siphon. When the pressure changes in the transmission, the liquid is displaced in the siphon, wherein the levels of liquid in the two containers move apart from one another. Water vapor condenses in the liquid, wherein a part thereof flows back into the transmission when the level of liquid has increased sufficiently or the pressure in the transmission has dropped. 
         [0012]    Previously to the initial operation, the siphon and the two containers are filled with such a large amount of liquid that their levels in this initial state reaches up to the orifice of the line. Due to this fact returning the condensate into the transmission becomes more even. 
         [0013]    The liquid in the siphon and in the two containers can be water, an agent lowering the freezing point, for example, glycol or a mixture of both. Depending on the conditions and requirements, the one or the other is to be preferred. If no temperatures below the freezing point are to be expected, water is preferable because in this way the composition of the lubricating liquid in the transmission remains unchanged. Otherwise the agent lowering the freezing point is more advantageous. 
         [0014]    In a further development of the invention, the space filled with liquid contains a porous material of high thermal conductivity. This improves heat transfer and condensation and prevents fluctuating of the levels of liquid in the two containers. As the porous material, in particular stainless steel wool has proved its worthiness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1 : schematic representation of the device according to the invention in a first embodiment; 
           [0016]      FIG. 2 : schematic representation of the device according to the invention in a second embodiment, in its initial position; 
           [0017]      FIG. 3  like  FIG. 2 , when the transmission is heated up; 
           [0018]      FIG. 4  like  FIG. 2 , when the transmission is cooled down; 
           [0019]      FIG. 5  schematic representation of the device according to the invention in a third embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In  FIG. 1 , a transmission is designated summarily with reference numeral  1  and a ventilation device according to the invention with reference numeral  11 . As a simplifying example, the transmission  1  herein contains a driving shaft  6  having a gear  5  and a driven shaft  8  having a gear V. Details which are common for transmissions, such as bearing and housing partition, are not elaborated on. In practice, it can be any transmission, such as the shift gearbox or the rear-axle transmission with differential of a motor vehicle. 
         [0021]    The lower part of the transmission contains a lubricating liquid  2  having a level  4 . The gear  7  immerges into the lubricating liquid  2 . The upper region of the transmission  1  has an opening  9 . Otherwise the transmission  1  is closed and thus has no flow connection with the environment. The lubricating liquid contains a considerable amount of water and is described in more detail in WO 2007/098523 A1. A line  10  leads steadily upwards from the opening  9  up to the ventilation device  11  according to the invention. In this case, the ventilation device  11  comprises a container  17 , inside of which there is a cooling pipe coil  15 . It is flown through by a preferably liquid cooling agent. Alternatively or additionally the container  17  can be provided with cooling ribs  16 . The container  17  has an opening  13  at its highest point, which, as a conventional ventilation opening, protects against the intrusion of dirt with a cap  14 . 
         [0022]    By way of this simplest embodiment of  FIG. 1 , the basis of the function can be recognized. The water evaporating due to friction and heating in the transmission  1  (and, if applicable, further liquids present in the lubricant) rises via the line  10  into the ventilation device  11 . There it condenses due to cooling, and the condensate flows back again into the transmission  1 . Since the ventilation device  11  is a separate unit, it can also be arranged at some distance from the transmission, such as at a place where it is exposed to the air stream. In this case the cooling ribs are useful; otherwise dissipation of the condensation heat is primarily effected by the cooling pipe coil  15 . Due to the opening  13  there is no change in pressure in the transmission or in the container  17 . 
         [0023]    In the embodiment of  FIG. 2  the ventilation device is formed by two containers, a first container  21  and a second container  22 , both essentially at the same geodetic height, thus side by side. The line  10  rising from the transmission opens into the first container  21  at a certain height (orifice  23 ). At its highest point the second container  22  has a conventional ventilation opening making the connection with the environment. The lowermost points  25 ,  26  of the two containers  21 ,  22  are interconnected via a U-shaped pipe  27 , so that they form vessels communicating for a liquid. 
         [0024]    The U-shaped pipe  27  and the lowermost regions of the two containers  21 ,  22  are filled with a liquid corresponding to the levels of liquid  30 ,  31 . The spaces  28  and  29 , respectively, on top of it contain air and water vapor. In the initial position shown in  FIG. 2  (previously to the first operation of the transmission), both levels  30 ,  31  are equally high. Herein the level  30  is somewhat below the orifice  23  of the line  10 . The liquid is one of the components of the lubricating liquid or the lubricant itself or a mixture of their components. 
         [0025]    In addition to the filling of liquid, the U-shaped pipe  27  and the lower regions of the two containers  21 ,  22  still contain another filling  32  made of a porous material, which takes up and conducts heat well. Though this filling  32  is to allow the flow of the liquid between the two containers  21 ,  22 , it dampens it. The main function of the filling  32  is to take up and dissipate heat, see further description of function hereinafter. Stainless steel wool has proven to be especially efficient for this purpose, not least due to its good thermal conductivity. The U-shaped pipe  27  can be additionally provided with cooling ribs  33 . 
         [0026]    In order to explain the function, at first the transition from the initial state of  FIG. 2  to the state of  FIG. 3  will be described: During operation the transmission heats up and with it the lubricant, wherein the water contained therein begins to evaporate and the air/vapor mixture expands in the space above the lubricant. This causes the mixture to rise through the duct  10  into the space  28 , in which the mixture at first displaces the liquid present there—causing the level  30  to decrease to  30 ′—and subsequently bubbles through the liquid in the U-shaped pipe  27 . Herein the water vapor condenses—especially fast due to the fact that it touches the filling  32 —, and the air reaches the second container  22 . The air rises therein and exits—just as the air displaced by the risen level  31 ′—through the ventilation opening  34  into the environment. If the liquid in the U-shaped pipe  27  and in the lower regions of the two containers  21 ,  22  contains a substance lowering the freezing point (glycol or another superior-grade alcohol), this substance absorbs the water vapor, wherein its volume is increased, and mixes with the condensate. Thus the condensing effect of this arrangement is a multiple one. 
         [0027]    In order to explain the function, furthermore the transition from the state of  FIG. 3  to the state of  FIG. 4  will be described: when the transmission is stopped, it cools down relatively quickly, and with it the air/vapor mixture present in the space  28  of the first container  21 . Due to this fact a vacuum develops in the space  28 , causing the level  30 ′ in the first container  21  to rise to the level  30 ″ and correspondingly the level  31 ′ to decrease to the level  31 ″. This is not impeded since air is able to flow in through the ventilation opening. However, the flow in the duct  27  is retarded by its filling  32  of steel wool. The rising level  30 ″ in the first container  21  finally reaches the opening  23 , in which the downwards leading line  10  opens into the container  21 . Due to the fact that the volume of the filling of liquid has increased by absorbing water and mixing with water, the additional amount flows back into the transmission  1  through the line  10 . This ensures that amount and mixing ratio of the total amount of liquid do not change. 
         [0028]    The embodiment of  FIG. 5  differs from the one of  FIG. 1  in that the line  110  in the container  117  is led downwards far enough for its open end  123  to immerge into the condensed water  132 . The end  123  lies below the level of liquid  130 , so that, when the transmission is cooled down, condensate is drawn back into the transmission. Due to this fact there is great freedom in arranging the container  117 . It can be placed in the vehicle such that it is exposed to the air stream, even near the ground. 
         [0029]    Within the framework of the invention, the described embodiments can be modified in various ways; in particular, individual features thereof can be combined with one another.