Abstract:
A transmission thermostat adapted to control the flow of automatic transmission fluid between a transmission and a transmission oil cooler including a temperature-sensitive mechanism. The temperature-sensitive mechanism includes at lease one bimetallic coil.

Description:
FIELD 
       [0001]    The present invention generally relates to a thermostat and more particularly, to a vehicular thermostat for transmission oil. The present invention employs a bimetal element to cause opening and closing the flow of oil through a heat exchanger rather than a wax motor. 
       BACKGROUND 
       [0002]    The automatic transmission of a motor vehicle requires cooling, which is achieved by flowing the automatic transmission fluid (ATF) through a transmission oil cooler. However, it is desirable to be able to control the flow of oil through said cooler, because cooling the ATF when it is cold enough can be detrimental, causing the oil to become too viscous for the proper operation of the transmission. It is desirable to allow the ATF to flow through the cooler only when cooling is actually required. At all other times the ATF should be recycled back to the transmission without flowing through the oil cooler. The present invention provides a reliable and cost-effective method to automatically bypass the ATF back to the transmission when the ATF does not require cooling. 
       SUMMARY 
       [0003]    Efforts have been made in the past to use a wax element to control the flow of ATF. However, this approach has reliability issues, because the high temperature and the high pressure under which an oil thermostat operates make it prone to failure. Even in more traditional uses, such as in water thermostats for engines, which typically operate a lower temperatures and lower pressures than the transmission, the wax thermostat is a failure-prone component because of the tendency of the wax to leak out of the wax capsule. In an oil application the use of a wax motor is even riskier. 
         [0004]    It is an object of present invention to provide a thermostat that eliminates the wax element failure mode. 
         [0005]    It is another object of present invention to provide a thermostat that is rugged and more reliable than conventional transmission thermostats. 
         [0006]    It is still yet another object of present invention to provide a transmission oil thermostat with all the above benefits. 
         [0007]    According to one particular aspect, the present teachings provide a transmission thermostat for controlling the flow of automatic transmission fluid between a transmission and a transmission oil cooler. The thermostat includes a temperature-sensitive mechanism. The temperature-sensitive mechanism may include at least one bimetallic coil. 
     
    
     
       DRAWINGS 
         [0008]      FIG. 1  is a view of a thermostatic control device constructed in accordance with the present teachings. 
           [0009]      FIG. 2  is a view of another thermostatic control device constructed in accordance with the present teachings, the thermostatic control device including a pressure-relief valve. 
           [0010]      FIG. 3  is a side view of the thermostatic control device of  FIG. 2 . 
           [0011]      FIG. 4  is a cross-sectional view of the thermostatic control device of  FIG. 2 . 
       
    
    
       [0012]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         [0013]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0014]      FIG. 1  shows a preferred embodiment of the invention. The transmission oil thermostat of  FIG. 1  regulates the oil flow between an automatic transmission (TRANS) and the transmission oil cooler (TOC). Oil flows from the automatic transmission to the thermostatic valve, then, depending on the temperature of the incoming oil, the valve either directs the oil to flow to the transmission oil cooler or directly back to the transmission. 
         [0015]    At the heart of the device of  FIG. 1  is a bi-metallic element that reacts to temperature changes of the incoming oil by opening and closing ports. In this embodiment the bimetal element is shaped like a coil. Bimetal coils experience a certain degree of winding or unwinding when subjected to a change in temperature. In other words, there is a relative rotation between the ends of the bimetal coil when the temperature changes. The angle of rotation is proportional to the temperature change and it is very consistent and repeatable. The rotary movement of the bimetal coil is used to open or close the ports of the device. Under cold conditions, the bimetal element senses the temperature of the incoming oil that it is exposed to and closes the ports that communicate with the transmission oil cooler while opening the by-pass ports that directly channel the cold oil back to the transmission. Under hot conditions, the bi-metal element opens the ports that allow flow to the transmission oil cooler while it closes the by-pass ports. 
         [0016]    The bi-metallic oil thermostat is a device that controls and directs the flow of oil from the transmission to either the transmission oil cooler, or back to the transmission when no cooling is required. 
         [0017]      FIG. 1  shows a preferred embodiment of the invention. The transmission oil enters the housing ( 1 ) through port ( 2 ). The oil flows over the bimetal coil ( 3 ) communicating its temperature to it. The bimetal coil is connected to shaft ( 4 ), which is connected to rocker arms  31  and  32 . Sealing plates  7 ,  8 ,  9  and  10  are attached to the ends of the rocker arms. The mission of the sealing plates is to open or close the orifices  21 ,  22 ,  23  and  24  on the capsule ( 6 ) in order to control and route the oil flow. 
         [0018]    Under a cold oil condition, the bimetal coil ( 3 ) rotates the shaft ( 4 ), causing plates ( 7 ) and ( 8 ) to open and plates ( 9 ) and ( 10 ) to close. When plates ( 7 ) and ( 8 ) are open, the oil flows back to the transmission (TRANS) through port ( 11 ) with minimal impedance. Therefore the thermostat is in by-pass mode, preventing the oil to reach the transmission oil cooler (TOC). 
         [0019]    As the oil temperature rises and the hot oil condition is reached, the bimetal coil ( 3 ) rotates the shaft ( 4 ) closing plates ( 7 ) and ( 8 ) while simultaneously opening plates ( 9 ) and ( 10 ). When orifices ( 21 ) and ( 22 ) are open, the oil communicates with port ( 12 ); thus flowing toward the transmission oil cooler (TOC). Oil then circulates through the heat exchanger, and returns to the housing ( 1 ) through port ( 13 ), then continues through port ( 11 ) with minimal impedance back to the pumping source. 
         [0020]    When orifices ( 23 ) and ( 24 ) are closed in a hot condition, no oil is bypassing the cooler circuit. When orifices ( 21 ) and ( 22 ) are closed in a cold condition, no oil is allowed to flow through the cooler circuit. In thermal transition, all plates ( 7 ),( 8 ), ( 9 ) and ( 10 ) are partially open allowing a modulated portion of oil to flow to the cooler circuit via port ( 12 ) and a portion to be bypassed through port ( 11 ). 
         [0021]      FIG. 2  shows a similar embodiment of the invention which also incorporates a pressure relief valve. Under excessive high pressure situations, which can happen when the oil is cold and takes on a gel-like consistency, a pressure relief valve ( 64 ) will open allowing oil to bypass the cooler circuit and flow to port ( 11 ) via relief orifice ( 65 ). 
         [0022]      FIG. 3  shows a side view of the transmission oil thermostat. 
         [0023]      FIG. 4  shows a cross-section of the thermostat with the closed sealing plates  41  and  42  and the open sealing plates  43  and  44 . A change in the temperature of the oil can cause a rotation of the bimetal coil  45 , causing a reversal of the status of the plates. The plates are clocked through the connecting shaft (not shown in  FIG. 4 ) so that either  41  and  42  are open and  43  and  44  closed, or vice versa. 
         [0024]    The plates are designed in a balanced way, so that for instance when the oil pressure tries to close plate  41 , it simultaneously tries to open plate  42 , creating a zero net torque on the connecting shaft. The balanced design is key to this thermostat, because it allows the bimetal coil to control the rotation of the shaft with the relatively small torque exerted by the coil, which would be insufficient to overcome the high oil pressure forces if such a balanced approach was not used. The balanced design cancels the forces created by the oil pressure, allowing the bimetal coil to rule. 
         [0025]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.