Abstract:
A torque transmitting mechanism has a hydraulically actuated piston that is slidably disposed in a cavity. An apply chamber, formed by the piston and cavity, is filled with hydraulic fluid when the torque transmitting mechanism is to be engaged. One or more gas permeable—liquid impermeable mats are secured in a passage between the chamber and a non-liquid area in a transmission. During initial engagement of the torque transmitting mechanism, any entrapped air is forced out of the chamber through the mat while the leakage of liquid is inhibited. In a rotating torque transmitting mechanism (clutch), the mats are installed near the innermost periphery of the chamber. In a stationary torque transmitting mechanism (brake) the mats are installed near the top outermost periphery of the chamber.

Description:
TECHNICAL FIELD 
     This invention relates to hydraulically actuated pistons for torque transmitting mechanisms and more particularly to such pistons having an air bleed structure. 
     BACKGROUND OF THE INVENTION 
     Torque transmitting mechanisms such as clutches and brakes employ a hydraulically operated piston to enforce frictional engagement between a plurality of friction plates that are alternately splined to a housing containing the piston and a hub member that is generally connected with a gear member. When the torque transmitting mechanisms are inactive during vehicle operation, a cavity or chamber formed between the piston and the housing remains filled with very low pressure oil. However, when the vehicle is stopped and the engine is not operated for a period of time, such as overnight, it is possible for the hydraulic fluid in the cavity to drain to the sump and the cavity becomes filled with air. 
     On a subsequent start-up of the vehicle, the air must be expelled from the cavity before consistent shift quality is attained. This may require five or more shift cycles which are objectionable. With today&#39;s electro-hydraulic controls, consistent hydraulic fluid fill times and volumes are necessary for shift quality consistency. The entrapped air in the cavity prevents the required consistency. The electronic controller of the electro-hydraulic control uses the data from the previous shift to calculate the optimum flow and pressurization rates to be employed during the current shift event. Since air is a compressible medium and hydraulic fluid is a relatively incompressible medium, if air is present in the cavity, it is difficult for the controller to determine the optimum flow and pressurization rates. During the first five or more shift events, the air is slowly bled from the cavity under the piston seals until the cavity is filled with hydraulic fluid only. 
     Some currently available power transmissions incorporating hydraulically operated torque transmitters utilize ball bleed valves and other such devices to provide a controlled passage through which the entrapped air can be evacuated. These devices often allow a significant amount of hydraulic fluid to leak from the cavity after the air bleed function is complete. The repeatability of these devices is inconsistent due to the variation in oil viscosity due to the operating temperature changes in the transmission. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved air bleed structure for the apply chamber of a torque transmitting mechanism. 
     In one aspect of the present invention, a hydrophobic material is secured in fluid communication between the apply chamber of a fluid operated torque transmitting mechanism and the interior of the transmission casing. In another aspect of the present invention, the hydrophobic material is disposed in either the piston or the housing of the torque transmitting mechanism. In yet another aspect of the present invention, a passage is provided between the apply chamber of a torque transmitter and the surrounding environment and a material permeable to vapor but substantially impermeable to liquid is secured in the passage. 
     Fluid operated disc type torque transmitting mechanisms can be either a rotating transmitter, such as a clutch, or a stationary transmitter, such as a brake. The entrapped air in a clutch is forced to the inner radius thereof as the clutch rotates since the heavier hydraulic fluid is centrifuged to the outer radius of the clutch. Therefore in a rotating torque transmitting mechanism, a vapor permeable-liquid impermeable material, such as a woven polytetrafluoroethylene (PTFE) is located in a passage at or near the inner radius of the clutch apply chamber. One such product found to be applicable to this environment is marketed by PALL Specialty Materials under the trademark Hydrolon®. 
     Generally, during the first engagement of the clutch after an idle period, entrapped air will have to be evicted from the clutch apply chamber. The air will be forced radially inwardly by the incoming hydraulic fluid and pass through the PTFE into the interior of the transmission from which it can be vented to atmosphere. 
     In a brake, the PTFE material is located in a passage formed at or near the top of the outer radius of the apply chamber. As the brake is applied, hydraulic fluid will force any entrapped air to the outer periphery of the brake apply chamber. The PTFE material will allow the air to escape to the interior of the transmission housing but prevent the leakage of any significant amount of hydraulic fluid. While the initial engagement of the clutch or brake may be slightly objectionable, after the entrapped air is evacuated, subsequent engagements of the brake or clutch will be within the desired design specifications for performance and pleasability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional elevational view of a torque transmitting mechanism incorporating the present invention. 
     FIG. 2 is an enlarged view of a portion of FIG.  1 . 
     FIG. 3 is a sectional elevational view of nested torque transmitting mechanisms incorporating the present invention. 
     FIG. 4 is a sectional elevational view of a stationary torque transmitting mechanism incorporating the present invention. 
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A rotating torque transmitter or clutch  10 , FIG. 1, which is a member in a power transmission, not shown, has a housing  12  drivingly connected with an input shaft  14  through a spline connection  16 . The housing  12  is rotatably mounted on a support sleeve  18  which may be part of a pump housing, not shown. The housing  12  has an annular cavity  20  in which is slidably disposed a piston  22 . The piston  22  has a seal  24  which sealingly engages an outer periphery  26  of the cavity  20 . An annular seal  28  that is disposed in an inner periphery  30  of the cavity  20  sealingly engages the piston  22 . The piston  22 , cavity  20  and seals  24  and  28  cooperate to form an apply chamber  32  for the clutch  10 . The chamber  32  is in selective hydraulic fluid communication with a conventional pressure source and control, not shown, through a passage  34  in the housing  12  and a passage  36  in the support sleeve  18  when engagement of the clutch  10  is requested. 
     A return spring assembly  38  is disposed between a wall  40  on the piston  22  and a locking ring  42  secured in a groove  44  in the housing  12 . The return spring assembly  38  is effective to urge the piston  22  leftward when the chamber  32  is not pressurized. A conventional ball dump valve mechanism, not shown, may be included to prevent centrifugal drift-on of the clutch  10 . 
     The housing  12  has a spline portion  46  in which is rotatably, drivingly engaged a plurality of friction plates  48  and a reaction plate  50 . The reaction plate  50  is limited in rightward movement by a locking ring  52  secured in a groove  54  formed in the spline portion  46 . Alternatingly spaced with the friction plates  48  is a plurality of friction plates  56  that are rotatably, drivingly connected with a spline  58  formed on an output hub  60 . The output hub  60  is drivingly connected with a gear or other transmission member, not shown, in a conventional manner. When the apply chamber  32  is pressurized, the piston  22  will move rightward to enforce frictional engagement between the friction plates  48  and the friction plates  56  to provide a torque transmitting path between the shaft  14  and the hub  60 . 
     The housing  12  has an opening or passage  62 , FIG. 2, which communicates between the apply chamber  32  and an interior portion  64  of the transmission. The opening  62  has a first bore portion  66  and a second bore portion  68  which is larger in diameter than the first bore  66  such that an annular shoulder  70  is formed in the opening  62 . The second bore  68  has a threaded outer surface  72 . A gas permeable—liquid impermeable mat  74  is disposed in the second bore  68  and held against the shoulder  70  by an annular plug  76  that is threaded into the bore  68  against a permeable or porous metal support  77 . The mat  74  is preferably a polytetrafluoroethylene (PTFE) material with a polyester cloth bonded to the surface adjacent the shoulder  70 . One such material is marketed as Hydrolon® PTFE by Pall Specialty Materials. While only one passage  62  and mat  74  are shown, a plurality of such assemblies can be spaced about the inner periphery of the housing  12 . 
     When the transmission is inactive for extended periods, such as overnight, the residual hydraulic fluid in the chamber  32  can drain to the transmission sump and be replaced by air. When the vehicle is placed in operation following the inactive period, the actuation of the clutch will be less pleasable than expected by the operator until the air in the chamber  32  is expelled. During the first engagement of the clutch  10 , the rotation of the clutch housing  12  will force the hydraulic fluid entering the chamber  32  radially outwardly thereby forcing the entrapped air inwardly. The mat  74  will permit the entrapped air to escape to the interior portion  64 . If the clutch is engaged for an extended period, after the initial engagement, the air will be expelled during the first engagement. If a ratio change occurs in the transmission prior to all of the air being expelled, two or more cycles may be needed, however this will be less than the currently available systems require. 
     In FIG. 3 a pair of nested rotating torque transmitting mechanisms or clutches  80  and  82  are shown. The clutch  82  is radially inward of the clutch  80 . The clutch  80  has a housing  84  which is rotatably mointed on a sleeve support  86 . The housing  84  is drivingly connected with an input shaft  88  through a spline connection  90 . The input shaft  88  also has a spline portion  92  that drivingly engages a plurality of friction plates  94  and a reaction plate  96  that are slidably disposed thereon. The housing  84  has an inner housing  98  secured therein. A piston  100  is slidably disposed in the housing  98 . The piston  100  and the housing  98  cooperate to form an apply chamber  102  which is in hydraulic fluid communication with a conventional transmission control through passages  104  and  106 . 
     The clutch  82  includes an output hub  108  with a plurality of friction plates  110  slidably disposed thereon in alternating arrangement with the friction plates  94 . A centrifugal balance chamber  112  is formed by a dam  114 , the housing  98  and a wall  116  of the piston  100 . A return spring assembly  117  is disposed between a locking ring  118 , secured to the housing  84 , and the piston  100  to urge the piston  100  to a disengaged position. Hydraulic fluid pressure in the chamber  102  will cause the piston to move rightward thereby enforcing engagement between the friction plates  94  and  110  to complete a torque transmitting connection between the input shaft  88  and the output hub  108 . 
     The housing  98  has a first bore  120  and a second bore  122  that provide communication between the chamber  102  and a low pressure space  124  between the housing  98  and a piston  126  of the clutch  80 . A gas permeable—liquid impermeable mat  128  is secured in the bore  120  by a threaded member  130  and a porous metal backing member  131 . The mat  128  is constructed similar to the mat  74  to permit the expulsion of entrapped gas from the chamber  102 . 
     The piston  126  of the clutch  80  is slidably disposed in the housing  84 . A pair of annular seals  132 ,  134  cooperate with the piston  126  and the housing  84  to form an apply chamber  136  which is in hydraulic fluid communication with the transmission control thorough a passage  138 . The housing  84  has a spline portion  140  in which is slidably disposed a plurality of friction plates  142  and a combination backing/transfer plate  144 . The plate  144  is secured in the housing  84  by a locking ring  146 . The plate  144  has a spline  148  adapted to connect with further input members downstream of the clutch  80 . A plurality of friction plates  150  are slidably disposed on and drivingly connected with a spline  152  formed on an output hub  154  which is connected with other transmission members such as gears, not shown. 
     The housing  84  has a first bore  156  and a second bore  158  that are interconnected to provide communication between the chamber  136  and a low pressure space  160 . A gas permeable—liquid impermeable mat  162  is secured in the bore  158  by a threaded member  164  and a porous metal member  165 . The construction of the bore  158 , threaded member  164 , porous metal member and mat  162  are similar to the structure of the components described in FIGS. 1 and 2. The mat  162  will permit the escape of entrapped air from the chamber  136  while significantly restricting the leakage of hydraulic fluid. As with the clutch described in FIG. 1, the air will be expelled during the first engagement of the clutches  80  and  82  unless a ratio change is incurred shortly after the initial engagement such that sufficient time is not available to completely exhaust the air from the chambers  102  and  136 . However, on the subsequent engagement of the clutch  80  or  82 , the air will generally be fully exhausted. 
     The clutch  80  also has a centrifugal pressure balance chamber  166  formed between the housing  84  and the piston  126 . The chamber is formed by an annular dam  168  and an annular end wall  170  of the piston  126 . The dam  168  is secured in the housing  84  by a locking ring  172 . The balance chamber  166  is supplied with fluid from the chamber  136  through a passage  174 . The fluid in the balance chamber  166  counteracts the centrifugal forces in the chamber  136  whenever hydraulic fluid is present in the chamber  136 . The balance chamber  112  is supplied with hydraulic fluid from the lube circuit through a passage  176 . The hydraulic fluid in the balance chamber  112  will balance the centrifugal forces generated in the chamber  102 . The chamber  102  may also be provided with a conventional ball dump valve to assist in counteracting the centrifugal forces when the clutch  82  is disengaged. 
     A stationary torque transmitting mechanism or brake  178  is shown in FIG.  4 . The brake  178  has a piston  180  slidably disposed in an annular cavity  182  formed in a transmission housing  184 . The piston  180  has assembled therewith a pair of annular seals  186  and  188  that cooperate with the cavity  182  to form an annular apply chamber  190 . A return spring assembly  192  is placed between the housing  184  and the piston  180  to urge the piston  180  leftward, as viewed in FIG. 4, to a disengaged position. Fluid pressure is supplied to the chamber  190  to urge the piston  180  rightward toward an engaged position. A plurality of friction plates  194  and a backing plate  196  are connected with the housing  184  through a spline  198 . The backing plate  196  is restrained from rightward movement by a locking ring  200 . A plurality of friction plates  202  are alternated with the friction plates  194  and connected with an output hub  204  through a spline  206 . The output hub  204  is connected with a gear member, not shown, to restrain rotation thereof when required by the operation of the transmission. 
     The housing  184  has a stepped bore  208  comprised of a large diameter portion  210  and a small diameter portion  212  formed near the top thereof. A PTFE mat  214  is secured in the large diameter portion by a threaded fastener  216  and porous metal backing member  218  in a manner similar to that shown in FIG.  2 . The small diameter portion  212  communicates with the apply chamber  190  near the top outer periphery thereof. The large diameter portion  210  communicates with the interior of the housing  184  to provide a gas permeable—liquid impermeable passage between the apply chamber  190  and the interior of the housing  184 . When the chamber  190  is initially pressurized after an extended period of vehicle shut down, air, which has collected in the chamber  190 , will be forced by the incoming hydraulic fluid to the top outer periphery of the chamber  190  and exhausted through the mat  214  to the interior of the housing  184 . If the brake  178  is engaged for an extended period during the initial engagement, all of the air will be exhausted during this period, however is the engagement period is short, during a quick ratio change, it may require two or perhaps three engagements of the brake  178  to fully exhaust the air from the chamber  190 . 
     While the above exemplary embodiments depict one vent opening per cavity, those skilled in the art will recognize that a plurality of such vents can be used in each cavity.