Abstract:
The present invention provides a three position hydraulic piston assembly for a gear ratio change mechanism for a transmission exhibiting reduced gear shift noise. The three position hydraulic piston assembly includes a master piston and motion retarding assemblies that act near both travel limits of the master piston. The master piston includes symmetrical passageways that provide hydraulic fluid flow to small chambers at each end of the piston that are closed off as the piston approaches its travel limits. Hydraulic fluid trapped in the chambers decelerates the piston and is bled off through an orifice allowing the piston to reach its travel limit and quickly and quietly engage a gear ratio.

Description:
FIELD 
     The present disclosure relates to a hydraulic actuator for transmissions for motor vehicles and more particularly to a hydraulic actuator having reduced noise for transmissions for motor vehicles. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
     In dual clutch transmissions (DCT&#39;s) and manual transmission automation (MTA), gear ratio changes are typically accomplished by double acting hydraulic cylinders. One side of a cylinder is pressurized with hydraulic fluid while the other side is exhausted to move a piston and associated shift rail, fork and synchronizer clutch in one direction to engage one gear and the opposite activity engages another gear. The double acting cylinder is configured to also provide a center or neutral position. 
     One of the acknowledged features and benefits of dual clutch transmissions is their ability to rapidly shift gears. Such rapid gear shifts actually are achieved when one of the dual clutches engaging the current gear ratio on one countershaft is released and the other clutch on the other countershaft is engaged, the newly selected gear ratio having been previously pre-engaged by the process described above. 
     Notwithstanding such gear ratio pre-selection and its attendant shift time reduction, there is still a desire and demand to even more rapidly change gear ratios. The speed of such gear ratio changes may be increased by many commonly known approaches: increasing the hydraulic pressure, increasing the flow rate of the hydraulic fluid and increasing the size of the hydraulic cylinder. Unfortunately, all of these approaches carry with them the essential certainty of increased noise of a very noticeable and objectionable type. The noise will not be, for example, a continuous or low frequency sound which might not be noticeable given the other sounds from the vehicle but will be a distinct and abrupt impact or clunk as the piston, shift rail and gear come to a sudden stop upon reaching the limit of travel and engagement. 
     This problem has not gone unrecognized and significant effort has been expended to rectify it. One of the more accepted solutions is referred to as apply pressure profiling. This involves controlling or adjusting the hydraulic pressure applied to a piston and reducing it as the piston approaches its travel limit to slow it in order to minimize the noise generated as it stops. Clearly this solution to gear ratio change noise is a compromise as it results in slower average piston travel and thus slower gear engagement. Furthermore, it significantly increases the complexity of the electronic control and driver circuitry as modulating control of the pressure of the hydraulic fluid must now be provided. Finally, if the travel of the hydraulic piston is very short, there is simply not enough time to achieve effective pressure profiling due to the inertia of the mechanical components of the system. 
     From the foregoing, it is apparent that noise reduction improvements in the art of gear ratio change mechanisms for dual clutch and other transmissions would be desirable and the present invention is so directed. 
     SUMMARY 
     The present invention provides a three position hydraulic piston assembly for a gear ratio change mechanism for a transmission which exhibits reduced gear shift noise. The gear change mechanism includes a synchronizer clutch engaged by a shift fork and rail which, in turn, is acted upon by a three position hydraulic piston assembly having a motion retarding assembly that acts near both travel limits of the hydraulic piston. The hydraulic piston includes symmetrical passageways that provide fluid flow to small chambers at each end of the piston that are closed off by small pistons as the hydraulic piston approaches its travel limits. Fluid trapped in the chambers decelerates the hydraulic piston and is bled off through an orifice allowing the piston to reach its travel limit and quickly and quietly engage a gear ratio. 
     Thus it is an object of the present invention to provide a hydraulic actuator for transmissions having reduced operating noise. 
     It is a further object of the present invention to provide a three position hydraulic actuator for transmissions having reduced operating noise. 
     It is a still further object of the present invention to provide a three position hydraulic actuator having a piston defining passageways and chambers which receive hydraulic fluid which decelerates the piston as it approaches its travel limits. 
     It is a still further object of the present invention to provide a three position hydraulic actuator having reduced operating noise for achieving gear shifts in a vehicle transmission. 
     Further objects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a side elevational view of a portion of a dual clutch transmission incorporating the present invention; 
         FIG. 2  is a top plan view of a hydraulic actuator of a dual clutch transmission incorporating the present invention in a center or neutral position; and 
         FIG. 3  is a top plan view of a hydraulic actuator of a dual clutch transmission incorporating the present invention in which a piston is approaching a gear engaging position. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     With reference now to  FIG. 1 , a portion of a dual clutch transmission is illustrated and generally designated by the reference number  10 . It should be appreciated that while the invention is described and illustrated in conjunction with a dual clutch transmission, the invention has broad application in other types of transmissions such as MTA applications and a broad array of other hydraulically actuated or controlled devices. The transmission  10  includes a housing  12  which surrounds, supports and protects various components such as a counter shaft or layshaft  14  which freely rotatably supports a pair of distinctly sized gears, a first, smaller gear  16  and a second, larger gear  18 . Disposed between the first gear  16  and the second gear  18  on the countershaft or layshaft  14  is a synchronizer clutch assembly  20  having an opposed pair of synchronizers  22  and opposed sets of face clutch or gear teeth  24  which mutually exclusively cooperate with face clutch or gear teeth  26  on the first gear  16  and the second gear  18 . An annular shift collar  30  includes a circumferential channel or groove  32  and a first detent mechanism  34 . The circumferential channel or groove  32  of the shift collar  30  receives a shift fork  36  which is secured to and translates with a shift rail  38 . The shift rail  38  is constrained for axial bidirectional movement in one or more openings or passageways  40  in the housing  12  (or a feature of the housing  12  such as a bracket or extension) and may be detented by one or a pair of second detent mechanisms  42 . Also attached to the shift rail  36  by, for example, cooperating grooves and snap rings  44  is an apply finger  46 . A Belleville or wave washer  48  may also be utilized to ensure a positive though slightly resilient connection between the shift rail  36  and the apply finger  46 . 
     Referring now to  FIGS. 1 and 2 , the apply finger  46  is bi-directionally translated by a three position hydraulic actuator assembly  50 . Specifically, the three position hydraulic actuator assembly  50  includes a housing  52 . For ease of manufacture and assembly, the housing  52  may comprise a cylindrical portion  54  defining a first or left inlet port  56 A and a second or right inlet port  56 B. It should be appreciated that while characterized as “inlet ports,” since that is their primary operational function, because there are no other passageways leading into or out of the housing  52 , the ports  56 A and  56 B also function as outlet or exhaust ports during certain phases of operation. The housing  52  also includes a first or left end plate  58 A and a second or right end plate  58 B. The end plates  58 A and  58 B may be identical and may be secured to the cylindrical portion  54  of the housing  52  by any suitable means such as, for example, threaded fasteners  62 . 
     The cylindrical portion  54  of the housing  52  includes an internal circumferential shoulder  64  that defines a stepped cylinder  66  that slidingly and sealingly receives a master piston  70 . The master piston  70  includes a centrally disposed radial passageway  72  that receives and engages the apply finger  46 . One end of the master piston  70  is stepped and defines an external circumferential shoulder  74 . The smaller diameter region of the master piston  70  adjacent the external circumferential shoulder  74  receives an annular neutral or center position piston  76 . The neutral or center position piston  76  cooperates with the master piston  70  to achieve, with suitable application of pressurized hydraulic fluid through the two inlet ports  56 A and  56 B, three positions of the master piston  70 : a position to the left, adjacent the first or left inlet port  56 A, a center or neutral position as illustrated in  FIG. 2  and a position to the right, adjacent the second or right inlet port  56 B. Inasmuch as those skilled in the art of hydraulic actuators will be familiar with such a configuration and its operation, this aspect of the three position hydraulic actuator assembly  50  will not be further described. 
     The end of the neutral or center position piston  76  proximate the left inlet port  56 A itself includes a first circumferential shoulder  78 A defining a first reduced diameter portion  82 A of the center position piston  76  and the adjacent end of the center position piston  76  includes a first plurality of radially oriented channels  84 A. Both the first reduced diameter portion  82 A and the first plurality of radially oriented channels  84 A facilitate rapid filling of a first or left chamber  86 A of the stepped cylinder  66  notwithstanding the leftmost disposition of the master piston  70  and the center position piston  76  which might otherwise momentarily interfere with fluid flow through the first or left inlet port  56 A and into the first or left chamber  86 A. 
     The end of the master piston  70  adjacent the second or right inlet port  56 B similarly includes a second circumferential shoulder  78 B defining a second reduced diameter portion  82 B of the master piston  70  and the adjacent end of the master piston  70  includes a second plurality of radially oriented channels  84 B. The second reduced diameter portion  82 B and the second plurality of radially oriented channels  84 B function as described directly above to facilitate rapid filling of a second or right chamber  86 B notwithstanding the rightmost disposition of the master piston  70 . 
     The master piston  70  also includes a first longitudinal passageway and port  90 A that provides fluid communication between the first or left chamber  86 A and a first retarding cylinder or chamber  92 A. Slidingly and sealingly received within the first retarding chamber  92 A is a first side pin assembly  100 A. The first side pin assembly  100 A cooperates with the first retarding chamber  92 A and functions as a piston. The first side pin assembly  100 A includes a first hollow cylindrical body  102 A having male threads  104 A on an enlarged portion of the first cylindrical body  102 A which are complementary to female threads  106 B in an opening  108 B in the second or right end plate  58 B. The first hollow cylindrical body  102 A receives a first end plug  110 A defining a first orifice  112 A sized to provide a controlled fluid flow as will be described subsequently. The first end plug  110 A is maintained in its position at the inner end of the first hollow cylindrical body  102 A by a first compression spring  114 A which, in turn, is retained within the first hollow cylindrical body  102 A by a first end cap  116 A which may be press fit into the first hollow cylindrical body  102 A or retained there by a snap ring (not illustrated). One or a plurality of first radial passageways  118 A provide fluid communication between the interior of the first hollow cylindrical body  102 A and the second or right chamber  86 B. 
     The three position hydraulic actuator assembly  50  is essentially symmetrical in both structure and operation. Thus it will be appreciated that the master piston  70  also includes a second longitudinal passageway and port  90 B communicating between the second or right chamber  86 B and a second retarding cylinder or chamber  92 B. Likewise, slidingly and sealingly received within the second retarding chamber  92 B is a second side pin assembly  100 B. The second side pin assembly  100 B cooperates with the second retarding chamber  92 B and functions as a piston. The second side pin assembly  100 B includes a second hollow cylindrical body  102 B having male threads  104 B which are complementary to female threads  106 B in an opening  108 B in the first or left end plate  58 A. The second cylindrical body  102 B receives a second end plug  110 B defining a second orifice  112 B sized to provide a controlled fluid flow as will be described subsequently. The second end plug  110 B is maintained in its position at the inner end of the second cylindrical body  102 B by a second compression spring  114 B which, in turn, is retained within the second cylindrical body  102 B by a second end cap  116 B. One or a plurality of second radial passageways  118 B provide fluid communication between the interior of the first hollow cylindrical body  102 B and the first or left chamber  86 A. 
     As noted above, operation of the three position hydraulic actuator assembly  50  is essentially symmetrical and thus only operation (translation) from its center or neutral position illustrated in  FIG. 2  to a position to the right as illustrated in  FIG. 3  to engage the second gear  18  (illustrated in  FIG. 1 ) will be described, it being understood that translation to the left involves the same operational steps. 
     To translate the master piston  70  to the right to engage the second gear  18 , pressurized hydraulic fluid is supplied to the first or left inlet port  56 A while the second or right inlet port  56 B and the second or right chamber  66 B is exhausted. Pressurized hydraulic fluid in the first or left chamber  66 A commences to translate the master piston  70  to the right in  FIG. 2  and it also flows through the first longitudinal passageway and port  90 A and fills and pressurizes the first retarding chamber  92 A. As the master piston  70  continues to translate to the right, the end of the first cylindrical body  102 A of the first side pin assembly  100 A will close off the first longitudinal passageway and port  90 A. Additional motion of the master piston  70  will increase the pressure of the hydraulic fluid in the first retarding chamber  92 A, thereby beginning to slow the master piston  70 . 
     The volume and thus the pressure of the hydraulic fluid in the first retarding chamber  92 A is controlled by the first orifice  112 A, specifically, its size. The size of the first orifice  112 A is chosen to essentially be a compromise between noise (clunk) reduction and shift speed, that is, a larger first orifice  112 A will allow shifts to be completed more rapidly whereas a smaller first orifice  112 A will result in greater noise reduction. 
     During certain operating conditions, typically at low temperatures, an otherwise desirable size of the first orifice  112 A may not provide sufficient hydraulic fluid flow, pressures may reach a high level and shifts may not be completed in what is considered to be an acceptable time. In such conditions, the hydraulic pressure will compress the first compression spring  114 A and the first end plug  110 A will move off its seat, allowing a rapid flow of hydraulic fluid into the interior of the first cylindrical body  102 A, out the first radial passageways  118 A and into the right chamber  86 B from which it is exhausted through the second or right inlet port  56 B. 
     It will thus be appreciated that the hydraulic actuator assembly  50  according to the present invention provides both rapid and quiet travel of the master piston  70  and gear engagement for a dual clutch transmission, in MTA applications or other transmissions. The actuator assembly  50  achieves this goal without complex electronic controls and modulatable control valves which have been utilized in the past to provide fluid pressure profiling to decelerate the actuator piston as it approaches the ends of its stroke. 
     It should also be appreciated that the hydraulic actuator assembly  50  according to the present invention and the associated shift rail  36 , the shift fork  34  and the synchronizer clutch assembly  20  will typically be utilized in groups of three or four in vehicle transmissions having, for example, five or more forward gears and reverse. 
     Finally, it should also be appreciated that although the hydraulic actuator assembly  50  according to the present invention having reduced operating noise has been described above as a three position (double acting) actuator having a defined center position and two end positions, the noise reduction feature of the present invention is equally suitable for use in a single acting actuator. In this instance, the master piston  70  would require only a single longitudinal passageway and port, for example, the first longitudinal passageway and port  90 A, as well as only one retarding cylinder or chamber, for example, the first retarding chamber  92 A and one slide pin assembly, for example, the first slide pin assembly  100 A. The annular neutral or center position piston  76  can, of course, be eliminated in a single acting device. The foregoing listing is not and is not intended to be exhaustive but rather to present the more important components necessary to achieve noise reduction in a single acting hydraulic piston and cylinder assembly. 
     The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the following claims.