Abstract:
In a hydraulic system with a dog clutch ( 23 - 25 ), a hydraulically operated dog clutch actuator is arranged to alternatively bring the dog clutch into a connect mode or a disconnect mode. The clutch actuator has a piston ( 27 ) reciprocally movable under hydraulic pressure between two end positions, corresponding to the two clutch modes. Pressurized hydraulic oil is supplied to the clutch actuator by a hydraulic power system comprising a pump ( 4 ) driven by an electric motor ( 3 ). Means are provided in the clutch actuator to decrease the hydraulic pressure acting on the piston ( 27 ) at the approaching by the piston of either of its end positions, so that a hydraulic signal is transmitted back to the hydraulic power system.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a hydraulic system with a dog clutch, wherein
       a hydraulically operated dog clutch actuator is arranged to alternatively bring the dog clutch into a connect mode or a disconnect mode,   the clutch actuator has a piston reciprocally movable under hydraulic pressure between two end positions, corresponding to the two clutch modes,   pressurized hydraulic oil is supplied to the clutch actuator by a hydraulic power system comprising a pump driven by an electric motor.       
 
       BACKGROUND OF THE INVENTION 
       [0005]    As is well known in the art, an AWD (All Wheel Drive) vehicle can be provided with at least one hydraulic disc coupling for distributing the driving torque from the vehicle engine to all of the vehicle wheels. Especially, such a coupling may be provided in the drive line between the front axle and the wheels of the rear axle, most often in the vicinity of the rear axle differential. 
         [0006]    Sometimes, it may be desirable to use the AWD vehicle in a FWD (Forward Wheel Drive) mode. In this case the coupling is disconnected, i.e. its discs are separated from each other. 
         [0007]    By operating the vehicle with the coupling in a disconnect mode, rotating masses in the vehicle are reduced, leading to a lower fuel consumption. 
         [0008]    In a practical case a propeller shaft of the vehicle, possibly including one or two bevel gear transmissions, has a hydraulic disc coupling at one end and a simple connect/disconnect clutch, for example a dog clutch, at the other end. In the disconnect mode both these couplings must be disconnected for obtaining the desired effect. 
         [0009]    The present invention relates to a hydraulic system with a dog clutch, having a hydraulically operated dog clutch actuator as set out above. 
         [0010]    A pump actuator system for an AWD hydraulic disc coupling is disclosed in WO 2011/043722. In such a system the supply of hydraulic oil to the coupling piston is governed by the rotative speed of a pump. A similar system may be utilized for the operation of the dog clutch actuator. 
         [0011]    It is of great importance that the disconnect/connect function is performed properly, so that the respective mode is reached as desired. There is thus a need to obtain a check in the form of a confirmation signal that the respective mode has been reached. 
       THE INVENTION 
       [0012]    This is according to the invention accomplished in that means are provided in the clutch actuator to decrease the hydraulic pressure acting on the piston at the approaching by the piston of either of its end positions, so that a hydraulic signal is transmitted back to the hydraulic power system. 
         [0013]    This hydraulic pressure decrease leads to a decreased current level to the motor in the hydraulic system. 
         [0014]    The design of the clutch actuator is preferably such that the piston together with its cylinder sleeve forms two working compartments each with an inlet port and that the piston is provided with two overflow holes each being arranged to get in contact with one of the working compartments, when the piston is in or close to one of its end positions. 
         [0015]    The clutch actuator is preferably arranged near the dog clutch coaxially around an axle to the dog clutch with the tubular piston surrounding the axle. 
         [0016]    The working compartments may be formed by the piston, the cylinder, a central sealing on the piston and two internal sealings in the cylinder sleeve, the latter sealings being positioned close to and axially outside of the inlet ports. 
         [0017]    Preferably, the piston is connected to a clutch ring of the dog clutch by means of longitudinal operating strips in axial grooves in the axle. 
         [0018]    A resilient positioning device may be attached to the piston and have resilient means for positioning engagement with either of two circumferential grooves in the axle. Hereby the piston will be positively held in either of its two end position. Also, the engagement between the resilient means and the respective groove can exert a force on the piston to assume its proper end position, so that the desired signal is transmitted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be described in further detail below under reference to the accompanying drawings, in which 
           [0020]      FIG. 1  shows a hydraulic scheme for a clutch according to the invention, 
           [0021]      FIG. 2  is an isometric view of a clutch and a clutch actuator according to the invention (without a clutch actuator housing), 
           [0022]      FIGS. 3 and 4  are sectional views from the left in  FIGS. 5-8  of the clutch in connect and disconnect mode, respectively, 
           [0023]      FIGS. 5 and 6  are two different longitudinal sections through the clutch (with the clutch actuator) in connect mode, 
           [0024]      FIGS. 7 and 8  are two corresponding, different longitudinal sections through the clutch (with the clutch actuator) in disconnect mode, 
           [0025]      FIGS. 9 and 10  are enlargements of important portions of  FIGS. 5 and 7 , respectively, and 
           [0026]      FIG. 11  is a sectional, schematic illustration of a piston sealing construction in the clutch. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    A drive system of an AWD (All Wheel Drive) vehicle is well known in the art. Typical examples are shown in WO 2011/043722. Such a system has an engine, a front axle with a differential, an intermediate shaft or cardan shaft, and a rear axle with a differential. In order to distribute the torque not only to the front axle but also to the rear axle in accordance with the driving conditions, an electronically controlled wet disc coupling is arranged in the driveline to the rear axle, often in the intermediate shaft or otherwise close to the rear differential. This wet disc coupling is for example described in WO 2012/125096. 
         [0028]    Only as possible examples two embodiments of rear axle architectures for an AWD vehicle are shown in  FIGS. 1 and 2  thereof. 
         [0029]    The function of the coupling when driving the vehicle in an AWD mode is described elsewhere, for example in the mentioned WO 2011/043722. 
         [0030]    When it is desired to drive the AWD vehicle in an FWD (Forward Wheel Drive) mode, the disc coupling is disconnected, i.e. its discs are separated for preventing them from transmitting any torque. The coupling may be said to be in a disconnect mode. For enhancing this separation effect, the oil normally provided in the coupling for lubricating and cooling its discs can be removed from the coupling. In order to reduce the acceleration of the rotating mass of the intermediate propelling shaft and to eliminate the drag torque in bearings and sealings therefor, a clutch, preferably close to the front axle differential, may be provided to bring the intermediate shaft  1  to a standstill in FWD mode of the vehicle. 
         [0031]    The present invention is concerned with such a clutch, which is a hydraulically controlled connect/disconnect dog clutch. 
         [0032]      FIG. 1  shows a hydraulic scheme for controlling the clutch according to the invention. 
         [0033]    The clutch is actuated by means of a piston  1  received in a cylinder  2 . The piston  1  will move to the left or right in the drawing depending on the hydraulic pressure applied to either side of the piston. 
         [0034]    An electric motor  3  drives a pump  4  via a drive shaft  5 . A pressure overflow valve  6  is controlled by a hydraulic pressure in the system and by a spring  8 . 
         [0035]    The motor  3  can be supplied with positive or negative current for rotation in either direction. Depending on the rotation direction—also of the pump  4 —hydraulic oil under pressure will be supplied to either side of the piston  1  by the hydraulic system, now to be described. 
         [0036]    Hydraulic oil for the hydraulic actuator system is contained in a reservoir  8 . If the motor  3  is rotated in a first direction, oil is sucked into the pump  4  through a hydraulic line  9 , provided with a one-way valve  10 , and is delivered therefrom towards the left side of the piston  1  through a hydraulic line  11 . If the motor  3  is rotated in a second direction, oil is sucked into the pump  4  through a hydraulic line  12 , provided with a one-way valve  13 , and is delivered therefrom towards the right side of the piston  1  through a hydraulic line  14 . 
         [0037]    Depending on the balance between the force of the spring  7  and the hydraulic pressure acting on the pressure overflow valve  6 , hydraulic flow for either side of the piston  1  is diverted through hydraulic lines  15 —provided with one-way valves  16 —through the overflow valve  6  and back to the reservoir  8 . The result is that the hydraulic pressure delivered to the cylinder  2  is governed by the spring  7 . 
         [0038]      FIG. 2  is an isometric view of a clutch and a clutch actuator according to the invention. The clutch is arranged between a flange arrangement  20  to be connected to a driving member of the vehicle, in which the clutch is to be arranged, and an axle  21 . The purpose of the clutch is to distinctly connect or disconnect the two members  20  and  21  to and from each other, respectively. The clutch actuator is lacking its housing in  FIG. 2 , which means that oil delivery holes  22  are visible. 
         [0039]      FIGS. 3 and 4  are sectional views through the mechanical dog clutch itself in connect mode and disconnect mode, respectively. The main portions of the dog clutch are an outer ring  23  in the flange arrangement  21 , an inner ring  24  on the axle  21 , and a clutch ring  25  between the two other rings  23 ,  24 . As illustrated, the rings are provided with meshing teeth. 
         [0040]    In  FIG. 3  the clutch ring  25  is arranged between the two rings  23 ,  24 , so that the axle  21  is connected to the flange arrangement  20 , whereas in  FIG. 4  the clutch ring  25  (not visible in  FIG. 4 ) has been brought out of engagement with the two rings  23 ,  24 , so that the axle  21  is disconnected from the flange arrangement  20 . 
         [0041]    The mechanical and hydraulic actuation or operation of the dog clutch will now be described with special reference to  FIGS. 5-10 . For the sake of clarity, reference numerals are only provided in these figures to the extent necessary for a proper understanding of what the respective figure is illustrating. 
         [0042]    The movement of the clutch ring  25  into and out of engagement with the two rings  23 ,  24  is accomplished—as is illustrated in FIGS.  6  and  8 —by means of longitudinal operating strips  26 . As illustrated in  FIG. 4 , the number of strips  26  may be three. Each strip  26  is at one end connected to the clutch ring  25  and at its other end to a piston  27  of a clutch actuator and is arranged in an axial groove  26 ′ in the axle  21 . 
         [0043]    Referring especially to  FIG. 5 , the clutch actuator is contained in a housing  28  and can be connected to a fixed part of the vehicle by means of a mounting flange  29  so as to be non-rotatable. Bearings  30 ,  31  may be provided between the flange arrangement  20  and the axle  21  and between the axle  21  and the clutch actuator housing  28 , respectively. Sealings  32  may be provided between the axle  21  and the clutch actuator housing  28 . 
         [0044]      FIGS. 5 and 6  show the clutch in the connect mode. The clutch ring  25  is in a right hand position in engagement with the outer ring  23  and the inner ring  24 . The piston  27  connected thereto by means of the operating strips  26  is also in its right hand position. 
         [0045]      FIGS. 7 and 8  show the clutch in the disconnect mode. The clutch ring  25  is in a left hand position out of engagement with the two rings  23 ,  24 . The piston  27  is also in its left hand position. 
         [0046]    The construction of the clutch actuator will now be described with reference to  FIGS. 9 and 10 . Shown here are the axle  21 , the clutch actuator piston  27 , and the clutch actuator housing  28 .  FIG. 9  illustrates the connect mode and  FIG. 10  the disconnect mode. 
         [0047]    The tubular piston  27  is attached to the operating strips  26  (which are not visible in  FIGS. 9 and 10 ) by means of spring rings  35 . It is provided with a central sealing ring  36  and at either sides thereof overflow holes  37  for a purpose to be described. 
         [0048]    A support sleeve  38  and a cylinder sleeve  39  are arranged in the housing  28 , sealed in relation thereto by sealings  40 . The cylinder sleeve  39  is provided with two inlet ports  41  and  42  for inlet of hydraulic oil for actuating the piston  27  into the connect mode shown in  FIG. 9  or into the disconnect mode shown in  FIG. 10 . The piston sealing  36  is always between the two inlet ports  41 ,  42  in the reciprocating movement of the piston  27 . 
         [0049]    The cylinder sleeve  39  is outside of each inlet port  41 ,  42  provided with an internal sealing  43 ,  44  in engagement with the piston  27 . The diameter of the piston  27  is somewhat decreased at either side of the piston sealing  27  for creating an active piston area for the supplied hydraulic pressure to act on and thus a pressurized working compartment. 
         [0050]    Referring to  FIG. 9 , the piston  27  moves to the right under the pressure supplied through the inlet port  41  in order to reach the shown connect mode. During this movement the left overflow hole  37  will not be in communication with the working compartment, until it has passed the sealing  43 , when the piston has reached its shown end position with the clutch connected. When the overflow hole  37 , which leads to the housing of the actuator and hereby indirectly to the oil reservoir, gets in communication with the working compartment, the pressure in the latter will drop. The decreased pressure level leads to a decreased current level to the motor  3 , providing a signal or indication that the connect mode has been properly reached and that the dog clutch has not been blocked in a teeth-to-teeth situation. 
         [0051]    A similar reasoning applies to the disconnect mode and the sealing  44  together with the right hand overflow hole  37 . 
         [0052]    The construction of the piston sealings  36 ,  43  and  44  is illustrated in  FIG. 11 . Shown here is specifically the right piston sealing  44  in the position of  FIG. 10  in order to illustrate the cooperation with the right overflow hole  37 . The piston sealing arranged in a circumferential groove in the cylinder sleeve  39  may comprise an O-ring  45  and a packing ring  46  of filled PFTE. When used in sealing constructions, PFTE is often provided with a filler material in order to improve its mechanical strength, stability, and wear resistance. The packing ring  46  may as shown have a flat surface in contact with the piston  27  and have beveled corners. Also the overflow hole  37  may for protective reasons be beveled or rounded at its top opening. 
         [0053]    Referring again to  FIG. 10 , a positioning device  50  is attached to the piston  27  by means of a looking ring  51 . To the left in the drawing the positioning device  50  may be sleeve shaped but towards the right be longitudinally slotted or separated into fingers  52  resiliently biased inwards by means of a spring ring  53 . The fingers  52  are internally provided with knobs for interaction with circumferential grooves  54  in the axle  21 , the left one for keeping the piston  27  in the position for connect mode and the right one for disconnect mode. Also, the interaction between the knobs and the respective groove  54  will assist in moving the piston  27  the last millimeter to its end position, when the respective overflow hole  37  opens. 
         [0054]    Modifications are possible within the scope of the appended claims.