Abstract:
An automatic coupling has two parts supported inside one another, and which are rotatable around a common longitudinal axis. The parts form an annular chamber. An annular piston axially divides the annular chamber into two compartments. The annular piston is connected to one of the parts in a rotationally fast and axially movable way and there is formed a shear channel which extends helically relative to the longitudinal axis. The shear channel connects the two compartments separated by the annular piston to one another. The end faces of the annular chamber are formed by the other one of the parts, with the annular piston being able to support itself on the end faces of the annular chamber while generating braking forces.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an automatic coupling comprising two parts which are supported inside one another, which are rotatable around a common longitudinal axis and which form an annular chamber filled with a highly viscous fluid, for the purpose of generating a locking effect between the two parts when the two parts rotate relative to one another. In referring herein to a “locking” effect between the two parts, it is to be appreciated that in practice the effect is one of an inhibition or braking of relative rotation between the parts rather than a complete prevention of relative rotation, and the term “locking” is to be interpreted accordingly. 
     Couplings of this type are known as viscous couplings and described in GB 1 357 106. These couplings are used in combination with differential drives or on their own in the drivelines of motor vehicles. In the former application they generate a differential-speed-dependent locking effect at the axle differentials or in central differentials. In the latter application they serve as so-called visco-transmissions which have the function of a differential-speed-dependent engageable coupling for the second driving axle which is normally the rear axle of motor vehicle with a permanent front wheel drive. 
     Furthermore, a coupling of this type is known from DE 37 25 103 C1 wherein the coupling plates of a viscous coupling operate in a highly viscous fluid and wherein a conveying worm also operating in the highly viscous fluid is intended to reduce the locking effect as a function of the fluid level in the region of the coupling plates in the case of a relative rotation. 
     Furthermore, DE 37 43 434 C2 describes a friction coupling operated by a pressure agent, which is combined with a viscous coupling, with the latter being included in the power flow between a housing and a hub when the friction coupling is loaded, whereas the parts rotate freely relative to one another when the friction coupling is not loaded. 
     Finally, P 43 43 307.3 proposes a Visco-Lok coupling wherein a highly viscous fluid in a chamber—as a result of shear processes in the highly viscous fluid—increases the pressure in such a way that there is displaced a piston which delimits the chamber and which loads a conventional multi-plate friction coupling in the sense of closing same. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a coupling of the initially mentioned type which, in the form of a viscous coupling, even at a small speed differential and after an extremely short reaction time, contributes towards building up a known locking effect and which, at a greater speed differential and thus with increased traction requirements, generates a greater locking effect. 
     The objective is achieved in that, in the annular chamber, there is arranged an annular piston which divides the annular chamber into two compartments and which, in the annular chamber, is connected to one of the parts in a rotationally fast and axially displaceable way by means of driving elements; and which, by means of a cylindrical outer face, closely fits into a cylindrical counter face of the other one of the parts; and that between the outer face and the counter face there is provided at least one shear channel which extends helically relative to the longitudinal axis and which connects the two compartments to one another, which compartments are separated by the annular piston; and that end faces of the annular chamber are formed by the other one of the parts and that the annular piston, by means of end faces, is able to support itself at least indirectly at the end faces of the annular chamber for the purpose of generating braking forces. 
     This coupling embodiment shows that when the annular piston is in a centered position, the coupling, in respect of design and function, corresponds to a viscous coupling and that if the annular piston is in a position of support in one compartment, the coupling becomes a friction coupling whereas, in the other compartment, it continues to operate as a viscous coupling with changed characteristics, with the effect of both couplings being added up. 
     In consequence, there are obtained three major characteristic curve portions which are advantageously adapted to and cover three operating conditions as follows: 
     small speed differential, determined entirely by the characteristics of a viscous coupling, initially with a low locking moment: suitable for tight cornering; avoids wind-up in the driveline. 
     higher speed differential, determined entirely by the characteristics of a viscous coupling, with higher, moderately increasing locking moment; suitable for normal driving conditions; no negative influence on vehicle handling. 
     high speed differential, largely determined by the characteristics of a friction coupling; progressively increasing locking moment as a starting aid in the case of wheel spin. 
     If there exists a relative speed between the two parts rotatable relative to one another, fluid shear takes place in the shear channel, as a result of which the fluid is conveyed from the one compartment into the other compartment, with the piston being axially displaced in the annular chamber. 
     If the annular piston is in a centered position in the annular chamber, the coupling has the locking effect and the advantageous vibration damping effect of a viscous coupling. In the case of a predetermined higher speed differential, the coupling additionally acts as a mechanical friction coupling. It is particularly advantageous that between the housing and hub there are provided only rotating seals and not also axially displaceable seals. 
     According to a preferred embodiment it is proposed that between the annular piston and one of the parts, there are arranged axially effective spring means which axially center the annular piston in the annular chamber and which, with a reproducible reaction time, do not allow the coupling to operate as a friction coupling until there exists a higher predetermined speed differential. 
     In this way it is ensured that the reaction behavior in both directions of relative rotation is always the same due to the annular piston being centered, and that it is not influenced by previous locking processes. When the annular piston is centered, pressure compensation takes place through the helical shear channel. 
     When eliminating such spring means, a similar effect can be achieved by a plurality of shear channels with a steep gradient in respect of the circumferential direction. In this case it is necessary to provide a higher speed differential to build up a piston pressure sufficient for closing the friction coupling. 
     According to a first embodiment it is proposed that the end faces of the annular piston and/or the end faces of the annular chamber are provided with friction linings and are able to contact one another directly. According to a further embodiment it is proposed that between the end faces of the annular piston and the end faces of the annular chamber, there are arranged sets of inner plates and outer plates which, in a rotationally fast and axially movable way, are alternately connected to the one and the other of the parts rotatable relative to one another, and which are able to contact one another directly. 
     By designing the friction linings in the two compartments in different ways or by providing different numbers of coupling plates in the two compartments, it is possible to achieve different characteristics as a function of the direction of relative rotation between the housing and the hub. 
     According to a preferred embodiment, the inner or outer plates directly contacting the end faces of the annular piston are connected in a rotationally fast way to the same one of the rotatable parts as is the annular piston. Any wear at the annular faces of the annular piston is thus avoided. According to a further embodiment it is proposed that the driving elements consist of longitudinal teeth at the one of the rotatable parts and of counter teeth at the annular piston, which teeth engage one another with a clearance fit. This measure ensures that there is neither friction nor wear between the cylindrical outer face of the annular piston and the cylindrical counter face of the housing. This means that the piston and possibly also the component providing the counter face can be made of plastics. 
     According to a preferred embodiment, it is proposed that the driving elements consist of longitudinal teeth at the one of the rotatable parts and of counter teeth at the annular piston, which teeth engage one another with a clearance fit. In this embodiment, the longitudinal teeth can simultaneously cooperate with counter teeth at the inner plates, whereas corresponding longitudinal teeth at the cylindrical part of the other one of the rotatable parts can cooperate with counter teeth at the outer plates. 
     To ensure that the seals have a small diameter, it is preferably proposed that a hub forms one of the rotatable parts to which the annular piston is connected and that a barrel-shaped housing forms the other one of the rotatable parts which forms the end faces of the annual chamber. 
     According to a first further embodiment, the shear channel extending helically relative to the longitudinal axis is provided in the form of a groove in the annular piston. According to a second further embodiment, the shear channel extending helically relative to the longitudinal axis is provided in the form of a groove in the inner face of the housing. In this respect it is particularly advantageous if the counter face at the other one of the rotatable parts, i.e. especially at the housing, is provided in a separately inserted sleeve. In this way it is possible to keep the basic components unchanged while being able to provide different groove shapes in respect of width, depth and gradient, simply by exchanging the sleeve. 
     According to a preferred embodiment, it is proposed that in the annular piston, there is provided at least one compensating chamber which is closed by a displaceable compensating piston. Such compensating chambers are necessary because of the temperature-related viscosity of the fluid and the need for the two compartments to be filled 100%. The compensating chamber which, in principle, can also be accommodated in housing parts is filled with a gaseous medium which, at ambient temperature, can also comprise negative pressure relative to the atmospheric pressure. In this embodiment, supporting springs for the compensating piston are to be provided. 
     The characteristics of the coupling in the first and second operating range are determined by the viscosity of the fluid used and the number and size of the coupling plates. The transition from the viscous coupling characteristics to combined viscous coupling and friction coupling characteristics is determined by the number of shear channels and the gradient of the at least one shear channel, especially when cooperating with the spring means for the purpose of centering the annular piston. The characteristics of the coupling in the third operating range, finally, are determined by the influencing factors already mentioned and also by the friction plates, the friction linings, the coupling plates and the characteristics of the compensating chambers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are illustrated in the drawings and explained in greater detail below. 
     FIG. 1 is half a longitudinal section through an inventive device having a piston which acts directly on end walls of the chamber. 
     FIG. 2 is half a longitudinal section through an inventive device having a piston which acts indirectly by means of friction plates on end walls of the chamber in a first embodiment. 
     FIG. 3 is half a longitudinal section through an inventive device having a piston which acts indirectly by means of friction plates on end walls of the chamber in a second embodiment. 
     FIG. 4 shows the characteristic curve of an inventive coupling with a locking moment T as a function of the speed differential Δn. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a device which, substantially, consists of a hub  11  and a barrel-shaped housing  12 , which two parts, together, form an annular chamber  13  accommodating an annular piston  14 . The hub  11  comprises a set of inner teeth  15  for establishing a connection with a shaft and forms the first of two parts which are rotatable relative to one another. The housing part  12  is welded together out of two cover parts  16 ,  17  and a cylindrical liner  18  and forms the second of two parts which are rotatable relative to one another. The cover parts  16 ,  17  form inner end faces  20 ,  21  of the annular chamber  13  and the cylindrical liner  18 , on its inside, forms a cylindrical counter face  19  for the annular piston  14 . The cover part  17  is provided with a set of circumferential teeth  22  for driving the other one of the two parts  12  rotatable relative to one another. In the cover part  16  there is formed a bore  23  which is closed by a ball  24  and serves to fill the annular chamber with a highly viscous fluid. The hub  11 , furthermore, comprises a set of outer teeth  25  which, in a rotationally fast and longitudinally displaceable way, cooperates with the inner teeth  26  of the annular piston  14 . The hub  11  also comprises a collar  27  on which there are supported plate springs  28 ,  29  which axially center the annular piston  14  in the annular chamber  13 . The annular piston  14  is provided with a securing ring  30  and an inner flange  31  which serve as holding means for the plate springs  28 ,  29 , with the inner teeth  26  being formed on to the inner flange  31 . 
     The annular piston  14  divides the annular chamber  13  into two compartments  32 ,  33  which communicate with one another entirely by means of a spiral-shaped groove  35  which is provided in the cylindrical outer face  34  of the annular piston  34  and which, from the outside, is closed by the cylindrical counter face  19 . The radial end faces  36 ,  37  of the piston  14  are covered by friction linings  38 ,  39  with different annular surfaces. In the annular piston  14  there is provided a compensating chamber  41  which is sealed by means of an axially displaceable compensating piston  42 . 
     The two rotatable parts  11 ,  12  are sealed relative to one another by annular seals  45 ,  46 , with the annular piston  14  being sealed relative to the collar  27  by means of a seal  47 . Finally, the compensating piston  42  is provided with annular seals  48 ,  49  for having a sealing effect relative to the annular piston  14 . 
     If the first and second rotatable parts  11 ,  12  rotate at the same speed, the annular piston  14  is axially centered within the annular chamber  13 . Due to the effect of the plate springs  28 ,  29 , pressure compensation can take place between the compartments  32 ,  33  through the groove  35 . When the two parts  11 ,  12  rotate relative to one another, the fluid shears inside the compartments  32 ,  33  between the respective end faces, as in a standard viscous coupling. Furthermore, fluid is conveyed in the groove  35 , which is due to the fluid shear between the faces of the groove base and the closing inner wall of the annular chamber  13 , which groove base faces and inner wall move relative to one another. A pressure increase in one of the compartments  32 ,  33 , which is sufficient to overcome the spring forces of the springs in the other one of the compartments leads to an axial displacement of the annular piston  14  towards the lower pressure until the respective friction lining  38  or  39  in the compartment with the reduced pressure contacts the respective end face  20  or  21 , so that the solid member friction between the parts rotatable relative to one another, which is aimed at, is effected in the same way as in a friction coupling. With an increasing relative speed and thus an increasing pressure in one of the compartments, the respective fluid shear in this compartment is also intensified. 
     FIGS. 2 and 3 each show a device which, substantially, consists of a hub  51  and a barrel-shaped housing  52 , which two parts, together, form an annular chamber  53  accommodating an annular piston  54 . The hub  51  comprises a set of inner teeth  55  for establishing a connection with a shaft and form the first of two parts which are rotatable relative to one another. The housing part  52  is welded together out of two cover parts  56 ,  57  and a cylindrical liner  58  and forms the second of two parts which are rotatable relative to one another. The cover parts  56 ,  57  form inner end faces  60 ,  61  of the annular chamber  53  and the cylindrical liner  58 , on its inside, forms a cylindrical counter face  59  for the annular piston  54 . The cover part  57  is provided with a set of circumferential teeth  62  for driving the other one of the two parts  52  rotatable relative to one another. 
     In the cover part  56 , there is formed a bore  63  which is closed by a ball  64  and serves to fill the annular chamber with a highly viscous fluid. The hub  51 , furthermore, comprises a set of outer teeth  65  which, in a rotationally fast and longitudinally displaceable way, cooperates with the inner teeth  66  of the annular piston  54 . 
     In the embodiment according to FIG. 2, the hub  51  comprises a collar  67  on which there are supported plate springs  68 ,  69  which axially center the annular piston  54  in the annular chamber  53 . The annular piston  54  is provided with a securing ring  70  and an inner flange  71  which serve as holding means for the plate springs  68 ,  69 , with the inner teeth being formed on to the inner flange  71 . 
     The annular piston  54  divides the annular chamber  53  into two compartments  72 ,  73  which communicate with one another entirely by means of a spiral-shaped groove  75  which is provided in the cylindrical outer face  74  of the annular piston  54  and which, on the outside, is closed by the cylindrical counter face  59 . 
     Between the radial end faces  76 ,  77  of the piston  54  and the inner end faces  60 ,  61 , there are provided inner plates  78  and outer plates  79  which are alternately arranged in the two compartments  72 ,  73 . The inner plates are secured to the outer teeth  65  in a rotationally fast and axially displaceable way, with the outer teeth  65  being interrupted by the collar  67 . The outer plates  79  are secured in inner teeth  90  in the cylindrical liner  58  in a rotationally fast and axially displaceable way, with the inner teeth  92  being interrupted by the cylindrical counter face  59 . In the annular piston  54  there is provided a compensating chamber  81  pointing towards the compartment  72  and sealed by an axially displaceable compensating piston  82 , and a compensating chamber  83  pointing towards the compartment  73  and sealed by an axially displaceable compensating piston  84 . The two rotatable parts  51 ,  52  are sealed relative to one another by annular seals  85 ,  86 . Furthermore, the annular piston  54  is sealed relative to the collar  67  by a seal  87 . Finally, the compensating piston  82  is provided with annular seals  88 ,  89  and the compensating piston  84  with annular seals  90 ,  91 , in both cases for the purpose of providing a sealing effect relative to the annular piston  54 . 
     If the first and the second of the rotatable parts  51 ,  52  rotate at the same speed, the annular piston  54  is axially centered within the annular chamber  53 . Due to the effect of the plate springs  68 ,  69 , pressure compensation can take place between the compartments  72 ,  73  through the groove  75 . When the two parts  51 ,  52  rotate relative to one another, the fluid shears inside the compartments  72 ,  73  between the respective plates, as in a standard viscous coupling. Furthermore, fluid is conveyed in the groove  75 , which is due to the fluid shear between the faces of the groove base and the closing inner wall of the annular chamber  53 , which groove base faces and inner end wall move relative to one another. A pressure increase in one of the compartments  72 ,  73 , which is sufficient to overcome the spring forces of the springs in the other one of the compartments leads to an axial displacement of the annular piston  54  towards the lower pressure until the respective inner plates  78  and outer plates  79  in the compartment with the reduced pressure contact one another and the respective end faces, so that the solid member friction between the parts rotatable relative to one another, which is aimed at, is effected in the same way as in a friction coupling. With an increasing relative speed and thus in increasing pressure in the other one of the compartments  72 ,  73 , the degree of fluid shear also increases in said compartment in which the plates are axially spaced. 
     In the embodiment according to FIG. 3, the annular piston  54  in the annular chamber  53  is designed so as to float freely. A sleeve  93  secured by bolts  94  in the cylindrical liner  58  is inserted into the housing  52 . 
     The annular piston  54  divides the annular chamber  53  into two compartments  72 ,  73  which communicate with one another through a spiral-shaped groove  95  in the sleeve  93 , which groove  95 , on its inside, is sealed by the cylindrical outer surface of the annular piston  54 . Between the radial end faces  76 ,  77  of the piston and the end faces  60 ,  61  of the annular chamber, there are provided inner plates  78  and outer plates  79  which are arranged alternately in the two compartments  72 ,  73 . The inner plates are secured to the outer teeth  65  of the hub  51  in a rotationally fast and axially movable way, with the outer plates  79  being secured in inner teeth  92  in the cylindrical liner  58  in a rotationally fast and axially movable way. In the annular piston  54  there is provided a compensating chamber  81  pointing towards the compartment  72  and sealed by an axially displaceable compensating piston  82 , and a compensating chamber  83  pointing to the compartment  73  and sealed by an axially displaceable compensating piston  84 . The two rotatable parts  51 ,  52  are sealed relative to one another by seals  85 ,  86 . The annular piston  54  is sealed relative to the hub  51  by a seal  87 , with the sleeve  93  being sealed relative to the cylindrical liner  58  by a seal  96 . Finally, the compensating piston  82  is provided with annular seals  88 ,  89  and the compensating piston  84  with annular seals  90 ,  91 , in both cases for the purpose of providing a sealing effect relative to the annular piston  54 . If the first and the second of the rotatable parts  51 ,  52  rotate at the same speed, the annular piston  54  is axially centered in the annular chamber  53 , which is due to the symmetric arrangement of the plates and the open connection between the compartments  72 ,  73 . If the two rotatable parts  51 ,  52 , rotate relative to one another, the fluid shears inside the compartments  72 ,  73  between the respective plates, as in the case of a standard viscous coupling. Furthermore, fluid is conveyed in the groove  95 , which is due to the fluid shear between the faces of the groove base and the closing wall of the annular piston  54 , which groove base faces and wall move relative to one another. A pressure increase in one of the compartments  72 ,  73  leads to an axial displacement of the annular piston  54  towards the lower pressure until the respective inner plates  78  and outer plates  79  in the compartment with reduced pressure contact one another and the respective end faces, so that the solid member friction between the parts rotatable relative to one another, which is aimed at, is effected in the same way as in a friction coupling. With an increasing relative speed and thus an increasing pressure in the other one of the compartments, the degree of fluid shear also increases in said compartment in which the plates are axially spaced. 
     FIG. 4 is a qualitative illustration of the characteristic curve of an inventive coupling for the locking moment (T) as a function of the speed differential (Δn). Three different differential speed ranges are marked, with ranges  1  and  2  being characterized by the function of a viscous coupling (V) and range  3  by the additional function of a friction coupling (V+R). The transition between  2  and  3  is marked by a circle. Range  1  is intended for tight cornering with a low locking moment at low differential speeds and range  2  for standard operating conditions which a locking effect acceptable for handling; range  3  is intended for increased speed range, and traction requirements with a progressively increasing locking effect at high differential speeds. As indicated by a pair of vertical arrows, the function of the viscous coupling can be varied by the number of plates for example. As indicated by a pair of horizontal arrows, the function of the friction coupling can be varied by the spring stiffness for example. Several curves in dashed lines constitute examples. 
     Preferred embodiments have been disclosed. The claims should be studied to determine the true scope and content of this invention.