Abstract:
A transmission parking lock device having a gearwheel with teeth and gaps and a pawl having a pawl tooth. The gearwheel rotates and the pawl pivots about axes that are parallel to each other. The gearwheel teeth each have a first contour and tooth flanks with respect to rotation of the gearwheel. The pawl tooth has a second contour and tooth flanks relative to the rotation of the gearwheel, and the gearwheel has an outer radius. The gearwheel and pawl are kinematically linked by a linear relationship between the pivoting and rotational angles. The first and/or second contour are at least partially circular with a radius of the outer radius of the gearwheel, and the teeth and/or the pawl tooth have, in the area of the front tooth flank of the gearwheel or parking lock pawl, a crown tip relief relative to the outer radius of the gearwheel.

Description:
This application claims priority from German patent application serial no. 10 2010 042 917.1 filed Oct. 26, 2010. 
     FIELD OF THE INVENTION 
     The invention concerns a parking lock device for an automotive transmission. 
     BACKGROUND OF THE INVENTION 
     Parking locks, which are designed to prevent a motor vehicle with an automatic transmission from inadvertently rolling away, are subject to various criteria, among which in particular a rejection condition at higher vehicle speeds must be fulfilled. In general a parking lock is designed for a particular drop-in speed, i.e. a limit speed of the vehicle at which the locking pawl can still just drop into a tooth gap so that the locked position is reached. Above this limit speed it must be ensured that the pawl tooth does not drop into a tooth gap, but is kept out by virtue of the teeth or tooth contour. The rejection of the locking pawl above the drop-in speed is known as “ratcheting” and, depending on the tooth geometry, results in a dynamic that damages the components. It has therefore already been proposed that by virtue of a suitable tooth carrier, the rejection of the locking pawl takes place as gently as possible. 
     EP 1 895 206 B1 discloses a parking lock device for a motor vehicle transmission, which comprises a parking lock gearwheel with circumferential teeth and a parking lock pawl with a pawl tooth or catch which engages in a tooth gap of the parking lock gearwheel to lock a transmission output shaft. At the crown of the pawl tooth or catch as viewed in the rotation direction of the parking lock gearwheel, the pawl tooth or catch has a rounded tooth contour which follows a defined logarithmic function or a defined exponential function. The shape of the logarithmic or exponential function is characterized by varying curvature, such that the curvature is most pronounced in the area of the forward tooth edge and least so in the area of its rear edge. Thus, as viewed in the rotational direction of the parking lock gearwheel the curvature of this tooth contour decreases with the width of the tooth. Due to this rounding of the tooth, above the drop-in speed and up to a maximum speed of the vehicle the locking pawl should be rejected with a low impulse magnitude, in particular with an approximately constant magnitude. Disadvantageously, however, the impulse exerted on the parking lock pawl during ratcheting depends very much on the exact position of the contact point at which the pawl tooth of the parking lock pawl meets a tooth of the parking lock gearwheel. The resulting scatter can be perceived by the driver of the vehicle as annoying. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention, with a parking lock device of the type mentioned at the beginning, is to achieve as gentle and material-sparing rejection as possible of the locking pawl above the drop-in speed and up to higher vehicle speeds, and thus to reduce the impact loading of the locking pawl that takes place during ratcheting. 
     The invention starts from a parking lock arrangement known from the prior art, with a parking lock gearwheel having teeth and tooth gaps and with a parking lock pawl having a pawl tooth, which for the purpose of arresting the parking lock gearwheel can pivot so as to engage in one of the tooth gaps of the parking lock gearwheel. In this arrangement the parking lock gearwheel rotates about its rotational axis and the parking lock pawl pivots about its pivot axis, which is arranged parallel to the rotational axis of the parking lock gearwheel. In a known manner the teeth of the parking lock gearwheel each have at the crown a first contour and, viewed in the rotational direction of the parking lock gearwheel, respective front and rear tooth flanks, whereas the pawl tooth of the parking lock pawl has at its crown a second contour and, relative to the rotational direction of the parking lock gearwheel, a front and a rear tooth flank. 
     The invention provides for a linear relationship between the pivot angle of the parking lock pawl and the rotational angle of the parking lock gearwheel, so that the pivoting speed of the parking lock pawl is constant. By contrast with the type-defining prior art, during ratcheting an impulse exerted on the parking lock pawl in the contact area of the crown contours of the pawl tooth and the tooth crown of the parking lock gearwheel is consequently independent of the exact point of contact. In this case the value of the pivoting speed, i.e. the change of pivoting angle, is a measure of the impulse. Thanks to the constant impulse on the parking lock pawl, the impact response during ratcheting, i.e. the loading of the components, is also reduced. 
     To approximate this linear kinematic behavior the invention provides that the first contour formed on the crown of the parking lock gearwheel&#39;s teeth and/or the second contour formed on the pawl tooth of the parking lock pawl conform at least partially with a mathematical function whose curvature differs by no more than 20% from the outer curvature of the parking lock gearwheel. In this case, in the area of the front tooth flanks of the parking lock gearwheel and/or the parking lock pawl, the teeth of the parking lock gearwheel and/or the pawl tooth of the parking lock pawl show a defined tip relief of the crown relative to the outer radius of the parking lock gearwheel. The outer curvature of the parking lock gearwheel is defined as the reciprocal value of the outer radius of the parking lock gearwheel. In the ideal case this geometry results in a linear relationship between the pivoting angle of the parking lock pawl and the rotational angle of the parking lock gearwheel and hence in a constant pivoting speed of the parking lock pawl, so that the impulse exerted on the parking lock pawl when the crown contours of the pawl tooth and the tooth crown of the parking lock gearwheel meet, does not depend on the exact point of contact. 
     Those with an understanding of the subject will be able to choose a suitable mathematical function that represents this (ideal) kinematic behavior sufficiently well for the accuracy demanded for the purposes of the application concerned, if necessary also taking into account relevant aspects of production technology. 
     A first example of such a mathematical function is a circle whose radius is no more than 20% different from the outer radius of the parking lock gearwheel. A second example of such a mathematical function is an ellipse whose semi-axes differ by no more than 20% from the outer radius of the parking lock gearwheel. The desired linear relationship between the pivoting angle of the parking lock pawl and the rotational angle of the parking lock gearwheel cannot be produced with sufficient accuracy either by the logarithmic function, or by the exponential function known from the prior art. 
     To approximate the linear kinematic behavior in design terms the invention provides that the contour of the teeth of the parking lock gearwheel (“first contour”) and/or the contour of the pawl tooth (“second contour”) are formed as a circular arc located in the area of the outer radius of the parking lock gearwheel. Thus, either the crown of the teeth of the parking lock gearwheel and/or the crown of the pawl tooth of the parking lock pawl are—at least partially—in the shape of circular-cylindrical surfaces, i.e. surfaces with a constant curvature. Furthermore, in the area of the leading tooth flank in the rotational direction a maximum crown tip relief on the teeth and/or on the pawl tooth is provided. Advantageously, the result of this crown tip relief is that above the drop-in speed the pawl tooth does not engage, or not too deeply, in a tooth gap of the parking lock gearwheel, which could otherwise lead to damage or failure. According to the invention, the crown tip relief varies in the direction of the rear tooth flank in accordance with a circular function. By virtue of this circular function, i.e. due to the constant curvature, in the contact area of the crown contours the pivoting angle of the parking lock pawl increases approximately proportionally to the rotational angle of the parking lock gearwheel. 
     In this case, in relation to the rotational axis, ideally the first contour is convex and the second contour of concave shape. As a variant of this, the convex contour of the teeth of the parking lock gearwheel can also co-operate with a flat or non-concave contour of the pawl tooth. Another variant can even provide that the pawl tooth is formed with a concave tooth contour which co-operates with a parking lock gearwheel having tooth contours without any crown tip relief, i.e. a cylindrical outer contour. 
     In a further preferred embodiment the crown tip relief is composed of a fraction for the wheel and a fraction for the pawl, such that the fractions preferably behave as the tooth widths of the wheel teeth and the pawl tooth. In this way the linear kinematic behavior is maintained throughout the contact area during the contact of the teeth, i.e. with a constant impulse and reduced dynamic loading of the parking lock. 
     According to a further embodiment, in the area of the forward and rear tooth flanks additional crown tip reliefs are provided, which decrease in accordance with a circular function in the direction toward the middle of the tooth. Thus, the tooth contour is formed by two circular arc sections with different mid-points. This embodiment is particularly advantageous for driving forward and in reverse, i.e. ratcheting in both rotational directions of the parking lock gearwheel, because the parking lock pawl is safely rejected in both rotational directions, and this, by virtue of the constant curvature of the tooth contour with a constant impulse. In relation to the tooth width the two circular arc sections can have different lengths, for example in the ratio 1:2 or 1:3. This division is provided having regard to the different forward and reverse driving speeds. 
     According to a further embodiment the radius of the circular arc forming the first and/or the second contour is in a range from 0.8 up to 1.2 times the outer radius of the parking lock gearwheel. In particular, the radius of the circular arc contour is equal to the outer radius of the parking lock gearwheel, i.e. the mid-point of the contour circle is in the area of and offset relative to the rotational axis of the parking lock gearwheel. A curvature radius for the first and/or second contour with this order of magnitude results in a relatively small, constant impulse, i.e. reduced dynamic loading of the parking lock. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of the invention are illustrated in the drawing and will be described in more detail below, so that further features and/or advantages can emerge from the description and/or the drawing, which shows: 
         FIGS. 1   a ,  1   b : Two engagement situations of a parking lock pawl and a parking lock gearwheel; 
         FIG. 2 : A graphical representation of a function of the pivoting angle of the pawl against the rotation angle of the gearwheel of a parking lock device of the prior art; 
         FIGS. 2   a ,  2   b ,  2   c : Various impact situations of the parking lock pawl on the parking lock gearwheel; 
         FIG. 3 : Selected functions of the pivoting angle of the pawl against the rotation angle of the gearwheel; 
         FIGS. 4   a ,  4   a   1 ,  4   b ,  4   b   1 ,  4   c ,  4   c   1 : Three contact situations of the parking lock gearwheel and the parking lock pawl, which are kinematically linked by a linear function of the pivoting angle and the rotation angle; 
         FIGS. 5   a ,  5   a   1 ,  5   b ,  5   b   1 ,  5   c ,  5   c   1 : Another example embodiment of the invention, with crown tip reliefs on both sides and linear kinematics; 
         FIGS. 6   a ,  6   a   1 ,  6   b ,  6   b   1 ,  6   c ,  6   c   1 : A further example embodiment of the invention, with an optimized wheel tooth contour and the corresponding kinematics; 
         FIGS. 7   a ,  7   a   1 ,  7   b ,  7   b   1 ,  7   c ,  7   c   1 : A further example embodiment of the invention, with an optimized pawl tooth contour and the corresponding kinematics; 
         FIG. 8 : A schematic representation of a wheel tooth with tooth contour according to the invention; and 
         FIG. 9 : A schematic representation of a pawl tooth with tooth contour according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1   a  and  1   b  show two positions of contact between a parking lock gearwheel  1  and a parking lock pawl  2  of a parking lock device (not shown in full), which can be used for example for an automatic transmission or even a dual-clutch transmission of a motor vehicle. As is known, the parking lock device serves to lock a transmission output shaft (not shown) mechanically, in order to prevent the vehicle from rolling away. The parking lock gearwheel  1  is connected to the transmission output shaft (not shown) in a rotationally fixed manner and undergoes rotational movement in the direction of the arrow D with a rotational angle φ and a rotational speed φ-dot (φ). The parking lock pawl  2  is mounted to pivot in a transmission housing (not shown) and, when the two tooth crowns rest against one another, undergoes a pivoting movement in the direction of the arrow P with the pivoting angle ω and the pivoting speed ω-dot (ω). The parking lock gearwheel  1 , also referred to in what follows as just the wheel  1  for the sake of brevity, has on its circumference a tooth array with teeth  3  and tooth gaps  4 , whereas the parking lock pawl  2 , or just pawl  2  for brevity in what follows, has a pawl tooth  5  which engages in a tooth gap  4  in order to block rotation of parking lock gearwheel  1 .  FIG. 1   a  shows a first position in which the tooth  3 , also called the wheel tooth  3 , which rotates counter-clockwise, makes contact with the pawl tooth  5  at a point A. Thus, the point A marks the beginning of a contact zone between the wheel tooth  3  and the pawl tooth  5 .  FIG. 1   b  shows a second position in which a point B marks the end of the contact zone between the wheel tooth  3  and the pawl tooth  5 . 
     A parking lock device is generally designed for a particular drop-in speed, i.e. a limit speed of the vehicle at which the pawl tooth  5  can just still drop into the tooth gap  4  and so produce a blocking position. However, a parking lock must also fulfill a so-termed rejection condition, i.e. if at a vehicle speed above the drop-in speed, the pawl is engaged—be it mistakenly or due to a defect—the pawl tooth  5  must not fall into the tooth gap  4 , since this would result in damage or failure. Rather, by virtue of suitable tooth geometry the pawl tooth  5  must be rejected, this generally taking place by a crown tip relief for example on the pawl tooth. This process of continual rejection at speeds above the drop-in speed is called ratcheting, and is generally associated with particular dynamic behavior, i.e. the locking pawl  2 , is continually accelerated and decelerated in the pivoting direction indicated by the arrow P and in the opposite direction. Here, this is also referred to as an oscillating pivoting motion. The impact loading that occurs during ratcheting will be examined and explained in more detail with reference to the diagrams that follow. 
       FIG. 2  shows the results of a study by the present applicant, namely the relationship between the pivoting angle ω of the locking pawl  2  and the rotational angle φ of the parking lock gearwheel  1 . The points A, B of the function ω=f(φ) correspond to the contact points A, B in  FIGS. 1   a  and  1   b . The shape of the function ω=f(φ) is characterized by a variety of slopes: at first the curve rises smoothly, then it enters a steeper zone and finally, at the point B, this becomes somewhat less steep again. The pivoting angle ω at the point B, i.e. at the end of the contact stretch, can also be referred to as the radial stroke of the pawl tooth  5  and corresponds to a crown tip relief δ at the wheel tooth  3  and the pawl tooth  5 . 
       FIGS. 2   a ,  2   b ,  2   c  show various impact situations of the pawl  2  on the parking lock gearwheel  1 , i.e. of the pawl tooth  5  on one of the wheel teeth  3 .  FIG. 2   a  shows an impact at a point A 1 , i.e. in an area where the slope of the curve is relatively shallow. A shallow slope in this case means that the pivoting angle ω of the pawl changes relatively slowly, i.e. the pivoting speed ω-dot is relatively small—so the pawl is subjected to a relatively slight (rotational) impulse.  FIG. 2   b  shows an impact of the pawl at a point A 2 , i.e. in an area where the slope is steeper than at point A 1 . This means that the pawl experiences a stronger impulse. The impulse is physically defined as the product of mass and speed, and in this case the speed is the pivoting speed ω-dot of the locking pawl. Finally,  FIG. 2   c  shows an arbitrary impact point A n . As mentioned, the curve shape shown in  FIG. 2  is based on contours of the wheel tooth and the pawl tooth according to the prior art. From this the inventors derived the knowledge that depending on the point of impact the locking pawl experiences different impulses, leading to the dynamics mentioned earlier. Accordingly, the inventors recognized that there is a direct relationship between the kinematic relation of the pawl and wheel in tooth contact, and the dynamics of the pawl&#39;s movement during ratcheting. 
       FIG. 3  shows various notional functions for the pivoting angle ω of the pawl, plotted against the rotation angle φ of the wheel, taking the above-mentioned knowledge into account. As examples three functions f 1 , f 2 , f 3  are represented, such that f 1  shows a progressive course, f 2  a degressive course and f 3  a linear course from the beginning up to the end of the contact between the teeth. For the degressive function f 2  a line g shows the gradient, i.e. the slope at a given point, which corresponds to the pivoting speed ω-dot (ω). The respective variable slope of the functions f 1 , f 2  brings about the undesired effect that the impact loading of the parking lock depends on the precise location of the tooth contact. In contrast, the linear course of the function f 3  with a constant slope leads to an impulse on the pawl which is at least to a large extent independent of the precise location of the tooth contact. 
       FIGS. 4   a   1 ,  4   b   1 ,  4   c   1  show on the one hand three positions for a tooth contact between the parking lock gearwheel  1  and the pawl  2 , the wheel tooth  3  and the pawl tooth  5  being designed in such manner that as shown in  FIGS. 4   a ,  4   b ,  4   c  a linear course is obtained for the function ω=f(φ). Regardless of whether the pawl tooth  5  encounters the wheel tooth  3  at the position as shown in the adjacent  FIGS. 4   a   1 ,  4   b   1 ,  4   c   1  there is in each case a uniformly increasing pivoting angle ω, i.e. a constant pivoting speed ω-dot (ω) and therefore a constant impulse is transmitted from the wheel  1  to the pawl  2 . Thereby, the dynamic during the process of ratcheting is considerably attenuated. 
       FIGS. 5   a   1 ,  5   b   1 ,  5   c   1  show a further development of the invention, such that a crown tip relief is formed on the forward and also on the rear tooth flank. The result of this is that the pawl  2  is rejected both when the parking lock gearwheel  1  is rotating forward and when it is rotating backward. The diagrams of to  FIGS. 5   a  to  5   c  show in each case on the left a linearly rising branch of the curve and on the right a linearly falling curve branch, indexed with the letters V, R. In the diagrams shown there is a symmetrical division for forward and backward ratcheting. However, the division can also be unsymmetrical, for example 1:2 or 2:3. This allows for the different speeds when driving forward or in reverse. 
       FIGS. 6   a   1 ,  6   b   1 ,  6   c   1  show another embodiment of the invention, such that only one of the two components involved, namely the contour of the wheel tooth  3 , is optimized in such manner that the functional shape shown in the  FIGS. 6   a   1 ,  6   b   1 ,  6   c   1  is obtained. The contour of the wheel tooth  3  is designed such that for the contact zone from  FIG. 6   a   1  to  FIG. 6   b   1  a linear course L of the function ω=f(φ) is obtained ( FIGS. 6   a ,  6   b ). In contrast, for the contact zone from the position in  FIG. 6   b   1  to the position in  FIG. 6   c   1  the course K of the pivoting angle ω is approximately constant, since the contour of the pawl tooth  5  is not optimized but rather, for example, consists of a flat tooth crown surface. 
     A course of the function ω=f(φ) similar to  FIGS. 6   a  to  6   c  is obtained if, according to the invention, the tooth crown of the parking lock pawl is made concave and co-operates with a parking lock gearwheel whose teeth have no crown tip relief, i.e. having a cylindrical outer contour. The corresponding example embodiment is illustrated in  FIGS. 7   a   1 ,  7   b   1  and  7   c   1 , and in this case only the contour of the pawl tooth  5  is optimized, in such manner that the ω=f(φ) function shape shown in  FIGS. 7   a ,  7   b ,  7   c  is obtained. 
       FIG. 8  shows a schematic representation of a tooth  13  of a parking lock gearwheel (not shown) with mid-point M R , through which the rotational axis of the parking lock gearwheel passes. The parking lock gearwheel has an outer radius r aR  and a radius r 1 , and the difference between these two radii is what is referred to as the crown tip relief δ R . The profile of the wheel tooth  13 , i.e. a section perpendicular to the rotational axis through the mid-point M R , has a convex tooth crown surface  13   a , also referred to in what follows as the contour  13   a , a front tooth flank  13   b  relative to the rotational direction indicated by the arrow D, and a rear tooth flank  13   c  relative to the rotational direction D. The front tooth flank  13   b  merges into the contour  13   a  at the point A and the rear tooth flank  13   c  merges with the contour  13   a  at point B. Point A lies on the radius r 1  while in contrast point B lies on the outer radius r aR , both centered around the mid-point M R . Thus the point A, which is in front relative to the rotational direction D, is set back by the amount of the crown tip relief δ R . As explained earlier, the crown tip relief δ R  serves to reject the pawl during ratcheting. According to the invention, the contour  13   a  between points A and B is in the form of a circular arc with outer radius r aR  about a mid-point M KR . Thus, the tooth crown surface  13   a  is a circular-cylindrical surface with constant curvature, with M KR  as the mid-point of the curvature and r K  the radius of the curvature. The width of the wheel tooth  13  is indicated by a sector φ ZR . The tooth width φ ZR  is decisive for calculating the crown tip relief δ R . 
       FIG. 9  represents schematically a pawl tooth  15  of a locking pawl (not shown). The pawl tooth  15  has a concavely shaped tooth crown surface  15   a , also called the contour  15   a  in what follows, which extends in the rotational direction D of the parking lock gearwheel (not shown here) from a forward point A to a rear point B. A tooth flank at the front relative to the rotational direction D is indexed  15   b  and called the forward tooth flank, and a tooth flank at the back relative to the rotational direction D is indexed  15   c  and called the rear tooth flank. Referring to  FIG. 8 , the point A of the wheel tooth  13  therefore first passes the point A of the pawl tooth  15 . The point B lies on the outer radius r aR  of the parking lock gearwheel around the mid-point M R . The tooth width is indicated by a sector φ ZK . Relative to the point B, the point A is offset in the radial direction outward by the amount of a crown tip relief δ K . It therefore lies on a radius r 2 , which is the sum of the outer radius r aR  and the crown tip relief δ K  for the pawl. The total crown tip relief δ is composed of the fraction δ R  for the wheel and the fraction δ K  for the pawl, so that:
 
δ=δ R +δ K  
 
     According to the invention, the contour  15   a  between points A and B is a circular arc with outer radius r aR , i.e. the tooth crown surface  15   a  is a hollow-cylindrical surface with constant curvature. The mid-point of the curvature circle is indicated as M KK  and the radius of curvature as r K . The fractions of the crown cut-backs for the wheel tooth  13  and the pawl tooth  15  behave as do their tooth widths, i.e. according to:
 
δ R :δ K =φ ZR :φ ZK  
 
     Starting from a specified total crown tip relief δ, the fractions δ R  for the wheel and δ K  for the pawl are then obtained from the following two equations:
 
δ R =δ·φ ZR /(φ ZR +φ ZK )
 
δ K =δ·φ ZK /(φ ZR +φ ZK )
 
     A parking lock gearwheel with the wheel tooth contour  13   a  (first contour) and a parking lock pawl with the pawl tooth contour  15   a  (second contour) result in a linear course of the function ω=f(φ), as shown in the diagrams of  FIGS. 4   a ,  4   b ,  4   c.    
     In the example embodiment represented in  FIG. 8  and  FIG. 9 , the radius of curvature r K  is equal to the outer radius r aR . However, it is within the scope of the invention to choose the radius of curvature r K  larger or smaller than the outer radius r aR , for example by up to 20% larger or smaller. Thus, in practical tests a variant for example with a difference of around 10% proved suitable. 
     INDEXES 
     
         
           1  Parking lock gearwheel 
           2  Parking lock pawl 
           3  Wheel tooth 
           4  Tooth gap 
           5  Pawl tooth 
           13  Wheel tooth 
           13   a  Contour 
           13   b  Front tooth flank 
           13   c  Rear tooth flank 
           15  Pawl tooth 
           15   a  Contour 
           15   b  Front tooth flank 
           15   c  Rear tooth flank 
         ω Pivoting angle 
         ω-dot, ω Pivoting rate 
         φ Rotation angle (wheel) 
         φ-dot, φ Rotation speed 
         φ ZR  Tooth width (wheel) 
         φ ZK  Tooth width (pawl) 
         δ Crown tip relief 
         δ R  Crown tip relief (wheel) 
         δ K  Crown tip relief (pawl) 
         A 1 , A 2  Impact point 
         A n  Impact point 
         D Rotation direction (wheel) 
         g Line (gradient) 
         M R  Mid-point of wheel 
         M KR  Mid-point of wheel/contour 
         M KK  Mid-point of pawl/contour 
         r aR  Outer radius (wheel) 
         r K  Radius of curvature (contour) 
         r 1  Radius 
         r 2  Radius