Abstract:
One-way clutches primarily employ a driving ring, a driven ring, and a number of identical connectors. The driving ring and the driven ring are rotationally detached, are concentric, and are configured to house the connectors between them. In one rotational direction, the connectors engage the driving and the driven rings, causing them to rotate in unison. In the opposite rotational direction, the connectors disengage the driving and the driven rings, causing them to freewheel. All existing one-way clutches are aided by means that bias the connectors toward engagement, introducing noise, wear, and heat while freewheeling. Additionally, existing one-way clutches that utilize relatively more efficient pawls as connectors, undergo relatively more backlash from freewheeling to engagement, introducing operational imprecision. The one-way clutch in the present invention utilizes pawls for maximum mechanical efficiency. Furthermore, this invention does not require any biasing means, and its backlash can be reduced to any desirable degree.

Description:
[0001]     This utility application claims priority from provisional application 60/779,546 filed on Mar. 6, 2006. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     One-way clutch mechanisms employ as their primary components a driving ring, a driven ring, and a plurality of identical connectors. The driving ring and the driven ring are rotationally detached, are concentric with respect to a rotation axis, and are configured in such a way as to house the connectors between them. The connectors act as one-way links between the driving ring and the driven ring. In one rotational direction, the connectors jam the driving ring and the driven ring, causing all components to rotate in unison and hence transmitting torque from the driving ring to the driven ring. In the opposite rotational directions the connectors are detached such that the driving ring cannot impart torque or rotation to the driven ring. When the connectors jam the driving ring and the driven ring, the clutch is said to be engaged. When the connectors are detached, the clutch is said to be disengaged or to freewheel. One-way clutch systems function in the two distinct modes of engagement and freewheeling.  
         [0003]     In existing one-way clutch systems, the change from the freewheeling mode to the engaged mode is aided by means that bias the connectors toward engagement. The bias is necessary for the inception of clutch engagement, but in the freewheeling mode it introduces undesirable noise, wear, heat, and the need for special lubrication and heat removal.  
         [0004]     The connector in existing one-way clutches is typically a roller, a sprag, or a pawl. Rollers and sprags are similar in the way they transmit torque. In the engaged mode, the rollers or sprags are wedged between corresponding converging surfaces on the driving ring and the driven ring, thereby transmitting torque by the nearly radial thrust in rollers or sprags. Consequently, large stresses and deformations develop in the mechanism, necessitating use of a large number of rollers or sprags, and requiring massive components with high material strength and hardness.  
         [0005]     In contrast to rollers or sprags, pawls transmit torque more efficiently. In the engaged mode, the pawls are disposed between corresponding notches in the driving ring and the driven ring, thereby transferring torque by the nearly tangential thrust in the pawls. The resulting stresses and deformations are relatively low, accommodating use of fewer pawls and lighter components with low material strength and hardness.  
         [0006]     Given the undesirable characteristics of roller or sprag clutches in terms of their torque transfer efficiency, they, however, have the desirable attribute of undergoing only negligible degree of backlash during engagement. In contrast, existing pawl clutches with their desirable torque transfer efficiency undergo a considerable degree of backlash during engagement. An additional undesirable characteristic of all existing one-way clutches is the biasing of the connectors toward engagement when the system is in freewheeling mode.  
         [0007]     The one-way clutch mechanism that is the subject of this invention falls within the pawl-type category of one way clutch systems with the added novelties that it does not require a biasing mechanism toward engagement, and its backlash can be reduced to any desirable degree. Consequently, in addition to its torque transmission efficiency in the engaged mode, it undergoes negligible backlash in the engaging process and it prohibits excessive noise, wear, and heat in the freewheeling mode of its operation.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The concept that underlies the present invention can be described with respect to the mechanism in  FIG. 1 . The mechanism is composed of a driving ring with a key on its outer cylindrical surface; a driven ring with a notch on its inner cylindrical surface; a pawl ring with a pivot hole; and a generally L-shaped pawl having therefore, an arm, a corner, and a stem. The arm includes a cutout near one end closest to the corner. The stem includes a pivot hole normal to plane of the pawl, and the corner end of the stem overlaps the corner end of the arm.  
         [0009]     In assembled configuration, the driving ring, the driven ring, and the pawl ring are concentric, the pawl arm is disposed tangentially on the outer cylindrical surface of the driving ring in the radial space between the driving ring and the driven ring such that the key on the driving ring is inserted in the pawl arm&#39;s cutout while the free end of the pawl stem is pivoted to the pawl ring through their respective pivot holes.  
         [0010]     When the pawl arm rests tangentially on the driving ring, as in  FIG. 1 , a clockwise rotation of the driving ring imparts the same rotation to the pawl and the pawl ring while the driven ring remains stationary. A counterclockwise rotation of the driving ring causes the pawl to rotate clockwise with respect to the pivot until the pawl arm contacts the inner surface of the driven ring. Further counterclockwise rotation of the driving ring maintains the said contact until the pawl arm in fully engaged with the notch on the driven ring. From this point on, the entire mechanism rotates counterclockwise in unison. Frames  1  to  6  in  FIG. 2 , as viewed through the dashed rectangular segment  2  in  FIG. 1 , sequentially depict the engagement process just described. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]      FIG. 1  Plan view of the essential features of the present invention including a driving ring with a key on its outer cylindrical surface, a driven ring with a notch on its inner cylindrical surface, a pawl ring with a pivot hole, and an L-shaped pawl composed of a stem with a pivot hole and an arm with a slant at one end and a cutout near the other end.  
         [0012]      FIG. 2  Sequential instances of the engagement process as viewed through the dashed rectangular segment  2  in  FIG. 1 .  
         [0013]      FIG. 3  Exploded view of a simple embodiment of the present invention where the three concentric rings include additional cylindrical features which enable them to support each other both rotationally and axially; and, to facilitate the assembly of the three concentric rings, the driven ring is cut in half axially to produce a driven ring consisting of two haves that are mirror images of each other; and, corresponding to the twin design of the driven ring, the pawl, still having an L-shaped configuration, is composed of two identical parallel arms; and, because of the pawl&#39;s symmetric twin-arm design, no cutouts in the arms are required, but instead the driving ring includes on its outermost cylindrical surface a pair of keys: one engaging key to sit behind the arms at their connection to the stem, and one disengaging key to sit in between the arms adjacent to the stem.  
         [0014]      FIG. 4  Isometric view of the pawl assembly consisting of the pawl pivoted to the pawl ring.  
         [0015]      FIG. 5  Isometric view of the driving assembly consisting of the pawl assembly fitted onto the driving ring in such a way that the pawl stem is positioned in the space between the keys on the driving ring.  
         [0016]      FIG. 6  Plan view of the driving assembly in  FIG. 5 .  
         [0017]      FIG. 7  Sequential instances of the concentric process of achieving the driving assembly as viewed through the dashed rectangular segment  7  in  FIG. 6 .  
         [0018]      FIG. 8  Plan view of the assembled system in freewheeling mode.  
         [0019]      FIG. 9  Plan view of the assembled system in the engaged mode.  
         [0020]      FIG. 10  Radial cross section of the system as viewed through section  10  in  FIG. 9 .  
         [0021]      FIG. 11  Isometric views of a double-stem pawl and its corresponding pawl ring.  
         [0022]      FIG. 12  Plan view of the assembled system in freewheeling mode utilizing a double-stem pawl.  
         [0023]      FIG. 13  Plan view of the assembled system in the engaged mode utilizing a double-stem pawl.  
         [0024]      FIG. 14  Radial cross section of the system with a double-stem pawl as viewed through section  14  in  FIG. 13 .  
         [0025]      FIG. 15  Enlargement of the dashed rectangular segment  15  in  FIG. 8 .  
         [0026]      FIG. 16  Sequential instances of the engagement process as viewed through the dashed rectangular segment  15  in  FIG. 8 .  
         [0027]      FIG. 17  Plan view of an embodiment of the present invention in freewheeling mode that includes a plurality of uniformly spaced pawls; and in which, the driving ring includes a plurality of uniformly spaced key pairs that are equal in number to the number of pawls while the driven ring includes a plurality of uniformly spaced notches that are equal in number to a whole multiple of the number of pawls.  
         [0028]      FIG. 18  Embodiment of  FIG. 17  in the engaged mode.  
         [0029]      FIG. 19  One variation of a compliant pawl that utilizes a pin and a rotational spring at the juncture of the stem with the arms.  
         [0030]      FIG. 20  Plan views of the assembled compliant pawl in  FIG. 19  in two limiting rotational configurations.  
         [0031]      FIG. 21  Another variation of a compliant pawl that utilizes a compliant stem fabricated from sheet spring material.  
         [0032]      FIG. 22  Plan views of the compliant pawl in  FIG. 21  showing the unloaded state and an instant of the deformed state.  
         [0033]      FIG. 23  Plan view of an embodiment of the present invention in freewheeling mode that utilizes compliant pawls of the type in  FIG. 19 .  
         [0034]      FIG. 24  Plan view of the embodiment in  FIG. 23  in the engaged mode.  
         [0035]      FIG. 25  Features and relative orientations of the pawl rings as used in the multi-ring embodiment of  FIG. 26 .  
         [0036]      FIG. 26  Plan view of the multi-ring embodiment of the present invention in the freewheeling mode.  
         [0037]      FIG. 27  Plan view of the embodiment in  FIG. 26  in the engaged mode.  
         [0038]      FIG. 28  Sequential instances of an engagement process for the multi-ring embodiment in  FIG. 26 , as viewed through the dashed rectangular segment  28  in  FIG. 26 .  
         [0039]      FIG. 29  Illustration of the layout procedure for the profiles of the driven ring notch and the free end of the pawl arms. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]     The present invention concerns a new one-way clutch mechanism in which a pawl is used as its connector.  FIG. 3  shows an exploded view of one simple embodiment of the present invention. The driving ring  50  has, at its outermost radial extremity, one pair of keys  51  and  52 . The driven ring  60  is composed of two halves  60   a  and  60   b  that are mirror images of each other. Each half includes one notch. The notch  61   a  corresponds to the driven ring half  60   a  while the notch  61   b  corresponds to the driven ring half  60   b . The pawl  70  includes two identical arms  71   a  and  71   b  that are rigidly joined to a stem  72 . The pawl stem  72  incorporates at its free end a pivot hole  73 . The pawl ring  80  includes a cutout  81  with two coaxial pivot holes  83 .  
         [0041]      FIG. 4  shows the pawl assembly  90  consisting of the pawl  70 , the pawl ring  80 , and the pin  84 . The free end of the pawl stem  72  is inserted in the cutout  81  from the inside of the pawl ring  80  and the pivot holes  73  and  83  are then aligned to admit insertion of the pin  84 . The pawl  70  can swing freely about the pin  84  in the central plane of the pawl ring  80 .  
         [0042]      FIG. 5  shows the driving assembly  100  consisting of the pawl assembly  90  and the driving ring  50 . The pawl stem  72  is positioned in the space between the keys  51  and  52  while the key  52  is positioned between the pawl arms  71   a  and  71   b .  FIG. 6  is a plan view of the driving assembly  100  in which the pawl arm  71   b  is not shown in order to make the key  52  on the driving ring  50  visible. The placement of the pawl assembly over the driving ring can be achieved in a systematic process during which the pawl ring and the driving ring remain coaxial.  FIG. 7  depicts four sequential instances of the driving assembly process as viewed through the dashed rectangular segment  7  in  FIG. 6 . As illustrated in frame  1  of  FIG. 7 , the coaxial placement of the pawl assembly over the driving ring would initially require tilting of the pawl toward the free end of the pawl arms in order to prevent the pawl from running into the key  52 . As can be noted in  FIG.7 , one edge of the pawl stem nearest to the key  52  is chamfered, in this particular case, to accommodate assembly.  
         [0043]     Referring to  FIG. 3 , the driven ring halves  60   a  and  60   b  form an axially symmetric enclosure for the driving assembly  100 . The two halves connect rigidly through their outermost flange by mechanical fasteners not shown. The cylindrical surfaces  53   a  and  53   b  of the driving ring  50  contact with, respectively, the cylindrical surfaces  63   a  and  63   b  of the driven ring  60  while the planar surfaces  54   a  and  54   b  of the driving ring  50  contact with, respectively, the planar surfaces  64   a  and  64   b  of the driven ring  60 . Furthermore, the cylindrical surface  82  of the pawl ring  80  contacts with the cylindrical surfaces  62   a  and  62   b  of the driven ring  60  while the planar surfaces  85   a  and  85   b  of the pawl ring  80  contact with, respectively, the planar surfaces  65   a  and  65   b  of the driven ring  60 . The contacting surfaces just identified are lubricated to incur practically negligible friction.  
         [0044]      FIGS. 8 and 9  show plan views of the assembled components without the driven ring half  60   b . The configuration in  FIG. 8  is termed “freewheeling” in the sense that the driving ring can rotate freely in a first rotational direction with respect to the driven assembly. The configuration in  FIG. 9  is termed “engaged” in the sense that the pawl arms are jammed between the driving ring key  51  and the driven ring notch an thus the entire mechanism rotates in unison in a second rotational direction. In order to provide another illustration of the positional relationships among components of the mechanism in the embodiment under consideration, a radial cross section through the pivot hole, as indicated by section arrows  10  in  FIG. 9 , is shown in  FIG. 10  in which the driven ring half  60   b  is also illustrated.  
         [0045]     A slightly different design for the pawl assembly  90  is shown in  FIG. 11 . In this particular construct, the pawl incorporates two stems  72   a  and  72   b  and the pawl ring includes just a pivot hole  83 . Plan and section views of the mechanism with the “double-stem stem” pawl assembly, are shown in FIGS.  12  to  14  which correspond, respectively, to FIGS.  8  to  10  for the “single-stem” pawl assembly. The radial cross section in  FIG. 14  corresponds to section arrows  14  in  FIG. 13  and includes the driven ring half  60   b . Comparing  FIG. 14  to  FIG. 10 , it can be noted that for the double-stem pawl the driven ring cross section near the pawl ring is also slightly modified.  
         [0046]     In the sequel, the freewheeling and engagement modes of operation are described with respect to the embodiment that utilizes a single-stem pawl. The description applies equally to the embodiment with a double-stem pawl. References to clockwise and counterclockwise rotations are therefore made with respect to the plan view in  FIG. 8 .  FIG. 15  is an enlargement of the dashed rectangular segment  15  of the plan view in  FIG. 8 . In  FIG. 15  the pawl arm  71   b  is not shown in order to make the key  52  on the driving ring  50  visible.  
         [0047]     The operation in the freewheeling mode can be described with reference to  FIGS. 8 and 15 . When freewheeling, the driving ring  50  rotates clockwise with respect to the driven ring  60 . Consequently, the key  52  contacts the front side  74  of the pawl stem  72  and rotates the pawl counterclockwise about the pin  84 . The said pawl rotation subsequently brings the pawl arms  71  into line contact with the driving ring  50 . The contact force exerted on the pawl  70  by the key  52  is illustrated symbolically by a horizontal arrow  1  in  FIG. 15 . The contact force exerted on the pawl  70  by the driving ring  50  is also illustrated symbolically by a vertical arrow  2  in  FIG. 15 . The two forces  1  and  2  cause a reaction force, indicated symbolically by the arrow  3 , exerted on the pawl  70  by the pin  84 . The combined effect of the three forces just described holds the pawl  70  stationary with respect to both the driving ring  50  and the pawl ring  80 . As a result, the driving ring  50 , the pawl  70 , and the pawl ring  80 , which constitute the driving assembly  100 , rotate clockwise in unison without transmitting torque to the driven ring  60 .  
         [0048]     It is evident that, while freewheeling, the mechanism does not require the pawl to be biased toward engaging the notch on the driven ring by any means which constitutes a unique feature of the present invention attributed firstly to the ability of the pawl arm to rotate about a pivot centered outside the arm&#39;s line of thrust and secondly to the said geometrical and kinematical relationships that govern the interplay among the constituent components of the mechanism.  
         [0049]      FIG. 16  illustrates sequential instances of one engagement process as viewed through the dashed rectangular segment  15  in  FIG. 8 . The operation in the engaging mode can be described with reference to  FIGS. 8, 15 , and  16 . When in the engaging mode, the driving ring  50  rotates counterclockwise with respect to the driven ring  60 . Consequently, the key  51  on the driving ring  50  contacts the backside  75  of the pawl  70  and rotates the pawl  70  clockwise about the pin  84 . The clockwise rotation of the pawl  70  continues until the edge  76  of the pawl arm  71  touches the surface  66  of the driven ring  60 . The operation up to this point is illustrated sequentially in frames  1  and  2  of  FIG. 16 , in which the pawl ring  80  remains stationary. Further counterclockwise rotation of the driving ring  50  causes the pawl  70  to undergo a counterclockwise rotation about the axis of the clutch system, which occurs in unison with a counterclockwise rotation of the pawl ring  80 , until, as depicted in frame  3 , the pawl edge  76  reaches the edge  67  of the notch  61 . From this point on further counterclockwise rotation of the driving ring  50  causes the pawl  70  to undergo a combination of two rotations. One rotation is counterclockwise about the axis of the clutch system, which occurs in unison with a counterclockwise rotation of the pawl ring  80 . The other rotation is clockwise about the center of the pin  84  on the rotating pawl ring  80 . The combined rotation of the pawl  70  continues until the pawl arm  71  completely engages with the notch  61 . Frames  4  and  5  in  FIG. 16  illustrate the combined rotation of the pawl  70  until engagement. Starting from the configuration in frame  5 , the system operates in an engaged mode where, as shown in frame  6 , the entire system rotates counterclockwise.  
         [0050]     The engagement process as illustrated in  FIG. 16  does not necessarily start with the configuration in frame  1 . A starting configuration that leads to the edge  76  of the pawl arm striking the surface  68  of the notch prior to full engagement is also possible. In this case, because of the slanted design of both the planar profile  77  at the free end of the pawl arm and the planar profile  68  of the surface of the notch, the edge  76  of the pawl arm will ride on surface  68  toward engagement. The said slanted design is illustrated in  FIG. 29  which shows partial plan views of the mechanism in the freewheeling and engaged configurations. The circular arc R is centered at the center of the pivot hole  83  and goes through the top end points  76  and  69  of, respectively, the profiles  77  and  68 . The said profiles are slanted equally to just fall on the inside of the arc R.  
         [0051]     A plan view of a more mechanically efficient embodiment of the present invention in freewheeling configuration is shown in  FIG. 17  in which the driven ring half  60   b  is left out for transparency.  FIG. 18  illustrates the engaged configuration. The embodiment as depicted in  FIGS. 17 and 18  utilizes a plurality of uniformly spaces pawls that are pivoted to the pawl ring in the same manner as described for a single pawl. Correspondingly, the driving ring includes a plurality of uniformly spaced key pairs  51  and  52  that are equal in number to the number of pawls; and the driven ring includes a plurality of uniformly spaced notches  61  whose quantity equals a whole multiple of the number of pawls. The assembly, freewheeling, and engagement processes as described previously apply equally to the embodiment in  FIGS. 17 and 18 .  
         [0052]     The engagement process involves a certain degree of backlash, that is, it involves a finite rotation of the driving assembly  100  with respect to the driven ring  60 . The degree of backlash depends on the relative rotational positions of the driving assembly  100  and the driven ring  60  at the inception of the engagement mode. The maximum degree of backlash is the sectoral extent A between two successive notches as illustrated in  FIG. 17 . Since reduction in backlash is desirable in certain applications, three distinct methods of backlash reduction are presented in the following paragraphs. The three methods of backlash reduction can also be used in combination. The most obvious method of backlash reduction is to increase the number of notches  61  on the driven ring  60 .  
         [0053]     Another method of backlash reduction is to employ a compliant variation of the rigid pawl  70 . For clarity of presentation, two views of one of various constructs of a compliant pawl  70  are shown in  FIG. 19 . In this particular construct, the pawl arms  71   a  and  71   b  connect to the pawl stem  72  by a pin and a rotational spring not shown. The pin holes on the arms and the stem, once axially aligned, accommodate the said connectivity. The pawl stem  72  includes two cutouts that are mirror images of each other. The surfaces  78   a  and  79   a  of the first cutout provide limits of rotation between the pawl stem  72  and the pawl arm  71   a  while the surfaces  78   b  and  79   b  of the second cutout provide the same limits of rotation between the pawl stem  72  and the pawl arm  71   b . Plan views of the assembled compliant pawl in two limiting rotational configurations are shown in  FIG. 20 . The top view in  FIG. 20  shows the compliant pawl  70  in its unloaded state where, by virtue of an initial torque in the said rotational spring, the pawl arms  71   a  and  71   b  are snugly fitted against, respectively, the surface  78   a  and  78   b . The bottom view in  FIG. 20  shows the compliant pawl  70  in its fully deformed state where the pawl arms  71   a  and  71   b  come in contact with, respectively, the surfaces  79   a  and  79   b  on the pawl stem. Another variation of a compliant pawl is shown in  FIG. 21  in which the pawl, instead of a rotationally compliant joint at the corner, utilizes a compliant stem fabricated from sheet spring material. The compliant stem is rigidly joined to the arms at one end and includes a circular feature at the opposite end to accommodate pivoting of the pawl to the pawl ring. Plan views of the compliant pawl in  FIG. 21  are shown in  FIG. 22  in which the top view shows the unloaded state whereas the bottom view shows an instant of the deformed state.  
         [0054]     The plan view of an embodiment of the present invention with compliant pawls of the type in  FIG. 19  is shown in  FIG. 23  where the driven ring half  60   b  is omitted for clarity of presentation. The embodiment in  FIG. 23  utilizes a plurality of pawls, not equal to a prime number, that are uniformly pivoted to a pawl ring. Correspondingly, the driving ring includes a plurality of uniformly spaced key pairs  51  and  52  that are equal in number to the number of pawls. The driven ring includes a plurality of uniformly spaced notches  61  whose quantity does not equal a whole multiple of the number of pawls; but does equal a whole multiple of a whole fraction of the number of pawls. In the particular arrangement in  FIG. 23 , the number of notches ( 15 ) does not equal a whole multiple of the number of pawls ( 6 ); but does equal a whole multiple ( 5 ) of a whole fraction ( 3 ) of the number of pawls ( 6 ). Consequently, as shown in  FIG. 24 , only a fraction (3:6=½) of the number of pawls undergo engagement; the rest deform to avoid binding with the driven ring. Upon activation of the freewheeling mode, the engaged pawls rotate back, and the deformed pawls rotate and retract back to their respective freewheeling configuration in  FIG. 23 . The embodiment in  FIG. 23  can be said to include M (an integer greater than 1) sets of N (and integer greater than 0) pawls of which only one set participates in engagement in any given engagement mode of operation. It is also evident that utilization of the compliant pawls of the type in  FIG. 21  produces the same result as described in this paragraph.  
         [0055]     Yet another method of backlash reduction is to employ M pawl assemblies, where a pawl assembly consists of a pawl ring and a set of N uniformly spaced pawls. For this purpose, a rigid double-stem pawl and its corresponding pawl ring, as shown in  FIG. 11 , are used.  FIG. 25  shows M=2 identical pawl rings with a plurality N=3 of round pivot holes  83  that are uniformly distributed in a circular pattern with the subtended angle of each pair of consecutive pivot holes equal to 2π/N=2π/3. Each pawl ring also includes a plurality N(M−1)=3 of sectoral slots  86  that are situated at the same radius as that of the round pivot holes  83 , and that divide into equal parts, 2π/MN=π/3, the angle subtended by each two consecutive round pivot holes  83 . Starting from the top pawl ring  80  in  FIG. 25  and proceeding sequentially downward, the next pawl ring  80  in shown rotated counterclockwise by 2π/MN=π/3 with respect to the previous pawl ring  80 .  
         [0056]     When the N=2 pawl rings  80  in  FIG. 25  are overlaid, each round pivot hole  83  in a given pawl ring  80  aligns with M−1=1 sectoral slots  86  in the remaining pawl rings. It is to this overlaid arrangement of the pawl rings  80  that the MN=6 pawls  70  are pivoted. Consequently, an individual pawl assembly can rotate with respect to the remaining M−1=1 pawl assemblies up to a limit imposed by the sectoral slots  86 .  
         [0057]     A plan view of the multi-ring embodiment of the invention without the driven ring half  60   b  is shown in  FIG. 26 . In the configuration of  FIG. 26  the number of notches is chosen according to the same procedure used for the compliant pawl embodiment with the result that the pawls in the same pawl assembly have the same positional alignment with the notches, whereas any two pawls belonging to different pawl assemblies have different positional alignment with the notches. The said alignment differences together with the ability of the pawl rings to have a finite rotational freedom with respect to each other facilitate an engagement mode of operation with reduced backlash. Upon engagement, the plan view in  FIG. 26  assumes the configuration in  FIG. 27 .  FIG. 28  illustrates sequential instances of one engagement process as viewed through the dashed rectangular segment  28  in  FIG. 26 . As evident from  FIG. 28  only the pawls on one pawl ring undergo engagement; the rest slide to avoid binding with the driven ring. Upon activation of the freewheeling mode, the engaged pawls rotate back, and the non-engaged pawls rotate and slide back to their respective freewheeling configuration in  FIG. 26 .