Abstract:
An automatic stroke adjustment device for a brake actuator having clutch rings ( 240, 340, 440, 540, 546, 640 ) making an irreversible screw engagement relatively rotatable for the amount of exceeding the backlash by releasing the clutch engagement to rotate when exceeding the backlash of reversible screw engagement and maintaining the clutch engagement when in releasing the brake actuator while the brake actuator is in an active state to improve the processability, reduce the weight, and prevent overadjustment of the shoe clearance during the thermal expansion of the brake drum.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an automatic stroke adjustment device for a brake actuator.  
           [0003]    2. Description of the Related Art  
           [0004]    A brake actuator with an automatic stroke adjustment device at a cylinder bore opening is disclosed in U.S. Pat. No. 4,852,702 (Japanese provisional patent publication number 63-285341). FIG. 13 generally illustrates the automatic stroke adjustment device of the &#39;702 patent.  
           [0005]    The automatic stroke adjustment device of the &#39;702 patent comprises a piston  21  slidably engaged with a cylinder bore  11   a  in a cylinder body  11  of a brake actuator  10 ; an adjustment nut  22  rotatably engaging a hollow in a top end of the piston  21  and abutting a stepped surface near the opening of the cylinder bore  11   a ; an adjustment bolt  30  having an externally threaded step  30   a  threadedly engaged with the internally threaded adjustment nut  22  so as to be non-rotatable with respect to the adjustment nut under an axial thrust, and having a bifurcated portion  30   b  at its top end holding a brake shoe  1  so as to restrict a rotation of the adjustment bolt  30 ; a clutch ring  40  having a clutching surface  40   a  making a clutch engagement with a clutching surface  11   b  at the opening side of the cylinder bore  11   a , and having an internal thread  40   b  threadedly engaged with an external thread  22   b  on the adjustment nut  22  with an axial backlash between the two; and an adjustment spring  50  biasing the clutch ring  40  into engagement with the clutching surface  11   b . Hereafter, “screw engagement threadedly engaged between the internal and external threads so as to be non-rotatable under an axial thrust” may be called “irreversible screw engagement,” while “screw engagement threadedly engaged between the internal and external threads so as to be rotatable under an axial thrust” may be called “reversible screw engagement.” 
           [0006]    Automatic stroke adjustment function of the conventional device is explained next. If the stroke of the piston  21  during operation of the brake actuator  10  is within a predetermined range, the adjustment nut  22  and the adjustment bolt  30  advance with the piston  21  within the backlash between the clutch ring  40  and the adjustment nut  22  and the clutch ring remains in clutching engagement with the clutching surface  11   b . If, for example, a lining on a brake shoe  1  (not shown in the figure) wears and the stroke of the piston  21  exceeds the predetermined range, the adjustment nut  22  moves with the piston beyond the backlash between the adjustment nut  22  and the clutch ring  40 . As a result of the force exerted on the clutch ring by the adjustment nut, the clutch engagement between the clutching surface  40   a  of the clutch ring  40  and a clutching surface  11   b  of the cylinder body  11  is released. The clutching ring  40 , under the force of the adjustment spring  50 , then slides on the clutching surface  11   b  of the cylinder body  11  and rotates along the external thread  22   b  on the adjustment nut  22 .  
           [0007]    When, the operation of the brake actuator  10  is released and the piston  21  returns due to a spring force by the shoe return spring, the adjustment nut  22  and the adjustment bolt  30  backstroke with the piston  21  across the backlash between the adjustment nut  22  and the adjustment bolt  30  without rotation and the clutch ring  40  engages the clutching surface  11   b  of the cylinder body  11  due to the spring force by the adjustment spring  50 , thereby restricting relative rotation between the clutch ring and the clutching surface.  
           [0008]    As the piston  21  continues to backstroke, until the stepped surface of the adjustment nut  22  engages the supporting section of the stepped surface of the cylinder bore  11   a  to restrict its rotation, the adjustment nut  22  rotates along the internal thread  40   b  of the clutch ring  40  as it backstrokes with the piston  21 . The rotation of the adjustment nut  22  with respect to the adjustment bolt  30  screws the adjustment bolt  30  outward to adjust the stroking amount of the brake actuator  10  in response to the brake lining wear.  
           [0009]    A brake actuator with an automatic stroke adjustment device at a bottom portion of a cylinder bore is disclosed in U.S. Pat. No. 5,713,437 (Japanese provisional patent publication number 9-229115). Particular sections of the automatic stroke adjustment device of the &#39;437 patent will be explained with reference to FIGS. 14 and 15.  
           [0010]    This automatic stroke adjustment device for a brake actuator  110  comprises a piston  121  slidably engaged with a cylinder bore  11   a  in a cylinder body  111  of a brake actuator  110 ; an adjustment bolt  130  having an externally threaded stem  130   c  at one end making irreversible screw engagement with an internal thread  121   c  in a hollow  121   b  on the back side of the piston  121 , and having a conical surface  130   d  at the other end making a clutch engagement with a first conical surface  111   d  formed on the partition wall  111   c  in the cylinder bore  111   a ; a clutch ring  140  having an internal thread  140   b  in reversible screw engagement with an external thread  130   e  formed on a large diameter axle portion of the adjustment bolt  130  with a backlash and a clutching surface  140   a  on the peripheral surface making a clutch engagement with a second conical surface  111   e  of the partition wall  111   c ; and an adjustment spring (drive ring spring)  150  biasing the clutch ring  140  to make a clutch engagement with the second conical surface.  
           [0011]    A piston head  123  is restricted in its own rotation by holding and securing the brake shoe  1  in a bifurcated portion  123   a  formed at a top of the piston head  123 , and the piston  121  is designed to be rotatable relative to the piston head  123  and the cylinder bore  111   a.    
           [0012]    Automatic stroke adjustment operation is explained next. If the stroke of the piston  121  during the operation of the brake actuator  110  is within a predetermined range, the adjustment bolt  130  advances with the piston  121  within the backlash between the clutch ring  140  and the adjustment bolt  130  and the clutch ring  140  remains in clutching engagement with the second conical surface  111   e.    
           [0013]    If a lining on a brake shoe  1  (not shown in the figure) wears and the stroke of the piston  121  exceeds the predetermined range, the adjustment bolt  130  moves beyond the backlash between the adjustment bolt  130  and the clutch ring  140 . As a result, as the piston  121  continues to move the clutch engagement between the clutching surface  140   a  on the peripheral surface of the clutch ring  140  and the second conical surface  111   e  is released, and the clutch ring  140  under the force of the adjustment spring  150  slides on the second conical surface  111   e  of the partition wall  111   c  and rotates with respect to the adjustment bolt  130  along the external thread  130   e  on the adjustment bolt  130 .  
           [0014]    When, the operation of the brake actuator  110  is released and the piston  121  backstrokes to return due to a spring force by the shoe return spring, the piston  121  and the adjustment bolt  130  backstroke across the amount of backlash at the reversible screw engagement without the rotation of the piston  121  or the adjustment bolt  130 , and the clutch ring  140  makes the clutch engagement with the second conical surface  111   e  of the partition wall  111   c  due to the spring force by the adjustment spring  150 , thereby restricting the clutch ring&#39;s rotation.  
           [0015]    As the piston  121  continues to backstroke, until the conical surface  130   d  of the adjustment bolt  130  engages with the first conical surface  111   d  on the partition wall  111   c  of the cylinder bore  111   a  to restrict its rotation, the adjustment bolt  130  rotates along the internal thread  140   b  of the clutch ring  140 . In response to the rotation of the adjustment bolt  130 , the piston  121  is screwed outward to adjust the stroking amount of the brake actuator  110  in compensation for the brake lining wear.  
           [0016]    Both of the above types of clutch rings  40  and  140  have lead angle internal threads  40   b ,  140   b  on their inner circumferential surfaces and need strength to rotate objects making irreversible screw engagement with another member under the spring force of the shoe return spring. In order to attain the above object, the conventional clutch rings  40 ,  140  are made of relatively thick plates, manufactured such as by machining or a combination of forging and machining.  
           [0017]    However, the above-described conventional automatic stroke adjustment device for a brake actuator has the following drawbacks.  
           [0018]    If the clutch ring is manufactured from a thick metal plate by all machining or combination of forging and machining, this method not only increases the material cost and manufacturing cost but also increases a weight of the device. Also, in consideration of the processability of the clutch ring such as by machining and forging, if soft materials such as brass are employed, there is a possibility of causing friction with an abutting surface of the adjustment spring, which reduces the durability. In order to improve the durability, a hard thin metal plate may be put between the adjustment spring and the surface abutting the adjustment spring. However, this method not only increases the number of components but also increases the manpower required to manufacture and assemble the device. Accordingly, employing the relatively thick plate has disadvantage in its cost and weight.  
           [0019]    Further, the clutch ring may be a monoblock made by a sintered metal. However, this method not only increases the cost but also complicates the structure of the die, which reduces the processability.  
           [0020]    An automatic stroke adjustment device must maintain its operation even if the brake drum experiences thermal expansion because of a frequent use on a long downhill. When the drum expands at elevated temperatures, the device adjusts the stroke to compensate. When the brake drum returns to normal temperatures, a clearance between the brake drum and the brake shoe reduces as a result of thermal contraction of the brake drum. To compensate for this contraction, the clearance under the normal temperature needs to be preset larger than optimal, which necessarily sacrifices a brake operation stroke.  
         SUMMARY OF THE INVENTION  
         [0021]    This invention is intended to remove the aforementioned drawbacks, and an object of this invention is to provide an automatic stroke adjustment device for a brake actuator with an excellent improved processibility and a reduced weight. Further, another object of this invention is to provide an automatic stroke adjustment device for a brake actuator enabling one to stop the adjustment operation under high temperatures and to set a small shoe clearance under normal temperatures, thereby reducing the brake operation stroke.  
           [0022]    The present invention is directed toward an automatic stroke adjustment device for a brake actuator, comprising a piston slidably fitting in a cylinder bore of a brake cylinder, a first screw engagement mechanism including a clutch ring and a first responsive member and allowing a relative rotation of the two due to an axial thrust, and a second screw engagement mechanism including the first responsive member and a second responsive member and inhabiting a relative rotation of the two under the axial thrust, in which if the piston overstrokes, a movement of the clutch ring of the first screw engagement mechanism exceeds a predetermined axial backlash between a female screw of the clutch ring and a male screw of the first responsive member, and a clutch engagement between the clutch ring and a third responsive member is released to allow a rotation of the clutch ring; if the overstroked piston then makes a back-stroke, when the backlash is filled up, the clutch engagement between the clutch ring and the third responsive member is activated making the two non-rotatable, and the first responsive member, making a screw engagement with the clutch ring, rotates and the second responsive member, making a screw engagement with the first responsive member, screws out until the first responsive member engages with a supporting member making the first responsive member nonrotatable, wherein the clutch ring is a monoblock ring formed by pressing a thin plate.  
           [0023]    Another embodiment of the present invention is directed toward an automatic stroke adjustment device for a brake actuator according to the first embodiment, wherein the first responsive member is an adjustment nut relatively rotatably fitting with the hollow at a top side of the piston and engaging with the supporting member at an opening side of the cylinder bore, the second responsive member is an adjustment bolt making a screw engagement with the female screw at an axis of the adjustment nut while the relative rotation therebetween due to an axial thrust is inhibited and the rotation is inhibited by a rotation regulator, a clutch ring makes a screw engagement with a male screw on a peripheral surface of the adjustment nut with an axial backlash therebetween while the relative rotation therebetween due to an axial thrust is allowed and makes the clutch engagement with the supporting member at the opening side of the cylinder bore, and an adjustment spring energizes the clutch ring in a direction to make a clutch engagement with the supporting member.  
           [0024]    Yet another embodiment of the present invention is directed toward an automatic stroke adjustment device for a brake actuator, wherein the first responsive member is the piston, the second responsive member is an adjustment bolt making a screw engagement with the female screw at an axis of the piston, while the relative rotation therebetween due to an axial thrust is inhibited and the rotation is inhibited by a rotation regulator, a clutch ring makes a screw engagement with a male screw on a peripheral surface at a tip end of the piston with an axial backlash therebetween while the relative rotation therebetween due to an axial thrust is allowed and makes the clutch engagement with the supporting member at the opening side of the cylinder bore, and an adjustment spring energizes the clutch ring in a direction to make a clutch engagement with the supporting member.  
           [0025]    Still another embodiment of the present invention is directed toward an automatic stroke adjustment device for a brake actuator, wherein the second responsive member is the piston being inhabited the rotation by a rotation regulator, the first responsive member is an adjustment bolt, one end of which has a male screw stem making a screw engagement with a female screw axially formed at a back end side of the piston while the relative rotation between the two due to an axial thrust is inhibited, and the other end of which makes a clutch engagement with a bottom of the cylinder bore, a clutch ring makes a screw engagement with a male screw formed adjacent to a clutch section of the adjustment bolt with an axial backlash therebetween while the relative rotation therebetween due to an axial thrust is allowed and makes a clutch engagement with the bottom of the cylinder bore; and an adjustment spring energizes the clutch ring in the direction to make a clutch engagement with the bottom of the cylinder bore.  
           [0026]    One particular embodiment of the present invention is directed toward a brake cylinder, wherein the clutch ring is made of a thermoreacting material which releases the clutch engagement of the clutch ring when it reaches a predetermined temperature in order to stop an automatic adjustment operation.  
           [0027]    According to embodiments of the present invention, a thin plate may be pressed to form the clutch ring, which reduces the cost of material and machining. An economic advantage increases with the mass production of the automatic stroke adjustment device. Further, the thin plate clutch ring improves the weight saving and the resources saving, thereby providing an excellent environmental advantage. In addition, a surface area of the clutching section of the clutch ring and a pith and a torsion angle (corresponding to a lead angle of the reversible thread) of plate teeth may easily be formed as desired regardless of the thickness of the thin plate, thereby increasing the degree of freedom in a layout of the automatic stroke adjustment device.  
           [0028]    Embodiments of this invention is applicable to a brake actuator in which the automatic stroke adjustment device is to be installed at an opening in a cylinder bore of a hydraulic operating means or mechanical operating means, thereby providing a great applicability.  
           [0029]    Embodiments of this invention is applicable to a brake actuator in which the automatic stroke adjustment device is to be installed at the bottom side of the cylinder bore.  
           [0030]    In addition, an automatic adjusting member can simply be a thermo deformable material such as a bimetal and a shape memory alloy so that the clutch engagement of the clutch ring may be released when the brake temperature reaches the predetermined degree, thereby eliminating an occasion for damaging the automatic stroke adjustment function. For example, over adjustment of the shoe clearance when the brake drum is expanded due to the heat may be prevented, and an operation stroke of the brake actuator under the normal temperature may be reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0031]    The above and other objects and the attendant advantages of the present invention will readily become apparent with reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:  
         [0032]    [0032]FIG. 1 is a diametric cross-sectional view of a brake actuator having an automatic stroke adjustment device in the cylinder bore opening according to one embodiment of the present invention;  
         [0033]    [0033]FIG. 2 is an enlarged view of a portion of the automatic stroke adjustment device of FIG. 1;  
         [0034]    [0034]FIG. 3 is a plan view of a clutch ring from the automatic stroke adjustment device of FIG. 1;  
         [0035]    [0035]FIG. 4 is a cross-sectional view of the clutch ring of FIG. 3, viewed along Section IV-IV;  
         [0036]    [0036]FIG. 5 is a development view schematically illustrating reversible screw engagement between the clutch ring and the adjustment nut of FIG. 1, viewed along Section V-V of FIG. 3;  
         [0037]    [0037]FIG. 6 is a plan view of a portion of a clutch ring according to another embodiment of the present invention;  
         [0038]    [0038]FIG. 7 is a cross-sectional elevation view of the clutch ring of FIG. 6 during use, viewed along Section VII-VII of FIG. 6;  
         [0039]    [0039]FIG. 8 is a development view schematically illustrating reversible screw engagement between the clutch ring of FIG. 6 and the adjustment nut, viewed along Section VIII-VIII of FIG. 6;  
         [0040]    [0040]FIG. 9 is a partially enlarged cross-sectional view of an automatic stroke adjustment device according to yet another embodiment of the present invention;  
         [0041]    [0041]FIG. 10 is the portion of the device of FIG. 9 showing the condition when the clutch ring is thermally deformed;  
         [0042]    [0042]FIG. 11 is a cross-sectional view of a brake actuator according to still another embodiment of the present invention;  
         [0043]    [0043]FIG. 12 is an enlarged view of a portion of a brake actuator having an automatic stroke adjustment device at the bottom side in the cylinder bore opening according to still another embodiment of the present invention;  
         [0044]    [0044]FIG. 13 is a cross-sectional view of a brake actuator having the automatic stroke adjustment device in the cylinder bore opening according to the prior art;  
         [0045]    [0045]FIG. 14 is a cross-sectional view of a brake actuator having an automatic stroke adjustment device in the bottom of the cylinder bore opening according to the prior art; and  
         [0046]    [0046]FIG. 15 is an enlarged view of a portion of the device of FIG. 14.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0047]    The present detailed description is generally directed toward automatic stroke adjustment devices for brake actuators. Several embodiments of the invention allow a brake actuator to automatically adjust as the brake lining wears. Many specific details of certain embodiments of the invention are set forth in the following description and in FIGS.  1 - 12  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or may be practiced without several of the details described in the following description.  
         [0048]    FIGS.  1 - 5  illustrate a brake actuator  210  having one particular embodiment of an automatic stroke adjustment device in a cylinder bore opening. Except for a clutch ring  240  and an adjustment spring  250 , the structure of this embodiment of the invention is almost identical to that illustrated and described in connection with FIG. 13. Therefore, the following explanation is mainly made as to the clutch ring  240  and the adjustment spring  250  to avoid redundancy.  
         [0049]    According to this embodiment of the invention, best illustrated in FIGS. 2 and 3, the clutch ring  240  is a monoblock made by pressing a thin metal plate, e.g., a thin steel plate, and comprises a disc ring body  241 , a clutching section  242  extending from a peripheral portion of the ring body  241 , and a toothed portion  243  composed of a plurality of plate teeth  243   a  spaced apart at predetermined intervals and projecting into a central opening of the ring body  241 . However, the ring body  241  is not a prerequisite of this invention and may be omitted if the clutching section  242  and the toothed portion  243  are designed to be continuous. The toothed portion  243  serves the same function as the internal thread  40   b  of the clutch ring  40  in the conventional art illustrated in FIG. 13, wherein each root section  243   b  (FIG. 3) as a thread groove is positioned between a plate tooth  243   a  and a neighboring plate tooth  243   a . The number of plate teeth  243   a  may be the same as the number of external threads  22   b  on the adjustment nut  22  of the conventional art or may be less, as long as it provides the required strength.  
         [0050]    As best illustrated in FIG. 2, the pitch between each small plate tooth  243   a  extending from the ring body  241  into an axial center in view of a plane direction can be equal to a pitch between each external thread  22   b  of the adjustment nut  22 , and the plate tooth  243   a  can be twisted to the same degree as the lead angle on the external thread  22   b  of the adjustment nut  22  (see FIG. 4). The clutch  242  can have a conical surface  242   a  corresponding with the clutching surface  11   b  in an opening of the cylinder bore  11   a , and the conical surface  242   a  can be biased toward the direction to make a frictional clutch engagement by the coiled adjustment spring  250 .  
         [0051]    [0051]FIG. 5 schematically shows a partial development view of the case when the clutch ring makes a screw engagement with the adjustment nut  22  while the brake actuator  21  (not shown) is in an inactive state. Two vertical broken lines in the drawing represent a plate surface of the thickness t of the ring body  241 , and the diagonal lines represent the screw thread of the external thread  22   b  on the peripheral surface of the adjustment nut  22  while the unmarked portions represent thread grooves. Each plate tooth  243   a  is positioned within a thread groove of the external thread  22   b  on the peripheral surface of the adjustment nut  22 , and adjacent plate teeth  243   a  are alternatively displaced toward the right and left relative to the plate surface of the ring body  241  shown in broken line. The plate tooth  243   a  is bent so that the distance between adjacent plate teeth  243   a  varies relative to the next distance between the adjacent plate teeth  243   a  (i.e., a side surface on the plate tooth  243   a  contacting the screw thread of the adjustment nut  22  varies relative to the next two side surfaces.  
         [0052]    One of a pair of adjacent plate teeth  243   a  contacts the flank at one side of one external thread  22   b , and the other of a pair of plate teeth  243   a  is spaced apart from and faces the flank on the opposite side of the one external thread  22   b , with a gap δ between the latter two. In other words, looseness or play in an axial direction (left-right in the illustration) is a backlash of the reversible thread. A backlash may be, when the adjustment nut  22  advances (i.e., moves toward the left in FIG. 5), the distance of the movement of the external thread  22   b  which fills up the gap  6  and reaches to abut against the plate tooth  243   a  bent toward the lower left side relative to the ring body  241 , or when the adjustment nut  22  backstrokes from the above state (i.e., moves toward the right in FIG. 5), the distance of the movement of the plate tooth  243   a  bent toward the lower left side relative to the ring body  241  which fills up the gap δ and reaches to abut against the external thread  22   b.    
         [0053]    Instead of the above arrangement, all plate teeth  243   a  may be arranged in the same direction relative to the ring body  241 . Especially as evident from FIG. 5, if each plate tooth  243   a  of the clutch ring  240  is configured to function as one screw thread, there is no need to thicken the plate thickness t even if the clutch ring  240  is a small size.  
         [0054]    Accordingly, if a side surface of each plate tooth  243   a  of the toothed portion  243  is aligned to be parallel to the flank of the screw thread on the external thread  22   b  on the adjustment bolt  22 , a larger contact surface of the reversible thread may be utilized even for a thin plate material, thereby providing a stable screw engagement when the reversible thread rotates either forward or backward.  
         [0055]    Further, the torsion angle of each plate tooth  243   a  may freely be set, thereby facilitating the press working regardless of the size of the lead angle of the external thread  22   b  on the adjustment nut  22 . In addition, by making an appropriate setting of the gap δ, the backlash can easily be changed.  
         [0056]    In one example of a manufacturing method for the clutch ring  240 , a two-step forming is conducted by blanking to snap a ring form to be the clutch ring  240  out from a coil material with a predetermined width and by drawing the clutching section  242  and bending all plate teeth  243   a  simultaneously. However, the manufacturing method and the number of process for the clutch ring  240  are not limited to what is described herein.  
         [0057]    Another embodiment of this invention is explained next. For the explanation of this embodiment, identical members from the first embodiment have the same reference numbers, except for the clutch ring  340 , and the explanation of identical members will be omitted here. FIGS.  6 - 8  show the clutch ring  340  with different shapes of plate teeth. The clutch ring  340  comprises a ring-shape clutch section  342  integrally formed from a thin metal plate material by press, and toothed portion  343  having plate teeth  343   a  projecting toward an axial center at predetermined intervals formed in a center opening of the clutch section  342 .  
         [0058]    Side surfaces  343   c  are located on both sides of the plate tooth  343   a , and are formed in a direction to cross the thread groove. Both side surfaces  343   c  are bent in different directions relative to an axial direction and in an angle generally parallel to the flank of the screw thread on the external thread  22   b . In this embodiment, FIG. 8 shows an engagement between one tooth  343  and the external thread  22   b  formed on a peripheral surface of the adjustment nut  22  during the inactive state of the brake actuator. The gap δ corresponds to the backlash of the reversible screw engagement section between both side surfaces  343   c  and the flank of the adjacent screw threads of the external thread  22   b  on the adjustment nut  22 . In addition, if strengthening the plate tooth  343   a  is desirable, an angle stiffening rib  343   d  may be formed at a center of the plate tooth  343   a.    
         [0059]    [0059]FIGS. 9 and 10 show an automatic stroke adjustment device according to another embodiment of the present invention, having a mechanism to prevent over-adjustment of a shoe clearance while the brake drum is thermally expanded.  
         [0060]    A shape of the clutch ring  440  is similar to those in the above-described embodiments, however, the clutch ring  440  is made of thermoreactive material deforming to reduce its diameter when reaching a predetermined temperature (under the temperature at which the brake drum expands). For the thermoreactive member, a bimetal, a shape memory alloy or other suitable materials may be used. In addition, if plural notched grooves  445  are formed on the clutching section  442  in a circumferential direction with predetermined intervals, the clutching section  442  tends to shrink inwardly easily and the deformation of the clutching section  442  is facilitated.  
         [0061]    [0061]FIG. 9 shows a condition of the inventive device prior to the brake drum reaching the predetermined temperature where the conical surface  442   a  of the clutch ring  440  makes a clutch engagement with the clutching surface  11   b  of the cylinder body  11 , thereby providing appropriate automatic adjustment similar to that described in connection with the above embodiments.  
         [0062]    [0062]FIG. 10 shows a condition of the inventive device when the brake drum is at or above the predetermined temperature. As the drum temperature increases, if the temperature within the brake reaches the predetermined temperature, the clutching surface  442  of the clutch ring  440  deforms to reduce in diameter, and if the conical surface  442  is completely separated from the clutching surface  11   b  of the cylinder body  11 , the bottom protruding peak of the clutching surface  442  makes a line contact with the stepped surface of the cylinder bore  11   a . The clutch ring  40  upon releasing the brake has a small rotational frictional force with the stepped surface of the cylinder bore  11   a , thereby rotating the clutch ring  40  without being able to rotate the adjustment nut  22 . Therefore, automatic stroke adjustment operation does not function, which prevents over-adjustment of the shoe clearance and, as a result eliminates the need to set an operation stroke of the brake actuator larger than optimal under the normal temperature.  
         [0063]    When the temperature reduces to return the brake drum to an initial diameter, the clutching surface  442  of the clutch ring  440  expands and again makes a clutch engagement with the clutching surface  11   b.    
         [0064]    [0064]FIG. 11 shows one particular embodiment of a brake actuator with a mechanical operating means  560 . The brake actuator  510  of this embodiment is explained in U.S. Pat. No. 4,887,697 and the explanation of which is limited to general information. Descriptions of the structure here are mainly made only as to other than what are described in &#39;697 patent.  
         [0065]    That is, the cylinder body  511  has a crossing hole  511   g  orthogonally aligned between the cylinder bores  511   a  and  511   f  rectilinearity communicating each other. Pistons  524 ,  525  rotatably slidably fit in the cylinder bores  511   a ,  511   f , where the pistons  524 ,  525  advance to depart from each other via rollers  561   a ,  561   a  by operation of wedge section  561  in the intersection hole  511   g.    
         [0066]    Reversible external threads  524   a ,  525   a  are formed at the top side of the pistons  524 ,  525 , and the clutch rings  540 ,  546  making a screw engagement with the external threads  524   a ,  525   a  with a backlash in the axial direction make clutch engagement with the clutching surface at the opening side of the cylinder bores  511   a ,  511   f  respectively. This structure utilizes a design concept of integrating the piston  21  and the adjustment nut  22  in the first embodiment of this invention above, having the same automatic stroke operation, the explanation of which will be omitted here.  
         [0067]    It can be easily considered that the automatic stroke adjustment device of this invention for example may be applicable to a brake actuator configuring both mechanical and hydraulic type as disclosed in the U.S. Pat. No. 4,535,875.  
         [0068]    The mechanical type of the brake actuator described in this embodiment is a pushing wedge type but may be applicable to a pulling wedge type as disclosed in the Japanese Utility Model Publication Number 2,590,929.  
         [0069]    [0069]FIG. 12 shows another embodiment of an automatic stroke adjustment device with the clutch ring  640  at the bottom side of the cylinder bore in the cylinder body  111 . A brake actuator in this embodiment of this invention is a device employing a design concept of one of the clutch rings  240 ,  340  or  440  in the first three embodiments above, applied in the brake actuator  110  of the conventional art illustrated in FIGS. 14 and 15. Therefore, the function of the brake actuator can be understood in light of the above detailed description, the explanation of which will be omitted here. The external thread  130   e  and the conical surface  130   d  of the adjustment bolt  130  may be reversed.  
         [0070]    By selecting an appropriate brake actuator as above, this invention may be applicable to two-leading (2L) type, duo two-leading (D2L) type, leading trailing (LT) type, and duo servo (DS) type drum brake devices. Further, the type of actuator may be one-side opening or both side opening as long as it has an automatic stroke adjustment device with the clutch ring. Other than the brake actuator for a drum brake, it can be easily understood that this invention may be applicable to a brake actuator for a disc brake device.  
         [0071]    Because of the above-described structure, this invention has the following advantages, among others:  
         [0072]    Employing a thin plate clutch ring, a continuous press working may be utilized, which not only reduces the cost of material and processing but also reduces weight.  
         [0073]    In addition to the above advantage, if the clutch ring is a thin steel plate, durability of the supporting surface on which the adjustment spring sits and the plate tooth is improved, and the shape of the plate tooth can be easily changes providing excellent designability.  
         [0074]    This invention can be employed to various types of brake actuators, drum brakes, and disc brake, which provides a wide range of applicability.  
         [0075]    If the clutch ring is made of a thermoreactive member annulling the clutch engagement of the clutching surface of the clutch ring when reaching the predetermined temperature, no additional part is necessary to accurately prevent the overadjustment of the shoe clearance during the brake drum thermal expansion, thereby allowing to set the operation stroke of the brake actuator small the under the normal temperature.  
         [0076]    It is readily apparent that the above-described has the advantage of wide commercial utility. It should be understood that the specific form of the invention hereinabove described is intended to be representative only, as certain modifications within the scope of these teaching may be apparent to those skilled in the art.  
         [0077]    Accordingly, reference should be made to the following claims in determining the full scope of the invention.  
         [0078]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.