Abstract:
An adjuster assembly for compensating for brake lining wear in a brake unit. The brake unit has a housing for accommodating the adjuster assembly, a brake lever journalled to the housing, and a force transmitter for transmitting displacement of the brake lever to at least one brake lining. The adjuster assembly includes a support shaft, sensing means for cooperation with the brake lever, a drive ring assembly cooperating with the sensor, and an adjuster gear. The adjuster gear cooperates with the drive ring assembly. To prevent unwanted release of the adjuster assembly, the engagement surface is provided with rotation a rotation preventer adapted for releasable engagement with engagement member cooperating with the housing for preventing rotation of the transmission ring assembly in one direction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This present application is a continuation patent application of U.S. patent application Ser. No. 09/428,260 filed Oct. 27, 1999 which is a continuation application of International Application Number PCT/SE98/00762 filed Apr. 27, 1998 which was published in English pursuant to Article 21(2) of the Patent Cooperation Treaty, and which claims priority to Swedish Application No. 9701589-5 filed Apr. 28, 1997. The full disclosure of said applications, in their entireties, are hereby expressly incorporated by reference into the present application. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Technical Field  
           [0003]    The present invention relates to an adjuster assembly for compensating for brake lining wear in a brake unit. The invention further relates to a brake unit comprising an adjuster assembly, as well as to a vehicle equipped with a brake unit comprising an adjuster assembly.  
           [0004]    2. Background of the Invention  
           [0005]    Braking systems for heavy goods vehicles are presently generally powered by compressed air. A typical braking system comprises a brake unit mounted on a hub of a wheel to be braked. A pneumatic actuator is arranged on the brake unit such that the actuator acts on a brake lever pivotally supported in the brake unit. Displacement of the brake lever causes friction linings to come into contact with a brake surface arranged on a component which rotates with the wheel to be braked. In an attempt to ensure reliable operation of braking systems, legislation dictates a maximum permissible displacement of the brake lever, i.e. the maximum stroke of the lever, to still obtain maximum braking effect.  
           [0006]    When new friction linings are fitted to a brake unit, the unit is adjusted so that a certain clearance is attained between the friction linings and the brake surface. In other words, a predetermined stroke of the brake lever will result in a maximum braking effect. Naturally, as the friction linings wear during use, the stroke of the brake lever will increase if the same braking effect is required. Once a certain degree of wear has taken place, the stroke can exceed the legislated maximum amount.  
           [0007]    Accordingly, it is known to provide braking systems with means to compensate for friction lining wear, hereinafter termed an adjuster assembly, to thereby ensure that the stroke of the brake lever is always maintained within a predetermined length during the entire useful working life of the friction linings.  
           [0008]    Known adjuster assemblies employ mechanical sensing means to determine the stroke of the brake lever. If the stroke exceeds a predetermined amount, the sensing means causes a take-up means to rotate to ensure that, in a released position, the brake linings sit closer to the brake surface than before rotation of the take-up means. To prevent the brake linings reverting to their original position when the brake lever is released, the sensing means communicates with the take-up means via a one-way clutch assembly so that the take-up means can only rotate in a direction which takes up excess play in the system.  
           [0009]    When the brake linings have exceeded their useful working life, they must be removed from the brake unit and new linings fitted thereto. This implies that the adjuster assembly must be “released” so that sufficient clearance is attained to allow the new linings to be fitted. In one braking system, this is achieved by providing a multiplate clutch between the one-way clutch-assembly and the take-up means. By overcoming friction in the multiplate clutch, the take-up means can be rotated back to its original position.  
           [0010]    Because it is necessary to be able to rotate the take-up means in one direction to take up play during operation of the braking system and in the opposite direction to release the adjuster assembly when new linings are to be fitted, a risk exists that the take-up means may be caused to rotate in the “release” direction, for example due to vibration, even when it is not intended to replace the linings. If this were to happen, the stroke of the brake lever would initially be longer than intended, though would adopt its operating length after several applications of the brakes.  
         SUMMARY OF INVENTION  
         [0011]    The present invention in its several disclosed embodiments alleviates the drawbacks described above with respect to brake lining wear adjuster assemblies. It is therefore an object of the present invention to provide an adjuster assembly in which the risk of improper operation is reduced at the same time that servicing of the brake unit is not impaired. The specific structures through which these benefits are delivered will be described in detail hereinbelow. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    The invention will now be described in greater detail in the following way, by example only, and with reference to the attached drawings, in which:  
         [0013]    [0013]FIG. 1 is a schematic, partially sectioned view of a brake unit incorporating an adjuster assembly according to the present invention;  
         [0014]    [0014]FIG. 2 is a schematic sectional view of one embodiment of an adjuster assembly according to the present invention mounted in a brake unit; and  
         [0015]    [0015]FIG. 3 is an enlarged view taken at circle A in FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0016]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale, some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention.  
         [0017]    Referring to the drawings, reference numeral  10  generally denotes a brake unit for use in a vehicle braking system. The brake unit  10  comprises a housing  12  in which an adjuster assembly, generally denoted by reference numeral  14 , is accommodated. The brake unit  10  further comprises a brake lever  16  journalled to the housing for rotation about an axis  18 . The brake lever is adapted to magnify an input load generated by a pneumatic actuator (not shown) associated with the brake unit to cause at least one brake pad  20 , and in the illustrated embodiment two brake pads  20 , to engage a brake surface on a brake disc  22 . Eccentric cam surfaces  24  on the brake lever  16  act on force transmitting means  26  arranged for slidable displacement in the housing  12 .  
         [0018]    In the illustrated embodiment, the brake lever  16  acts onpair of force transmitting means  26 , with each force transmitting means comprising a pressure plate for transmitting the applied force to an internally threaded outer cylinder  30 . An externally threaded inner cylinder  32  intermeshes with the outer cylinder  30  to transmit the applied force to a head piece  34 . The inner cylinder  32  is axially displaceable along a rod  36  connected to the pressure plate  28 . The pressure plate  28 , rod  36  and inner cylinder  32  are locked against rotation. A gearwheel  38  is press-fitted to the outer cylinder  30  and the outer cylinder is arranged to be rotatable with respect to the pressure plate, rod and inner cylinder. Rotation of the gearwheel  38  in one direction will cause the inner cylinder to move away from the pressure plate  28  thereby displacing the brake pad towards the brake disc  22 . Rotation of the gearwheel  38  in the opposite direction will cause the inner cylinder  32  to retract within the outer cylinder  30 , thereby increasing clearance between the brake pad and the brake disc.  
         [0019]    Rotation of the gearwheel  38  is affected by an adjuster gear  40  forming a part of the adjuster assembly  14  according to the present invention.  
         [0020]    With particular reference to FIG. 2, the adjuster assembly  14  according to the present invention comprises a support shaft  42  which is adapted to be journalled for rotation about a rotational axis  44  in the housing  12 . Sensing means  46 , in the form of a cylindrical sleeve  48  mounted concentrically with the support shaft  42 , is adapted for rotation about the rotational axis  44 . The sensing means  46  is adapted for cooperation with the brake lever  16  via a drive pin  50  and, as shown, a sliding block  52  carried in an axially extending groove  54  in the cylindrical sleeve  48 . In this manner, a displacement of the brake lever will be translated into a rotation of the sensing means about the rotational axis  44 . Naturally, it will be evident to the skilled person that rotation of the sensing means due to displacement of the brake lever may be achieved in other ways. For example, the brake lever may be connected to the cylindrical sleeve via a gearwheel arrangement.  
         [0021]    The sensing means  46  cooperates with a drive ring assembly  56  via a one-way clutch  58 . In the illustrated embodiment, the one-way clutch is in the form of a helical spring overlying adjacent axially extending flange portions  60 ,  62  of the cylindrical sleeve and drive ring assembly, respectively. It is to be understood that any form of one-way clutch, for example a unidirectional roller bearing, may be employed for this purpose. In a manner which will be explained hereinbelow, the drive ring assembly  56  is adapted for rotation about the rotational axis  44  and, to a limited degree, about the support shaft  42 .  
         [0022]    As mentioned above, the adjuster assembly  14  further comprises an adjuster gear which is adapted to effect adjustment of the force transmitting means  26  via rotation of the gear wheel  38 . The adjuster gear  40  cooperates with the drive ring assembly  56  via a torque-limiting clutch  66 . For reasons which will be explained later, the drive ring assembly  56  is allowed to rotate a small amount, about 1 to 5 degrees, before clearance between the drive ring assembly and the torque-limiting clutch  66  is taken up. By the expression “torque-limiting clutch”, it is hereby meant that, once the clearance has been taken up, rotation of the drive ring assembly  56  will be imparted to the adjuster gear  40  until a predetermined torque level is reached, at which level the clutch  66  will begin to slip to thereby allow the drive ring assembly  56  to rotate without rotating the adjuster gear  40 . Advantageously, the torque-limiting clutch  66  is a multiplate clutch with one set of plates presenting radially protruding tangs  68  which are accommodated with said clearance in axially extending slots  70  in the drive ring assembly.  
         [0023]    The adjuster gear  40  cooperates with the support shaft  42  via a transmission ring assembly  72 . In the illustrated embodiment, the transmission ring assembly  72  is in the form of a threadless nut keyed to the support shaft  42  via a spigot  74 . At least one shaped internal surface  75  on the adjuster gear  40  engages with a correspondingly shaped surface on the nut to ensure that the adjuster gear and the transmission ring assembly co-rotate. The spigot  74  advantageously passes diametrically through the support shaft  42  and respective free end regions of the spigot  74  are accommodated with clearance in diametrically opposed slots  76  in the transmission ring assembly  72 .  
         [0024]    As is clearly apparent from FIGS. 1 and 2, each slot  76  extends at an angle to the rotational axis  44 , which angle is non-perpendicular to the rotational axis. Thus, and as will be explained in greater detail below, the transmission ring assembly  72  is capable of executing an axial displacement along the support shaft  42  when the support shaft is rotated in one direction.  
         [0025]    Biasing means in the form of a spring member  78  acts between the adjuster gear  40  and the transmission ring assembly  72  to bias the transmission ring assembly away from the adjuster gear and towards engagement means  80  in the form of an insert non-rotatably affixed in the housing and concentric with the support shaft  42 .  
         [0026]    In accordance with the present invention, the transmission ring assembly  72  is provided with an engagement surface  82  forming an angle to the rotational axis  44 . Preferably, the engagement surface  82  is perpendicular with respect to the rotational axis so that the engagement surface may contact a substantial portion of the engagement means  80 . The engagement surface  82  is provided with rotation prevention means  84  adapted for releasable engagement with the engagement means  80 . As is most clearly apparent from FIG. 3, the rotation prevention means  84  cooperates with the engagement means in a manner such that rotation of the transmission ring assembly  72  is possible in only one direction when the transmission ring assembly is in the position shown in FIG. 2; i.e. with the engagement surface  82  biased against the engagement means  80 .  
         [0027]    At its end remote from the transmission ring assembly  72 , the support shaft  42  is provided with a free end region adapted to project out of the housing  12  and having means, for example a hexagonal head  86 , to allow rotational forces to be applied to the shaft  42 .  
         [0028]    Operation of the adjuster assembly  14  in accordance with the teachings of the present invention will now be explained. Initial fitting of the brake pads  20  takes place with the adjuster assembly  14  and the force transmitting means  26  in the respective positions shown in FIG. 2. Accordingly, the inner cylinder  32  of the force transmitting means is withdrawn into the outer cylinder  30  thereby creating as large a clearance space as possible between the brake disc  22  and the head piece  34 . Once new brake pads have been fitted, the support shaft  42  of the adjuster assembly is rotated clockwise as shown in FIG. 2 by the application of a suitable socket or wrench to the hexagonal head  86 .  
         [0029]    Clockwise rotation of the support shaft  42  is translated into clockwise rotation of the transmission ring assembly  72  due to interaction between the spigot  74  and the lower end region of the slot  76  in the transmission ring assembly. Due to the form of the engagement means  80  and the rotation prevention means  84 , as well as the clearance for the spigot  74  in the slot  76 , during clockwise rotation, the rotation prevention means  84  will ride over the engagement means  80  due to resilience in the spring member  78 , thereby allowing the adjuster gear  40  to rotate with the support shaft  42 . Due to the provision of the one-way clutch  58 , clockwise rotation of the support shaft  42  will not be translated into rotation of the sensing means  46 .  
         [0030]    Clockwise rotation of the adjuster gear  40  imparts anticlockwise rotation to the gearwheel  38  of the force transmitting means  26 . Since the inner cylinder  32  of the force transmitting means is prevented from rotation, anticlockwise rotation of the outer cylinder causes the inner cylinder to progress outwardly from the outer cylinder, thereby causing the brake pads  20  to approach the brake disc  22 .  
         [0031]    Clockwise rotation of the support shaft  42  is performed until a desired clearance between the brake pads and the brake disc is attained, whence rotation is ceased and the brake unit  10  is properly adjusted for normal operation.  
         [0032]    During normal operation, displacement of the brake lever  16  causes the sensing means  46  to effect a clockwise angular displacement about the rotational axis  44 . Due to the one-way clutch  58 , the clockwise displacement of the sensing means  46  is transmitted to the drive ring assembly  56 . The slots  70  in the drive ring assembly accommodate the tangs  68  of the torque-limiting clutch  66  with such clearance that the drive ring assembly  56 , and hence the sensing means  46 , are allowed to rotate through a certain angle corresponding to the permitted stroke of the brake lever  16  before the clearance is taken up. When the brake lever is released, its return motion causes the sensing means, and hence the drive ring assembly, to return to their initial positions.  
         [0033]    After a certain amount of use, the brake pads will have worn to an extent such that the stroke length of the brake lever  16  imparts an angular displacement to the sensing means  46  which exceeds the amount of play between sidewalls of the slots  70  of the drive ring assembly and the tangs  68  of the torque-limiting clutch  66 . In these circumstances, the drive ring assembly  56  imparts clockwise rotation via the torque-limiting clutch  66  to the adjuster gear  40 . Although the adjuster gear  40  is coupled to the support shaft  42  via the transmission ring assembly  72 , and the engagement surface  82  of the transmission ring assembly is in contact with the engagement means  80  affixed to the housing  12 , the rotation prevention means  84  on the engagement surface  82  only prevents rotation in the anticlockwise direction. Hence, the adjuster gear  40 , the transmission ring assembly  72  and the support shaft  42  are rotated clockwise.  
         [0034]    As explained in connection with the initial fitting of the brake pads  20 , clockwise rotation of the adjuster gear  40  results in the head piece  34  of the force transmitting means  26  approaching the brake disc  22 . Accordingly, some of the clearance caused by wear to the brake pads is taken up between the brake pads and the brake disc.  
         [0035]    When the brake lever  16  reverts to its initial position, the drive pin  50  rotates the sensing means  46  anticlockwise to its initial position. Anticlockwise rotation of the sensing means  46  causes the drive ring assembly  56  to be rotated anticlockwise due to friction between the flanges  60  and  62  of the cylindrical sleeve  48  and the drive ring assembly  56 , respectively. This friction is overcome once the sidewalls of the slots  70  in the drive ring assembly contact the tangs  68  of the torque-limiting clutch  66 , causing the one-way clutch  58  to slip, thereby allowing the sensing means  46  to revert to its initial position without continued rotation of the drive ring assembly  56 .  
         [0036]    Because of the interaction between the engagement surface  82  and the engagement means  80 , there is little risk that the adjuster gear  40  can be caused to rotate anticlockwise even if the adjuster assembly  14  is subjected to tremendous vibration. Accordingly, the force transmitting means  26  cannot inadvertently be caused to impart a greater clearance between the brake pads  20  and the brake disc  22 .  
         [0037]    During very heavy braking, the brake pads  20  may become compressed to such a degree that the sensing means  46  of the adjuster assembly  14  detects that the stroke of the brake lever  16  has exceeded the predetermined maximum value. If, however, the adjuster assembly under these circumstances were to respond by taking up clearance between the brake pads and the brake disc, there would be a very real risk that once the brake lever returned to its initial position, the brake pads would bind on the brake disc.  
         [0038]    This above-described risk is avoided in the present invention in the following manner. When a very heavy braking force is applied via the brake lever  16  to the force transmitting means  26 , the threads on the inner and outer cylinders  30 ,  32  are clamped together with such a force that they are effectively locked against rotation. Thus, the sensing means  46  will be rotated clockwise by the drive pin  50 , causing the drive ring assembly  56  to be co-rotated until the clearance between the slots  70  of the drive ring assembly and the tangs  68  of the torque-limiting clutch  66  is taken up. Since the adjuster gear  40  is locked against rotation by the lock-up of the threads in the force transmitting means  26 , continued application of torque by the drive ring assembly  56  on the torque-limiting clutch  66  will cause the clutch to slip, therefore permitting the drive ring assembly to continue rotation with the sensing means  46  without affecting rotation of the adjuster gear  40 . In this manner, no take up of clearance between the brake pads  20  and the brake disc  22  will occur, even though the sensing means  46  detects a stroke of the brake lever which is greater than the predetermined maximum.  
         [0039]    After an incidence of very heavy braking, the adjuster assembly reverts to its initial position in the same manner as described above.  
         [0040]    When it becomes necessary to change the brake pads  20  due to excessive wear, it is necessary to retract the inner cylinder  32  of the force transmitting means  26  into the outer cylinder  30  to thereby obtain maximum clearance between the head piece  34  and the brake disc  22 . This is achieved in the following manner. A suitable socket or wrench is applied to the hexagonal head  86  of the support shaft  42 . Upon initial anticlockwise rotation of the support shaft, the spigot  74  acts on the transmission ring assembly  72 , via the inclined slots  76 , to cause the transmission ring assembly to be axially displaced along the support shaft  42  in a direction away from the engagement means  80  to compress the spring member  78 . In this manner, the engagement surface  82  of the transmission ring assembly is freed from the engagement means  80 .  
         [0041]    Anticlockwise rotation of the support shaft  42  is resisted by the sensing means  46  since the sensing means is coupled to the brake lever  16  via the drive pin  50 . Nevertheless, anticlockwise rotation of the support shaft is possible since the torque applied to the torque-limiting clutch  66  by the adjuster gear  40  is sufficient to cause the clutch to slip, thereby permitting the adjuster gear  40  to co-rotate with the support shaft  42  while the drive ring assembly  56  and the sensing means  46  remain stationary.  
         [0042]    Accordingly, continued anticlockwise rotation of the support shaft  42  will result in anticlockwise rotation of the adjuster gear  40 . This, in turn, causes the outer cylinder  30  of the force transmitting means  26  to be rotated clockwise. In this manner, the inner cylinder  32  is retracted into the outer cylinder  30 , thereby increasing the clearance between the head piece  34  and the brake disc  22 .  
         [0043]    To prevent possible damage to any of the components making up the adjuster assembly and the force transmitting means as a result of violent forces being applied to the support shaft  42  during retraction of the inner cylinder  32 , for example if the support shaft were to be rotated by a pneumatic wrench, in a preferred embodiment of the invention the spigot  74  is designed to shear should a torque be applied to the free end region of the support shaft above a predetermined value.  
         [0044]    The invention is not restricted to the embodiments described above and illustrated in the drawings, but may be varied within the scope of the appended claims. For example, although the invention has been described in connection with a disc brake unit, it will be evident to the skilled person that the adjuster assembly of the present invention may equally be applied to a drum brake system. Indeed, the adjuster assembly of the present invention could conceivably be employed in any circumstance in which it is desirable to compensate for wear in a brake system. Furthermore, although the rotation prevention means  84  on the engagement surface  82  have been shown in the form of a saw-tooth pattern, it will be appreciated that any mechanical engagement means may be employed which permits rotation in one direction only. In addition, it will be apparent to the skilled person that the invention may be practiced with the spigot  74  formed in the transmission ring assembly  72  and the slot  76  in the support shaft  42 . Alternatively, the support shaft  42  may present a threaded portion which cooperates with a groove in the transmission ring assembly or vice-versa. Naturally, the adjuster assembly of the invention will still operate as intended by combining the actions of the one-way clutch and the torque limiting clutch in one unit.