Abstract:
A method and apparatus for selectively coupling members in a brake actuating mechanism. A coupling member is located between an outer clutch sleeve and a clutch cap. The coupling member exerts a pre-determined amount of force on both the outer clutch sleeve and the clutch cap. The force couples the outer clutch sleeve and the clutch cap. The coupling member allows the outer clutch sleeve and the clutch cap to decouple if a torque acting on the clutch cap exceeds the force exerted by the coupling member.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a torque limiting sleeve for an air disc brake actuator clutch mechanism and a method of using the same. 
   BACKGROUND OF THE INVENTION 
   Torque limiting devices in brake actuator clutch mechanisms are well-known. The known devices can be divided into four basic categories. The first category includes devices utilizing a tightly wrapped coil spring to act as a torque limiting device. This design uses a precision machined, or formed, spring that acts on the internal diameter of a clutch sleeve. Devices in this category are disadvantageous since they typically require both a rotational movement and a linear movement to install and they are comparatively bulky in radial cross section. 
   The second category of devices are those that utilize a clutch pack assembly as the torque limiting device. This design uses a series of clutch plates, a splined inner and outer clutch sleeve, a compression spring to develop the clamping force on the clutch pack, and a variety of retaining and assembly parts. This design is disadvantageous since it typically requires a high number of components and it requires a large amount of space. 
   The third category of devices uses a ball spring torque limiting device. The ball spring torque limiting device is a highly machined and precisely designed structure with complex components, complex assembly and complex operation. 
   The last general category of devices uses a slip sleeve to wear surface torque limiting device. A compression spring produces a force that acts on a hardened sleeve that is in contact with a wear surface located on the rear of the tappet sleeve. The compression spring is retained by a custom-made quarter twist internal hex locking nut. The nut locks in a machined slot of hex cross section in the tappet sleeve. Devices in this category are known to have limited reliability. 
   Various embodiments of the devices in the above-described categories have been patented. For example, U.S. Pat. No. 4,351,419 teaches an adjustable mechanism having a first coil clutch and a second coil clutch. The clutches act together to permit adjustment of the mechanism to a predetermined torque, but slip if the predetermined torque is exceeded. More specifically, the clutches permit selective rotation of a nut member and an intermediate sleeve. 
   U.S. Pat. No. 4,394,890 teaches an adjustable clutch mechanism similar to the &#39;419 patent in that two clutches are used. One of the clutches is a force limiting coil clutch that transmits clockwise torque from the intermediate sleeve to the driven sleeve. When the friction linings are not in contact with the disc, the torque transmitting capacity of the clutch is greater than that required to rotate the nut member. But, when the friction linings are in contact with the disc, the torque transmitting capacity is less than the torque required to rotate the nut member. The coil clutch thus acts as a torque limiting coupling for clearance sensing purposes in the low torque transmitting direction of rotation. 
   U.S. Pat. No. 4,527,683 provides for a torque limiting helical coil clutch drivingly connecting a first rotatable member and a second rotatable member. The coil clutch is right-hand wound to radially expand when transmitting a clockwise rotation and to radially contract when transmitting a counterclockwise rotation. The first member includes a bore in which one end of the coil clutch is received. The second member defines an outer periphery about which the other end of the coil clutch is received. In this orientation, the clutch will selectively slip or grip the first or second member based on the direction of rotation. 
   In light of the disadvantages of the prior art discussed above, it would be advantageous for a coupling device in a brake actuator mechanism to be cost effective, compact and simple to install. Additionally, the device should provide substantially constant torque resistance. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward a coupling device for a brake actuator mechanism. At least a portion of a clutch cap is coaxial with an outer clutch sleeve of the brake actuator mechanism. A coupling device is located between the clutch cap and the outer clutch sleeve for selectively coupling the clutch cap and the outer clutch sleeve. 
   The present invention is also directed toward a method of selectively coupling the clutch cap and the outer clutch sleeve with the coupling device. More specifically, the coupling device is disposed between an inner surface of the outer clutch sleeve and an outer surface of the clutch cap. The inner surface of the clutch cap is connected to a clutch drive shaft. The drive shaft is connected to a brake tappet and the outer clutch sleeve is connected to a brake lever. The clamping action of the brake generates a resistive torque between the brake tappet and a tappet sleeve that is passed through the mechanism to the coupling device. If the torque is greater than the designed torque capability of the coupling device, the outer clutch sleeve will rotate independently of the clutch drive shaft. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description when considered in the light of the accompanying drawings in which: 
       FIG. 1  is a cut-away side view of one embodiment of the present invention; and 
       FIG. 2  is a schematic, perspective view of the embodiment depicted in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless the claims expressly state otherwise. 
   Referring now to  FIG. 1 , a preferred embodiment of a torque limiting device for a brake actuator mechanism  10  is depicted. The mechanism  10  comprises a lever  12  connected to a brake actuating device (not shown) in the operator&#39;s compartment of a vehicle (not shown), as known to those skilled in the art. The lever  12  is connected to an outer clutch sleeve  14  mounted for rotation in the mechanism  10 . A connector portion  16  of the outer clutch sleeve  14  is preferably connected to the lever  12  with one or more pins (not shown). 
   The outer clutch sleeve  14  has a tubular portion  18  extending axially from the connector portion  16 . The tubular portion  18  has an inside surface  20  and an outside surface  22 . The inside surface  20  and the outside surface  22  are substantially parallel to one another. 
   Spaced a pre-determined distance apart from the inside surface  20  of the outer clutch sleeve  14  is an outer surface  24  of a clutch cap  26 . The outer surface  24  of the clutch cap  26  is preferably located along a first leg  28  of the clutch cap  26 . The outer surface  24  of the clutch cap  26  extends substantially parallel to the inside surface  20  of the outer clutch sleeve  14 . 
   A coupling device  30  is preferably located in a gap  32  between the inside surface  20  of the outer clutch sleeve  14  and the outer surface  24  of the clutch cap  26 . In a preferred embodiment, the coupling device  30  is a rolled sheet metal ring with at least one contour rolled into it. The contour is designed to contact the inside surface  20  of the outer clutch sleeve  14  and the outer surface  24  of the clutch cap  26 . 
   In the embodiment depicted in  FIGS. 1 and 2 , the contour is at least one corrugation  34  having a peak  36  in contact with the inside surface  20  of the outer clutch sleeve  14  and a valley  38  in contact with the outer surface  24  of the clutch cap  26 . Those skilled in the art will appreciate that additional corrugations  34  may be used without departing from the scope or the spirit of the present invention. It should also be appreciated that the peak  36  and the valley  38  can be located anywhere between the outer clutch sleeve  14  and the clutch cap  26  and that the coupling device  30  can be of any length. 
   At least one peak  36  of the corrugation of the coupling device  30  is designed to exert a pre-determined amount of force substantially normal to the inside surface  20  of the outer clutch sleeve  14 . At least one valley  38  of the corrugation  34  of the coupling device  30  is designed to exert a pre-determined amount of force substantially normal to the outer surface  24  of the clutch cap  26 . Preferably, the coupling device  30  exerts a substantially constant torque resistance on the clutch cap  26  and the outer clutch sleeve  14  sufficient to selectively couple the clutch cap  26  and the outer clutch sleeve  14  during a braking action. 
   At least one peak  36  and at least one valley  38  of the coupling device  30  are dimensioned to at least partially compress when installed in the gap  32  between the inside surface  20  of the outer clutch sleeve  14  and the outer surface  24  of the clutch cap  26 . The coupling device  30  is linearly installed between the outer clutch sleeve  14  and the clutch cap  26 . 
   On one end of the clutch cap  26 , a second leg  40  extends radially inward substantially perpendicular to the first leg  25 . A third leg  42 , connected to the second leg  40 , extends in a direction normal to the second leg  40  axially inward. The third leg  42  has an inside surface  44  and an outside surface  46 . The first leg  28 , the second leg  40  and the third leg  42  form a C-shaped structure. 
   Preferably, at least a first portion  48  of a one-way spring clutch  50  is connected to the inside surface  44  of the third leg  42  of the clutch cap  26 . As shown in  FIG. 1 , it is preferred that at least a second portion  52  of the one-way spring clutch  50  is also connected to a portion  54  of a clutch drive shaft  56 . 
   The clutch drive shaft  56  is connected to a gear train  60 , as shown in  FIGS. 1 and 2 . The gear train  60  preferably comprises five gears, only three of which are shown in the figures as the other two have been cut-away from the figures for clarity. 
   As best seen in  FIG. 2 , a first gear  62  is connected to the clutch drive shaft  56 . A second gear  64  is connected to a tappet sleeve  68 . And a third gear  66  is drivingly connected between the first gear  62  and the second gear  64 . 
   Those skilled in the art will appreciate that other structures than the gear train  60  may be used to synchronize the clutch drive shaft  56  with the tappet sleeve  68 . The other structures include, but are not limited to, chains and/or cables. Of course, gear trains having greater or fewer gears than those described and depicted herein are within the scope of the invention also. 
   The tappet sleeve  68  is connected to a tappet  70  by any means known to those skilled in the art for axially moving the tappet  70 . The means includes, but is not limited to, one or more cams and/or interlocking helical splines (not shown). 
   As shown in  FIG. 1 , the tappet  70  extends toward a center portion  72  of a rotor  74 , to effect braking as known by those skilled in the art. The tappet  70  can also move away from the rotor  74  to reduce, or eliminate, braking, or to adjust the mechanism  10  to account for brake lining wear or any excessive clearance between the brake pad (not shown) and the rotor  74 . 
   In a preferred embodiment, a fourth gear (not shown) is connected to a second tappet sleeve (not shown). A fifth gear (not shown) is drivingly connected between the fourth gear and the first gear  62 . The second tappet sleeve is connected to a second tappet (not shown) in the same manner the tappet sleeve  68  is connected to the tappet  70 . 
   A preferred method of operating the mechanism  10  comprises engaging the lever  12  to rotate the outer clutch sleeve  14  during a braking action. The outer clutch sleeve  14  is selectively coupled, through the coupling device  30 , to the clutch cap  26  to rotate the clutch cap  26 . The clutch cap  26  in turn rotates the one-way spring clutch  50  and the spring clutch  50  rotates the clutch drive shaft  56 . The clutch drive shaft  56  rotates the gear train  60  causing rotation of the tappet sleeve  68 . The rotation of the tappet sleeve  68  causes the threaded tappet  70  to screw towards the rotor  74 , as known to those skilled in the art. The second tappet sleeve and tappet are similarly simultaneously controlled. 
   The braking force to the rotor  74  continues as the lever  12  motion continues. The increase in the brake clamp force creates a corresponding increase in the rotation drag torque of the tappet sleeve  68 . The rotation drag torque is transmitted through the tappet sleeve  68 , the gear train  60 , the clutch drive shaft, the one way spring, and the clutch cap to the coupling device  30 . If the drag torque is greater than the designed torque capacity of the coupling device  30 , a rotational slip will occur between the coupling device  30  and the outer clutch sleeve  14 . The slip allows the outer clutch sleeve  14  to rotate independently from the clutch drive shaft  56 . The slip decouples the lever  12  motion from the tappet sleeve  68  rotation, preventing continued movement of the tappet  70  and excessive loading of the linkage between the motion of the lever  12  and the tappet sleeve  68  rotation. The reverse rotation of the outer clutch sleeve  14  is decoupled from the adjustment mechanism through the one-way spring clutch  50  to prevent forced reverse adjustment. 
   In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.