Abstract:
An adjuster arrangement for maintaining first and second brake shoes of a brake between a minimum and a maximum clearance with respect to a drum. The adjuster arrangement has a pawl located on a pivot pin on the first brake shoe and an arm that engages a star wheel of an expandable strut. A first leg of a thermo clip is held against the first brake shoe by the pivot pin and receives thermal energy generated during a brake application. During a brake application, the pawl pivots on the pin and imparts a rotary torque to the star wheel to expand the strut to create a maximum clearance while at the same time a predetermined temperature generated during a brake application acts on the thermo clip to moves the pawl out of engagement with the star wheel to prevent rotating the star wheel and thereby sustain a minimum clearance.

Description:
This invention relates to a thermo clip that acts on a pawl of an adjustment mechanism in a drum brake to limit an increment of adjustment when a temperature generated during a brake application exceeds a predetermined value to assure a minimum running clearance is present after a brake application. 
   BACKGROUND OF THE INVENTION 
   U.S. Pat. Nos. 4,220,227 and 4,502,574 disclose adjuster mechanisms for use in a drum brake to control the extension of a strut member and define a running clearance between a friction surface on brake shoes and a drum member. In particular, the adjuster mechanism includes a pawl carried by a first brake shoe wherein a spring is connected to the pawl and the other brake shoe such that movement of the first brake shoe toward the drum causes the pawl to pivot on the first brake shoe. When a predetermined movement of the brake shoe occurs as when lining wear occurs, the pawl pivots to an extent that a first edge moves past a tooth on a star wheel and when the brake application terminates, the first edge again engages the tooth and rotates the star wheel to cause an extension in the strut member. This type adjuster mechanism functions in an adequate manner for most operations, however, should the drum brake be operated under extreme conditions such as when the thermal energy generated during a brake application caused the drum to expand and now when an input force is applied to moves the friction surface on the brake shoe into engagement with the drum it is possible that the first edge on the pawl will move past several teeth on the star wheel. Now on return of the pawl to a rest position, a second edge engages the star wheel and rotates the star wheel through an arc corresponding to several teeth such that a desired running clearance is largely reduced or non-existent. Thereafter when the drum cools, it contracts and if the running clearance is totally eliminated, the brake will drag until the friction lining is worn away through continued engagement with the drum. Thus, it is important to control the rotation of the star wheel so that the extendable strut is not over adjusted and the brake shoes are maintained with a desired running clearance with respect to the drum. 
   In order to prevent such over adjustment caused by thermal expansion, a bimetallic member has been placed in the expandable strut as disclosed in U.S. Pat. No. 4,390,086 and on an end member to engage the pawl as disclosed in U.S. Pat. No. 6,691,838 and a leg of the pawl as disclosed in U.S. Pat. No. 4,385,681 to move the pawl out of engagement with the star wheel when a predetermined temperature is generated during a brake application. While brakes having such bimetallic members function in an adequate manner it has been observed that under some conditions, over adjustment may still occur as the thermal energy generated during a brake application is not directly conducted into the bimetallic strip but passes though several members and as a result a delay occurs in the disengagement of the pawl and the star wheel. 
   SUMMARY OF THE INVENTION 
   A primary object of the present invention is to provide a v-shaped thermo clip that is held against a web of a brake shoe that directly receives thermal energy generated during a brake application to provide a force that moves a pawl out of engagement with a star wheel of an adjuster arrangement when a predetermined temperature is reached during a brake application. 
   This invention is used with a brake having a first web with friction lining attached thereto and a second web with friction lining attached thereto that engage a friction surface on a drum to effect a brake application. Each engagement of the friction linings with the friction surface of the drum generate thermal energy and as a result the drum may experience some expansion that requires an actuator member to correspondingly move an additional distance during a brake application. During each brake application, some friction lining is worn away and in order to maintain a desired running clearance between the friction lining and the friction surface on the drum, an extendable strut is located between the first web and the second web. The extendable strut includes a first member that is connected to the first web and separated from a second member connected to the second web by an adjuster mechanism that includes a star wheel with a first shaft located in a first bore in the first member and a second shaft with threads thereon that are mated with a threaded bore of the second member and the pawl. A first leg of the pawl is connected by a spring to the second web and a second leg is connected to the star wheel such that during a brake application the pawl pivots and when such pivoting exceeds a predetermined amount the second leg rotates past a tooth on the star wheel and when the brake application is terminated the spring acts on the first leg such that the second leg engages the tooth to move the threaded shaft out of the threaded bore and incrementally extend the extendable shaft such that a maximum running clearance is maintained. In the present invention, a first leg of the v-shaped thermo clip is held against the first web by the pivot pin while a second leg engages the pawl such that thermal energy generated during braking is directly conducted into the first leg of the v-shaped thermo clip. As the temperature of the thermo clip increases the second leg responds by expanding and after overcoming a component force of the spring acts on, moves and holds the pawl away from the star wheel when a predetermined temperature develops during a brake application. When the brake application terminates, the spring moves the pawl to a position of rest however, the second leg does not engage the tooth of the star wheel and thus the thermal expansion of the drum is not a factor in the expansion of the strut and a running clearance that was present prior to the brake application is sustained for a subsequent brake application. 
   An advantage of this invention resides in locating a thermo clip directly on a web of a brake shoe such that an immediate response to a predetermined temperature resulting from the generation of thermal energy moves a pawl out of engagement with a star wheel of an expandable strut to essentially eliminate any effect of thermal expansion on an adjustment of the length of the strut. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a drum brake for a vehicle having a v-shaped thermo clip, according to the present invention, retained on a web by a pivot pin for a pawl and connected to a star wheel of an adjuster arrangement; 
       FIG. 2  is a perspective view of the thermo clip for use in the brake of  FIG. 1 ; 
       FIG. 3  is a view of the drum brake of  FIG. 1  showing the attachment of the thermo clip to a web of a brake shoe; 
       FIG. 4  is a sectional view taken along lines  4 - 4  of  FIG. 1  and  FIG. 4   a  is a sectional view taken along lines  4   a - 4   a  showing a relationship the pivot pin, v-shaped thermo clip, web, pawl and star wheel when the temperature experienced in the drum brake is below a predetermined level; and 
       FIG. 5  is a sectional view taken along lines  4 - 4  of  FIG. 1  and  FIG. 5   a  is a sectional view taken along  4   a - 4   a  showing a relationship the pivot pin, v-shaped bimetallic clip, web, pawl and star wheel when the temperature experienced in the drum brake is above a predetermined level. 
   

   DETAILED DESCRIPTION 
   The drum brake  10  shown in  FIG. 1  for use in a vehicle includes an actuator assembly  12  for effecting a brake application. The drum brake  10  is of a general type as disclosed in U.S. Pat. No. 4,502,574 wherein a first brake shoe  14  is mounted on the backing plate  16  by a first spring clip and pin  20  and a second brake shoe  22  is mounted on the backing plate  16  by a second spring clip and pin  24 . The first brake shoe  14  has a web  15  with a first engagement end  26  located in a guide slot on a projection of an anchor  28  on the backing plate  16  and a second engagement end  30  that is connected to wheel cylinder  32  included in the actuator assembly  12  and a notch that receives a first body or section  34  of an extendable strut  36 . The second brake shoe  22  has a web  23  with a first engagement end  38  located in a guide slot on projection of the anchor  28  and a second engagement end  40  that is connected to wheel cylinder  32  and receives a second body or section  35  of the extendable strut  36 . A first spring  50  attached to webs  15  and  23  urges the first engagement ends  26 ,  38  toward anchor  28  while a second spring  52  is attached to pawl  70  of an adjuster mechanism  100  and web  23  urges the second engagement ends  30 ,  40  toward the wheel cylinder  32  such that the first  14  and second  22  brake shoes are positioned with respect to drum  46  to define a running clearance “C” when the actuator assembly  12  is in a position of rest. The running clearance “C” for the first  14  and second  22  brake shoes is determined by the length of the extendable strut  36  that is located between webs  15  and  23 . 
   The extendable strut  36  is defined by a star wheel  80  that is located between a first body  34  and a second body  35 . The first body  34  is essentially a cylinder having a first end  37  with a slot for receiving web  15  of brake shoe  12  and a second end defined by a smooth a bore or socket  39  therein for receiving a corresponding smooth shaft  82  of the star wheel  80 . The star wheel  80  in addition to smooth shaft  82  has a threaded shaft or stem  84  that is mated with a threaded bore  86  in the second body  35 . The second body  35  is essentially a cylinder with a slot  87  for receiving web  23  of brake shoe  22 . The star wheel  80  has a plurality of teeth  81 , 81 ′ . . .  81   n  having a uniform pitch wherein each tooth represents an equal arcuate segment or increment that establishes a limit for modifying the running clearance “C” during a single brake application. 
   The pawl  70  along with the extendable strut  36  is part of the adjuster mechanism  100  and is defined a base or first leg  90  having a first slot  92  for receiving a pivot pin  94  that is fixed to web  15  for located the base  90  on web  15  to permit pivotal movement of the pawl  70 , an opening or groove  96  for receiving a first end of spring  52  that is connected to web  23 , a second slot  98  for receiving the first end  37  of the first body  34  of the extendable strut  36  and an second leg or arm  102  that extends from the base or first leg  90  to align a blade  104  thereon in perpendicular alignment with a first tooth  81  of the plurality of teeth on the star wheel  80 . A thermo clip  200  as best shown in  FIGS. 2 and 3  that is made of a bimetallic material is located on the pivot pin  94  between the pawl  70  and web  15  to control the engagement of the blade  104  with the teeth  81 , 81 ′ . . .  81   n  on star wheel  80  as a function of the temperature experienced by or present in web  15 . 
   In more detail, the thermo clip  200  as illustrated in  FIG. 2  has a v-shape defined by a first leg  202  hinged to a second leg  204 . The first leg  202  has a first opening  206  therein while the second leg  204  has a second opening  208  therein with the first opening  206  having a smaller diameter than the second opening  208  such that a shoulder  95  on pivot pin  94  passes through the second opening  208  and engages the first leg  202  as the end  93  on pivot pin  94  passes through the first opening  206  to hold the first leg  202  tight against the first web  15  as shown in  FIGS. 4 and 4   a . Spring  52  provides an axial force that urges the first leg  90  of pawl  70  into engagement with the second leg  204  of the thermo clip  200  that is retained on web  15  by pivot pin  94  such that the temperature as present in the web  15  is directed into the thermo clip  200 . 
   With the actuator mechanism  100  in a rest position and the temperature experienced by brake  10  is below a predetermined level, the pawl  70  and star wheel  80  have a relationship as illustrated in  FIGS. 1 and 4  wherein blade  104  on leg  102  of pawl  70  engages a first tooth  81  and the legs  202  and  204  of the thermo clip  200  are essentially parallel. Spring  52  acts on the first leg  90  of pawl  70  to bias the extendable strut  36  into engagement with webs  15  and  23  and the second engagement ends  30  and  40  into engagement with wheel cylinder  32  to define the running clearance “C”. 
   The length of the extendable strut  36  may be altered by rotating star wheel  80  on the first body  34  with respect to this second body  35  to establish and maintain a predetermined running clearance “C” between a first friction surface  13  on brake shoe  14 , a second friction surface  21  on brake shoe  22  and friction surface  45  on drum  46 . The first brake shoe  14  is urged toward the second brake shoe  22  by the action of the first spring  50  urging the ends  26 ,  38  toward the anchor post  28  and the second spring  52  urging the second ends  30 ,  40  toward wheel cylinder  32  with the length of the extendable strut  36  located between and webs  15  and  23  limiting the movement of the second ends  30 ,  40  toward the wheel cylinder  32 . 
   Mode of Operation 
   A hydraulic input force from actuator assembly is supplied to wheel cylinder  32  and acts on the second engagement ends  30  and  40  of webs  15  and  23  of the first  14  and second  22  brake shoes. After overcoming the force of springs  50  and  52 , the input force moves the first  14  and second  22  brake shoes through the running clearance “C” to bring friction surfaces  13  and  21  into engagement with the friction surface on drum  46  to effect a brake application. As web  15  moves from the position of rest, the first leg  90  of pawl  70  pivots on pin  94  such that blade  104  slides toward the apex of a first tooth  81 . If the distance that the web  15  moves exceed a predetermined distance, the first leg  90  pivots sufficiently such that the first blade  104  moves past the apex of tooth  81  and toward a second tooth  81 ′. When the hydraulic input supplied to the wheel cylinder  32  terminates, the input force applied to the second engagement ends  30 ,  40  also terminates and springs  50  and  52  return the first  14  and second  22  brake shoes to a rest position as illustrated in  FIG. 1 . As web  15  returns to the position of rest, spring  52  acts on the first leg  90  of the adjustment mechanism  100  to rotate pawl  70  to a position of rest such that the first blade  104  now engages tooth  81  and rotates the star wheel  80  to a position where tooth  81 ′ is perpendicular to the blade  104  rather than tooth  81 . The rotation of the star wheel  80  is defined an increment of the pitch of tooth  81  and as a result adjustment of the extendable strut  36  in establishing a running clearance “C” resulting from an hydraulic input force supplied to wheel cylinder  32  is limited during a brake application. 
   During a brake application, thermal energy is generated through the engagement of friction surfaces  13  and  21  with the friction surface  45  on drum  46 . The thermal energy is directly carried into web  15  and with the first leg  206  of the thermo clip  200  is tight against the web  15  by shoulder  95  on pivot pin  94 , the same temperature present in the web  15  is also present in the thermo clip  200 . 
   The thermo clip  200  being a bimetallic member responds to temperature change such that the second leg  208  pivots about the hinge to develop an axial force that acts on the first leg  90  of pawl  70  and after overcoming the axial component of the spring force  52  moves the first leg or base  90  and correspondingly the second leg or arm  102  away from the web  15  as illustrated in  FIGS. 5 and 5   a . When the second leg  208  has moves about 15 degree with respect to web as a result of a temperature of about 200 degree Fahrenheit being generated during a brake application, the second leg  208  will have moved sufficiently that blade  104  is moved out of contact with tooth  81  on the star wheel  80  as illustrated in  FIG. 5  and an thermal expansion of the drum  46  that could effect the pivoting motion of pawl  74  is rendered in effective. On termination of the actuation force, the return spring  52  acts to return the first brake shoe and second brake shoe to a position of rest but the blade  104  does not engage tooth  81  and a minimum running clearance present before the brake application remains between the friction surfaces  13  and  21  and the friction surface  45  on drum  46 .