Abstract:
An annular disk brake assembly having a housing mounted to a vehicle and a rotor disk mounted to a wheel of the vehicle. Annular brake pads extend parallel to the rotor disk within the housing and are mounted thereto with at least one brake pad being movable axially by an oil applied bladder mounted to the housing to move the first brake pad axially against the rotor disk. The rotor disk is adapted to slide axially to engage the second brake pad when pressure is applied to the rotor disk by the first brake pad and the bladder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part application of International Application No. PCT/CA97/01014 filed on Dec. 30, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to disk brakes and more particularly to improvements in large area contact disk brakes for vehicles. 
     2. Description of the Prior Art 
     The disk brake of the present invention is a disk brake of the type described in U.S. Pat. No. 5,330,034 issued Jul. 19, 1994 and U.S. Pat. No. Re. 35055 issued Oct. 10, 1995 referring to full annular disk brakes for larger vehicles such as trucks. The concept of the full annular disk brake is now proposed for automobiles and light trucks and the present invention relates to a structure of a full annular disk brake for such vehicles. 
     There are obvious advantages in having a complete annular array of friction pads contacting an annular disk on both sides of the disk. The braking or thermal energy distribution is related directly to the thermal resistance associated with both sides of the interface where the heat is generated. In a full annular brake there is a large area to distribute the braking energy more efficiently. 
     It has also been found that vibrations between the inner and outer pads are the major causes for brake squeal. 
     The analysis of vibration response is of considerable importance in the design of brakes that may be subjected to dynamic disturbances. Under certain situations, vibrations may cause large displacements and severe stresses in the brake. The velocity of a vibrating system is in general, proportional to its frequency and hence a viscous damping force increases with the frequency of vibration. Forces resisting a motion also arise from dry friction along a non-lubricated surface. It is usually assumed to be a force of constant magnitude but opposed to the direction of motion. In addition to the forces of air resistance and external friction, damping forces also arise because of imperfect elasticity or internal friction, called hysteric damping, within the body. The magnitude of such a force is independent of the frequency but is proportional to the amplitude of vibration or to the displacement. 
     In a brake system, dynamic loading produces stresses and strains, the magnitude and distribution of which will depend not only on the usual parameters encountered previously but also on the velocity of propagation of the strain waves through the material of which the system is composed. This latter consideration, although very important when loads are applied with high velocities, may often be neglected when the velocity of application of the load is low. Since dynamic loading is conveniently considered to be the transfer of energy from one system to another, the concept of configuration (strain energy) as an index of resistance to failure is important. One of the important concepts is that the energy-absorbing capacity of a member, that is, the resistance to failure is a function of the volume of material available, in contrast to the resistance to failure under static loading, which is a function of cross-sectional area or section modulus. 
     One of the main problems in adapting the technology of a full annular brake system of the type described in the above mentioned patents is the consideration of weight and cost. It would be unrealistic, no matter what the advantages, to assume that the a new full annular brake system would be accepted on the market at a price substantially higher than present day disk brakes. Furthermore any increase of weight compromises the fuel consumption. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide a brake system, especially for automobiles, that has improved heat distribution properties, and reduces the occurrence of wear. 
     It is a further aim of the present invention to provide a brake system that reduces low frequency brake squeal. 
     It is still a further aim of the present invention to provide an annular disk brake system where the maximum brake performance is obtained. 
     A construction in accordance with the present invention comprises a disk brake assembly for a vehicle wheel wherein the wheel includes a hub journaled to an axle on the vehicle, the disk brake assembly comprises a housing mounted to the vehicle and at least an annular rotor disk within the housing and means mounting the disk to the wheel. The rotor disk has at least a first radial planar friction surface and the housing includes a first annular brake shoe provided adjacent the first planar friction surface of the disk and movable axially towards and away from the first friction surface. Means are provided for restraining the first brake shoe from rotating with the disk. The housing also includes an annular radial wall parallel to the first brake shoe, and an annular fluid expandable bladder extends between the first annular brake shoe and the radial wall, whereby upon expansion of the bladder the first brake shoe moves axially to frictionally engage the friction surface of the disk, means for disengaging the first brake shoe from frictional contact with the rotor disk upon release of the fluid from the expandable bladder. 
     In a more specific embodiment of the present invention the radial disk is provided with a second annular friction surface, parallel to the first and on an opposite side of the rotor disk wherein the first and second friction disks have different radii, and a second annular brake shoe adjacent the second annular friction disk wherein brake squeal will be reduced. 
     In a still more specific embodiment of the present invention, the means for retaining the first brake shoe includes a brake shoe backing plate having an annular periphery and the housing includes a concentric wall having an internal surface radially adjacent the periphery of the first brake shoe while the inner surface of the concentric wall and the periphery of the first brake shoe have mating interdigital elements which allow axial movement of the first brake shoe relative to the concentric wall but prevents peripheral movement of the first brake shoe relative to the concentric wall of the housing. 
     In a still more specific embodiment of the present invention, the means for disengaging the first brake shoe from the first friction surface of the rotor disk is at least one rolling seal provided between axially generated adjacent surfaces of the annular radial wall of the housing and the first brake shoe. 
     The features of the present invention can be utilized for large trucks as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in detail having reference to the accompanying drawings in which: 
     FIG. 1 is an exploded fragmentary perspective view of an embodiment of the disk brake in accordance with the present invention; 
     FIG. 2 is a fragmentary radial cross-section taken through the assembled disk brake; 
     FIG. 3 is a radial cross-section similar to FIG. 2 but including further elements; 
     FIGS. 4 a  and  4   b  are enlarged fragmentary cross-section taken along the same section as FIG. 3 but showing the elements in a different operative position; 
     FIG. 5 is a fragmentary radial cross-section similar to FIG. 3 but showing another embodiment; 
     FIG. 6 is a fragmentary perspective view, partially in cross-section, of another embodiment of the present invention; 
     FIGS. 7 a  and  7   b  are enlarged fragmentary radial cross-sections of the embodiment of FIG. 6 showing certain elements in different operative positions; 
     FIG. 8 is a fragmentary perspective view, partly in cross-section, of the embodiment shown in FIGS. 6 and 7; 
     FIG. 9 is an exploded fragmentary perspective view of yet another embodiment of the present invention; 
     FIG. 10 is a fragmentary enlarged radial cross-section of the embodiment shown in FIG. 9; 
     FIG. 11 is a fragmentary perspective view partly in cross-section of another embodiment of the present invention; 
     FIG. 12 is a fragmentary perspective view of a detail of the embodiment shown in FIG. 11; 
     FIGS. 13 a  and  13   b  are fragmentary perspective exploded views taken from opposite sides of yet a further embodiment of a detail of the present invention; 
     FIGS. 14 a  and  14   b  are respectively a fragmentary perspective view and an axial cross-section of a still further embodiment of a detail of the present invention; and 
     FIG. 15 is a fragmentary perspective view of a detail of the embodiment shown in FIGS. 14 a  and  14   b.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and more particularly to FIGS. 1 to  4   b,  a disk brake assembly  10  for an automobile is illustrated having a housing in the form of a shell  12 . The shell  12  has a cylindrical wall  14  with a corrugated inner surface  16  having valleys  16   a  and ribs  16   b.  The housing  12  includes a radial annular wall  18  provided with an annular brake pad lining  20 . The ribs  16   b  are relatively flat and represent valleys  17   b  on the outer surface  17  while ribs  17   a  correspond to valleys  16   a.    
     The cylindrical wall  14  also includes a radial flange  15 . 
     The shell  12  also includes an annular radial wall  22  to which is mounted an annular cylindrical corrugated rim  24  adapted to fit within the corrugated inner surface  16  of the wall  14  and is retained therein by flange  15 . The ribs  24   a  of the corrugated rim  24  fit in the valleys  16   a  of surface  16  while the valleys  24   b  correspond to the ribs  16   b  of the housing wall  14 . Thus, the shell  12  will be locked against circumferential movements relative to the radial wall  22 . The radial wall  22  has a hub portion  26  which can be bolted to a flange on an axle (not shown) of the vehicle. The radial wall  22  also includes an annular radial planar wall portion  28  and a cylindrical flange  30  as shown in FIG.  2 . 
     An indented detent  70  (FIG. 2) is provided in the housing wall  14  in order to lock the shell  12  against axial movement relative to the radial wall  22 . The detent  70  protrudes inwardly to engage the edge of rim  24 . 
     An annular rotor disk  32  includes radial planar friction surfaces  34  and  36  and a cylindrical annular rim  38  having an inner corrugated concentric surface  40  with ribs  40   a  and valleys  40   b.  A hub adapter  42  includes a radial wall portion  44  adapted to be mounted to a vehicle wheel (shown in the embodiment of FIG. 8) and a cylindrical corrugated wall  46 . The wall  46  has ribs  46   a  and valleys  46   b  which are adapted to fit within the inner surface  40  of the rim  38  of rotor disk  32 . Thus, the rotor disk  32  will be locked against rotational movement relative to the hub adapter  42  but is slidable axially thereon. Since the hub adapter  42  is mounted onto a vehicle wheel the rotor disk  32  will rotate with the wheel. The rotor disk  32  is ventilated and therefore has radially extending ventilation passages  48  communicating with openings  49  in housing wall  14 . As shown in FIGS. 1,  2  and  3 , there are axial opening  48   a  that intersect radial openings  48  so as to ensure that as much air as possible passes through the rotor disk  32 . 
     An annular brake shoe  50  includes brake linings  52  and a backing plate  54 . The brake shoe  50  includes a corrugated peripheral edge  51  engaging the inner surface  16  of the cylindrical wall  14 . Thus, the brake shoe  50  can slide axially but is retained against rotational movement relative to the shell  12 . 
     An annular inflatable bladder  56  is provided between the wall portion  28  of radial wall  22  and the backing plate  54 . When fluid such as oil is fed into the inflatable bladder  56  it will expand, moving the brake shoe  50  axially towards the friction surface  36  of rotor disk  32 . The rotor disk  32  will also slide axially on the hub adapter  42 , in response to the force exerted by the inflatable bladder  56 , and the radial friction surface  34  will come in frictional contact with the brake linings  20 . Thus, when it is necessary to apply the brakes, the inflatable bladder  56  is expanded. However, to release the brakes the oil is allowed to drain from the inflatable bladder  56 , thereby releasing the axial force on the brake shoe  50 , allowing the disk rotor  32  to rotate freely within the shell  12 . However, in one aspect of the present invention, means are provided for retracting the brake shoe  50  from the rotor  32  and likewise the rotor  32  from the brake lining  20 . 
     These means are shown in FIGS. 3,  4   a  and  4   b,  that is the rolling seals  62 ,  64  which will now be described. A pair of rolling seals  62  are located, in the present embodiment, on the outer surface of corrugated wall  46  of the hub adapter  42  and are formed to the contour of the corrugated surface. Pairs of circumferentially extending grooves  46   c,    46   d  are defined in wall  46  to receive the rolling seals  62   a  and  62   b  respectively. As shown in FIG. 3, the pair of rolling seals  62   a  and  62   b  are pre-compressed when inserted between the hub  42  and the rim  38  of the rotor disk  32 . Retainer ring  63  may be provided to hold seal  62   a  in place. Retainer ring  63  is formed with convexly curved surface  63   b  to support seal  62   a  and control the deformation of the seal  62   a  as will be described. Likewise the groove wall  65  of groove  46   d  is also formed with convexly curved surface  65   b  to control the deformation of seal  62   b.    
     When the rotor disk  32  slides on the hub adapter  42 , as previously described, the rolling seals  62   a  and  62   b  will be deformed in the direction of the path of the rotor disk  32 , as illustrated by the arrow in FIG. 4 b,  when force is exerted by the inflated inflatable bladder  56  on the brake shoe  50 . When the brakes are released, the rolling seals  62   a,    62   b  will be restored because of the energy stored therein, and will return to the shape as shown in FIG. 4 a,  thereby moving the rotor disk  32  and thus drawing the friction surface  34  away from the brake pad  20 . 
     The rolling seals  62   a  and  62   b  can be selected to provide the right amount of clearance to avoid the drag which might occur when the rotor disk  32  remains in contact with the friction pad  20 . It is important that only a slight clearance be provided in order to avoid undue pedal movement. 
     In the same manner, rolling seal  64  which is located in circumferential groove  30   a  on the flange  30  (FIG. 3) engages the flange of backing plate  54  on the brake shoe  50 , and will act to return the brake shoe  50  away from the friction surface  36  of the rotor disk  32  when the fluid is drained from the inflated bladders  56 , in order to eliminate drag of the brakes. Wiper  66  on the housing  14  seals the brake shoe from debris and dust and supplements the action of rolling seal  64 . 
     Referring back to FIG. 1, the wall  28  is adapted to receive strain sensor  60 . These strain sensors  60  may be the type known under Trademark MULTIDYN and described in U.S. Pat. No. 5,522,270 issued Jun. 4, 1996 to THOMSON-CSF. The strain sensor  60  can provide valuable information on the braking efficiencies and the wear of the brake shoes. 
     The strain sensor  60  extends somewhat tangentially to the wall  28  and can, therefore, monitor the torque being applied between the hub  26  and the cylindrical flange  30  of spider  22 . With the information which can be obtained from strain sensor  60 , the temperature of the brakes can be monitored by means of suitable micro processors. For instance, when the brakes are applied, the pressure is known, and if the heat should increase the torque will be reduced. Increased temperature of the brakes will normally signal brake deterioration or malfunction. 
     Other criteria can also be determined logically from the known pressure, and the torque information provided by the strain sensor  60 . 
     Referring now to FIG. 5 there is shown a modification to the brakes of the present invention. The elements which in FIG. 5 are similar to those in FIGS. 1 to  4  have been raised by  100 . 
     More specifically, the housing  112  is a shell having a cylindrical wall  114  that now includes a smooth cylindrical portion  155  adjacent the corrugated portion  116 . Likewise, the radial wall  122  has a smooth cylindrical wall portion  160  adjacent the corrugated peripheral wall  124 . Thus, when the radial wall  122  is received within the shell or housing  112  the smooth wall portion  160  of radial wall  122  will fit in the smooth cylindrical wall portion  155  of the housing  112 . A ledge  155   a  is formed between the corrugated wall portion  114  and the smooth wall portion  155  which acts as a stopper for the radial wall  122  having complementary peripheral surfaces, that is between the corrugated portion  124  and the smooth portion  160 . This will eliminate the need for indents  70  as shown in the embodiment of FIGS. 1 to  4 . 
     The cross-section of FIG. 5 is taken through the radial wall  122  at exactly the position where the bleed openings  170  and  172  for the bladder  156  are located. 
     A further embodiment of the present invention is disclosed in FIGS. 6 to  8 . The reference numerals in these figures, designating elements which correspond to similar elements in the embodiment of FIGS. 1 through 4, have been raised by  200 . 
     The disk brake  210  is shown mounted to the hub H of a wheel W (FIG.  8 ). Thus, the hub adapter  242  is mounted to the hub H by means of studs. The hub adaptor  242  includes a corrugated wall  246  (FIGS. 6,  7   a  and  7   b ) including ribs  246   a  and valleys  246   b  which mate with the corrugated inner surface  240  of rim  238  which is an integral part of the rotor disk  232 . 
     FIG. 6 illustrates the various elements of this embodiment but without the rotor disk  232 . The rotor disk  232  is illustrated in FIGS.,  7   a,    7   b  and  8 . 
     As previously described, the rotor disk  232  is restrained against circumferential rotation relative to the hub adapter  242  but the rotor disk  232  can slide axially relative to the hub adapter  242 . The rim  238  is notched along each edge thereof to receive rolling seal housings  263  and  265  respectively. Each rolling seal housing  263  and  265  is made of thin wall stamping and is formed as an annular channel having a lateral width which is greater than the diameter of the rolling seals  262   a  or  262   b  respectively. The area of the channel is represented by the numeral  263   b  and  265   b  in FIGS. 7 a  and  7   b.  The bight portion of the channel forms a ramp which is sloped downwardly from left to right in FIGS. 7 a  and  7   b.  Thus, when the rotor disk  232  is slid from right to left to engage the brake shoe represented by brake pad  220 , the rim  238  and rolling seal housings  263  and  265  will move towards the left from the position shown in FIG. 7 a  to the position shown in FIG. 7 b.    
     Observing the position of the rolling seals  262   a  and  262   b,  in FIG. 7 b,  one would recognize that the rolling seals are somewhat squeezed by ramps of the channels  263  and  265 . Thus, the rolling seals have stored energy which can overcome the forces applied to the rotor disks  232  by the bladder  256  when the fluid is released from the bladder  256 , as will be described. Thus, the rolling seals  262   a  and  262   b  will draw the rotor disk  232  away from the brake pad  220  to a position shown in FIG. 7 a.  The rolling seals  262   a  and  262   b  will slide on surface  246  in order to compensate for wear of the brake pad  220 . The rolling seals  262   a  and  262   b  also serve as a suspension to dampen the vibrations between the rotor disk  232  and the hub adaptor  242 . 
     In the present embodiment, the housing shell  212  represented by cylindrical wall  214  and radial wall  218  is a thin wall stamping. A skirt  218   a  is formed at the inner edge of the wall  218  to allow the brake pad  220  including a backing wall  221  to be snapped into position within the housing as shown in FIGS.,  7   a  and  7   b.  The shell  212  may be assembled from the left end side of FIGS. 7 a  and  7   b,  with the portion  255  extending over and concentric with the cylindrical wall portion  224  of the radial wall  222 . A cap  283  which may be hinged in two parts surrounds the enlarged collar portion formed by the extension  255  and has a radial skirt on each edge thereof to form a channel to lock the wall  224  of the radial wall  222  within the housing  212 . 
     FIG. 6 shows how the two-part cap  283  with short extensions  283   a  and  283   b  overlap each other. A coupling member  284  extends over the joint so formed by the ends of the hinged cap  283 . The coupling member  284  includes openings  286  through which pins  288  can pass. These pins are shaped and pass in an area coincident with the valleys in the cap  283 . 
     The bladder  256  is shown here with a U-shaped membrane  256   a  having leg portions which are inserted into slots  276  and  278  within the radial wall  222 . Reinforcement rings  280  and  282  are also placed in these slots to prevent the membrane  256   a  from expanding radially. 
     The brake shoe  250  including the brake pad  252  and backing plate  254 , have a T-shaped configuration with the foot of the T  251  folding back the membrane  256   a  to form an M, as shown in FIGS. 7 a  and  7   b.  Thus, when fluid such as oil is injected through the inlet  272  as shown in FIG. 7 a,  the bladder  256  will expand in the axial direction as shown in FIG. 7 b.    
     A further ring  230  (corresponding to the flange  30  in FIGS. 1 to  4 ) is also inserted into the groove  278  but extends axially from the radial wall  222  to support a rolling seal  264 . The backing plate  254  is provided with a channel shaped groove  257  having the same construction as that described with respect to channels  263  and  265  herein. Thus, when the bladder  256  is expanded, the brake shoe  250  moves towards the left in the drawings of FIGS. 7 a  and  7   b,  applying an axial force against the rotor disk  232  by means of the brake pad  252 , frictionally engaging the friction surface  236 , and further pressing against the rotor disk  232  such that the friction surface  234  engages the brake pad  220 . Once oil is released from the bladder  256 , the rolling seal  264  which has been somewhat compressed as shown in FIG. 7 b,  will overcome the reduced axial force, thereby retracting the brake shoe  250  from the friction surface  236  of rotor disk  232 . Simultaneously, the rolling seals  262   a  and  262   b  will retract the rotor disk  232  from frictional engagement with the brake pad  220 . 
     A wiper  268  is shown mounted to the backing plate  254  to prevent debris from entering into the rolling seal area  264 . Similar wipers (see wiper  66  in FIG. 3) can be provided at other practical locations such as between the backing plate  254  and the cylindrical housing wall  214 . 
     A further embodiment is shown in FIGS. 9 and 10. Reference numerals corresponding to elements which correspond to elements shown in the embodiment of FIGS. 1 through 4 have been raised by  300 . The rotor disk  332  has friction surfaces  234  and  236  at different radial distances from the axis of rotation of the rotor disk. As seen in FIG. 10 more clearly, the opposed friction surfaces  334  and  336  are staggered. The corresponding brake pads  320  and  352  are also constructed to correspond to the radially staggered friction surfaces  334  and  336 . 
     The housing wall  314  is accordingly formed in order to accommodate this difference in radius. It has been found, that the amplitude and difference in amplitude of the vibration between pads such as pads  20  and  52  in the embodiment of FIGS. 1 through 4 were the major factors contributing to the generation of brake squeal. Brake squeal has been found to be a result of self induced vibration phenomena of the various parts. Under certain situations, vibrations may cause large displacements and severe stresses in the brake. The velocity of a vibrating system is, in general, proportional to its frequency and enhance a viscous stamping force increases with the frequency of vibration. 
     It has been found that by having the brake pads  320  and  352  as well as the corresponding annular friction surfaces  334  and  336  on the rotor disk  332  at different radii, these vibrations are at different frequencies and thus reduce the chances of harmonics which helps to reduce the brake squeal and stresses which might occur in the disk brake. 
     Another embodiment is illustrated in FIGS. 11 and 12. In this embodiment the numerals which correspond to numerals in respect of earlier embodiments are the same but have been raised by  400 . 
     Thus the hub adapter  442  now includes a stepped cylindrical wall  446 . A pair of circumferential grooves  446   c  and  446   d  are defined in the outer surface of the cylindrical wall  446 . These circumferential grooves  446   c  and  446   d  correspond to grooves  46   c  and  46   d  in the embodiment described in respect of FIG.  3 . 
     Rolling or rocking seals  462   a  and  462   b  sit within the circumferential groove  446   c  and  446   d,  respectively. The rocking seals  462   a  and  462   b  will be described in more detail. 
     A brake shoe  450  mounting a brake liner  452  is mounted on the circumferential flange  430  which extends from radial wall  422 . The brake shoe  450  is shaped to include a backing plate  454  for the brake liner  452  which is also provided with a lip  451  providing the reverse contour of bladder  456 . The brake shoe  450  is also provided with a peripheral groove  453  adapted to receive an annular rocking seal  464  as will be described further. 
     The radial wall  422  is provided with a cylindrical flange  428 . The radial wall  422  is also provided with a series of radial winglets or fins  475  for the purpose of absorbing and removing heat from within the bladder  456 . The bladder  456  includes hook shaped ribs  456   a  and  456   b  adapted to be inserted in similar shape grooves in the respective portions of rim  424  and flange  428 . The brake assembly in accordance with the embodiment shown in FIG. 11 will operate similarly to the brake assembly in previously described embodiments. 
     It has also been discovered that an antifreeze liquid such as Prestone (Prestone is a trademark for antifreeze of Prestone Products Corporation) could be used as a fluid for the bladder. 
     The rocking seals  462   a,    462   b  and  464 , shown in FIGS. 11 and 12, are an alternative to the rolling seals  62   a,    62   b  and  64  illustrated in FIG.  3 . As shown in FIG. 12 rocking seal  464  includes relatively rigid annularly aligned arcuate segments  465  each with a circumferential rib  465   a.  The segments  465  are attached to an elastomeric body  467  by way of adhesive. The elastomeric annular body  467 , in accordance with the present embodiment, has circumferential concavities  469  on three sides of the body leaving convex ribs on the corners of the body. The rigid circumferential segments  465 , of rocking seal  464 , engage the horizontal surface of the flange  430 . 
     When the brake shoe  450  moves towards the friction surface on rotor  432 , the elastomeric material  467  will be slightly deformed. Once the pressure is released on the brake shoe  450 , the rocking seal  464  under the influence of the resilience of the elastomeric body  467  will cause the brake shoe  450  to move slightly away from the friction surface on the rotor  432 . 
     The rocking seals  462   a  and  462   b  are similar in construction to the rocking seal  464  but the rigid portions  465  thereof are on the outer periphery in order to engage the rim  438 . 
     When pressure is applied to brake shoe  450  by the bladder  456  the brake shoe  450  moves towards the rotor  432 . The brake shoe  450  slides laterally on the horizontal wall  446  until the friction surface  434  engages the brake pad  420 . When the pressure is released on the brake shoe  454  the rocking seals  462   a  and  462   b  will act, similarly to rocking seals  464 , to retract the rotor  432  from engagement with the brake pad  420 . 
     A space  465   b  is illustrated between two rigid segments  465 . Thus, the elastomeric body  467  exerts pressure against the rigid segments  465  to frictionally engage the surface on which the rigid segments are to be in contact with. In this case, the rigid segments  465  are in tight contact with the surface of the flange  430  as shown in FIG.  11 . As discussed, in respect of the embodiments shown in FIGS. 3,  4   a,  and  4   b,  the grooves  453 , in FIG. 11, has a lateral width which is greater than the lateral width of the rigid portion  465  in order to allow relative axial movement of the brake shoe  450  in this case relative to the position of the rigid segments  465 . 
     There are several further embodiments of the so-called rocking seal and the bladder construction. 
     FIGS. 13 a  and  13   b  show an embodiment which is similar in construction to the embodiments shown in FIGS. 6 through 12. The reference numerals which correspond to elements in those embodiments as illustrated in the drawings, have been raised by  500 . The backing plate  554  of the brake shoe  550  is provided in this embodiment with a cylindrical flange  557  and a rocking seal  564  is mounted to the flange  557  within the groove  553  formed in the cylindrical wall extension  530 . 
     A further cylindrical flange  551  extends from the backing plate  554  to which is mounted an insulating annular member  551   a  which engages the inverted U of the membrane  556 . The membrane  556  is provided with enlarged annular beads  556   a  and  556   b  sitting in grooves  576  and  578 , respectively, of the radial wall  522 . A bladder support member  575  is located in a position as shown in FIGS. 13 a  and  13   b  and defines spaced-apart fluid inlet openings  571 . 
     In the present embodiment the inlet  571  is obround in cross-section and is adapted to receive an obround tubular extension  572  extending from the fluid plenum  590 . A recess portion  594  in the tube  572  receives the seal  592  when the parts are assembled. 
     In addition to the rocking seal  564  a plurality of coil springs  596  are attached at one end to the cylindrical flange  557  of the backing plate  554  and at the other end to the wall  522  in order to retract the brake shoe  550  from the friction surface  536  on the rotor  540 . 
     A further embodiment is shown in FIGS. 14 a  to  15 . The reference numerals in these figures have been raised by  600  compared to corresponding reference numerals in FIGS. 1 through 4 b.    
     The disc rotor disk rotor  632  is shown as a solid cast annular rotor without air passages as shown in the previous embodiments. Rather the rotor is cooled by the provision of a series of heat exchange fins  636  extending in the neck formed between the periphery of the rotor and the rim  638 . 
     Likewise, the flange  657  of the backing plate  654  and the backing plate  621  are provided with cooling fins  657   a  and  621   a  respectively. 
     Also shown in this embodiment is the enhanced construction of the annular skirt  618  extending from the housing  612 . The skirt is formed with a reinforced bead centrally of the backing plate  621  such that the bead will apply pressure in the central portion. Cooling fins  621   a  extends from the backing plate and an elastomeric pad  623  is seated on the backing plate  621  and engages the bead  618   a  of the skirt  618  in order to reduce vibrations. 
     The rocking seal  664  is more clearly illustrated in FIG.  15 . 
     In this embodiment, the rocking seal  664  is made of elastomeric material and is preformed to have a somewhat frusto-conical shape with the tip  664   b  closest to the brake shoe  650  and the remote portion of the base  664   c  being closest to the other side of the groove  653  but mounted on the flange  657  of the backing plate  654 . Thus, when the brake shoe  650  is moved towards the rotor  632  the rocking seal  664  will be compressed within the groove  653 , particularly along an axis extending between the tip  654   b  and the remote base portion  664   c.  Once the fluid is released from the bladder  656  the stored energy within the frusto-conically shaped rocking seal will be effective to retract the brake shoe from the rotor. 
     It goes without saying that the rocking seal  662   a  and  662   b  could be operated in a similar manner as rocking seal  664 .