Abstract:
A brake caliper that has the same performance as a brake caliper with multiple pistons despite its compact structure, in which drag resistance between a brake pad and a brake disk is minimized, that is less likely to produce non-uniform wear of brake pad and brake squealing, and whose weight and cost are suppressed. Pistons ( 23 ) are formed in a circular hollow cylindrical shape, and apply pressing force to a disk rotor ( 1 ) by circular pressing parts. Piston-containing holes ( 21 ) recessed in a circular shape and containing the pistons ( 23 ) so that they can slide in the axial direction of the pistons are formed in caliper bodies ( 100, 105 ).

Description:
[0001]    This is a continuation of International Application PCT/JP2007/070279 (published as WO 2008/047840 A1) having an international filing date of Oct. 17, 2007, which is based on and claims priority from JP 2006-284594 filed on Oct. 19, 2006, JP 2007-013658 filed on Jan. 24, 2007 and JP 2007-137353 filed on May 24, 2007, the contents of which is incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a brake caliper of a disk brake that is used in a vehicle such as an automobile or a motorcycle. 
       DESCRIPTION OF THE RELATED ARTS 
       [0003]    There are two types of brake caliper; a brake caliper of the first type has at least one piston on only one side of a brake disk (a disk to be braked), while a brake caliper of the second type has at least one piston on each side of a brake disk; in the case of the second type, a piston on one side and a piston on the other side face each other across the brake disk. Many performance cars adopt a disk brake of the second type; some racing cars or rally cars (either of which is a typical performance car) are provided with a brake caliper having three or more pistons on each side of a brake disk, for a total of six or more pistons. 
         [0004]    An example structure of a conventional brake caliper is shown in  FIGS. 13 to 15 . 
         [0005]      FIG. 13  shows a perspective view that depicts an outline fitting arrangement as to a conventional brake caliper. A brake caliper  2  generally comprises an outer piece  3 , an inner piece  4 , and a bolt  5  fastening the pieces  3  and  4 ; whereby, the brake caliper  2  is arranged so that the caliper  2  sandwiches a disk rotor  1  with the pieces  3  and  4 . 
         [0006]      FIG. 14  shows a cross section that depicts a configuration as to the conventional brake caliper  2 . The outer piece  3  comprises: an outer caliper body  100 ; a hole  101  for housing a piston (a brake piston)  6  so that the cylindrical piston  6  can be guided by the hole  101  as well as slide along an axis of the hole  101 ; thereby, a hydraulic chamber  7  is demarcated (implemented) between the bottom wall of the hole  101  and the head (top) wall of the piston  6  so that hydraulic oil (brake fluid) which is pressurized by a master hydraulic cylinder (not shown) is supplied to the hydraulic chamber  7  through a feed oil pipe (not shown). Further, a square seal  8  as a seal element is provided between an outer periphery of the piston  6  and a cylindrical periphery wall of the hole  101 , so that the brake fluid does not leak; and, a dust seal  9  is provided so as to prevent intrusion of the impurities from outside (the brake fluid supply side). 
         [0007]    In the outer piece  3 , a brake pad  10  is configured so as to be adhesively fixed to a backing plate  104 ; the brake pad  10  is pressed against the disk rotor  1  via the backing plates  104  in response to the movement of the piston  6 , so that braking force is generated. 
         [0008]    In the inner piece  4 , the situation is the same as in the outer piece  3 ; the inner piece  4  comprises: an inner caliper body  105 , and a hole  101  for housing a piston  6 . Thus, the outer piece  3  and the inner piece  4  are integrated into the brake caliper  2  so as to configure the caliper  2 . 
         [0009]      FIG. 15  shows a front view that depicts a relative arrangement as to the brake pad  10  and the piston  6 , a part of the view showing a cross section. As shown in  FIG. 15 , in contrast with the brake pad  10 , the piston  6  is considerably smaller; a region that is not pressed by the piston  6  is rather broad; namely, the piston  6  cannot sufficiently press most of a region on the outside of a circle (the piston area) of the piston  6 , in particular, on the vehicle center side; where  FIG. 15  is concerned, the vehicle center side corresponds to the lower part of  FIG. 15 . This situation causes distortion of the backing plates  104  and becomes a factor behind uneven wear of the brake pad  10 . Further, a free vibration of the region on which the piston  6  cannot sufficiently press becomes a factor behind brake squealing. 
         [0010]    In order to prevent the uneven wear of the brake pad  10  or the brake squealing, many kinds of brake calipers are known, the calipers having three or more pistons on each side of a brake disk; for instance, the patent reference 1 discloses a brake caliper having three pistons on each side of a brake disk so as to press more evenly on the backing plate and thence on to the brake pad. 
         [0011]    On the other hand, many kinds of contrivances are known whereby a square seal is provided on a circumference surface of a brake piston; after hydraulic pressure is released, by means of the restoring force of the seal, the brake piston returns to a position where the piston is placed before the piston is pressed by the hydraulic pressure against a brake disk via a backing plate and a brake pad; on the other hand, the piston is re-activated by a generated hydraulic pressure so as to press the brake disk via the brake pad. 
         [0012]    For instance, the patent reference 2 discloses an ideal characteristic quadratic curve as to piston return-back displacement with respect to hydraulic pressure during pressure release, the curve being defined as a superposition of a caliper deformation curve and a brake pad compression curve. In the disclosed contrivance, two parameters are varied; namely, they are the rubber hardness of the square seal, and the size of the chamfer volume as to the corner of the groove for the square seal; and, it is shown that a solution that approximates the ideal curve characteristics can be obtained by selecting pertinent levels for the two parameters. 
       QUOTED REFERENCES 
     References 
     Patent Reference 1: JP2002-213502 
     Patent Reference 2: JP1998-325432 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0013]    The patent reference 1 discloses a brake caliper provided with three pistons on the outer piece side and on the inner piece side, respectively; however, the structure of the brake caliper is complicated, and the number of manufacturing man-hours is increased; further, because of the extra space needed for housing an increased number of pistons, the rigidity of the caliper body is reduced; for the purpose of compensating the lowered rigidity, the caliper body has to be reinforced; as a result, disadvantages of increased caliper mass and increased cost are incurred. 
         [0014]    In general, many performance cars have disk brakes in which two or more pistons are provided on each side of their brake disks thereby a pair of pistons on each side face each other across the brake disk; on the other hand, the area of the brake pad is determined in response to vehicle mass, regardless of the number of the pistons; in other words, the brake pad area is increased in proportion to vehicle mass. Thus, for instance, in a case where at least two pistons are provided on each side of the disk rotor, a region of the brake pad that is pressed by the pistons extrudes toward the neighboring regions around the pressed region, which causes uneven wear and/or brake squealing. 
         [0015]    In order to significantly reduce the distortion of the backing plate and thence the brake pad when pressed by the piston, a manner providing at least three pairs of pistons can be taken into consideration in view of a brake pad structure; however, since it becomes necessary that additional pistons be manufactured, and the space housing the additional pistons be provided in the brake caliper; accordingly, this manner is likely to incur mass and/or cost increases. Further, according to this manner, the neighboring pistons are placed close to each other; thus, the rigidity of the brake caliper is lowered, when a high brake fluid pressure is applied; as a result, for the purpose of compensating the lowered rigidity, the parts surrounding the pistons have to be reinforced; therefore, further mass and/or cost increases are incurred. 
         [0016]    On the other hand, the patent reference 1 discloses an ideal characteristic quadratic curve as to piston return-back displacement with respect to hydraulic pressure during the pressure release, the curve being defined as a superposition of a caliper deformation curve and a brake pad compression curve; the reference 1 discloses a contrivance that a characteristic corresponding to the ideal curve is approximately obtained by means of adjusting two factors, namely, the rubber hardness of the square seal, and the size of the chamfer volume as to the corner of the groove for the square seal. However, further improvements are required so as to obtain an ideal characteristic that satisfactorily resembles the ideal curve as to the piston return-back displacement. 
         [0017]    There are two kind of problems in finding the two factors at the same time; one problem relates to securing for the driver good brake pedal ‘feel’ during low brake fluid pressure when the brake pedal is lightly stepped on as well as during high brake fluid pressure under such a condition that an ABS (Anti-lock Braking System) works; the other problem relates to resolving a dragging phenomenon between the brake pad and the brake disk, during normal driving. 
         [0018]    In general, a brake caliper generates a brake force in a manner that at least one piston on each side of the brake disk is provided so as to form at least one pair of pistons; the pistons of each pair face each other across the brake disk; an applied brake fluid pressure thrusts the pair of pistons toward the brake disk so that the pair of brake pads sandwiches the brake disk from both sides. 
         [0019]    Further, on the outer periphery of the piston, a square seal is provided so as to prevent the brake fluid from leaking outward, as well as, so as to return the piston back to the original position where the piston is placed before a braking action, in response to the release of the braking action or the brake fluid pressure. 
         [0020]    Regardless of the provision of the square seal, the brake fluid pressure varies every moment; at some point in time, the pressure is high, and at another point in time, the pressure is low; further, as the brake pad wears down, the original position of the piston before the brake pedal is stepped on moves closer to the brake disk in response to the wear of the pad; accordingly, the deformation of the square seal in shape varies with each repeated braking action; thus, in a conventional technology, it is difficult to provide a square seal that prevents brake fluid leakage under all the braking conditions such as a low brake fluid pressure condition or a high brake fluid pressure condition; it is also difficult to provide a square seal that satisfies the requirement as to the piston return-back characteristic. 
         [0021]    In view of the situation as described above, the present invention aims to provide a brake caliper: being of a compact structure as well as having the same level of performance as in a conventional brake caliper with one or more pairs of pistons; reducing the dragging resistance between the brake pad and the brake disk to a minimum level; avoiding uneven wear or brake squealing as to the brake pad; doing away with mass and/or cost increases; and securing a good brake pedal ‘feel’. 
       Means to Solve the Problem 
       [0022]    A first aspect of the present invention as a measure to overcome the difficulties as described above is a brake caliper for a disk brake, the caliper being provided with a pair of brake pistons so that the pair of pistons face each other across a disk rotor, wherein, the piston forms a shape of a thick-walled cylinder, namely, a cylinder with a hollow co-cylinder therein, thereby an annular ring shaped pressing part of the piston thrusts a force against the disk rotor; an annular ring shaped hole space for housing the piston is incorporated into the caliper body, in response to the thick-walled cylinder shape of the piston, so that the piston slides in the hole space, along the center axis of the piston, being guided by the hole. 
         [0023]    According to the above first aspect of the present invention, the pressing part of the piston is of an annular ring shape while the pressing part of a conventional piston is of a solid cylinder shape; thus, the outer diameter of the piston area, namely, the outer diameter of the piston pressing part in the present invention can be larger than the diameter of the piston pressing part in the conventional piston, even though the piston area of the piston of the present invention and the piston area of the conventional technologies are the same. 
         [0024]    For instance, in view of a conventional brake caliper having two pairs of pistons, a region of the brake pad that is pressed by the pistons extrudes toward the outside of the pressed region, especially toward the vehicle wheel axis of rolling rotation; thus the brake piston in the conventional technology suffers from uneven wear or brake squealing as to the brake pad; on the contrary, in this invention, an extruded region can be substantially reduced in size thanks to the increased outer diameter of the piston area; accordingly, the brake pad can be pressed with an even surface pressure distribution pattern over the whole brake pad; a clearance gap between the brake pad and the disk rotor can be narrower; as a result, an excellent braking force can be achieved, and the problem of brake squealing can be removed. 
         [0025]    Further, in the first aspect of this invention, the problem as to the extrusion of the brake pad can be settled without increasing the pairs of pistons in number to more than three pairs; thus, manufacturing of additional pistons, machining of additional piston-housing-hole spaces, reinforcing of the parts around the additional spaces can be dispensed with; as a result, the disadvantage of mass and/or cost increases can be avoided. 
         [0026]    In the next place, preferably in the described first aspect, as a mode thereof, a center protrusion part is provided so as to form the inner diameter periphery wall of the annular ring shaped hole space for housing the piston; and, the center protrusion is formed as a member separated from the caliper body. 
         [0027]    According to the structure (as the mode of the first aspect) just described above, in contrast to a structure whereby the center protrusion is provided as a part of the caliper body, the brake caliper in this mode can dispense with a complicated boring-machining of the hole space for housing the piston; therefore, more cost-effective mass production can be achieved. 
         [0028]    Further, in a conventional brake caliper, an inner piece and an outer piece of the brake caliper body are manufactured as divided members, so that the two members are fastened with a bolt; thus, it is difficult to achieve a sufficient level of rigidity as to the whole caliper, in consideration of a case when a high brake fluid pressure is applied; however, in this mode of the first aspect, the inner piece and the outer piece are combined into a single piece, in spite of the incorporation of the annular ring shaped hole space for housing the piston; and, the caliper rigidity is not reduced under a high brake fluid pressure condition, partly because of the elimination of the fastening bolt that reduces the stiffness of a brake caliper; as a result, a brake caliper of lower mass and of enhanced rigidity and excellent braking performance can be realized. 
         [0029]    In the following place, a variation of the above mode is preferably the brake caliper (the caliper-protrusion separation structure), whereby the center protrusion part forms a part of the caliper body, and the center protrusion part is separated from the remainder of the caliper body in the direction towards the center axis of the piston. Another variation of the above mode is preferably the brake caliper, whereby a top protrusion part of the center protrusion part is separated from the center protrusion part itself; in this variation, the center protrusion part includes the top protrusion part and a base protrusion part. 
         [0030]    In the case where the center protrusion part forms a part of the caliper body, and the center protrusion part is separated from the remainder of the caliper body in the direction of the center axis of the piston, in consideration of machining the annular ring shaped hole space for housing the piston, a complicated boring-machining of the hole space can be eliminated, thereby making the caliper body much easier to mass produce. 
         [0031]    In the other case where the top protrusion part as to center protrusion part is separated from the rest of the center protrusion part, the annular ring shaped hole space can be easily machined, since a machining jig for a complicated boring-machining of the hole space is not required, thereby eliminating any possible risk of the machining jig&#39;s colliding with the top protrusion part of the center protrusion; as a result, because of the elimination of this risk, enhanced machining accuracy can be secured with enhanced rigidity of the jig, and enhanced productivity can be obtained as well. It is noted that the remaining part described just above means the center protrusion part from which the top protrusion part is removed; namely, the remaining part is a base (center) protrusion part. 
         [0032]    Regarding the separated structure between the center protrusion part and the caliper body, or, between the top protrusion part and the base protrusion part, a fastening method such as a screw mechanism, a friction welding, a diffusion bonding, a welding or an adhesive bonding is preferably applied so that the separated structure is combined into a single piece. 
         [0033]    Since the separated two parts are fastened with a screw mechanism, or another method such as a friction welding, the center protrusion part can be formed without significantly changing a machining operation or process from that in a conventional brake caliper in which a center protrusion part dose not exist. 
         [0034]    In the separated structure as described above, an inner seal groove for housing an inner seal that slides on and comes in contact with the inner periphery wall for the piston is preferably provided on the side of the outer diameter periphery wall of the center protrusion part or the top protrusion part of the center protrusion part. 
         [0035]    Thus, since the inner seal groove for housing an inner seal that slides on and comes in contact with the inner periphery wall for the piston is formed in a member separated from the caliper body, the inner seal that slides on and comes in contact with the inner periphery wall for the piston can be placed in a determined position with accuracy, only with providing the center protrusion part or the top protrusion part of the center protrusion part each of which has the accurately pre-machined inner seal groove. 
         [0036]    In the case where the top protrusion part of the center protrusion part is separated from the remainder of the center protrusion part, the outer diameter of the top protrusion part is preferably larger than the outer diameter of the remainder (the base protrusion part) of the center protrusion part. 
         [0037]    In the structure as described above, since the outer diameter of the top protrusion part is larger than the outer diameter of the base protrusion part, a fine machining for the outer diameter of the base protrusion part can be dispensed with; thereby, what is required is only installing the top protrusion part, which is accurately pre-machined, into the brake caliper. 
         [0038]    As a result, the inner seal that slides on and comes in contact with the inner periphery wall for the piston can be easily provided in the brake caliper. 
         [0039]    Further, preferably in the first aspect of the present invention, an outer clearance is provided between the outer periphery wall of the piston and the outer diameter periphery wall of the piston-housing hole, as well as, an inner clearance is provided between the inner periphery wall of the piston and the inner diameter periphery wall of the piston-housing hole; whereby a difference in amount between both clearances is made. 
         [0040]    According to the structure of the brake caliper as just described above, the outer clearance is provided between the outer periphery wall of the piston and the outer diameter periphery wall of the piston-housing hole, as well as, the inner clearance is provided between the inner periphery wall of the piston and the inner diameter periphery wall of the piston housing hole; and, a difference in amount between both clearances is made; this point brings a structural advantage in regard to strength, as to the center protrusion part that is formed with the inner diameter periphery wall of the piston housing hole, because the center protrusion part can be less prone to be pushed by a force such as causes a bending moment to the center protrusion part; a detail as to this advantage will be explained in the later description of the third embodiment. 
         [0041]    Moreover, a piston can surely return back to an original position where the piston is placed before a braking action, when the brake fluid pressure is released. 
         [0042]    In addition, preferably in the first aspect of the present invention, the caliper having the piston of the annular ring type is provided with a hollow space inside of the ring itself. It is noted that the piston in the present invention is of a shape of a thick-walled cylinder; therefore, the shape of the piston is also of an annular ring type. 
         [0043]    With the structure as above, since the ring itself as the piston shape is not of a solid body type but a hollow body type, the piston can be of a lower mass; moreover, the thermal capacity of the piston can be optimized so that the piston endures the heat generated during a braking action. 
         [0044]    A second aspect of the present invention as a measure to settle the difficulties in conventional technologies is a brake caliper for a disk brake, the caliper being provided with a pair of brake pistons so that the pair of pistons face each other across a disk rotor, 
         [0000]    wherein, 
         [0045]    the piston forms a shape of a thick-walled cylinder, thereby an annular ring shaped pressing part of the piston thrusts a force against the disk rotor; 
         [0046]    an annular ring shaped hole space for housing the piston is formed in a caliper body, in response to the thick-walled cylinder shape of the piston, so that the piston slides in the hole space, along the center axis of the piston, being guided in the hole space; 
         [0047]    an outer seal is provided between the outer periphery of the piston and the outer periphery of the annular ring shaped hole space for housing the piston, as well as, an inner seal is provided between the inner periphery of the piston and the inner periphery of the annular ring shaped hole space for housing the piston. 
         [0048]    According to the above second aspect, since not only the outer seal on the outer periphery of the piston but also the inner seal on the inner periphery of the piston are provided, the piston can be surely returned back to an original position before a braking action, thanks to the enhanced deformation restoring capability as to each seal, when the brake fluid pressure is released; thus, a dragging phenomenon between the brake disk and the brake pad can be less prone to be caused. 
         [0049]    Another preferable mode of the above second aspect is that the outer seal has a higher stiffness than the inner seal. 
         [0000]    By specifying a stiffness characteristic of each seal as is described above, an ideal seal characteristic for restoring the piston displacement or the seal deformation can be achieved by means of combining the different characteristics of the seals, namely, by means of combining the higher shearing stiffness of the outer seal around a larger diameter periphery with the lower shearing stiffness of the inner seal around a smaller diameter periphery. 
         [0050]    In other words, according to the above mode of the second aspect, an ideal seal characteristic can be achieved so as to ideally restore the piston displacement to a proper position, or so as to ideally restore the seal deformation to a proper shape; whereby, a vehicle driver cannot feel any backplay (backlash) when stepping on the brake pedal after a surplus return-back of the brake piston; further, a dragging phenomenon between the brake pad and the brake disk can be avoided during a low brake fluid pressure under a condition that the brake pedal is lightly stepped on as well as during a high brake fluid pressure under such a condition that an ABS (Anti-lock Braking System) works. 
         [0051]    According to the first aspect of the present invention, the pressing part of the piston is of an annular ring shape while the pressing part of a conventional piston is of a solid cylinder shape; thus, the outer diameter of the piston area, namely, the outer diameter of the piston pressing part in the present invention, can be larger than the diameter of the piston pressing part in the conventional piston, even though the respective piston areas are the same; thus, the brake caliper in the present invention can be of a compact structure, and has the same level of performance as in a conventional brake caliper with at least two pairs of pistons. 
         [0052]    In view of a conventional brake caliper having, for instance, two pairs of pistons, a region of the brake pad that is pressed by the pistons extrudes toward the outside of the pressed region, especially toward the vehicle wheel center axis; thus the brake piston in the conventional technology suffers from uneven wear or brake squealing as to the brake pad; on the contrary, in this invention, an extruded region can be significantly reduced in size thanks to the increased outer diameter of the piston; accordingly, the brake pad can be pressed with an even surface pressure distribution pattern over the whole brake pad; the clearance gap between the brake pad and the disk rotor can be reduced; as a result, an excellent braking force can be achieved, and the problem of brake squealing can be removed. 
         [0053]    Further, in the first aspect of this invention, the problem as to the extrusion of the brake pad can be settled without increasing the pairs of pistons in number to more than three pairs; thus, manufacturing of additional pistons, machining of additional piston-housing hole spaces, reinforcing of the parts around the additional spaces can be dispensed with; as a result, any disadvantages of mass and/or cost increases can be avoided. 
         [0054]    According to the above second aspect of the present invention, since not only the outer seal on the outer periphery of the piston is provided, but also the inner seal on the inner periphery of the piston is provided, the piston can be surely returned back to its original position before a braking action, thanks to the enhanced deformation restoring capability as to each seal, when the brake fluid pressure is released; thus, a dragging phenomenon between the brake disk and the brake pad is significantly less likely. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0055]    In what follows is described a non-limiting example of a preferred embodiment which is visualized in the accompanying drawings, in which: 
           [0056]      FIG. 1  shows a cross section that depicts a brake caliper according to a first embodiment of the present invention; 
           [0057]      FIG. 2  shows a front view that depicts a fitting arrangement as to the brake caliper according to the first embodiment of the present invention, a part of the view showing a cross section; 
           [0058]      FIG. 3  shows a cross section that depicts a brake caliper according to a second embodiment of the present invention; 
           [0059]      FIG. 4  shows a cross section that depicts how a square seal is located, the seal sliding on a surface of a piston while coming into contact therewith; 
           [0060]      FIG. 5  shows a cross section that depicts how the square seal deforms while the piston is being activated; 
           [0061]      FIG. 6  shows characteristic curves as to the relationships between piston displacements (or return-back displacements) and hydraulic pressures (brake fluid pressures), while the hydraulic pressures are being released; 
           [0062]      FIG. 7  shows a cross section that depicts the clearances between a caliper body and the piston as well as between a center protrusion of the caliper and the piston according to the third embodiment of the present invention, whereby the caliper body as well as the protrusion slides on and comes in contact with the piston through the clearances or the square seals; 
           [0063]      FIG. 8  shows a cross section that depicts the brake caliper according to a fourth embodiment of the present invention; 
           [0064]      FIG. 9  shows a cross section that depicts the brake caliper according to a fifth embodiment of the present invention; 
           [0065]      FIG. 10  shows a cross section that depicts the brake caliper according to another mode of the fifth embodiment of the present invention; 
           [0066]      FIG. 11  shows a cross section that depicts a detailed center protrusion according to a sixth embodiment of the present invention; 
           [0067]      FIG. 12  shows a cross section that depicts the brake caliper according to a seventh embodiment of the present invention; 
           [0068]      FIG. 13  shows a perspective view that depicts an outline fitting arrangement as to a conventional brake caliper; 
           [0069]      FIG. 14  shows a cross section that depicts a configuration as to a conventional brake caliper; 
           [0070]      FIG. 15  shows a front view that depicts a fitting arrangement as to a conventional brake caliper, a part of the view showing a cross section. 
       
    
    
       [0071]    The items with the numerals in the figures are explained as follows:
         1  a disk rotor;     2  a brake caliper;     3  an outer piece     4  an inner piece     5  a bolt;     6  a piston (a brake piston);     7  a hydraulic chamber;     8  a square seal;     9  a dust seal;     10  a brake pad;     21  a hole for housing the piston, a piston-housing hole, or a piston containing hole;     22  a center protrusion, or a center protrusion part;     23  a piston, or a brake piston;     24  a hollow space;     25  a hydraulic chamber;     27  a force pressing part, or a thrusting part;     29  an engaging hole;     31  a square seal;     33  a dust seal;     36  a square seal;     38  a chamfer;     40  an end plug;     43  a machined (machining) hole for housing the element  40 ;     45  a fastening screw, or a screw mechanism;     47  a step;     49  a fitting surface;     50  a guide member;     52  a base protrusion part, or a base (part);     54  a thread part;     55  pressure welding;     56  a recess;     58  an inner seal groove;     60  a dust boot;     100  an outer caliper body;     101  a hole for housing the piston, a piston-housing hole, or a piston containing hole;     104  a backing plate;     105  an inner caliper body.       
 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0109]    Hereafter, the present invention will be described in detail with reference to the embodiments shown in the figures. However, the dimensions, materials, shape, the relative placement and so on of a component described in these embodiments shall not be construed as limiting the scope of the invention thereto, unless especially specific mention is made. 
       A First Embodiment 
       [0110]    Hereby, a first embodiment according to the present invention is explained based upon  FIGS. 1 and 2  where the same numeral symbols are used as those in  FIGS. 13 to 15  as to the conventional technology, in the case where a numeral symbol is common to the present invention and the conventional technology. 
         [0111]    The whole configuration of a brake caliper  2  in  FIGS. 1 and 2  is basically the same as the fitting arrangement of the brake caliper  2  in  FIGS. 13 to 15 . The brake caliper  2  is configured so as to sandwich a disk rotor  1  with an outer piece  3  and an inner piece  4 ; and, the brake caliper comprising: the outer piece  3 , the inner piece  4 , and a bolt  5  fastening the pieces  3  and  4 . 
         [0112]    As shown in  FIG. 1 , the outer piece  3  comprises: an outer caliper body  100 ; a hole  21  for housing a piston (a brake piston)  23  so that the piston  23  can be guided in the hole  21  as well as sliding along an axis of the hole  21 ; thereby, a hydraulic chamber  25  is demarcated (implemented) between the bottom wall of the hole  21  and the head wall of the piston  23  so that hydraulic oil (brake fluid) which is pressurized by a master hydraulic cylinder (not shown) is supplied to the hydraulic chamber  7  through a feed oil pipe (not shown). 
         [0113]    The piston  23  is configured so as to form an annular tubular shape (or an annular ring shape, or a shape of a cylinder with a hollow co-cylinder, or a shape of a thick-walled cylinder), and the piston is provided with a thrusting part  27  of an annular shape in the piston itself; thereby, the thrusting part  27  transfers pressing forces toward the disk rotor via a brake pad  10 . And, the outer caliper body  100  is provided with the hole  21  of an annular groove shape for housing the piston  23 ; thereby, the annular-tubular-shaped piston  23  can be guided by the hole  21  incorporated in the outer caliper body  100  with an annular groove shape, as well as sliding along an axis of the hole  21 . 
         [0114]    In the outer piece  3 , a brake pad  10  is configured so as to be adhesively fixed to a backing plate  104 ; the brake pad  10  is pressed against the disk rotor  1  via the backing plate  104  in response to the movement of the piston  23 , so that a braking force is generated. 
         [0115]    In the inner piece  4 , the situation is the same as in the outer piece  3 ; in the inner piece  4  as in the outer piece  3 , the inner caliper body  105  is provided with the hole  21  for housing the piston  23 . The outer piece  3  and the inner piece  4  are integrated into the brake caliper  2 . 
         [0116]    According to this embodiment as to the brake caliper  2  as shown in  FIG. 2 , a center protrusion  22  is formed in the middle part of the hole  21  for housing each piston  23  of the outer piece  3  and the inner piece  4 ; thereby, the protrusion  22  forms an inner periphery wall for the piston  23 . Further, as shown in  FIG. 2 , in the middle part of the piston  23  in response to the protrusion  22 , an engaging hole  29  is provided. 
         [0117]    In other words, the outer periphery wall of the piston  23  slides on and comes in contact with the outer diameter periphery wall of the hole  21  for housing the piston  23 , while the inner periphery wall of the piston  23  slides on and comes in contact with the outer diameter periphery wall of the protrusion  22 . More specifically, the piston  23  is configured so as to form an annular tubular shape; the geometry of the piston  23  is as if it was cut from a short length of thick-walled circular-section pipe. 
         [0118]    In addition, as shown in  FIG. 2 , the inner piece  4  is fastened to the outer piece  3  by a bolt  5 . 
         [0119]    Further, the piston may be configured so as to form an annular tubular shape, as shown in  FIG. 1 . 
         [0120]    In the clearance space between the outer periphery wall of the piston  23  and the outer diameter periphery wall of the hole  21 , a square seal  31  (an annular ring seal member having a square cross section orthogonal to the ring hoop direction) is provided; the square seal  31  is also called an outer square seal  31 , and the seal  31  prevents the leakage of brake fluid. On the other hand, as shown in  FIG. 1 , for preventing foreign matter such as dust from entering the clearance space, a dust seal  33  is provided at the open end of the clearance space, as shown in  FIG. 10 ; further, for preventing foreign matters such as dust or mud from entering inside of the clearance, a dust boot  60  is fitted on the top side of the protrusion  22 . 
         [0121]    In consideration of a conventional piston of the same outer diameter as the piston  23 , the area to be pressed by the piston  23  is reduced by the cross section area of the engaging hole  29  that corresponds to the center protrusion  22 , in contrast with the piton area of the conventional piston; hereby, it is noted that the center protrusion  22  in either of the outer piece  3  and the inner piece  4  is depicted as shown in  FIG. 1 , while the piston  23  area to be pressed by brake fluid is depicted as the area of the large circle of diameter D 2  minus the area of the small circle of diameter P, as shown in  FIG. 2 . 
         [0122]    Since a reduced braking force is not desirable, it is required that the above-described decrease in the area be compensated for with an enlargement of the outer diameter as to the piston  23 . For instance, as shown in  FIG. 2 , a greater diameter D 2  ( FIG. 2 ) increased in comparison with a conventional lesser diameter D 1  ( FIG. 15 ) can be adopted. Thus, the region of the brake pad  10  that is not pressed by the piston  23  can be automatically reduced with the above-described configuration. 
         [0123]    In addition, it is preferable that the ratio of the inner diameter P to the outer diameter D 2  as to the annular thrusting part  27 , namely, the ratio P/D 2  is within 0.35 to 0.65; when the ratio is equal to or less than 0.35, then a sufficient effect on a piston area enlargement with an increased outer diameter cannot be expected; and, when the ratio is equal to or more than 0.60, then an even thrusting (contact) pressure distribution over the brake pad cannot be expected because of a reduced (contact) pressure distribution around the piston center. 
         [0124]    In addition, a rectangular cross section orthogonal to the hoop direction as to the annular piston  23  may be of a solid core type; however, the rectangular cross section of a hollow type is provided in this embodiment. 
         [0125]    Since the annular piston  23  in this embodiment has a hollow space  24 , the piston  23  is of lower mass, which contributes to a reduction in the whole brake caliper mass. Thus, even in a case where an impressing speed as to a brake fluid pressure, a sufficient follow-up movement of the piston can be achieved thanks to the reduced mass of the piston  23 . 
         [0126]    In addition, since the piston  23  has the hollow space  24 , the mass of the piston can be varied by varying the volume of the hollow space  24 ; and, thereby the thermal capacity of the piston can be optimized so that the piston endures the heat generated while the brake is being activated. In a case where the piston  23  is made from metal, the piston having the hollow space  24  can be formed as a united piece manufactured from two divided parts, by means of a method such as welding, soldering, friction-welding or the like. Were the piston to be made from resin, the piston can be formed as a united piece manufactured from two divided parts, for instance, by means of a method such as solvent welding. 
         [0127]    According to the above-detailed embodiment, since the outer diameter of the piston  23  of the brake caliper can be increased without disturbing the balance of the braking forces between the front wheels and rear wheels as to a vehicle, the region of the brake pad  10  that is not pressed by the piston  23  can be reduced; thus, the occurrence of uneven wear or brake squealing in relation to the brake pad  10  can be prevented. 
         [0128]    Further, according to the above-detailed embodiment, the brake pad  10  can be more evenly pressed toward the disk rotor, without the conventional requirement such as shown in  FIG. 15  whereby the number of the pistons  6  is inordinately increased. Thus, a distance from a piston  23  to an adjacent piston  23  can be greater than a distance from a piston  6  to an adjacent piston  6  ( FIG. 15 ); therefore, the brake caliper stiffness can be sufficient; in conclusion, this embodiment does not incur a potential cost or mass increase as does the conventional method. 
       A Second Embodiment 
       [0129]    Hereby, a second embodiment according to the present invention is explained based upon  FIGS. 3 to 6 . 
         [0130]    In the first embodiment, a square seal  31  is provided as an outer seal (a piston outer seal) between the outer diameter periphery wall of the hole  21  and the outer periphery wall of the piston  23 , whereas, in the second embodiment, a square seal  36  is additionally provided as an inner seal (a piston inner seal) between the outer diameter periphery wall of the hole  21 . 
         [0131]    It goes without saying that there is no problem, from a functional point of view, in providing the square seal  36  not on the center protrusion  22  side but on the piston  23  side, by providing a groove for the square seal  36  on the piston  23  side. 
         [0132]    As shown in  FIG. 3 , the piston  23  comes in contact with the square seal  31  on the outer periphery wall of the piston  23 , as well as with the square seal  36  on the inner periphery wall of the piston  23 .  FIG. 4  illustratively shows how the square seal  31  is keeping in contact with the outer periphery wall of the piston  23  when the brake is not being activated during an incipient stage. When the brake pedal (not shown) is stepped on, the pressure in a brake fluid circuit (not shown) is increased, then the increased pressure in the hydraulic chamber  25  acts on the piston  23 ; as a result, the piston  23  moves left in  FIG. 4 , and the square seal  31  elastically deforms as shown in  FIG. 5 ; thereby, the elastic deformation depends on the size of a chamfer  38 , the material of the square seal, and so on. This action-mode as to the square seal  31  also applies to the action-mode as to the square seal  36  (as shown in  FIG. 7 ) that slides on and comes in contact with the inner periphery wall of the piston  23 . 
         [0133]      FIG. 6  shows characteristic curves as to the relations between piston displacements (or return-back displacements) and hydraulic pressures (brake fluid pressures), while the hydraulic pressures are being relieved; thereby, the horizontal axis denotes the brake fluid pressure as a pressure parameter, while the vertical axis denotes the piston displacement as a return back displacement parameter. 
         [0134]    In  FIG. 6 , the curve A shows the relationship between the necessary minimum piston return-back displacement and the brake fluid pressure; the curve B shows the relationship between the ideal piston return-back displacement and the brake fluid pressure; the curve C shows the relationship between the piston return-back displacement and the brake fluid pressure, in the case when a conventionally designed square seal is used for this embodiment; the curve D shows the relationship between the brake caliper deflection and the brake fluid pressure; the curve E shows the relationship between the brake pad compression deformation amount and the brake fluid pressure. Hereby, it is noted that the curve A is obtained by superposing the curve D and the curve E. 
         [0135]    As shown in  FIG. 6 , in a region where the brake fluid pressure is close to the pressure P 2 , the piston return-back displacement according to the curve C is approximately equal to that according the curve B; however, if the square seal characteristics of the curve C are applied, then the necessary minimum piston return-back displacement of the curve A falls short of the actual piston return-back displacement of the curve C, by a displacement difference δ 1 , in a region where the brake fluid pressure P 1  is lower than the pressure P 2 ; thus, after the brake fluid pressure is released, the piston returns back with an excess displacement of the difference δ 1 , so as to pass the original position before the brake pedal is stepped on. Therefore, when the bake pedal is next pressed, a driver feels as if the free play of the brake pedal were increased in relation to the difference δ 1 . 
         [0136]    On the other hand, in a region where the brake fluid pressure P 3  is higher than the pressure P 2 , the actual piston return-back displacement of the curve C falls short of the necessary minimum piston return-back displacement of the curve A, by a displacement difference δ 3 ; thus, after the brake fluid pressure is released, the piston returns back with a displacement shortage by the difference δ 3 , so as to stop a part of the way back. Therefore, even with the release of the brake pedal (brake pressure), the brake pad remains under a condition such that the brake pad is pressed against the brake disk; accordingly, what is known as a dragging phenomenon occurs, causing an early stage excessive wear of the brake pad and increased fuel consumption due to the increased running resistance. 
         [0137]    According to this embodiment, since the square seals  31  and  36  of different properties are provided on the outer periphery wall of the piston  23  and the inner periphery wall for the piston  23  respectively, the ideal piston return-back displacement characteristic of the curve B in  FIG. 6  can be substantially obtained by superposing the characteristics of the square seals  31  and  36  of the different properties; in this way, this embodiment can prevent a driver from feeling a sense of anomaly as to the brake pedal manipulation, and as well, it can realize a brake caliper that is free from the dragging phenomenon. 
         [0138]    Since the square seal  31  that slides on and comes in contact with the outer periphery wall of the piston  23  has a larger size because of the outer diameter of the piston, the elastic stiffness of the square seal  31  can be sufficient; further, by means of selecting a material of higher hardness for the seal  31 , and/or by means of adopting a larger size of the chamfer for the seal groove, a characteristic such that is approximately equivalent to the curve D in  FIG. 6  can be rather easily obtained, in conjunction with the relationship between the brake caliper deflection and the brake fluid pressure. On the other hand, since the square seal  36  that slides on and comes in contact with the inner periphery wall of the piston  23  has a smaller size because of the inner diameter of the piston, the elastic stiffness of the square seal  31  can be restrained; thus, by means of selecting a material of lower hardness for the seal  36 , and/or by means of adopting a smaller size of the chamfer for the seal groove, a characteristic that is approximately equivalent to the curve E in  FIG. 6  can be obtained, whereby the curve E is regarded as a characteristic of a quick response case. 
         [0139]    In this way, it is preferable that a combination of a shape and material as to the seals  31  and  36  is designed so that the shear stiffness of the outer square seal  31  is higher than that of the inner square seal  36 ; thus, by means of installing square seals of different properties in the brake caliper, a seal characteristic substantially equivalent to the characteristic of the curve B that is a to-be-targeted curve as shown in  FIG. 6  can be obtained. 
       A Third Embodiment 
       [0140]    Hereby, a third embodiment according to the present invention is explained based upon  FIG. 7 . This embodiment relates to a determination or a designing of the clearance between the piston  23  and the hole  21  for housing the piston  23 . 
         [0141]      FIG. 7  depicts the arrangement of the piston  23  and the hole  21  in detail; the piston  23  is placed in the hole  21  with a clearance δ 1  between the piston  23  and the caliper body  100  or  105  and a clearance  62  between the piston  23  and the center protrusion  22 ; in this embodiment, it is basically assumed that the clearance δ 1  is not equal to the clearance δ 2 . In other words, different clearances are applied to the outer side clearance and the inner side clearance as to the annular shaped piston  23  so as to design the brake caliper  2 . 
         [0142]    As shown in  FIG. 7 , the piston  23  strongly presses the disk rotor  1  via the brake pad  10 , when a brake fluid pressure is applied in the hydraulic chamber  25  of the caliper body  100  or  105 . 
         [0143]    On the other hand, since the disk rotor moves in a direction vertical to the paper surface of  FIG. 7 , the brake pad and in turn the backing plate and the piston  23  are forced to move slightly in the same direction; then, the clearance δ 1  around the outer periphery of the piston gets closer to zero in the described direction. There are two clearance-designing methods (concepts) in determining the clearances δ 1  and δ 2 ; the methods are explained as follows. 
         [0144]    In the mentioned first method, it is basically assumed that the clearance δ 1  on the outer periphery of the piston  23  greater than the clearance  62  on the inner periphery of the piston  23 . Thus, even if the clearance δ 1  becomes zero, namely, even if the piston  23  gets in contact with the caliper body  100  or  105 , the clearance  62  is still greater than zero; accordingly, the (positive) clearance between the piston  23  and the center protrusion  22  can be secured (in other words, the center protrusion  22  can not get in contact with the piston  23 ). Thus, the center protrusion  22  can be free from being bent by the force that the piston  23  receives in the direction vertical to the paper surface of  FIG. 7 ; this gives the brake caliper a structural advantage in regard to strength. 
         [0145]    On the other hand, in the above-mentioned second method, it can be assumed that the clearance δ 1  on the outer periphery of the piston  23  is smaller than the clearance δ 2 . In the case of this context, even if the clearance δ 2  becomes zero, namely, even if the piston  23  gets in contact with the center protrusion  22 , the clearance δ 1  is still greater than zero, and; accordingly, the (positive) clearance between the piston  23  and the caliper body  100  or  105  can be secured. In other words, the piston  23  can not get in contact with the caliper body  100  or  105 . Thus, when the brake fluid pressure is released, the piston can return back to an original position where the piston is placed before the brake fluid pressure was applied, by means of the deformation restoring force of the square seals  31  and  36 . Accordingly, a clearance between the disk rotor  1  and the brake pad  10  can be secured; this prevents the disk rotor  1  in  FIG. 1  from dragging the brake pad  10  in  FIG. 1  even under a normal running condition of the vehicle. 
         [0146]    Hereby and hereafter, the fourth to seventh embodiments according to the present invention are explained. In the embodiments, the center protrusion  22  is configured not as a part of caliper body  100  or  105 , but as a member separated from the caliper body. 
       A Fourth Embodiment 
       [0147]    First, the fourth embodiment is explained with reference to  FIG. 8 . 
         [0148]    The brake caliper is not divided into two members; namely, the caliper is not formed with an outer piece  3  and an inner piece  4 , but with a single piece into which an outer piece  3  and an inner piece  4  are combined. Needless to say, it is no longer necessary to have a bolt fastening the pieces  3  and  4 . 
         [0149]    According to this embodiment, in an inner caliper body  105 , for the purpose of achieving simplified manufacturing, a machined hole  43  for housing an end plug  40  is provided; thereby, the end plug  40  includes a center protrusion  22  (in  FIG. 8 ), not as a part of the inner caliper body, so that the center protrusion  22  can be pulled out from the inner caliper body. The configuration of this embodiment is the same as the configuration of the brake caliper  2  in the second embodiment explained with  FIG. 3 , except that the machined hole  43  for housing the end plug  40  is provided. 
         [0150]    Also in the brake caliper of this embodiment, as is in the brake caliper according to the above-described first or second embodiment, a center protrusion is formed, at a place in accordance with the place of the protrusion in the first or second embodiment, in an outer caliper body  100  of an outer piece  3 , as well as, in an inner caliper body  105  of an inner piece  4 . 
         [0151]    However, the center protrusion  22  of the inner caliper body  105  is formed as a part of the end plug  40  that is screwed into the inner caliper body  105 , with a fastening screw mechanism  45  in the case of  FIG. 8 . Needless to say, a fastening mechanism other than the screw mechanism can be applied to this embodiment. 
         [0152]    As shown in  FIG. 8 , the fastening mechanism  45  is furnished on the outer periphery side of the end plug, as well as, on an (outer) end-face side of the end plug; a step  47  is provided on the outer periphery of the end plug, between an end of the fastening screw and the brake pad  10 ; a fitting surface  49  of the end plug  40  that faces the inner caliper body is formed with an outer periphery of a larger diameter part as to the step  47 ; a vertical step-surface that is vertical to the center axis of the end plug and parallel to the end face of the end plug is used as a seating surface so that the end plug can be positioned in the inner caliper body, and the end plug does not come out of the inner caliper body. 
         [0153]    According to the fourth embodiment as detailed above, it is not necessary that the center protrusion  22  be manufactured as a part of the inner caliper; this lowers the manufacturing cost of a brake caliper in view of machining and assembling. 
         [0154]    Further, the brake caliper is not formed with an outer piece  3  and an inner piece  4 , but with a single piece into which an outer piece  3  and an inner piece  4  are combined; therefore, the reliability of the brake caliper can be enhanced. In addition, since the brake caliper  2  can have enhanced rigidity, the disk rotor can be more strongly pressed, and a braking force of enhanced stability can be achieved. 
         [0155]    Still further, as shown in  FIG. 8 , since a machining jig for machining the hole  21  for housing the piston can be inserted through the machining hole  43  prior to installing the end plug  40 , the machining of the hole  21  for housing the piston in the outer piece  3  side can be simplified; therefore, the machining of the hole  21  can be easily performed even in the case where an outer piece  3  and an inner piece  4  are combined into a single piece body. 
       A Fifth Embodiment 
       [0156]    Hereby, a fifth embodiment according to the present invention is explained based upon  FIGS. 9 and 10 . 
         [0157]    In this fifth embodiment, a guide member  50  is provided so that the guide member forms a tip (top) part of the center protrusion  22  that is placed in the middle part of the hole  21  for housing each piston  23 ; namely, the guide member  50  forms a tip part of the center protrusion  22 , the tip (top) part being manufactured as a different part from the center protrusion as well as being attached to the center protrusion. 
         [0158]    Incidentally, regarding the elements already described in the first to third embodiments, the same numeral symbols are used as in these embodiments; and, repeated explanations of the elements are hereby omitted. 
         [0159]    As shown in  FIG. 9 , a center protrusion  22  that is formed in the middle part of the inner caliper body  105  is configured with the guide member  50  and a base protrusion part  52 ; and, the member  50  and the part  52  are fastened together through a screw mechanism  54 . In addition, as shown in  FIG. 10 , a fastening mechanism other than the screw mechanism  54 , for instance, a friction welding fastening can be applied to this embodiment. Needless to say, alternative friction mechanisms such as diffusion bonding, welding, or an adhesive bonding can be used. 
         [0160]    On a head part of the guide member  50 , a recess  56  is depressed so that a fastening jig for fastening the member  50  and the part  52  through a screw fastening or a pressure welding can be set. 
         [0000]    Further, on the outer periphery of the guide member  50 , an inner seal groove  58  for housing the square seal  36  is provided so that the square seal  36  is fitted into the inner seal groove. 
         [0161]    The guide member  50  provided with the square seal  36  that slides on and comes in contact with the inner periphery wall for the piston  23  is combined with the base protrusion part  52 ; thus, from the inner caliper body  105  in connection with the machining of the hole  21  for housing the piston  23 , a machining of the part corresponding to the outer periphery of the guide member  50  can be eliminated; accordingly, not only the machining can be simplified but also a risk as to a possible collision of a machining jig with the center protrusion  22  can be eliminated; as a result, enhanced machining accuracy can be achieved, together with enhanced rigidity of the jig, and enhanced productivity can be achieved as well. 
         [0162]    In addition, only by means of installing the guide member  50  that is machined separately from the other parts, with a high degree of accuracy, both outer diameters and inner diameters as to the part housing the piston  23  are realized also with a high degree of accuracy; thus, enhanced accuracy can be achieved, with no decrease in productivity. 
         [0163]    In relation to the clearances between the outer periphery wall of the piston  23  and the inner diameter periphery wall of the piston-housing hole  21  in the inner caliper body  105 , as well as, between the inner periphery wall of the piston  23  and the outer periphery of the guide member  50 , the variations (deviations) as to the clearances have to be confined within an absolute minimum so as to provide maximum performance as designed in relation to the square seals  31  and  36 . 
         [0164]    In order to confine the variations (deviations), a quality control technique is adopted; whereby, machined pistons  23  within tolerance are classified, for example, into three layers according to the finished outer diameter measurements of the piston; in response to a result as to the inner periphery wall diameter measurement of a piston-housing hole  21  in an inner caliper body  105 , it is determined which layer of pistons is assigned to the piston-housing hole  21 . Hereafter, this technique is called a fit-selection control. 
         [0165]    Since the piston  23  is configured so as to form an annular tubular shape, the piston itself has a hollow space around the center axis in the running direction; thus, the piston has two engaging running surfaces: the outer periphery wall surface and the inner periphery wall surface. As a result, it is required to perform the above-mentioned fit selection control as to the outer periphery side and also the inner periphery side; however, this dual-sided fit-selection control can be simplified according to this fifth embodiment as is described in a later paragraph. 
         [0166]    If the guide member  50  and the inner caliper body are made as one piece from the beginning, namely, if the center protrusion  22  is made as a part of the inner caliper body, then the above-mentioned fit selection control, for example, of the three-layer clearance-classification is required so that clearance measurements have to be performed on both the clearances as to the inner periphery wall surface side of the piston-housing hole  21  and the outer diameter periphery wall surface side of the protrusion  22 ; thus, 9 (3×3) layers have to be prepared in advance. In fact, such a control with many layers is not realistic. 
         [0167]    According to this fifth embodiment, the guide member  50  is separated from the center protrusion  22  that faces the inner periphery wall surface side of the piston  23 , so that the guide member  50  and the base protrusion part  52  configure the center protrusion  22 ; 
         [0168]    First, an inner periphery wall diameter of a piston-housing hole  21  in an inner caliper body  105  is measured; in response to the measurement result, a piston  23  is selected from one of the three layers in which machined pistons  23  have been classified in accordance with piston outer diameter measurements in advance, so as to make a pair of a piston  23  and an inner caliper body  105 . Thus far, the manner is the same as a manner in the conventional fit-selection control; yet, in this embodiment, the inner periphery wall diameter of the selected piston  23  is measured; further, in response to the measurement result, a guide member  50  is selected from one of the three layers in which machined guide members  50  have been classified in accordance with guide member outer diameter measurements in advance; and, the selected guide member  50  is fastened to the base protrusion part  52  of the center protrusion  22 , by means of a screw mechanism or through friction welding; the piston  23  that can provide allowable outer and inner clearances is selected so as to be assembled. 
         [0169]    In this way, the 9 (3×3) layer fit-selection control such as is complicated as mentioned can be avoided; thus, desired performance as to the square seals  31  and  36  can be obtained without a decrease in productivity. 
       A Sixth Embodiment 
       [0170]    Hereby, a sixth embodiment according to the present invention is explained based upon  FIG. 11 . 
         [0171]    In this sixth embodiment, as shown in  FIG. 11 , an outer diameter d, of the guide member  50  is set-up so as to be larger than an outer diameter d 2  as to the base protrusion part  52  of the center protrusion  22 ; this point distinguishes the sixth embodiment from the fifth embodiment. 
         [0172]    Incidentally, regarding the elements already described in the fifth embodiments, the same numeral symbols are used as are in the embodiments; and, repeated explanations of the elements are hereby omitted. 
         [0173]    As shown in  FIG. 11 , the outer periphery of the base protrusion part  52  of the center protrusion  22  is conical in shape, namely the generating line is tapered along the center axis thereof toward the guide member  50 ; the diameter of a base root as to the base part  52  is d 3 , while that of a head part as to the base part  52  is d 2 . Further, the outer diameters of the base protrusion part  52  including d 2  and d 3  are smaller than the outer diameter d, as to a head part of the guide member  50 . In addition, for preventing outside foreign matter such as dust or mud from entering inside, a dust boot  60  is fitted on the top side of the guide member  50 . 
         [0174]    According to this sixth embodiment, the outer diameter d 2  as to the base protrusion part  52  of the center protrusion  22  is smaller than the outer diameter d, of the guide member  50 ; thus, owing to the widened clearance around base protrusion part  52 , the possible collision contact between the piston  23  and the base part  52  can be less likely to happen, even in a case of piston-slap; it is noted that the piston-slap is defined as a relative movement between the piston  23  and the piston-housing hole  21  whereby the piston is shaken in the piston-housing hole as if the piston is rotated around an axis vertical to the piston center axis (X-X in  FIG. 11 ). In other words, it is meant that the center protrusion  22  or the base protrusion part  52  swings along the Y-direction as shown in  FIG. 11 , rotating around an axis vertical to the drawing paper surface, in the hollow space of the piston center middle part. 
         [0175]    Therefore, the base protrusion part  52  of the center protrusion  22  can be free from a force (a collision-contact force) such as causes a bending moment to the base protrusion part  52 ; incidentally, it is noted that there is no structure or strength problem, even though the outer periphery wall of the base protrusion part  52  is slightly removed because of the taper machining. Further, the outer diameter as to the base protrusion part  52  of the center protrusion  22  is smaller than the outer diameter of the guide member  50 ; it becomes unnecessary to finish the outer periphery surface of the base protrusion part  52 . Thus, the outer surface of the base protrusion part  52  can remain the surface of raw materials such as diecast material. As a result, the production cost can be reduced. 
         [0176]    Moreover, the guide member  50  incorporating the square seal  36  that slides on and comes in contact with the inner periphery wall of the piston  23  is a member separated from the base protrusion part  52 ; and, the outer diameter of the guide member  50  is larger than outer periphery diameter of the base protrusion part  52 . Thus, the guide member can house the square seal  36  with a sufficient space, and without enhanced machining accuracy in contrast to the machining accuracy around the base protrusion part. In this way, only with moderate machining accuracy, the guide member can be manufactured; only with incorporating this guide member into the brake caliper, the square seal  36  that slides on and comes in contact with the inner periphery wall of the piston  23  can be easily realized. 
       A Seventh Embodiment 
       [0177]    Hereby, a seventh embodiment according to the present invention is explained based upon  FIG. 12 . 
         [0178]    In this sixth embodiment, the position of the square seal  31  as an outer seal member in the fifth embodiment as shown in  FIGS. 9 and 10  is shifted from a place in the inner caliper  105 , to a place in the piston  23 ; namely, the location of the seal  31  in the inner caliper body  105  is shifted to the location of the seal  61  in the piston  23  as shown in  FIG. 12 . 
         [0179]    In the case of providing the square seal  61  in the seal groove formed on the outer periphery wall side of the piston  23 , the same level of sealing performance as is in the fifth embodiment can be achieved; yet, since machining of the seal groove for the inner diameter periphery wall of the_piston-housing hole  21  can be dispensed with, the machining of the inner caliper can be simplified. 
       INDUSTRIAL APPLICABILITY 
       [0180]    According to the present invention, the pressing part of the piston is of an annular-ring shape while the pressing part of a conventional piston is of a solid cylinder shape; thus, the outer diameter of the piston area, namely, the outer diameter of the piston pressing part in the present invention can be larger than the diameter of the piston pressing part in the conventional piston, even though the respective piston areas are the same; thus, the brake caliper in the present invention can be of a compact structure, and has the same level of performance as in a conventional brake caliper with at least two pairs of pistons. 
         [0181]    As a result, the brake pad can be pressed with an even surface pressure distribution pattern over the whole brake pad; the clearance gap between the brake pad and the disk rotor can be reduced; as a result, an excellent braking force can be achieved, and the problem of brake squealing can be eliminated. 
         [0182]    Further, since not only the outer seal on the outer periphery of the piston is provided, but also the inner seal on the inner periphery of the piston is provided, the piston can be surely returned back to an original position before a braking action, thanks to the enhanced deformation restoring capability as to each seal, when the brake fluid pressure is released; thus, a dragging phenomenon between the brake disk and the brake pad is significantly less likely to happen. 
         [0183]    In conclusion, the present invention is applicable to a brake caliper that is used in a vehicle such as an automobile or a motorcycle.