Disc Brake Device

The present invention addresses the problem of providing a disc brake device for which sliding at the boundary surface between a piston and a piston seal is suppressed, and dragging is reduced. The present invention is provided with a cylinder, a piston housed in the cylinder, an inner brake pad opposing a disc rotor, an inner circumferential groove formed in a cylinder inner circumference, and a piston seal that is provided in the inner circumferential groove and contacts the piston. The inner circumferential groove is provided with a wall, a wall on the opposite side from the wall, a bottom wall connecting the wall and the wall, and a curved surface expanding the inner circumferential groove at the wall. The bottom wall is formed such that the distance to the piston gradually increases from the wall toward the wall. The curved surface is provided with a curvature starting point on the side closer to the piston seal and a curvature endpoint on the opposite side from the curvature starting point with the curved surface therebetween, and the curvature endpoint is positioned farther outside than the cylinder inner circumference.

TECHNICAL FIELD

The present invention relates to a disc brake device arranged in an automobile and the like.

BACKGROUND ART

A disc brake device used for an automobile and the like operates a piston arranged within a bore (cylinder) of a brake caliper by hydraulic pressure and the like, presses a brake lining (brake pad) to a friction ring (disc rotor), and obtains a braking force. The piston slides within the bore. A groove is formed in a part of the inner circumferential surface within the bore where the piston slides, and a seal ring preventing leakage of the pressure medium is arranged in the groove.

When the brake is operated and the piston is moved to the direction of the brake lining, the seal ring deforms following up the piston. When the brake is released, the piston is taken back by a restoring force of the seal ring having deformed, and, accompanying it, the brake lining moves to the direction of departing from the friction ring.

In order to improve the action of taking back the piston, there is a technology that the groove bottom portion of the groove is made to incline so as to approach the center axis of the cylinder (bore axis) as it goes toward the pressing direction of the piston. As such technology, a technology described in Patent Literature 1 is proposed.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, in the technology described in Patent Literature 1, although the seal ring having deformed tried to return to the original position by the restoring force, in returning, the seal ring collided on the side surface of the groove formed on the opposite side of the brake lining (brake pad), and such state occurred that movement of the seal ring was restricted and the piston did not return sufficiently. As a result, there was a problem that the brake lining (brake pad) did not depart from the friction ring (disc rotor) sufficiently, the brake lining and the friction ring dragged each other in a contact state even when the brake was not operated, and the fuel economy was deteriorated.

The object of the present invention is to provide a disc brake device solving the problem described above, suppressing sliding at the boundary of the piston and the piston seal, and reducing dragging.

Solution to Problem

In order to achieve the object described above, the present device is a disc brake device including a cylinder, a piston housed in the cylinder, and an inner brake pad arranged on one side of the piston and opposing a disc rotor, and is featured that an inner circumferential groove formed in an inner circumference of the cylinder and a piston seal that is provided in the inner circumferential groove and contacts the piston are provided, the inner circumferential groove includes a wall on the inner brake pad side, a wall on the opposite side of the inner brake pad, a bottom wall connecting the wall on the inner brake pad side and the wall on the opposite side, and a curved surface expanding the inner circumferential groove at the wall on the opposite side, the bottom wall is formed such that the distance to the piston gradually increases from the wall on the inner brake pad side toward the wall on the opposite side, the curved surface includes a curvature starting point on the side closer to the piston seal and a curvature endpoint on the opposite side from the curvature starting point, the curved surface being located between the curvature starting point and the curvature endpoint, and the curvature endpoint is positioned farther outside than the inner circumference of the cylinder.

Advantageous Effects of Invention

According to the present invention, it is allowed to provide to provide a disc brake device suppressing sliding at the boundary surface between the piston and the piston seal, and reducing dragging.

DESCRIPTION OF EMBODIMENTS

Embodiments related to the present invention will be hereinafter explained in detail based on the drawings.

First Embodiment

A basic configuration of the disc brake device of the present embodiment will be explained usingFIGS. 1 to 3.FIG. 1is a cross-sectional view of a disc brake device related to the first embodiment of the present invention. Also, a caliper body8is shown in a simplified construction.FIG. 2is a perspective view of a rotation/linear motion conversion mechanism of the disc brake device related to the first embodiment of the present invention. Also, in order to explain the internal construction of the rotation/linear motion conversion mechanism11, a nut roller34is not illustrated.FIG. 3is a perspective view of a piston of the disc brake device related to the first embodiment of the present invention.

As shown inFIG. 1, a disc brake device1includes a pair of inner brake pad2and outer brake pad3arranged on both sides in the axial direction sandwiching a disc rotor12that is attached to a rotational portion of a vehicle, the caliper body8, and the rotation/linear motion conversion mechanism11. A pair of the inner brake pad2and the outer brake pad3and the caliper body3are supported by a bracket so as to be movable in the axial direction of the disc rotor12, the bracket being fixed to a non-rotational portion of the vehicle. On one side (opposite side of the disc rotor) of the inner brake pad2, a projection portion26is arranged. The projection portion26has a function of engaging with a recessed portion24arranged on the other end side surface of a piston18and preventing rotation of the piston18.

For convenience of explanation, hereinafter, the right side (the opposite side of the caliper claw portion) of the drawing is expressed one end side, the left side (the caliper claw portion side) is expressed the other end side, the lower side is expressed the open side, and the upper side is expressed the root side.

The caliper body8includes a cylinder6arranged on the inner brake pad2side (one end side), a caliper claw portion4arranged on the outer brake pad3side (the other end side), and a disc path portion (saddling portion)5positioned between the cylinder6and the caliper claw portion4.

A bore portion9opening to the inner brake pad2side is formed in the cylinder6, and a hole portion10is arranged in a bottom wall6bof the bore portion9positioned on one end side. The piston18is housed in the inner circumferential surface of the bore portion9. The inner brake pad2is provided on one end side of the piston18.

The disc path portion5is positioned on the root side of the cylinder6, is extended to the other end side (the caliper claw portion4side) toward a rotation axis70direction of a spindle75, straddles the disc rotor12, and connects the cylinder6and the caliper claw portion4to each other. That is to say, the caliper claw portion4is supported by the cylinder6in a cantilever style by way of the disc path portion5. The caliper claw portion4is positioned on the opposite side of the cylinder6side of the disc path portion5, and is configured to extend to the direction perpendicular to the rotation axis70and to oppose the outer brake pad3. That is to say, the caliper claw portion4is arranged on the opposite side of the piston18with respect to the disc rotor12, and an inner surface (cylinder opposing surface)7of the caliper claw portion4and an inner surface (caliper claw portion opposing surface)6aof the cylinder6oppose each other through the outer brake pad3, the disc rotor12, and the inner brake pad2. The inner surface7of the caliper claw portion4is in a flat surface shape, and is perpendicular to the rotation axis70. Also, the inner surface7of the caliper claw portion4opposes a flat surface portion22aof the piston18through the outer brake pad3, the disc rotor12, and the inner brake pad2.

In the disc brake device1, when an ordinary hydraulic brake is operated, the piston18is made to advance to the disc rotor12side by a brake fluid supplied to a hydraulic chamber21within the bore portion9, the inner brake pad2is pressed by this piston18, the disc rotor12is sandwiched along with the outer brake pad3, and thereby a thrust force that is a brake force is generated.

The piston18is inserted into the bore portion9of the cylinder6so as to be slidable in the rotation axis70direction, and a bottom portion22is arranged so as to oppose a surface on one end side of the inner brake pad2as shown inFIG. 1. As shown inFIGS. 1 and 3, the piston18is formed into a bottomed cup shape including the bottom portion22and a cylindrical portion23. When the piston18advances to the disc rotor12side, a piston seal43loaded to an inner circumferential groove44formed in the inner wall (cylinder inner circumference51) of the cylinder6contacts the piston18, elastically deforms by friction against the boundary surface between the piston18and hydraulic pressure, and follows up the piston18. When the hydraulic brake is released, elastic deformation of the piston seal43is released, and the piston18returns by a restoring force of the piston seal43to the position of the time before the hydraulic brake is operated. Gaps are generated between the disc rotor12, the inner brake pad2, and the outer brake pad3, and the brake force is released.

The flat surface portion (end surface portion)22aon the other end side of the piston bottom portion22is a flat surface perpendicular to the rotation axis70and extending in parallel to the disc rotor12. On the other hand, a flat surface portion (end surface portion)25on one end side of the piston bottom portion22, namely the flat surface portion25opposing the rotation/linear motion conversion mechanism11, has a shape inclining with respect to the rotation axis70as shown inFIG. 1, and thickness of the bottom portion22becomes thicker toward the opening side. In the present embodiment, the flat surface portion25inclines by 3° (θ=3°) with respect to the line perpendicular to the rotation axis70so as to open toward the open side. Also, as shown inFIG. 3, the recessed portion24is arranged by one position on the outer circumferential side of the other end surface opposing the inner brake pad2of the piston bottom portion22. This recessed portion24engages with the projection portion26of the inner brake pad2, and executes rotation prevention in the rotational direction and position determination of the piston18. With respect to the position in the circumferential direction of the recessed portion24, the recessed portion24is arranged at a position where the piston bottom portion22becomes thinnest. With respect to the disposal position in the circumferential direction of the piston18, the piston18is arranged so that the recessed portion24comes to the root side as shown inFIG. 1. In this case, the flat surface portion25of the piston inner surface inclines so that the open side approaches the cylinder side (namely one end side). That is to say, the flat surface portion25of the piston inner surface inclines so that, compared to the root side, the open side approaches the rotation/linear motion conversion mechanism11or the opening side of the piston18.

Next, explanation will be made on the rotation/linear motion conversion mechanism11. The rotation/linear motion conversion mechanism11shown in the present embodiment is a mechanism featured to use a roller42, and will be hereinafter referred to as a roller type mechanism.

The rotation/linear motion conversion mechanism11converts rotation of an electric motor not illustrated to motion in the linear direction (will be hereinafter referred to as linear motion), imparts a thrust force to the piston18, and holds the piston18at the braking position. The rotation/linear motion conversion mechanism11is housed between the bottom wall6bof the cylinder6and the flat surface portion25of the piston inner surface. That is to say, the rotation/linear motion conversion mechanism11is supported by the cylinder6of the caliper body8along with the piston18. Explanation will be hereinafter made on configuring components.

A plate base31is fixed in the bottom wall6bof the cylinder6by a pin not illustrated, and is prevented from rotation with respect to the nut roller34. The plate base31is formed into a disc shape, and a hole portion31ais worked at the center in the radial direction of the disc shape, the spindle75being installed in the hole portion31a.

The spindle75is configured as a rotation transmission member to which rotation of the electric motor is transmitted and is supported so as to be rotatable with respect to the cylinder6and the plate base31, and rotational motion from the electric motor is transmitted to the spindle75through a gear unit not illustrated. A thread portion76is formed on the outer circumferential surface on the other end side of the spindle75, and is screw-fitted to a shaft roller35, a thread portion35abeing formed on the inner circumferential surface of the shaft roller35. By rotation of the spindle75to the applying direction, the shaft roller35having been screw-fitted advances to the direction of the other end side.

On one end side of the spindle75, a polygonal shape portion77is formed. By connection of this portion to the gear unit not illustrated, rotational torque of the electric motor can be transmitted.

The roller42has an annular mountain shape, is fitted to an annular groove portion on the outer circumferential surface of the shaft roller35in the annular mountain portion of the roller42, and is held so as to be rotatable in the axial direction. Also, the roller42is fitted to a thread mountain portion on the inner circumferential surface of the nut roller34in the annular mountain portion of the roller42, and is held so as to be rotatable in the axial direction. The roller42is disposed by plural number of pieces in the circumferential direction of the outer circumferential surface of the shaft roller35.

The nut roller34is fitted to the plate base31in the radial direction, and is prevented from rotation. The inner surface of the nut roller34is subjected to threading work, and the roller42is held at this threaded portion. A cage roller36is disposed on the outer circumferential surface of the shaft roller35, and includes plural number of pieces of elongated hole portion36a. The roller42is disposed in this elongated hole portion36a. The end surface on the other end side of the elongated hole portion36aand the end surface of the roller42contact each other, and a spring load described below is transmitted to the roller42. The elongated hole portion36acontacts the contour portion of the roller42in the circumferential direction.

The end surface on the other end side of the cage roller36slides against a plate spring37. With respect to the plate spring37, the left end surface contacts a spring38, and the right end surface contacts the cage roller36. The plate spring37has a function of transmitting precompression of the spring38to the cage roller36. The spring38is positioned on the outer circumferential surface (the outer circumference side) of the shaft roller35, and imparts precompression to the cage roller36in the axial direction.

In the shaft roller35, the inner surface portion is subjected to threading work, and the outer circumferential portion is subjected to annular groove work. Here, the inner surface portion is screw-fitted to the spindle75, and the annular groove of the outer circumferential portion is fitted to the annular mountain portion of the roller42. A groove portion for ball thrust is formed on the other end side of the shaft roller35, and holds a retainer thrust40and a ball thrust39in a gap against a plate thrust41. The roller42is held in the axial direction by the annular groove and is made turnable, an axial force from the ball groove portion is transmitted to the roller42in applying, and a reaction force from the roller42is transmitted to the thread portion in releasing.

The annular mountain portion of the roller42described above is formed as an annular mountain portion (projection portion) on the outer circumferential surface of the roller42, and the annular groove of the shaft roller35described above is formed as an annular groove portion (recessed portion) on the outer circumferential surface of the shaft roller35. The annular mountain portion of the roller42and the annular groove of the shaft roller35have the width and the interval which allow mutual engagement.

A one end side ball thrust32is positioned between a ball groove portion75aof the spindle75and the plate base31, and transmits an axial force from the spindle75to the plate base31while it rotates. The other end side ball thrust39is positioned between the plate thrust41and the shaft roller35, and rotates the shaft roller35. Also, the other end side ball thrust39has a function of transmitting a thrust force from the plate thrust41to the shaft roller35side.

A one end side retainer thrust33is positioned between the ball groove portion75aand the plate base31, and holds the one end side ball thrust32. The other end side retainer thrust40is positioned between the ball groove portion and the plate thrust41, and holds the other end side ball thrust39.

Next, a motion mechanism in operating the electric brake device will be explained usingFIG. 1.

When the brake is applied using the electric motor, an ECU drives the electric motor and rotates various gears. By this rotation of the gears, rotation of the electric motor is transmitted to the spindle75. Next, by rotation of the spindle75to the applying direction, the shaft roller35advances toward the inner surface side (the bottom portion22side) of the piston18along the direction of the rotation axis70. As a result, the other end side ball thrust39, the distal end side retainer thrust40, and the plate thrust41advance toward the inner surface portion of the piston18along the direction of the rotation axis70in an integral manner, and a pressing portion41aof the plate thrust41abuts upon the inner surface portion of the piston18. By this abutment, the piston18advances and the flat surface portion (end surface portion)22aon the other side of the piston18abuts upon the inner brake pad2.

Further, when rotation drive of the electric motor to the applying direction is continued, the piston18presses the inner brake pad2by movement of the shaft roller35, sandwiches the disc rotor12along with the outer brake pad3, and thereby generates a thrust force that is a braking force. When the piston18advances, the piston seal43loaded to the inner circumferential groove44of the cylinder6elastically deforms by friction against the boundary surface between the piston18and follows up the piston18.

When the hydraulic brake is released, elastic deformation of the piston seal43is released, and the piston18returns to the position of the time before the hydraulic brake is applied. Compared to the case of the hydraulic brake, in the case of the electric brake, since hydraulic pressure is not applied to the piston seal43, the piston seal43hardly follows up the piston18and hardly deforms elastically. Compared to the state after releasing the hydraulic brake, in the electric brake, the restoring force generated in the piston18from the piston seal43after releasing the electric brake is smaller, and the piston18hardly returns to the position of the time before the brake is applied. As a result, the gaps generated between the disc rotor12, the inner brake pad2, and the outer brake pad3become smaller. When these gaps are narrow, there is a problem that the disc rotor12, the inner brake pad2, and the outer brake pad3drag each other while they are in contact with each other, and the fuel economy is deteriorated. Countermeasures for solving it will be explained usingFIG. 4.

FIG. 4Ais a cross-sectional view of the piston seal, the inner circumferential groove of the cylinder, and the piston of the disc brake device related to the first embodiment of the present invention.FIG. 4Bis an enlarged view of the portion A inFIG. 4A.

The disc brake device includes the cylinder6, the piston18, the inner brake pad2, and the outer brake pad3, the piston18being housed in the cylinder6, the inner brake pad2and the outer brake pad3being arranged on one end side of the piston18and opposing the disc rotor12.

In the boundary surface of the inner wall of the cylinder6(the cylinder inner circumference51) against the piston18, the inner circumferential groove44is arranged. In the inner circumferential groove44, the piston seal43is housed, the piston seal43being wound around the piston18and energizing the piston18to the opposite side of the outer brake pad3. The inner circumferential groove44includes a wall45on the inner brake pad side, a wall46on the opposite side (cylinder bore bottom side) of the inner brake pad, and a bottom wall47.

The bottom wall47is formed so that the distance to the piston18(the cylinder inner circumference51) gradually increases from the wall45on the inner brake pad side toward the wall46on the opposite side of the inner brake pad. To the contrary, it is formed so that the distance to the piston18(the cylinder inner circumference51) gradually reduces from the wall46on the opposite side of the inner brake pad toward the wall45on the inner brake pad side.

The wall46on the opposite side (the cylinder bore bottom side) of the inner brake pad includes a curved surface50that expands the cylinder inner circumferential groove44.

The curved surface50includes a curvature starting point48on the side closer to the piston seal43and a curvature endpoint49on the opposite side from the curvature starting point48with the curved surface50therebetween, and is formed so that the curvature endpoint49is positioned farther outside (outer circumferential side) than the cylinder inner circumference51.

While the electric brake is operated, the piston seal43moves so as to approach the wall45on the inner brake pad side while the piston seal43is shear-deformed by a friction force generated at the boundary surface between the piston seal43and the piston18. Since the bottom wall47is formed so that the distance to the cylinder inner circumference51gradually reduces from the wall46on the opposite side of the inner brake pad toward the wall45on the inner brake pad side, as the piston seal43moves toward the wall45, the compression force applied in the radial direction of the piston seal43increases, the friction force increases, and therefore the piston seal43easily follows up the piston18.

According to the first embodiment, since the piston seal43easily follows up the piston18, even after releasing the electric brake, the restoring force generated from the piston seal43to the piston18can be increased, and the piston18can easily return to the position of the time before the brake is applied.

Also, according to the first embodiment, since the curved surface50includes the curvature starting point48on the side closer to the piston seal43and the curvature endpoint49on the opposite side from the curvature starting point48with the curved surface50therebetween and is formed so that the curvature endpoint49is present farther outer circumferential side than the cylinder inner circumference51, the deformation allowance of the piston seal43in being restored toward the wall46on the opposite side increases (the piston seal43goes beyond the position of the wall46on the opposite side) after releasing the electric brake, and therefore the piston18can more easily returns to the position of the time before the brake is applied.

As described above, according to the first embodiment, it is allowed to provide a disc brake device controlling elastic deformation of the piston seal, suppressing sliding at the boundary surface between the piston and the piston seal, and reducing dragging.

Second Embodiment

Next, the second embodiment of the present invention will be explained usingFIG. 5.FIG. 5Ais a cross-sectional view of a piston seal, an inner circumferential groove of a cylinder, and a piston of a disc brake device related to the second embodiment of the present invention.FIG. 5Bis an enlarged view of the portion A inFIG. 5A. A configuration same to that of the first embodiment will be marked with a same reference sign, and detailed explanation thereof will be omitted.

In addition to the configuration of the first embodiment, the second embodiment is configured as described below. When the piston seal43is viewed in the cross section as shown inFIG. 5B, the bottom wall47is formed to incline so that one half of difference of the outside diameter of the piston18and the average diameter of the bottom wall47becomes smaller than natural length in the radial direction of the piston seal43by equal to or greater than 10%.

When it is formed so that one half of difference of the outside diameter of the piston18and the average diameter of the bottom wall47becomes smaller than natural length in the radial direction of the piston seal43by equal to or greater than 10%, the compression force applied in the radial direction of the piston seal43can be increased exponentially. Therefore, according to the second embodiment, the friction force between the piston18and the piston seal43is maintained high, and the piston seal43can easily follow up the piston18.

Third Embodiment

Next, the third embodiment of the present invention will be explained usingFIG. 6.FIG. 6Ais a cross-sectional view of a piston seal, an inner circumferential groove of a cylinder, and a piston of a disc brake device related to the third embodiment of the present invention.FIG. 6Bis an enlarged view of the portion A inFIG. 6A. A configuration same to that of the first embodiment and the second embodiment will be marked with a same reference sign, and detailed explanation thereof will be omitted.

In addition to the configuration of the first embodiment and the second embodiment, in the third embodiment, it is formed so that the angle between the bottom wall47and the cylinder inner circumference51becomes equal to or greater than 2 degrees.

According to the third embodiment, since it is formed so that the angle between the bottom wall47and the cylinder inner circumference51becomes equal to or greater than 2 degrees, when the electric brake is operated, as the piston seal43moves toward the wall45on the inner brake pad side, the compression force can be increased more efficiently, and the piston seal43can easily follow up the piston18.

Fourth Embodiment

The fourth embodiment of the present invention will be explained usingFIG. 7.FIG. 7Ais a cross-sectional view of a piston seal, an inner circumferential groove of a cylinder, and a piston of a disc brake device related to the fourth embodiment of the present invention.FIG. 7Bis an enlarged view of the portion A inFIG. 7A. A configuration same to that of the first to the third embodiments will be marked with a same reference sign, and detailed explanation thereof will be omitted.

In addition to the configuration of the first to the third embodiments, in the fourth embodiment, it is formed so that the distance between the curvature endpoint49and the outermost circumference of the piston18becomes equal to or greater than 0.3 times of the difference of the maximum radius of the bottom wall47and the radius of the outermost circumference of the piston18.

According to the fourth embodiment, since it is formed so that the distance between the curvature endpoint49and the outermost circumference of the piston18becomes equal to or greater than 0.3 times of the difference of the maximum radius of the bottom wall47and the radius of the outermost circumference of the piston18, after the electric brake is released, the deformation allowance of the piston seal43in being restored toward the wall46on the opposite side increases, and the piston18can return more efficiently to the position of the time before the brake is applied.

Fifth Embodiment

Next, the fifth embodiment of the present invention will be explained usingFIG. 8.FIG. 8Ais a cross-sectional view of a piston seal, an inner circumferential groove of a cylinder, and a piston of a disc brake device related to the fifth embodiment of the present invention.FIG. 8Bis an enlarged view of the portion A inFIG. 8A. A configuration same to that of the first to the fourth embodiments will be marked with a same reference sign, and detailed explanation thereof will be omitted.

In addition to the configuration of the first to the fourth embodiments, in the fifth embodiment, it is formed so that the radius R of the curved surface50becomes equal to or greater than 0.2 mm.

According to the fifth embodiment, since it is formed so that the radius R of the curved surface50becomes equal to or greater than 0.2 mm, such event can be suppressed that the stress is concentrated when the piston seal43contacts the curved surface50, and the piston18can return more efficiently to the position of the time before the brake is applied after the electric brake is released.

Sixth Embodiment

Next, the sixth embodiment of the present invention will be explained usingFIG. 9.FIG. 9Ais a cross-sectional view of a piston seal, an inner circumferential groove of a cylinder, and a piston of a disc brake device related to the sixth embodiment of the present invention.FIG. 9Bis an enlarged view of the portion A inFIG. 9A. A configuration same to that of the first to the fifth embodiments will be marked with a same reference sign, and detailed explanation thereof will be omitted.

In addition to the configuration of the first to the fifth embodiments, in the sixth embodiment, an opening portion52having a tapered shape is formed between the wall45on the inner brake pad side and the cylinder inner circumference51. The opening portion52inclines so as to expand to the inner brake pad side from the wall45on the inner circumferential groove44over to the cylinder inner circumference51.

According to the sixth embodiment, since the opening portion52having a tapered shape is formed between the wall45on the inner brake pad side and the cylinder inner circumference51, the deformation allowance of the piston seal43is added to the inner brake pad side, and the piston seal43can follow up the piston more easily while the electric brake is operated.

Seventh Embodiment

Next, the seventh embodiment of the present invention will be explained usingFIG. 10.FIG. 10Ais a cross-sectional view of a piston seal, an inner circumferential groove of a cylinder, and a piston of a disc brake device related to the seventh embodiment of the present invention.FIG. 10Bis an enlarged view of the portion A inFIG. 10A. A configuration same to that of the first to the fifth embodiments will be marked with a same reference sign, and detailed explanation thereof will be omitted.

In addition to the configuration of the first to the fifth embodiments, in the seventh embodiment, a curved surface53expanding the inner circumferential groove44is formed between the wall45on the inner brake pad side and the cylinder inner circumference51. The curved surface53curves so as to expand to the inner brake pad side from the wall45of the inner circumferential groove44over to the cylinder inner circumference51.

According to the seventh embodiment, since it is configured to form the curved surface53expanding the inner circumferential groove44between the wall45on the inner brake pad side and the cylinder inner circumference51, the deformation allowance of the piston seal43is added to the inner brake pad side, and therefore the piston seal43can follow up the piston easily while the electric brake is operated. Also, according the seventh embodiment, such event can be suppressed that the stress is concentrated when the piston seal43contacts the curved surface50.

LIST OF REFERENCE SIGNS