Disc brake device

A disc brake device comprising:

• a pad;
• a caliper including a cylinder having an opening on a pad side;
• a piston fitted to the cylinder and configured to press the pad toward a rotor;
• and
• a rotary-to-linear motion conversion mechanism configured to convert a rotary motion of a drive source into a linear motion to push the piston toward the rotor, wherein
• a braking force of a service brake is generated by feeding brake oil into the cylinder, and a braking force of a parking brake is generated by operating the rotary-to-linear motion conversion mechanism,
• the piston is divided into two parts that are a piston main body and a piston cap in an axial direction,
• the rotary-to-linear motion conversion mechanism includes:
• a rotary member configured to be driven to rotate by the drive source;
• and
• a linear motion member screwed to the rotary member, disposed inside the piston main body, engaged with the piston main body so as not to be rotatable relative to the piston main body, and configured to press the piston main body in the axial direction;
• the disc brake device includes a unidirectional rotation regulating portion provided between the piston main body and the piston cap, configured to regulate relative rotation of the piston main body in a forward rotation direction with respect to the piston cap when the rotary member is driven to rotate in the forward rotation direction to move the linear motion member to a rotor side, and configured to allow relative rotation of the piston main body in a reverse rotation direction with respect to the piston cap when the rotary member is driven to rotate in the reverse rotation direction to move the linear motion member to a side opposite to the rotor,
• the unidirectional rotation regulating portion is configured to transmit an axial force between the piston main body and the piston cap,
• the unidirectional rotation regulating portion includes a main body-side sliding contact surface provided on the piston main body and a cap-side sliding contact surface provided on the piston cap and opposed to the main body-side sliding contact surface in the axial direction,
• at least one of the main body-side sliding contact surface and the cap-side sliding contact surface is subjected to surface processing for increasing a friction coefficient between the main body-side sliding contact surface and the cap-side sliding contact surface, or includes a friction member,
• when the rotary member is driven to rotate in a forward rotation direction, the main body-side sliding contact surface and the cap-side sliding contact surface are frictionally engaged so as not to rotate relative to each other with an increase in an axial force acting on the cap-side sliding contact surface from the main body-side sliding contact surface, and
• when the rotary member is driven to rotate in a reverse rotation direction, the main body-side sliding contact surface relatively rotates in the reverse rotation direction with respect to the cap-side sliding contact surface with a decrease in the axial force acting on the cap-side sliding contact surface from the main body-side sliding contact surface.