As brake systems for land transport vehicles typified by railway vehicles, there are block brake, drum brake, disc brake etc. Among them, the use of disc brake has been increasing recently in association with an increase in speed and size of vehicles.
The disc brake is a device which obtains a braking force by friction between a brake disc and a brake lining. Usually, in the brake disc, a braking force is generated by pushing a brake lining against a frictional surface of a ring-shaped brake disc which is fixed to an axle or a wheel with bolts. With the braking force, the rotation of the axle or wheel is regulated to control the vehicle speed.
Super high-speed railway vehicles such as the Shinkansen have been further improved in speed and an operation thereof at a speed exceeding 300 km per hour is demanded. For further speed-up of railway vehicles, reduction of vehicle weight is necessary, and a brake disc in which a plate portion serving as a frictional surface is directly fixed to a wheel is suited. This is because this frictional-surface-fixed-type brake disc does not need a devoted part that solely serves for the fixing and therefore it is possible to attain the reduction of weight, in comparison with the conventional inner periphery fixing type brake disc.
In the conventional frictional-surface-fixed-type brake disc, for the purpose of suppressing deformation such as warpage caused by frictional heat during braking, a concave portion to be closely fitted with a convex portion formed on a member to be fixed (e.g., wheel) is formed on the brake disc surface for fixing (Patent Literature 1). There have been conventional frictional-surface-fixed-type brake discs for which a total volume ratio between fixing parts and counter portions to be fixed is prescribed (Patent Literature 2).
In such conventional frictional-surface-fixed-type brake discs, the problem of an increase in noise, to which no consideration has been conventionally given, is becoming an issue in such a high-speed traveling condition as exceeding 300 km per hour.
The brake disc rotates at high speed together with the associated wheel. With this rotation, the air around the brake disc is sucked in from the inner periphery side and is discharged to the outer periphery side. Consequently, a high-speed air flow occurs on the back side of the brake disc.
The air flow plays an important role for cooling the brake disc, but in such a high-speed range as exceeding 300 km per hour, the high-speed air flow not only cools the brake disc but also generates noise, which is called “aerodynamic noise.”
For reducing the aerodynamic noise it is effective to suppress the flow of air between the back side of the brake disc plate portion as well as fins formed thereon and a plate portion of wheel.
The present applicants have proposed such brake disc 1 as shown in FIG. 10 in which ribs 1c interconnected circumferentially are provided in spaces defined by the back side of a plate portion 1a and fins 1b, both being constituents of the brake disc 1, and a plate portion 2a of wheel 2, to minimize a total opening area in a circumferential direction (Patent Literature 3). In FIG. 10, numeral “1d” denotes a gap between a tip of each rib 1c and the plate portion 2a of the wheel 2, the tip of rib being configured so as to generate the gap. Numeral “2b” denotes a rim portion of the wheel 2. Numeral “2c” denotes a boss portion of the wheel 2, numeral “3a” denotes a bolt for fixing the brake disc 1, and numeral “3b” denotes a nut for fixing the brake disc 1.