Brakes – Elements – Brake wheels
Reexamination Certificate
1998-09-15
2001-07-31
Butler, Douglas C. (Department: 3613)
Brakes
Elements
Brake wheels
C188S25100R, C188S25100R, C188S21800R
Reexamination Certificate
active
06267209
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed generally to a brake unit having a nonmetallic brake disc of ceramic and a hub.
RELATED TECHNOLOGY
A brake unit and a manufacturing method are known, for example, from German Patent No. 44 38 455 C1 and from German Patent Application No. 44 38 456 A1. German Patent Application No. 44 38 456 A1 also describes a frictional unit, a frictional body, and a core body, the core body being able to have an extension prolongation with connection orifices in the manner of a nonmetallic hub. The frictional body is made of a C—C/SiC material, while the core body is constituted of a carbon fiber-reinforced carbon. The core body and the frictional body are joined by an interconnection layer containing essentially silicon carbide.
Brake units are also known, whose brake discs are constituted of carbon over their entire cross-section. Special measures are required to adapt a toroidally-shaped disc of this kind to the wheel hub. Provision has been made in known methods for a hat- or pot-shaped metal hub. This hub is mechanically coupled by its free rim to the brake disc. This mechanical coupling is achieved using a wreath-type rim of mounting bolts and corresponding entrainment bores or radial slots in the area of the inner rim or the inside of the brake disc, which produce a mechanically loadable form-locking between the brake disc and metal hub. Provision is usually made for six or eight mounting bolts.
What is problematic here is the difference between the expansion coefficient specific to the brake disc (about 3-5×10
−6
/K) and that specific to the metal hub (about 15-20×10
−6
/K). For that reason, one has to prevent the parts from sticking, as can occur due to the tension produced by this type of attachment. Rather, radial expansion must still be allowed between the two parts, in spite of the mechanical coupling. Therefore, a suitable play is intentionally permitted between the entrainment bores and the bolt in the radial direction, whereas a precise as possible fit with the bolt diameter is aspired to in the circumferential direction.
The predefined, hub-side position of the bolts, on the one hand, and the disc-side position of the entrainment bores, on the other hand, must, therefore, conform as precisely as possible. However, in spite of all efforts to achieve manufacturing precision, mathematical agreement can not be achieved with any kind of realistic cost outlay. Rather, in practice, it may not be possible to entirely avoid positional tolerances with respect to the bolts, on one hand, and the entrainment bores, on the other hand. As a result, merely two, at the most three, of all the provided bolts are load-bearing in response to mechanical stressing of the brake, whereas a play exists between the remaining bolts and the entrainment bores, preventing these bolts from bearing load.
After a plastic or wear-related deformation has taken place in the area of the bearing bolts, it is possible for the play existing in the area of the other bolts to be reduced, enabling these bolts to be load-bearing as well. When working with carbon discs, sufficient plasticity is given in the area of the entrainment bores, depending on the material used, so that in this case, at the latest following the first extreme mechanical stressing of the brakes, the initially sole three bearing bolts have pressed into the inside face of the corresponding entrainment bores, plastically deforming these bores, eliminating the play between the remaining bolts and their entrainment bores, thus rendering them load-bearing as well.
It is a different situation, however, with rigid ceramic discs. In the just described mechanical coupling between the brake disc and the metal hub, play may be eliminated for the non-bearing bolts, due to wear, or because of a plastic deformation of the bearing bolts; but this can take a very long time.
A solution to this problem, as described in the German Patent Application No. 44 38 456 A1, is to revert to a nonmetallic hub, which would lessen the differences with respect to the expansion coefficients. What is problematic, however, is securing the hub to the brake disc, due to the high mechanical and thermal stressing experienced during braking maneuvers.
SUMMARY OF THE INVENTION
An object of the present invention is to improve a brake unit having a nonmetallic disc of ceramic and a metal hub so that in spite of an inflexible ceramic disc, all of the bolts provided will be able to participate virtually uniformly in the transmission of the braking torque occurring at the brake disc, even without the bolts experiencing any plastic or wear-induced deformations. It is also an object of the present invention to provide an additional method for attaching a nonmetallic hub, as well as to provide a method for manufacturing brake units of this kind.
The present invention provides a brake unit (
1
) having a nonmetallic brake disc (
2
) of ceramic and a metallic hub (
10
,
10
′), which radially overlap one another with their mutually facing rims and are mechanically joined by a wreath-type rim of mounting bolts (
15
) that axially penetrate the overlapping rims, in the area of its inner rim (
3
), the brake disc (
2
) having corresponding entrainment bores (
5
,
5
′) or radial entrainment slots to receive the mounting bolts (
15
), wherein the entrainment bores (
5
,
5
′) or entrainment slots are lined with a sleeve (
20
) of plastically deformable material, or the disc material is conceived to be plastically deformable to specific areas in this region. In the brake unit of the present invention having a metal hub, it is the region of the brake disc representing the inside face of the entrainment bore that is designed to be plastically deformable. This can be achieved, on the one hand, with the aid of a plastically deformable sleeve, or by using a plastic form of the normally rigid ceramic material. The effect of the plastic deformability is to compensate for the above-mentioned positional deviation of the bolts, on the one hand, and of the entrainment bore, on the other hand, in that the sleeve undergoes a deformation in response to mechanical stress, so that all bolts are load-bearing and undesired play no longer occurs.
Provision is made for a brake unit of the present invention having a nonmetallic hub for the hub to be molded into an inner region of the ceramic brake disc and doped with boron.
A soft metal sleeve may be used as a plastically deformable lining for the inside bore face. Sleeves of copper or aluminum, for example, are suitable. Instead of a metal sleeve, a sleeve of carbon fiber-reinforced plastic (CFK material) or carbon fiber-reinforced carbon (C/C material) may also be used. A nonmetallic CFK or C/C material of this kind is sufficiently plastically deformable. The sleeve is preferably slotted at one peripheral location over the entire length and fits with mechanical prestressing on the inside face of the bore or of the slot. The longitudinal slot makes the sleeve radially elastic, so that even under conditions of considerable temperature variations, accompanied by noticeable expansion or contraction, it fits with prestressing on the inner bore face.
The wall thickness of the sleeve is a function of the size of the brake disc or of the diameter of the bores. The wall thickness may typically be {fraction (1/20+L )} of the bore diameter. In the case of motor vehicle brake discs having a bore diameter of, for example, 15 mm, the wall thickness is about 0.75 mm; in the case of railroad brake units having a bore diameter of, for example, 200 mm, the wall thickness is about 10 mm.
It is a feature of a further embodiment of the present invention that the area immediately along the inner face of the entrainment bores or of the entrainment slots contains boron and/or boron-containing compounds and/or boron-releasing compounds in a locally limited fashion, this area being made of a non-ceramic C/C material, while the ceramic brake disc is made of C/C—SiC material. This embodiment is based
Gross Gerhard
Haug Tillman
Rebstock Kolja
Schwarz Christian
Butler Douglas C.
Daimler-Benz Aktiengesellschaft
Kenyon & Kenyon
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