Note: Descriptions are shown in the official language in which they were submitted.
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WEIGHT RED~Ç~P BRAKE ROTOR
Backg~ound_of ~he I~ve~tio~
The present invention relates to an improved dlsc brake
rotor for automotive applications which achieves the same
structural rigidity of rotors of conventional construction with a
substantially reduced weight, together with improved cooling
characteristics.
While useful in other applications, the present
invention is disclosed as being applied to disc brake rotors of
the type used in automotive vehicles. The standard brake rotor
presently in commercial use for this purpose is constructed as a
one piece casting having a hub integral with a first annular
plate like braking member. A second annular plate like braking
member is integrally joined to the first braking member by a
plurality of fins which extend between the two opposed faces of
the braking members to support the second braking member in
axially spaced coaxial relationship to the first braking member.
The fins are spaced from each other to provide for air flow
between the opposed faces of the braking members to disslpate the
frictional heat generated by application of the vehicle brakes.
In the presently employed standard configuration, the
fins which ~oin the two braking members to each other lie in
general planes which extend radially from the rotor axis with the
radial dimension of the individual fins substantially exceeding
the thickness of the fins circumferentially of the rotor axis.
The number, shape and size of the fins employed to join
the two braking members to each other must be such as to provide
a~equate structural strength to transmit braking forces applied
to the rib supported braking member to the hub supported braking
member while at the same time leaving enough open space between
the ribs to accommodate sufficient air flow between the two
members to provide adequate cooling. The industry standard
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radial fin brake rotor has been used for many years, and its
design details and techniques for manufacturing the rotor have
been exhaustively refined. While many alternative fin
arrangements have been proposed, few, if any, have enjoyed any
s substantial commercial s~ccess, typically because any improved
performance achieved by alternative designs has been outweighed
by increased manufacturing costs.
The present invention is directed to a brake rotor
having fins dimensioned and configured in a manner such that less
than half of the material required to form the fins of a standard
radial fin rotor of comparable dimensions and strength need be
employed. In addition to reducing the overall material cost, a
substantial reduction in weight without a corresponding reduction
in structural rigidity is achieved. The new rib configuration
and arrangement can be manufactured by the same manufacturing
techniques employed to manufacture radial fin rotors, and the
improved fin arrangement also achieves better cooling air flow
through the rotor than is obtained with the standard radial fin
arrangement.
SUMMARY OF THE INVENTION
In accordance with the present invention, the fins
which interconnect the two annular braking members of a disc
brake rotor are arranged in unif~rmly circumferentially spaced
relationship along two circles of different radius centered on
the rotor axis. The fins are elongated circumferentially of the
rotor axis. The rotor may be formed as an integral casting
utilizing the sam~ tooling techniques as those employed in the
casting of the standard radial finned rotor. Alternatively, the
rotor hub may be formed as a sheet metal stamping with the two
circular series of ribs projecting axially from the hub and from
uniformally spaced spokes formed on the hub. This sheet metal
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stamping is utilized as an insert which is cast in place or
otherwise assembled to the two annular braking members.
Other objects and features o~ the invention wlll become
apparent by reference to the following specification and to the
drawings.
Brief Des~ri~tion of the Drawinqs
Fig. 1 is an end view of an automotive disc brake rotor
oE standard prior art constructlon, with certain parts broken
away or shown in section;
Fig. 2 is a cross sectional view of the prior art rotor
of Fig. 1 taken on line 2-2 of Fig. 1;
Fig. 3 is an end view of one form of rotor embodying
the present invention, with certain parts broken away or shown in
section;
Fig. 4 is a detailed cross sectional view of the rotor
of Fig. 3 taken on line 4-4 of Fig. 3;
Figs. 5a and Sb are respectively schematic side and
plan views of a prior art fin of the type shown in Figs. 1 and 2;
Figs. 6a and 6b are respectively side and plan views of
a fin embodying the present invention;
Fig. 7 is an end view of a portion of a disc brake
rotor, with certain parts broken away or shown in section, of an
alternative form of fin embodying the present invention;
Fig. 8 is an end view of a portion of a disc brake
rotor, with certain parts broken away or shown in section, of
another form of brake rotor embodying the present invention; and
Fig. 9 is a cross sectional view of ths rotor of Fig. 8
taken on line 9-9 of Fig. 8.
A radially finned disc brake rotor of standard prior
construction is shown in Figs. 1 and 2 for purposes of comparison
to rotors embodying the present invention and to identify certain
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fin dimensions utilized in comparing the prior art rotor to those
of the present invention.
The prior art rotor of Figs. 1 and 2 is typically
constructed as a one piece casting formed with a central hub
portion 20 integrally joined at its outer periphery to an annular
plate like first braking member 22. A second annular plate like
braking member 24 is integrally joined to the first braking
member 22 by a plurality of fins 26 integrally joined to and
extending between the opposed faces 28, 30 respectively of
members 22 and 24 to support braking member 24 in axially spaced
relationship to member 22 with the plate like annular braking
members 22 and 24 lying in spaced parallel generally planes in
coaxial relationship to each other and to the axis A of rotation
of the rotor.
The fins 26 lie in respective axial general planes
extending radially from the rotor axis A and are uniformly
angularly spaced about axis A. For purposes of comparing the
fins of the standard prior art rotor of Figs. 1 and 2 to fins
conformed in accordance with the present invention, each fin 26
of the prior art rotor of Figs. 1 and 2 may be considered to be a
rectangular solid having a dimension bl measured radially of axis
A, a tangential dimension hl measured tangentially of axis A and
a dimension al (Fig. 2) measured parallel to axis A.
Referring now to Figs. 3 and 4, one form of disc brake
rotor embodying the present invention may be formed as a one
piece cast~ng which includes a hub portion 200, a first annular
braking member 220 integrally formed on the outer periphery of
hub 200 and a second plate like annular braking member 240
integrally joined to member 220 by a plurality of fins 260, 262.
The dimensions of the hub portion 200, first annular braking
member 220 and second braking member 240 may be assumed to be
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identical to the corresponding dimensions of the hub portion 20,
first braking member 22 and second braking member 24 of the
standard prior art rotor shown in Figs. 1 and 2.
It is believed that apparent that the difference
between the prior art rotor of Figs. 1 and 2 and the rotor
illustrated in Figs. 3 and 4 resides in the size, shape and
arrangement of the fins 260, 262 as compared to the size, shape
and arrangement of the fins 26 of the prior art rotor of Figs. 1
and 2. In the arrangement of Figs. 3 and 4, the fins are divided
into two groups, a first group of fins be~ng the fins 260 which
are uniformly spaced about the rotor axis A along a clrcle of
radius R1 centered at axis A and the remaining or second group of
fins 262 which are similarly unlformly spaced along a second
circle of radius R2 also centered at the rotor axis A. For
purposes of comparison with the prior art fin arrangement of
Figs. 1 and 2, the fins 260 and 262 may be considered as a
substantlally rectangular solid having a dimension b2 radially of
axis A, a dimension h2 tangentially of axis A and a dimension a2
(Fig. 4) parallel to axis A.
The primary function of the fins 26 of the prior art
Fig. 1 rotor and the fins 260, 262 of the rotor of Fig. 3 is to
transmit braking forces applied to the second braking member 24
or 240 to the first braking member 22 or 220 which is integrally
attached to the hub 20 or 200 which in a turn will be fixedly
attached to the wheel being braked. In order to transmit this
braking force, the fins must be capable of resisting the stresses
induced by the applied forces. In the case of the radially
finned prior art rotor, the applied forces are schematically
indicated in Figs. ~a and 5b.
In the prior art radially finned rotor of Figs. 1 and
2, a braking force F applied to the second braking member 24 acts
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in a direction tangentially of the rotor axis and may be
considered to be applied to the rib 26 at the edges of the ribs
26 at which each rib is joined to the braking member 24. As
indicated in Fig. 5a, the force ~ F as a portion of F is applied
to the rib 26 at a distance al from the location at which rib 26
is integrally joined to braking member 22, hence the applied
force F will induce a bending stress in rib 26. The bending
stress at this latter point is equal to the bending moment (~Fal)
divided by the section modulus which, for a rectangular section
is equal to blhl2/6 which may be algebraically expressed as
stress S = 6~Fal/blhl2.
In Figs. 6a and 6b, similar schematic diagrams
representing the application of a braking force to braking member
~ 240 of the Fig. 3 embodiment to a rib 262 are shown. In this
case, the stress S - 6~Fa2/b2h22.
To compare the effectiveness of a single radial fin 26
of Fig. 1 to a single tangentially extending rib 262 of Fig. 3,
the dimensions of a rib 262 can be compared to the dimensions of
a radial rib 26 by analyzing the situation in which the same
braklng force ~F applied as indicated in Figs. 5a and 6a would
induce the same bending stress in fin 26 as it would induce in
fin 262. To simplify this comparison, it will be assumed that
the axial dimensions al and a2 of the fins 26 and 262 are equal
and that the smallest dimension of the respective fins 26 and 262
- that is the fin thickness hl of the fin 26 is equal to the
thickness b2 of the fin 262. Thus, the relative size of the two
fins can be compared by comparing their longest respective
dimensions bl and h2.
With the foregoing assumptions, if the two above
equations for the bending stress of a fin 26 and the bending
stress for a fin 262 are made equal to each other, the dimension
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h2 of fin 262 is found to be equal to the square root of the
cross sectional area hlbl of the fin 26.
To reduce this comparison to a numerical example, in a
typical prior art radial fin rotor as shown in Figs. 1 and 2, the
dimension al of a fin 26 may be taken as lOmm, the dimension hl
as 6mm and the dimension bl as 45mm, with the rotor as a whole
having forty-eight fins 26. If a rib 262, as assumed above, has
a dimension a2 of lOmm (al=a2), and a dimension b2 of 6mm
(hl=b2), then its dimension h2 can be computed as 16.4mm (the
square root of hlbl which is the square root of 6 X 45 or the
square root of 270). Effectively, in a disc brake rotor, a rib
262 or 260 oriented as shown in Fig. 3 and having dimensions of 6
X 10 X 16.4 is as strong as a rib 26 oriented in Fig. 1 having
dimensions of 6 X lo X 45 measured in the same units. A rotor
constructed in the standard configuration of Fig. 1 having forty-
eight ribs 26 of dimensions of 6mm X lOmm X 45mm would require
939 grams of cast iron to form the forty-eight ribs 26. A rotor
constructed as in Figs. 3 and 4 having twenty-four ribs 262 and
twenty-four ribs 260 all of dimensions of 6mm X lOmm X 16.4mm
would require only 3~2 grams of cast iron material to form all
forty-eight ribs. This represents a weight saving of 597 grams
per rotor for a rotor of equal strength.
In addition to the weight saving advantage set forth
above, the rotor of Fig. 3 provides a more efficient flow of
cooling air through the rotor as compared to the radial fin
arrangement of the prior art rotor of Fig. 1. Because a
substantially smaller volume of the space between the two braking
members is occupied by the fins in the Fig. 3 arrangement, it is
believed apparent that the Fig. 3 arrangement provides a
substantially greater path of flow of air through the space
between the two braking members than does the prior art radial
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fin arrangement of Fig. 1. Further, by staggering the locations
of the fins 260 and 262 so that the fins 262 are radially aligned
with the spaces between the fins 260, preferred flow paths of air
through the fins for low, medium and high speeds of rotation of
the rotor are indicated at vl, v2 and v3 respectively in Fig. 3.
It will be noted that at higher speeds, the flow path length
through the rotor increases, thus enabling more heat to be
extracted from the braking members by the flowing air because the
air sweeps a greater area of the braking members during its
passage through the rotor.
One frequently proposed solution for improving the air
flow through the prior art rotor of Fig. 1 is to incline or curve
the fins to create an outwardly spirally flow paths. In addition
to requiring more complex tooling, this solution requires the
rotors to be made with left and right handed fin configuration to
achieve similar air flow for rotation in either direction. The
fin arrangement of Fig. 3 generates the same air flow in either
direction of rotation.
In that embodiment of the invention disclosed in Figs.
3 and 4, the fins 260 of the first group of fins and the fins 262
of the second group of fins are all assumed to be of equal
dimension and further, each fin 260 and 262 has been assumed to
be a structural equivalent of a single fin 26 of the prior art
rotor of Figs. 1 and 2. Further, in the Figs. 3 and 4
2~ embodiment, the number of fins 260 on the innermost circle is the
same as the number of fins 262 on the outermost circle. While as
a general rule for to the present invention, the number of fins
on the inner circle will be made equal to the number of fins on
the outer circle and the fins on the inner circle being
symmetrically arranged at opposite sides of a radial line
bisecting the space between two adjacPnt fins on the outer circle
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to achieve the desired bi-directional air flow paths, it is not
necessary that the radial and tangential dimensions of the inner
fins 260 be the same as the corresponding dimensions of the fins
262 on the outer circle.
In Flg. 7, one alternative arrangement is shown in
which the tangential dimension h3 of the fins 260a is less than
the tangential dimension h4 of the fins 262a located on the outer
circle. Optimum dimensioning of the fins involves consideration
of other dimensions of the rotor and the magnitude of the braking
forces for which the rotor is designed. The general arrangement
in which the fins are elongated tangentially of the rotor axis
and arranged in two concentric circular groups allows a
substantial degree of flexibility in selecting the tangential and
radial dimensions of the fins to match the requirements of a
specific rotor application.
The rotors of Figs. 3 and 4 and Fig. 7 ~re of a one
piece cast constxuction. In Figs. 8 and 9, the invention is
shown applied to a rotor of modified construction in which a hub
designated generally 300 is formed as a sheet metal stamping
having a plurality of spoke like arms 302 projecting radially
outwardly from the outer periphery of the central portion of the
hub. The arms 302 are uniformly angular]y spaced about the rotor
axis. A first group of fins 30~ project axially from the central
portion of hub 300 between adjacent arms 302, while a second
group of fins 306 project axially from the outer ends of the
respective arms 302. The hub 300 may be cast in place with a
pair of annular braking members 308, ~10 or alternatively may be
assembled with a pair of preformed braking members having
appropriately located recesses to receive the arms and fins which
are then welded in place. A steel sheet metal stamping such as
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the stamping 300 has a substantially higher strength to welght
characteristic than a corresponding cast iron structure.
While various embodiments of the invention have been
described in detail, it will be apparent to those skilled in the
S art that the disclosed embodiments may be modified, therefore,
the foregoing description is to be considered exemplary rather
than limiting, and the true scope of the invention is that
defined in the following claims.
