Brake Rotor With Vibration Harmonic Suppression, and Method of Manufacture

The present invention is directed to brake rotors and methods of manufacture, and more particularly to a ventilated disc brake rotor structured for suppression of harmonic vibration during braking.

Vibration of a brake rotor during brake operation can cause a loud and irritating "squeal" if permitted to resonate at audible harmonic frequencies. This problem can occur with conventional drum and disc brake systems. Prior art attempts to address this problem have included the use of sound deadening inserts or fillers in the rotor (drum or disc) structure, and special material compositions in the rotor and/or brake pad. In the specific environment of ventilated disc brake rotors, the Day U.S. Pat. No. 3,298,476 and Hagiwara et al No. 4,379,501 disclose techniques for varying the pattern of cooling fins around the rotor circumference to break up the vibration harmonic resonance patterns. In Day, the rotor circumference is divided into equal angular segments, and the spacing between cooling fins in each segment is random or non-repetitive. Hagiwara et al contemplates irregular or non-uniform spacing between and thickness of cooling fins around the rotor.

A general object of the present invention is to provide an improved technique for damping vibration harmonics, and thereby reducing irritating squeal, in both drum and disc brake rotors.

A further object of the invention is to provide a brake rotor with enhanced vibration damping capacity while maintaining desired balance, strength and thermal dissipation capacity.

Yet a further object of the invention is to provide a method of manufacturing such a rotor.

Another and more specific object of the invention is to provide a ventilated disc brake rotor which is structured to reduce or eliminate brake squeal.

The invention, together with additional objects, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawing in which:

FIG. 1 is an axial elevational view of a ventilated disc brake rotor;

FIG. 2 is a sectional view taken along the line 2--2 in FIG. 1;

FIGS. 3-7 are schematic illustrations of circumferential patterns of disc ventilation fins in accordance with five presently preferred embodiments of the invention;

FIGS. 8 and 9 are graphic illustrations useful in understanding one aspect of the invention;

FIG. 10 is a schematic illustration of another disc brake fin pattern in accordance with the invention; and

FIG. 11 is a graphic illustration useful in explaining the structure of FIG. 10.

FIGS. 1 and 2 illustrate a ventilated disc brake rotor 20 as comprising a pair of flat annular friction rings or cheeks 22,24 spaced from each other by a circumferential array of radially extending angularly spaced ventilation fins 26. A central mounting surface 28 is integral with friction ring 22 and is provided with suitable apertures 32,34 for mounting rotor 20 on a vehicle. Rotor 20 may be of composite or one-piece cast construction of suitable composition. To the extent thus far described, rotor 20 is of conventional construction. In operation, rotor 20 is suitably mounted on a vehicle so that the axially facing surfaces of friction rings 22,24 may be selectively engaged by friction pads on one or more caliper assemblies (not shown) for slowing or braking an associated vehicle wheel. The present invention is directed to selective control of the length of and/or spacing between ventilation fins 26 for enhanced vibration damping during such braking operation by suppression of vibration-harmonic resonances. With harmonic resonances so suppressed, vibration energy is dissipated as "white noise", which is substantially unnoticed by vehicle occupants.

In general, in accordance with one important aspect of the present invention for frequency modulation of vibration harmonics a total number of fin cycles is selected, with each fin cycle consisting of a fin 26 and a space between that fin and the next-adjacent fin in the array. The circumferential array of fin cycles is then divided into a plurality of nominally unequal fractional angular segments. The nominal fractional size of the angular segments and the number of fin cycles in each segment are determined by modification of the techniques disclosed and claimed in the Landers U.S. Pat. No. 4,327,792 issued to The Goodyear Tire & Rubber Company, parent to the assignee of this application, in a manner to adapt the same for brake rotor applications in accordance with the principles of the present invention. The disclosure of this patent is incorporated herein by reference. The total number of fin cycles in the circumferential array, the number of fractional segments and the nominal fractional size of each fractional array segment preferably are prime numbers in accordance with the present invention. The sequence of fractional angular segments in the circumferential fin array is in non-sequential order by size in the embodiments of FIGS. 3, 6 and 7 of the present disclosure, or is in order of increasing segment size (FIGS. 4 and 5). The angular dimension and sequence of the fin cycles in each segment are in cyclic patterns within each segment in the embodiments of FIGS. 3 and 4 so that the wavelength of the predominant vibration frequency for each segment substantially corresponds to the circumferential length of that segment, or in non-cyclic and non-repetitive progressively increasing patterns within each segment (FIGS. 5, 6 and 7). This nominally unequal segmentation of and selected spacing between ventilation fins so modulates the frequencies of vibration as to suppress harmonic resonances.

In accordance with a second important aspect of the invention disclosed herein, radial lengths of the ventilation fins are also selected and controlled to modulate and dampen the amplitude of vibration, and to balance the rotor about its axis of rotation. This is accomplished by locating the center of gravity of a particular rotor design assuming that all fins are of equal length, and then shortening the fin length at alternate fin postions around the entire array as an exponential function of angle.

More specifically, in providing a ventilated disc brake rotor in accordance with the principles of the present invention, the number of cooling fins and the limits of angular spacing between fins are normally dictated by mechanical strength and manufacturing capabilities. That is, the desired number of fins and the maximum allowable spacing between fins are selected to obtain desired strength and rigidity in the overall structure under worst-case stress and temperature conditions. In accordance with an important feature of the preferred embodiments of the invention (FIGS. 3-7), the number of fins is selected to be a prime number. In the embodiments of FIGS. 3-6, twenty-three fins are illustrated. In the embodiment of FIG. 7, twenty-nine fins are employed. The embodiments of FIGS. 3-7 (and FIG. 10) find particular application in 220 mm OD automotive passenger car disc brake rotors. The minimum spacing between fins is typically dictated by manufacturing capabilities. A maximum spacing of 1.75 inches (44.45 mm) and a minimum spacing of 0.25 to 0.5 inches (6.35 to 12.7 mm), measured in the circumferential direction, have been found suitable in automotive passenger car applications.

As a next step in provision of the preferred embodiments of the invention (FIGS. 3-7), the circumferential dimension of the fin array is divided into fractional segments. In accordance with the preferred technique of the present invention, prime numbers are employed where selection permits. Such use of prime numbers is considered to enhance the frequency modulation feature of the invention. Thus, the number of segments is preferably a prime number, five in the embodiments of FIGS. 3-7. Likewise, a nominal fraction associated with each segment is selected such that the fraction denominators are prime numbers. Thus, in FIGS. 3-7, the fractional angular segments nominally comprise segments of 1/5, 1/7, 1/11, 1/13 and 1/17. It will be noted that the segment fractions sum to 0.57, which is substantially less than unity.

The number of fin cycles within each segment is then determined as a function of the total number of fin cycles multiplied by the ratio of the nominal fraction for each segment to the sum of all segment fractions. Thus, the number of cycles in the "1/5" segment of FIG. 3 is a function of the total number of fins (23) multiplied by the ratio of the segment fraction (1/5) to the sum of all fractions (0.57).

More specifically, for the embodiments of FIGS. 3-6, selection of the number of fin cycles within each fractional segment proceeds per the following table:

First, a "multiplier base" is found by multiplying the denominators of the nominal segment sizes. This base is 85085 in the example given. The nominal segment sizes (X in column 1) are then each multiplied by the base (85085), and the results added in column 2 to arrive at a common fraction base (48457). The figures of column 2 (Y) are then divided by the fraction base to arrive at the adjusted fraction or decimals (Z) in column 3 associated with each segment. These adjusted segment fractions are then multiplied by the total selected number of fin cycles (23) and rounded off to arrive at the number of "cycles" per segment in column 5.

For the embodiment of FIG. 7, the last steps of the foregoing procedure are varied slightly, per the following table:

That is, the number of fin cycles per segment is rounded off to the closest prime number in the embodiment of FIG. 7, rather than to the closest number as in the embodiment of FIGS. 3-6. Note that this results in one extra cycle in the 1/5 nominal segment, and one less cycle in the 1/13 nominal segment. Again, this design decision is in keeping with the general consideration in accordance with the preferred aspects of the invention to employ prime numbers wherever possible.

The next step in providing the preferred embodiments of the invention (FIGS. 3-7) is to determine the sequence of fractional segments around the fin array. In the embodiments of FIGS. 3, 6 and 7, the segments are arranged in non-sequential order by size--i.e., in the order 1/5, 1/17, 1/11, 1/13, 1/17 in the clockwise direction in FIGS. 3, 6 and 7. In the embodiments of FIGS. 4 and 5, the order of decreasing segment size is employed--i.e., 1/5, 1/7, 1/11, 1/13, 1/17.

The size and sequence of fin cycles within each segment must then be determined. In all preferred embodiments of the invention, all fins 26 are of equal angular dimension. Thus, size of the fin cycles is varied by varying the size or angular dimension of the space between the fin of one fin cycle and the fin of the next-adjacent fin cycle. In general, the size and sequence of fin cycles in the embodiments of FIGS. 3-7 are selected as a predetermined junction of angle within each segment.

More specifically, in the embodiments of FIGS. 3 and 4, the size and sequence of fin cycles within each segment are arranged so that the wavelength of the predominant vibration frequency for that segment substantially corresponds to the circumferential length of that segment. This is illustrated by the following table:

The dimension "A" is the minimum spacing between fins, or 0.5 inches (12.7 mm) in the embodiments discussed. The dimension "C" is the maximum spacing between fins, or 1.75 inches (44.45 mm) in the embodiments discussed. The dimension "B" is halfway between "A" and "C", or 1.125 inches (28.575 mm) in these embodiments.

In the embodiments of FIGS. 5-7, the fin cycles within each segment are arranged in sequential order by length, with the lengths of the successive inter-fin spaces increasing at uniform increments substantially between the minimum and maximum design limits. Specificially, the first inter-fin space in each segment of these embodiments (in clockwise order) is selected to be the minimum design (0.5 inches or 12.7 mm), and the last space in each segment is initially assumed to be at the maximum design dimension (1.75 inches or 44.45 mm). The intervening spaces are then dimensioned to increase uniformly from minimum to maximum. For example, the inter-fin spacings in the embodiments of FIGS. 5 and 6 are given by the following table:

It will be noted that it is not always possible using simple fractional increments to have the last space equal to the maximum dimension. The spacing dimensions and sequence within each segment in the embodiment of FIG. 7 is determined in an analogous manner.

In accordance with a second important aspect of the invention, rotor balance about its axis of rotation is obtained, and amplitude modulation (damping) of the vibration harmonics is also accomplished, by varying the length of alternate fins--i.e., every other fin--as a function of angle around the array. More specifically, the radial dimension or length of alternate fins is shortened or decreased from the length of the intermediate fins (which are at equal length) as an exponential fuction of angle around the fin array. The procedure employed will be described in detail in connection with the embodiment of FIG. 7, with it being understood that the embodiments of FIGS. 3-6 are designed in an analogous fashion (as is the embodiment of FIG. 10 to be described).

FIG. 9 illustrates alternate fins A-O in the embodiment of FIG. 7 distributed around the circle 40, which is at a radius r from the rotor axis x corresponding to the radius of the weight-center of the fins. After the locations and spacing of the several fins have been determined per the foregoing discussion, the center of gravity CG and the location and amount of unbalance associated with the center of gravity CG are determined either analytically or empirically. A straight line passing through CG and x intersects circle 40 at the point CG/(r.sub.1 -r.sub.2), which represents the point at which weight must be removed, and the amount of weight which must be removed, to balance the rotor.

FIG. 8 graphically illustrates the amount of material which must be "removed" from alternate fins A-O in order to balance the rotor and at the same time provide the desired vibration amplitude modulation previously described. In FIG. 8, the abscissa is in degrees around circle 40, with the relative locations of fins A-O being placed accordingly. The ordinate of FIG. 8 represents the amount by which each fin A-O is to be foreshortened as compared with the intervening fins, which are all of identical length. The angle of point CG/r.sub.1 -r.sub.2, which fell next to fin J in FIG. 9, is placed in FIG. 8 at a position corresponding to an angle of 120.degree. from the shortest fin--i.e. fin A. In accordance with this aspect of the present invention, the length of alternate fins A-O is then determined as a function of the exponential equation y=ax.sup.b in FIG. 8, where y is fin length, x is angle in radians, and a and b are constants. As a starting point, b is made equal to unity. However, it has been found necessary in practice of the invention to vary the constant b upwardly or downwardly from unity because of the limited number of fins.

In one example of the invention in accordance with the embodiment of FIG. 7, the nominal fin length is 1.44 inches, and the lengths, of alternate fins A-O are given in inches by the following table:

It will be noted that several fins designated by the asterisk (*) are not in sequential order by length. Because of the limited number of fins, it is sometimes possible only to approximate balance by the foregoing method. The lengths of individual fins are then selectively varied to bring the rotor into balance, while still maintaining a substantially exponential length function as previously described. It will also be noted that, because an odd number of fins are employed, adjacent fins A and O are foreshortened.

FIG. 10 illustrates a ventilation fin pattern in accordance with another embodiment of the present invention. In the embodiment of FIG. 10, the design begins with a uniformally spaced array of fin locations, in this case twenty-eight loctions A-BB. Fins are then deleted or left out of locations at spacings which increase step-wise around the array. That is, one fin at location B separates "empty" locations A and C, two fins at locations D and E separate "empty" locations C and F, etc. This staggered location of empty fin locations inhibits reinforcement of vibration harmonics. To obtain amplitude modulation, and to balance the rotor, the fins at alternating locations are foreshortened as an exponential function of angle as shown in FIG. 11, starting with fin location E and proceeding around the array to fin location BB. (It will be appreciated that there are no fins at locations F, J, D and U to be shortened.). The fins in the embodiment of FIG. 10 have equal angular dimension.

Ventilated disc brake rotors having the configuration of FIG. 10 have been constructed and compared in terms of vibration damping to a disc brake rotor of conventional construction and identical size and composition. The conventional rotor employed for comparison purposes is used on the Chrysler K car and embodies 28 equally spaced fins of identical angular dimension. The radial dimension of the fins alternate between long and short around the array, with all long fins being of equal length and all short fins being of equal length. The damping coefficient for the conventional rotor was between 0.25 and 0.29, while the damping coefficient of the embodiment of FIG. 10 was 0.60.

Thus, a first important aspect of the present invention, disclosed in connection with FIGS. 3-7, contemplates a method of manufacturing brake rotors wherein a circumferential array of cooling ribs or fins is divided into a plurality of unequal nominal fractional segments. Preferably, the number of fins, the number of segments and the nominal segment fractions are prime numbers. The actual size of each fractional segment in terms of the number of inter-fin spaces disposed therein is then determined as a function of the ratio of segment of fraction to the numerical sum of all segment fraction and rounded off to the closest number (FIGS. 3-6) or to the closest prime number (FIG. 7). Note that application of the latter technique in the embodiment of FIG. 7 resulted in equality of actual array fractional segments, even though nominal fractional segments were unequal, i.e., the 1/13 and 1/17 nominal segments, because of the limited number of fins and inter-fin spaces in the overall array. The sequence of fractional segments is in non-sequential order by size (FIGS. 3, 6 and 7) or in order of decreasing size (FIGS. 4 and 5). The sizes and sequence of spaces within each array segment are in a substantially cyclic pattern having a pattern wavelength which substantially corresponds to the circumferential length of the associated segment (FIGS. 3 and 4), or are in a sequential incrementally increasing pattern between design minimum and maximum limits (FIGS. 5-7).

In accordance with a second aspect of the present invention, which is preferably used in combination with the first aspect as described in detail in connection with FIGS. 7-9, but may also be employed separately per FIGS. 10 and 11, the length of the fins at alternate fin positions is varied as an exponential function of angle around the array.

In accordance with a third aspect of the invention described in connection with FIG. 10, fin locations are uniformly spaced or positioned around the rotor, and fins are selectively deleted at location spacings which increase step-wise around the array.

The first and third aspects of the invention, which provide inter-fin spaces at psuedo-random locations and/or of psuedo-random dimension, achieve frequency-modulated damping or suppression of vibration harmonics in the rotor. Selective foreshortening at alternating fin locations as a function of angle around the array obtains amplitude modulation of vibration harmonics, and also is employed to balance the rotor.

It will be appreciated that, although the invention has been disclosed in detail in connection with ventilated disc brake rotors, the principles of the invention find equal application in the manufacture of drum brake rotors--i.e. brake drums--wherein spacing between and/or axial length of cooling fins around the drum periphery may be varied to obtain reduced brake squeal in accordance with the invention.