GB2232449A - A brake assembly in which brake squeal is alleviated - Google Patents

Abstract

A friction brake assembly comprises a rotor, such as a disc or drum to be braked by the engagement of friction material therewith, which has an array of effective loads 12, 13 or X, Y, Z which rotate with it and are peripherally spaced equally about the rotational axis. In the absence of the loads, the brake assembly has characteristics which, during a braking operation, causes the rotor to exhibit vibration from which brake squeal emanates, the vibration comprising a standing wave in the rotor which is stationary relative to the friction material and provides an even number of nodal lines of substantially zero vibrational amplitude. The number of effective loads applied to the rotor is equal to the number of said nodal lines divided by an integer and where said division results in an integer. The effective loads can be formed integrally with the rotor or by discrete masses attached thereto. With a ventilated disc rotor the effective loads X, Y, Z can be provided by the form and distribution of webs within the disc which partly define the ventilation ducts. <IMAGE>

Description

TITLE "A friction brake assembly and a method of alleviating brake squeal noise in such an assembly" TECHNICAL FIELD & BACKGROUND ART The present invention relates to a friction brake assembly and a method of alleviating brake squeal noise in such an assembly. During a braking operation a friction brake, such as a disc brake or a drum brake, often becomes dynamically unstable whereby the brake rotor, typically a disc or drum, exhibits vibration upon engagement of the friction material to an extent which causes it to emit audible sound known as brake "squeal". It is recognised that such brake squeal is undesirable and it is an object of the present invention to provide a friction brake assembly in which brake squeal is alleviated and also a method for alleviating brake squeal in a friction brake assembly.

STATEMENTS OF INVENTION & ADVANTAGES According to the present invention there is provided a friction brake assembly comprising a rotor rotatable about an axis and which rotor is to be braked by the engagement of friction material therewith, said rotor having an array of effective loads which rotate therewith and are peripherally spaced equally about the axis; the brake assembly having characteristics which, in the absence of said loads and during engagement of the friction material with the rotor in a braking operation, causes the rotor to exhibit a vibration mode from which brake squeal noise emanates, said vibration mode comprising a specific standing wave in the rotor which is stationary relative to the friction material and provides an even number of nodal lines of substantially zero vibrational amplitude that are peripherallyspaced equally about the axis, and wherein the number (m) of said loads applied to the rotor is equal to the number (n) of said nodal lines divided by an integer and said division results in an integer (that is It = an integer).

Further according to the present invention there is provided a method of alleviating brake squeal noise in a friction brake assembly having a rotor rotatable about an axis and which is braked by the engagement of friction material therewith which comprises determining characteristics of a vibration mode of the rotor during the development of brake squeal noise when the rotor is being braked by the friction material, said vibration mode comprising a standing wave which is stationary relative to the friction material and provides an even number of nodal lines of substantially zero vibrational amplitude which are peripherally spaced equally about the axis, and modifying the rotor to provide an array of effective loads which are peripherally spaced equally about the axis, the number (m) of said effective loads being equal to the number-(n) of said nodal lines divided by an integer and where said It division is an integer (that is ~ an integer).

It has been determined that when a friction brake squeals during the engagement of its friction material with a braking surface of its rotor (such as a disc or drum) the rotor exhibits a particular pattern of vibration (its "mode shape") which has well defined characteristics. The mode shape is typically indicated by the positions of its "nodes" or nodal lines, that is notional lines along which the vibration amplitude is zero. Typically the characteristics of a brake disc when squealing provide an even number of radially extending nodal lines across its braking surface (over the rubbing path of that surface with the friction material). Similarly a brake drum when squealing is found to have an even number of nodal lines parallel to its axis across its rubbing path.It has been determined that when a friction brake is squealing the aforementioned nodal lines that comprise a specific standing wave do not rotate with the rotor but remain fixed relative to the friction material (or other non-rotational - stationary part - of the brake assembly). This behaviour of the nodal lines is believed possible only because the rotor may be regarded as rotationally symmetrical about its axis so that the orientation of the mode shape, relative to the rotor, is arbitary.In developing the present invention it was determined that the aforementioned modes of vibration of the rotor can be fixed to a specific angular position on the rotor by modifying the rotor mass structure to produce appropriate rotational asymmetry whereby the aforementioned fixed modes of vibration rotate with the rotor thus destroying the required inter-action between the friction material and the rotor which would result in brake squeal. The aforementioned asymmetry has been achieved with the present invention by the provision of the array of effective loads to the rotor.

When a friction brake is squealing, the number of nodal lines on the brake rotor is not the same for all brake assemblies or characteristics of brake squeals which emanate from those assemblies. In practice the number of nodal lines has been found to be any even number above four, the higher number being associated with higher frequency squeal and thus the upper limit may be determined by the upper frequency limit of human hearing. By the present invention it has been determined that the structural asymmetry pattern required to fix the vibration modes relative to the rotor and alleviate brake squeal must be related to the number of nodal lines on the rotor when the brake is squealing.If, for a given brake assembly which is intended to be reproduced commercially for use, say, in a vehicle, it is determined that the rotor exhibits squeal upon engagement by the friction material in a braking operation there are an even number of nodal lines, then the squeal can be alleviated by modifying the rotor design or structure for commercial production whereby the rotor is provided with a peripherally spaced array of effective loads or masses which are equally spaced about the axis according to the formula: rL nL~ a positive integer where n is the number of nodal lines and m is the number of equally spaced masses.

By way of example, if a friction brake assembly is found to develop squeal with ten nodal lines being produced in the rotor, such squeal may be eliminated or substantially reduced by modifying the rotor to provide two or five or ten effective loads or masses which are fixed relative to the rotor and are peripherally spaced equally about its axis. The effectiveness of the reduction in noise or squeal has been found to improve as the masses of the effective spaced loads which are provided for thê rotor are increased. The masses provided by the effective loads on the rotor will be symmetrically distributed and preferably each effective load has substantially the same mass. It is appreciated that the physical dimensions of each effective load will often be restricted by the design characteristics of the brake assembly and for this reason it is likely that a higher rather than a lower number of effective loads will be provided on the rotor (within the perameters of the aforementioned formula) so that each effective load can be relatively small physically although the total mass of all of the effective loads is relatively large.

In addition to fixing the position of the vibration mode relative to the rotor, the effective loading of the rotor in accordance with the present invention is also believed to replace a single frequency of the mode with two frequencies closely related. The difference between these frequencies may be regarded as a measure of the degree of asymmetry in the rotor structure and is preferably as large as possible for maximum effectiveness against squeal.

The effective loading of the rotor in accordance with the present invention can be achieved by adding discrete masses around the rotor or by locally modifying the rotor structure so that the effective loads are integral with that structure (for example by varying the wall thickness or casting to form localised masses on the rotor).

Usually the effective loads will be located around the outer peripheral edge of the disc for a disc brake assembly or around the outer peripheral surface of the drum for a drum brake assembly so that the loads do not interfere with the braking surfaces of the respective rotors. With ventilated discs where radially extending ventilation ducts are provided between opposed side plates of the disc, such ducts are usually formed by webs of material which interconnect the side plates and in such case the effective loading of the disc in accordance with the present invention may be achieved by suitable design and disposition of the effective masses provided by the webs.

In the art of friction brake assemblies it is recognised that rotors may be categorised as being symmetrical or asymmetrical, the former being where the rotor is of constant and uniform axial section around the axis of the rotor, typically for a solid annular disc or cylindrical drum. With an asymmetric rotor the aforementioned axial section will vary around the axis of the rotor, for example as in the case of a ventilated disc where ventilation ducts extend radially therethrough or where a disc may be provided with an array of holes which extend therethrough parallel to its axis or where a drum may be provided with an array of random protrusions on its outer periphery.While asymmetric rotors have hitherto been proposed and as such may be regarded as having an array of effective loads distributed around their axis, the number and distribution of such effective loads has not been determined in accordance with the requirements of the present invention and as such the prior proposals have been ineffective in the alleviation of brake squeal noise.

DRAWINGS Embodiments of friction brake assemblies constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying illustrative drawings in which: Figure 1 diagrammatically illustrates a typical disc brake assembly in which the disc is shown (much exaggerated) exhibiting a vibration mode from which brake squeal noise emanates; Figures 2 and 2A diagrammatically illustrate a typical drum brake assembly and drum respectively in each of which the drum is shown (greatly exaggerated) exhibiting a vibrational mode from which brake squeal noise emanates; Figure 3 shows a disc similar to that of Figure 1 and modified in accordance with the invention by the application of effective loads thereto to alleviate brake squeal;; Figure 4 shows the drum of Figure 2 modified in accordance with the present invention to provide effective loads in the form of discrete masses which are attached thereto to alleviate brake squeal; Figure 5 shows an alternative modification of the brake drum of Figure 2 in accordance with the present invention whereby effective loads are formed integral with the drum to alleviate brake squeal; Figure 6 is a plan view of a ventilated disc which would have a similar mode of vibration to that shown in Figure 1 but which is modified to provide effective loads in accordance with the present invention to alleviate brake squeal by appropriate distribution and design of webs forming radially extending ventilation ducts in the disc, and Figure 7 is a side elevation of part of the disc shown in Figure 6 to illustrate the ventilation ducts.

DETAILED DESCRIPTION OF DRAWINGS The brake assembly of Figure 1 has a conventional annular disc 1 with opposed braking surfaces 2 and 3. The disc 1 is secured for co-axial rotation about an axis 4 with a shaft 5 which is to be braked. Braking is effected in conventional manner by opposed pads 6 of frictional material which are brought into engagement with the braking surfaces 2 and 3. For convenience of description, the pads 6 have been shown diagrammatically and the calipers and associated components for mounting the pads have been omitted, it being realised that these are well known in the art. During a braking operation when the pads 6 are brought into engagement with the braking surfaces, the disc 1 may develop a pattern of vibration resulting in brake squeal.With the disc rotating relative to the brake pad it is found that the aforementioned pattern of vibration has a mode shape which comprises a standing wave that is stationary relative to the friction pads 6 and which wave provides an even number of notional nodal lines 7 of zero vibrational amplitude that extend radially from the axis 4 and over the braking surfaces 2 and 3 which is rubbed by the pads of friction material 6. The wave form in the vibration mode of the disc during brake squeal is shown greatly exaggerated in Figure 1 to illustrate the nodal lines 7 and in this example twenty nodal lines 7 are developed during brake squeal.In accordance with the present invention therefore the brake squeal may be alleviated by modifying the disc 1 to provide an array of effective loads which rotate with-the disc and are peripherally spaced equally about the axis 4 and where the number of said loads which is applied to the rotor is equal to the number of nodal lines divided by an integer provided that an integer results from said division. In the example aforementioned there are twenty nodal lines so that the number of effective loads which may be applied to the disc can be selected from the numbers two, four, five, ten and twenty.

The brake assembly of Figure 2 includes a conventional cylindrical brake drum 8 which is coaxially secured to a shaft which is to be braked from rotation about its axis 4A. The drum 8 has an interior cylindrical braking surface 9 and braking is effected by the application of a layer of friction material indicated at 10 against the surface 9. The friction material 10 will usually be carried by a brake shoe and controlled for its engagement and dis-engagement with the surface 9 hydraulically or mechanically - the means for mounting and displacing the friction material in the brake assembly of Figure 2 are well known in the art and, for convenience, have not been illustrated. During a braking operation when the friction material 10 engages the surface 9 of the rotating drum, the drum can exhibit a pattern of vibration, the mode of which can result in brake squeal. For this latter occurence the vibration mode comprises a standing wave in the rotor which is stationary relative to the friction material 10; the extent of the vibration mode which may occur for the drum in Figure 2 is shown greatly exaggerated by the deformation indicated at 8A and 8B at the mouth of the drum. The standing wave pattern in Figure 2 provides an array of notional nodal lines or nodes 11 along which the vibration amplitude is zero. These nodal lines 11 are peripherally spaced equally about the axis 4A of the drum and extend parallel to that axis across the rubbing path between the layer of friction material 10 and the braking surface 9.

In the example of Figure 2 there are six nodal lines.

Consequently, in accordance with the present invention the squeal which emanates from the brake assembly of Figure 2 can be alleviated by modifying the drum with the application thereto of an array of effective loads which rotate with the drum and are peripherally spaced equally about the axis 4A provided that the number of said effective loads is equal to the number of the aforementioned nodal lines 11 divided by an integer (and where said division results in an integer). In the example of six nodal lines on the drum the number of effective loads which are applied to the periphery of the drum can therefore be selected from the numbers two, three and six.

Figure 2A shows a drum suitable for use with the assembly of Figure 2 and better illustrates the standing wave of vibration mode (which drum mode was produced by computerised finite element modelling).

The disc 2 shown in Figure 3 has been modified in accordance with the present invention and it will be seen that six effective loads are located on the outer circumferential edge of the disc to be equally spaced around that peripheral edge. The loads 12 may be regarded as discrete masses in the form of projections or bosses which are cast and machined integral with the disc. The six effective loads provided for the disc 2 in Figure 3 are appropriate for a disc brake assembly in which the disc (without the loads 12) would, during squeal in a braking operation, have a number of nodal lines corresponding to six times an integer. It will be appreciated that the disc loaded as shown in Figure 3 would not be suitable for a disc having a vibration mode with twenty nodal lines as shown in Figure 1.In practice clearance must be provided between the loads 12 and the components of the brake assembly (such as the calipers for the brake pads 6) relative to which the disc rotates. However, it is desirable for an effective and efficient reduction in brake squeal that the aggregate mass of the effective loads 12 is as large as practically convenient - since in practice the physical dimensions of each load 12 is restricted by the space available for clearance it is desirable that the number of loads should be as large as possible and, ideally, the disc plate 2 in Figure 3 will have six nodal lines.

The brake drum 8 of Figure 2 is shown modified in Figure 4 by having attached thereto three discrete equal masses 13 which are circumferentially spaced equally about the drum. The masses 13 provide effective loads which are bolted or otherwise fixedly secured to the outer peripheral surface of the drum adjacent to its mouth. The drum in Figure 2 had six nodal lines in its vibration mode during squealing and accordingly the modification of the drum by the application of the three effective loads 13 satisfies the requirements of the present invention to alleviate squeal.

The drum 8 shown in Figure 2 can also be modified as shown in Figure 5 where two equal effective loads 14 are equally spaced on the outer circumferential surface of the drum to alleviate squeal. The loads 14 comprise ribs which are cast integral with the drum and appropriately machined to extend parallel with the drum axis. Again the requirements of the present invention are satisfied where the unmodified drum shown in Figure 2 has six nodal lines and the modified drum shown in Figure 5 has two effective loads. It will be appreciated that the effective loads which are applied to a particular drum can be achieved by a combination of the techniques mentioned with reference to Figures 4 and 5.

In the assembly of Figures 6 and 7 there is an annular disc 1A of the ventilated type having opposed braking surfaces 2A-.and 3A against which the pads 6 of friction material are applied. The disc comprises opposed plate parts 15 and 16 which are interconnected by a spaced array of webs 17 (conveniently shown by broken lines in Figure 6) which are integral with the plate parts. The webs and plate parts define ducts 18 which extend generally radially through the plate 1A to permit air flow and cooling of the disc in use. It will be seen from Figure 6 that the webs 17 are disposed in three identical patterns in part annular regions X, Y and Z around the disc. The regions X, Y and Z of the webs are peripherally spaced equally on the disc and the webs in each region are similarly orientated with respect to the axis 4 of the disc and with regard to the corresponding webs in the other two regions.

The webs are therefore distributed and disposed to provide an array of three effective loads in the asymmetric structure of the disc. The three loads provided by the symmetrical array of web regions X, Y and Z of the ventilated disc will therefore, in accordance with the present invention, alleviate squeal in a disc which prior to modification, will have at least six nodal lines provided that the total number of such lines when divided by three provides an integer.

Claims (17)

1. A friction brake assembly comprising a rotor rotatable about an axis and which rotor is to be braked by the engagement of friction material therewith, said rotor having an array of effective loads which rotate therewith and are peripherally spaced equally about the axis; the brake assembly having characteristics which, in the absence of said loads and during engagement of the friction material with the rotor in a braking operation, causes the rotor to exhibit a vibration mode from which brake squeal noise emanates, said vibration mode comprising a specific standing wave in the rotor which is stationary relative to the friction material and provides an even number of nodal lines of substantially zero vibrational amplitude that are peripherally spaced equally about the axis, and wherein the number (m) of said loads applied to the rotor is equal to the number (n) of said nodal lines divided by an integer and said division results in an integer (that is t = an integer).

2. An assembly as claimed in claim 1 in which the effective loads are formed integral with the rotor.

3. An assembly as claimed in either claim 1 or claim 2 in which the effective loads are discrete masses secured for rotation with the rotor.

4. An assembly as claimed in any one of the preceding claims in which each effective load itself comprises a spaced array of loads in the rotor.

5. An assembly as claimed in any one of the preceding claims in which the rotor is a disc and the effective loads are located at the outer peripheral edge of said disc.

6. An assembly as claimed in any one of the preceding claims in which the rotor is a disc of the ventilated kind in which opposed plate parts of the disc are interconnected by webs and the effective loads comprise said webs.

7. An assembly as claimed in any one of claims 1 to 4 in which the rotor is a cylindrical drum and the effective loads are located on the outer peripheral surface of said drum.

8. An assembly as claimed in claim 7 in which the drum has an axial end thereof secured for rotation with a shaft which is to be braked and a mouth end axially remote from said shaft and the effective loads are located adjacent to said mouth end.

9. A method of alleviating brake squeal noise in a friction brake assembly having a rotor rotatable about an axis and which is braked by the engagement of friction material therewith which comprises determining characteristics of a vibration mode of the rotor during the development of brake squeal noise when the rotor is being braked by the friction material, said vibration mode comprising a standing wave which is stationary relative to the friction material and provides an even number of nodal lines of substantially zero vibrational amplitude which are peripherally spaced equally about the axis, and modifying the rotor to provide an array of effective loads which are peripherally spaced equally about the axis, the number (m) of said effective loads being equal to the number (n) of said nodal lines divided by an integer and where said division is an integer (that is ~ = an integer).

10. A method as claimed in claim 9 which comprises modifying the rotor by securing thereto discrete masses to provide said array of effective loads.

11. A method as claimed in either claim 9 or claim 10 which comprises modifying the rotor by integrally forming therewith said array of effective loads.

12. A method as claimed in any one of claims 9 to 11 in which the rotor is a disc and said method comprises locating the array of effective loads at the outer peripheral edge of the disc.

13. A method as claimed in any one of claims 9 to 12 in which the rotor is a disc and which comprises forming the array of effective loads by the location and distribution of webs in the disc which webs partly define ventilation ducts in the disc.

14. A method as claimed in any one of claims 9 to 11 in which the rotor is a substantially cylindrical drum and which comprises locating the effective loads on the external circumferential surface of the drum.

15. A friction brake assembly as claimed in claim 1 and substantially as herein described with reference to the accompanying illustrative drawings.

16. A method of alleviating brake squeal noise in a friction brake assembly as claimed in claim 9 and substantially as herein described.

17. A friction brake assembly in which brake squeal noise is alleviated by the method as claimed in any one of claims 9 to 14 and 16.