MEANS FOR CARRYING A FRICTION PAD

Abstract: The invention concerns a means (12) for carrying a friction pad (13), which is a part of a system for retarding rotational movement, e.g. at a rotating shaft. The carrier means (12) is intended for distributing the press force from a brake mechanism via a contact surface to the friction pad (13) mounted upon said means (12). In its active position, the pad cooperates with the surface (10, 18) connected to the object of rotation wherein the carrier means (12) with the friction pad (13) is oriented extending along a sector of the rotation diameter of the contact surface. The carrier means (12) is so designed that the value of the local natural angle frequency varies between one end of said sector of the rotation diameter of the contact surface and the other end of said sector.

Description: Means for carrying a friction pad FIELD OF THE INVENTION The present invention refers to means for carrying a friction pad, which is a part of a system for retarding rotational movement, e.g. at a rotating shaft, said carrier means being intended for distributing the press force from a brake mechanism via a contact surface to the friction pad mounted upon said means, which in its active position cooperates with a surface connected to the object of rotation wherein the carrier means with the friction pad is oriented extending along a sector of the rotation diameter of the contact surface, BACKGROUND OF THE INVENTION Sometimes, a so called brake squeal occurs when a vehicle is retarded, which phenomenon occurs both at disc and drum brakes. Typical frequencies for this noise lies in the area of some kHz, and lies within the sensitivity range of the human hearing. It is true that the noise which is generated decreases with the distance from the source of the noise, but said noise may amount to about 140 dB close to the brake. Especially disturbing are town busses stopping at short intervals within densely populated regions. Brake squeal is a common cause of complaint regarding new cars , trucks and busses. Even if brake squeal does not affect the efficiency of the brakes and the security, the problem is regardlessly very large and has to be solved. The problem also occurs on vehicles having so called ABS-brakes. Many analyses of the problem of brake squeal has been made, and many solutions have been tested. For example, tests have been made where the components of a brake system has been enlarged in order to get a different natural oscillation. This has led to increasing costs, without getting any guarantee for not getting the brake squeal phenomenon under certain circumstances. Further, GB 2143916 shows a disc brake pad with a back plate for a friction pad which varies in thickness along a line in the direction of rotation. This means that the thickness of the pads varies in the direction of rotation, which leads to negative consequences. For example, the life expectancy is reduced because the entire pad volume may not be used. Besides, the wear will be larger on one side, which will be the cause of local increases in temperature with thermal stress, which in the worst case will cause formation of cracks in the drum or the disc, perhaps leading to breakdown. Further, an unfavourable distribution of friction between different brakes may lead to the brakes pulling sideways, i.e. a brake fault which affects the steering. Also, there is a risk for shearing off the brake pad, which may lead to a total loss of breaking power or to locking of some wheel. It is obvious that no measure which is intended to reduce the problem of brake squeal, may lead to a reduction of break efficiency. THE TECHNICAL PROBLEM One object of the present invention is therefore to provide a carrier means for a friction pad, which in a simple and efficient way eliminates the problem of break squeal, without decreasing the efficiency of the brakes. THE SOLUTION For this object, the invention is characterized in that the carrier means is so designed that the value of the local natural angle frequency varies between one end of said sector of the rotation diameter of the contact surface and the other end of said sector. This can be achieved in that either the back plate or the spar web, or both these elements are designed for varied natural angle frequency. Advantageous variants for providing such variations are illustrated by the following subclaims. The background to the expression local natural angle frequency Instable oscillations (self-oscillation) are by nature wave motion. Wave velocity for bending waves are proportional to the square root of the quotient between the bending stiffness, EI(φ) and the distribution of mass, m(φ), according to: v EI ( φ ) ffl(φ) The natural angle frequency is proportional to the square root of the quotient between the bending stiffness, El, and the mass, M according to: Let us therefore define "local natural angle frequency", so that v ~ ^ ( φ ) . The distribution of mass, m(φ), has the unit kg/m and is the product of cross sectional area, A, and density, ζ, according to m(φ) = A(φ) • ζ(φ) i . e . m( φ ) ~ A( φ ) . The history of a brake pad (that is to say the wear that the brake pad has been subjected to during its life time) has a large significance upon the distribution of pressure which the brake pad exerts upon the drum. Also brake pressure and generation of heat during breaking has a great influence upon the distribution of pressure. The different distributions of pressure makes different parts of the brake shoe being active at different times. If the pressure is large at the sides of the shoe, the carrier plate for the pad (back plate) is more active than the spar web, while if the pressure is substantially located to the middle of the brake shoe, the spar web is more active than the back plate. Accordingly, the natural angle frequency is varied in the carrier means by special design of either the back plate or the spar web, or by designing these two elements for varied natural angle frequency. Analysis of drum brakes has also shown that unstable oscillations in that area of frequency where brake squeal occurs must contain wave movement. If this wave movement is prevented, instability is reduced. It is known that a part of a wave is reflected when transiting into local natural angle frequency. When a wave is reflected in this way, a standing wave is created together with a reduced moving wave before the point of reflection and a reduced moving wave after the point of reflection. Thus, the reflection results in a decreasing moving wave. Because of this, instability is also reduced. Transitions in local natural angle frequency may for example be provided by step-by-step alterations. One step is here defined by that the local natural angle frequency for the section cuts the local natural angle frequency mean value for the entire section of the carrier means, in such a way, that the received step lengths, 1, meet the condition l>3t where t is themean thickness value of the spar web. ω 2 _.2>3t l 2 >3t l <3t If the local natural angle frequency of the carrier means is not altered in the form of a step, the maximum-/minimum values shall be used within each step length according to the definition. Both during continual and step-by-step alteration between two adjacent steps is applicable: or roi+l where ω ± and ^ are two adjacent local natural angle frequencies or two local natural angle end frequencies. The brake shoe is divided into the segments A = back plate, B = spar web and AB = the total section. For these segments shall be applicable: q A < 0,7q B < 0,7q M < 0,8 DESCRIPTION OF THE DRAWINGS The invention will be described hereinafter with reference to embodiments that are shown in the enclosed drawings, wherein Fig. 1 shows parts of a drum brake with a shoe according to a first embodiment of the invention, in view from the side, Fig. 2 is a section along the line II-II in Fig. 1, Fig. 3 is a section along the line III-III in Fig. 1, Fig. 4 shows in a corresponding way as Fig. 1 a drum brake shoe according to a second embodiment of the invention, Fig. 5 is a section along the line V-V in Fig. 4, Fig. 6 is a section along the line VI-VI in Fig. 4, Fig. 7 shows in a corresponding way as Fig. 1 a drum brake shoe according to a third embodiment of the invention, Fig. 8 is a section along the line VIII-VIII in Fig. 7, Fig. 9 is a section along the line IX-IX in Fig. 7, Fig. 10 shows in a corresponding way as Fig. 1 a drum brake shoe according to a fourth embodiment of the invention, Fig. 11 is a section along the line XI-XI in Fig. 10, Fig. 12 is a section along the line XII-XII in Fig. 10, Fig. 13 shows in a corresponding way as Fig. 1 a drum brake shoe according to a fifth embodiment of the invention, Fig. 14 is a section along the line XIV-XIV in Fig. 13, Fig. 15 is a section along the line XV-XV in Fig. 13, Fig. 16 shows in a corresponding way as Fig. 1 a drum brake shoe according to a sixth embodiment of the invention, Fig. 17 shows a disc brake provided with back plates in accordance with the invention, Fig. 18 shows the disc brake in a plane view, Fig. 19 shows a drum brake shoe according to a seventh embodiment of the invention, and Fig. 20 shows a drum brake shoe according to an eighth embodiment of the invention. DESCRIPTION OF AN EMBODIMENT In Fig. 1 a part of a drum brake is shown with the drum 10, which rotates in the direction of the arrow 11 and with a brake shoe 12 which forms a carrier for a pad 13. The shoe 12 mediates the force from a not shown hydraulic or mechanic brake mechanism to the pad 13 and is provided with a spar web 14 which is directed substantially perpendicular out from the reverse side of the surface which carries the friction pad. The shoe 12 is designed in a conventional way with the friction pad 13 oriented so that it extends along a sector of the rotation diameter of the contact surface. Experiments have shown that it is possible to eliminate brake squeal at such a drum brake by designing the shoe 12 so that the value of the local natural angle frequency which is proportional to the square root of the quotient between the bending stiffness and the mass, increases from a first value at one end of said sector of the rotation diameter of the surface to a second value at the other end of the sector. As is illustrated by Fig. 2 and 3, the height of the spar web 14 varies from a large dimension adjacent the normal front edge of the friction pad 13, to a small dimension ad acent the normal back edge of the friction pad, according to the rotation direction defined by the arrow 11. By this design of the spar web 14 both the bending stiffness as well as the distribution of mass is affected along the above described sector, therefore providing altered natural angle frequency along the sector. Fig. 4 to 6 shows a second embodiment of the invention, where the spar web height varies in the same way as in the preceding embodiment, but with a constant or slightly varied distribution of mass caused by a slot 15 which has been milled into each side of the spar web 14. In this way, a greater difference in natural angle frequency is achieved than in comparison with the embodiment shown in Fig. 1 to 3. Fig. 7 to 9 shows a third variant of the invention where the spar web height and the same time the bending stiffness is constant. On the other hand, mass distribution is varied in that the spar web is provided with a waist 16 which increases gradually i volume along the above mentioned sector. Fig. 10 to 12 shows a fourth embodiment of the invention where distribution of mass and spare web height is constant, but bending stiffness is varied along the sector length, by gradually increasing the width of the spar web 14 along the edge which abuts the reverse side of the surface which carries the friction pad and at the same time decreasing it along the opposite edge. Fig. 13 to 15 shows a fifth variant of the invention where the spar web height is constant, but both bending stiffness and mass distribution is varied along the sector length. This is achieved by providing the spar web 14 with one or more angle flanges 17 extending along said sector and with gradually increasing thickness along the sector at the same time as the distance from the reverse side of the surface which carries the friction pad, decreases gradually in the same direction. This means that mass increases in one direction, while the bending stiffness increases in the other direction. Fig. 16 shows still another embodiment, where the spar web 14 is provided with apertures 18 with gradually increasing size along the sector length. This means that mass increases in one direction, without appreciably affecting the bending stiffness. The apertures 18 may be designed oval, as shown, or round. The oval apertures may vary in size, either only with regard to length, or both with regard to length and width. The disc brake shown in Fig. 17 comprises a disc 19 which normally rotates in the rotation direction defined by the arrow 20. The back plates 12 carrying the pads 13 are wedge- shaped in such a way, that the bending stiffness varies continuously along a sector of the rotation diameter of the surface which carries the pad, without affecting the actual volume of the pad. The plates 12 are provided with flanges 21 which are inversely wedge-shaped towards the back plates 12, so that the back sides of the plates are parallel with the pad side. Fig. 19 and 20 shows two further embodiments of drum brake shoes, of which Fig. 19 shows a drum brake shoe the back plate 12 of which is designed as a tubular beam having a continuously narrowing section. A number of local masses 22 are attached to this back plate. The design according to this embodiment provides both altered bending stiffness and altered mass along the brake shoe. Fig. 20 illustrates a drum brake shoe with a back plate that has been made in two steps 12a, 12b, which provides a step- by-step altering of the bending stiffness along the brake shoe. This step-by-step alteration also causes a change in the natural angle frequency, which reduces the occurrence of brake squeal. Naturally, the invention is not limited to the above described embodiment, instead more variants are conceivable within the scoop of the following claims. Naturally, other means may be used for varying bending stiffness and mass distribution. For example, the back plate of the carrier means may be designed so that the natural angle frequency is altered by varying bending stiffness along the bow length of the back plate. Alternatively, the local natural angle frequency of the back plate may be altered by varying the mass along the length of arc of the back plate. Naturally, both bending stiffness and mass may vary along the length of arc of the back plate. In all cases, the above described variations in bending stiffness and mass may vary gradually or step-by-step.

Claims: CLAIMS 1. Means (12) for carrying a friction pad ( 13), which is a part of a system for retarding rotational movement, e.g. at a rotating shaft, said carrier means (12) being intended for distributing the press force from a brake mechanism via a contact surface to the friction pad (13) mounted upon said means ( 12) , which in its active position cooperates with a surface (10, 18) connected to the object of rotation wherein the carrier means ( 12) with the friction pad ( 13) is oriented extending along a sector of the rotation diameter of the contact surface, c h a r a c t e r i z e d in that the carrier means is so designed that the value of the local natural angle frequency varies between one end of said sector of the rotation diameter of the contact surface and the other end of said sector. 2. Carrier means according to claim 1, c h a r a c t e r i z e d in that it comprises at least one spar web ( 14) extending along said sector and being directed substantially perpendicular out from the reverse side of the surface which carries the friction pad (13) . 3. Carrier means according to claim 1 or 2, c h a r a c t e r i z e d in that the value of the local natural angle frequency varies by variation of the bending stiffness along said sector. 4. Carrier means according to claim 3, c h a r a c t e r i z e d in that the bending stiffness is varied by varying the height of the spar web (14) gradually or by steps along said sector, and that the mass is substantially constant along said sector, by successive or by steps enlargement of the width of slots (15) or the diameter of apertures (18) being milled or drilled in connection with the neutral layer of the carrier means. 5. Carrier means according to claim 3, c h a r a c t e r i z e d in that the bending stiffness varies by varying the width of the carrier means (12) gradually or by steps along the edge which abuts the reverse side of the surface which carries the friction pad and simultaneously decreases along the opposite edge. 6. Carrier means according to claim 3, c h a r a c t e r i z e d in that the bending stiffness is varied by varying the width of the spar web (14) gradually or by steps along that edge which abuts the reverse side of the surface which carries the friction surface and simultaneously decreases along the opposite edge. 7. Carrier means according to claim 2, c h a r a c t e r i z e d in that the natural angle frequency varies by alteration of the mass along said sector. 8. Carrier means according to claim 1 or 6, c h a r a c t e r i z e d in that the mass varies in that the spar web (14) is provided with a waist (16) which varies by volume gradually or by steps along said sector. 9. Carrier means according to claim 7, c h a r a c t e r i z e d in that the stiffness is substantially constant along said sector, and that the mass varies along the sector, by successive or by steps enlargement of the width of slots ( 15 ) or the diameter of apertures (18) having been milled or drilled in connection with the neutral layer of the carrier means. 10. Carrier means according to claim 1 or 2, c h a r a c t e r i z e d in that the natural angle frequency varies by alteration of both bending stiffness and mass along said sector. 11. Carrier means according to claim 10, c h a r a c t e r i z e d in that the bending stiffness and mass is varied by successive or by steps variation of the spar web (14) height along said sector. 12. Carrier means according to claim 10, c h a r a c t e r i z e d in that the bending stiffness varies by varying the height of the spar web ( 14) gradually or by steps along said sector, and that mass varies in the opposite direction against the increase in stiffness along said sector, by successive or in steps enlargement of the width of slots (15) or the diameter of apertures (18) having been milled or drilled in connection with the neutral layer of the carrier means. 13. Carrier means according to claim 1 or 2, c h a r a c t e r i z e d in that the spar web (14) is provided with one or more cross flanges (17) extending along said sector and the thickness of which is varied gradually or by steps along the sector at the same time as the distance from the reverse side of the surface which carries the friction pad ( 13 ) varies gradually or by steps in the same direction.

Inventors: HULTEN JOHAN, HULTEN, JOHAN
Applicants: HULTEN JOHAN, VOLVO AB, AB VOLVO, HULTEN, JOHAN
Application year: 1996
Main classification: F16D65
IPC codes: F16D65