Shim for preventing brake squeal in a disk brake

Abstract: In order to provide a shim for preventing brake squeal in a disk brake, which can decrease high frequency noise (squealing phenomena) produced at braking, a rubber coat metal, in which a rubber layer (2) is disposed on one surface of a metal plate (1) is used and a shim structure is so formed that a pressure sensitive adhesive layer (3) is disposed directly on the other surface of the metal plate (1), where there is no rubber layer or through a primer layer thereon. The pressure sensitive adhesive layer (3) of this shim structure serves as an adhesive portion to be stuck to a backplate of a disk brake pad. Since the pressure sensitive adhesive layer is disposed on the metal plate, the pressure sensitive adhesive layer is never peeled off from the rubber coat metal and its damping action (brake squeal preventing action) is maintained.

Description: The present invention relates to a shim for preventing brake squeal in a disk brake for an automobile and, in particular, to an improved damping shim for reducing high frequency noise (squealing phenomena) produced during braking. Heretofore a shim for preventing brake squeal in a disk brake for an automobile has been fabricated by using a rubber coated metal (hereinbelow abbreviated to RCM), in which thin rubber layers 2 are formed on both surfaces of a thin metal plate 1 made of iron (cold pressed steel plate), stainless steel, copper, etc., as a starting material, as illustrated in Fig. 8; forming a shim structure A, in which a pressure sensitive adhesive layer 3 is disposed on the surface of one of the rubber layers in the starting material; and sticking the shim structure to a backplate 4 in a disk pad consisting of the backplate 4 and a frictional material 5 at the surface of the pressure sensitive adhesive layer 3. This is illustrated in Fig. 9 and results in a restraint type damping structure consisting of RCM, pressure sensitive adhesive layer 3, backplate 4, the combination producing shearing deformation between the backplate 4 and the RCM to obtain a vibration damping action (brake squeal preventing action) by this shearing deformation. In Fig. 9, reference numeral 6 represents a brake piston member. In the shim structure, an age resister and an agent for preventing deterioration due to ozone contained in the interior of the rubber layers 2 are segregated and migrate towards the surfaces of the rubber layers (called blooming phenomena) due to heat produced by friction and the thrusting force exerted by the piston member when the disk brake is in operation, which gives rise to a segregated substance layer at the interface between the rubber layer 2 and the pressure sensitive adhesive layer 3. Since the age resister and the agent for preventing deterioration due to ozone (wax component), which have migrated to the interface between the rubber layer and the pressure sensitive adhesive layer in this way, melt at 60 to 75°C, the pressure sensitive adhesive layer is peeled off from the RCM. In such a state, since the pressure sensitive adhesive layer is not restrained between the backplate and the RCM, no shearing deformation takes place and the damping action (brake squeal preventing action) decreases. WO 93/13329 relates to a vibration and noise damping insert for vehicle brakes, and a method of producing a web for the manufacture of such inserts. GB-A-2185547 relates to an operating member for a disc brake. The present invention has been made in order to remove the drawback of the prior art shim structure, in which the pressure sensitive adhesive layer is disposed on the RCM and the main object thereof is to provide a shim for preventing brake squeal in a disk brake, in which no migration of the segregated substance, which lowers strength of the interface between the RCM and the pressure sensitive adhesive layer, takes place and the pressure sensitive adhesive layer is not peeled off from the RCM so that the damping action(brake squeal preventing action) is maintained. Accordingly, the present invention provides a shim for preventing brake squeal in a disc brake, the shim comprising a metal plate having first and second surfaces, a rubber layer, and a pressure sensitive adhesive layer, characterised in that the rubber layer is disposed on only the first surface of the metal plate, the pressure sensitive adhesive layer is disposed on the second surface of the metal plate, an optional low friction substance layer is disposed on the surface of the rubber layer facing away from the first surface of the metal plate, wherein, in use, the pressure sensitive adhesive layer is adapted to adhere to a backplate of a disc brake pad and either the surface of the rubber layer facing away from the first surface of the metal plate or the optional low friction substance layer is adapted to contact a brake piston member. For disposing the pressure sensitive adhesive layer on the metal plate, there are a method, for coating the metal plate directly with the pressure sensitive adhesive layer and further another method for forming a first primer layer on a surface of the metal plate and thereafter it is coated with the pressure sensitive adhesive layer. In this way it is possible to improve adhesiveness between the metal plate serving as a base plate and the pressure sensitive adhesive layer. They may be suitably selected according to a required performance level. For the primer layer phenol resin can be cited at first, but resin or coupling agent of epoxy, acryl, etc. can be widely utilized. By the shim structure thus constructed, the pressure sensitive adhesive layer is never peeled off from the RCM owing to the fact that the pressure sensitive adhesive layer is disposed directly or through the first primer layer on the surface of the metal plate constituting the RCM and is stuck to the backplate, and therefore the damping action (brake squeal preventing action) can be maintained. Here a second primer layer may be disposed also between the metal plate and the rubber layer. Further the shim structure may be so constructed that a low friction substance layer is disposed on the surface of the rubber layer in the construction according to the present invention. Graphite, molybdenum bisulfide grease, etc. may be used for the low friction substance layer. By the shim structure thus constructed it is possible to obtain an effect that the low friction substance layer disposed on the surface of the rubber layer decreases friction between the rubber layer and the brake piston member to intercept vibration causing "brake squeal". For the pressure sensitive adhesive layer cited, for example, acryl resin or silicone resin can be cited. In a second aspect, the present invention also provides a method for fabricating a shim for preventing brake squeal in a disc brake, which method comprises providing a metal plate having first and second surfaces, disposing a rubber layer on only the first surface of the metal plate, coating the second surface of the metal plate with an adhesive solution, drying the adhesive solution and thereafter heating it for a predetermined period of time to form a pressure sensitive adhesive layer, and optionally disposing a low friction substance layer on the surface of the rubber layer facing away from the first surface of the metal plate, wherein, in use, the pressure sensitive adhesive layer is adapted to adhere to a backplate of a disc brake pad and either the surface of the rubber layer facing away from the first surface of the metal plate or the optional low friction substance layer is adapted to contact a brake piston member. In a third aspect, the present invention provides a method for fabricating a shim for preventing brake squeal in a disc brake, which method comprises providing a metal plate having first and second surfaces, disposing a rubber layer on only the first surface of the metal plate, disposing a primer layer on the second surface of the metal plate, coating the primer layer with an adhesive solution, drying the adhesive solution and thereafter heating it for a predetermined period of time to form a pressure sensitive adhesive layer, and optionally disposing a low friction substance layer on the surface of the rubber layer facing away from the first surface of the metal plate, wherein, in use, the pressure sensitive adhesive layer is adapted to adhere to a backplate of a disc brake pad and either the surface of the rubber layer facing away from the first surface of the metal plate or the optional low friction substance layer is adapted to contact a brake piston member. Figs. 1a and 1b are cross-sectional views of shims for preventing brake squeal in disk brakes, which are two embodiments of the present invention; Fig. 2 is a cross-sectional view showing a state, in which the shim indicated in Fig. 1b is mounted on a disk brake pad; Fig. 3 is a cross-sectional view of a shim for preventing brake squeal in a disk brake, which is another embodiment of the present invention; Fig. 4 is a cross-sectional view showing a state, in which the shim indicated in Fig. 3 is mounted on a disk brake pad; Fig. 5 is a graph indicating variations in the number of brake squeals in a brake squeal dynamo test of a shim structure; Fig. 6 is a graph showing a relation between length of the adhesion portion of the pressure sensitive adhesive layer and damping property obtained by a beam test; Fig. 7 is a diagram showing the main part of a beam test apparatus; Fig. 8 is a cross-sectional view of a prior art shim for preventing brake squeal in a disk brake; and Fig. 9 is a cross-sectional view showing a state, in which the shim indicated in Fig. 8 is mounted on a disk brake pad. Figs. 1a, 1b and 2 show two embodiments of the present invention, in which members identical or similar to those indicated in Figs. 8 and 9 are referred to by same reference numerals. In Figs. 1a and 1b, B represents a shim structure, in which reference numeral 1 is a metal plate and 2 is a rubber layer disposed on one surface thereof. On the other surface of the metal plate 1, where there is no rubber layer, a pressure sensitive adhesive layer 3 is disposed directly, as indicated in Fig. 1a, or through a first primer layer P, as indicated in Fig. 1b. For either one of the constituent materials restriction on the thickness is unnecessary. However, in particular in case where it is used as a brake shim, since there is a limitation on the clearance between the pad and the brake piston member, in general following thickness regions are preferable. For the metal plate 1 it is preferable to use a cold pressed steel plate or a stainless steel plate 0.4 to 0.6mm thick. A thickness comprised between 20 and 200µm is used for the rubber layer 2 and a thickness comprised between 50 and 200µm is used for the pressure sensitive adhesive layer 3 are preferable. Fig. 2 shows a state, in which the shim structure B indicated in Fig. 1b is stuck to the backplate 4 in the disk pad. According to the construction of the shim structure B, since the pressure sensitive adhesive layer 3 is stuck directly or through the first primer layer P to the metal plate 1, there is no elution of the substance segregated from the rubber layer, which decreases strength of the interface of the pressure sensitive adhesive layer, as produced by the prior art shim, and therefore the pressure sensitive adhesive layer is never peeled off from the RCM. Consequently the damping action (brake squeal preventing action) is maintained. Fig. 5 is a graph representing variations of the number n of brake squeals in function of the number N of braking cycles, when two shims to be tested, which are the prior art shim structure A indicated in Fig. 8 and the shim structure B according to the present invention indicated in Fig. 1, are subjected to a brake squeal dynamo test. This test was effected by using an experimental apparatus for evaluating "brake squeals" produced at stopping a really rotating tire by means of a disk brake. It is believed that the number of brake squeals is generally smaller than 3 for 1000 brakings. The test was effected by repeating 10 times a cycle of 270 brakings (2700 brakings in total). 3/2700 squeal was detected for the shim structure B according to the present invention, while 60/2700 squeal was detected for the prior art shim structure A. Fig. 6 is a graph showing a relation between length ℓ of the adhesion portion of the pressure sensitive adhesive layer and damping property obtained by a beam test, in which (60/240), (120/240) and (180/240) mean (length ℓ of the adhesion portion / overall length L) and Fig. 7 shows the construction of a beam test apparatus. B represents a shim structure using an RCM (240mm long and 15mm wide); 3 is a pressure sensitive adhesive layer; 7 is a beam 3.0mm thick and 15mm wide; and 8 is a non-adhesive portion. In case where it is supposed that brake squeals are caused by the bending vibration mode of the brake pad (backplate), when the beam 7 is replaced by a backplate, brake squeal preventing property is equal to attenuating capacity in the beam test. As indicated by the graph obtained by the beam test in Fig. 6, it was confirmed that the damping property increases with increasing length of the adhesion portion of the pressure sensitive adhesive layer. This beam test is obtained by reproducing peeling-off of the pressure sensitive adhesive layer in a model and by measuring its attenuating capacity and as a result it can be said that greater peeling-off is produced by smaller length of the adhesion portion. In other words this indicates that better brake squeal preventing results can be obtained by a shim according to the present invention, by means of which a good adhesion state is obtained, even if all the materials are same as those used in the prior art shim. Through Figs. 5 and 6 it can be seen that one rubber layer in the prior art shim is never efficient. Figs. 3 and 4 show another embodiment of the present invention and members identical or similar to those indicated in Figs. 1b and 2 are referred to by same reference numerals. In Figs. 3 and 4, B' is a shim structure; 1 is a metal plate; 2 is a rubber layer disposed through a second primer layer P' on one surface of the metal plate 1; 3 is a pressure sensitive adhesive layer stuck through a first primer layer P to another surface of the metal plate 1, where there is no rubber layer, a low friction substance layer 8 made of graphite, molybdenum bisulfide grease, etc.; 4 is a backplate; 5 is a frictional material; and 6 is a brake piston member. In the construction described above, since a low friction substance layer 8 is disposed on the surface of the rubber layer 2, friction between the rubber layer 2 and the piston member 6 is decreased and an effect can be obtained that vibration causing "brake squeals" is intercepted. For example, it was confirmed that, for a rubber layer, to which graphite is applied, the static friction coefficient against a clean and flat metal surface is decreased by 30 to 50% with respect to a-rubber layer, to which no graphite is applied and that the number of brake squeals is decreased by 5 to 10% in the brake squeal dynamo test. For fabricating a shim for preventing brake squeal, in which a pressure sensitive adhesive layer is superposed on a metal plate in an RCM, there are known heretofore a method, wherein an adhesive sheet formed previously in the form of a film is transposed on the metal plate and stuck thereto to superpose the adhesive, or a method, wherein an adhesive solution, in which adhesive solved in solvent and bridging agent are mixed, is applied thinly and uniformly to the metal plate and dried so that bridging takes place therein to superpose the pressure sensitive adhesive layer on the metal plate, etc. However, by the methods described above for superposing the pressure sensitive adhesive layer on the metal plate, superposition is terminated in a state, where bridging degree in the adhesive is saturated at a certain level. Consequently, if the bridging degree in the adhesive is low, since there is a tendency that the adhesive is easily softened at a high temperature and heat resistance decreases, if the pressure sensitive adhesive layer is superposed on the metal plate by a prior art method, the adhesive is softened by heat produced by friction of the disk brake, which gives rise a possibility that the function as a shim for preventing brake squeal is lost. As a general method for increasing the bridging degree in the pressure sensitive adhesive layer, there are known a method, wherein the content of functional groups is increased in the adhesive and further the amount of the bridging agent is increased to be mixed or a method, wherein the bridging mode is changed, etc. However, in case where the bridging degree in the pressure sensitive adhesive layer is increased by the methods described above, together therewith, influences on other properties of matter, e.g. initial adhesion (tack), damping property, etc., are great and performance required as a shim for preventing brake squeal becomes unsatisfactory. Therefore at fabricating a shim for preventing brake squeal according to the present invention, it is preferable to use a following method for superposing a pressure sensitive adhesive layer on a metal plate. That is, a method is preferably used, in which it is made possible to increase the bridging density in the adhesive and to improve the heat resistance of the adhesive acting as a shim for preventing brake squeal, by subjecting it to a suitable temperature processing for heating it in a predetermined period of time after having coated the metal plate in the RCM with an adhesive solution obtained by solving an adhesive in an solvent and dried it. TABLE 1 indicates various properties of matter, when the bridging degree in the adhesive is increased by prior art methods and the method according to the present invention, in which a corresponds to a prior art method, wherein an acrylic base polymer is used for the adhesive and the amount of functional groups and bridging agent is increased; b to a standard method different from one mentioned lastly, and c to the method according to the present invention, wherein the metal plate is subjected to a thermal hysteresis by heating it at a temperature of 120°C in 10 minutes after having coated it with the adhesive solution and dried it. a b c SOFTENING TEMPERATURE (°C) 189 100 192 TACK (g) 400 900 870 In TABLE 1 the softening temperature is one at which the adhesive is softened by heat, which is used as an index for heat resistance, and the tack is a value of load measured when a probe is pressed to the adhesive with a small pressure and thereafter pulled therefrom, which indicates the initial adhesiveness. According to TABLE 1 it can be seen that it is possible to superpose a pressure sensitive adhesive layer on a metal plate in an RCM having good properties of matter as a shim preventing brake squeal by heating the adhesive, with which the metal plate is coated, in a predetermined period of time to give it a suitable temperature hysteresis. The temperature for heating is suitably selected in a range between 100°C and 180°C according to the kind of the adhesive and the period of time for heating is preferably comprised between 5 and 30 minutes. The adhesive solution may be applied directly to the metal plate in the RCM or the method described above may be used after having coated it with a first primer layer. As described above in detail, according to the present invention, since the shim structure is formed by using an RCM, in which a rubber layer is disposed only on one surface of a metal plate and by disposing a pressure sensitive adhesive layer on the opposite surface of the metal plate, it is possible to obtain a shim for preventing brake squeal in a disk brake, in which a damping action (brake squeal preventing action) is maintained in a long term. In addition, when the shim is so constructed that a low friction substance layer is disposed on the surface of the rubber layer in the shim structure, which is brought into contact with a brake piston member, it is possible to obtain a shim for preventing brake squeal in a disk brake, which can intercept vibration causing "brake squeal" and decrease the number of brake squeals owing to the vibration intercepting action. Further, by the method according to the present invention, heat resistance of the pressure sensitive adhesive layer is improved and it is possible to prevent deterioration of the shim for preventing brake squeal due to heat produced by a disk brake and to avoid cost-up by effecting drying of the adhesive and subsequent heat processing thereof.

Claims: A shim for preventing brake squeal in a disc brake, the shim comprising a metal plate (1) having first and second surfaces, a rubber layer (2), and a pressure sensitive adhesive layer (3), characterised in that the rubber layer (2) is disposed on only the first surface of the metal plate (1), the pressure sensitive adhesive layer (3) is disposed on the second surface of the metal plate (1), an optional low friction substance layer (8) is disposed on the surface of the rubber layer (2) facing away from the first surface of the metal plate (1), wherein, in use, the pressure sensitive adhesive layer (3) is adapted to adhere to a backplate (4) of a disc brake pad and either the surface of the rubber layer (2) facing away from the first surface of the metal plate (1) or the optional low friction substance layer (8) is adapted to contact a brake piston member (6). A shim according to claim 1, further comprising a first primer layer disposed between the metal plate (1) and the pressure sensitive adhesive layer (3). A shim according to claim 2, further comprising a second primer layer disposed between the metal plate (1) and the rubber layer (2). A shim according to any one of claims 1 to 3, wherein the optional low friction substance layer (8) is made of graphite or molybdenum bisulfide grease. A shim according to any one of claims 1 to 4, wherein the metal plate (1) is 0.2 to 1.0 mm thick, the rubber layer (2) is 20 to 200 µm thick, and the pressure sensitive adhesive layer (3) is 50 to 200µm thick. A disc pad comprising a shim as defined in any one of claims 1 to 5 and a backplate (4), wherein the pressure sensitive adhesive layer (3) of the shim is adhered to the backplate (4). A method for fabricating a shim for preventing brake squeal in a disc brake, which method comprises providing a metal plate (1) having first and second surfaces, disposing a rubber layer (2) on only the first surface of the metal plate (1), coating the second surface of the metal plate (1) with an adhesive solution, drying the adhesive solution and thereafter heating it for a predetermined period of time to form a pressure sensitive adhesive layer (3), and optionally disposing a low friction substance layer (8) on the surface of the rubber layer (2) facing away from the first surface of the metal plate (1), wherein, in use, the pressure sensitive adhesive layer (3) is adapted to adhere to a backplate (4) of a disc brake pad and either the surface of the rubber layer (2) facing away from the first surface of the metal plate (1) or the optional low friction substance layer (8) is adapted to contact a brake piston member (6). A method for fabricating a shim for preventing brake squeal in a disc brake, which method comprises providing a metal plate (1) having first and second surfaces, disposing a rubber layer (2) on only the first surface of the metal plate (1), disposing a primer layer on the second surface of the metal plate (1), coating the primer layer with an adhesive solution, drying the adhesive solution and thereafter heating it for a predetermined period of time to form a pressure sensitive adhesive layer (3), and optionally disposing a low friction substance layer (8) on the surface of the rubber layer (2) facing away from the first surface of the metal plate (1), wherein, in use, the pressure sensitive adhesive layer (3) is adapted to adhere to a backplate (4) of a disc brake pad and either the surface of the rubber layer (2) facing away from the first surface of the metal plate (1) or the optional low friction substance layer (8) is adapted to contact a brake piston member (6). A method as claimed in claim 7 or claim 8, further comprising adhering the pressure sensitive adhesive layer (3) of the shim to a backplate (4) of a disc brake pad.

Inventors: NIWA TAKAHIRO, SAKURADA SEIJI, YANO KUNIHIKO, YOSHIHARA MASAKI, NIWA, TAKAHIRO, SAKURADA, SEIJI, YANO, KUNIHIKO, YOSHIHARA, MASAKI
Applicants: NICHIAS CORP, NICHIAS CORPORATION
Application year: 1996
Main classification: F16D65
IPC codes: F16D65