GB2030665A - Disc Brake Device for Vehicles - Google Patents

Abstract

Disc brake device for vehicles comprising a brake disc 2 adapted to be rotated with the wheel of the vehicle and a pair of friction pads 3 adapted to be forced into frictional engagement with the brake disc, the brake disc 2 being made of a corrosion resistant steel and having a surface hardness of 28 to 45 on the Rockwell "C" Scale and the friction pads being made of a copper-based sintered alloy containing at least 55% by weight of copper. The brake device provides a braking effort which does not significantly decrease even under wet conditions, a layer of an oxide of copper being produced by heat generated during braking to increase the friction coefficient. Pad also contains metallic materials such as Sn, Pb, Fe, Mn and Si specific compositions are given. <IMAGE>

Description

SPECIFICATION Disc Brake Device for Vehicles The present invention relates to brake device for vehicles and more particularly to disc brake devices.

Conventionally, disc brake devices include a brake disc adapted to be rotated with a wheel of a vehicle and friction pads carried by a stationary part of the vehicle and adapted to be forced into frictional engagement with the opposite sides of the brake disc. Such disc brake devices have been widely used, however, they have disadvantages in that braking efforts are significantly decreased in wet conditions. In fact, it has been recognized that the braking efforts are decreased in wet conditions less than one-half of those in dry conditions. In the case of motor-cycles, since the brake devices are totally exposed, the problem is particularly acute.

Where the friction pads are made of metallic materials, an effect takes place wherein films of water on the disc surfaces are broken up by the friction pads when they are engaged with the disc surfaces so that it becomes possible to prevent any significant decrease of braking effort under wet conditions. However, in general, metallic materials are considered as being unsuitable f6r braking pads because they have relatively low coefficient of friction. Furthermore, where the friction pads are of the same material as the brake disc, the pads may stick to the brake disc due to the heat which may be produced when the brake device is actuated.

It is therefore an object of the present invention to provide a disc brake device which has no significant decrease in the braking effort under wet conditions.

Another object of the present invention is to provide a disc brake device having friction pads made of a metallic material which can produce an adequate braking effort.

According to the present invention, the above and other objects can be accomplished by a disc brake device for a vehicle comprising a brake disc adapted to be rotated with a wheel of the vehicle, and friction pads disposed on opposite sides of the brake disc and having friction surfaces adapted to be brought into frictional engagement with the brake disc, the brake disc being made of a corrosionresistant steel and having opposite side surfaces for engagement with the friction pads, wherein the opposite sides surfaces of the brake disc have a hardness of between 28 and 45 on the Rockwell "C" scale and the friction pad are made of a copper-based sintered alloy comprising at least 55% by weight of copper. The copper content in the friction pad is effective to increase the friction coefficient since it produces copper oxide by the heat which is generated during braking.It has in fact been recognized that the friction coefficient of such copper oxide is twice as large as that of copper. Where the copper content is less than 55% by weight, an adequate frictional coefficient cannot be ensured. However, the friction pad must contain various metallic materials such as Sn, Pb, Fe, Mn and Si for the purpose of providing an adequate frictional coefficient in low temperatures, with lubrication, for preventing smoothing of the disc surfaces and for suppressing squeaking noises. Thus, the present invention proposes to form the friction pad from a sintered alloy containing one or more of such materials in addition to not less than 55% by weight of copper. It is also important to manufacture the brake disc with a corrosion-resistant steel because otherwise corrosion on the brake disc may cause sticking of the friction pads.With the hardness of the brake disc less than 28, the disc surfaces will be rapidly worn by the copper oxide in the friction pads, however, where the hardness is greater than 45, unacceptably loud squeaking noises will be produced during the application of the brake. Thus, according to the present invention, there is provided a disc brake which shows no significant decrease in braking effort under wet conditions and provides an adequate braking effort both in dry and wet conditions.

The above and other objects and features of the present invention will become more apparent from the following descriptions of a preferred embodiment and examples taken with reference to the accompanying drawings, in which: Figure 1 is a fragmentary sectional view of a disc brake device to which the present invention can be applied; Figure 2 is a diagram showing the brake distances in dry and wet conditions; Figure 3 is a diagram showing the relationship between the coefficient of friction and the copper content; Figure 4 is a diagram showing the relationship between the wear and the hardness of the disc surface; and Figure 5 is a diagram showing the effect of the brake disc surface on the squeaking noise.

Referring now to the drawings, particularly to Figure 1, the disc brake device shown therein includes a calliper body 1 which is adapted to be mounted on a stationary part (not shown) of a vehicle and a brake disc 2 which is adapted to be secured to a wheel (not shown) of the vehicle. The calliper body 1 has a pair of cylinder bores 1 a located at the opposite sides of the brake disc 2 and pistons 5 are respectively received in the cylinder bores 1 a for slidable movement. Each of the pistons 5 carries a support plate 4 at an end adjacent to the brake disc 2 and a friction pad 3 is provided on the plate 4 at a side facing the brake disc 2.A pressure chamber 6 is defined in the calliper body 1 behind each piston 5 so that when hydraulic pressure is applied to the chamber 6 the piston 5 is displaced toward the brake disc 2 to bring the friction pad 3 into contact with the brake disc 2.

Conventionally, the brake disc 2 has been made of a heat-treated corrosion-resistant steel and the friction pad 3 has been of a so-called resin-moulded type containing asbestos with organic or inorganic additives and cemented by a resinous binder such as phenolic resin. When this type of disc brake device is used in rain or on a wet road, water is deposited on the brake disc and forms a film thereon. This water film cannot be easily broken even when the friction pad is forced against the brake disc so that the braking effort is significantly decreased as previously described. In a motorcycle, since the brake device is exposed, the problem is acute. In the case of an automobile, the brake disc is not exposed so that the aforementioned problem may be relatively small.However, even in an automobile, there is a risk of a decrease in braking effort when the automobile passes through a water trap.

Referring to Figure 2, there are shown test results obtained from a motorcycle having a 650 cc engine. In Figure 2, the reference A shows the braking distance with the disc brake in dry conditions while B-1 and B-2 show the braking distances in wet conditions, all with the conventional disc brake device. In all cases, the braking distance has been measured as a distance required for stopping the motorcycle from an initiai speed of 100 km/h. Comparing the braking distances B-1 and B-2 which have been measured with different amounts of water deposited on the brake disc, it will be noted that there is a certain amount of difference in the braking distance in accordance with the amount of water deposited.

According to the present invention, the material and surface hardness of the brake disc and the material of the friction pad are selected so that any adverse effect of water can be substantiaily decreased or eliminated. More specifically, the brake disc is comprised of a heat-treated corrosion resistant steel having a surface hardness of 28 to 45 HRC (Rockwell Hardness C Scale). The friction pad co-operating with the brake disc is comprised of a copper-based sintered alloy containing not less than 55% by weight of copper.

Referring to Figure 2, there are shown by the references A' and B' the braking distances under dry and wet conditions for a 650 cc motorcycle having a disc brake in accordance with the present invention. It will be noted that there is little difference in braking distance between the dry and wet conctitions. It will further be noted that the braking distance as obtained with the disc brake in accordance with the present invention is comparable to that with the conventional disc brake in dry conditions.

According to the present invention, the friction pad comprises not !ess than 55% by weight of copper as described above. It has been found that the copper content in the friction pad produces a layer of an oxide of copper by the heat generated during operation of the brake device and the oxide layer functions to brake the water film and provide an increased frictional coefficient. However, the friction pad must also include several metallic components such as Sn, Pb, Fe, Mn and Si for the purpose of increasing the frictional coefficient under low temperatures, for facilitating lubrication and for suppressing the generation of squeaking noises, and for preventing smoothing of the disc surfaces.

Thus, the copper content in the friction pad is apt to be decreased. However, as shown in Figure 3, the frictional coefficient between the brake disc and the friction pad changes in accordance with the copper content in the pad. In Figure 3, the curve I represent the frictional coefficient in dry conditions whereas the curve II is for wet conditions with the friction pad materials as shown in the Table. It will be noted in Figure 3 that with a copper content of less than 55% by weight there is a noticeable decrease in the frictional coefficient so that the braking distance will be accordingly increased.

Table % in weight Cu 30 40 50 55 60 70 80 90 Sn 15.4 13.2 11 9.9 8.8 6.6 4.4 2.2 Pb 17.5 15 12.5 11.3 10 7.5 5 2.5 Si 2.1 1.8 1.5 1.4 1.2 0.9 0.6 0.3 Al 3.5 3 2.5 2.3 2 1.5 1 0.5 Fe 4.9 4.2 3.5 3.2 2.8 2.1 1.4 0.7 Mn < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 C 23.1 19.8 16.5 14.9 13.2 9.9 6.6 3.3 Others 3.49 2.99 2.49 1.99 1.99 1.49 0.99 0.49 Where this copper-based sintered alloy is used for the friction pad, a specific material must be used for the brake disc. According to the present invention, there is proposed to use a corrosionresistant steel. If corrosion is produced on the brake disc, the friction pad will possibly stick to the disc, thus preventing satisfactory operation of the brake device. Thus, the corrosion-resistant steel is the most preferred material for the brake disc. One example of such a corrosion-resistant steel is JIS (Japanese industrial Standard) G 4305 SUS 420J2. Other steel material such as carbon steel which has often been used for brake disc in conventional devices is not suitable because, even if it is subjected to a corrosion-resistant treatment, such a treated layer is relatively quickly worn-off and there will soon be a problem of corrosion.

The surface hardness of the brake disc is important from the point of view of wear and squeaking noise. If the hardness is less than 28, there will be rapid wearing of the brake disc, but if the hardness is greater than 45 a problem of squeaking noise will be produced. In Figure 4, there is shown a relationship between the surface hardness of the brake disc and the amount of wear. Each measurement was performed after 1000 brake applications. In each of the brake applications, a pad piece of 15 cm2 was applied to a disc rotating at a speed of 12 m/sec with a moment of inertia of 2.5 kg.m.sec2. The pad was applied under a force of 500 kg to the brake disc with its centre to a circle of 250 mm in radius. In Figure 4, it will be noted that with a surface hardness of less than 28 HRC there is a noticeable increase in wear.This is understood as being caused by the fact that, in accordance with the present invention, the friction pad has a hard layer of an oxide of copper so that the disc is rapidly worn unless it has an adequate hardness.

In a disc brake device having a friction pad of a copper-based sintered alloy, squeaking noises are apt to be produced during application of the brake. This tendency is also dependent on the hardness of the brake disc. Referring to Figure 5, there are shown results of tests performed on a motorcycle having a 650 cc. engine. It will be noted in Figure 5 that the squeaking noise becomes unacceptably loud when the surface hardness of the brake disc is greater than 45 HRC.

From the foregoing descriptions, it will be understood that, in accordance with the present invention, there is proposed a combination of a brake disc made of a corrosion-resistant steel and having a hardness of 28 to 45 on the Rockwell "C" Scale, with a friction pad made of a copper-based sintered alloy containing not less than 55% by weight of copper. This combination provides a disc brake in which the braking distance is not noticeably increased even under wet conditions. Further, the wear of the brake disc and the squeaking noise can be suppressed within practically acceptable limit.

Claims (2)

Claims

1. A disc brake device for a vehicle comprising a brake disc adapted to be rotated with a wheel of the vehicle and friction pads disposed on opposite sides of the brake disc and having friction surfaces adapted to be brought into frictional engagement with the brake disc, the brake disc being made of a corrosion-resistant steel and having opposite side surfaces for engagement with the friction pads, wherein the opposite side surfaces of the brake disc have a hardness of between 28 and 45 on the Rockwell "C" Scale, and the friction pad are made of a copper-based sintered alloy comprising at least 55% by weight of copper.

2. A disc brake device as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawings.