Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Noninterengaged fiber-containing paper-free web or sheet...
Reexamination Certificate
2000-02-22
2002-07-02
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Noninterengaged fiber-containing paper-free web or sheet...
C428S323000, C428S328000, C428S333000, C428S344000, C428S372000, C106S036000
Reexamination Certificate
active
06413622
ABSTRACT:
The present invention relates to non-asbestos friction materials made by molding and curing compositions comprising a fibrous base, an inorganic filler, an organic filler and a binder. More specifically, it relates to non-asbestos friction materials which can prevent undesirable jerkiness in low-speed braking, and are thus highly suitable as friction materials for use in large vehicles such as buses and trucks.
BACKGROUND OF THE INVENTION
A sharp increase in braking performance or effectiveness at low speed, especially in large vehicles such as buses and trucks, results in jerky low-speed braking characterized by sudden stopping of the vehicle accompanied by large, lurching movements. This makes for an uncomfortable ride, and sometimes even causes passenger injuries due to falls inside the vehicle. Hence, there exists a need for a way to prevent such jerkiness in low-speed braking.
At the same time, regulatory in Japan has led to an increased demand for higher braking effectiveness. The following improvements in friction materials have been proposed as ways to achieve better effectiveness.
(1) Add a large amount of metal powder to the friction material.
(2) Include a large amount of glass fibers in the friction material (e.g., at least 10% by volume, based on the overall composition).
(3) Increase the average particle size of the abrasive used in the friction material and suitably adjust the content of the abrasive. A typical example is the use of zirconium silicate or magnesium oxide having an average particle size of at least 10 &mgr;m.
However, each of these prior-art methods for increasing braking effectiveness has a number of associated problems. For instance, the first approach, according to which a large amount of metal powder is added to the friction material, causes “metal catch,” resulting in such undesirable effects as scoring of the brake drum and uneven braking action which causes the vehicle to pull to one side during braking.
The second approach, which involves adding at least 10% by weight of glass fibers to the friction material, does improve braking effectiveness. However, this advantage is offset by an increase in the “speed spread,” defined herein as the absolute value of the difference between effectiveness at 50 km/h and effectiveness at 100 km/h, and also larger and undesirable changes over time in both the braking effectiveness and the speed spread.
In the third approach mentioned above, an abrasive such as zirconium silicate or magnesium oxide having an average particle size of at least 10 &mgr;m is included in the friction material. This solution is indeed effective for enhancing braking effectiveness, yet it too has a number of shortcomings. Undesirable results include a larger speed spread, decreased effectiveness at high speed, and large and undesirable changes over time in both the speed spread and braking effectiveness. In addition, the abrasive scores the brake drum and is a cause of jerkiness during low-speed braking.
Moreover, owing to the large content of the above added components, these friction materials (1) to (3) cause a higher than necessary degree of wear on the mating surface (e.g., drum or disc surface), shortening the brake life.
Hence, prior-art friction materials all have significant drawbacks. Not only do they fall short of the requirements for such materials, they are unable to prevent undesirable jerkiness during low-speed braking.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a non-asbestos friction material which has a high braking effectiveness in ordinary use, a small speed spread, and minimal change over time in both the braking effectiveness and speed spread, does not give rise to a morning effect, and can prevent jerky movement during low-speed braking. “Morning effect,” as used herein, refers to an increase in the braking effectiveness from the initial effectiveness during cold-temperature operation.
We have found that, unlike earlier attempts to enhance braking effectiveness, adding to the friction material composition a specific amount of an inorganic filler having a high hardness and small particle size,.and preferably adding also a smaller than customary amount of chopped glass strand has the surprising and unanticipated effect of providing an outstanding non-asbestos friction material which is endowed with a good braking effectiveness under normal use yet does not cause undesirable jerkiness during low-speed braking.
That is, we have discovered that the incorporation, in a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, an inorganic filler, an organic filler and a binder, of 0.1 to 10% by volume, based on the overall composition, of an inorganic filler having a 90% particle size of 0.1 to 8 &mgr;m (as opposed to the particle size of at least 10 &mgr;m typical of the prior art) and a Mohs hardness of 6 to 8, and preferably the further incorporation of the lower than conventional amount of 1 to 6% by volume of chopped glass strand, based on the overall composition, elicits synergistic effects between these constituents and other constituents of the friction material. By virtue of these effects, there can be obtained outstanding non-asbestos friction materials which have a high braking effectiveness in normal use (generally about 50 km/h), a small speed spread, and an undiminished braking effectiveness at high speeds, undergo minimal change over time in effectiveness and speed spread, do not give rise to a morning effect, can prevent jerkiness during low-speed braking, cause minimal drum surface roughness and drum wear depth following continuous use, and have outstanding durability and a longer service life.
The reasons for the excellent properties of the inventive friction material are not well understood. However, a likely explanation is that molding the friction material composition in a state where the inorganic filler having a high hardness and a specific 90% particle size is uniformly blended with preferably a small amount of chopped glass strand allows each ingredient to exhibit its full capabilities. This makes it possible to achieve a friction material which, unlike prior-art friction materials that contain only an abrasive having a large average particle size or have a high glass fiber content, has a high braking effectiveness in normal use, and also has a small speed spread, does not undergo a decline in braking effectiveness at high speeds, experiences minimal change in the effectiveness and speed spread over time, can prevent undesirable jerkiness during low-speed braking, causes minimal drum surface roughness and drum wear depth following continuous use, and has excellent durability and a longer service life.
Accordingly, the present invention provides a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, an inorganic filler, an organic filler and a binder, wherein the inorganic filler has a 90% particle size of 0.1 to 8 &mgr;m, a Mohs hardness of 6 to 8, and accounts for 0.1 to 10% by volume of the overall composition.
The invention additionally provides a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, an inorganic filler, an organic filler and a binder, which friction material has a difference ratio between the braking effectiveness at 5 km/h (TP1) and the braking effectiveness at 30 km/h (TP2), as determined by low-temperature low-speed braking tests in accordance with Japan Automobile Technology Association standard JASO C407-87 and expressed as (TP1−TP2)/TP1×100, of at most 40%.
DETAILED DESCRIPTION OF THE INVENTION
The non-asbestos friction material of the invention can be made by molding and curing a composition composed primarily of a fibrous base, an inorganic filler, an organic filler and a binder. However, to achieve the objects of the invention, it is critical that, of these components, the amount and type of inorganic filler be selected such that a specific proportion of an inorganic filler
Dixon Merrick
Nisshinbo Industries Inc.
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