Wet disc brake

Details Wet disc brake Wet disc brake

1999-09-02

2001-02-13

Oberleitner, Robert J. (Department: 3613)

Brakes

Wheel

Axially movable brake element or housing therefor

C188S26400E, C188S26400E, C188S25000B

Reexamination Certificate

active

06186285

ABSTRACT:

The present invention relates to improvements in disc brake assemblies and components, particularly discs and friction pad configurations.
Conventional vehicle brakes most commonly comprise a rotor disc with a pair of friction material pads respectively acting on opposed braking surfaces of the rotor disc in a dry environment. Such systems have a number of significant disadvantages in that both the disc and particularly the friction material pads wear during use allowing wear particles to be released into the atmosphere which is polluting and may well be a health risk. Moreover the performance of such brakes is not uniform and varies considerably over time, particularly, as a result of friction pad wear and external factors such as whether water or other liquids get onto the braking surfaces during use.
Forms of wet disc brake are also known where a plurality of friction material discs are provided within a sealed brake housing containing oil. Such wet disc brakes have usually utilised oils of 7-8 or higher centistokes mm
2
/sec at 100° C. viscosity. Such oils normally also had a low viscosity index of 100 which caused substantial viscosity alterations as the oil temperature changed. The discs are provided such that some carrying grooved friction material rotate and other plain discs are stationary and braking effect is achieved by axially forcing the discs together. The oil is provided primarily as a coolant and for this purpose oil circulates through the grooves in the rotating discs. Oil of 7-8 (or higher) centistokes was, however, considered necessary to promote hydrodynamic lubrication and resistance to pressure “squeeze out” in the interengaged friction zone to suppress stick slip, wear, and localised overheating of the brake friction discs and plates. The drawback of utilising hydrodynamic lubrication regime is that a lower friction coefficient results in the interengaged region. Thus, because these oils cause an adverse effect on friction levels, it is necessary to have the much greater braking surface area provided by the plurality of discs. Moreover the configuration of these systems are generally considerably different to those of the dry friction systems described in the preceding paragraph making it difficult to interchange the respective assemblies.
In an earlier filed International Patent Application No. PCT/AU92/00540, there is also disclosed a brake assembly for a vehicle of the type comprising a single rotor disc adapted to rotate with an axle end and a stationary housing enclosing the rotor disc with one or more seals acting to provide a substantially sealed but pressure equalised zone surrounding at least a brake effect region of said rotor disc. At least one pair of friction pads is provided adapted to be engaged against opposed braking faces of a peripheral zone of the rotor disc, and the housing is formed so that it can contain a liquid in at least part of the braking zone. In one part of its disclosure, the aforesaid International Application indicates the liquid may be maintained at a level above the seals so as to prevent ingress of contaminants into the interior of the housing and particularly in the area of the braking surfaces and friction pads. The aforesaid International Patent Application further discloses friction pads including backing plates with friction material secured thereto, the friction material having rotor disc engaging faces with one or more grooves formed therein of particular configuration. The aforementioned brake arrangement will, however, benefit from a minimum of liquid or lubrication and the grooves are provided for the purpose of oil skimming and draining from the braking surfaces in contrast to other oil immersed brake systems where grooves are provided to assist with oil circulation.
The objective of the present invention is to provide improved friction material products, particularly brake pads and other components, capable of use in brake assemblies of the type shown in the aforesaid International Patent Application.
A further preferred objective is to provide an improved brake arrangement capable of advantageously using the aforesaid friction material products and other brake assembly components. It is also a preferred objective that the brake arrangement according to the present invention be such that it is relatively easily interchangeable with the dry friction disc brake assemblies now commonly in use.
With regard to the friction material products, it is recognised in the aforesaid International Patent Application that the friction material needs to be provided with skimming and drainage grooves to enable liquid removal from the braking surfaces, however, the general positioning of these grooves has been found to be important in relation to braking performance. Moreover, it has been found that optimum performance is generally a balance between the number and width of grooves relative to the remaining friction surface areas or lands between the grooves. That is, there must be sufficient land areas remaining to achieve desired braking characteristics. Used throughout this specification “leading” and “trailing” or similar have reference to the brake pad and the direction of rotation of the rotor disc.
According to a first aspect of the present invention, there is provided a brake pad including a friction material having a braking surface adapted, in use, to contact a rotor disc of a brake assembly, said braking surface having a leading edge relative to rotation of said rotor disc, an opposed trailing edge, and inner and outer circumferentially extending edges joining said leading and trailing edges, said friction material further including a plurality of grooves formed in said friction material with said grooves opening onto said braking surface and being angled from a leading end of the grooves rearwardly relative to a radial line, at least one of said grooves having a said leading end opening onto the leading edge of said braking surface and exiting from said friction material at either said trailing edge or said outer circumferential edge, said at least one of said grooves having a widened mouth portion at the leading edge of said braking surface and a zone of decreasing width inwardly from said leading edge of the braking surface. Conveniently, the leading edge is substantially radially disposed in use. The trailing edge may also be radially disposed in use for the sake of convenience in manufacture. Preferably, at least two said grooves are provided with a leading end opening onto the leading edge of the friction material. Conveniently, each said groove having a leading end opening onto the leading edge has a substantially uniform width from said zone of decreasing width to its exit from the friction material. Conveniently, a plurality of said grooves are provided spaced over the length of the brake pad from the leading edge to the trailing edge with said grooves being equally spaced. In one preferred arrangement, the leading ends of the grooves having leading ends located rearwardly of the leading edge of the brake pad, are located radially outwardly of the inner peripheral edge of the friction material, each of these grooves having trailing ends opening either on the outer peripheral edge or the trailing edge of the brake pad friction material. In a second preferred arrangement, the leading ends of the grooves having leading ends located rearwardly of the leading edge of the friction material of the brake pad are located opening onto the inner peripheral edge of the friction material. Conveniently, each of the grooves are substantially parallel to one another and are curved when viewed perpendicular to the braking surface, the curvature being such as to present a convex side of the grooves towards the leading edge of the friction material.
The advantage that is achieved by arranging one or more grooves leading from a leading edge of the pad friction material and exiting from preferably the outer peripheral edge is that an improved flow of liquid away from the braking surface region is achieved and thereby improv

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