Brakes – Elements – Shoes
Reexamination Certificate
2001-05-03
2003-01-07
Butler, Douglas C. (Department: 3683)
Brakes
Elements
Shoes
C188S26400E, C188S25100R, C192S10700R
Reexamination Certificate
active
06502674
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention concerns a friction engagement device, in particular a brake or a clutch for motor vehicles. The friction engagement device contains at least one first component and at least one second component, that can move relative to each other, in that, as a rule, one of the components rotates and the other component remains at rest. The first component is provided with at least one contact surface and consists, at least in its region bordering on the contact surface, of a material with high heat conductivity, such as aluminum or an aluminum alloy. The second component contains a support component to which at least one friction lining is applied with adhesive, riveting or the like. In order to perform a friction engagement for braking or engaging a clutch, the friction surface of the friction lining can be pressed against the contact surface.
The rapid removal of heat from the region of friction contact during the deceleration (braking, clutch engagement) is critically important for the durability and load capacity of a brake or clutch of a motor vehicle, for example, a tractor or passenger vehicle, in order to keep the peak temperature below a specific critical temperature at which the brake or clutch could be damaged and lose its effectiveness or even fail completely.
Due to increasing operating speeds, particularly in the case of tractors, ever greater amounts of braking energy are applied to service and auxiliary brakes in the form of heat. In view of the increasing load density and the associated temperature problems, the durability of the components, particularly the brake lining, is increasingly endangered or its life reduced.
Commonly steel or cast iron are applied in brakes as opposing material to the friction linings of brake bands or opposing disks. This leads to poor heat flow from the friction contact zone and to great weight which is largely undesirable, in particular in future light-weight vehicle concepts.
Beyond that, multi-component brake drums are known, in which a friction ring as a separate component is fastened, fixed against rotation, to the radially inner side of a pot-shaped brake drum. In that way US-A-3,005,259 discloses a brake drum of aluminum with a friction ring of a cast iron material, where the friction ring is fastened to the brake drum, fixed against rotation, by means of radial undercuts. For reasons of weight reduction the brake drum is made of aluminum. But the friction ring consists of a cast iron material and cannot guarantee a rapid flow of heat from the friction contact surface.
EP-A-0 879 975 describes a brake drum component group, consisting of a pot-shaped brake drum of aluminum, that is fastened, fixed against rotation, to the hub of a vehicle axle, as well as a friction ring of metal matrix composite material (MMC) fastened, fixed against rotation, to the brake drum. Due to the MMC material these brake drum component groups enjoy a very high load capacity and thermal resistance. Since the brake drum consists of aluminum it also provides advantages in weight reduction. Due to the use of the MMC material, however, these brake drum component groups are very costly and complicated to manufacture.
Furthermore DE-A-42 43 516 describes a brake lining for brakes with aluminum brake disks. In order to avoid strength problems in the aluminum brake disks at temperatures above 450° C., the friction material is to be selected in such a way that its friction coefficient remains constant up to a temperature of 400° C., and declines at higher temperatures. However, brakes of this configuration are recommended only for relatively low braking forces.
U.S. Pat. No. 5,339,931 describes a friction material made of phenolic resin; an organic friction modifier such as rubber scrap, cellulose, latex, cork, or cashew particles; an aramid fiber; a carbonaceous material such as graphite, synthetic graphite, carbon and coke; glass fiber; inorganic friction modifiers such as barytes, whiting, talc, rottenstone, vermiculite or suzorite mica; abrasive particles such as silica, magnesia, zircon, zirconia, mullite, alumina or iron oxides; a lubricant, and 2-18 percent porous copper powder. This friction material is specially formulated for use with aluminum metal matrix composites and can operate at temperatures that approach 450° C.
The problem underlying the invention is seen as that of defining a friction engagement device of the type defined above, that is able to absorb high braking energy levels, exhibits high durability life together with very good heat conductivity out of the friction contact zone, can be manufactured cost effectively and is sufficiently rigid to absorb and transmit high forces and torques.
It has been found, in particular for friction engagement devices in which high energy levels are encountered, that the combination of aluminum or aluminum alloy contact surfaces on the one hand, with friction linings of an organic material on the other hand, can fulfill the requirements imposed. The power level absorbed by the friction engagement device can be increased considerably compared to previous devices. On the one hand, the increase in the peak temperature in the friction contact zone is reduced by approximately 30% by the use of aluminum or an aluminum alloy in place of the otherwise usual materials such as steel and cast iron. Thereby the friction engagement device can absorb a higher load during a friction engagement, for example, during braking, or its durability is clearly increased at a given load. This permits a limited-time durability to be converted into a long-term durability. On the other hand, cost effective but temperature sensitive organic materials that could burn in the case of poor heat conduction can be applied without their susceptibility to high temperature being detrimental. Increased size friction contact surfaces and a resulting larger space requirement and increased weight for the friction engagement device can be avoided.
Heavy, high speed utility vehicles, particularly agricultural tractors, that are being designed for vehicle speeds of 50 km/h and above, require powerful brakes. As a rule, generally there is only a relatively small space available for this purpose, so that the brakes must be designed for high power densities. For example, with such tractors the parking brake (hand brake) should not exceed an average thermal density of 3 W/mm
2
. In the case of the service brake (foot brake) this value is closer to 1 W/mm
2
. These values can be exceeded with a friction engagement device according to the invention.
As used herein for the present invention, the friction material is predominantly made of carbon based compounds. A preferred formulation uses mainly cellulosic fibers mixed with synthetic thermally stable fibers such as Kevlar™ or graphite fibers bound in a resin. For reasons of cost, it is advantageous to generally use paper linings for friction linings. For example, a friction lining of the type HM200E of the company Miba, Austria, is appropriate for a high static and dynamic friction value. The friction coefficient of the friction lining is appropriately greater than 0.1.
In dry brakes, as a rule, abraded material of the lining appears in the contact zone. The abraded material can become embedded in the friction contact surface, particularly in the aluminum surface, and increase its roughness, which can lead to increased wear and finally to the destruction of the friction surface and the friction lining. According to a particularly preferred further development of the invention, these wear phenomena can be effectively avoided by immersing the rotating first or second component at least partially in a lubricating oil sump, so that the friction surface is moistened constantly.
The lubricant here preferably is a lubricating oil, particularly a usual hydraulic oil, a brake oil or a gear oil. Many vehicles already contain lubricant circulating systems. The lubrication of the friction engagement device can be incorporated into these circulating systems to great adva
Elsner Wolfram
Reis Viktor
Rheinheimer Gunter
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