1. Field of Invention
This invention relates to clutches for motor vehicles and, in particular, to an improved design and application of frictional linings for enhanced performance of automotive clutches.
2. Brief Statement of the Prior Art
The typical automotive clutch has a clutch disc fixedly secured to the drive shaft and supported between the flywheel and a pressure plate. The clutch cover, which is bolted onto the flywheel includes a conical (Belleville) spring that is actuated by a clutch throw out mechanism and hydraulics to release the Belleville spring force against the pressure plate, clamping the clutch disc between the pressure plate and flywheel. The clutch disc supports frictional linings which usually have a metallic base ring on the theory that the high heat conductivity of a metal base ring would enhance heat transfer and thus reduce the tendency of the clutch linings to overheat.
The design of the present day automotive clutch is the result of a compromise of complex and competing design factors. Because of limitations in size and accessibility, and the requirement for a moderate pedal pressure, the maximum spring load which can be used with most clutches is from about 800 to about 2,000 pounds. The demand for increased performance of automobiles has increased the torque which is transmitted through the clutch and, accordingly, manufacturers provide clutches with clutch discs having diameters as large as practical to achieve moment arms adequate to handle the large torques transmitted by the clutches.
Most of the car manufacturers have elected to line the entire or substantially the entire face of the clutch disc with frictional linings, apparently on the theory of the more the better. This has resulted in the unit pressures applied to the clutch disc facings being approximately 25-30 psi.
Until the concern over the potential hazards to the public, asbestos-containing facings were the most commonly used facings. The phase out of asbestos facings has spurred development of new materials including in particular organic composite facings. These linings frequently have optimum unit pressures from about 35 to about 65 psi, or greater, which exceed the unit pressures of current clutch designs. This has caused a less than optimum performance of clutches, often observed as a chattering of the clutch, and high wear and maintenance of the clutch disc and its frictional facings. Further, the organic composite linings are particularly sensitive to high temperatures, loosing much of their performance when their temperatures exceed about 450 degrees F.
It is possible to improve clutch performance by increasing the size and strength of the Belleville spring, thus applying a greater load to the clutch assembly. This approach, however, is not practical for a number of reasons. The clutches used in vehicles currently on the market have hundreds of differently sized and shaped Belleville springs and a conversion of even a small portion of the vehicles on the road and presently being manufactured would require many hundreds of thousands of dollars in tooling costs. Additionally, the higher loads imposed on the clutch assembly by the increased Belleville spring force would lead to higher thrust and would require substantially complete redesigning of the clutch throw out mechanism and hydraulics. Accordingly, the approach of increasing the load applied to a clutch is not a practical solution for retrofitting the millions of vehicles currently on the road or being marketed.
Car manufacturers also seek higher and higher performance, leading to a new type of car, the Street Legal Performance (SLP) car which has performance characteristics which rival those of the dragsters and race cars of recent years past. The traditional approach by car manufacturers has been to increase horsepower and torque dramatically and to retool the drive train including the clutch to handle the greater power and torque. Usually clutches have been upgraded by increasing the clutch diameter and/or clamp load. This approach is expensive and usually requires retooling of related mechanical equipment.
There exists, today, a need for a simple and efficient conversion to increase the performance of clutches, specifically by increasing the unit pressure applied to clutch facings to the optimum value which provides maximum performance of the particular frictional facings used in the clutch. There also exists a need for a clutch lining having a design and formed of materials which protect the facing from overheating.