1. Field of the Invention
This invention relates generally to the friction clutches, and particularly to centrifugal friction clutches that engage at a predetermined speed of rotation of a drive member.
2. Background
Centrifugal friction clutches are well known in the art. For example, U.S. Pat. No. 4,111,291 discloses a centrifugal friction clutch having at least one input or pressure plate, at least one fixed plate, and at least one output or driven disc concentric with the pressure and fixed plates. At a predetermined speed of input rotation, the pressure plate and output disc or discs are forced to move axially into frictional engagement by centrifugal weights or cam mechanisms on the pressure plate. These cam mechanisms are constrained by springs, such that the speed of frictional engagement can be modified by adjusting the spring force.
The most significant design challenge with respect to centrifugal friction clutches is the control and dissipation of heat generated by the frictional engagement of the plates and discs. In prior art clutches, frictional surfaces of the friction discs contact directly with the pressure plate and the fixed plate. The resulting heat builds up in the plates and eventually causes distortions in the surfaces of the plates. These distortions are amplified by the opposing bending stresses induced in the pressure plate by the cam mechanisms and springs.
In prior art clutches, the resulting distortions in the pressure plate reduce frictional engagement between the discs and plates, reducing the efficiency of the frictional engagement and generating additional unwanted heat. This cycle eventually causes permanent distortion of the pressure and/or fixed plates, causes the clutch to fail, and requires the replacement or repair of the pressure and/or fixed plates and other related components.
Efforts to address the problems caused by waste heat in such clutches have been unsatisfactory. For example, U.S. Pat. No. 6,823,975 discloses the use of a plurality of “air impeller vanes” or grooves in the exterior side of the pressure and/or fixed plates, opposite the interior or contact friction side of the plates, to provide air cooling and promote heat dissipation. However, due to the small size of the clutch, the limited air intake, and turbulent air flow patterns, this approach provides only limited benefits. The result is some delay, but not prevention, in the distortion of the pressure and fixed plates.
The present invention addresses this problem by separating the surfaces used for frictional engagement from the surfaces of the pressure and fixed plates. This separation provides for dramatically improved heat dissipation, reduced heat transfer to the pressure and/or fixed plates, and the elimination or reduction of associated distortions to the surface of the pressure and fixed plates. It further provides for improved frictional engagement even in situations where there is either temporary or permanent distortion of the pressure and/or fixed plates. The present invention further provides for the use of impeller vanes on the interior side of the pressure and fixed plates to further dissipate heat generated during clutch operation.