Friction clutch for a motor vehicle, and an elastic diaphragm for such a clutch

A friction clutch comprising:

• a cover plate;
• a pressure plate;
• and a resilient diaphragm between the said cover plate and pressure plate, the said diaphragm having a frustoconical shape in a free state and comprising an outer peripheral portion of substantially annular shape and a series of resilient fingers extending radially inwardly from the said outer peripheral portion, the said fingers having free ends, the cover plate, pressure plate and diaphragm defining an axis of the clutch, the said free ends of the diaphragm fingers being arranged for cooperation with a clutch manoeuvring member displaceable parallel to the said clutch axis, the said outer annular peripheral portion of the diaphragm having two opposed faces, the pressure plate defining a first engagement element in facing relationship with the diaphragm, and the cover plate having an internal face defining a second engagement element in facing relationship with the diaphragm,
• wherein the said outer annular peripheral portion of the diaphragm defines a first annular rib and a second annular rib radially inward of the first annular rib and concentric therewith, the said ribs projecting axially and defining respective convexities oriented in opposite directions, the said first and second engagement elements being surface portions of the pressure plate and cover plate respectively, with each said rib cooperating with a respective one of the said surface portions, whereby the said outer annular peripheral portion of the diaphragm has a profile substantially in a form of a letter S or Z in its cross section taken through an axial plane thereof.