Method and apparatus for cold-working annular workpieces

An apparatus for cold working an annular workpiece having an internal cavity by applying thereto an internal fluid pressure to cause diametric enlargement of said annular workpiece through plastic deformation, said apparatus comprising:

• a pair of diametrically expansible packings of ring form, one of said packings to be fitted in an annular groove formed in an opening at each axially opposite end of said annular workpiece
• a pair of presser plates, each said presser plate covering one said opening of said annular workpiece and adapted to be held in radially slidable pressed contact with one of said packings
• a main pressure source connected to a main pressure chamber defined by said internal cavity of said annular workpiece;
• a cylinder device operable for moving said presser plate axially of said annular workpiece, said cylinder device internally defining an auxiliary pressure chamber
• and
• an auxiliary pressure source connected to said auxiliary pressure chamber, further comprising an arithmetic unit connected to said main auxiliary pressure sources and adapted to maintain the contacting surface pressure of each said packing and presser plate at a predetermined value by detecting and calculating values indicative of a main pressure acting in said main pressure chamber, an auxiliary pressure acting in said auxiliary pressure chamber and variations in the shape of said annular workpiece.