A door closer to be affixed to a door or door frame for closing said door after said door is opened, said door closer comprising:
- (a) a housing body and a cylinder inside said housing body, said cylinder having first and second cylinder ends, and first, second, and third sections, each of said sections of said cylinder having a different predetermined diameter, said cylinder being sealed at both ends thereof so as to contain a compressed fluid;
- (b) a piston rod penetrating through said first end of said cylinder and being in a sealed but movable relationship therewith, said piston rod being connected to a link via a connecting rod outside of said cylinder, said link being pivotably affixed to said housing body;
- (c) turning means sealingly provided at said second end of said cylinder for turning said cylinder;
- (d) first and second speed adjusting mechanisms sleeved on said piston rod, said first speed adjusting mechanism comprising a first rotatable piston head and a first fixed piston head, and said second speed adjusting mechanism comprising a second rotatable piston head and a second fixed piston head, said first rotatable piston head being constructed so as to be in a sealingly frictional contact with said first section of said cylinder via a first O-ring, and said second rotatable piston head being constructed so as to be in a sealingly frictional contact with said third section of said cylinder via a second O-ring;
- (e) said first rotatable piston head having a first radially extending groove and a first axially extending graded groove, both of which being in communication therebetween, and said first fixed piston head having a first serpentined O-ring, wherein said serpentined O-ring contains a spiral portion whose disposition relative to said axially extending graded groove is adjusted by a relative rotational movement between said first rotatable piston head and said first fixed piston head so as to control an effective opening in said axially extending graded groove through which said compressed fluid may flow;
- (f) said second rotatable piston head having a second radially extending groove and a second axially extending graded groove, both of which being in communication therebetween, and said second fixed piston head having a second serpentined O-ring, wherein said serpentined O-ring contains a spiral portion whose disposition relative to said axially extending graded groove is adjusted by a relative rotational movement between said second rotatable piston head and said second fixed piston head so as to control an effective opening in said said axially extending graded groove through which said compressed fluid may flow;
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- (g) further wherein said cylinder and said first and second speed adjusting mechanisms are so structured such that only said first rotatable piston head is in a sealingly frictional contact with said first section of said cylinder when said door is opened for no greater than a predetermined angle, and only said second rotatable piston head is in a sealingly frictional contact with said third section of said cylinder when said door is opened for greater than said predetermined angle;
- (h) whereby a turning of said turning means causes said cylinder to turn, which, in turn, causes either said first rotatable piston head or said second rotatable piston head to turn, thus changing portions of said first axially extending graded groove or said second axially extending graded groove blocked by said first or second serpentined O-ring, so as to control a flow rate of said compressed fluid through said first or second axially extending graded groove and control a door closing speed.