Scroll-type fluid displacement apparatus having spiral start portion with thick base and thin tip

A scroll-type fluid displacement apparatus comprising;

• a rear housing having an open end and an inlet port and outlet port;
• a front housing closing said open end;
• a fixed scroll having a first end plate and a spiral element formed on and extending from a first side of said first end plate, and attached to said rear housing;
• an orbiting scroll, having a second end plate and a spiral element formed on and extending from said first side of said second end plate, each of said spiral elements interfitting with the other at an angular and a radial offset to form a plurality of line contacts defining at least one pair of sealed-off fluid pockets;
• a driving mechanism including a drive shaft rotatably supported by said front housing to effect an orbital motion of said orbiting scroll member by rotation of said drive shaft to thereby change a volume of said fluid pockets;
• and
• a rotation preventing mechanism means preventing said orbiting scroll from rotating;
• wherein an interior wall of a widened-starting portion of each of said spiral elements is inclined, such that a thickness of a base surface of said widened-starting portion is greater than a thickness at a tip surface and the thickness of said widened-starting portion gradually decreases towards said tip surface of said widened-starting portion, and wherein a first transition line between said interior wall and said tip surface comprises a first upper arc connecting to an upper interior involute wall starting point and second upper arc connecting to an upper exterior involute wall starting point, wherein a second transition line between said interior wall and said base surface comprises a first lower arc connecting to a lower interior involute wall starting point and a second lower arc connecting to a lower exterior involute wall starting point, and wherein each of said elements is defined by the following equations and relationships:
• Rb=ru+Ror;Ru=rb+Ror;
• ru<rb;
• Ru>Rb;
• and
• φPi−φPo=180°,
• where:
• Rb is a radius of the first lower arc;
• rb is a radius of the second lower arc;
• Ru is a radius of the first upper arc;
• ru is a radius of the second upper arc;
• Ror is an orbital radius of said orbiting scroll;
• φPi is a widened-starting angle of an interior involute wall;
• and
• φPo is a widened-starting angle of an exterior involute wall.