1. Field of the Invention
The present invention relates in general to compressor screw rotors for feeding compressible gas or fluid while compressing or expanding them and, more particularly, to an improvement in tooth profiles of helical or screw rotors having lands and grooves meshing each other in a compressor casing for improving the operational performance of the compressor. The rotor's tooth profiles are generated using a quadratic function with optimized constants as generation parameters.
2. Description of the Prior Art
Conventionally, a gas compressor for feeding compressible gas or fluid while compressing or expanding them includes a pair of asymmetric screw rotors, that is, male and female screw rotors. The major portions of the female screw rotor are positioned in the inside of its pitch circle, while the major portions of the male screw rotor are positioned in the outside of its pitch circle.
More recently, the tooth profile of screw rotors for compressors have been actively studied. For example, U.S. Pat. No. 4,412,796 and U.K. Patent Nos. 1,197,432 and 2,092,676 disclose screw rotors suitable for improving the operational performance of the compressor.
That is, the above patents disclose use of asymmetric male and female screw rotors instead of conventional symmetric screw rotors and thereby improving the operational efficiency of the compressor. In the screw rotors disclosed in the above patents, the tooth profiles of the male and female screw rotors are asymmetric relative to the radial lines extending from the rotor's centers of rotation and passing through the lowest positions of the grooves.
However in the above male and female screw rotors, the deddendum of each groove of the male screw rotor is relatively larger than the outer diameter of the female screw rotor. Additionally, the addendum of each land of the female rotor is relatively larger than the outer diameter of the female screw rotor. Such larger deddendum and addendum of the male and female rotors provide advantage in that they not only increase the working space volume but also improve the drive conditions of the female rotor. However, the rotors having the above larger deddendum and addendum are problematic in that both the addendum and deddendum enlarge the blow hole and thereby reduce volume efficiency as well as adiabatic efficiency.
Additionally, the screw rotors disclosed in the U.K. Patent No. 1,197,432 have a portion with pressure angle of 0.degree.. This portion causes a bad cutting condition in a hob milling process for producing the rotors. In the screw rotors disclosed in either the U.K. Patent No. 1,197,432 or the U.S. Pat. No. 4,412,796, the tooth profile of the following rotor has a point-generated portion which is difficult to be precisely machined. Additionally, the above point-generated portion of the following rotor is severely abraded during operation of the rotors and thereby cause considerable damage to the tooth surface of the rotor. The point-generated portion also increases the trapped pocket volume.