Summary: | 博士 === 國立中正大學 === 機械工程學系暨研究所 === 103 === The twin-screw dry vacuum pump is widely used in low and medium-low vacuum applications. Its core element consists of a pair of rotors rotating in opposite direction on parallel axes. Usually one side of the rotor profile is a concave cycloid curve to prevent the inter-lobe leakage. Screw rotors are typically finished by the form grinding method. However, such concave rotor profile is almost impossible to grind on a conventional thread grinder by one tool due to severe undercutting and secondary enveloping problems. In this paper we propose a novel Tilt Form Grinding (TFG) method to overcome the problem of concave profile grinding. By using this proposed machine arrangement, the concave rotor profile can be ground without undercutting and secondary enveloping. Further, the right-hand and left-hand screw rotor can be manufactured by the same tool when only one rotational direction in tilt angle. Numerical examples are presented to validate the proposed TFG method. The rotor of twin-screw vacuum pump with uniform pitch can be ground by form grinding tool, but the one with variable pitch not. Thus, manufacturing the screw rotor with the variable pitch by CNC Lathes is proposed. The initial position and trajectory of turning tools and the manufactured profile error of screw rotor are derived and verified numerically. According to the simulation results, the proposed CNC turning process is feasible. However, further experiments are still required to check the tool life and machining accuracy.
The screw rotor of vacuum pump works rely on a pair of timing gears, some clearances between the rotors are required. The lock-up and interference between the rotors are caused by the gear fatigue failure. So the priority will be the calculation of contact stress of the gears. In this paper, the Hybrid Finite Element Method (HFEM) combined with the Tooth Contact Analysis (TCA) was applied to solve the static contact stress problem of the meshing cylindrical gear set. Using the examples in this study, the contact stress of the meshing gears was calculated and compared with the result of ANSYS in order to validate the proposed HFEM method. Furthermore, the proposed HFEM in this study can also be used to determine the amount of tooth surface modification.
Keywords:Screw rotor, Variable pith, Tilt form grinding method, Hybrid finite element method, Compliance matrix
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