Study On The Profile Of The Gradational Lead Screw Rotor In Dry Screw Vacuum Pump

The oil-free screw pump has widely been used in the semiconductor?medicine food? chemical and many other industry areas, because of its advantages, such as wide pumping speed range, simple and compact structure, no friction between pumping components, long life, low energy consumption, oil-free, etc. The most important component in a oil-free screw vacuum pump is a pair of screw rotors as the pumping parts. Which directly influence the working performance and the working life, Its manufacturing costs account for about half the cost of the entire screw pump. The profile design is the most critical technology of the design of screw vacuum pump. Where, the single-thread and changing-pith rotor, compared with equal pitch screw rotor, because of its internal pre-compression way of exhaust, which has the outstanding advantages of energy saving of noise reduction. Dry Twin Screw Vacuum Pump has won the good graces and much concern of designers and users in recent yearsAiming at the screw rotors with single-threaded and gradational-lead used in dry screw vacuum pump, some detail research about the gradational lead screw rotor is carried out in this paper.First, the end face profiles of oil-free screw vacuum pump are classified:trapezoidal tooth and rectangular tooth, all the profile equation of each the end face profile are given, and discuss the advantages of this type profile. Second, the study of gradational lead screw rotor axial surface profile (second power profle?third power profile and the profile of the lead is proportional to the axial coordinate lines). First?The form of the gradational pitch axis equation. Described by the gradational pitch coefficients and the initial or final lead; second? the form of the pumping volume. The pumping volume is described by the angle and the coordinate; third?The analysis geometric pumping speed; fourth?The gas thermodynamic analysis. Respectively from the volume calculation, thermal process equation, gas compression calculation, the change about temperature?internal energy?enthalpy and entropyThrough the analysis above, addendum width changes to narrowed gradually along with the lead(pitch) of screw rotor becomes narrowed gradually from the fluid inlet towards the fluid outlet-This kind of structure will lead to the following unreasonable situation directly: the addendum width of tooth profile needless increases in the fluid inlet, which decreases volumetric efficiency of the pump; on the contrary, owning to large pressure differences in each levels, the addendum width of tooth profile opposite to decrease in the fluid outlet,which increases the regurgitant volume. The consequences will make ultimate vacuum come down and pumping speed of screw vacuum pump decline.Aiming at these problems above, two solving ways are introduced in this paper.The first: a profile of the gradational lead screw rotor with equal axial addendum width is introduced in the article (certainly, the dedendum width will remain constant correspondingly), a gradational lead screw rotor, when the screw lead of the rotor gradually get larger and larger along with the rotor axis from outlet towards inlet, but the addendum width remains the same; only the axial length of rapezoid surface gradually become larger. Therefore, which can increase the suction volume in the fluid inlet, reduce the gas reflux in the fluid outlet, so as to improve the pumping speed of pump and ultimate vacuum.The second:a profire of variable cross-section and gradational lead screw rotor is introduced in this paper, along with the process that the lead changed from the suction to the exhaust gradually, cross-section end face also changed all the time.thus, not only can improve the pump leakage blocking ability, but also improve the compression ratio of the pump greatly.