| The research subject of the dissertation stems from the National Natural ScienceFunds (NNSF)"Research on key technical and new design method of rotor profile forhigh efficiency three screw pump (50875268)" and "Research on key technical andbasic theory of multistage variable speed screw pump (51175522)".Screw pumps have been widely used in the offshore drilling platforms, ships,precision machine tools, petrochemical industry and other fields due to its superioradaptability and economic practicability."The Central Committee of CPC onformulating the12th five-year plan for economic and social development suggestions"clearly pointed out that the high-end equipment manufacturing is the core position ofseven strategic emerging industries and will be highly supported by the government inthe important fields. As a key component of the high-end equipments, the developmentof screw pumps will power the high-end equipment manufacturing industry, and supportthe strategic emerging industry. Meanwhile, during the12th five-year, the governmentwill invest300billion in marine equipment, including offshore drilling platforms,offshore vessels and submarines, etc. Therefore, screw pump as an importantfundamental part of the high-end equipment have huge demands and broad prospects.However, screw pump, which is a high precision mechanical product, is sensitive to theprecision of the design and manufacturing. The products that are manufactured in ourcountry usually have various defects, including low volumetric efficiency, unstable flowcharacteristic, serious tooth surface wear, large power consumption, because of the lackof the understanding of the working mechanism and the lack of the comprehensivetechnique in design and machining. As a result, the high-end screw pumps basically relyon imports for a long time. In order to improve the internationalization level of high-endequipment manufacturing industry in our country and reduce the dependence on foreignhigh-end equipment parts such as screw pump, we need to carry out the innovationstudy in design principles and methods of screw pump system and solve the keytechnical problems of the design and manufacture of screw pump. Such breakthroughwill be socially and economically crucial to the independent innovation of machineryequipment in our country and have great practical value in engineering.In this paper, screw rotors that are the core parts of the screw pump are the researchobjects. Fluid dynamics, leak theory, contact theory, spatial envelope theory, etc., have been used as tools to analyze the flow kinematics and dynamics of the pump chamber,to study the structure of screw motors and the design principles and methods onparam-pattern between the volumetric efficiency of screw pump and the configurationprinciples, to establish the screw tooth profile model basing on the meshing clearance,and to form the precision profile of forming cutter of the screw tooth surface. The maincontents of this dissertation are shown as follows:①Based on the definition of suction capacity of positive displacement machine,the screw pump suction capacity calculation model has been set up. The impacts ofscrew speed, screw helix angle, the transmission medium, etc., on the screw pumpssuction capacity have been analyzed in detail. In addition, the matching relationshipbetween the key design parameters and the suction capacity of screw pump have beenobtained by studying the volumetric efficiency characteristics influence on its suctioncapacity. Furthermore, the improving method for the capability of screw pump’s suctionhas been proposed.②Based on theory of hydrodynamic boundary layer separation, the path line andthe separated points have been calculated by in-depth analyzing the capacity of screwpump’s suction and the flow around a cylinder. Then, the optimization model of theboundary layer separation flow around cylinder has been established and solved byusing graphics and analytical methods. Moreover, the optimized3D model of thestreamline body has been obtained through the cross section geometry of the screw rotor.Finally, the feasibility of the optimization model has been validated by the theoreticalanalysis method.③Clearance leakage is the main cause of pump leakage. For the design andcontrol of the key clearance which determines the performance of screw pump such asclearance of barrel wall and meshing area, the model of differential pressure flow andshear flow of clearance leakage in pump cavity has been established. Furthermore, themathematical expressions of leakage from different clearance are obtained by analyzingthe leakage for clearance of barrel wall and meshing area. Then, the influence of therotation speed and pressure difference between import and export on flowcharacteristics and volumetric efficiency characteristics of screw pump has beenrevealed by theoretical calculation and experimental analysis of twin-screw pump (2heads for main and3heads for follower, respectively).④Aiming at the problem of the nonuniform meshing clearance that is caused bythe complicated design and the highly difficult process of the rotors which are the key components of screw pump, a new optimized design method of screw tooth profilebased on the given clearance is proposed by analyzing the formation mechanism ofscrew clearance and using the space analytic geometry theory. The transformationequations related to the feed path, screw tooth profile and the clearance in screwmanufacturing process are then established. Furthermore, the influence of feed directionsuch as axial, radial and normal on the meshing clearance and the tooth profile deviationof rotors has been analyzed in detail. Thus, in combination with the isometric surfaceconfiguration of differential geometry, the screw cross tooth profile with thepredetermined clearance is obtained.⑤Let’s take the processing of screws for screw pumps as an example. Based uponthe principle of gearing mesh we can use the cubic spline interpolation method to obtainthe tooth profile of the screw forming cutter according to the tooth profile at any crosssection of screw composed of discrete points. Furthermore, a design method isinnovatively proposed for the screw forming cutter based on discrete points, namelyForm-Position Geometric Method (FPGM). This method is obtained by combining thespace enveloping and geometric characteristics between the screw and cutter during theprocessing and the shape of the contact line which is generated by the cooperativemothion of the machine tool, screw and cutter, as well as the spatial location parameters.After comparing the proposed method with the principle of gear meshing, it can beobserved that the cutter-workpiece enveloping solution model which is simplified by theFPGM can overcome the key technical difficulty. For example, it is difficult toaccurately calculate the cusp of the tooth curve using the cutter-workpiece model, butthe proposed cutter design method can improve the machining precision of the screwtooth profile. Finally, the feasibility and superiority of FPGM is verified byexperiments. |
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