Computer aided design software package for conjugate helical surfaces

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ELSEVIER

Journal of Materials ProcessingTechnology61 (1996) 72-77

Materials Processing Technology

C o m p u t e r aided design s o f t w a r e package for conjugate helical surfaces" Da Zhun Xiao and Ke Bing Li Department of Mechanical Engineering, Dalian University of Technology, Dalian 116024, C/u'na Zhi Qiang Wang and Dian Min Liu Dalian Refrigeration Co., Ltd., Dalian 116033, C/u'na

Abstract

A computer aided design software package for conjugate helical surfaces applied to the twin screw compressor is developed to establish an integrated multitasking environment for the tooth profile design, the performance analysis and the cutter profile design. The background, function, structure and interface of the package is presented with eye--catching illustrations in this paper. The package is believed to be significant to the industrial application. Keywords : CAD, Software package, Conjugate helical surfaces

1. Introduction

The conjugate helical surfaces with complex transverse tooth profile are widely applied to fluid compression or pumping. A typical application is in the twin screw compressor whose tooth profile design of screw rotor and performance analysis are both complicated in nature and fundamental to the development of the new type of product. Computer aided design and performance analysis for the twin screw compressor have been attracting much research interest and have made a great progress in recent years I-1--53. The reason for this lies in the intrinsic feature of the field in consideration in addition to the general availability and super power of the computer application. For development of new type of product and practical industrial application, an integrated software package is necessary for the tooth profile design, the performance analysis that covers geometry analysis, thermodynamic proce~ simulation and mechanics analysis, and the cutter profile design. Such a package possesses complete functions and is very much friendly towards the user. For this reao~n, the software package is developed to establish an integrated multitasking environment for the mentioned

* The ProjectSupportedby National Natural ScienceFoundation of China 0924-0136/96/$15.00© 1996ElsevierScience S A All rights reserved Pll 0924-0136(96)02468-5

requirements. The package is organized with OOP (Object--Oriented Programming) technique and programmed with language of Borland Cq--+- under Microsoft Windows 3. 1 environment, with Windows MDI (Multiple Document Interface) used as the interface design standard. The function, structure and interface of the software package is presented in this paper, with some illustrations attached. It is believed that the software package is significant to both the research and the industrial application.

2. Functions of the package

The functional modules of the software package are shown in Fig. 1, which includes three parts: the tooth profile design, the performance analysis and the cutter profile design. The tooth profile design module is used for design of the transverse tooth profiles of both the male rotor and the female rotor. Different types of conjugate tooth profiles are provided for designer' s selection. The conjugate points on the helical surfaces and the contact points on the contact line are determined and their coordinates are stored in respective data files that are accessible for other modules. Two types of tooth profiles of the twin screw compressors are shown in Fig. 2.

D.Z. Xiao et al./Journal of Materials Processing Technology 61 (1996) 72-77

The performance analysis module contains three s u b - modules, i.e. , the geometry analysis, the compression process simulation and the mechanics analysis. The geometry analysis covers calculation of such quality indexes as the available area coefficient, the blow hole area, the length of the contact line and the average comprehensive radius of curvature (ACRC), etc. , which are related to the displacement capacity and fluid leakage characteristic of the twin screw compressor. The compression process simulation includes the thermodynamic process simulation , volume and pressure variation in the higher pressure chamber (see Fig. 3), and the animation of tooth meshing process, i.e. , the motion process of

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the contact line in both transverse plane and axial plane (see Fig. 4) . The mechanics analysis contains the torque analysis for the male and female rotors based o~a the above thermodynamic process simulation, especially the analysis for negative torque on the driven female rotor that has passive impact on the machine efficiency (see Fig. 5). The cutter profile design module produces the data and drawings of the milling cutter profile for both male and female rotors. Both analytical and numerical approaches for this calculation are included in this module that is essential to industrial application. Two designed profiles of the milling cutter for male and female rotors are shown in Fig. 6.

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D.Z. Xiao et al./Journal of Materials Processing Technology 61 (1996) 72-77

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D.Z Xiao et al./dournal of Materials Processing Technology 61 (1996) 72-77

3. Structure of the package

The software package is developed for establishing an integrated environment based on Windows platform to carry out the above functions. The application programs for different functions were individually developed before, lacking of unified structure and language. In order to minimize the code amount and to make the software convenient to maintenance, extension and code reuse, the advanced object--oriented programming(OOP) technique is adopted for developing the software package. The core of OOP is the CLASS, a u s e r - defined data type. Differing from the conventional data type, the class specifies the implementation of the dam, the member functions and the accessibility of the data that is classified as public, protected and private categories. The public member functions show the functions of the calss and act as external interfaces interacting with other classes, while the protected or private member data or functions are prevented from arbitrary access. Only the member functions within the class can access them, which is called encapsulation of the class that protects the privacy of the class and prevents those protected from illegal modification. To programmers, the encapsulated class looks like a "software chip" that greatly facilitates the software development and the code reuse. Another important feature of the class is its inheritance that refers to that all the descendant classes may inherit the characteristic and functions of the parent class. By means of inheritance, a new class can be constructed based on the existing classes, adding some functions and stating the difference between the new class and the existing class. Re--programming from the beginning is no longer necessary, which greatly simplifies the software development and makes it easy to be extended. In our developed software package, first of all, the tooth profile of the screw rotors in the twin screw compressors is extracted and a profile class is construtted. The data members of this class include some basic parameters such as the nominal diameter, the number of teeth, the center distance, etc. , that are grouped in a " protected" category to prevent them from illegal modification and to be accessed by the member functions of the descendant classes. The functions of the profile class contain determining of conjugate points on the helical surfaces and contact points on the contact line, calculation of the available area coefficient, the blow hole area and the length of the contact line, and drawing of the tooth profiles in the transverse plane. The member functions grouped in a "public" category in the c h ~ carry out those functions. Some commonly--used types of conjugate tooth profiles are provided in the profile class that can

be extended through a new descendant class from this class. Through inheritance, the class system is established in the software package based on the profile class. The software package is programmed by means of langugage of Borland C + + 3.1 for Windows that is best suitable for OOP technique.

4. Interface of the Package

Microsoft W'mdows, a multitasking operating environment added to MS--DOS, is well received by broad software users and software developers due to its powerful functions and e y e - catching graphical interface between the user and the computer. The Windows interface has become a standard interface for software development. In view of requring display of a lot of graphical inforrrmtion in our software package, e. g . , the tooth profile, the contact line, the volume and pressure curves,etc. , the Windows MDI (Multiple Document Interface) is adopted for our interface design that is shown in Fig. 7. There is a main menu bar at the top of the frame window. All the functions of the software package can be selected from this menu bar. The client window within the frame window provides various child windows with display background and controls their generation or removal. The child windows displayed in the client window show different graphics that can be printed in accordance with user's requirement. Each child window can also be extended, contracted or translated. At the bottom of the client window is a prompt bar on which a brief prompt for user's operation is given. The Windows MDI makes it possible to watch several performance curves of one type of tooth profile, or the same performance curve of different types of tooth profiles, concurrently. These graphical functions will help the user identify the advantages or disadvantages of different tooth profiles quickly, conveniently and intutively. The menu system of the package is of pop--up type that facilitates user's control of the execution of the program. The dialog boxes are also provided to interact with the user. Therefore, the software package is very friendly towards the user and very easy in operating.

5. Conclusions

A CAD software package for conjugate helical surfaces applied to fluid compression, the twin screw

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D.Z. )(iao et al./Journal of Materials Processing Technology 61 (1996) 72-77

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compressors, is developed to establish an integrated multitasking environment for the tooth profile design, the performance analysis and the cutter profile design. The package is organized with OOP technique and programmed with language of Borlandc C q-q- under Microsoft Windows operating environment, with Windows MDI used as an interface design standard. As a resuk, the package is friendly towards the user, easy to be operated, and favorable for maintenance, extension and code reuse. Such a package is believed to be new to the literature and to be significant to the industrial application. Acknowledgment

The financial support of the National Natural Science Foundation of China under Grant No. 59375171 is greatly appreciated.

References

[-1-] B. Sangfors, Proceedings of the 1984 International Compressor Engineering Conference at Purdue, USA, 1984, pp. 528--535. [-2] M. Fujiwara et al, Proceedings of the 1984 International Compressor Engineering Conference at Purdue, USA, 1984, pp. 536--543. [-3] P. J. Singh et al, Proceedings of the 1984 International Compressor Engineering Conference at Purdue, USA, 1984, pp. 544--553 [-41 M. Fujiwara et al, Proceedings of the 1990 International Compressor Engineering Conference at Purdue, USA, 1990, pp. 51--61 [-5-] D. Z. Xiao et al, Proceedings of sixth International Conference on C A P E , London, UK, 1990, pp. 181--188