COMPARATIVE STUDY OF COMMANDS TO GENERATE GEARS FROM DIFFERENT SOFTWARE DESIGN

Headnote

Abstract: The paper aims to approach commands for the automatic generation of cylindrical gears with right teeth and involute profile that can be used to obtain further cylindrical gears. The following software is investigated: NX 7.5 and Inventor Professional 2011. The author presents in detailed analysis if the software itself contains instructions for generating gears or require an additional patch. The paper presents the results of applying those commands and direct or indirect possibilities of obtaining a cylindrical gear with right/straight teeth. After a thorough study, the conclusions are drawn about the strengths and weaknesses of each software in use, and recommendations are made in accordance with the author's opinions.

Key words: Design, CAD, modelling, Autodesk, gear.

1. THEORETICAL ASPECTS

Cylindrical gears are machine parts made of rotating cylindrical bodies provided with external or internal teeth. They allow transmission of torque and rotational motion at a constant transmission ratio. The theoretical part on the cylindrical /spur gears that includes both graphical representation and sizing and reliability calculations is extremely vast and represents the object of activity of many disciplines. Any spur gear is composed of three main parts, namely: hub, disc and crown. The lateral surfaces of the teeth between the head/upper and the foot/bottom cylinder are called flanks, one is called the right flank and the other the left flank

The flanks that are in contact and belong to a wheel forming a gear are called conjugate flanks. Two gears having the same module can form an external or internal spur gear. These, under the mechanical solutions in transmitting the rotation movement, have the following advantages: reliability, long life, reduced size, high efficiency and providing a wide range of powers, speeds and transmission ratios. In terms of disadvantages mention should be made first of the advanced processing technology, the need for precision machine tools, and the generation of noise and vibration during operation at high rotation speeds. Therefore, the gear was, is and will be one of the most precious machine parts, both in terms of manufacturing technology and in terms of functionality. The importance of gears led to the development of software components for threedimensional generation and a wide variety of machine tools for their processing, to communicate by various methods with the virtual model developed by the designer

Initial data for the calculation of gear are: angular velocity or speed of the machine shaft, the shaft power and torque. When calculating a cylindrical right teeth gear, the gear nominal forces are determined by the motor torque on the pinion shaft. Consequently the normal force on the tooth, denoted by Fn, applied at the point of intersection of the gear line with the pitch circle is decomposed into a tangential force Ft and a radial one Fr at the pitch circle. Based on the findings that the losses inside the gear are relatively small, it can be considered that forces in the gear acting on the two wheels are equal and of opposite sign in the pitch circle.

In the manufacture of gears, as in fact in all mechanical sectors, international competitiveness caused the design to climb a qualitatively new stage in the sense that it was passed to a parametric design, which allows obtaining 3D models that can be further analyzed with FEA software or can be subjected to a manufacturing process simulation. In the field of machine building, the production of gears represents a priority direction, which requires performant software and highly skilled professionals.

2. METHOD OF GENERATING SPUR GEARS BY NX 7.5

The NX application allows the establishment of a complete and integrated design solution [2] for the mechanical field, including tools and highly methods for:

· Performance exhaustive modelling allowing continuous use of the most productive modelling approaches - from explicit modelling of surface and solids to direct parametric modelling specific to the process and modelling not based on specific historical features using models from any CAD system.

· Creating an active layout and overall design that allows for interactive work with massive assemblies, while involving the application of both the main assembly management and the technical tools for modelling singular benchmarks.

· 3D annotation and drawing according to standards, which simplifies the creation of product documentation by directly using the basic 3D model.

When making an extremely succinct characterization of the NX software from Siemens it must be said that [3]:

· NX software is a fully integrated CAD / CAE / CAM of latest generation.

· NX has a user friendly interface and is applied in many fields of the mechanical design, including the design of systems and equipment for agriculture.

· NX allows opening both transfer files of the type IGES Step and the files created by other CAD systems.

· NX observes the principles of concurrent engineering

· NX provides production-based tools for documenting the designs as 3D products or 2D drawings acc. to standards and annotations on manufacturing information.

· All NX modules related to CAE and CAM functions are integrated in the same interface.

· The powerful capabilities for creating layout and assembly of the solution NX design provides for a diverse range of interactive capabilities that accelerate overall design and engineering processes.

· The JT format and technology allow loading assemblies with up to a million pieces with an exceptional interactive performance.

Regarding the basic package that works with files of Model, Assembly, Shape Studio, NX Sheet Metal type finds that does not contain a module for modelling gears, which does not mean that there is no such thing as a separate module which can be installed later. However for current needs use can be made of a file called gear_model2.dll which accessed in some way generates a 3D gear model, without having the ability to achieve a gear. Opening this file is made by NX Open command from the pull-down File as shown in Figure 2.

The parameters required for launching this file are shown in Figure 3 and are as follows: Pitch Diameter, No of Teeth, Tooth length. For the set values in Figure 3 a gear will be obtained on which multiple commands of Extrude type will be applied for generating the corresponding shoulder of the gear, for reducing weight and getting the groove, leading to the final result shown in Figure 4.

3. METHOD OF GENERATING SPUR GEARS BY INVENTOR 2011

The software product called INVENTOR produced by Autodesk Inc. USA is the improved successor of the conventional version of Mechanical Desktop. Unlike its predecessor, this is full 3D software very easy to use thanks to personalized ribbon with as many orders in sight. It also brings to install an extensive range of standard objects such as screws, nuts, washers, cable, tubing, feathers assembly profits, the spline gears, etc. Although it is very easy to use, it is particularly powerful because it allows the management of large projects (over 10,000 pieces). The first version appeared in USA on September 20. 1999 and took the name "Mustang" and up to the 2005 version had 2-3 issues per year and was named the FORD cars (Thunderbird, Camaro, Corvette, Cherokee, etc.), while in 2005 took the new name that contains the year of publication, eventually followed by names of famous inventors (Faraday, Tesla, Hopper, etc.). It should be noted that the first two versions were intended solely for the American continent. The designer from the mechanical field receives from Inventor a lot of advanced tools which have an contextual applicability. Inventor essence lies in its ability to design parameters which starts from scratch and rated constrained parametric allowing any time modification intervention both at the sketch and 3D features made.In addition to these features, Inventor comprises a plurality of modules for design in the mechanical field [4], which is part of the so-called Accelerator Design mode, presented as the ribbon in Figure 5, with all its components.

Launching command Spur Gears will open the window in Figure 6, in which the designer has the option to choose one of the design variants below:

· Module and Number of Teeth - calculates the module and number of teeth of the gears based on the distance between the axes imposed by the project theme and other input parameters.

· Number of teeth - calculates the number of teeth starting from the distance between the axes and module imposed by the project theme.

· Center distance - distance between axes calculated based on all other input parameters.

· Total Unit Correction - calculates the total profile displacement coefficient using the distance between the axes as imposed along with other input parameters.

· Module - Calculate the module based on the distance between the axes and other input parameters.

It should be noted that all Autodesk products, starting with Aut°CAD [5] and continuing with Inventor have outstanding capabilities of visualization in threedimensional space. Figure 6 shows the corresponding window for the calculation method called Module and Number of Teeth. If the box next to the words "Desired Gear Ratio" has been checked, we could have obtained an external and internal gear. The previous Box allows the possibility to preview, by clicking Preview button, the geometrical parameters imposed or calculated by the command "Spur Gears". The label Calculation containing the values imposed to the external loads, mechanical properties, the life of gear and results obtained from calculations performed by software after pressing "Calculate" button.

Particularly impressive is the window that contains the library of materials in Figure 7 of installing Inventor, which can add any content from within any national standard. The Inventor superiority compared to other software in general is that the window, besides the proper list, also views the mechanical properties of each material.

A detail of particular importance is that at the end of the gear generation, the software will let you know where each gear and the subassembly were saved. The indisputable advantage to NX 7.5 is the default centering of the flanks of the two gears while meeting the gearing theory. Figure 8 shows the gear obtained after the Spur Gears command followed by a series of commands to customize the geometric shapes. As a conclusion we should remember the following:

· Design by Design Accelerator provides the ability to control the outcome by very accurate engineering calculations and the preview windows.

· Access to modelling tools for spur gears can be done only within file of assembly type, with the extension *.iam.

· All modelling and generation tools are grouped in the Design tab of the ribbon specific to this software.

· If launching a command from this tab is simultaneous with pressing CTRL, the effect will not take into account the last application settings but will be reset to the default values of "factory".

· Editing any gear or gear subassembly existing in the left browser of the working interface can be done by pressing the right button of the mouse on that item and from the browser and selecting the option "Edit Using Design Accelerator".

4. CONCLUSIONS

Although released in 2010, NX 7.5 is a high range software, unfortunately inferior to Inventor 2011 in terms of gears generation, the latter's basic variant containing a module for generating gears, allowing also the calculation of the whole spur gear. This does not mean that add-ons to NX are not permitted so as to compensate this disadvantage. Doing a thorough comparison, the Inventor indisputable superiority as to the basic version is obvious as a result of the following arguments:

· Inventors simultaneously achieve each gear and the related spur gear, the three files being saved distinctly in the same location.

· Inventor allows three-dimensional modelling starting from the very accurate strength calculations.

· Inventor generates the conjugated gears depending on the choice made by the designer on the calculation method chosen.

· Inventor has a library of materials that can be improved by adding new materials in accordance with the standards of each country.

· Inventor solves the problem of the relative positioning of the gears in compliance with the conditions of the tangent at flank level.

· Inventor is particularly different from the technical viewpoint, in that it meets graphically the aspects arising from the calculations.

· Inventor allows viewing the name of material together with the mechanical properties of each material.

References

6. REFERENCES

[1] Filipoiu, I.D., Tudor A. (2001). Proiectarea Transmisiilor Mecanice, Publisher BREN, ISBN 973-8143-26-8, Bucharest.

[2] Siemens PLM Software, available at: http://plm.automation.siemens.com/en_us/, Accessed by. 2014.07.08.

[3] Goanta, A.M. (2014). Siemens PLM Solution Applied to the Design of Agriculture Facilities and Equipment, The Annals of ''Dunarea de Jos'' University of Galati, Fascicle XIV, Mechanical Engineering, Issue Vol. 1, pp. 43-46, ISSN 12245615.

[4] Stancescu, C-tin. (2014). Modelare parametrica si adaptiva cu Inventor, Publisher Fast, ISBN 978973-86798-8-7, Bucharest.

[5] Ion, E.E., Haraga, G., Ion, G.C. (2005). The drawing of the Object in perspective using one, two and three vanishing points, Proceedings of International Conference on Engineering Graphics and Design, pp.355-358, ISBN 973-648-471-8, Bucharest, 06. 2005, Publisher BREN, Bucharest.