Dynamic and thermal analysis of the feed drive system.

Varban, Ramona Cristina ; Parpala, Radu Constantin ; Predincea, Nicolae 等

1. INTRODUCTION

In a feed drive system, the ball screw plays an important role as a power transmission unit and a linear scale. When a large speed of motion is demanded, the rate of feed of the ball screw transmission is much increased. Much heat is produced at both ends of the bearing support and the nut because of a greater speed of rotation (Kim & Cho, April 1997).

The accumulated heat causes the temperature to rise in these areas. Then the ball screw deforms thermally and has a seriously negative effect on machine accuracy. Several types of analysis must be performed for a complete study of machine tools behavior. The structural behavior under thermal and dynamic loads is evaluated in order to obtain an optimized design of the feed drive.

Because of the impact that feed drives imply over the finite piece quality the design demands are very high so accurate analysis must be performed in order to assure a very good behavior of the whole machine tools.

2. COMPUTER AIDED DESIGN

One of the first steps in virtual prototyping is building the CAD model. The 3D model of the feed drive must be designed in order to be accepted as input by various software suits that will be used for further analyses (Altintas et al., 2005).

During the design phase simplified simulation models are used to estimate the impact of design parameters over the machine performance. These simplified models are also used in order to improve the time needed for calculation and to eliminate the computing errors. It is well known that a very complex model can generate errors during the FEM analysis; because of this many features from the 3D model are eliminated. Also some details that are essential for FEM analysis are not so important for Kinematical analysis and must be ignored. A CAD model must be easily redesigned so it's important to use parameters to define all the key elements of the model. The 3D model of the feed drive was designed by using the CATIA V5 CAD software mainly because of his good integration with the ANSYS software which was used for statically and dynamical FEM analyses.

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All 3D part where fully parameterized in order to optimize needed parameters in FEM analysis.

In order to generate all the contacts between surfaces it's necessary to correctly design the 3D assembly, it's also very usefully to check all the clashes and clearances within the CAD environment. Using the information provided by the CAD software we can set the correct tolerances for the automatic contact generation.

The CATIA software provides some very usefully tools for space analysis. In fig. 2 it's an example of a misplaced component. In this case the error is detectable by a visual inspection of the model. Because we need to use the 3D model for further analyses it's a good practice to define also material properties by using predefined model from the CATIA library or by defining new materials. By defining material properties we can also check important aspects of the assembly like volume, mass, moment of inertia.

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3. ANSYS FEM ANALYSIS

3.1. Dynamic analysis for the feed drive system

The dynamic response of a machine, structure, or system can be determined by superposing it's natural modes of vibration when the amplitudes of motion are small. Thus a complete dynamic description of the machine requires the determination of the modal frequencies, mode shapes, and the system parameters-equivalent mass, stiffness, and damping ratio. The procedure determining this information of a system is called Modal Analysis (Zaeh & Oertli, 2004).

The Finite Element Method (FEM) is widely used to perform a Modal Analysis. FEM is extremely useful for complicated devices and structures with unusual geometric shapes. The frequencies that are calculated by the program (Table 1) can be further used for other verifications.

3.2. Thermal analysis for the feed drive system

The thermo-elastic machine behavior, i.e. the load dependent temperature distribution and the resulting deformation of the machine tool, is influenced by a variety of constructional and thermotechnical parameters (Schmitt, 1996).

The legend of figure 4: 1--uniaxial feed drive system; 2 table; 3--servomotor; 4--ballscrew; 5--thermocouple; 6 displacement gauge; 7--data processing software; 8--data collection system.

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The objectives of the experiments were to establish a system to measure the ball screw transmission system thermal errors, to determine a suitable model for the thermal error data, and to predict (and to compensate) thermal error according to the determined model.

The thermocouples were attached to positions Tb T2 and T3 as shown in fig. 4 to measure temperature increases of the front bearing, nut and rear bearing as key heat sources, respectively. The fourth thermocouple was used to measure the environmental temperature (Predincea et al., 28-30 May 1998).

The variation of temperature rises of positions T2 and the thermal error is shown in fig. 5.

4. CONCLUSION

By using ANSYS Workbench one is able to optimize the design process by changing one or more of the initial parameters; those parameters are automatically updated into the CATIA 3D CAD model. By analyzing the calculation result in the post-processing program the designers can evaluate the machine properties during the design stage.

Today the main problem in checking structures consists in importing and preprocessing the CAD model. It is well known that the geometry of the model can dramatically change FEM results. The multiple regression analysis is adequate to predict the ball screw thermal errors with variation of table speed and temperature history. The predicted thermal error data can be used to correct the error with a suitable numerical control route.

The principle and the method used in this work can be used to solve a system of multiaxis feed drives.

5. REFERENCES

Altintas, Y.; Brecher C.; Weck M. & Witt S. (2005). Virtual machine tool, Annals of the CIRP, 54/2: 651-669.

Kim, S.K. & Cho, D.W. (April 1997). Real-time estimation of temperature distribution in a ball-screw system, International Journal of Machine Tools and manufacture, Vol. 37, No. 4, pp. 451-464.

Predincea, N.; Pupaza, C. & Toma, O. (28-30 May 1998). Analiza deformatiilor termice ale mecanismului surubpiulita, (Analysis of thermal deformations of ball-screw system), The VIII-th Conference of Managerial and Technological Engineering, pp. 499-506, Timisoara, Romania.

Schmitt, T. (1996). Modell der Warmeubertra-gungsvorgange in der mechanischen Struktur von CNC-gesteuerten Vorschubsystemen, (Model of the heat transfer in the mechanical structure of a CNC feed drive system), ISBN 38265-1476-9.

Zaeh, M. & Oertli, Th. (2004). Finite Element Modeling of Ball Screw Feed Drive Systems, Annals of the CIRP, 53/1: 289-292.

Table 1. Natural frequency.

Mode Frequency

 1 22,29 Hz
 2 22,3 Hz
 3 61,41 Hz
 4 61,43 Hz
 5 120,18 Hz
 6 120,23 Hz
 7 198,15 Hz
 8 198,23 Hz
 9 294,94 Hz
 10 295,07 Hz