Dynamic feed axis with ball screw drive

Transcription

1 Dynamic feed axis with ball screw drive Thorsten Frank and Erich Lunz INA reprint September 1997

2 Dynamic feed axis with ball screw drive Dipl.-Ing. Thorsten Frank and Erich Lunz This paper describes the design and development of a Dynamic feed axis with ball screw drive and the potential for innovative drive mechanisms for machine tool axes based on proven technology. The system comprises a ball screw spindle rigidly located at both ends and a driven nut. The design features and operating characteristics achieved with the test device are described. 1 Introduction In recent years, the German machine construction sector has been affected by strong international competition [1]. As a result, companies in the NC machine tool sector are under constant pressure to increase both the productivity and machining quality of their machines. In addition to reduced costs in the production process, new machine concepts are required which allow flexible and cost-effective handling and interlinking functions. Increasing both the machining speed and machining quality puts high demands on the dynamic and thermal behaviour of the feed systems of modern machines. This trend is supported by considerable progress in the development of control systems and dynamic drive motors [2]. In contrast, the mechanical components in the drive are often the weakest link when seeking improvements in dynamic behaviour. The classical NC axis with a locating/non-locating bearing arrangement is currently limited to a speed range of 30 to 60 m/min and acceleration up to 10 m/s2 [3]. One possibility for overcoming these technical limitations is the linear drive, whose particular features offer a clear increase in dynamic characteristics. This system converts electrical energy directly into motion without the use of mechanical elements. Test drives of this type have achieved acceleration values of 10 to 12 times acceleration due to gravity and speeds up to 100 m/min [4]. In contrast to these general benefits, however, drives of this type have lower dynamic rigidity than electromechanical feed systems, greater thermal sensitivity to the machine structure, significant sensitivity to load parameter fluctuations and, in particular, significantly higher system costs; the linear drive cannot be seen as the ideal solution for all applications [3, 5]. It is therefore considered mainly for light machining applications. It is not feasible for the manufacture of large, heavy components requiring high machining forces. 2

3 Legend: 1. Polymer concrete bed 2. Locating housing 3. Linear guidance system RUE 45, INA 4. Ball screw drive (40 x 40), length 2 m, Deutsche Star 5. Bearing housing with double row ball bearing ZKLF, INA 6. Motor flange and toothed belt drive 7. Table for SIEMENS synchronous servomotor (FT6) and AMK asynchronous servomotor (SKB 10) Figure 1 Overview of test device, Institute for Machine Tools and Production Sciences (wbk) of University of Karlsruhe Figure 3 An important component of the test device: the INA double row axial angular contact ball bearing ZKLF It is therefore desirable to develop a system which is competitive in dynamic terms and based on proven feed system technology. In a joint project promoted by the federal state of Baden-Württemberg, established machine tool manufacturers and component suppliers have worked with the Institute for Machine Tools and Production Sciences of the University of Karlsruhe to design and build an innovative feed kinematic system comprising a spindle rigidly located at both ends and a driven nut. 2 Structure of test device of Dynamic feed axis with driven nut In electromechanical feed axes, the rotary motion of the servomotor is converted by a ball screw drive to give linear table motion. The classical NC axis design comprises a driven feed spindle and a nut fixed rigidly to the table. The Dynamic feed axis with ball screw drive is a variant on this design: the spindle is rigidly clamped in two housings and the nut is driven by a servomotor (Figure 1). The investigation considered two drive variants. Figure 2 shows (left) a standard servomotor driving the nut via a toothed belt and (right) a direct drive system with a hollow shaft servomotor. In both cases, the dynamic motors of the latest design provide 100 Nm for table acceleration and achieve speeds up to 4000 min 1. Furthermore, the table mass can be increased in steps up to half a tonne. Digital control systems with a linear scale for positional feedback are used. In order to achieve a system with high dynamic characteristics, the complete rotary drive system was designed in order to achieve a compact structure which would minimize the moments of inertia (Figure 2). The bearing arrangement was based on the INA double row axial angular contact ball bearing ZKLF (Figure 3) [7], giving rigid connection of the nut with the table. In order to allow accurate comparison of the two drive variants, the same bearing was fitted in the hollow shaft servomotor. Despite the mass of the servomotor to be moved, the total moment of inertia of the prototype with a 2 metre spindle is identical to that of a driven spindle the same length. For a longer spindle, the inertia gives an even more favourable situation. Figure 2 Drive variants: standard servomotor / toothed belt (left), direct drive with hollow shaft servomotor (right) 3

4 50 Axial rigidity, c a [N/µm] Rigidity of locating/non-locating arrangement, c a1 Rigidity of locating/locating arrangement, c a2 Rigidity ratio between locating/non-locating and locating/locating arrangements *100 [%] Rigidity ratio, c a1 /c a2 Acceleration**, a [m/s 2 ] Servomotor SIEMENS FT-6 with ball screw drive (d=40mm, p=40mm) Servomotor SIEMENS FT-6 with ball screw drive (d=40mm, p=20mm) * Rotary and translational masses ** Based on maximum motor torque of 120 Nm 0 0,00 0,25 0,50 0,75 1,00 1,25 1,50 1,75 2,00 Spindle nut position, l s [m] Moving mass*, m t [kg] Figure 4 Rigidity of location/non-locating and locating/locating spindle arrangement as a function of axis position with 2 m spindle length Figure 5 Calculated acceleration of dynamic feed axis at maximum torque 3 Increased productivity through new kinematic system The characteristics of the new feed kinematic system are well matched to the high motor performance. Since the spindle is located rigidly at both ends, the axial rigidity of the complete system far exceeds that of a spindle with a locating/ non-locating bearing arrangement. Figure 4 shows the relationship between axial rigidity and spindle nut position. While the axial rigidity of a conventional spindle is lowest at the non-locating end, the rigidity with a locating/locating arrangement is lowest in the centre of the traverse range, minimizing the rigidity variation over the whole length. In terms of maximum traverse speed, the driven nut gives clear advantages since it avoids the problem of critical whirling speed. Frequency analyses of the test device indicate that the lower axial natural frequency of the driven system is over 120 Hz. Even at high drive speeds, the system always runs in the subcritical range. The traverse speed is therefore limited only by the maximum rolling speed of the screw drive. The screw drives in the tests were of 40 mm diameter and 40 mm pitch, giving a traverse speed of 120 m/min at the maximum possible speed of 3000 min 1. For a table mass of 100 kg, acceleration of 30 m/s 2 was achieved; no attempt was made to reach the theoretical limit of 45 m/s 2, in order to preserve the operating life of the drive (Figure 5). Increasing the productivity of a machine tool significantly increases the thermal load, since almost all the energy is converted into heat [8, 9]. The ongoing increase in speed and velocity gives a continuous increase in drive system temperature. The measurement curves in Figure 6 show the temperature equilibrium of the feed axes for traverse speeds between 40 and 100 m/min. The highest temperatures occur in the area of the nut and spindle, while significantly lower temperatures occur in the area of the INA bearing arrangement, motor and guidance systems. Similar values are obtained for the direct drive and indirect drive variants. When the table mass is increased to the 600 kg maximum, the temperature values increase by 10 %. The influence of the speed on spindle elongation is shown in Figure 6. Thermal expansion of the spindle due to heating causes a large length change in the spindle at high speeds. Pretensioning the spindle to compensate for this expansion would require forces of about 60 to 100 kn and is not therefore feasible. 4

5 Geschwindigkeit, v [m/min] Temperature, T [ C] Expansion, δ [µm] 1000 Nut temperature in direct drive Nut temperature in indirect drive Spindle temperature in indirect drive External INA bearing arrangement in indirect drive Spindle elongation of indirect drive 30 Travel distance: 0,84 m 200 Acceleration: 15 m/s Table mass: 100 kg Measurement time: 120 min Ground spindle 40x Speed, n [min 1] Figure 6 Temperature and expansion curves at characteristic points of feed axis with table mass m1 = 100 kg In order to achieve constant operating behaviour of the ball screw drive, a preload adjustment device is required which can compensate for thermal expansion by briefly opening a simple clamping device without reducing the clamping location rigidity. A further possibility for a rigidly tensioned spindle is the use of a hollow spindle with internal cooling by an agent. Variations in torsional rigidity can effectively be ignored and, due to the stationary spindle, no eccentric mass effect should be assumed. The spindle coolant circuit can be integrated in the machine coolant system. Long term studies [10] showed a reduction in differential spindle temperature from 32 to 4 K. Figure 7 Investigation of dynamic behaviour of direct drives 3.1 Positional accuracy with the aid of dynamic drive investigations In addition to high acceleration and traverse speed, the development programme also has the objective of optimized transmission behaviour of the feed kinematic system with positional control (Figure 7). In order to give the required motion, the motion axis must conform to the specified positional guidance values with the minimum possible distortion. In an actual system, this can only be achieved in approximate terms due to the intrinsic dynamics of the positional control with its energy conversion system and mechanical components. The objective is therefore to achieve system behaviour which allows transmission of high acceleration forces to the feed kinematic system without causing significant mechanical vibrations. This is a basic precondition for achieving adequate path accuracy even at high feed rates [11, 12]. In order to determine the transmission behaviour, the skip response of the speed control circuit was recorded on the table using an acceleration sensor. The table mass can be increased by steps up to 600 kg. Figure 8 (top of page 6) shows the system response to a disruption due to a nominal value skip from 0 to 10 m/min, corresponding for example to forces in milling or turning with interrupted cut. The ripple times of the direct drive are about 80 m/s and therefore in an acceptable range for machine tools. Increasing the table mass from 100 kg to 600 kg does not lead to an increase in times. The indirect drive gives even shorter ripple times. The excellent dynamic behaviour of the system can be attributed to the fact that the transmitted torques of the driven nut are absorbed on both sides of the spindle. Since the spindle is clamped at both ends, this reduces the torsional tendency of the spindle. The high axial rigidity of the system allows the motor torque to be converted directly into axial acceleration without excessive energy storage by spindle torsion. Furthermore, the system has good damping with maximum values of about 0,04 in the central area of the spindle travel. 5

6 20-640,000 Acceleration, a [m/s 2 ] Ripple curve with table mass of 100 kg Ripple curve with table mass of 600 kg Position, X [mm] -645, ,000 Actual position Nominal position Time, t [ms] Disruption behavior [nominal value skip v = 10 m/min] 0,30 0,33 0,35 0,38 0,40 0,43 0,45 0,48 0,50 0,53 Time, t [s] Accurate stop [v = 100 m/min] Figure 8 Skip response of direct drive Figure 9 Positional behavior at a (NC) = 15 m/s2, K v = 4000/min, table mass of 410 kg and a travel distance of 400 mm For a defined positioning process, this means a significant reduction in travel time without a reduction in accuracy. This is shown in Figure 9 by a positional skip with accurate stop for a distance of 400 mm and a speed of 100 m/min. These characteristics are also favoured by the significant increase in the natural frequencies of the complete system compared to conventional axes. With a single frequency of 170 Hz, the direct drive shows clear single-mass vibration behaviour. The axial dynamic rigidity of the preloaded spindle is almost independent of the table position. Single-mass vibration behaviour offers good controllability of the axis, which can be further improved by use of an appropriate control system. In addition to its high axial natural spindle frequency of 130 khz in the lower range, the indirect drive has other natural frequencies which can be attributed to the toothed belt. This frequency can be favourably influenced by adjusting the preload of the toothed belt or the rigidity of the vibration chain. The dynamic drive investigations showed that, due to the rigid arrangement, the new NC motion axis type has good dynamic behaviour, not only for small table masses but also for larger masses, one of the main weak points of the linear motor. 4 Increased speeds through modified bearing technology 4.1 Bearing arrangement of feed spindle nut The improved dynamics of the whole drive system obviously requires further development of the individual components. The bearing arrangement of the spindle and nut, comprising a preloaded, double row axial angular contact ball bearing has been further developed to give a clear increase in performance of these proven bearings. In particular, the individual components of the bearing have been modified to allow higher speeds. Production of the raceways was improved by modern manufacturing technology, and considerable progress was made by the use of lubricant compositions matched to the specific load spectrums. Comprehensive tests at INA have confirmed this progress, with the result that a dimensionally identical bearing can now be offered with no increase in costs giving a 30% increase in speed (Figure 10). While the conventional bearing arrangement met the requirements of the test device and the tests, it was known to be close to the upper limits of the performance range. The traverse speed of 120 m/min and the 40 mm ball screw lead require a bearing speed of 3000 min 1. This speed was beyond the limiting speed n G of 2400 min 1 given in the catalogue for the bearing ZKLF Z then fitted in the test device. It was found that this speed limit could be increased to 3000 min 1 with the modified bearing. Where the traverse speed of 120 m/min was previously outside the permissible limiting speed, the new bearing is within the permissible limit values. 4.2 Hybrid bearings The use of ceramic balls as mentioned earlier has also been brought to fruition. On INA test rigs, a hybrid bearing comprising steel rings and ceramic rolling elements showed large increases in speed, reaching speeds of 6500 min 1 without difficulty. The significantly lower thermal expansion coefficient of 2,9 µm/m/k of the ceramic balls is of positive benefit here. If the steel rings (inner ring and larger outer ring) expand by a certain amount due to increasing temperature, the ceramic balls expand by a smaller amount. This reduces the preload in the bearing, reducing the specific rolling element load and the friction; from a bearing technology viewpoint, a traverse speed of 260 m/min could be achieved with a 40 mm pitch ball screw. This seems far-fetched of course, but it is clear that there are no obstacles to further progress in dynamic feed axes with ball screw drive and that bearing components with an appropriate level of bearing technology are already available. 6

7 Hybrid special bearing with ceramic balls 250% Modified standard bearing Conventional standard bearing 100% 130% Figure 10 Bearing arrangement of feed spindle nut with double direction axial angular contact ball bearing ZKLF. Speed increase with modified bearing = 130 %, with hybrid special bearing with ceramic balls = 250 %. Fitting example according to wbk design. 5 Summary and prospects The feed system developed at the Institute for Machine Tools and Production Sciences opens up new prospects for the development of new generations of dynamic machines based on the proven ball screw drive technology. With traverse speeds up to 120 m/min and acceleration up to 30 m/s 2, this technology represents an alternative to linear drives in terms of equivalent performance capacity. Furthermore, this dynamic behaviour is not limited to small table masses but is particularly suitable for large table masses. It can therefore be installed in existing machine designs with relatively little additional work. The continuing development of the INA bearing arrangement and, perhaps most important, the development of a ball screw drive with a high speed rotating nut promises significant performance increases for this design principle in the future. Literature [1] Weule, H.: Die Bedeutung der Produktentwicklung für den Industriestandort Deutschland, VDI Entwicklung, Konstruktion, Vertrieb, Jahresbericht 1997, Düsseldorf [2] Schmitt, Thomas: Modell der Wärmeübertragungsvorgänge in der mechanischen Struktur von CNC-gesteuerten Vorschubsystemen., Dissertation. TH Darmstatt, 1995 [3] Pritschow, G.; Fahrbach, C.; Scholich-Tessmann, W.: Elektrische Direktantriebe im Werkzeugmaschinenbau, VDI-Z 137 (1995) Nr. 3/4, S [4] Rehsteiner, F.; Zirn, O.: Schnelle Vorschub-Antriebssysteme an Werkzeugmaschinen, Werkstatt und Betrieb 128 (1995), S , Hanser-Verlag [5] Hopper, E.: Linearmotoren synchron oder asynchron, Antriebstechnik 33 (1994) Nr. 6, S [6] Ebert, Jürgen: INA Wälzlager Schaeffler ohg: The tensioning of feed spindles supported by rolling bearings; Der Konstrukteur; ASB/95; Seite [INA-95] [7] INA Wälzlager Schaeffler ohg: Bearings for screw drives, Herzogenaurach [INA 92] [8] Fischer, H.: Beitrag zur Untersuchung des thermischen Verhaltens von Bohrund Fräsmaschinen; Dissertation, TU Berlin 1970 [9] Kersten, A.: Geometrisches Verhalten von Werkzeugmaschinen unter thermischer und statischer Last; Dissertation, TH Aachen 1983 [10] N.N.: Präzisionsmaschinenelemente, Linearbewegungstechnik; NSK Corporation, Pr.No. GK PME [11] Zirn, O.; Weikert, S.; Rehsteiner F.: Design and Optimization of Fast Axis Feed Drives Using Nonlinear Stability Analysis, Annals of Chirp Vol.45/1/1996; Seite [12] Pritschow, G.: Zum Einfluß der Geschwindigkeitsverstärkung auf die dynamische Bahnabweichung, wt-produktion und Management 86: 1996, S About the authors: Dipl.-Ing. Thorsten Frank is a scientist at the Institute for Machine Tools and Production Sciences (wbk) of the University of Karlsruhe. Erich Lunz is an application engineer in the Industrial Sector Management for Machine Tools of INA Wälzlager Schaeffler ohg, Herzogenaurach (Germany) 7

8 INA Wälzlager Schaeffler ohg D Herzogenaurach Telephone ( ) 82-0 Fax ( ) Art.Nr /HVK GB-D Printed in Germany