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Double nut ball screw with improved operating characteristics
G Model
CIRP-1239; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Double nut ball screw with improved operating characteristics A. Verl (2) a b
a,b,
*, S. Frey a, T. Heinze a
Institute for Control Engineering of Machine Tools and Manufacturing Units (ISW) – University of Stuttgart, Germany Frauenhofer Institute for Manufacturing Engineering and Automation (IPA) – Stuttgart, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Ball screw Design optimization Preloading
The performance and availability of production machines is strongly affected by the operating characteristics of the installed ball screws. In this context, the value of preloading is an important property greatly influencing the quality of the feed motion as well as the expected service life. Dynamic applications often require ball screws with high preloading values. As a result, such systems typically suffer from high friction values with the involved wear and heat generation. This paper presents a novel design principle for ball screws, allowing for a considerable reduction of the preloading and hence an overall improvement of the operating characteristics. ß 2014 CIRP.
1. Introduction
Next to these, numerous adaptronic approaches have been presented in literature [6–8], in which additional actuator-sensorsystems allow for an acquisition and adjustment of the preloading according to the actual requirements. Most of the solutions developed, however, are still quite cost-intensive and with certain restrictions for the application. On this account, adaptronic ball screw systems are not used in industrial applications up until today. Within this paper, a novel design principle for high performance ball screws is presented. By means of an appropriate ball screw nut design and the use of additional spring elements, the value of preloading can be reduced for a large number of applications, leading to an overall improvement of the operating characteristics.
Ball screws are the components most frequently used for transforming rotational into linear motion. The operating characteristics and the availability of this component thereby have a decisive impact on the productivity of modern production machines. The term operating characteristics in this case refers to all properties affecting the motion quality and the efficiency of a ball screw. Next to accuracy and uniformity of the feed motion, issues such as wear, heat generation and thermal stability have a dominant impact on the quality of the feed motion and the attainable service life accordingly. One fundamental parameter in this context is the value of preloading [1]. In order to eliminate backlash and provide the required rigidity for dynamic processes, most applications in the field of production technology use preloaded ball screws. For high demanding applications, such as in feed drives for machine tools, typically ball screws with 2-point-contact are being used [2]. While the induced pretension eliminates backlash and increases the rigidity of the component, the friction characteristics change significantly with the value of preloading: bore and sliding friction accumulate, the uniformity of the motion diminishes, and wear and heat generation increase, compromising the quality of the feed motion as well as the attainable service life. In order to overcome this inherent trade-off between rigidity and friction characteristics, numerous approaches have been presented in literature. Amongst others, new design principles with improved lead geometry and increased contact angles [3] have been proposed. Others have used ceramic balls or PVD coatings [4] to reduce friction and improve the operating characteristics of ball screws. In addition to that, most ball screw manufacturers provide forced cooling options for the shaft and/or the ball screw nut in order to cope with thermal issues as discussed in [5].
* Corresponding author.
1.1. Preloading value A graphical tool that gives insight into the conditions of a double nut system at different load conditions is the forcedisplacement diagram depicted in Fig. 1 [9]. The two curves represent the prevailing load on each half of the double nut plotted against the axial displacement of the flange. In a first instance, the preload value is mechanically set as part of the assembling process. For a double nut system, this is usually achieved by inserting a spacer between the two halves of the nut or by simply tightening the nuts against each other. In this manner, the clearance between balls and tracks is eliminated and a 2-pointcontact is established. The actual value of preloading determines the axial stiffness of the ball screw: the higher the value of preloading, the greater the stiffness of the component. According to the theory of Hertzian contact stress, the relationship between internal preloading force FP and the resulting stiffness of the ball screw nut CN can be described as follows: CN
1 2 F : 3 P
(1)
The total stiffness of a ball screw assembly, however, does not exclusively depend on the rigidity of the ball screw nut. Instead, it
http://dx.doi.org/10.1016/j.cirp.2014.03.128 0007-8506/ß 2014 CIRP.
Please cite this article in press as: Verl A, et al. Double nut ball screw with improved operating characteristics. CIRP Annals Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.128
G Model
CIRP-1239; No. of Pages 4 A. Verl et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
2
Extended load span
Normal load span
Force in N
Left Nut
Extended load span
Right Nut
In particular dynamic applications with large acceleration forces often require high preloading values. As a result, the ball screw is operated at high preloading throughout the entire operation, even though this additional tension is only needed for a short period of time, e.g. during acceleration. The high preloading values commonly used in the field of production machines, cause a significant additional stress to the component, greatly affecting the efficiency and the quality of the feed motion.
State of the art Prototype
2. Functional principle and prototype
Preload
Point of decompression
Relative displacement in µm
Fig. 1. Force-displacement of left and right nut within a preloaded double nut system.
The driving question of the presented design optimization was how to bypass axial load peaks into a mechanism that enables the system to run with moderate preloading values, hence avoiding excessive contact stress and thereby improving the operating characteristics. The idea lead to a system which remains unchanged to the state of the art double nut design, however, with an additional mechanism to prevent from overload. Fig. 3 shows the functional principal of the designed double nut for different load conditions: under moderate load conditions, e.g. during the machining process, the system behaves like a standard double nut with the known contact properties, operation characteristics and rigidity values.
Fig. 3. Novel double-nut design under moderate and extensive axial load.
Fig. 2. Single and combined rigidity of a ball screw in axial direction.
is defined by the serial combination of the individual components: stiffness of the nut CN, stiffness of the shaft CS and stiffness of the supporting bearing CB (see Fig. 2). The data plotted in Fig. 2 corresponds to the actual values of a mid-sized feed drive for machine tools, also used for further examinations. As illustrated in Fig. 2, increasing the value of preloading above a certain level has little impact on the overall effective stiffness. The axial stiffness of a typical ball screw assembly is primarily defined by the geometry of the ball screw shaft [10]. Hence, for most applications, the rigidity of the ball screw nut is of secondary importance when choosing the value of preloading. For a large number of applications, the relevant boundary condition is maintaining a certain contact pressure between the balls and the tracks throughout all operating states. If an axial load is applied to the ball screw, the load distribution within the nut changes. Considering the force-displacement diagram in Fig. 1, the contact pressure between balls and tracks on the one side of the nut increases, while on the other side the contact pressure is reduced. If the axial load reaches the level of approximately 2.83 times the preloading value, one side of the nut passes the point of decompression [11]. In this state, the preloading is fully ceased and the balls roll in an undefined manner. This is an undesirable mode of operation, which causes a sudden change in the contact conditions, possible disruptions of the oil film and an excessive increase in wear. In order to avoid such a mode of operation, the preloading value is usually set to a level which can cope with the axial loads while still maintaining a certain contact pressure on the balls.
In case of an extensive axial load, the decompressed part of the double nut goes into a free-floating state in which an additional spring mechanism sustains the load compression required for a proper rolling condition. This guarantees a defined contact between the balls and the track, independent of the applied axial load. Note that the actual load is constantly supported by the opposing and preloaded part of the ball screw nut. This guarantees high rigidity values in both directions and a constant symmetrical behaviour throughout the entire mode of operation. The corresponding characteristic of the new double nut design is also illustrated in Fig. 1. Based on the basic functional principle [12] a proper design for an actual prototype has been developed and realized. Fig. 4 shows the assembled prototype already installed on a feed drive test bench.
Fig. 4. Prototype of a double nut ball screw with improved operating characteristics.
Please cite this article in press as: Verl A, et al. Double nut ball screw with improved operating characteristics. CIRP Annals Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.128
G Model
CIRP-1239; No. of Pages 4 A. Verl et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
3
Table 1 Specification of the selected ball screw. Description
Value
Nominal diameter ball screw Ball screw lead Nominal ball diameter Number of ball circles Dynamic load capacity Nominal rigidity at 4400 N preloading
40 mm 20 mm 6 mm 24 44,000 N 800 N/mm
The developed prototype represents a technical implementation of the functional principle on an industrial ball screw. The overload mechanism is realized using specially designed functional elements which incorporate numerous tasks:
Provide the spring force during overload. Enable an axial displacement between nut and shaft. Provide the possibility to initially adjust the preloading. Maintain the initial preloading value after it has been set.
The individual functional elements are equally arranged in grooves around the circumference of the ball screw nut which leads to minimal restrictions for the component mounting. The system is designed for versatile examinations. An additional sensor system, properly integrated into the ball screw nut, provides the possibility to measure the actual load conditions during operation. Table 1 states the most relevant parameters of the ball screw used for the technical implementation.
3. Experimental setup and measurement results In the course of the examination, the developed ball screw is compared to a state of the art double nut ball screw with identical dimensions and specifications. Fig. 5 illustrates the experimental setup used for the further measurements.
Fig. 5. Experimental setup.
The ball screw is integrated into a feed drive test bench and used to move a machine table with variable load mass. A highly rigid piezoelectric sensor platform, installed between the flange and the nuts of the ball screw, allows for a measurement of the axial load during operation. The load measurement is performed for each nut separately, while a linear encoder detects the displacement of the machine table. Furthermore, an additional linear direct drive, firmly attached to the table, provides the possibility to induce axial load forces of up to 14 kN onto the ball screw. With the experimental setup depicted in Fig. 5, numerous static and dynamic measurements have been performed. In a first step, a static load was applied to the state of the art system as well as to the newly designed ball screw. The experimental setup provides
Fig. 6. Measured force-displacement diagram for a state of the art double nut and the developed prototype.
the possibility to actually verify the force-displacement diagram derived from theory with experimental data. Fig. 6 exemplarily shows a measured force-displacement diagram for the state of the art ball screw without functional elements as well as for the developed prototype. According to common practice, the value of preloading for the state of the art double nut was set to approximately 10% of the dynamic load capacity. For the newly designed prototype, the overload state and the associated loss of pressure between the balls and the track is far less critical. Therefore, a considerable lower preloading value of about 1800 N was selected which, in accordance to Fig. 2, still provides the desired rigidity of the ball screw assembly. With the applied static axial load, the state of the art ball screw reaches the point of decompression at about 11 kN. This roughly corresponds to the predicted value of 2.83 FP. In the same manner, the force-displacement diagram of the new double nut design has been measured. The functional elements were designed to provide a spring force of 500 N in case of an overload situation. The measurement results in Fig. 6 clearly state, how the progression of the force-displacement curve is shaped by the additional spring mechanism. Independent of the axial load applied, the preloading level maintains at about 500 N and the point of decompression is never reached. Considering the significant reduction of preloading with almost identical rigidity and even better overload condition, the newly developed prototype provides much better operating characteristics than the state of the art system. The static load analysis verifies the functional principle of the ball screw design and furthermore confirms the internal load distribution generally derived from theory. In a next step, the prototype was exposed to dynamic load scenarios and the results were compared to those of the state of the art ball screw. For this purpose, high-speed motion tests with different load mass and operating conditions were performed. First of all, the state of the art design was exposed to acceleration tests to verify whether the point of decompression is actually reached during normal operation. In order to compare the results, the value of preloading was set to about 2000 N for both ball screw configurations. Fig. 7 shows the force-over-time plot for a highspeed motion trajectory with an acceleration of 10 m/s2 and a feed rate of 50 m/min. Starting from the same preloading level, it can clearly be seen, that the compression within the state of the art design ceases fully during acceleration, leading to the already mentioned critical mode of operation. The new double nut design, on the other hand, transmits the high acceleration forces while still maintaining a minimum load on the opposing side of the ball screw nut. This guarantees defined contact conditions between balls and tracks and a proper rolling characteristic of the ball screw throughout the entire operation. In order to maintain a certain pressure on all balls and avoid a complete loss of contact during acceleration, the state of the art
Please cite this article in press as: Verl A, et al. Double nut ball screw with improved operating characteristics. CIRP Annals Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.128
G Model
CIRP-1239; No. of Pages 4 A. Verl et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
4
significant reduction of preloading for numerous applications. In principle, different competing issues concerning the level of ball screw preloading are discussed and guidelines for a proper selection are derived. It is shown, that in many applications ball screws with 2-point-contact are operated with an exaggerated level of preloading most of the time, leading to excessive friction, wear and heat generation. In order to overcome this limitation and improve the operating characteristics of ball screws for high demanding applications, a new design is presented. The functional principle is explained and a first prototype is introduced. Experimental results of static and dynamic operations with the new design are presented along with measurements of a state of the art ball screw. The conducted examinations prove the functionality of the proposed design and demonstrate the effectiveness of the new double nut ball screw, in particular for dynamic applications. In further studies, the mechanical design will be optimized and adapted for long-term tests in industrial applications. Acknowledgement The authors would like to thank the August Steinmeyer GmbH & Co. KG for their support of this project.
Fig. 7. Measured force within the ball screw during high speed motion testing.
ball screw requires much larger values for the preloading. The conducted measurements have shown that for the given motion profile, a preloading value of roughly 4000 N is needed. This again has a dominant impact on the overall effective load and the operating characteristics of a ball screw. Comparing the newly developed double nut system with the state of the art ball screw, the reduction of preloading leads to minimized stress on the components, lower friction values, reduced wear and heat generation and all together to a significant improvement of the efficiency and the motion characteristics of the ball screw. The developed double nut design is in particular suitable for dynamic applications, where high acceleration forces determine the value of preloading and substantially affect the overall load. 4. Conclusion and outlook In this paper, a conceptual ball screw nut design with improved operating characteristics is presented. The key aspect of the new design is a passive overload mechanism, which allows for a
References [1] Verl A, Frey S (2010) Correlation Between Feed Velocity and Preloading in Ball Screw Drives. Annals of CIRP 59(1):429–432. [2] Frey S, Walther M, Verl A (2010) Periodic Variation of Preloading in Ball Screws. Production Engineering 4(2):261–267. [3] Weule H, Dambacher U, Mussa S (2001) Neue Gewindegeometrie fu¨r den Kugelgewindetrieb. Wt-Online 8:519–522. [4] Spath D, Rosum J, Haberkern A (1995) Kinematics, Frictional Characteristics and Wear Reduction by PVD Coating on Ball Screw Drives. Annals of the CIRP 44(1):349–352. [5] Mayr J, Jedrzejewski J, et al (2012) Thermal Issues in Machine Tools. Annals of CIRP 61(2):771–791. [6] Golz HU (1990) Analyse, Modellbildung und Optimierung des Betriebsverhaltens von Kugelgewindetrieben, (Dissertation) WBK, Karlsruhe104–115. [7] Pritschow G, Verl A, Frey S (2010) Adaption der Vorspannung in Kugelgewindetrieben, Adaptronik in Werkzeugmaschinen: 978-3-8322-9809-8207–225. [8] Fleischer J, Herder S (2012) Adaptronic Ball Screw for the Enhancement of Machine Precision. Procedia CIRP 1:621–626. [9] Spieß D (1970) Das Steifigkeits- und Reibungsverhalten Unterschiedlich Gestalteter Kugelschraubantriebe mit Vorgespannten und Nicht Vorgespannten Mutternsystemen, Dissertation, TU-Berlin61–73. [10] Frey S, Dadalau A, Verl A (2012) Expedient Modelling of Ball Screw Feed Drives. Production Engineering 6(2):205–211. [11] N N (2008) Ball Screws Part 4: Static Axial Rigidity, DIN ISO 3408-4, Beuth-Verlag, Berlin. [12] Frey, S., 2009, Kugelgewindemutter mit nachgiebigem Element im Kraftnebenschluss, Patentnr. DE 10 2009 049 936 A1/EP 2 312 184 A1.
Please cite this article in press as: Verl A, et al. Double nut ball screw with improved operating characteristics. CIRP Annals Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.128
CIRP-1239; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Double nut ball screw with improved operating characteristics A. Verl (2) a b
a,b,
*, S. Frey a, T. Heinze a
Institute for Control Engineering of Machine Tools and Manufacturing Units (ISW) – University of Stuttgart, Germany Frauenhofer Institute for Manufacturing Engineering and Automation (IPA) – Stuttgart, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Ball screw Design optimization Preloading
The performance and availability of production machines is strongly affected by the operating characteristics of the installed ball screws. In this context, the value of preloading is an important property greatly influencing the quality of the feed motion as well as the expected service life. Dynamic applications often require ball screws with high preloading values. As a result, such systems typically suffer from high friction values with the involved wear and heat generation. This paper presents a novel design principle for ball screws, allowing for a considerable reduction of the preloading and hence an overall improvement of the operating characteristics. ß 2014 CIRP.
1. Introduction
Next to these, numerous adaptronic approaches have been presented in literature [6–8], in which additional actuator-sensorsystems allow for an acquisition and adjustment of the preloading according to the actual requirements. Most of the solutions developed, however, are still quite cost-intensive and with certain restrictions for the application. On this account, adaptronic ball screw systems are not used in industrial applications up until today. Within this paper, a novel design principle for high performance ball screws is presented. By means of an appropriate ball screw nut design and the use of additional spring elements, the value of preloading can be reduced for a large number of applications, leading to an overall improvement of the operating characteristics.
Ball screws are the components most frequently used for transforming rotational into linear motion. The operating characteristics and the availability of this component thereby have a decisive impact on the productivity of modern production machines. The term operating characteristics in this case refers to all properties affecting the motion quality and the efficiency of a ball screw. Next to accuracy and uniformity of the feed motion, issues such as wear, heat generation and thermal stability have a dominant impact on the quality of the feed motion and the attainable service life accordingly. One fundamental parameter in this context is the value of preloading [1]. In order to eliminate backlash and provide the required rigidity for dynamic processes, most applications in the field of production technology use preloaded ball screws. For high demanding applications, such as in feed drives for machine tools, typically ball screws with 2-point-contact are being used [2]. While the induced pretension eliminates backlash and increases the rigidity of the component, the friction characteristics change significantly with the value of preloading: bore and sliding friction accumulate, the uniformity of the motion diminishes, and wear and heat generation increase, compromising the quality of the feed motion as well as the attainable service life. In order to overcome this inherent trade-off between rigidity and friction characteristics, numerous approaches have been presented in literature. Amongst others, new design principles with improved lead geometry and increased contact angles [3] have been proposed. Others have used ceramic balls or PVD coatings [4] to reduce friction and improve the operating characteristics of ball screws. In addition to that, most ball screw manufacturers provide forced cooling options for the shaft and/or the ball screw nut in order to cope with thermal issues as discussed in [5].
* Corresponding author.
1.1. Preloading value A graphical tool that gives insight into the conditions of a double nut system at different load conditions is the forcedisplacement diagram depicted in Fig. 1 [9]. The two curves represent the prevailing load on each half of the double nut plotted against the axial displacement of the flange. In a first instance, the preload value is mechanically set as part of the assembling process. For a double nut system, this is usually achieved by inserting a spacer between the two halves of the nut or by simply tightening the nuts against each other. In this manner, the clearance between balls and tracks is eliminated and a 2-pointcontact is established. The actual value of preloading determines the axial stiffness of the ball screw: the higher the value of preloading, the greater the stiffness of the component. According to the theory of Hertzian contact stress, the relationship between internal preloading force FP and the resulting stiffness of the ball screw nut CN can be described as follows: CN
1 2 F : 3 P
(1)
The total stiffness of a ball screw assembly, however, does not exclusively depend on the rigidity of the ball screw nut. Instead, it
http://dx.doi.org/10.1016/j.cirp.2014.03.128 0007-8506/ß 2014 CIRP.
Please cite this article in press as: Verl A, et al. Double nut ball screw with improved operating characteristics. CIRP Annals Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.128
G Model
CIRP-1239; No. of Pages 4 A. Verl et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
2
Extended load span
Normal load span
Force in N
Left Nut
Extended load span
Right Nut
In particular dynamic applications with large acceleration forces often require high preloading values. As a result, the ball screw is operated at high preloading throughout the entire operation, even though this additional tension is only needed for a short period of time, e.g. during acceleration. The high preloading values commonly used in the field of production machines, cause a significant additional stress to the component, greatly affecting the efficiency and the quality of the feed motion.
State of the art Prototype
2. Functional principle and prototype
Preload
Point of decompression
Relative displacement in µm
Fig. 1. Force-displacement of left and right nut within a preloaded double nut system.
The driving question of the presented design optimization was how to bypass axial load peaks into a mechanism that enables the system to run with moderate preloading values, hence avoiding excessive contact stress and thereby improving the operating characteristics. The idea lead to a system which remains unchanged to the state of the art double nut design, however, with an additional mechanism to prevent from overload. Fig. 3 shows the functional principal of the designed double nut for different load conditions: under moderate load conditions, e.g. during the machining process, the system behaves like a standard double nut with the known contact properties, operation characteristics and rigidity values.
Fig. 3. Novel double-nut design under moderate and extensive axial load.
Fig. 2. Single and combined rigidity of a ball screw in axial direction.
is defined by the serial combination of the individual components: stiffness of the nut CN, stiffness of the shaft CS and stiffness of the supporting bearing CB (see Fig. 2). The data plotted in Fig. 2 corresponds to the actual values of a mid-sized feed drive for machine tools, also used for further examinations. As illustrated in Fig. 2, increasing the value of preloading above a certain level has little impact on the overall effective stiffness. The axial stiffness of a typical ball screw assembly is primarily defined by the geometry of the ball screw shaft [10]. Hence, for most applications, the rigidity of the ball screw nut is of secondary importance when choosing the value of preloading. For a large number of applications, the relevant boundary condition is maintaining a certain contact pressure between the balls and the tracks throughout all operating states. If an axial load is applied to the ball screw, the load distribution within the nut changes. Considering the force-displacement diagram in Fig. 1, the contact pressure between balls and tracks on the one side of the nut increases, while on the other side the contact pressure is reduced. If the axial load reaches the level of approximately 2.83 times the preloading value, one side of the nut passes the point of decompression [11]. In this state, the preloading is fully ceased and the balls roll in an undefined manner. This is an undesirable mode of operation, which causes a sudden change in the contact conditions, possible disruptions of the oil film and an excessive increase in wear. In order to avoid such a mode of operation, the preloading value is usually set to a level which can cope with the axial loads while still maintaining a certain contact pressure on the balls.
In case of an extensive axial load, the decompressed part of the double nut goes into a free-floating state in which an additional spring mechanism sustains the load compression required for a proper rolling condition. This guarantees a defined contact between the balls and the track, independent of the applied axial load. Note that the actual load is constantly supported by the opposing and preloaded part of the ball screw nut. This guarantees high rigidity values in both directions and a constant symmetrical behaviour throughout the entire mode of operation. The corresponding characteristic of the new double nut design is also illustrated in Fig. 1. Based on the basic functional principle [12] a proper design for an actual prototype has been developed and realized. Fig. 4 shows the assembled prototype already installed on a feed drive test bench.
Fig. 4. Prototype of a double nut ball screw with improved operating characteristics.
Please cite this article in press as: Verl A, et al. Double nut ball screw with improved operating characteristics. CIRP Annals Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.128
G Model
CIRP-1239; No. of Pages 4 A. Verl et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
3
Table 1 Specification of the selected ball screw. Description
Value
Nominal diameter ball screw Ball screw lead Nominal ball diameter Number of ball circles Dynamic load capacity Nominal rigidity at 4400 N preloading
40 mm 20 mm 6 mm 24 44,000 N 800 N/mm
The developed prototype represents a technical implementation of the functional principle on an industrial ball screw. The overload mechanism is realized using specially designed functional elements which incorporate numerous tasks:
Provide the spring force during overload. Enable an axial displacement between nut and shaft. Provide the possibility to initially adjust the preloading. Maintain the initial preloading value after it has been set.
The individual functional elements are equally arranged in grooves around the circumference of the ball screw nut which leads to minimal restrictions for the component mounting. The system is designed for versatile examinations. An additional sensor system, properly integrated into the ball screw nut, provides the possibility to measure the actual load conditions during operation. Table 1 states the most relevant parameters of the ball screw used for the technical implementation.
3. Experimental setup and measurement results In the course of the examination, the developed ball screw is compared to a state of the art double nut ball screw with identical dimensions and specifications. Fig. 5 illustrates the experimental setup used for the further measurements.
Fig. 5. Experimental setup.
The ball screw is integrated into a feed drive test bench and used to move a machine table with variable load mass. A highly rigid piezoelectric sensor platform, installed between the flange and the nuts of the ball screw, allows for a measurement of the axial load during operation. The load measurement is performed for each nut separately, while a linear encoder detects the displacement of the machine table. Furthermore, an additional linear direct drive, firmly attached to the table, provides the possibility to induce axial load forces of up to 14 kN onto the ball screw. With the experimental setup depicted in Fig. 5, numerous static and dynamic measurements have been performed. In a first step, a static load was applied to the state of the art system as well as to the newly designed ball screw. The experimental setup provides
Fig. 6. Measured force-displacement diagram for a state of the art double nut and the developed prototype.
the possibility to actually verify the force-displacement diagram derived from theory with experimental data. Fig. 6 exemplarily shows a measured force-displacement diagram for the state of the art ball screw without functional elements as well as for the developed prototype. According to common practice, the value of preloading for the state of the art double nut was set to approximately 10% of the dynamic load capacity. For the newly designed prototype, the overload state and the associated loss of pressure between the balls and the track is far less critical. Therefore, a considerable lower preloading value of about 1800 N was selected which, in accordance to Fig. 2, still provides the desired rigidity of the ball screw assembly. With the applied static axial load, the state of the art ball screw reaches the point of decompression at about 11 kN. This roughly corresponds to the predicted value of 2.83 FP. In the same manner, the force-displacement diagram of the new double nut design has been measured. The functional elements were designed to provide a spring force of 500 N in case of an overload situation. The measurement results in Fig. 6 clearly state, how the progression of the force-displacement curve is shaped by the additional spring mechanism. Independent of the axial load applied, the preloading level maintains at about 500 N and the point of decompression is never reached. Considering the significant reduction of preloading with almost identical rigidity and even better overload condition, the newly developed prototype provides much better operating characteristics than the state of the art system. The static load analysis verifies the functional principle of the ball screw design and furthermore confirms the internal load distribution generally derived from theory. In a next step, the prototype was exposed to dynamic load scenarios and the results were compared to those of the state of the art ball screw. For this purpose, high-speed motion tests with different load mass and operating conditions were performed. First of all, the state of the art design was exposed to acceleration tests to verify whether the point of decompression is actually reached during normal operation. In order to compare the results, the value of preloading was set to about 2000 N for both ball screw configurations. Fig. 7 shows the force-over-time plot for a highspeed motion trajectory with an acceleration of 10 m/s2 and a feed rate of 50 m/min. Starting from the same preloading level, it can clearly be seen, that the compression within the state of the art design ceases fully during acceleration, leading to the already mentioned critical mode of operation. The new double nut design, on the other hand, transmits the high acceleration forces while still maintaining a minimum load on the opposing side of the ball screw nut. This guarantees defined contact conditions between balls and tracks and a proper rolling characteristic of the ball screw throughout the entire operation. In order to maintain a certain pressure on all balls and avoid a complete loss of contact during acceleration, the state of the art
Please cite this article in press as: Verl A, et al. Double nut ball screw with improved operating characteristics. CIRP Annals Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.128
G Model
CIRP-1239; No. of Pages 4 A. Verl et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
4
significant reduction of preloading for numerous applications. In principle, different competing issues concerning the level of ball screw preloading are discussed and guidelines for a proper selection are derived. It is shown, that in many applications ball screws with 2-point-contact are operated with an exaggerated level of preloading most of the time, leading to excessive friction, wear and heat generation. In order to overcome this limitation and improve the operating characteristics of ball screws for high demanding applications, a new design is presented. The functional principle is explained and a first prototype is introduced. Experimental results of static and dynamic operations with the new design are presented along with measurements of a state of the art ball screw. The conducted examinations prove the functionality of the proposed design and demonstrate the effectiveness of the new double nut ball screw, in particular for dynamic applications. In further studies, the mechanical design will be optimized and adapted for long-term tests in industrial applications. Acknowledgement The authors would like to thank the August Steinmeyer GmbH & Co. KG for their support of this project.
Fig. 7. Measured force within the ball screw during high speed motion testing.
ball screw requires much larger values for the preloading. The conducted measurements have shown that for the given motion profile, a preloading value of roughly 4000 N is needed. This again has a dominant impact on the overall effective load and the operating characteristics of a ball screw. Comparing the newly developed double nut system with the state of the art ball screw, the reduction of preloading leads to minimized stress on the components, lower friction values, reduced wear and heat generation and all together to a significant improvement of the efficiency and the motion characteristics of the ball screw. The developed double nut design is in particular suitable for dynamic applications, where high acceleration forces determine the value of preloading and substantially affect the overall load. 4. Conclusion and outlook In this paper, a conceptual ball screw nut design with improved operating characteristics is presented. The key aspect of the new design is a passive overload mechanism, which allows for a
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Please cite this article in press as: Verl A, et al. Double nut ball screw with improved operating characteristics. CIRP Annals Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.128