Development of an automated system for designing ball screws

. The purpose of the study is labor saving, productivity gain of work and ball screws quality design pursuant to the use in numerical control machines. The article is devoted to the problem of automation of the designing of ball screws with return ports. These ball screws are an important element of the mechatronic modules of numerically controlled machines, as they affect the accuracy of linear displacement. Research methods: system-oriented analysis, structural analysis, methods of functional and object-matching programming, simulation. The novelty of the work: a method of three-dimensional parametric simulation of nuts of ball screws and a computation algorithm for a ball screw based on the synthesis of well-known techniques, has been developed. Research results: a software product consisting of modules that together automate the design process of nuts, screws, rigid supports, tilting saddles and ball screw coupling elements, has been developed. Conclusions: the developed methods for calculation of basic ball screw parameters can be used as the algorithmic basis of software products. Since the process for designing ball screw nuts with return ports has been automated, there are grounds for developing automation methods for other types of nuts.


Introduction
Numerical control (CNC) machines, currently and abandantly used in mechanical engineering, make it possible to automate manual labor with simultaneous improvement of the accuracy of the resulting products.The high quality of manufactured products on CNC machines is provided by mechatronic systems, including ball screws (BS), which promote the accuracy of CNC machine slide linear displacements.Since high accuracy is a requirement provided by BS, it is essential to perform its calculations on a deeper scale and produce three-dimensional models that can be used for these ball screws manufacture later on.A specialized system that automates the necessary procedures can provide high-quality design solutions.A ball screw is a mechanism designed to convert rotary mode into forward running.BS consists of a nut with threaded grooves, a male screw with reciprocal roots and balls located between them.In this study, ball screws with return ports are viewed since they have ups of both static and dynamic load capacities.In these ball screws the balls move along the return ports, which form a closed loop, redirecting the balls from the end-point to the start point.For ports latch-up, is used.It is located on the nut flattened surface, where it is attached using a fastener joint.Also in ball screws a preload pressure is delivered for increasing the accuracy of displacement.In this study, we go over ball screws with an allowance realized by changing the flank of the track using a point-to-point section material, since this solution provides a smaller contact mark, resulting in a relatively small heating of the parts [1].For the development of a specialized system designed to automate the calculations of nuts in ball screws with return ports and the construction of their parametric three-dimensional models the following main tasks have been assigned: 1. Identify the main components of a specialized software module and develop its structural and functional scheme.2. Develop parametric models in a computer-aided design system.3. Develop algorithms for the functioning of modules to automate the task and develop a software product based on them.

Materials and methods
Ball screw design centers around geometric parameters computation (nominal diameter, thread pitch, ball diameter, flank of screw thread) of a nut and a threaded member, which will ensure operability under specified conditions.While analyzing ball-screw design publications, two thesis research were found [2,3].Four thesis works on transmission by rolling were also studied [4,5,6].In these works, mathematical models were developed, but they did not bring up issues of design automation.In the analysis of existing studies on the automation of ball screw design there was a work found that had a computerized analysis of ball screws.As the insight into the CAE (computer-aided engineering) system "APM Screw" [7,8] was provided, several disadvantages of it were uncovered: 1.The geometric characteristics of the transmission, that are to be calculated beforehand, are used as input data.2. Lack of integration with CAD systems for building three-dimensional coupling engagement models.3. The issues of design automation of other ball-screw parts such as screw, screw supports, etc., are not viewed.During the analysis of existing ball screw design techniques, it was revealed that each of them had different initial data.Therefore only a part of the required characteristics was calculated.In this study, it was decided to use the synthesis of the methods of Anuryev V.I. [9], Egorov O.D. [10], HIWIN company [11] and the work "Calculation of ball screws of recirculating ball screw-and-nut" [12], that will make it possible to compute all the necessary geometric parameters of the ball screw.According to the suggested methodology, the calculation of the ball screw parameters will begin with the determination of the dynamic load capacity, which can be calculated two-way: taking into account the adjustment factors and making allowance for the required resource condition.For further calculations, the highest value of the dynamic load capacity is selected to ensure the largest margin of ball screws safety.Further on, within the cycle, the search for the nearest large values will take place according to All-Union Standard 2 P31-5-89 and pursuant to the catalog of the company "HIWIN" [11,13], among which the minimum value will be selected.Later accordantly with the calculated dynamic load capacity obtained, the typical parameters of the ball screws can be found.Then the calculation of the internal diameter of the screw is performed, while the working stresses conditions are tested.If the engineering stresses are less than working stresses, a screw bending analysis is carried out and a critical slenderness ratio check is made.To continue the calculations, a critical slenderness ratio must be larger than the calculated one.The last check comes down to the determination of the critical speed of rotation [14,15].If all the tests are survived successfully, the screw can be considered stable, subsequently the rest of the parameters are calculated.If at least one of the tests results in the negative, it is necessary to increase the requirements for the value of the dynamic load capacity and carry out the calculations again.If the calculated load capacity is greater than the maximum permissible one within the framework of this work, then it is necessary to reduce ball screw requirements.
Since the ball screw with return ports has a complex design of the threaded part, it was decided to develop a methodology for designing three-dimensional models of the ball screw nut [16,17].According to this technique, details can be modeled using the main sketch, which will contain all the necessary parameters.To obtain a preload, one idle scroll is used.If there are more than two contours, then some of these idle scrolls should be constructed without changing a thread pitch, since they will become a part of a group with the same contact line of the ball with a screw and a nut.To determine the position of the differentiating transition turn, it is necessary to determine the total number of closed profiles and divide by 2.Then, after constructing the first part of the contours, it is necessary to add a transition turn with a slight increase in the thread pitch by the value of the axial displacement of the profile multiplied by 4. The rest of the closed contours will grow in the same way as the first part.If the number of closed loops is odd, then there will be 1 more contour in one contact group than in the other.Next, a holding down device, return ports, a sketch of the location of the balls and the remaining elements of the nut are designed.The number of details of return ports, arrays of points for balls will depend on the number of closed contours.Within the framework of this study, the Autodesk Inventor 2021 system is used as a CAD, since it has a COM interface (communication port interface) and the ability to set parameters of parts.The ability to create three-dimensional spiral sketches, the ability to create adaptive sketches and support in a two-dimensional sketch such compatibility constraints as perpendicularity, parallelism, convergency, alignment, concentricity, collinearity, horizontality, verticality, equality and symmetry became also important selection criteria.
Python was chosen as the programming language for system development.The reason for the choice is dynamic typing of the language, availability of pools that automate work with files and a special library for interacting with CAD "Autodesk Inventor".For the development of the user graphical interface, the "Qt Designer" framework is used, which supports Python programming.Among the important selection criteria was the ability to create modal windows and develop event-driven handlers inside the application.

Results
As part of this study, a system was developed to automate the design of ball screw nuts with two return ports, a screw, screw supports and a coupling.The block diagram of this system is shown in Figure 1.
The initial data for the design is presented in the form of a requirements specification, which will be sent to the control module.In the requirements specification, it is necessary to choose the required resource, the survival probability, the depth of a screw, the estimated total weight of both the machine bench and the workpiece, and the dynamic loop factors.The control module will send the necessary data to the calculation module to perform the calculation of the main parameters of the ball screw.The calculation module can interact with component selection modules and data processing, increasesing efficiency of its operation.Coupling, nut, screw, floating support and rigid support modules are designed to show the results of the calculation module, as well as to fetch CAD "Autodesk Inventor" in order to reconfigurate parametric parts.To interact with the CAD software interface "Autodesk Inventor", a COM module was developed that transmits data, using the appropriate technology.The report generation module is designed for a user screen dataoutput of the data received in the design.Microsoft Excel is used as a database because of small amount of data, processed in this implementation and represented mainly by parts characteristics given in the reference literature.Functions for saving and uploading calculation results into text files were also implemented.The system's trouble-free operation is ensured by error handling functions in user's datainputting, as well as in case of both system failures and problems when interoperating with CAD.Three-dimensional parametric models of nut, screw, rigid support, floating support and coupling parts were developed.In this system, the geometric parameters, found in the calculation module, are beamed to the CAD "Autodesk Inventor", where they are inserted into the parametric table.The use of dependencies in the development of sketches ensures absence of errors in the sketch reference lines navigating.Component drawings, which can be changed in case of their solid-state models reconfiguration, were also developed.

Discussion
In the course of this study, a design policy for calculating basic geometric characteristics of a ball screw based on the synthesis of known techniques, has been developed.A new technique describing the process of three-dimensional modeling of a ball screw in computeraided design systems, has been produced.Growth prospects for the development of this study are to work out a system for automating buffle type nuts design, end ball return systems creation, as well as methods for making a preload.It is also possible to expand the range of designed products, such as gides, mounting modules, bodies.

Conclusion
These methods are true for nuts having return ports.As a result of the research, a system for automating ball screw nuts with two return ports has been developed, which solves the problems of calculating ball screw parameters and three-dimensional modeling.The developed system can be implemented at machine-building enterprises designing mechatronic systems or using them for manufacturing their own products.The penetration into the enterprise will allow conducting a study of the economic efficiency of the software product and revealing flaws during operation.

Fig. 1 .
Fig. 1.Structural diagram of the automated ball screw design system [Compiled by the authors].