MF2077 Machine Design Advanced Course II School of Industrial Engineering and Management Linas Capas, Teodor Hidén, Joseph Montuoro, Matilda Svensson, Maryam Tarik Hamad Final report Test Rig for a Load Sharing Unit in a Ball Screw Transmission KTH Royal Institute of Technology, Stockholm, Autumn 2019 [Page intentionally left blank] 1 KTH Abstract The project goal was to reassemble and test an existing load sharing unit, working on 3 ball nuts and a ball screw. The load sharing unit should be tested for “load sharing”, and the ball nuts on ball screw should be tested for efficiency. To be able to perform these tests, a test rig is needed, where different amount of force could be applied repeatedly. All this in a short period of time, only 16 weeks. This project was originally started in 2016 and was then put on hold until 2019. The test rig that was originally planned to be used had been scrapped and recycled in the time on hold. The load sharing unit was completely taken apart and reassembled. The piping was redone and it was no longer leaking oil. Possibility of properly bleeding the system was added. The test rig was designed with the scope of keeping as many of the existing parts as possible. To apply the force to the system (power in) the most feasible solution was used, a rope - pulley and weight drop system. The weight that was dropped could be increased or decreased. The test rig and ball screw was also fitted with a brake to control the speed that the load sharing unit was traveling at for different weights. In order to measure the load sharing between the ball nuts and the efficiency of the linear ball screw transmission, multiple load cells, a force sensor, a torque sensor and one encoder were used. The load cells showed the load that each ballnut had to take. The force sensor could together with the encoder show the power in, and the torque sensor could together with the encoder show the power out. The difference in power in and out gave the efficiency. The complete measuring system could not be finalized due to time constraints. The efficiency was never managed to be tested and the non-complete load sharing test gave results that were hard to analyze. Two of the six load cells stopped working and could not be used in the data acquisition. Future work with testing of the load sharing and efficiency should be done. 2 KTH [Page intentionally left blank] 3 KTH Foreword We would like to thank Pontus Karlsson with colleagues at Cascade Drives for a educational cooperation, discussion and support. We also want to thank Stefan Björklund and Kjell Andersson for the discussions that have been ongoing during the whole project. We also thank the KTH staff that has been helping us with answering questions, testing and building. We especially want to thank Tomas Östberg that has been manufacturing parts for the test rig and has been helpful in all ways and Mikael Hellgren who has helped us with the electronic components and setups. A big thanks to KTH Prototype Center for letting us borrow your soldering station, tin and your Picoscope. Thanks / Joe, Linas, Maryam, Matilda and Teodor 4 KTH [Page intentionally left blank] 5 KTH Table of Contents 1. Introduction 10 1.1 Background 10 2. Literature Review 12 2.1 Load Sharing Unit 12 2.1.1 Ball nut 12 2.1.2 Hydraulics 14 2.1.3 Ball Screw 15 2.2 Test Rig 16 2.2.1 Force Applying System 17 2.2.2 Frame 19 2.2.3 Measuring system 20 3. Methodology 22 3.1 Project Management Methods 22 3.2 Product Development Methods 22 3.2.1 Concept Evaluation 22 3.2.2 CAD 23 3.3 Test plan 23 3.3.1 Test Procedure 24 3.3.2 Hypothesis 24 3.3.3 Safety and Uncertainties 25 4. Design Results 26 4.1 Load Sharing Unit 26 4.1.1 Hydraulics 26 4.1.2 Ball Screw 26 4.1.3 Mounting 27 4.2 Test Rig 27 4.2.1 Steel Frame 28 4.2.2 Power Input and Output 28 4.2.1 Measuring system 29 5. Test Results 34 5.1 Tests 34 5.1.1 Load sharing 35 5.1.2 Efficiency 39 5.1.3 Uncertainties 39 6. Discussion 40 7. Future work 42 References 43 Appendix A: Project charter 44 Appendix B: Links to components of choice 47 Appendix C - Work Breakdown Structure Chart 52 6 KTH Appendix D - GANTT chart 53 Appendix E - Torque Sensor 54 Appendix F - Doughnut Load Cells Data Sheet 56 Appendix G - Arduino Data Sheet 59 Appendix H - S-Shaped Force Sensor Data Sheet 63 Appendix I - Encoder Data Sheet 64 Appendix J - Ball Screw Data Sheet 75 Appendix K - Matlab Code 76 Appendix L - Arduino Code 78 Appendix M - Matlab Code for Test Data 81 [Page intentionally left blank] 7 KTH List of Abbreviations Arduino Open-source microcontroller used for data acquisition CAD Computer Aided Design KTH Kungliga Tekniska Högskolan (Royal Institute of Technology) LSU Load Sharing Unit WBS Work Breakdown Structure 8 KTH [Page intentionally left blank] 9 KTH 1. Introduction This chapter provides an overall background to the project and highlights the purpose and the requirements as well as the delimitations of the project. 1.1 Background In 2016 a student project started with the aim of investigating load sharing in a linear ball screw transmission with the use of a load sharing unit (LSU). CorPower was the stakeholder and aimed for using this type of transmission for wave power purposes. Using load sharing higher load capacity can be reached. In this unit, six hydraulic cylinders enable the load sharing by spreading the load over the three ball nuts. Figure 1 illustrates how they are assembled. There was no time for the project to conduct tests on how the load sharing was performing or how efficient the transmission was in 2016. Figure 1: Overview of the assembly of the load sharing unit. [1] In 2019 the project was given to our group with a new stakeholder, Cascade Drives, an affiliate company of CorPower and a new scope. Since the previous project hadn’t done any tests the aim of the new project was to test the load sharing of the unit and measure the efficiency of the transmission. Everything that was left from previous projects, including the load sharing unit, had been lying in a box at the department of machine design at KTH and some of the parts that were supposed to exist according to their report did not. Considering that the test rig that the previous project had intended to use for testing had been scrapped by CorPower which meant that the project had to be extended to cover the design, manufacturing and assembling of a test rig. 10 KTH 1.2 Problem Definition and Purpose The project is part of the course Machine Design Advanced Course part II, MF2077, where the focus is to do detailed design, manufacture and assemble a product. The main goals of the project are to test if the load is shared between the three ball nuts of the load sharing unit (LSU) and find the efficiency transmission. To reach these goals the project also has to find and repair the oil leakage of the load sharing unit as well as mount it correctly on the ball screw. Since the planned test rig was scrapped, one of the scopes the project had to cover design, manufacturing and assembly of a test rig where the load sharing and the efficiency could be tested. 1.3 Delimitations One delimitation of this project is the time since the project is only due to span during the course MF2077, meaning from August 26 to December 12. Because of the time constraints it was imperative to make the design of the test rig as simple and feasible as possible. 11 KTH 2. Literature Review This chapter provides the information search made before getting started and how the project was divided into subsystems in order to finish the project on time. The project was divided into two subsystems: load sharing unit and test rig. The main source of information was the final report of the previous group. 2.1 Load Sharing Unit The load sharing unit (LSU) consists of three ball nuts connected with hydraulic cylinders that are used to evenly distribute the load among the ball nuts (see Figure 2). The cylinders are double acting to allow the system to handle bidirectional movement. A major reason that multiple ball nuts are used to share the load evenly is the low load capacity of the ball screw transmission. [1] Figure 2. Overview of the load sharing unit with the hydraulics. [1] ​ The ball nuts are attached to a plate which is then connected to an attachment plate to the test rig. In the ball screw assembly the screw and nuts need to have matching helical grooves. [2] 2.1.1 Ball nut In a ball nut ball bearing balls roll between the helical grooves of the ball screw which converts rotational motion into linear motion thanks to recirculating ball bearing balls. As the screw rotates or nut travel linearly the balls are deflected by a deflector into a ball return system in a continuous path. [2] Ball return system There are two different ball return systems: external and internal ball return system. For the external ball return system the return tubes are located outside the ball nut body while for the internal the balls return inside the nut based on tangential deflection (see Figure 3).