PROBLEMS OF DESIGNING AND SELECTING TOOLS FOR EXPERIMENTAL RESEARCH IN VARIABLE TRANSMISSION RATIO DRIVES

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1 KOMISJA BUDOWY MASZYN PAN ODDZIAŁ W POZNANIU Vol. 27 nr 2 Archiwum Technologii Maszyn i Automatyzacji 2007 PIOTR KRAWIEC* PROBLEMS OF DESIGNING AND SELECTING TOOLS FOR EXPERIMENTAL RESEARCH IN VARIABLE TRANSMISSION RATIO DRIVES The article deals with the problems of selecting and designing tools for experimental research in special belt transmission systems. A distinctive feature of transmission systems under consideration is a periodically changeable transmission ratio obtained thanks to the application of elliptical pulleys and circular, eccentrically mounted pulleys. The article includes technical conditions of designing measuring tools for conducting experiments regarding belt transmission systems with variable kinematic and dynamic characteristics. Key words: belt transmission system, measuring tools 1. INTRODUCTION In recent years there has been a dynamic development of belt transmission systems, where forces are transmitted through frictional contact and gear coupling thanks to the use timing belts. This development has occurred thanks to the successful cooperation of science and technology which combined their efforts to design, manufacture and test new experimental belt transmission systems. It was also possible thanks to the scientific description of deformation processes occurring in multi-molecule materials such as polymers and elastomers as well as several physical phenomena occurring when belts and pulleys interact. The use of new materials for toothed belts and designing their new geometrical characteristics have made it possible to use belt transmission systems in new drives. Consequently, it was possible to develop new designs of toothed belt transmission systems, namely transmission systems with variable transmission ratios, where a certain degree of speed variability is obtained by the use a belt transmission system with wheels having shapes of ellipses, ovals or non-circular disks [2, 3]. The problem of selecting their geometrical and kinematic characteristics was * Dr inż. Chair of Basics of Machine Design, Poznan University of Technology.

2 104 P. Krawiec discussed by the author in paper [3]. In order to use such drives on a large scale they must undergo a stage of experimental verification. That is why a need to design and build a test stand appeared. The main objective of the research was to find out, whether a toothed belt transmission system, having specific wheel shapes, would work in accordance with the assumed kinematic and functional criteria. It was also important to check whether drive s cyclycity would be maintained. Moreover, test stand trials were to check calculated kinematic and dynamic characteristics of variable-speed transmission systems. 2. TORQUE METER DESIGN Torque is one of basic qualities responsible for characteristics of power transmission systems. As this statement is true, there is a need to create tools and methods for precise and efficient measurements of instantaneous torque. Simultaneously, it should be remembered, that attaching a torque measuring apparatus (torque meter) to the examined power transmission system introduces changes to the way it operates. It is particularly important during dynamic measurements. The inspiration to design and then construct the torque meter in question was a need to conduct tests of classical and special belt transmission systems. Special belt transmission systems shall be understood here as variable-ratio toothed belt transmission systems. The basic rules of design and principle of operation of transmission systems of that type are discussed in papers [2, 3]. Procedures concerning development of measuring methodology were started with analysing the design solutions of torque meters available on the market. The following assessment criteria were assumed: precision of measurements, possibility of adapting to different drive systems, price, measurement automation. As a result of conducted comparative analysis of torque meters design characteristics, the solution suggested by Hottinger Baldwin Messtechnik has been found quite interesting. However, that solution requires the so called building up. Its price is also considerable. Hence an attempt to develop a new design and build an improved torque meter. For precise torque measurements in power transmission systems and equipment, torsion meters are most frequently employed, in which under influence of a load, an elastic strain of a mechanical flexible converter occurs. Measuring signals are shaped in torque meter s electrical converter. In rotation torque meters, the torsional element is directly a part of the rotating assembly. Hence, direct measuring of a torque transmitted by the power

3 Problems of designing and selecting tools. 105 transmission system s shaft is possible. As a result of analysis of available torque meters, their characteristics and prices a need appeared to design a set of torsion torque meters, where signals would be transmitted to stationary coils installed in torque meter housings without any actual physical contact (Figure 1). Fig. 1. View of the universal torque meter on a special transmission system Rys. 1. Widok uniwersalnego momentomierza przekładni specjalnej The design of the torque meter is universal and makes it possible to use it for measurements both in special transmission systems and other drive assemblies. 3. MEASURING ANGLES OF ROTATION OF WHEELS IN A VARIABLE-SPEED TRANSMISSION SYSTEM Another task which the designed test stand is to perform is measuring angles of rotation (angular velocities) of the driving and driven shaft of a given variable-speed transmission system. Instantaneous angles of rotation of shafts in a tested transmission system are registered by rotation converters (Fig. 2) attached to shaft ends with bellow type couplings. In order to determine initial angular position of a shaft, the converters are equipped with sensors which make it possible to start the measuring process of angles of rotation for shafts in the initial position at a 0 degree angle.

4 106 P. Krawiec Fig. 2. View of the encoding sensor for measuring shafts angle of rotation Rys. 2. Widok czujnika obrotowo-kodowego do pomiaru kątów obrotu wałów przekładni specjalnej 4. TEST STAND FOR SPECIAL TRANSMISSION SYSTEMS The test stand for testing non-classical toothed belt transmission systems is presented in Figure 3, and the measuring diagram of a reducing/multiplying transmission system is presented in Figure 4. The test stand is used for conducting kinematic and dynamic tests of variable-speed two-wheel transmission systems with torque load transmitted or not (then torque meters record only instantaneous moment of inertia). The test stand consists of a basis 1. On the basis there is a driving shaft 4, layshaft 5, driven shaft 6 and brake 9. On the basis a motor 2 is mounted which is powered through a frequency converter 3. Shafts with attached torque meters are placed in radial ball bearings on supports. The brake 9 is used to apply load to a tested transmission system. The installed reducing gear 7 is directly coupled with the multiplying gear 8. Signals from torque meters and encoding sensors 12, 13, 14 are transmitted to amplifier 10 and further on to computer 11. Control unit 15 is used to adjust load applied by the brake.

5 Problems of designing and selecting tools. 107 Fig. 3. View of the special transmission system test stand Rys. 3. Widok stanowiska badawczego przekładni specjalnej Fig. 4. Measuring diagram of a variable speed belt transmission system (description of numbers is in the text) Rys. 4. Schemat układu pomiarowego nierównobieżnej przekładni pasowej (opis w tekście)

6 108 P. Krawiec Fig. 5. Magnetorheological brake Rys. 5. Hamulec magnetoreologiczny The design of the magnetorheological brake requires constant cooling with the liquid from tank 16. The magnetorheological brake has been specifically developed for its intended usage. The device uses changes in fluid s viscosity under the influence of external electrostatic or magnetic field, which is a highly original solution. Fluid s rheological properties are controlled in such a way that their precise fine-tuning is possible, thus allowing for changes in torque under varying loads. Torque changes are effected by means of electric control signals. Consequently, they can be transmitted to computers or PLC s (Fig. 5). The measuring procedure is presented below. After starting the motor, a required driving speed is set by means of the frequency converter. Then the brake is set to apply a required load torque. Having set the load and temperature, angular movements and torques are measured. Signals from extensometric and encoding sensors are digitized by a multichannel A/C card and then directly transmitted via the DMA channel into computer's RAM memory. Measurements are conducted automatically with a program written in C ++ language. The program makes it possible to start measurements at a specific angular position of the driving wheel, with a selected frequency and duration, which in most cases is equal to one revolution of the driven wheel. Results of the measurements are saved in a text file, which is then properly processed with MS Excel and Statistica. On the basis of conducted measurements the system calculates transmission system wheels velocities and accelerations. Results of the calculations are presented in a graphical form in charts. The test stand makes it possible to perform measurements of: torque within [ 20 20] Nm range (symbol stands for load s direction), angles of rotation of the driven and driving wheels (registered each 0.14 revolution of the driving wheel). It was assumed that the rate of rotation of the driving wheel shall be within the range (0 2000) rpm (for the specific test stand in question).

7 Problems of designing and selecting tools. 109 Any wheel of the transmission system can be the driving wheel in any configuration. The modular design of the test stand makes it possible to conduct trials of belt transmission systems working in various configurations. 5. RESULTS OF RESEARCH ON A TWO-WHEEL TRANSMISSION SYSTEM Tests of a two-wheel reduction gear were carried out for the average reduction ratio u sr = 2. The driving wheel was a non-circular one, the driven wheel had an elliptical envelope. As a result of experimental research a table was produced stating angles of rotation of pulleys and torques. Next a diagram showing relationship between the characteristics of transmission ratio as a function of a degree of rotation of wheel 1 was made (Figure 6). The following diagrams (Figure 7) illustrate the character of changes of angular velocity ω of the driven wheel and characteristics of changing acceleration a of the driven wheel against the degree of rotation of the driving wheel 1 (Figure 8). Figure 9 illustrates torque changes on each shaft of the transmission system. u 0,8 0,7 0,6 0,5 0,4 0, f ( ) Fig. 6. Transmission characteristics of a two-wheel transmission system with a given ω 1 = 20 rad/s, with a load torque M h = 0 N m, where: u transmission ratio Rys. 6. Charakterystyka przełożenia przekładni dwukołowej przy prędkości ω 1 = 20 rad/s, moment hamowania M h = 0 N m, u przełożenie przekładni

8 110 P. Krawiec Fig. 7. Characteristics of angular velocity as a function of a degree of rotation of driving wheel of a two-wheel transmission system with a given ω 1 = 20 rad/s, with a load torque M h = 0 N m, where: ω 1 angular velocity of shaft one, ω 2 angular velocity of shaft two, ω 3 angular velocity of shaft three Rys. 7. Charakterystyka prędkości kątowej w funkcji kąta obrotu wału czynnego dla przekładni dwukołowej przy ω 1 = 20 rad/s, przekładnia obciążona momentem M h = 0 N m, ω 1 prędkość kątowa wału pierwszego, ω 2 prędkość kątowa wału drugiego, ω 3 prędkość kątowa wału trzeciego 0, 2 a[rad/s^2] 0,15 0, 1 0,05 0-0,05-0, 1-0,15-0, 2 a2 a a3 φ [ ] Fig. 8. Acceleration characteristics of a two-wheel transmission system with a given ω 1 = 20 rad/s, with a load torque M h = 0 N m, where: a 1 acceleration of shat one, a 2 acceleration of shaft two, a 3 acceleration of shaft three Rys. 8. Charakterystyka przyspieszenia dla przekładni dwukołowej przy ω 1 = 20 rad/s, przekładnia obciążona momentem M h = 0 N m, gdzie: a 1 przyspieszenie wału pierwszego, a 2 przyspieszenie wału drugiego, a 3 przyspieszenie wału trzeciego

9 Problems of designing and selecting tools. 111 M [N m] M[Nm] 0,1 0,09 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0 M2 M1 M Fig. 9. Torque characteristics as a function of angle of rotation φ of a two-wheel transmission system with a given ω 1 = 20 rad/s, with a load torque M h = 0 N m, where: M1 torque on shaft one, M2 torque on shaft two, M3 torque on shaft three Rys. 9. Charakterystyka momentów w funkcji kąta obrotu φ dla przekładni dwukołowej przy ω 1 = = 20 rad/s, przekładnia obciążona momentem M h =0 N m, M1 moment na wale pierwszym, M2 moment na wale drugim, M3 moment na wale trzecim φ[ ] 6. SUMMARY On the basis of conducted experimental and theoretical research the following conclusions can be formulated: the conducted experimental research have justified the use of the designed measuring equipment, the universal character of developed research methods and equipment makes it possible to apply them in various configurations, variable-speed transmission systems are suitable for producing required movement parameters with generally small useful loads. Nevertheless, measurements with an external torque applied were performed as well. The presented results show that external loads are combined with the torques, in particular, at higher speeds, on the basis of conducted experiments and calculations the range of permissible speeds of such transmission systems has been determined. The speed should not exceed n 2000 rpm for a two-wheel transmission system working as reduction gear and n 1000 rpm in case of a transmission system working as a multiplying gear, the recorded torques of shafts with aluminum alloy wheels were comparable with their useful torques,

10 112 P. Krawiec connecting several variable-speed transmission systems in a row makes it possible to influence the final variability characteristics of the whole system. [1] [2] [3] [4] REFERENCES Dudziak M., Przekładnie cięgnowe, Warszawa, PWN Dudziak M., Krawiec P., Meler F., Analiza nierównobieżnej przekładni cięgnowej, Special Issue No 5 Computer Simulation in Machine Design, p Krawiec P., Nieklasyczna przekładnia cięgnowa z pasem zębatym, Przegląd Mechaniczny, 2005, vol. 4, p Krawiec P., Variable-speed toothed belt transmission systems with periodically variable transmission ratios, Archives of Mechanical Technology and Automation, 2004, vol. 24, nr 2, p Praca wpłynęła do Redakcji Recenzent: prof. dr hab. inż. Bogdan Branowski PROBLEMATYKA PROJEKTOWANIA I DOBORU NARZĘDZI DO BADAŃ EKSPERYMENTALNYCH NIERÓWNOBIEŻNYCH PRZEKŁADNI CIĘGNOWYCH S u m m a r y W artykule omówiono problematykę doboru i projektowania narzędzi do badań eksperymentalnych przekładni cięgnowych specjalnych. Charakterystyczną cechą tej przekładni jest okresowo zmienne przełożenie uzyskane dzięki zastosowaniu kół o nieokrągłym zarysie wieńca oraz kół okrągłych mocowanych mimośrodowo. Omówiono zaprojektowane i wykonane narzędzia oraz zespoły pomiarowe. Wskazano uwarunkowania techniczne projektowania narzędzi pomiarowych do badań przekładni cięgnowych o okresowo zmiennych cechach kinematycznych i dynamicznych. Słowa kluczowe: przekładnia cięgnowa, narzędzia pomiarowe