PROTOTYPE OF LOAD CELL APPLICATION IN TORQE MEASUREMENT

Transcription

1 IMEKO 21 TC3, TC5 and TC22 Conferences Metrology in Modern Context November 22 25, 21, Pattaya, Conburi, Tailand PROTOTYPE OF LOAD CELL APPLICATION IN TORQE MEASUREMENT Tassanai Sanponpute 1, Cokcai Wattong 2 1 National Institute of Metrology Tailand, Patumtani, Tailand, Tassanai@nimt.or.t 2 National Institute of Metrology Tailand, Patumtani, Tailand, Cokcai@nimt.or.t Abstract Torque laboratory of National Institute of Metrology Tailand as designed and developed te prototype of torque transfer wrenc and torque wrenc calibrator by using load cell as main device in order to provide low cost torque transfer standard. Loading point of load cell was designed to directly connect to torque measurement axle. Moreover, loading point and base of load cell were supported by elastic part acting as flexure bearing. Torque standard macine and commercial torque transfer wrenc were used to compare torque measured by te prototypes, torque transfer wrenc and torque wrenc calibrator respectively. Simulation from finite element analysis confirms tat te design can be used as torque measuring device. Metrological properties are investigated according to standard DKD 37 and DKD 38. Te experiment and calibration result sows tat uncertainty is witin ±.8% for torque transfer wrenc and witin ±.3% for torque wrenc calibrator. 2. TORQUE TRANSDUCER DESIGN Tree models of torque measuring devices, Model A Model B and Model C, were designed and built. Model A is torque wrenc calibrator wile Model B and C are torque transfer wrenc. Te working principles oll tree models are similar. Load cell is used to measure force F, wic is te result of torque moment T, acting on loading point of te load cell at te distance r from te rotation axis as in te working principle scematic in figure 1. Te pivot is made from elastic part working as flexure bearing. Capacity of load cell can be calculated from fundamental equation as in (1). Keywords : Torque wrenc calibrator, Torque transfer wrenc, load cell 1. INTRODUCTION Force measurement and torque measurement, bot are mass related quantity. Altoug bot measurements seem to be te same field, in fact measurand and influence quantities of bot measurements are different. Force measuring instrument, load cell, is designed to measure perpendicular load acting on te loading point of load cell. Eccentric loading, te load acting out of te loading point, causes moment on te load cell, and tis moment is te influence quantity of force measurement. Torque measuring device is designed to measure torque around measurement axis under te condition tat no force is acting on te axis or constant force is acting on te axis. Tis force or fluctuation of force is te influence quantity of torque measurement. Because of te significantly iger cost of torque measuring device tan one of load cell, tis study is intended to use load cell as a major instrument to build torque measuring device tat as caracteristics consistent wit DKD R 37 standard for torque transfer wrenc[1] and DKD R 38 standard for torque wrenc calibrator[2]. Fig. 1. Working principle scematic. T = F r were T is torque moment F is load acting on load cell r is distance from pivot to te loading point of load cell In te real design, lengt of load cell is a part of te lever. Tus, anoter elastic part besides pivot was also added between base and load cell in order to more clearly define te lever distance from center of pivot to te center of anoter elastic part. In tis researc, all tree models of te devices used te same load cell model and capacity in order to be comparable. Singlepoint typed load cells made from anodized aluminium by TedeaHuntleig model: 142, rate Page 285

2 TC3 : FORCE, MASS, TORQUE & DENSITY capacity: 5 kg, rate output signal: 2 mv/v, are used in tis study. Torque wrenc calibrator model A was designed to use flexure bearing made oluminium and installed in te position tat makes load acting perpendicularly to te line of flexure bearing installation as in figure 2 (or flexure bearing under compressive stress). 3. FINITE ELEMENT ANALYSIS To confirm te designs, commercial finite element program, COSMOSWorks, was used to analyze te design, Figure 5 sows stress result of torque transducers model A, B, and C. Te stress of load cell was used as an output signal of torque measurement. Fig. 2. Torque wrenc calibrator model A. Model A was modified to torque transfer wrenc model B by lengtening te base enoug to be te lever of wrenc as in figure 3. Tis modification causes te line of cross force parallel to te line of flexure bearing installation (or flexure bearing under sear stress). In model B, flexure bearing was still made from 2 mm 1 mm aluminium, wile steel grade DC53 was used for torque transfer wrenc model C to make bearing more robust. Besides material of bearing, te major cange in model C is tat te line of flexure bearing installation is perpendicular to te line of loading as illustrated in figure 4. Fig. 3. Torque transfer wrenc model B. Fig. 5 Stress analysis from finite element of torque transducer Model A, Model B, Model C For torque wrenc calibrator model A, base of transducer was restrained. Normal force was applied on te simulated wrenc, wic is connected to te measurement axis in order to simulate torque around te axis. However, in torque transfer wrenc model B and C, measurement axis were restrained. Input torque is generated from te load acting on te lever of te wrenc. Magnitude of load and position of load acting on te lever were varied to determine nonlinearity and varied cross force error as sown in figure 6. Fig. 4. Torque transfer wrenc model C. Page 286

3 Te preliminary study started from study of effect of flexure bearing, zero drift, full load drift and ysteresis of load cell and torque standard as sown in figure 7. Tis test sows tat flexure bearing is te factor increasing creep and stress relaxation oll tree models of torque measuring devices. Fig. 6. Nonlinearity and varied cross force error Te errors of tree models are under.1%, wic is consistent wit DKD R 37 and DKD R 38 standard. Model C gave te lowest error. Tis could be te result from different material of elastic part. Steel was used as bearing in model C, so it can support cross force better tan aluminium bearing. 4. EPERIMENTAL RESULT Figure 6 sows te fabricated torque as te model designed in tis researc. Fig. 7. Zero drift, Full load drift, Hysteresis error After preliminary test, torque wrenc calibrator model A was calibrated according to DKD R 38 by using commercial torque transfer wrenc as calibration standard wit uncertainty ( 2σ ) ±.2% and torque transfer wrenc model B and C, were calibrated according to DKD R 37 by torque standard macine as calibration standard wit uncertainty ( 2σ ) ±.3%. Te calibration results of transducers model A,B and C are sown in figure 8,9, and 1 respectively. Fig. 6. Fabricated torque transducer Model A, Model B, Model C Page 287

4 TC3 : FORCE, MASS, TORQUE & DENSITY Fig. 8. Calibration result of transducer model A. Fig. 8. Calibration result of transducer model B. Table 2. Metrological properties of transducer Model B. b f E N m % % % % % % Table 3. Metrological properties of transducer Model C. Fig. 8. Calibration result of transducer model C. Metrological properties according to DKD standard, repeatability b, reproducibility due to mounting position b, reproducibility due to varied cross force b l, zero error f, ysteresis error, and interpolation error, were calculated and sown in table 1,2, and 3. Table 1. Metrological properties of transducer Model A. b f E N m % % % % % % According to te result, tese inouse manufactured torque measuring instruments can be classified as follows. Model A: Class 1, Model B: Class.5, Model C: Class.2 wit expanded uncertainty ( 2 σ ) ±.3% (linear fitting function),.2± % (cubic fitting function), and ±.8% (cubic fitting function) respectively. f E N m % % % % % CONCLUSION Load cell can be applied as torque measurement instrument in tis researc. Creep stress relaxation and ysteresis error of torque standard depends on te property of load cell and flexure bearing. Magnitude of varied cross force error depends on te line of flexure bearing installation and its material. Te installation line tat is perpendicular to te cross force gives lower varied cross force error. Besides lower cost tan any commercial torque standard, tese models metrological properties conforms wit DKD R 37 and DKD R 38 wit measurement uncertainty ( 2σ ) ±.3%,±.2%, ±.8% for model A, model B, and model C Page 288

5 respectively. Anoter advantage of building torque standard by load cell is we can set protection level to be IP 67 or IP 68 by selecting load cell as required IP protection, wic is ard to find in commercial torque measuring device at tis time. REFERENCES [1] DKDR 37, Static calibration of reference torque wrences, October 23 [2] DKDR 38, Static calibration of torque wrenc calibration devices, October 23 Page 289