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Korea Soil fax: +82 42 823 6246. E-mail address: sochungcnu.ac.kr (S.-O. Chung). 0167-1987/$ see front matter C223 2013 Elsevier B.V. All rights reserved. http:/dx.doi.org/10.1016/j.still.2013.07.013 Effects of gear selection of an agricultural PTO load during rotary tillage Yong-Joo Kim a , Sun-Ok Chung b, *, Chang-Hyun Choi a Machinery Technology Group, Advanced R Van et al., 2009) for efficient and optimum design of a tractor (Han et al., 1999). Most studies on the load analysis have focused on the transmission since it makes up about 30% of the total tractor costs (e.g., Kim, 1998). For analysis of the transmission load, researchers analyzed torque load acting on the transmission input shaft and the driving axle shafts of the tractor during field operations such as plow tillage (Kim et al., 2001; Nahmgung, 2001). The load on the transmission input shaft and the driving axle shafts increased with plowing speed in most of the field conditions. Some research considered load on PTO shafts during rotary tillage and baling operations. Kim et al. (2011b) analyzed power consumption of a tractor with a rated engine power of 75 kW during baler operation and reported that ratios of the power consumption to the engine power were 5075% for all PTO gear levels. Also, Kim et al. (2011a) analyzed power requirement of major components (driving axles, PTO shaft, and hydraulic pumps) of a 30 kW agricultural tractor during plow tillage, rotary tillage, and loader operations. Rotary tillage required the greatest power, and the PTO shaft experienced the greatest amount of the power among the components during the process. Summarizing the findings above, considerable amount of the load was applied on PTO shaft during rotary tillage. However, research related to the effects of transmission (i.e., operation speed) and PTO gear selection on the tractor load during field operations has not been reported. This study was an attempt to provide guidelines for the optimum gear setting, considering both field efficiency and load severeness of the major power transmission parts. The purpose of this study was to analyze effects of gear selection on loads acting on the transmission and PTO shafts of a 75 kW agricultural tractor during rotary tillage. 2. Materials and methods 2.1. Measurement system A 75-kW agricultural tractor (L7040, LS Mtron Ltd., Korea) was used in this study. The tractor had a total mass of 3260 kg and dimensions of 4077 mm C2 2000 mm C2 2640 mm (length C2 width C2 height). The rated engine power and the PTO power of the tractor at an engine revolution speed of 2300 rpm were 75 kW and 65 kW, respectively. The tractor was equipped with a Synchro-mesh type manual transmission composed of two direction-gears, four main-gears, and four sub-gears. The 16 forward and 16 backward ground speeds of the tractor were determined by combination of the gear settings. The PTO rotational speeds of the tractor at P1, P2, and P3 settings were 540 rpm, 750 rpm, and 1000 rpm, respectively. Fig. 1 shows the torque transducers and radio telemetry systems fitted on the transmission and PTO input shafts for load measurement. The transmission and PTO input shafts were connected directly to the engine crankshaft; therefore, the speed ratio between the engine crankshaft and the shaft was 1:1. The load measurement system was installed inside of the clutch housing. The load measurement system was constructed with strain-gauge sensors (CEA-06-250US-350, MicroMeasurement Co., USA) to measure torque, radio telemetry I/O interfaces (R2, Manner, Germany) to acquire the sensor signals, and an embedded system to analyze the load. For load measure- ment of the transmission, a strain-gauge with a rotor antenna was installed on the transmission input shaft, and a stator antenna was installed on the shaft case. For PTO load measurement, a strain- gauge was installed on the flywheel-sleeve, and a rotor antenna and a stator antenna were installed on the flywheel and engine case, respectively. The embedded system had the maximum resolution of 24 bits. Load signal from the strain gauges of the calibrated torque transducers was digitized with a sampling rate of 19.2 kHz at a 24-bit resolution and stored in the embedded system (MGC, HMB, Germany). A program to measure the load signal was developed based on Labview software (version 2009, National Instrument, USA). 2.2. Experimental methods The load acting on the tractor during field operation depends on many factors such as soil condition and driver skill. Because taking all of these factors into consideration was not practical (Nahm- gung, 2001), effects of these factors were minimized in the study to focus on effects of ground speed and PTO rotational speed on the load through gear selection. Rotary tillage was conducted at three ground speeds and three PTO rotational speeds in upland field sites located at 35859 0 23 00 and 35859 0 26 00 North and 127812 0 56 00 and 127813 0 3 00 East. The soil type was sandy, the average water content was 22.3%, and the average cone index was 1236 kPa, over the depths of 0250 mm. The tillage depth was set as 20 cm. The transmission gear was set to at L1, L2, and L3 gears to match with PTO gears of P1, P2, and P3, respectively. The gear settings were selected based on the results of a survey for the annual usage ratio of tractor reported by Kim et al. (2011a). The ground speeds of the tractor at L1, L2, and L3 were 1.87 km h C01 , 2.64 km h C01 , and 3.77 km h C01 and the PTO rotational speeds at P1, P2, and P3 were 540 rpm, 750 rpm, and 1000 rpm, respectively. The rotary tillage tool was a heavy duty rotavator (WJ220E, WOONGJIN, Korea), and the required rated power, total mass, tillage width, and dimensions were 75 kW, 750 kg, 2220 mm, and 1050 mm C2 2390 mm C2 1380 mm (length C2 width C2 height), respectively. 2.3. Load analysis Procedures to analyze the tractor load would be different, depending on the purpose. Many researchers have used simple statistics such as average, maximum, and minimum values in order to represent the load. This method extracts representative values to show the difference of amplitude, but this simplification prohibits characterizing the whole load profiles since the field load is irregular. Effects of gear setting on the transmission and PTO load, One-Way ANOVA and the least significant difference test (LSD) were conducted with SAS (version 9.1, SAS Institute, Cary, USA). Also, it is difficult to represent effects of the load on the tractor since the load causes damages to the tractor, and fatigue of the tractor parts also needs to be investigated. Fatigue of a tractor is defined as the damage sum from repeated loadings (Lampman, 1997). Severeness, another method of load representation proposed by Kim et al. (1998, 2000), is defined as the ratio of the damage sum at each operation to the minimum damage sum from all the operations. Severeness would be inversely proportional to fatigue life. When load severeness is greater, fatigue life would be shorter. Kim et al. (1998) measured the loads acting on the transmission input shaft and analyzed the load severeness during plow tillage, rotary tillage, and transportation operations. They found that the severeness during transportation operation was similar to that during plow tillage, but the severeness during rotary tillage was about 63 times greater than that during the transportation operation. Later, Kim et al. (2000) analyzed the severeness of the transmission input shaft during rotary tillage at four speed combinations of the tractor ground speeds (2.9 km h C01 and 4.1 km h C01 ) and PTO rotational speeds (588 and 704 rpm) using a tractor with a rated engine power of 30 kW. The load severeness increased by 2.32.6 times when the PTO speed increased with the Y.-J. Kim et al. / Soil Nguyen et al., 2011). The SN curve was obtained for the material of the input shaft, SCM 420H, using the ASTM standard (2004) in Eq. (3). The ASTM standard has been widely used for fatigue analysis of materials (Wannenburg et al., 2009; Mao, 2010). N 10 6C06:097logS=223 (3) where N is the number of cycles, S is shear stress (MPa). To calculate damage sum, equivalent torque of load spectrum was converted to stress (Rahama and Chancellor, procedures of severeness evaluation. input Y.-J. Kim et al. / Soil Petracconi et al., 2010). The diameters of the transmission and the PTO input shafts were 28 mm and 26.5 mm, respectively. S 16T pd 3 (4) where S is stress (MPa), T is equivalent torque (Nm), and d is diameter of the shaft (mm). The damage sum was calculated based on the Miners rule (Miner, 1945) in Eq. (5). Miners rule is a procedure for estimating the number of cycles of loading to failure (Miner, 1945; Robson, 1964; Renius, 1977). The number of cycles (n) was derived from an equivalent torque of the load spectrum. The fatigue life cycles (N) was derived from the SN of SCM 420H. The damage (D) was calculated by dividing the number of the fatigue life cycles into the number of cycles. D t X k i1 n i N i (5) where D t is damage sum, n i is number of cycles, and N i is fatigue life (cycles). 3. Results and discussion 3.1. Transmission and PTO loads by gear selection Fig. 3 shows examples of torque loads on the transmission and PTO input shafts for the ground speed at L1 and the PTO rotational speed at P2 during the rotary tillage operation. The rotary tillage operation consisted of a preparing period to descend the 3-point Fig. 3. Example of torque loads on the transmission and PTO hitch, an operating period to till the soil, and a completion period to ascend the 3-point hitch. The measured torque on the transmission and PTO input shafts showed steep increasing in the preparing period and decreasing in the completion period, and torque on these components showed irregular fluctuation patterns in the Table 1 Average torque (Nm) on the transmission and PTO input shafts by gear setting during Transmission input shaft P1 P2 P3 L1 29.8 C6 2.8 Aa 1,2,3 35.9 C6 4.2 Ba 38.7 C6 4.7 Ba L2 43.5 C6 4.7 Ab 51.3 C6 3.3 Bb 58.5 C6 3.1 Cb L3 64.9 C6 3.2 Ac 71.5 C6 3.5 Bc 82.1 C6 4.9 Cc 1 Average C6 standard deviation. 2 Means with different superscript (A, B, C, D) in each row are significantly different 3 Means with different superscript (a, b, c, d) in each column are significantly different 4 Values in the parentheses are the ratios of the torque on the PTO input shaft to the operating period. The magnitude and range of the measured torque on the PTO input shaft were greater than those on the transmission input shaft in the operating period. Table 1 shows torque levels on the transmission and PTO input shafts by speed combination of the ground speeds (L1, L2, L3) and the PTO rotational speeds (P1, P2, P3). The average torque was calculated only for the operating period data, not for the preparing and completion periods. The averaged torque levels of the PTO input shaft were greater than those of the transmission input shaft at all gear levels during the rotary tillage. These results were similar to the results by Kim et al. (2011a) that the PTO required the greatest amount of power among the major components during rotary tillage. The average torque on the transmission input shaft was considerably and significantly increased as the ground speed increased from L1 to L3 at the same PTO rotational speeds. Load increases on the transmission and driving shafts with the speed increase of plow tillage were also found by Kim et al. (2011a,b) and Nahmgung (2001). Also, the averaged load on the transmission input shaft increased as the PTO rotational speed increased, except that the load values were not significantly different between L1P2 and L1P3. The average torque on the PTO input shaft increased as the ground speed and PTO rotational speed increased. The increases were statistically significant for the PTO rotational speed, but not significant for the ground speed. 3.2. Severeness evaluation Figs. 4 and 5 show load spectra of the transmission and the PTO input shafts by gear setting during the rotary tillage, respectively. shafts torque at the L1P2 selection during rotary tillage. The spectra were constructed considering the entire life of the tractor, and the numbers of cycles were in the ranges from 10 3 to 10 7 . The range of the maximum torque ratio of the transmission input shaft was 0.71.5 for the speed-combinations and the greatest torque ratio was found at L3P1 as shown in Fig. 4. In rotary tillage. 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