Power tiller vibration transmitted to hand and body of operator in transportation mode on asphalt rural road Seyed Reza Hassan-Beygi1*, Hossein Ahmadian1, Barat Ghobadian2 1
Dept. Agro-technology, Abouraihan College, University of Tehran, Pakdasht, Tehran, Iran.
2
Dept. Mechanics of agricultural machinery engineering, Tarbiat Modarres University, Tehran, Iran. *Corresponding author. E-mail:
[email protected]
Abstract Power tillers are used for field applications as well as on transportation of agricultural products and human beings on rural roads. These machines generate high levels of vibrations due to lack of suspension system. The power tiller operators work in harsh environment and then their ergonomic characteristics has a great importance. In the present paper, the vibration transmitted to hand-arm and whole-body of a power tiller operator was evaluated. The vibration acceleration signals for evaluation were obtained in a field experiment using a 13-hp power tiller. Experiments were conducted at five levels of engine speed, four levels of transmission gear ratio, two positions of accelerometer, and in the lateral, longitudinal and vertical directions in transportation mode on asphalt rural road. Number of three accelerometers was used for measuring the vibration acceleration. An analogue output of the accelerometers was converted to digital ones by using A/D converter. The time domain vibration acceleration signals were stored in laptop computer hard disk. The recorded digital signals in time domain were converted to frequency domain using a developed fast Fourier transform (FFT) computer program. The amount of vibration damage on operator's body and the exposure limits were calculated in accordance with the ISO standards. The results showed that the vibration acceleration was increased with increasing the engine speed at all gear ratios and directions for both positions of handle and seat. The vibration acceleration was the highest values at vertical direction for all of experiments. For 8 hours daily exposure was observed that the vibration transmitted to the hand was more than 11 m/s2 and then after 2.4 year exposure in 10% of operators of the power tiller various diseases and disorders, and depreciation on their fingers may occur. As the results the equivalent vibration transmitted from the power tiller seat to body of operator was in the range of 0.5 to 1 m/s2 (uncomfortable range). So the exposure time should be lower than 8 hours/day. It is necessary to reduce the vibration transmitted to the user’s hand and body by designing and developing adequate insulating systems or isolators. Keywords: tractor, vibration, power tiller, transportation 1. Introduction Power tillers or two wheel tractor are the main sources of power supply in small and medium size farms. Beside on field applications they also engaged on transportation of agricultural products and human beings on rural roads. These machines equipped with single cylinder diesel engine which was assembled on chassis without any vibration absorber and suspension system so they generate high levels of sound and vibrations. The power tiller operators work in harsh environment and then their ergonomic characteristics has a great importance. Long term of continues working with these machines has led to movement disorders, damage to various
body organs including the ear, spine and gastrointestinal disorders, neurological diseases, decreased of work efficiency and increased risk of hazards. To reduce the risk of using such machines, some regulations have been developed by international organizations to limit working hours and duration of vibration exposure. The International Standard Organization (ISO) for example developed ISO 5349 (1986) and ISO 2631 (1997) respectively for hand-arm and whole body exposure to vibration. Pervious research work showed that vibration transmitted to human body from power tiller was higher than ISO standards limits (Sam and Kathirvel, 2006; Dewangan and Tewari, 2009; Salokhe et al., 1995; Tewari et al., 2004). Literature survey showed that there is not published study concerning vibration of a 13-hp power tiller on transportation mode. In this research, vibration transmitted from of a 13-hp power tiller to hand-arm and whole-body of an operator was evaluated on transportation mode in different levels of engine speed and gear ratio. The obtained data can be used to power tiller structure design in order to reduce harmful effect of vibration on the operator with designing a vibration insulting system. 2. Material and methods In this research vibration acceleration of a 13-hp power tiller (Mitsubishi CT-82), at handle grip and trailer seat was measured. In order to simulate practical use of the power tiller at transportation, load of 9000 N was placed inside attached trailer (Fig. 1).
FIGURE 1: Power tiller and attached trailer Experiments was conducted at five levels of engine speed (1400, 1600, 1800, 2000, and 2200 rpm), at four levels of gearbox gear ratios (2 and 3 light, 2 and 3 heavy), and in three directions (lateral, longitudinal, and vertical) on asphalt rural road. For measuring vibration of the power tiller at position of trailer seat and handle grip, three CTC-AC192 type accelerometer were used. The required power for set up was supplied from two 12-volt battery and an electronic circuit. Using an A/D convertor which is recognized and controlled by LABVIEW software program, the accelerometer analog output voltage was converted to digital ones with 40000 Hz sampling rate and recorded on lap-top computer (Fig. 2a). These recorded time domain digital signal were converted to frequency domain narrow band signals by fast Fourier transform (FFT) algorithm using MATLAB software program (Fig. 2b). The narrow band frequency domain signals were converted to 1/3rd octave frequency band signals (Fig. 2c). The amount of vibration damage on operator's body and the exposure limits were calculated in accordance with the ISO 5349 for hand-arm vibration and ISO 2631 for whole body vibration standards. The accelerometers mounted on the right handle grip attached trailer seat of the power tiller using suitable adapter (Fig 3). The operator was seated on the seat of trailer and holding the handles to control the power tiller.
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FIGURE 3: Mounting accelerometers, (a) trailer seat, (b) handle grip 3. Results and discussion 3.1. Vibration transmitted from the power tiller handles and trailer seat in time domain Fig. 4 showed the total RMS value of vibration acceleration (sum of x, y, z directions) or total unweight acceleration versus engine speed for different gear ratios. As depicted from Fig. 4a with increasing engine speed from 1400 to 2000 rpm, the mean values of RMS at handle position were increased in the range of 14 m/s2 to 26 m/s2 which was contributed to increase number of combustion courses and piston blows per unit time. However, this increasing trend was not observed at 2200 rpm engine speed that can be contributed to the fact that 2200 rpm is the nominal engine speed of this power tiller and the engine and its components are in fairly good dynamic balance at this speed. It could stated that the power tiller handle act like a
cantilever beam and with increasing vibration at the root of handle the vibration at the free end is increased (Salokhe et al., 1995). The obtained results in this study was in accordance with similar investigations have been carried out in different working conditions (Sam & Kathirvel, 2006; Dewangan & Tewari, 2009).
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FIGURE 4: Total unweighted acceleration, (a) at the handle grip and (b) at the trailer seat The total RMS values of vibration acceleration at the trailer seat position are shown in Fig 4.b. It could be also observed from this figure that with increasing engine speed from 1400 to 2000 rpm, the mean values of RMS at the trailer seat position was increased in the range of 2.8 m/s2 m/s2 to 4.7 m/s2. Comparing Fig. 4 and b revealed that the RMS values of vibration at the seat position were lower than the handle grip position because of a part of vibration energy was damped by structure and lower part of vibration energy transmitted to the trailer seat position. The result of pervious research works showed that the power tiller vibrations on transportation condition increase with increasing forward speed as well as engine speed (1, 2, and 8). However, the forward speed of power tiller dependent on engine speed and gear ratio. In this study the effects of engine speed and gear ratio were investigated independently. It could be stated that in farm machineries due to low levels of forward speed, the influence of engine speed on the amount of vibration is more than the gear ratio. The maximum change in the vibration RMS values due to gear ratio variation was 2 m/s2 (Fig 4). As the results, comparing the RMS acceleration values in the vertical, lateral and longitudinal directions revealed that in all experiments, the acceleration in the vertical direction is the maximum and the lateral acceleration was greater than in the longitudinal direction. 3.2. Hand-arm vibration transmitted from the power tiller handles in frequency domain The envelope curve of total weighted acceleration of the power tiller handle at different engine speeds and gear ratios was illustrated in Fig. 5. According to this figure, the vibration acceleration value, in the frequency range of 8 to 80 Hz was higher than exposure limit, 2 m/s2, for hand-arm vibration exposure for 8-hour working per day. The total weighted acceleration has the maximum value at the frequency of 31.5 Hz with the vibration amplitude reach up to 11 m/s2 (Fig. 5). However, the mean value of total vibration acceleration in frequencies between 6.3 to 16 Hz, which is the most sensitive frequency of hand-arm system, has considerable values. At frequencies above 100 Hz for all of the gear ratios and engine speeds, the acceleration value is less than 1 m/s2. In this study it was found that hand-arm acts like a low pass filter and high frequency range of vibration energy, decreased by the fingers and wrist joints and the amplitude
of vibration is reduced. This trend is also observed by Sam and Kathirvel (2006), while they were studding 7.5-Hp power tiller with an empty trailer on transportation.
FIGURE 5: The 1/3rd octave envelope curves of total weighted acceleration for different engine speeds and gear ratios at the power tiller handle position Fig 6. Shows the allowable exposure time for hand-arm vibration transmitted from the power tiller handle in various engine speed and gear ratios. According to this figure, it can be seen that with increasing engine speed from 1400 rpm to 2000 rpm in all gears, the exposure time is reduced, but at 2200 rpm engine speed the allowable exposure time increases, it could be related to the dynamic balance of the power tiller engine at 2200 rpm which is the engine nominal speed. For various combinations of gear ratios and engine speeds, the allowable exposure time vary from 2.32 to 5.7 years. Investigations on power tillers vibration have shown that with increasing in forward speed and engine speed, the allowable exposure time decreased. In these studies the allowable exposure time was reported between 1.2 year for tillage and 13 years for transportation with empty trailer (Taghizade et al., 2006, Sam and Kathirvel, 2006; Tewari et al. 2009).
FIGURE 6: Allowable exposure time for hand-arm transmitted vibration from the power tiller 3.3. Whole body vibration transmitted from the trailer seat
The 1/3rd octave envelope curves of total weighted acceleration at the trailer seat for different engine speeds and gear ratios were shown in Fig. 7 in the frequency range of 1 to 80 Hz. As depicted from this figure the total weighted acceleration has its maximum value at the frequency of 31.5 Hz as same as for the handle position. However, the maximum amplitude of vibration at the seat position, 0.68 m/s2, is lower than the handle position due to dissipation of vibration energy in the path of transfer from the tractor engine to the trailer seat. Comparing the envelope curve with ISO guidelines for whole body exposure, revealed that vibration acceleration between the frequency range of 3.15 to 6.3 Hz was 0.3 to 0.4 m/s2 which was within a little uncomfortable limit, and for the frequency range of 25 to 40 Hz was between 0.5 to 0.67 m/s2, which was within almost uncomfortable limit based on ISO 2631-1997. 1/3 octave spectrom- total weighted acceleration-seat
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FIGURE 7: The 1/3rd octave envelope curves of total weighted acceleration at the trailer seat for different engine speeds and gear ratios 4. References: Dewangan, K.N., & Tewari, V.K. (2009). Characteristics of hand transmitted vibration of a hand tractor used in three operational modes. International journal of industrial ergonomics, 39: 239-245 Salokhe, V.M., Majumder, B., & Islam, M.S. (1995). Vibration characteristics of a power tiller. Journal of Terramechanics, 32:181-196. Sam, B., & Kathirvel, K. (2006). Vibration characteristics of walking and riding type power tillers. Biosystem Engineering, 95:517–528. Taghizadeh, A. 2007. Experimental investigation of a power tiller vibration. Msc. Thesis, Dept. Mechanics of Agricultural Machinery Engineering, Tarbiat Modarres Univeristy, Tehran, Iran (In Persian) Tewari, V.K., Dewangan, K.N., & Karmakar, S. (2004). Operator’s fatigue in field operation of hand tractors. Biosystems Engineering, 89:1-11
