Ergonomic characteristics and operator body fatigue against two-wheel tractor vibration

Transcription

1 International Journal of Agriculture and Crop Sciences. Available online at IJACS/2013/5-4/ ISSN X 2013 IJACS Journal Ergonomic characteristics and operator body fatigue against two-wheel tractor vibration B. Heidary 1*, S. R. Hassan-beygi 1, B. Ghobadian 2 1. Department of Agriculture Technical Engineering, College of Abureihan, University of Tehran, Tehran, Iran. 2. Department of Mechanics of Agricultural machinery Engineering, College of agriculture, University of Tarbiat Modarres, Tehran, Iran. *Corresponding author Bahareh_celestial@yahoo.com ABSTRACT: Power tillers are very simple and versatile machines with large scale diffusion in developing countries, where they are commonly used both for agriculture and for transportation purposes. The operators of these kinds of machinery are exposed to high levels of hand transmitted vibration. Long time working with this machinery cause dynamic disorders, damaging different part of the body such as: earache, spine and digestion disorders and vascular disease,investigated throughout experiments that conducted at five engine speeds(1400, 1600, 1800, 2000 and 2200 rpm) and three axes (vertical (z), lateral (y) and longitudinal (x)) in three position on engine, handle grip and operators body. The recorded digital time domain signals, were converted to frequency domain and then 1/3 octave spectrum of vibration acceleration was calculated. The amount of vibration damage on operator's body were determined. The results showed that the vibration acceleration increased with increasing the engine speed. The vibration acceleration was the highest values at vertical direction for all of the experiments. The total weighted acceleration has the maximum value at the frequency of 16, 31.5 and 60 Hz with the vibration amplitude reach up to 7.3, 6.95 and 5.8 m/s 2 respectively in handle grip and at the frequency of 6.3, 16 and 31.5 with the vibration amplitude reach up to 0.06, 0.16 and 0.07 m/s 2 respectively in chest of operator. The mean value of total vibration acceleration in low frequencies, which is the most sensitive frequency of hand- arm system, has considerable values. At frequencies above 40 Hz in operator chest position for all engine speeds, the acceleration values are less than 1 m/s 2 and in handle position frequencies above 100 Hz the acceleration values are less than 1 m/s 2. The handle vibration at specific frequencies and at high engine speeds was greater than the allowable amount for the Comfort limit and some desease and symptoms like headache, Influence on speech and Lump in the throat could be appeared. Keywords: Power tiller, operator's body, hand- arm system, frequency. INTRODUCTION The human body is both physically and biologically a system of an extremely complex nature. When looked upon as a mechanical system it can be considered to contain a number of linear as well as non-linear elements, and the mechanical properties are quite different from person to person. Biologically the situation is by no means simpler, especially when psychological effects are included. In considering the response of man to vibrations and shocks it is necessary, however, to take into account both mechanical and psychological effects. Knowledge about comfort and fatigue-decreased proficiency is based on statistical data collected under practical and experimental conditions. Power tillers are very simple and versatile machines with large scale diffusion in developing countries, where they are commonly used both for agriculture and for transportation purposes. The operators of these kinds of machinery are exposed to high levels of hand transmitted vibration. Long time working with this machinery cause dynamic disorders, damaging different part of the body such as: earache, spine and digestion disorders and vascular disease (Sam and Kathirvel, 2006; Salokhe, 1995; Tiwari and Gite, 2002). Furthermore, it causes decrease in efficiency and work the quality downfall (Tewari et al., 2004). The vibration transmission characteristics of the hand-arm system are strongly related to the magnitude and dominant directions of vibration (Reynolds et al., 1977). However, how the damages on the operator bodies were not still clear. Some studies have been stated that

2 spinal movements caused by vibration were the main reason for these destructive injuries to the body, but the exact reason not provided so far (Wang et al., 2004). In an investigation regarding the ergonomic conditions of an 8-hp power tiller, 200 farmers and 100 extension workers were studied. The study revealed that noise and vibration of the power tiller played an important role in damages experienced by them (Kang et al., 1988). Many experiments are done for investigating the power tillers vibration and tractors vibration. Taghizadeh- Alisaraei (2007) was assessed vibrations of a 7.5-hp walking power tiller. Experiments were conducted at stationary condition, plowing operation and transporting. The vibration acceleration was measured at locations of chassis and handle of the power tiller as well as the arm and chest of the driver at the vertical, lateral and longitudinal directions. The results revealed that the vibration RMS values increased in the all of location with increasing engine speed. It was observed that the dominant frequency of vibration in all directions and locations were equal to engine speed (revolution per second). The acceleration values were the maximum in the vertical direction at all of the locations. The vibration RMS values were decreased when transferring from the power tiller handle to the driver's chest (Taghizadeh, 2007). Sam and kathrivel, examined the Vibration Characteristics of Walking and Riding Type Power Tillers. The results indicate that machine vibration increased with an increase in engine speed and major excitation of the vibration of the power tiller was the unbalanced inertial force of the engine. The walking type power tiller showed higher hand transmitted vibration than the riding type during rototilling, whereas the riding type power tiller exhibited higher hand transmitted and whole body vibrations during the transport mode. It was observed that, if the power tiller is used at least 4 h per day at the forward speed of 2.4kmh -1, these disorders would appear in 4 yr, for 10% of the operators, under usual working conditions (Sam and Kathirvel, 2006). Some other similar studies have also been reported by other researchers on power tillers ; Tiwari and Gite;. The literature reviews, show that the research on operators of power tiller body fatigue has not been reported so far. Therefore, this study was conducted to measure the ergonomic characteristics of power tiller. METHODS AND MATERIALS In this research vibration acceleration of a 13-hp power tiller was measured at engine and handle. The power tiller specifications were given in Table 1. Table 1. Specifications of the power tiller under test. Engine Specifications: Internal combustion system, Indirect injection Number of cylinder: Single Stroke cycle: 4 Stroke Air intake system: Naturally aspirated Cooling system: Water cooled Rated engine speed: 2200 rpm Power at rated speed: 13 hp Other Specifications: Type of clutch Dry, multi-plates Speed: Forward 5 stage Experiments was conducted at five levels of engine speed (1400, 1600, 1800, 2000, and 2200 rpm), six levels of fuel blends (D, B5, B10, B15, B20 and B100), and in three directions (lateral, longitudinal and vertical) on stationary mode. To measure the vibration of the power tiller at position of engine, handle grip and chest of operator by three CTC-AC192 type accelerometers were used. The required power to set up was supplied from a 24-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 Hz sampling rate and recorded on laptop computer hard disk. Fig. 1 shows the instrumentation setup used in this study. The power tiller vibration assessment in time domain was carried out by using the Root Mean Square (rms) The basis of machine vibration is the use of root mean square (rms), Mathematically, r.m.s. can be expressed as: 1/2 1 T 2 a RMS = a ( t ) dt T 0 (1) Where, a rms is the acceleration root mean square (rms) (m/s 2 ); a (t) is measured acceleration domain (m/s 2 ) and T is measured acceleration period (s) (Mansfield, 2005). The vibration total value a total was described as total rms of three component's value and shows the total vibration acceleration of three axes. A total, was determined as below: (Dewangan and Tewari,!!"; "#$%. 371

3 Intl J Agri Crop Sci Sci.. Vol., 5 ((4), , 2013 a total [ = ax + ay + az ] 1/2 (2) 2 Where, the ax is total vibration acceleration in x axis in (m/s ), ay is total vibration acceleration in y axis in (m/s2) and az is total weighted vibration acceleration in z axis in (m/s2) (Mansfield, 2005). 2005) Weighted acceleration value ahw, was determined as bel below: ow: (ISO , 2001;; Dewangan and tewari, 2009; 2009 Goglia et al., 2006) (3) For a detailed investigation of the vibration signals and evaluation of operator health it is necessary to analysis the vibration signals in the frequency domain. The recorded time domain digital signals were converted to frequency domain narrow band signa signals ls by Fast Fourier Transform (FFT) algorithm using MATLAB software program. Due to sudden change and uncertainty range of the narrow band signals, the narrow band frequency domain signals were converted to 1/3rd octave frequency band signals by using a su subroutine broutine computer program. The accelerometers mounted on the power tiller engine, right handle grip and chest of the operator (Fig. 2). The operator was picked up and held the handles to control the power tiller. Figure 1 1.Vibration Vibration measurement and data data acquisition set up up. (a) (b) (c) Figure 2 2. Mounting accelerometers: (a) engine (b) handle grip and (c (c) chest of operator. operator Experiments were conducted at five levels of engine speed speeds (1400, 1600, 1800, 2000 and 2200 rpm) and in lateral, longitudinal tudinal and vertical directions a accordin ccording g to Iso 5349 standard. standard The test matrix was shown in table 2. From a shock and vibration point of view the low frequency range may be considered more important. Vibration at frequencies below 1 Hz occurs in man many y forms of transport and produces effects, e.g. Kinetosis (motion sickness), which are completely different in character from those produced at higher frequencies. These effects cannot be simply related to the three parameters of the exciting motion, inten intensity, sity, duration, and frequency, as has been possible in the range from 1 Hz to 80 Hz. In addition, human reaction to vibration below 1 Hz is extremely variable and seems to depend on a large number of external factors which have nothing to do with the motio motion, n, e.g. age, sex, vision, activity, odors. Symptoms for vibration exposure at frequencies of 1 to 20 Hz are shown in Table 1. The frequency ranges given are those where the symptoms are most predominant. 372

4 Table 1. Frequency ranges and the symptoms (bruel and kjear,1982). Symptoms Frequency General feeling of discomfort 4-9 Head symptoms Lower jaw symptoms 6-8 Influence on speech Lump in the throat Chest pains 5-7 Abdominal pains 4-10 RESULTS AND DISCUSSION The experimental design for the measurement of vibration was considered as 5 levels of engine speeds balanced factorial experiment on the basis of completely randomized design with 3 replications. The vibration acceleration rms were also statistically analyzed. Identification of differences in response of the vibrations to the fuel blends and engine speeds and axes are based on the outcome of these tests. Additional analysis was carried out in order to determine the interaction between the position and engine speeds and axes for each treatment separately. time domain spectrum analysis The vibration acceleration at the engine, handle grip and chest of operator positions of 13-hp power tiller are shown in Fig.3. Regarding contraction, the vibration acceleration spectrum by diesel fuel at engine speed of 2000 rpm is shown in figure 3, 4 and 5 in three axis of lateral, longitudinal and vertical respectively. The vibration acceleration is dominating in the vertical axis and its domain is between ±120 m/s 2 in engine position, ±40 m/s 2 in handle grip position and ±2 m/s 2 in the chest of operator. Also the vibration acceleration in longitudinal axis in most part of the spectrum is more than lateral, the longitudinal axis domain is between ±100 m/s 2 in engine position, ±20 m/s 2 in handle grip position and 0 to 1.5 m/s 2 in the chest of operator. In lateral axis domain is between ±80 m/s 2 in engine position, ±20 m/s 2 in handle grip position and ±1 m/s 2 in the chest of operator. As the tests showed, with increasing engine speed from 1400 to 2200 rpm, the mean values of handle RMS acceleration was increased which is because of in-creasing number of combustion courses and piston blows per unit time but the vibration acceleration at 1800 rpm engine speed was higher in handle grip and this can be refer to the fact that 1800 rpm is the nominal engine speed of this power tiller and the engine and handle grip and its component are in a more dynamic equilib-rium. It should be noted 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 (salokheh, 1995) Comparing the RMS acceleration values in three vertical, lateral and longitudinal directions, became clear that in all experiments, the accelera-tion in the vertical direction is maximum and the lateral acceleration was greater than in the longi-tudinal direction. As the figure 4, 5 and 6 illustrated, the vibration acceleration in vertical axis was higher than other two axis and sever vibration acceleration was seen in handle grip and chest of the operator and according to Iso 5349 and 2631 standards, this magnitude of vibration (more than 2 m/s 2 ) cause damage to operator body and limited the safe operator vibration exposure time. frequency spectrum analysis Human response to vibration is dependent on the frequency of the vibration. According to ISO 5349 (2001) recommendations so the effects of variation in engine rotational speed on 1/3 octave spectra of total weighted accel-eration (sum of three direction), at engine, handle grip and chest of operator positions, and between 2.15 to Hz was showed in Fig.6. According to this figure in all engine speeds at the frequency of 31.5 Hz, vibration acceleration has a maximum value and In general. At frequencies above 100 Hz in all engine speeds, in chest of operator position, the acceleration value is less than 1 m/s 2, But at other frequencies, changes in the Total vibration acceleration has not been uniform and the effect of changes in engine speeds in the frequencies of 16, 31.5 and 60 Hz on acceleration value is higher. According to table 1 the frequency range of 4 to 20 Hz damage to different parts of operator's body. The results showed that vibration acceleration peaks were observed between 10 and 100 Hz and around 1000 Hz and The first peak vibration occurred in engine combustion frequencies 20, 23.33, 26.66, 30, 33.33, and Hz in 1200, 1400, 1600, 1800, 2000 and 2200 rpm respectively. It was observed that an increase in forward speed in three axes, vibration acceleration increased at almost all the frequencies. However, vibration acceleration increased more in engines high speeds but in 1800 rpm the vibration acceleration of handle increase so much and the reason was the resonance happened in handle gripe so using this engine speed is so dangerous for operators and between two engine speeds of 2000 and 2200 rpm the engine speed of 2000, the engine produce less 373

5 vibration acceleration and this speed is more safe. Also it is observed that the amount of vibration acceleration in vertical axis is much more than other two axes and in longitudinal axis is more than lateral axis. The reason of the high amount of acceleration in vertical axis is the vector of engine knocks. Dewangan showed that the axes have a prominent effect on measured vibration (Dewangan et al., 2009). Also other researchers showed the same results && and tewari, 2009). Risk assessment for hand-transmitted vibrations is in most countries based on the International Standard ISO 5349 (International Standard ISO5349, 2001). The measurements of vibration according to the standardd are expressed in terms of the frequency-weighted acceleration. The total weighted acceleration has the maximum value at the frequency of 16, 31.5 and 60 Hz with the vibration amplitude reach up to 7.3, and 5.8 m/s 2 respectively in handle grip (Fig. 7) and at the frequency of 6.3, 16 and with the vibration amplitude reach up to 0.06, 0.16 and 0.07 m/s 2 respectively in chest of operator. However, the mean value of total vibration acceleration in low frequencies, which is the most sensitivee frequency of hand- arm system, has considerable values. At frequencies above 40 Hz in chet position for all engine speeds, the acceleration values are less than 1 m/s 2 and in handle position frequencies above 100 Hz the acceleration values are lesss than 1 m/s 2. So the vibration acceleration in handle grip was high in the range of Hz and the vibration acceleration in chest of user justified that vibration transmitted to operator's chest mostly in the range of Hz. In this frequency range the symptoms of headache, Influence on speech and Lump in the throat could be appeared. The frequency less than 1 Hz is approximately negligible so the motion sickness doesn t appear in operators of 13 Hp power tiller. 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 13 Hp power tiller with an empty trailer on transportation. Figure3. Time domain vibration spectrum- lateral rpm engine speed (a) engine, (b) handle grip (c) chest of the operator Figure4. Time domain vibration spectrum- longitudinal rpm engine speed (a) engine, (b) handle grip (c) chest of the operator Figure5. Time domain vibration spectrum- vertical rpm engine speed (a) engine, (b) handle grip (c) chest of the operator 374

6 Figure 6. Total unweighted vibration acceleration at power tiller in 2000 rpm engine speed, (a) chest of operator, (b) handle grip and (c) engine. Figure 7. Total weighted vibration acceleration at power tiller in 2000 rpm engine speed, (a) chest of operator, (b) handle grip. CONCLUSIONS This study was carried out for investigating the vibration of power tiller vibration, its influence on operators and the ergonomics characteristics of power tiller. For this reason the time and unweighted and wheited frequency domain signals were analyzed in five engine speeds(1400, 1600, 1800, 2000 and 2200 rpm) and three axes (vertical (z), lateral (y) and longitudinal (x)) in three position on engine, handle grip and operators body. The experiments indicated that magnitude of vibration for the power tiller engine depends on axis of measurement (the vibration was the most in vertical axis and was least in lateral and longitudinal axis) and engine speed (with increasing engine speed, the vibration increased). Also the results showed that Vibration transmitted from the handle was much higher than standard limit. The handle vibration at specific frequencies and at high engine speeds was greater than the allowable amount for the Comfort limit and some desease and symptoms like headache, Influence on speech and Lump in the throat could be appeared. ACKNOWLEDGEMENTS The authors wish to thank the University of Tehran for provided the opportunity to complete this project and Tarbiat Modares University for providing the laboratory facilities to carry out tests. REFERENCES Bruel, Kjear Human body vibration, Technical review, ISSN Dewangan KN, Tewari VK Characteristics of vibration transmission in the hand arm system and subjective response during field operation of a hand tractor, International Journal of biosystem engineering, 100, Dewangan KN, Tewari VK Characteristics of hand transmitted vibration of a hand tractor used in three operational modes. International journal of industrial ergonomics, 39: Goglia V, Gospodaric Z, Filipovic D, Djukic I Influence on operator shealth of hand-transmitted vibrations from the handles of a singleaxle tractor. Annals of Agricultural and Environmental Medicine Griffin MJ Hand Book of Human Vibration. Academic Press, London. ISO Mechanical Vibration Guidelines for the Measurement and Assessment of Human Exposure to Hand-transmitted Vibration. Part 2: Practical Guidance for Measurement in the Workplace. International Standard Organization, Geneva. 375

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