M. Feyzi; A. Jafari; H. Ahmadi
Abstract
Introduction Tree felling is an important part of forest exploitation. According to the condition of Iranian forests, mechanization in these forests has not been grown as it should be. Therefore, a main part of tree felling operations are performed by chainsaw. This machine can cause high level hand-arm ...
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Introduction Tree felling is an important part of forest exploitation. According to the condition of Iranian forests, mechanization in these forests has not been grown as it should be. Therefore, a main part of tree felling operations are performed by chainsaw. This machine can cause high level hand-arm vibration which is one of the main reasons of white finger syndrome. This syndrome affects the nerves, blood vessels, muscles, and joints of the hand, wrist, and arm. Reducing and controlling chainsaw vibrations are unlikely to be achievable without awareness of effective factors of it. So, the effects of various factors should be studied. It seems that, single cylinder engine and the interaction of the chain with wood can be the main reasons of chainsaw vibration. Therefore, in the current study the effects of engine rotary speed and wood type on the vibration acceleration of a common chainsaw have been investigated. In contrast to the previous studies, no-cutting condition as a control sample was added to experiments and the effects of cutting itself were evaluated. Experiments were also conducted in three different engine speeds to investigate the effects of interaction between wood species and engine speed. Materials and Methods The vibration acceleration was measured on three orthogonal axes on the front handle of Stihl-070 chainsaw. The measurements were conducted at three levels of engine speed (6000, 7720, and 8630 RPM) during four operations. The operations include cutting three types of Iranian woods (Beech, Hornbeam, and Alder) and no-cutting as a control sample. The experiments were conducted in split-factorial design where the operation was taken as main plot and different speeds and directions as factors. A single-axis piezoelectric accelerometer (VMI-192) was used to sense the vibration. Vibration signals were received and analyzed by a portable data acquisition system (Easy Viber). The RMS vibration acceleration at one-third octave frequency bands in the center frequency range between 6.3Hz to 1250Hz were calculated from acceleration-frequency vibration spectra. Then, the amounts of frequency-weighted vibration acceleration (ahw) were computed based on international standards. SAS software was used to analyze the data statistically. Results and Discussion The results of ANOVA showed that the effect of operation on frequency-weighted acceleration was insignificant, but the effects of engine speed and vibration axis were significant at the 1% level. When the chainsaw was free of cutting, the vibration acceleration values were lower in lower frequencies and higher in higher frequencies compared to cutting wood operations. There was a significant difference between the vibration acceleration in 6000 RPM engine speed and other two speeds, but the differences between 7720 RPM and 8630 RPM engine speeds was insignificant. However, an increase in engine rotary speed increased the value of frequency-weighted vibration acceleration. The acceleration-frequency vibration spectra had peaks at frequencies in accordance with the engine combustion frequencies (100 Hz in 6000, 125 Hz in 7720, and 160 Hz in 8630). Maximum value of unweighted vibration acceleration was observed during no-cutting operations in 6000, 7720, and 8630 RPM engine speeds to be 77.2, 138.6, and 139.0 m s-2, respectively. Vibration acceleration was highest along the Xh (perpendicular to the palm area) equal to 12.05 m.s-2 followed by Yh axis (along the third metacarpal bone) with the value of 9.12 m s-2. Altogether, the results of these tests indicated that the vibration level of employed chainsaw in this study is very higher than other machines has been investigated by other researchers. Conclusion The effects of wood species and engine speed on the hand-transmitted vibration of chainsaw were evaluated. The operation has not significantly affected the frequency-weighted vibration acceleration. However, cutting operations, increase and decrease the vibration acceleration values in lower and higher frequencies, respectively. The frequency-weighted vibration acceleration had higher values in higher engine speeds. Also, it is concluded that the risk of white finger syndrome among the operators of this type of chainsaw is very high.
M. Feyzi; A. Jafari; H. Ahmadi
Abstract
Introduction: Nowadays most of the agricultural and industrial tasks are performed using different machines and almost any people are exposed to the vibration of these machines. Just as sound can be either music to the ear or irritating noise, human vibrations can either be pleasant or unpleasant. Whole-body ...
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Introduction: Nowadays most of the agricultural and industrial tasks are performed using different machines and almost any people are exposed to the vibration of these machines. Just as sound can be either music to the ear or irritating noise, human vibrations can either be pleasant or unpleasant. Whole-body vibration and hand-arm vibration are two main types of unpleasant vibration. The hand-arm transmitted vibration can cause complex vascular, neurological and musculoskeletal disorder, collectively named as hand-arm vibration syndrome. The chainsaw is a portable machine, powered by a two-stroke engine. This machine is used by tree surgeons to fell trees, remove branches, and other activities such as prune trees. The chainsaw exposes own operators to high level of hand-arm vibration which can lead to problems such as vibration white finger syndrome and Raynaud's phenomenon. White finger syndrome affects the nerves, blood vessels, muscles, and joints of the hand, wrist and arm. It is clear that before trying to control the vibrations, the level of vibrations should be identified. Therefore, an investigation on the vibration level of this machine is crucial.
Materials and Methods: The Stihl-MS230 chainsaw was selected in this study. The size of this type of chainsaw is middle and it is equipped with anti-vibration system. According to the ISO-7505 standard, vibration must be measured at three speed level of engine. First at idling speed, second at nominal speed and third at 133% of the nominal speed or maximum speed of engine whichever is less (Racing). So 2800, 10000, and 13300 RPM Engine speed were selected. One of the employed accessories was ARMA ETI-TACHO tachometer which had been fabricated in Taiwan. The vibrations were measured and analyzed using the portable data acquisition system (Easy Viber). During the measurements, data acquisition system was powered by internal batteries. The vibrations were sensed by the piezoelectric accelerometer (VMI-192). The accelerometer mounted on an adapter inserted between the handle and accelerometer. The experiments were conducted in split plot completely randomized design. Ninety tests in two handles, three speeds of engine, three perpendicular axes and five repeats were conducted. The vibration acceleration at various conditions was measured and the root mean square of vibration acceleration was calculated based on acceleration-time spectrum. To investigate the characteristics of vibration in different speeds, the vibration spectrums in time domain were converted to spectrums in frequency domain. The frequency weighted RMS acceleration at 1/3rd octave bands from 6.3Hz to 1250Hz and the vibration total value was calculated from frequency spectrum. To analyze the obtained data, SAS software was used. Furthermore, the Duncan's multiple range tests were used to compare the RMS values.
Results and Discussion: Main source of vibration of chainsaw was single cylinder engine. The acceleration spectra of employed chainsaw had peaks in frequencies in accordance with the speed of engine. These peaks in 2800 rpm, 10000 rpm and 13300 rpm speeds of engine occurred in 46.5Hz, 166.5Hz and 221.5Hz, respectively. To achieve a safe design for handle of portable tools, identifying the frequency which leads to the maximum value of vibration acceleration is very useful. To avoid the resonance phenomenon, the natural frequency of handle must be far from dominant frequency of engine. The results of ANOVA showed that the RMS acceleration in different axes and different speeds were significant at 1% level. The maximum value of vibration acceleration, at idling engine speed, occurred in the lateral axis. In addition, the mentioned variable was maximized in normal and axial axes at nominal and racing speeds, respectively. The total value of vibration was increased when the speed of engine moving away from nominal speed. This increase in rear handle is very larger than front handle. The total value of vibration was maximized under rear handle and idling engine speed conditions. So the exposure to white finger disorder in right hand of operator is more probable. Total daily duration of exposure was assumed to be equal to 2h. According to the international standard (ISO 5349), 10% of the users can expect developing the symptoms of white finger syndrome less than seven years.
Conclusions: In chainsaw, strokes caused by combustion are the main source of vibration. On the front handle, the maximum value of vibration acceleration, at 2800, 10000 and 13300 RPM engine speed, occurred in the lateral, normal and axial axes respectively. The vibration acceleration value was maximized under rear handle and racing engine speed conditions caused by large value of vibration acceleration at 400Hz center of 1/3rd octave bands. The total value of vibration in idling and racing engine speeds, on rear handle are more than front handle significantly but in nominal engine speed, on rear handle is less than front handle.
