Studies on the Bush Cutter equipped with a Fixed Blade (2). Effect of the Shape of the Fixed Blade on the Mowing Performance and the Surface Electro-Myogram of Operator.:Effect of the Shape of the Fixed Blade on the Mowing Performance and the Surface Elec
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The mowing efficiency, resistance force imposed on the bush cutter control shaft, and burden on the operator's muscle were investigated and compared with conventional blades to discover a shape of the fixed blade which is suitable for mowing weeds and grass. The following is a summary of the results of the investigation.1) Mowing efficiency of the 23-tooth fixed blade is higher and more stabilized than that of 19-tooth and 45-tooth fixed blade, with engine speed of over 6, 500rpm in operation.2) The pitch of the cutting edge of the 45-tooth fixed blade is too small to cut all the grass and inevitable inching operation results in a conspicuous decline of mowing efficiency, especially in a higher range of running speed of the rotary blade.3) The fixed blade having 19 teeth has a cutting edge of a wide pitch, so that it takes in a large amount of grass stems. However, it supports only part of the stems, and most of them are pushed down by the rotating cutting blade before they are cut. To prevent this, it is necessary to reduce the amount of grass for the fixed blade to take in. For this reason, mowing is done by slowing the travel speed of the bush cutter. Consequently, the mowing efficiency did not increase in this test, too.4) The degree of mowing resistance inflicted on the control shaft in directions of right and left, and up and down was measured. With the fixed blade, the resistance on the right and left side was on the lowest level with engine speed of about 6, 500rpm in operation and the resistance increased with the engine speed below and above 6, 500rpm. This phenomenon is reverse to that of mowing efficiency. The phenomenon can be explained as follows. (1) While a lower engine speed allows the cutter to take in more grass stems, the cutting capability can not catch up with the fast speed of the cutter movement which is set in the experiment. (2) A faster engine speed accelerates the rotary speed of the blade which causes some grass stems to be pushed by the outer periphery of the blade. The above phenomenon was more conspicuous with 19-tooth and 45-tooth fixed blades than with 23-tooth fixed blade.5) The degree of mowing resistance on the control shaft in directions of up and down was the same on different types of the fixed blade. With the fixed blades, the resistance was on the puls (up) side, the reason being that the grass stems, after being cut, worked to push the bottom of the rotary blade upward. With an ordinary cutting blade, the resistance was on the minus (down) side, the reason being that the grass was cut down and left on the cutting blade. The grass accumulated on the blade worked to push the blade downward.6) Surface electro-Myogram on 8 points of the operators shoulders and arms was measured. With both of the fixed blade and ordinary blade, a strong electric emission was recognized on the left shoulder muscle (M. trapezius), where the suspension belt is placed and on the arms muscles (M. branchioradialis), the area to support cutter's handle operation. The electric emission signifies muscle strain.7) Electric emission from the left M. trapezius and left M. branchiordialis in using an ordinary blade, was less than that in using the fixed blade, because the weight of cutting head of the ordinary blade was lighter than that of the fixed blade by about 0.5kg. But, the emission from the right side muscles, in using the ordinary blade, was more than that in using the fixed blade. The increased electric emission from the right side muscles in judged to be attributed to the increased mowing resistance (in directions of up and down) caused by the grass clippings accumulated on the blade, and to bending and twisting for arms to throw the grass clipping on the blade. To ascertain the judgment, the weight of the cutting head of the ordinary blade was adjusted to the same weight as of the fixed blade, to make comparison in the electro-myogram.
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