固定刃付き刈払機刈刃に関する研究 第１報 作業性の実態

Abstract

For the purpose of preventing frequent stone accidents in using bush cutter, we have developed a new cutting system equipped with a fixed blade upper the rotary blade, which is designed to catch grass and cut the way a pair of shears do. Experiments were carried out to know the mowing efficiency and the physical stress on the operators in mowing work. The results were compared with conventional bush cutters (8-cutter blade) without a fixed blade. The major results obtained are summarized as follows.1) In test mowing of oat grass, with a new cutter equipped with the fixed blade, a slightly mowing efficiency was obtained. The lower rate of mowing efficiency can be attributed to the following factors. (1) Diameter of the rotary blade is smaller than that of 8-teeth blade by 4mm. (2) The cutter head including gear case and blades is heavier than that of 8-teeth blade by about 70%, which may affect mowing operation adversely. (3) Engine was set at high speed of 8, 000rpm, which might have caused the cutter blades to catch less grass, resulting in slower movement of the bush cutter.2) With the new cutting system, higher mowing efficiency was obtained with engine speed 6, 000rpm (peripherals speed of the rotary blade 21m/sec) than with 8, 000rpm (29m/sec). The lower engine speed allowed the cutter to catch more grass and cut grass with better performance. The result was in contrast to the phenomenon with ordinary bush cutters.3) Relation between a degree of abrasion on the rotary blade and mowing efficiency was investigated. A used blade with abrasion of about R 0.5 allowed the cutter to catch more grass and to yield higher mowing efficiency than with a new blade.4) Ordinary cutter blade yield higher mowing efficiency in mowing grass with low cutting load. However, the efficiency declined steeply in mowing thick grass in high density and with high moisture. In Contrast, the new cutting system maintained a stabilized mowing efficiency without being affected by the degree of grass density.5) The effect of using the new cutting system on the extent of operator's physical stress was also studied. Weight of the cutter head is heavier than that of conventional machines by about 70%, which caused slightly heavier pyhsical stress on the operator.6) In test mowing with machines equipped with cutter heads of the same weight, heart rates with an ordinary blade increased by 144%, and that with the new cutting system by 138%. Lower rate as observed for the latter was believed to have resulted from moment of inertia and self rotating angular velocity which were less than for an ordinary blade by about 1/2 and 1/3 respectively, corresponding to reduction of about 1/6 in gyroscopic couple force generated on the new cutting system blade. Reduced gyroscopic couple made it easier to operate the machine and reduced pysical stress on the operator.7) Rate of increase in heart rates in operating the cutting system to mow along concrete wall fence was smaller. Rotary blade of the system, smaller in diameter than that of the fixed blade on top, worked to prevent itself to hit concrete wall and to spatter concrete debris. Also, it worked to prevent the machine to get swung back on hitting the wall. All this resulted in reducing the degree of pyhsical and mental stress on the operator.