林道路面の良否判定に関する研究 (II) : (車両の振動加速度と測定諸条件との関係)

Abstract

車両の振動現象を利用し, 車両に生ずる衝撃荷重の大きさを尺度として, 林道路面の良否判定が可能であることは, 文献 (15) においてすでに記述したが, 本稿では, 路面の良否判定法の尺度である振動加速度をとりあげ, それが各測定条件に対しいかなる関係を有するか, その性質をつぎの7つに分けてそれぞれ明らかにするとともに, 測定に際してとられるべき条件の検討を行った｡ 以下, これについての内容を簡単に記述する｡ 1. 測定位置の変化と振動加速度との関係を論じたのが1章の内容であるが, 勿論, その目的は振動加速度の測定場所として, どこが適当であるかを求めることにあった｡ 筆者は, 測定の実用的立場から, 車両の荷台上を測定場所とし, その中で6個所の位置を選び, もっとも振動加速度の小さくなる位置 (重心にもっとも近いと考えられるとする) を求めこれを考察した｡ その結果, 軸距の中央, 荷台上のNo･1の位置を測定の基準とすることが適当であるという結論を得た｡ (1･4･3参照) 2. 測定車両の走行速度を変数 (20km, 30km, 40m, 50km/hの4水準測定) として, そのさい車両に生ずる振動加速度の変化を, 実験を基にして考察を行ったのが2章の内容である｡ その結果, 上下･左右の振動加速度値は速度の増大とともに増加し (2･3参照), とくにgHは, (速度の増大につれて) gVに対して相対的な増加が著しくなることがわかった｡ こうした考察から, 測定条件として, 走行速度はできるだけ小さい方がよく, 具体的には, (林道における走行条件, 測定能率を考慮すると) 大体20km/hを適当とすることが推奨される｡ (2･4･2参照) 3. 測定車両の荷台上に木材を積込む場合, その積載量の変化と振動加速度との関係を検討してみた｡ その結果, 上下, 左右の振動加速度は積載量の増大と共に減少するという傾向を見出した｡ この結果から, 路面判定のための振動加速度の測定は, 積載量条件を一定にしておくことが重要であること, および, 空車走行による測定が有利であることを導いた｡ 4. 4章では, 車両タイヤ圧をとりあげ, タイヤ圧の変化と, そのさい車両に生ずる振動加速度との関係を追究し, 振動加速度の増大はタイヤ圧の増大にほぼ比例することを見出した｡ この事実により, 測定にさいして, タイヤ圧を無視することはできないこと, および, 測定中タイヤ圧の一定 (少なくとも左右両輪のバランスがとれているべきこと) を条件とすべきことを知り得た｡ 5. つぎに5章では, 車両がカーブ路線を通過するさい, 車両に生ずる振動加速度がカーブによって, とくに小回りのカーブでどのような影響を受け, また, 路面判定にさいして, カーブ路線をいかに取扱うべきか, ということを中心として論じた｡ 測定の結果, カーブは左右振動加速度の増大に関係をもち, rの小さい (r≦30において) カーブ程, その影響の大きいことがわかった｡ したがって, r≦30のカーブ路線については, 路面判定にさいして, αVと共にαHを考慮する必要がある｡ (5･4･4参照) 6. 上記の5つの条件の外に, 振動加速度の測定にさいして, 一応考えておくべきつぎの条件をとりあげ, 1), 車両の種類2), 路面の勾配の大小, 3), 運転者の技能度, 等と測定される振動加速度との関係を述べた｡ 7. ついで上記測定条件の中で, 振動加速度に対する主なる作用因子として, 走行速度, タイヤ圧, 測定位置をとりあげ, 振動加速度に対する作用因子間の交互作用につき検討を行った｡ その結果, 上記3因子の間には, 交互作用のないことを確認した｡ このことから, 振動加速度の測定にさいして, われわれが測定条件をかなり弾力的に (必ずしも特定の条件に拘束されることなく, 現場に応じて) 決めうることをえた｡ (7･4参照) 本研究における詳細な考察･論議の結果, および文献 (15) に基礎をおくことにより, 車両に生ずる衝撃荷重の大きさ (具体的には, 振動加速度の係数αH, αV の大きさ) を尺度として, 林道路面の良否を判定する一つの方法が確立された｡ なお, 本研究の主なる適用･応用性はつぎのように要約できる｡ (1). 林道路面の良否に対する直感判断に科学的な根拠を与えることができ, 比較の基準がえられる｡ (2). 林道路面改善への一つの基礎となり得る｡ (3). 路面の良否の数値的な把握によって, それを基にした科学的な路面の補修, 管理を可能にし, また林道における車両の運転上の安全性向上に役立て得る｡ (4). 判定法の尺度である衝撃荷重の大きさは, とくに悪路での走行を要求される林業用車両の損耗を理解する上に重要な意味をもち, その方面への研究に役立たせることができる｡

The main purposes of this paper were to make clear the relation between the characteristics of the vibro-acceleration and the measuring conditions, and also to find the best condition for the estimation of the quality of forest roads as in Report No. 1515). Therefore, the followings were considered and discussed in chapters 1~7 respectively. 1. It can be generally pointed out that the vibro-acceleration measured by means of a vibrograph which is fixed on a motor-truck will be the smallest at the position of its center of gravity, C. G. and may be different at each position on which it can be measured. In practice, however, it was very difficult to measure the vibro-acceleration at C.G., so that I determined to select, on practical grounds, the best place for measuring it on the loading deck of the motor-truck. In this case, the following problem will appear to us : what position is suitable for tests. In order to find the most suitable position for the measurement of vibro-acceleration, each position, Nos. 1~6, as shown in Fig. 9 of Report No. 15 was experimentally discussed in Chapter 1. From the results, it was seen that the value of vibro-acceleration was a minimum at the position No. 1 and that this position was the optimum for the measurement of vibro-acceleration on the loading deck of the motor-truck. 2. In chapter 2, the relation between the increase of the speed and the variation of the vibro-acceleration was experimentally studied at the 4 speed levels 20, 30, 40 and 50km/h. It was made clear from results that with increasing the speed, the values of the vibro-acceleration, gH and gV increased, but in these cases at higher speed the value of gH became relatively greater than that of gV. Therefore, if we consider the measuring efficiency and effect, it is recommended that the speed at which the relation of gH<gV can be satisfied be selected for this investigation, and especially in rough forest roads, average speed 20 km/h is best used for this purpose. This is because the roughness of road surfaces is closely related to the vibration in a vertical direction. 3. It is generally considered that the vibration occuring in the running motor-truck may not be independent of the amounts of carrying timber on its loading deck. In this chapter, the test was to investigate the relation between the amount of the carrying timber and the variation of the vibro-acceleration, and also it was made to find a suitable measuring condition for estimating the quality of forest roads. From the results, we can draw the conclusion that the values of the vibro-acceleration decrease with an increase of amounts of the carrying timber. Thus, it is necessary to keep constant the loading condition in order to measure the vibro-acceleration and also in this case I consider that measurements using an unloaded motor-truck are more suitable than those obtained by using a loaded motor-truck for the purpose of this study. 4. The relation between the tire-pressure of motor-trucks their vibro-acceleration was investigated in chapter 4. This chapter had two purposes; the first was to establish an optimum measuring condition for the object of this study, and the second was to investigate the effects of the tire-pressure. The following facts became clear from the results; first the values of vibro-acceleration were nearly in proportion to variations of the tire-pressure, and secondly the tire-pressure could not be neglected in measuring the vibro-acceleration. 5. Driving on curved roads, a given motor-truck is affected by centrifugal force. From this point of view, the effect of curve on vibro-acceleration was investigated in this chapter. From the results, it became obvious that the value of GH in horizrntal direction increased with the decreasing of the radius (r) of curvature in the range of r≦30 m, but that of gv in vertical direction was not noticeably influenced by the value of r. Thus, I consider that the r of a curved road is an important factor which is impossible to be neglected when estimating the quality of forest roads, and in this case especially it is important how many curves (r≦30) are included in a given road, because the effect of r based on centrifugal force is found only in the range of r≦30, when the average speed is 20 km/h. 6. The following conditions which could not be pointed out in previous chapters were discussed on the vibro-acceleration in chapter 6; (1) types of given motor-truck; (2) average slope of a road; (3) the driver of a motor-truck. 7. It is easily seen that the followings among the measuring conditions are very important factors in determining the vibro-acceleration; the speed, the tire-pressure, the measuring position on which a vibrograph is fixed and amounts of loaded timber. In chapter 7, the interaction between the measuring conditions was investigated on the basis of the first three factors. From the test results, it was seen that there was not any interaction between the speed and the measuring position, and also between the speed and the tire-pressure. Therefore, it is seen from these facts that we can select the measuring conditions (example : speed, tire-pressure, position of vibrograph etc.) easily reached at a testing place, even if it is difficult to use conditions (the best value or the best position) recommended in chapters 1~4 for the purpose of this method. This is available for our selections. On the basis of the considerations described in chapters 1~7 and Report No. 1515), I have established a ṡcientific method for estimating the quality of forest roads by means of the vibro-acceleration occuring in a running motor-truck, that is, in practice by means of its each coefficient, αH and αV. The applicative estimation of this method is made as follows. Assuming that forest roads, A and B, are measured by a motor-truck T1, for instance, when αH and αV of forest road A are smaller than those of B, it is judged by the above-mentioned way that the former is better than the latter in regard to the quality of forest road surfaces. In this case, when tests are made on straight roads, according to my experiments, it is clearly seen that the mere comparison of each αV is enough to estimate the quality of road surfaces. This reason is based on the fact that the impact load increases in proportion to the coefficient of the vibro-acceleration. According to my opinion, a way of universalizing this method is as follows. Suppose that two tested roads A and B are geographically far apart and can not be measured by means of a same moto-truck, T1. Furthermore, suppose that the value of αV is the coefficient of vibro-accel eration on road A measured by T1 and that the value of αV′ is it on road B measured by T2 (T1≠T2). If the value of the vibro-acceleration at various sizes of concaves as seen in Fig. 6 of Report No. 1515) are given by T2 as well as by T1, and also the curves showing the variation of the vibro-acceleration against various sizes of concaves are obtained by graphs for T1 and T2 respectively, on the bases of these values (or from two given curves) it may be possible to make an indirect comparison between αV and αV′, that is, between road A and road B. As described above, the characteristic of this paper lies in the point where the value of the impact load was used for estimating the quality of forest roads. Therefore, I consider this investigation may be possible to contribute not only to the estimation of the quality of road surfaces but also to a study on the degree of fatigue of the motor-truck.