Abstract of Technical Note (TN-76)

National Institute of Occupational Safety and Health, Japan

The Study on 'Safety Distance' of Power Press

TN-76-5

Kiyoshi FUKAYA, Noboru SUGIMOTO and Yoshinobu SATO

: This study comprises two parts ; a simulation of power press operation to study the velocity of hand ; an industrial survey of stop-time, that is, time interval between the moment that emergency stop button was pushed (t₀), and the moment that the ram stopped ; both were essential in order to calculate "Safety distance".
In this simulation two types of maximum velocities were gained ; one of them was for the movement whose initial velocity was zero, and the other was for the movement whose initial velocity was not zero. The former was 1.43 m/s and the latter was 1.84m/s. They correspond Two-hand trip and Presence sensing device respectively. But further research is necessary to get standard of velocity in press operation.
Stop-time consisted of delay time of brake (T₁) and effective brake time (T₂). As to the same press, T₁ was constant. T₁ ruled after-run-length, that is, length that ram moved after the emergency stop button was pushed. T₂ had been expected to depended on the crank angle θ₀ at T₀, but in some Presses the influence of θ₀ was cancelled by counterbalance. There was a press whose stop-time was 85 ms, on the other hand there were presses with loose brake whose stop-time were more than half-cycle.

Transition to and Decaying of Detonations in Acetylene-Air Mixtures --Effects of Perforated Plates in Tubes on Flame Travelling--

TN-76-6

Toshihiro HAYASHI

: It has been well known that flames occurred in a tubular vessel containing explosive gas mixtures could transit to detonations after travelling some distance, provided the composition of the fuel-air mixture and the geometry of vessel are in adequate ranges. Because of the high destructive power of a detonation, it would be important, from the viewpoint of safety, to prevent the transition to detonation or, when the the transition has been attained, at least to interrupt the detonation before it travels a long distance. When attempting to prevent the transition to detonation, informations are necessary for various conditions on the detonation induction distance (D.I.D), along which flames should have travelled after an ignition in order to transit to detonation, and also on methods of delaying the transition to detonation.
    This report describes about D.I.D for acetylene-air mixtures in 1- and 2-inch enclosed tubes, and about effects on D.I.D. of such factors as acetylene contents and initial pressures of mixtures, and length of tubes through which flames travel.
    Effects of various perforated plate, inserted in the test tube against to flames, are also studied, and the result is obtained which proves that any perforated plate does never accelerate a flame propagation, but, on the contrary to the traditional opinions, delays the flame development and thus the transition to detonation.
    Further experiments are carried out to obtain some knowledge on behaviours of a detonation when it comes upon any perforated plate in its travelling way. It is shown that detonations could be decayed temporarily through the perforated plate with holes of relatively small diameter. Although detonations may be interrupted directly using such quenching element as sintered metal, they would be more easily quenched after their temporary decayings. Those results mentioned above indicate that the design of a detonation arrester may be possible, which is composed of a perforated plate (i.e. a detonation decaying part) and some form of quenching element, the latter should be positioned before the point of re-transition to detonation of decayed flames.

Accident Investigation in Tunnel Construction Sites (2nd Report) --The case of New Sanyo Line Tunnels of Hiroshima and Yamaguchi Prefectures--

TN-76-7

Ikuo MAE and Shigeo HANAYASU

: It is well known that injury rates (frequency rates) in the construction industry ate higher than to many other industries. Accidents associated with tunnel construction have a very high frequency rate and also are of extreme severity in the construction industry.
    Furthermore, due to such considerations as noise pollution and difficulties in purchasing real estate, the trend has been toward increased construction of tunnels and other underground structures.
    Thus, it is necessary to conduct accident investigation in this field. This paper deals with accident investigation for the purpose of providing a better understanding of the nature of recent accidents at tunnel construction sites.
    New Sanyo Line Railway (from Osaka to Hakata) including many tunnels (142 in number, total length 275 km) was constructed during the period 1967-1975. The tunnels under investigation at this time are part of the New Sanyo Line Railway tunnels belong to Hiroshima and Yamaguchi prefectures respectively. (88 in number, total length 158 km) The available accident data (Reports on Industrial accidents) were gathered from Hiroshima And Yamaguchi Prefectural Labour Standard Offices and, also information concerning the construction areas was given by the Japan National Railway. The data of this reports were coded and then analyzed and listed by computer. (FACOM 230-35)
    The conclusions in this investigation are as follows ;
  (1) The average age of injured workers was approximately 40. This reflects the fact that the age of workers in the construction sites has been increasing. The fewer the years of experience of the workers, the more occurrences of accidents. This would reveal that workers cannot adequately accept the rapid transition in technology of tunnel construction. (Fig.1, 2, 3, Table 1, 2)
  (2) The characteristics of injuries such as severity and the part of body injured depend on the kind of occupation, working activities and agents involved. (Fig.4, Table 3, 4, 5, 6)
  (3) The frequency distribution of accidents with regard to time shows the peaks after 3 or 4 hours from beginning work or from rest periods similarly to other industrial accidents. (Fig.5)
  (4) The cross classification of work activities and agents involved in the injury expresses that about half of the accidents occur in excavation work, 24% are in transportation work 15% are in concrete lining work. The most frequent agents of injury are falling rocks. Transportation equipment is involved in a large percentage of fatal accidents. The average number of days charged for one work injury was 26.5 days. (Table 7, 8, 9 )
  (5) In excavation work the highest risk area is Heading and the most dangerous activity is steel arch support setting. On the other hand, drilling and chopping of rock are the most dangerous activities in Upper half cut. (Table 10, 12, 13)
  (6) Large scale collapses of roof rarely occurred, rather most of the injuries related to rocks resulted from rocks of less than 30 cm in diameter falling less than 3 m. The injury rate due to rock falls was not influenced by the rock mechanical properties of the rock strata. It was approximately 3.0 accidents / km except in soil I. Also the injury rate was influenced not by the length of tunnel but by the method of excavation. (Fig.6, 7, Table 11, 14)
  (7) One of the typical accidents caused by transportation equipment in the tunnel is being struck by or caught by locomotive, wagon, and other movable equipment or struck by any other construction materials moved by the contact with transportation equipment. The latter cause, i.e. struck by materials, includes about 40 % of all contact accidents related to transportation equipment. Material handling in transportation work should be given much caution to prevent accidents because many injuries occur in connection with it. (Fig.8, 9, Table 15, 16)
  (8) The classification of the location in tunnel where accidents related to transportation take place suggests that considerable numbers of accidents occur at the surface and entrance of a tunnel. They are more frequent than the accidents in heading. Tunnels which have plural entrances indicate higher injury rates than tunnels that have one entrance. (Table 17, 18)
  (9) The injury rate with respect to transportation accidents is influenced by the systems of transportation such as rail systems or truck systems. The truck system's injury rate is about 1/3 less than that of the rail system's. Also, the injury rate is influenced by the method of excavation and the length of the tunnel. The longer the tunnel excavated, the higher the injury rate indicated. (Fig.10, Table 19, 20)

RBalance of Human Body Working on Electric Pole

TN-76-8

Eiki YAMANO, Yasuyuki TABATA and Masayoshi TERASAWA

: Accidents often result from the unbalance of human body working on the electric pole. A temporary platform, however, fitted on a pole may contribute to the balance of such worker's body. In the present research, the problems that on what factors and how the balance of body on the platform depends, are investigated.
    First, an external force tolerable for static balance is theoretically analyzed, on the assumptions that the human body is rigid and the shoes are not slippery on the platform. Stability of standing posture on the platform is also discussed on the same assumptions.
    The tolerable force is a function of its direction, body weight, position of feet, centre of gravity of body, and point of force action. Force in opposite direction to the safety rope is proportional to the tension of safety rope. The stability of standing posture is further influenced by the height of the centre of gravity and the length of a safety rope.
    Experiments have been made on the maximum strengths of subjects in various conditions and the results of them showed a good agreement with the theoretical ones.
    Secondly, in case where the shoes may be slippery, the friction between the shoes and the platform was taken into account.
    Using a theoretical model which is constituted by rigid segments and has movable hip joints allowing the feet to move respectively, the value of coefficient of friction required to keep balance has been estimated to be about 0.6-1 in typical conditions.
    Measurements of the coefficient of friction were made on site in various conditions, and the majority of the data was observed in the range of 0.6 to 1.
    It can be considered that the platform or the circumstances of labor leaves something to be improved. The results in this article is available to improve it.

Research on Flame and Detonation Arresters (First Report) --Quenching of Acetylene-Air and Propane-Air Flames in 2-and 4-inch Pipes--

TN-76-9

Toshihiro HAYASHI

: It has been recommended that any pipeline in which an ignition of explosive gas mixture may arise should be equipped with flame arresters in order to arrest flames in their earlier stage of development so that the wide spread of explosion effect could be avoided. Although, in fact, available informations are not yet satisfactory for the design of arresters. The author has reported on quenching of flames by wire gauges and others in 1-inch pipes. But, as flame behaviours depend largely on geometric dimensions of the pipe in which flames travel, quenching behaviours in pipes with larger diameters should also be considered to differ from those in smaller ones. Effects of several factors on flame quenching in 2- and 4-inch pipes are studied in this report.
Experiments are carried with stoichiometric mixtures of acetylene-air and propane-air. Each test enclosure is composed of ignition chamber, whose diameter and length are varied, and a protected chamber. An arrester housing containing quenching elements (wire gauges or sintered metals) is set between those two chambers. Premixed mixture of a desired pressure is ignited in the ignition chamber and then it is observed if an explosion occurs in the protected chamber. Increasing the initial gas pressure by step, a minimum pressure, beyond which flames always pass through the quenching element, is determined and this pressure is used as a measure to evaluate a quenching ability of the element under test.
Propane flames are far more easily quenched than those of acetylene, and the transition to detonation should be taken into account for the latter flames. Effects of the following factors on flame quenching are discussed : the length and diameter of the ignition chamber, mesh size and thickness of quenching element, spacing distance of wire gauze layers, and the distance between a igniter and quenching gauges. It is also shown that whether ends of chambers are opened to atmosphere affects largely on flame quenching behaviours.

On Study of Particuler Frame Pipe Scaffolding --Frame pipe scaffolding with narrow open-end frame--

TN-76-10

Yoshitada MORI, Katsunori OGAWA and Yoshimasa KAWAJIRI

: Recently frame pipe scaffolding, of which open-end frame is narrower than that of usual type, have used in construction sites.
    They are divided into two types, according to sites which they are used, one is what is chiefly used in construction sites of reinforced concrete or steel structures (type A), another is what is mainly used in sites of wooden structures as dwelling house (type B).
    Now, in order to clear structural stability of these scaffoldings, we carried out the efficiency test of open-end frames and buckling test in actual size.
    The results of this experimental study may be summarized as follows.
    About type A
  (1) Structure, dimensions and materials of open-end frame are as Fig.11.
  (2) Allowable load of open-end frame shall be less than 3,500 kg / 1 frame.
  (3) Superimposed load shall be less than 350 kg / 1 span.
  (4) Scaffoldings shall be anchored to wall and others every 5 floors, 4 spans.
    About type B
  (1) Structure, dimensions and materials of open-end frame are as Fig.15.
  (2) Allowable load of open-end frame srame shall be less than 1,500 kg / 1 frame.
  (3) Superimposed load shall be less than 250 kg / 1 span.
  (4) Scaffoldings shall be anchored to wall and others every 3 floors, 3 spans.
  (5) Scaffoldings shall be less than 5 floors in height.

JNIOSH