Abstract of Technical Note (TN-82)

National Institute of Occupational Safety and Health, Japan

Safety problems with Industrial Robots and FTA

TN-82-1

Kunitomo KAWAGUCHI, Noboru SUGIMOTO, Yoshinobu SATO and Teizo HAKAMAZUKA

: Considering that Robots take over dangerous job, it seems ironic to say that the robot also poses certain safety problems. Today more than 90,000 robots are used in Japan. We have already experienced some accidents caused by robots. And much more incidents occurred.
    These experiences show that robots have not been enough reliable and it sometimes leads to accidents, and that there always exist many kinds of manual operation done by workers around robots and peripheral equipments, in spite of robots as tools of labor-saving.
    In this study, we have first classified manual operation related to robots, namely,
  (a) Transport, Installation
  (b) Preparation
  (c) Programming (Teaching)
  (d) Testing
  (e) Starting
  (f) Job during an automatic operation
  (g) Trouble-shooting
  (h) Maintenance, Adjustment
    Secondly, we have identified the possible hazards involved in these manual operations by means of an effective check-list for inductively pinpointing the type of accidents. Table 6 gave a result which make it possible to examine the accidents that may occur in a man-robot operation system.
    Thirdly, the causes of accident were deductively analyzed, and Fault Tree Analysis (FTA) shown in Fig.3 has been obtained as a result. By this FTA, it has become possible to grasp clearly the causes of the accident that may occur in a man-robot operation system.

Accudent Prevention on Walkways and Stairways (4th Report) --Analysis of Annual Seaths Xaused by Falling Accidents on Stairways, Floors, Ladders and Scaffolding--

TN-82-2

Hisao NAGATA

: According to the vital statistics of Japan, 4420 persons were killed by falling accidents over 1980. And the annual deaths is increasing every year. In this report, the tendency of the annual deaths by falling accidents was analyzed. The reason why the number of deaths is increasing was made clear, and the future trend was estimated from the results of analyses.
    The results of the accidental deaths such as on stairways, floors, ladders and scaffolding were taken as the subjects of a survey in this report. Except for the fatal accidents on scaffolding, the death ratio markedly increased for ages over 70. Considering that the aged population is gradually increasing, it follows that the number of deaths will continue to increase in proportion to the aged population.
    The number of laborers' fatal deaths has been decreasing over the past 10 years, and now the trend is slowing down. It was clearly estimated that laborers' fatal accidents will increase according to the rising trend of the aged population and the job retirement age.
    Other characteristic points of each accident are referred to in this report such, as monthly trends, the nature of injury, the sites of accidents etc. The chief results were as follows.
  1) Annual male deaths of stair-accidents increased from 65 in 1950 to 451 in '80 (6.9 times); female deaths from 36 to 217 (6.0 times). As for floor-accidents they increased from 268 to 615 in males; from 121 to 806 (6.7 times) in females. Most of the sufferers received injuries at home.
  2) Deaths suffered mostly by intracranial injury and fracture of skull and face bone except for floor-accidents of females, in which they were mostly injured by fractures of lower limbs.
  3) In 1980, 9.1% of the population were over the age of 65. In the year 2025, the aged over 65 will constitute about 21.3% of the population. In regard to stair-accidents, estimated deaths during the year based on the death ratio of 1979-80, will be from 451 to 825 in males; from 217 to 637 in females. In the case of floor-accidents, from 615 to 1844 in males; from 806 to 3414 in females.

Accident Analysis --Failure of U-type Hangers made of Reinforcing Steel bar Fixed to Reinforced Concrete Beams--

TN-82-3

Masazumi TANAKA and Yutaka MAEDA

: In the afternoon of the 1st of February 1981 an accident happened on a building construction field in Tokyo. A couple of U-type hangers made of reinforcing steel bar and fixed to the ceiling concrete beams of first story were suddenly broken, when workers of a transport company, using those hangers, were hanging down a load of 4 ton weight (air conditioner) onto the floor of basement 2, through a hatch opened in the first floor. Two workers operating chainblocks on the load fell down together with it on the floor of basement 2 about 6 meters below, and were hardly wounded.
    In this report, the causes of this accident are investigated through spot inspection, fractographic analysis of the fractured surfaces of hangers, estimation of loading condition of hangers, and certification test of their strength containing Charpy V-notch test and so on.
    The main results of this investigation are summarized as follows ;
  (1) Though a reinforcing steel bar has fairly high tensile strength as a material, it is originally poor in ductility when compared with usual structural steels and loses its strength remarkably through embrittlement if cold worked. Therefore, it is very dangerous to use this kind of steel in the cold worked condition for main load-sustaining members.
  (2) The stress condition of U-type hanger is harder when obliquely loaded with so large angle from the vertical direction as in this case (about 60°) than when vertically loaded. The tensile strength of the hanger is highest in vertical direction, and decreases remarkably with the increase of the loading angle.
  (3) The fracture accident of this case occurred due to the strength shortage of hangers, because the hangers were used under the oblique loading condition which is hard for them, and in such low strength state through cold working as mentioned above.
  (4) Such situation came arise from the fact that the user of those hangers had not shown the oblique loading condition, and the designer had adopted the reinforcing steel as hanger material, not knowing such strength property of this steel as mentioned in (1).
  (5) By the way, in many construction fields, the reinforcing steel bars are even now used easily as the materials for hagers and anchors similar to those in this case, in the state where their strength is not certified. It is necessary for this dangerous state to be improved immediately.

Flameproofness of the Single-hole Packing Type for Leading Conductor into a Flameproof Enclosure

TN-82-4

Katsuhiro SAKAMUSHI

: The flameproof packing type is one of the methods used for leading electrical conductors into a flameproof enclosure. In this type, a portion of the wall of flameproof enclosure is provided a packing box, packing, pass-through conductors, washers and packing gland to ensure the flameproofness.
    In practice, the above flameproofness is ensured in such a way that when the packing gland is compressed and deformed, sufficient tightness will develop between the surfaces of packing and those of packing box and conductor.
    From the viewpoint of flameproofness, the more strongly the packing is compressed, the better it will be; but from the viewpoint of avoiding excessive deformation of the sheath of conductor, the packing shall be compressed rather moderately.
    In order to find out appropriate conditions for compression of the packing in terms of relations between the packing compression rate, the deformation of the sheath of conductor and the flameproofness, visual inspection, X-ray test and explosion test for flameproofness have been carried out on a model of a flameproof, single-hole packing type.
    As the result, it has been concluded through the tests that appropriate compression rate should be in the range of 5-10 percent to meet the requirement for the flameproofness and the deformation of the sheath of conductor.