Abstract of Special Research Report (SRR-84-1)

National Institute of Occupational Safety and Health, Japan

Special Research of Integrated Preventive Countermeasures for Overturnning Accidents Transportation Machinery


Ikuo MAE

: Movable cranes and fork lift trucks have been widely used in many industrial places because of their efficient mobility. Accordingly overturnning accidents of these transportation machinery have occasionally happened due to their unstable conditions. The number of accidents associated with cranes have been decreasing, yet they still account more than 6,500 accidents. Among them, overturnning is the major cause of accidents which account for 10% of all movable crane accidents. Overturnning of movable crane have a high potential of risk that will induce serious accidents not only to workers in a workplace but to pedestrians in a city area.
    One type of safety measure against overturnning accidents is fastening a safety device as a moment-limiter to a movable crane. This was regulated in 1972 and have played an important role in decreasing overturnning accidents. This safety device, however, is concerned only to the operation on a firm ground so that safe operation on a soft, weak ground mainly depends on skill of the workers.
    So far as a fork lift truck concerned, this machine is more likely to be unstable compared with other transportation machinery due to its shape characteristics. Hence many overturnning accidents also take place during its operation.
    Considering the situation of movable cranes and fork lift trucks, it is necessary to conduct research on safety measurement against overturning accidents.
    The main objective of this research project is to develop a safety device that will prevent movable cranes from overturnning on a soft ground and to develop a protective device that will prevent the operator from getting injury. For this purpose, comprehensive research work have been carried out as follows :
  1) A study of evaluation of ground characteristics and of dynamic behaviour of movable cranes on a slope to establish the requirement of the safety devices.
  2) Developing an outrigger for the soft ground use and evaluate its effectiveness from a safety point of view.
  3) Clarify the mechanism of overturning by making use of an actual fork lift truck and develop a safety device to protect a operator from ejection and moderate the impact on to the ground.

On The Load-Settlement Characteristics of Soft Ground (Kanto-Loam)

Noriyuki HORII and Etsuji YOSHIHISA

: Accidents of mobile cranes due to overturning have been increasing as mobile cranes are widely used in construction sites and construction in soft ground increases. It is considered that overturning accidents of mobile cranes in soft ground are caused by large settlement of outrigger.
    Therefore, it is necessary to establish the effective countermeasures against overturning accidents in soft ground.
    The purpose of this study is to investigate the geotechnical characteristics of soft ground, load-settlement relationship etc., which may provide the fundamental understanding of soft ground behavior.
    A series of loading tests are carried out both in model ground and in-situ.
    The main results obtained in this study are as follows.
  1) Load(load intensity) -settlement curves of Kanto-loam ground may be approximated by hyperbolae with high degree of accuracy.
  2) Assuming hyperbolic relationship between load and settlement, settlement of outrigger may be estimated easily.
  3) From the loading tests using ring type plates, when the gaps between rings are relatively small, it is found that same bearing capacity may be expected in spite of decrease of contact area.
  4) In case of "a" type plate, vertical stress in soil concentrates both at central part of loading plates and in the shallow depth with load intensity increasing.
  5) "d" type plate, however, stress distribution still remains uniform.
  6) Undrained shearing strength τf of Kanto-loam may be expressed as follows,
    τf = Cu + σ tan φu
      where Cuu : Mohr-Coulomb strength parameters.
             σ: normal total stress.

Prevention of Overturning of Mobile Crane (Analysis on the Movement of Crane Slewing on the Slightly Inclined Ground) (Trialmanufacture of Outriggers for Prevention from Overturning)

Yutaka MAEDA

: Overturning is one of the most typical accidents of mobile cranes. Although cranes have been designed under the assumption that the grounds they are working on are absolutely horizontal and firm, but the grounds on which cranes actually work are not always on such conditions.
    This report deals with at first the dynamic stability of crane. The condition for that maximum swinging angle of the load gives the maximum falling moment is clarified that the maximum swinging angle (θ0) should be less than 37 degrees and less than ψ/4, where ψ is the angle of elevation of jib top at the fulcrum for falling down.
    And the differential equations of motion for the crane slewing on the slightly inclined ground are introduced from Lagrange's equation and are numerically solved with digital computer. And as for the moment of inertia of upper slewing body, it can be determined only by measuring oil pressure of slewing motor and angular acceleration of slewing.
    Finally, two types of outrigger-floats that will prevent cranes from overturning are made by trial and the competence of them are investigated through the experiment using 11 tonf hydraulic truck crane.

Prevension of disaster caused by upset of forklift trucks

Kiyoshi FUKAYA and Noboru SUGIMOTO

: A forklift truck is a useful machine. But it has a defect, i.e. it is unstable essentially. Because of the defect, upset accidents of forklift trucks occur. To protect an operator of a truck from such accidents, there is a need for safety devices.
    A mathematical model of a forklift truck was made up and the degree of hazard of lateral upset during turning on the inclined ground was estimated. And the upset time was estimated to be 0.5-1.5 second.
    The cab mockup with a man was inclined from 0° to 50° and the escape action of men was studied.
    Several ways of safety devices were compared with each other. And one of them, i.e. air bag system was manufactured by way of trial. This system consists of air bag deploying subsystem and sensor of upset. This works in the following way. When a fork lift truck upsets, the air bag will deploy and close the sidespace of truck and prevent the operator's ejection or runaway out of the truck. The air bag also works as cushion for the operator.
    To detect the upset, the pendulum-type sensors were tested. The sensors detected inclination well. The sensors picked up false signals due to the vibration of forklift truck, but these were eliminated to usable level by use of lowpass filter.
    There are many ways to deploy the air bag. In this research the compressed air was used. In this method the deployment time were 0.5 - 1.0 second. This is comparable to the time of upset and air bag will deploy until the end of upset.
    In the experiment, the air bag reduced the shock of dummy's strike on the ground to the human tolerance level.
    In this research it was certified the air bag is useful to protect operator from injury.

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